Deep-water nutrients mediated glacial-interglacial diatom productivity in the Indian Polar Front Zone of the Southern Ocean

Deep-water nutrients mediated glacial-interglacial diatom productivity in the Indian Polar Front Zone of the Southern Ocean | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Deep-water nutrients mediated glacial-interglacial diatom productivity in the Indian Polar Front Zone of the Southern Ocean Aditi Nautiyal, Sunil Kumar Shukla, Rahul Mohan This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7470794/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract High diatom blooms characterize the Crozet and Kerguelen Plateaus of the Southern Ocean (SO) due to the natural iron fertilization despite the co-limitation of iron and light in the Polar Front Zone (PFZ). However, information on glacial-interglacial diatom productivity and associated biological carbon pump from this region is sparse. We present a diatom productivity record to decipher the palaeoceanographic changes for the past 40 ka using sediment core SN 2 (47°S and 57°30’E) amidst the Crozet and Kerguelen Plateau. Our results show the highest diatom productivity during early Marine Isotope Stage (MIS) 2 (29.5–23 ka), lowest during the deglacial-Holocene periods (18 − 7.8 ka), with lower intermediate diatom productivity during late MIS 3 (40-29.5 ka) and the Last Glacial Maximum (LGM, 23 − 18 ka). The abundances of the permanent open ocean zone (POOZ) group diatoms covaried with diatom productivity and showed an inverse correlation with water stratification group diatoms. The patterns of diatom productivity and the POOZ group diatoms do not strongly correlate with the fluxes of dust and iron. Based on the inverse correlation between the diatom groups from the PFZ (core SN 2 ) and the Antarctic zone, we suggest that higher diatom productivity during early MIS 2 could be due to the availability of nutrient-rich southern waters through SO upwelling as a result of the northward Antarctic Polar Front (APF) migration. Conversely, the lower intermediate diatom productivity during late MIS 3, the LGM, and the deglacial-Holocene periods could have resulted from the unavailability of southern nutrient-rich waters due to inefficient APF migration and weaker SO upwelling. We propose that the Antarctic Circumpolar Current-driven APF migration and the intensity of upwelling possibly resulted in heterogeneous diatom productivity in the Indian PFZ of the SO. Consequently, the availability of deep-water nutrients (iron and silicate) might have controlled diatom productivity and was responsible for the strengthening/weakening of the biological carbon pump. Geology Oceanography Paleoecology Climatology Antarctic Circumpolar Current Frontal Migration Upwelling Diatom Community Composition Principal Component Analysis Heterogeneous glacial diatom productivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction The co-limitation of iron and light controls the primary productivity in the Southern Ocean (SO) (Blain et al., 2007 ; Boyd et al., 2007 ; de Baar et al., 1995 ; Martin, 1990 ; Martin et al., 1990 ). Among the phytoplankton, diatoms are dominant organisms, which significantly contribute to the burial of biogenic silica and organic carbon to the deep sea and help in lowering atmospheric carbon dioxide (Buesseler, 1998 ; Buesseler et al., 2001 ; Tréguer et al., 2021 ; Smetacek et al., 2012 ). Apart from iron, nitrate and silicate are also required by the diatoms to grow, which show a north-south gradient in the SO. For instance, nitrate is mostly constant from the Sub-Antarctic Zone (SAZ) to south of the Antarctic Polar Front (APF) until the sea-ice zone, whereby silicate availability is more in the south of the APF and decreasing towards the north of the APF until the SAZ (Brzezinski et al., 2001 ; Nelson et al., 2001 ; Quéguiner and Brzezinski, 2002 ). Thus, iron and silicic acid concentration co-limits diatoms’ growth and abundances in the SAZ of the SO (Boyd et al., 2001 ; Lannuzel et al., 2011 ). However, the Polar Front Zone (PFZ), a region located between the SAF and APF (Park et al., 2019 ), is demonstrated with the northward silica supply during the glacial periods that led to increased diatom abundances and lower atmospheric CO2 (Matsumoto et al., 2002 ). The sediment trap samples from the PFZ indeed suggest diatoms’ dominance and carbon export compared to the SAZ, where calcareous organisms are preponderant (Rigual-Hernández et al., 2015 ). High diatom blooms characterize the Crozet and Kerguelen Plateaus in the SO due to the availability of natural iron fertilization (Blain et al., 2007 ; 2008 ; Planquette et al., 2011 ; Bowie et al., 2015 ). Diatom blooms were recorded explicitly from the north of the Crozet (Salter et al., 2007 ) and east of the Kerguelen Plateaus (Blain et al., 2007 , 2008 ). However, the diatom blooms from the Kerguelen Plateau were reported due to the availability of iron and major nutrients in the surface waters supplied from iron-rich deep water (Blain et al., 2007 , 2008 ; Park et al., 2008 , 2014 ). The inferred diatom community composition from the Kerguelen Plateau water samples similarly suggested a role of silica in conjunction with iron for defining diatom bloom (Armand et al., 2008a ; Lafond et al., 2020 ). The surface sediment samples, on one hand, from the northeastern Kerguelen Plateau, revealed the correlation of diatoms with iron, and on the other hand, from the off-Plateau samples near the Polar Front, with diatom species of Permanent Open Ocean Zone with insignificant role of dust-bearing iron (Armand et al, 2008b ). Despite these crucial studies, which have implicated the role of diatoms and the drawdown of atmospheric carbon dioxide, the glacial-interglacial diatom productivity records from the Crozet and Kerguelen Plateaus are sparse and represented by a few studies that are near the north of the Crozet Plateau (Nair et al., 2015 , 2019 ). The studies from the Indian sector of the SO have suggested increased diatom productivity during the glacial periods (Marine Isotope Stage, MIS 2–3) in the sub-antarctic zone (SAZ) and related it to the Southern Hemisphere Westerly Winds (SHWW) driven northward migration of the APF (Nair et al., 2015 , 2019 ; Ghadi et al., 2020 ). However, lower diatom productivity during MIS 2–3 is suggested from the SAZ, which resulted from the ineffective northward APF migration and silica supply (Shukla et al., 2023 ). Such regional differences in the diatom productivity were attributed to the bottom topographic variations in the Indian sector of the SO, where lesser interactions of fronts limit the supply of nutrients (Civel-Mazens et al., 2021a , b ), resulting in lower diatom productivity (Shukla et al., 2023 ). The phytoplankton productivity in the PFZ and SAZ of the Atlantic and Pacific sectors of the SO was higher during the glacial periods and strongly correlated with dust-bearing iron availability. Conversely, low phytoplankton productivity was due to reduced dust-bearing iron (Anderson et al., 2014 ; Anderson et al., 1998 ; Jaccard et al., 2013 ; Kohfeld et al., 2005 ; Lamy et al., 2014 ; Martínez-Garcia et al., 2011 ; Martínez-Garcia et al., 2009 ; Shoenfelt et al., 2018 ; Watson et al., 2000 ; Ziegler et al., 2013 ). Although the southwest Indian sector of the SO is too far to receive dust-bearing iron from South America/South Africa (Lamy et al., 2014 ), a study from the east of the Kerguelen Plateau recorded higher glacial lithogenic/iron fluxes, and attributed it to the South Africa and/or Kerguelen Plateau (Thöle et al., 2019 ). However, despite higher iron fluxes, the export production of organic carbon was lower during the glacial periods and therefore showed a muted response (Thöle et al., 2019 ). In this vein, we present a diatom productivity record from the PFZ of the Indian sector of the SO for the past 40 ka using a sediment core (47°S, 57.3°E), located between the south of the Crozet and the west of the Kerguelen Plateaus. Our study aims to explore 1) whether glacial-interglacial diatom productivity in the PFZ of the Indian sector of the SO correlates with dust-bearing iron, 2) the effect of natural iron fertilization on overall diatom productivity and diatom community composition, and 3) the role of frontal migration in driving the glacial-interglacial diatom productivity. 2. Materials and Methods We analysed diatoms preserved in a sediment core SN 2 (47°S and 57°30’E; length – 3.8 m; water depth − 3470 m), which was retrieved from the Indian sector of the Southern Ocean, on board ORV Sagar Nidhi . The sediment core is located in the Polar Front Zone (PFZ), a zone between the Antarctic Polar Front (APF) at the south and the Sub-Antarctic Front (SAF) at the north (Park et al., 2019 ). The core location is amidst two major bathymetric features: Crozet Island to the left and Kerguelen Plateau to the right. However, the core location is away from both these bathymetric features and facing the corridor of the APF, which is straight in front (Park et al., 2019 ). Although the PFZ is generally considered to be iron and silica-deficient (Blain et al., 2007 ), the nutrient-rich Antarctic surface water might be responsible for high productivity when the APF shifts northward during the glacial periods (Shetye et al., 2014 ). The age model of core SN 2 is based on radiocarbon dates and was used after Shetye et al. ( 2014 ). A total of 75 samples were analysed for the diatom study at a 2 cm interval of sediment core (time resolution ranged between ~ 280 years and ~ 600 years) spanning the last ~ 40 ka. The diatoms were extracted from the sediment following the method of Crosta et al. ( 2020 ). By following this method, 1–2 gm of sediment sample (depending on the abundances of diatoms after examining smear slides) was treated with 20 ml Oxidative solution (870 ml H 2 O 2 at 35% + 130 ml distilled water + 42 g Na 4 P 2 O 7 .10H 2 O for 1 litre solution) to remove the organic material and then with 20ml of 30% HCl for the removal of carbonates. The sample was washed thrice with distilled water through centrifugation, and then the final sample was diluted to 50 ml. A known amount of sample was pipetted onto a coverslip and, after drying, mounted onto the slides with NOA61 mounting medium. Diatoms were counted following the method described in Crosta and Koç ( 2007 ) using an Olympus BX50 light microscope with an oil immersion lens (x1000 magnification). The preservation of diatoms in all the samples was excellent, so a minimum of 300 diatoms in each sample was counted. The relative abundances of all the diatoms were used for the Principal Component Analysis (PCA), which allowed the identification of the co-variation of the species over the studied time period and, therefore, significant diatom species in the entire diatom assemblage. Total diatom abundances (absolute abundances) were calculated using the equation detailed in Crosta et al. ( 2008 ). Principal Component Analysis (PCA) and One-way ANOVA tests were applied to the diatom productivity data of sediment core SN 2 . PCA was applied to the relative abundances of diatoms to identify the significant species that co-varied through time, whereas one-way ANOVA tests were applied to total diatom abundances and relative abundances of significant diatom species to identify the dis(similarities) across different time periods for the last ~ 40 ka. All the data are provided in (Supplementary file A). 3. Results 3.1 Total diatom abundances The total diatom abundances in core SN 2 ranged between 5.47 x 10 6 valves/g and 448.05 x 10 6 valves/g of dry sediment (mean of 157.44 x 10 6 valves/g) for the studied time interval of 39.21–7.86 ka (Fig. 2 A). Diatom abundances were highest during early Marine Isotope Stage - MIS 2 (29.5–23 ka, mean of 253.61 x 10 6 valves/g, range 120.97 x 10 6 − 448.05 x 10 6 valves/g, n = 23), lowest during deglacial to Holocene period (18-7.86 ka, mean of 64.69 x 10 6 valves/g, range 5.47 x 10 6 − 153.79 x 10 6 valves/g, n = 17), and intermediate values were recorded during late MIS 3 (39.21–29.5 ka, mean of 146.46 x 10 6 valves/g, range 104.99 x 10 6 − 232.57 x 10 6 valves/g, n = 26) and during the Last Glacial Maximum – LGM (23–18 ka, mean of 118.62 x 10 6 valves/g, range 43.74 x 10 6 − 154.49 x 10 6 valves/g, n = 9) (Fig. 2 A). One-way ANOVA tests confirm this variability and are graphically shown in Fig. 3 . The millennial-scale variability of diatom abundances was highest during the deglacial to Holocene period. Conversely, early MIS 2 and the LGM periods, although represented with much lesser variability of diatom abundances compared to the deglacial and Holocene periods, were still higher than late MIS 3. These observations suggest heterogeneity in diatom abundances among the different glacial periods, such as late MIS 3, early MIS 2, and the LGM. 3.2 Diatom Community compositions The diatom community in the core SN 2 is composed of a total of 31 species/species groups. To identify the major diatom species that co-varied through time and contributed to the diatom productivity, the principal component analysis was applied, which yielded six major species, namely, Fragilariopsis kerguelensis , Thalassiosira lentiginosa , Thalassiosira antarctica , Eucampia antarctica , Chaetoceros resting spores, and Thalassiothrix antarctica . The initial three principal components (PC1, PC2, and PC3) contributed to > 90% of the data variance. PC1 is represented by 69.71% variance and comprises F. kerguelensis with the highest positive factor loadings, whereas T. lentiginosa , E. antarctica , and Thalassiosira antarctica have significant negative factor loadings. PC2 is represented with 14.15% variance, including Thalassiosira antarctica with positive factor loadings, whereas T. lentiginosa and Thalassiothrix antarctica have significant negative factor loadings. PC3 is represented with 6.73% variance and included Thalassiosira antarctica and T. lentiginosa with positive factor loadings, while Chaetoceros resting spores and Eucampia antarctica had significant negative factor loadings. PC1 mimics the pattern of total diatom abundances in core SN 2 , indicating the substantial contribution of F. kerguelensis , T. lentiginosa , E. antarctica , and Thalassiosira antarctica. 3.3 Diatom species’ absolute abundances and diatom groups Based on the PCA, the absolute abundances of the six significant diatom species that mainly co-varied through time and ultimately superimposed on the diatom productivity are presented. In core SN 2 , both F. kerguelensis and T. lentiginosa absolute abundances mimic the pattern of total diatom abundances and were highest during early MIS 2, lowest during the deglacial to Holocene period, and were intermediate during late MIS 3 and the LGM (Fig. 2 C and 2 D). The absolute abundances of Thalassiothrix antarctica also showed a similar pattern to that of F. kerguelensis and T. lentiginosa , except for the LGM, where its abundances remained high, like early MIS 2 (Fig. 2 H). The other three species, namely, Thalassiosira antarctica , Eucampia antarctica , and Chaetoceros resting spores’ absolute abundances showed inverse correlation with F. kerguelensis , T. lentiginosa , and Thalassiothrix antarctica , as their abundances were higher during late MIS 3, whereas lower during early MIS 2, LGM, and deglacial to Holocene (Fig. 2 E, 2 F and 2 G) . Among the above-noted species, F. kerguelensis , T. lentiginosa , and Thalassiothrix antarctica are the permanent open ocean zone (POOZ) group diatoms. In contrast, Thalassiosira antarctica , Eucampia antarctica , and Chaetoceros resting spores are water stratification group diatoms. Both POOZ and water stratification diatom groups showed strong inverse correlation from late MIS 3 to the Holocene (Fig. 4 ). 4. Discussion In the core SN 2 from the Polar Front Zone (PFZ) of the Indian sector of the Southern Ocean (SO), the total diatom abundances were highest during early MIS 2 and conversely lowest during the deglacial-Holocene periods, with lower intermediate values during late MIS 3 and the LGM periods. Such patterns of diatom productivity in core SN 2 are confirmed with the first principal component PC1 (Fig. 5 A) and one-way ANOVA tests (Fig. 3 A). If high diatom productivity during the glacial periods results from the northward APF migration that supplied increased nutrients through upwelling (Nair et al., 2019 ; Ghadi et al., 2020 ; Shukla et al., 2023 ) or through increased dust-bearing iron (Shukla et al., 2013 ; Nair et al., 2020 ), then increased diatom productivity should have been found throughout the glacial period of MIS 2–3 in core SN 2 . However, the magnitude of the diatom productivity is much larger during early MIS 2 compared to the lower intermediate diatom productivity found during late MIS 3 and the LGM periods. The heterogeneous diatom productivity during the glacial periods may have been related to non-synchronous northward APF migration or variable dust-bearing iron availability. We, therefore, explore the possible reasons for the heterogeneous diatom productivity patterns found in the PFZ core SN 2 from late MIS 3 to LGM. 4.1 Correlation of diatom Groups from Polar Front Zone (Core SN 2 ) and Antarctic Zone (Core PS2606-6): POOZ diatoms vs water stratification group diatoms The total diatom abundances in core SN 2 co-varied with the POOZ group diatoms from late MIS 3 to the deglacial-Holocene periods (Figs. 5 A and 5 B). Conversely, POOZ group diatoms were inversely correlated with water stratification group diatoms in core SN 2 (Fig. 4 ). An inverse correlation of both groups of diatoms from core SN 2 is also found when comparing with the Antarctic zone sediment core PS2606-6 from the Conrad Rise at 53°S (Jacot des Combes et al., 2008 ) over the studied period (Figs. 5 B and 5 C). Consequently, the POOZ group diatoms in core SN 2 were higher during early MIS 2, when core PS2606-6 showed lower values (Fig. 5 B). Conversely, the POOZ group diatoms in core SN 2 were lower during late MIS 3 and the deglacial-Holocene periods, when core PS2606-6 showed higher values. Interestingly, both cores showed almost comparable POOZ group diatoms during the LGM (Fig. 5 B). The water stratification group diatoms are also inversely correlated between the two cores, with higher abundances during late MIS 3, the LGM, and deglacial-Holocene periods in core SN 2 , when core PS2606-6 showed lower abundances (Fig. 5 C). Conversely, lower abundances were found during early MIS 2 in core SN 2 , when core PS2606-6 showed higher abundances (Fig. 5 C) Thus, it seems that the abundances of the POOZ and water stratification group diatoms in core SN 2 from late MIS 3 to the Holocene wax and wane through the northward APF migration due to the variable oceanographic conditions and the related nutrient availability, which are needed to be explained. 4.2 Correlation of diatom productivity with nutrient availability The studies from the SAZ of the Atlantic and Pacific sectors of the SO suggested an imperative role of dust-bearing iron-availability, which resulted in increased productivity during the glacial periods (Anderson et al., 2014 ; Jaccard et al., 2013 ; Martínez-Garcia et al., 2009 , 2011 , 2014). However, a muted response of export production in the Indian SAZ was observed despite high iron fluxes through the dust from the Kerguelen Plateau (Thole et al., 2019). The Indian SAZ similarly showed low diatom productivity during the glacial periods of MIS 2–3, which was related to frontal shifts and silica availability (Shukla et al., 2023 ). The high diatom productivity of core SN 2 during the early MIS 2 period, when the POOZ group diatoms ( F. kerguelensis , T. lentiginosa , and Thalassiothrix antarctica ) were high, reveals a strong correlation with high fluxes of dust (Lambert et al., 2008 ) and iron (Thole et al., 2019). The higher δ 15 N values (Ai et al., 2020 ) and lower values of δ 30 Si (Dumont et al., 2020 ) hint towards the high nitrate utilization in presence of iron-repleted conditions, which could have resulted in high diatom productivity (Fig. 5 D). Conversely, low diatom productivity during the deglacial-Holocene periods, when the POOZ group diatoms were low, is in concordance with the lower δ 15 N values (Ai et al., 2020 ) and higher values of δ 30 Si (Dumont et al., 2020 ), when both dust and iron fluxes were low (Lambert et al., 2008 ; Thole et al., 2019) (Fig. 5 ). However, late MIS 3 and the LGM periods showed lower intermediate diatom productivity in core SN 2 , when the POOZ group diatoms were low despite higher fluxes of dust and iron, and δ 15 N (Lambert et al., 2008 ; Thole et al., 2019; Ai et al., 2020 ) (Fig. 5 ). The higher surface nitrate consumption during late MIS 3 and the LGM suggests stratified waters (Ai et al., 2020 ) that might have promoted higher abundances of water stratification group diatoms than the POOZ group diatoms in core SN 2 (Francois et al., 1997; Sigman et al., 2004 ). The δ 30 Si records from the Indian SAZ (Beucher et al., 2007; Dumont et al., 2020 ) showed lower silicic acid availability during the glacial period and therefore suggest the dependency of diatoms on higher nitrate availability, when iron-repleted conditions prevailed (Ai et al., 2020 ). Nonetheless, higher nitrate utilization during the glacial periods compared to the deglacial and Holocene periods is evident from both the Antarctic zone and SAZ of the SO (Shemesh et al., 2002 ; Crosta et al., 2005 ) and suggests increased stratification conditions with reduced upwelling (Francois et al., 1997; Ai et al., 2020 , 2024 ). Although the opal fluxes in the Indian SAZ were higher during late MIS 3 to the LGM, suggesting greater export production linked to the high dust-bearing iron availability, the organic carbon fluxes were contrarily low, especially during MIS 3 and the LGM (Thole et al., 2019). Therefore, our data of lower intermediate diatom productivity patterns during late MIS 3 and the LGM coincide with the lower export production in the Indian SAZ (Thole et al., 2019). 4.3 Diatom productivity and frontal migration A sediment core PS2606-6 at 53°S from the POOZ of the Conrad Rise showed SST between 0°C and 2°C from late MIS 3 to LGM, whereas SST between 2°C and 5°C occurred during the deglacial-Holocene periods (Fig. 6 A) (Xiao et al., 2016 ; Civel-Mazens et al., 2024 ). The cooler SSTs of 3°C during late MIS 3 to LGM compared to the deglacial-Holocene periods could have resulted from the northward migration of the Southern Antarctic Circumpolar Current Front (SACCF – Ghadi et al., 2020 ). The subsequent migration of the APF could potentially lead to the cooler SSTs of 3–5°C in core DCR-1PC (at 46°S from the Del Cano Rise) during late MIS 3 to early MIS 2, compared to 5–8°C SSTs during the deglacial-Holocene periods (Fig. 6 A) (Crosta et al., 2020 ; Shukla et al., 2021 ). Although low-resolution, the SST range 5 to 8°C in core DCR-1PC during the LGM does not show the northward migration of the APF. Based on the SST data, although the APF migration was evident during late MIS 3 and early MIS 2 (Crosta et al., 2020 ), the diatom productivity was lower during both periods in core DCR-1PC, and this was attributed to the ineffective APF migration and silica supply (Shukla et al., 2023 ). Such lower productivity is also mirrored by the lower abundances of the pelagic SO diatom species found in the Agulhas Plateau (Romero et al., 2015 ). These contrasting evidence for the northward positioning of the APF during the glacial periods prompted us to present an APF index based on the cumulative relative abundances of F. kerguelensis, T. lentiginosa , and Thalassiothrix antarctica , as these three species are the permanent open ocean zone diatom species (Crosta et al., 2005 ; Armand et al., 2008), and can help track the past APF position (Cortese and Gersonde, 2007 ; Shukla et al., 2013 , 2016 ; Nair et al., 2019 ) at the location of PFZ for core SN 2 . The APF index applied here, including the POOZ group diatoms ( F. kerguelensis , T. lentiginosa , and Thalassiothrix antarctica ), was > 70% in core SN 2 is found only during early MIS 2, when the APF index in core PS2606-6 was < 70% (Fig. 6 B), suggesting the northward APF migration, when southern nutrient-rich Antarctic zone surface water allowed greater diatom productivity in core SN 2 by leaving nutrient-deficient and thereby lower diatom productivity in core PS2606-6. However, the APF index during late MIS 3 is 70% in core PS2606-6, suggesting no northward frontal migration (Fig. 6 B). The APF index was also 70% in core PS2606-6, suggesting no effective southward frontal migration. The LGM is an exception when the APF index in both cores shows nearly comparable values and is < 70% (Fig. 6 B), and suggests no effective frontal migration in the Indian sector of the SO. Therefore, our data suggests that early MIS 2 was only characterized by the efficient northward APF migration, and late MIS 3 and the LGM were characterized by inefficient APF migration at 47°S in the vicinity of Crozet and Kerguelen islands. Our results agree with Wu et al. ( 2021 ), who suggested the weakening of the ACC during the glacial periods of MIS 2–4. The inferences drawn for the LGM also corroborate those of Sime et al. ( 2013 ) and Ai et al. ( 2024 ), who suggested more southerly positions of the frontal systems during the LGM, which might have resulted in the lower diatom productivity found in core SN 2 . 4.4 Diatoms and Palaeoceanographic Conditions in the Indian Polar Front Zone The higher abundances of water stratification group diatoms when abundances of the POOZ group diatoms decreased during late MIS 3 and the LGM in our data suggest more stratified conditions during both these periods. Conversely, increased abundances of the POOZ group diatoms in core PS2606-6 during late MIS 3 and the LGM could have resulted from the increased surface nutrients through upwelling (Anderson et al., 2009 ; Shukla et al., 2013 , 2016 ; Xiao et al., 2016 ; Gottschalk et al., 2020 ; Sigman et al., 2021 ; Shukla et al., 2023 ). These observations suggest that there might be no efficient northward APF migration during late MIS 3 and the LGM, which led to decreased POOZ group diatoms in the PFZ core SN 2 and, conversely, increased POOZ diatoms south of the APF in core PS2606-6. The increased abundances of water stratification group diatoms in core SN 2 during late MIS 3 and the LGM suggest increased stratification conditions. Thus, the absence of ample nutrients in surface waters from the deep waters through upwelling during late MIS 3 and the LGM could have resulted in decreased POOZ group diatoms and overall lower diatom productivity in core SN 2 (Gottschalk et al., 2020 ; Sigman et al., 2021 ). The relationship of upwelling in the Indian sector of the SO is related to the intense interaction of the ACC with local bathymetry, based on which upwelling hotspots have been identified (Tamsitt et al., 2017 ). The east of the Kerguelen Plateau is proposed as an upwelling hotspot, with higher mean particle transport. Conversely, west of the Kerguelen Plateau receives lesser mean particle transport and is therefore characterized by reduced upwelling (Tamsitt et al., 2017 ). The core SN 2 from the west of the Kerguelen Plateau, where less interaction of the ACC with local bathymetry may have resulted in less surface nutrient availability and consequently lower diatom productivity. Further, the high opal fluxes during late MIS 3 and the LGM in the SAZ core located at the east of the Kerguelen Island (Thole et al., 2019) could be attributed to the contribution of iron-rich shelf waters from the Heard Island (53°S) into the surface and subsurface waters of east of the Kerguelen Island (Park et al., 2014 ) and related to the intense upwelling (Tamsitt et al., 2017 ). However, an opposite scenario persists west of the Kerguelen Plateau, where reduced upwelling conditions prevail (Park et al., 2019 ). Therefore, the observed lower diatom productivity in core SN 2 during late MIS 3 and the LGM could be attributed to the lack of nutrient-rich deep waters through reduced upwelling in the west of the Kerguelen Islands (Ai et al., 2020 , 2024 ; Civel-Mazens et al., 2024 ). The latitudinal shifting of the ACC in the SO is proposed to be more northward during the glacial periods and conversely ~ 6 degrees southward during the interglacial periods due to the strength and latitude range of the SO upwelling (Ai et al., 2024 ). Although the latitudinal shifts of the ACC were concluded not to be strongly correlated during the MIS 2–4 due to iron fertilization in the SAZ, the ACC was found to be shifted ~ 2 degrees northward during MIS 2–3, while 0 to 4 degrees southward during the LGM (Ai et al., 2024 ). Thus, there is a possibility that the location of the northern flank of the APF was at 49°S during late MIS3 and the LGM, currently located at 51°S. As a result, the absence of the APF at the location of core SN 2 of 47°S during late MIS 3 and the LGM might have resulted in lower diatom productivity, as evidenced through decreased POOZ group diatoms and conversely increased water stratification group diatoms. A recent study from the Pacific sector of the SO similarly observed the absence of the APF during the LGM based on the lack of open ocean diatom species (Oliva et al., 2024 ). Therefore, we assume that the shifting patterns of ACC and SO upwelling somehow controlled the oceanographic conditions during the late MIS 3 and the LGM, and indeed the northward migration of the APF, which was probably ineffective during both these periods. A southward shift of SHWW has been recorded in the SO during the LGM (Toggweiler et al., 2006 ; Kohfeld et al., 2013 ), through which the PFZ waters were possibly pushed into the Antarctic zone, where nutrient-poor waters led to the low diatom productivity due to strong stratification (Francois et al., 1997; Sigman et al., 2021 ), and decreased wind-driven upwelling (Ai et al., 2020 ; Civel-Mazens et al., 2024 ). A similar condition could have prevailed during late MIS 3, when obliquity-driven SHWW were either more poleward or less mixing of the surface and deep waters (reduced upwelling) could have resulted in lower diatom productivity (Ai et al., 2024 ). Thus, based on the modern data (Blain et al., 2007 ; Pollard et al., 2009 ; Robinson et al., 2016 ) and down-core studies (Ai et al., 2020 , 2024 ; Civel-Mazens et al., 2024 ), we propose that the lower diatom productivity during late MIS 3 and the LGM in the PFZ of the Indian sector of the SO was possibly controlled by the co-limitation of deep-water iron and macro-nutrient (mainly silica) in the absence of efficient northward APF migration and intense upwelling. 4.5 Implications for glacial-interglacial atmospheric CO 2 A 5–10 ppmv decrease in atmospheric CO 2 during 40 − 18 ka (Bereiter et al., 2015 ) has been proposed due to the strengthening of the biological pump in the SO (Kohfeld and Chase, 2017 ). Compared with the atmospheric CO 2 records, our new diatom productivity data show coherence with lower diatom productivity during late MIS 3 when atmospheric CO 2 increased by an average of ~ 10 ppmv compared to the early MIS 2 (Bereiter et al., 2015 ), having a larger magnitude of the diatom productivity (Figs. 5 A and 5 F). Nonetheless, both the early MIS 2 and the LGM showed comparable values of atmospheric CO 2 , and each of these periods showed heterogeneous diatom productivity in the PFZ of the SO. Conversely, lower diatom productivity during the deglacial-Holocene periods coincides with the higher atmospheric CO 2 records and agrees with the higher deglacial-Holocene diatom productivity in the Antarctic zone of the SO, when ACC shifts southward (Anderson et al., 2009 ). The lower diatom productivity during late MIS 3 and the LGM could be attributed to the more stratified conditions evidenced through the higher abundances of water stratification group diatoms and lower abundances of the POOZ group diatoms. Such stratified surface ocean conditions during late MIS 3 and the LGM, although preventing the release of deep carbon into the atmosphere through upwelling (Anderson et al., 2009 ; Sigman et al., 2021 ), might have resulted in the lower POOZ diatom species, especially F. kerguelensis , a major species of carbon sequestration in the PFZ (Riguel-Hernandez et al., 2015). Thus, the supply of deeper nutrients to the surface through upwelling somewhere counteracts the SO as increased nutrients strengthen the biological carbon pump, while deeper nutrients also carry carbon-rich deep waters (Toggweiler and Samuels, 1995 ) that eventually raise the atmospheric CO 2 (Anderson et al., 2009 ). Therefore, the reduced upwelling and more stratified conditions may have helped lower atmospheric CO 2 during glacial periods in the Indian sector of the SO (Ronge et al., 2020 ). Our diatom productivity data and inferred oceanographic conditions provide essential insights into diatoms’ role in controlling the biological carbon pump. Moreover, the new diatom productivity data suggest the role of the ACC-driven frontal migration and availability of micro (iron) and macro-nutrient (silica) in shaping the diatom productivity in the PFZ of the SO and therefore show a strong linkage with atmospheric CO 2 (Ai et al., 2024 ; Gottschalk et al., 2016 ). 5. Conclusions We have presented a high-resolution diatom productivity record from the Indian PFZ of the SO, spanning the last 40 ka. The total diatom abundances and diatom community compositions were used to assess diatom productivity and reconstruct the palaeoceanographic conditions in the Indian sector of the SO. Our results demonstrate that the diatom productivity was highest during early MIS 2. In contrast, it was lowest during the deglacial-Holocene periods, with lower intermediate values during late MIS 3 and the LGM periods. The abundances of the POOZ group diatoms covary with the total diatom abundances and show an inverse correlation with the water stratification group diatoms. The higher abundances of POOZ group diatoms during early MIS 2 indicate the effective northward migration of the APF, due to which southern nutrient-rich Antarctic zone waters and stronger SO upwelling might have resulted in high diatom productivity. In contrast, lower intermediate diatom productivity during late MIS 3 and the LGM reflects a less efficient northward migration of the APF, when increased water stratification and reduced upwelling might have limited the supply of deep-water nutrients to the surface. Overall, our results demonstrate that the diatom productivity in the PFZ showed weaker correlation with the fluxes of dust and iron. The observed heterogeneous glacial diatom productivity highlights the variable oceanographic conditions that prevailed in the Indian sector of the SO, where deep-water nutrients (mainly iron and silica) controlled the diatom productivity. Thus, the ACC-driven frontal migration in the Indian sector of the SO and the availability of deep-water iron and silica through the upwelling possibly mediated the diatom productivity. The SO upwelling can be counter-intuitive, in that on one hand, deep-water iron and silica boost the diatom productivity and sequester the atmospheric carbon; on the other hand, deep-water carbon raises the atmospheric carbon dioxide. Therefore, our diatom productivity data provide an essential insight for the strengthening/weakening of the biological carbon pump during glacial-interglacial periods. Declarations Declaration of Competing Interest There is no known competing interest. Supplementary File A The First four principal components (PC1, PC2, PC3, and PC4) are provided for all diatom species of sediment core SN 2 . Acknowledgements This research was funded by Anusandhan National Research Foundation (ANRF), New Delhi, Core Research Grant (ANRF Project No. CRG/2023/003120). The Director, BSIP, is thankfully acknowledged for the necessary laboratory and infrastructural facilities. We also sincerely thank the Director, NCPOR, for all the encouragement. The research presented in this manuscript is part of AN's ongoing Ph.D. research work. References Abelmann A, Gersonde R, Cortese G, Kuhn G, Smetacek V (2006) Extensive phytoplankton blooms in the Atlantic sector of the glacial Southern Ocean. https://doi.org/10.1029/2005pa001199 . 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Q Sci Rev 64:104–120 https://doi.org/10.1016/j.quascirev.2012.12.008 Smetacek V, Klaas C, Strass VH, Assmy P, Montresor M, Cisewski B, Savoye N, Webb A, d’Ovidio F, Arrieta JM, Bathmann U, Bellerby R, Berg GM, Croot P, Gonzalez S, Henjes J, Herndl GJ, Hoffmann LJ, Leach H, Losch M, Mills MM, Neill C, Peeken I, Röttgers R, Sachs O, Sauter E, Schmidt MM, Schwarz J, Terbrüggen A, Wolf-Gladrow D (2012) Deep carbon export from a Southern Ocean iron-fertilized diatom bloom. Nature 487:313–319. https://doi.org/10.1038/nature11229 Tamsitt V, Drake HF, Morrison AK, Talley LD, Dufour CO, Gray AR, Griffies SM, Mazloff MR, Sarmiento JL, Wang J, Weijer W (2017) Spiraling pathways of global deep waters to the surface of the Southern Ocean. Nat Commun 8. https://doi.org/10.1038/s41467-017-00197-0 Thöle LM, Amsler HE, Moretti S, Auderset A, Gilgannon J, Lippold J, Vogel H, Crosta X, Mazaud A, Michel E, Martínez-García A, Jaccard SL (2019) Glacial-interglacial dust and export production records from the Southern. Indian Ocean Earth Planet Sci Lett 525:115716. https://doi.org/10.1016/j.epsl.2019.115716 Toggweiler JR, Russell JL, Carson SR (2006) Midlatitude westerlies, atmospheric CO 2 , and climate change during the ice ages. https://doi.org/10.1029/2005pa001154 . Paleoceanography 21 Toggweiler JR, Samuels B (1995) Effect of Sea Ice on the Salinity of Antarctic Bottom Waters. J Phys Oceanogr 25:1980–1997 Tréguer PJ, Sutton JN, Brzezinski M, Charette MA, Devries T, Dutkiewicz S, Ehlert C, Hawkings J, Leynaert A, Liu SM, Llopis Monferrer N, López-Acosta M, Maldonado M, Rahman S, Ran L, Rouxel O (2021) Reviews and syntheses: The biogeochemical cycle of silicon in the modern ocean. Biogeosciences 18:1269–1289. https://doi.org/10.5194/bg-18-1269-2021 Watson AJ, Bakker DCE, Ridgwell AJ, Boyd PW, Law CS (2000) Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Nature 407:730–733. https://doi.org/10.1038/35037561 Wu S, Lembke-Jene L, Lamy F, Arz HW, Nowaczyk N, Xiao W, Zhang X, Hass HC, Titschack J, Zheng X, Liu J, Dumm L, Diekmann B, Nürnberg D, Tiedemann R, Kuhn G (2021) Orbital- and millennial-scale Antarctic Circumpolar Current variability in Drake Passage over the past 140,000 years. Nat Commun 12. https://doi.org/10.1038/s41467-021-24264-9 Xiao W, Esper O, Gersonde R (2016) Last Glacial - Holocene climate variability in the Atlantic sector of the Southern Ocean. Q Sci Rev 135:115–137. https://doi.org/10.1016/j.quascirev.2016.01.023 Ziegler M, Diz P, Hall IR, Zahn R (2013) Millennial-scale changes in atmospheric CO2 levels linked to the Southern Ocean carbon isotope gradient and dust flux. Nat Geosci 6:457–461. https://doi.org/10.1038/ngeo1782 Zielinski U, Gersonde R (1997) Diatom distribution in Southern Ocean surface sediments (Atlantic sector): Implications for paleoenvironmental reconstructions. Palaeogeogr Palaeoclimatol Palaeoecol 129:213–250 https://doi.org/10.1016/s0031-0182(96)00130-7 Additional Declarations The authors declare no competing interests. Supplementary Files SupplementaryFileA.pdf The First four principal components (PC1, PC2, PC3, and PC4) are provided for all diatom species of sediment core SN 2 . Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7470794","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":506387879,"identity":"ea4b07af-2c34-4768-bd74-1694c357cdda","order_by":0,"name":"Aditi Nautiyal","email":"","orcid":"","institution":"1Birbal Sahni Institute of Palaeosciences, Lucknow – 227 007, India 2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India","correspondingAuthor":false,"prefix":"","firstName":"Aditi","middleName":"","lastName":"Nautiyal","suffix":""},{"id":506391942,"identity":"3094fd55-2a34-4bd6-bf1e-4d90a91b8202","order_by":1,"name":"Sunil Kumar Shukla","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCElEQVRIie2QMUvEMBSAXyh0ap0DFvoXIg6HIOSvvC51OZ0dHAIHcbq9h+JvuOnA7ZXAufQHZLhBl5tuyKhQ0YsVESGnboL5hhDC++B7AYhE/ih0TwACUkZwTpC+v2Y7FRwUIOh+qMCHwjR9H1U2py1hB3LEa6Snm5Xc21epcz0Uo4Ai7BkSWqhum5ra6WJd6YKS2UxDdqQCCh8LQgco7ImifGEw5ZgkuYJMBCLLZlCkV9rnayPflL4PK2C9YoHNbU0mV4Zpr2z/IqiIbiP8+tW8W6Mpln6XdsKmmofDLseHzi23YXf1wcPmYiXLq4mBx/5YBsMGXj7dOVP+3Dn/hV8NRyKRyH/gFR0ZZa/k7afmAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-7517-5564","institution":"1Birbal Sahni Institute of Palaeosciences, Lucknow – 227 007, India 2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India","correspondingAuthor":true,"prefix":"","firstName":"Sunil","middleName":"Kumar","lastName":"Shukla","suffix":""},{"id":506391943,"identity":"a1aaa418-f8cd-411c-8318-c1f7f2db1cec","order_by":2,"name":"Rahul Mohan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIiWNgGAWjYFACHgaGBwwMCQwMzAeQRA0IaEkAI7YEkrXw4FWGAObtZ49JJObY5PHP7vn24OcOm3wG6cOHPzAU3MGpReZMXppE4ra0Yok7Z7cb9p5Js2zgS0uTYDB4hlOLBEOOGVDL4cSGG7nbpBnbDhsw8PCYAf1yGLcW/jcQLfNv5DyDauH//AGvFgmoLRtu5LDBbAHajVfLG2MLoF8SN95IM5ME+sWAjYfNTCIBr8NyDG983GaTOO9G8jMJYIgZ8PMwP/7w4Q9uLaiAsQEYoyBGApEaIFpGwSgYBaNgFKADAAYPUAMRSQAvAAAAAElFTkSuQmCC","orcid":"","institution":"National Centre for Polar and Ocean Research, Ministry of Earth Sciences, Headland Sada, Vasco-da-Gama, Goa – 403 804, India","correspondingAuthor":true,"prefix":"","firstName":"Rahul","middleName":"","lastName":"Mohan","suffix":""}],"badges":[],"createdAt":"2025-08-27 10:40:55","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-7470794/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7470794/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":90100773,"identity":"22212f3d-ad60-4d4f-94de-84b11caa69e7","added_by":"auto","created_at":"2025-08-28 13:12:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3428940,"visible":true,"origin":"","legend":"\u003cp\u003eThe location of the studied sediment core SN2 (red star) is shown from the Polar Front Zone of the Indian sector of the Southern Ocean along with other sediment cores (black dots) used for the comparison. Positions of the subtropical front (STF – red line), the Sub-Antarctic Front (SAF – orange line), the Antarctic Polar Front (APF – dark blue), and the southern Antarctic Circumpolar Current Front (SACCF – light blue) are also shown after Park et al. (2019). The map is prepared using the Ocean Data View software (Schlitzer, 2025).\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/7d5353964f1b37663b1f07f8.png"},{"id":90100774,"identity":"10109a35-2d40-4351-a61b-51e2c38de9ed","added_by":"auto","created_at":"2025-08-28 13:12:48","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1210624,"visible":true,"origin":"","legend":"\u003cp\u003eDiatom productivity record in sediment core SN\u003csub\u003e2 \u003c/sub\u003efor the past 40 ka, which depicts the total diatom abundances (A), principal components PC1 and PC2 obtained through the principal component analysis applied to diatom species assemblages (B), and absolute abundances of major diatom species \u003cem\u003eFragilariopsis\u003c/em\u003e \u003cem\u003ekerguelensis\u003c/em\u003e (C), \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003elentiginosa\u003c/em\u003e (D), \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (E), \u003cem\u003eEucampia\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (F), \u003cem\u003eChaetoceros\u003c/em\u003e resting spores (G), and \u003cem\u003eThalassiothrix\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (H). The highlighted periods show late Marine Isotope Stage - MIS 3 (gray), early MIS 2 (green), and the Last Glacial Maximum (cyan).\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/60ef19d17fa8c86420cbe96d.png"},{"id":90101237,"identity":"40166501-5a10-459c-b5b0-dbf1c289a556","added_by":"auto","created_at":"2025-08-28 13:20:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":561492,"visible":true,"origin":"","legend":"\u003cp\u003eStatistical variation of diatom productivity data in core SN\u003csub\u003e2\u003c/sub\u003e is shown through box and whisker plots for late Marine Isotope Stage (MIS) 3, early MIS 2, the Last Glacial Maximum, and the deglacial-Holocene periods, which depict the absolute abundances of total diatoms (A), and major diatom species, namely, \u003cem\u003eFragilariopsis\u003c/em\u003e \u003cem\u003ekerguelensis\u003c/em\u003e (B), \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003elentiginosa\u003c/em\u003e (C), \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (D), \u003cem\u003eEucampia\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (E), \u003cem\u003eChaetoceros\u003c/em\u003e resting spores (F), and \u003cem\u003eThalassiothrix\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e (G).\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp; \u0026nbsp;\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/c22efba8b82a8540d39d9c65.png"},{"id":90100776,"identity":"d607aad0-043c-42e7-bf0d-5a3ed11517cb","added_by":"auto","created_at":"2025-08-28 13:12:48","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1032711,"visible":true,"origin":"","legend":"\u003cp\u003eCumulative relative abundances of the Permanent Open Ocean Zone (POOZ) group diatom species (\u003cem\u003eFragilariopsis\u003c/em\u003e \u003cem\u003ekerguelensis\u003c/em\u003e, \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003elentiginosa\u003c/em\u003e,\u003cem\u003e \u003c/em\u003eand\u003cem\u003e Thalassiothrix\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e), and water stratification group diatom species (\u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e, \u003cem\u003eEucampia\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e, and \u003cem\u003eChaetoceros\u003c/em\u003e resting spores) in core SN\u003csub\u003e2\u003c/sub\u003e for the past 40 ka (A). The POOZ and water stratification group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e show a linear inverse correlation for the past 40 ka (B). The highlighted periods show late Marine Isotope Stage - MIS 3 (gray), early MIS 2 (green), and the Last Glacial Maximum (cyan). \u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/6f1029be912a3cca5d787174.png"},{"id":90100782,"identity":"28f7f28a-fcec-417e-9deb-c3f7a4bc4ce3","added_by":"auto","created_at":"2025-08-28 13:12:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1391169,"visible":true,"origin":"","legend":"\u003cp\u003eThe total diatom abundances of core SN\u003csub\u003e2 \u003c/sub\u003ealong with the principal component PC1 (A) are compared with the POOZ group diatoms (B: - magenta curve – this study, and blue curve - core PS2606-6, Jacot des Combes et al., 2008), water stratification group diatoms (C: black curve – this study, and gray curve - core PS2606-6, Jacot des Combes et al., 2008), diatom-bound δ\u003csup\u003e15\u003c/sup\u003eN (D – brown curve, Ai et al., 2020) and δ\u003csup\u003e30\u003c/sup\u003eSi data (D – black curve, Dumont et al., 2020), fluxes of opal (E – black curve) and Fe (E – green curve) after Thöle et al. (2019). EPICA Dome C dust flux (F – black curve, Lambert et al., 2008) and CO\u003csub\u003e2 \u003c/sub\u003e(F – red curve, Bereiter et al., 2015) data are also shown. The highlighted periods show late Marine Isotope Stage - MIS 3 (gray), early MIS 2 (green), and the Last Glacial Maximum (cyan).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/a81ebe987ffcb5455ae6a597.png"},{"id":90100784,"identity":"7f477aec-767d-4f51-92af-12824ff5d177","added_by":"auto","created_at":"2025-08-28 13:12:49","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1863459,"visible":true,"origin":"","legend":"\u003cp\u003eThe sea-surface temperatures of the sub-Antarctic Zone (SAZ) core DCR-1PC (A – orange curve, Crosta et al., 2020) are compared with the Antarctic Zone (AZ) core PS2606-6 (A – blue curve, Civel-Mazens et al., 2024) for the past 40 ka. The shaded band shows the presence of the Antarctic Polar Front (APF), when SST ranged between 3°C and 5°C in core DCR-1PC during late MIS 3 and early MIS 2, whereas in the AZ core PS2606-6 during the deglacial-early Holocene periods. The APF-index (comprising the POOZ group diatoms - \u003cem\u003eFragilariopsis\u003c/em\u003e \u003cem\u003ekerguelensis\u003c/em\u003e, \u003cem\u003eThalassiosira\u003c/em\u003e \u003cem\u003elentiginosa\u003c/em\u003e,\u003cem\u003e \u003c/em\u003eand\u003cem\u003e Thalassiothrix\u003c/em\u003e \u003cem\u003eantarctica\u003c/em\u003e) is shown for SN2 core (B – magenta curve, this study) compared with the AZ core PS2606-6 (B – blue curve, Jacot Des Combes et al., 2008). The shaded band shows the presence of the APF, when APF-index \u0026gt;70% in PFZ core SN2 during early MIS 2, whereas in the AZ core PS2606-6 during late MIS 3, and the deglacial-Holocene periods. The highlighted periods show late Marine Isotope Stage - MIS 3 (gray), early MIS 2 (green), and the Last Glacial Maximum (cyan).\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/71123595b195ff56b6e9e335.png"},{"id":90102518,"identity":"a8d0a54f-fd55-4298-9967-74b06a73f24e","added_by":"auto","created_at":"2025-08-28 13:36:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9110403,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/86a1982c-91a9-4ce2-98dc-408a848b1da6.pdf"},{"id":90101238,"identity":"d8184f61-af0a-44b6-b5e7-b1a286538c6a","added_by":"auto","created_at":"2025-08-28 13:20:48","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":526398,"visible":true,"origin":"","legend":"\u003cp\u003eThe First four principal components (PC1, PC2, PC3, and PC4) are provided for all diatom species of sediment core SN\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"SupplementaryFileA.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7470794/v1/641c008707a88ced1c425c41.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eDeep-water nutrients mediated glacial-interglacial diatom productivity in the Indian Polar Front Zone of the Southern Ocean\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe co-limitation of iron and light controls the primary productivity in the Southern Ocean (SO) (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Boyd et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; de Baar et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Martin, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Martin et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). Among the phytoplankton, diatoms are dominant organisms, which significantly contribute to the burial of biogenic silica and organic carbon to the deep sea and help in lowering atmospheric carbon dioxide (Buesseler, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Buesseler et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Tr\u0026eacute;guer et al., \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Smetacek et al., \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Apart from iron, nitrate and silicate are also required by the diatoms to grow, which show a north-south gradient in the SO. For instance, nitrate is mostly constant from the Sub-Antarctic Zone (SAZ) to south of the Antarctic Polar Front (APF) until the sea-ice zone, whereby silicate availability is more in the south of the APF and decreasing towards the north of the APF until the SAZ (Brzezinski et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Nelson et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Qu\u0026eacute;guiner and Brzezinski, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Thus, iron and silicic acid concentration co-limits diatoms\u0026rsquo; growth and abundances in the SAZ of the SO (Boyd et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lannuzel et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). However, the Polar Front Zone (PFZ), a region located between the SAF and APF (Park et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), is demonstrated with the northward silica supply during the glacial periods that led to increased diatom abundances and lower atmospheric CO2 (Matsumoto et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). The sediment trap samples from the PFZ indeed suggest diatoms\u0026rsquo; dominance and carbon export compared to the SAZ, where calcareous organisms are preponderant (Rigual-Hern\u0026aacute;ndez et al., \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHigh diatom blooms characterize the Crozet and Kerguelen Plateaus in the SO due to the availability of natural iron fertilization (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Planquette et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Bowie et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Diatom blooms were recorded explicitly from the north of the Crozet (Salter et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) and east of the Kerguelen Plateaus (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). However, the diatom blooms from the Kerguelen Plateau were reported due to the availability of iron and major nutrients in the surface waters supplied from iron-rich deep water (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Park et al., \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The inferred diatom community composition from the Kerguelen Plateau water samples similarly suggested a role of silica in conjunction with iron for defining diatom bloom (Armand et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008a\u003c/span\u003e; Lafond et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The surface sediment samples, on one hand, from the northeastern Kerguelen Plateau, revealed the correlation of diatoms with iron, and on the other hand, from the off-Plateau samples near the Polar Front, with diatom species of Permanent Open Ocean Zone with insignificant role of dust-bearing iron (Armand et al, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2008b\u003c/span\u003e). Despite these crucial studies, which have implicated the role of diatoms and the drawdown of atmospheric carbon dioxide, the glacial-interglacial diatom productivity records from the Crozet and Kerguelen Plateaus are sparse and represented by a few studies that are near the north of the Crozet Plateau (Nair et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe studies from the Indian sector of the SO have suggested increased diatom productivity during the glacial periods (Marine Isotope Stage, MIS 2\u0026ndash;3) in the sub-antarctic zone (SAZ) and related it to the Southern Hemisphere Westerly Winds (SHWW) driven northward migration of the APF (Nair et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Ghadi et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, lower diatom productivity during MIS 2\u0026ndash;3 is suggested from the SAZ, which resulted from the ineffective northward APF migration and silica supply (Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Such regional differences in the diatom productivity were attributed to the bottom topographic variations in the Indian sector of the SO, where lesser interactions of fronts limit the supply of nutrients (Civel-Mazens et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003eb\u003c/span\u003e), resulting in lower diatom productivity (Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe phytoplankton productivity in the PFZ and SAZ of the Atlantic and Pacific sectors of the SO was higher during the glacial periods and strongly correlated with dust-bearing iron availability. Conversely, low phytoplankton productivity was due to reduced dust-bearing iron (Anderson et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Anderson et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Jaccard et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Kohfeld et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Lamy et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Mart\u0026iacute;nez-Garcia et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Mart\u0026iacute;nez-Garcia et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Shoenfelt et al., \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Watson et al., \u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Ziegler et al., \u003cspan citationid=\"CR98\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Although the southwest Indian sector of the SO is too far to receive dust-bearing iron from South America/South Africa (Lamy et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), a study from the east of the Kerguelen Plateau recorded higher glacial lithogenic/iron fluxes, and attributed it to the South Africa and/or Kerguelen Plateau (Th\u0026ouml;le et al., \u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). However, despite higher iron fluxes, the export production of organic carbon was lower during the glacial periods and therefore showed a muted response (Th\u0026ouml;le et al., \u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In this vein, we present a diatom productivity record from the PFZ of the Indian sector of the SO for the past 40 ka using a sediment core (47\u0026deg;S, 57.3\u0026deg;E), located between the south of the Crozet and the west of the Kerguelen Plateaus. Our study aims to explore 1) whether glacial-interglacial diatom productivity in the PFZ of the Indian sector of the SO correlates with dust-bearing iron, 2) the effect of natural iron fertilization on overall diatom productivity and diatom community composition, and 3) the role of frontal migration in driving the glacial-interglacial diatom productivity.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eWe analysed diatoms preserved in a sediment core SN\u003csub\u003e2\u003c/sub\u003e (47\u0026deg;S and 57\u0026deg;30\u0026rsquo;E; length \u0026ndash; 3.8 m; water depth \u0026minus;\u0026thinsp;3470 m), which was retrieved from the Indian sector of the Southern Ocean, on board \u003cem\u003eORV Sagar Nidhi\u003c/em\u003e. The sediment core is located in the Polar Front Zone (PFZ), a zone between the Antarctic Polar Front (APF) at the south and the Sub-Antarctic Front (SAF) at the north (Park et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The core location is amidst two major bathymetric features: Crozet Island to the left and Kerguelen Plateau to the right. However, the core location is away from both these bathymetric features and facing the corridor of the APF, which is straight in front (Park et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Although the PFZ is generally considered to be iron and silica-deficient (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), the nutrient-rich Antarctic surface water might be responsible for high productivity when the APF shifts northward during the glacial periods (Shetye et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The age model of core SN\u003csub\u003e2\u003c/sub\u003e is based on radiocarbon dates and was used after Shetye et al. (\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eA total of 75 samples were analysed for the diatom study at a 2 cm interval of sediment core (time resolution ranged between ~\u0026thinsp;280 years and ~\u0026thinsp;600 years) spanning the last\u0026thinsp;~\u0026thinsp;40 ka. The diatoms were extracted from the sediment following the method of Crosta et al. (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). By following this method, 1\u0026ndash;2 gm of sediment sample (depending on the abundances of diatoms after examining smear slides) was treated with 20 ml Oxidative solution (870 ml H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at 35% + 130 ml distilled water\u0026thinsp;+\u0026thinsp;42 g Na\u003csub\u003e4\u003c/sub\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.10H\u003csub\u003e2\u003c/sub\u003eO for 1 litre solution) to remove the organic material and then with 20ml of 30% HCl for the removal of carbonates. The sample was washed thrice with distilled water through centrifugation, and then the final sample was diluted to 50 ml. A known amount of sample was pipetted onto a coverslip and, after drying, mounted onto the slides with NOA61 mounting medium. Diatoms were counted following the method described in Crosta and Ko\u0026ccedil; (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) using an Olympus BX50 light microscope with an oil immersion lens (x1000 magnification). The preservation of diatoms in all the samples was excellent, so a minimum of 300 diatoms in each sample was counted. The relative abundances of all the diatoms were used for the Principal Component Analysis (PCA), which allowed the identification of the co-variation of the species over the studied time period and, therefore, significant diatom species in the entire diatom assemblage. Total diatom abundances (absolute abundances) were calculated using the equation detailed in Crosta et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e\u003cp\u003ePrincipal Component Analysis (PCA) and One-way ANOVA tests were applied to the diatom productivity data of sediment core SN\u003csub\u003e2\u003c/sub\u003e. PCA was applied to the relative abundances of diatoms to identify the significant species that co-varied through time, whereas one-way ANOVA tests were applied to total diatom abundances and relative abundances of significant diatom species to identify the dis(similarities) across different time periods for the last\u0026thinsp;~\u0026thinsp;40 ka. All the data are provided in (Supplementary file A).\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Total diatom abundances\u003c/h2\u003e\u003cp\u003eThe total diatom abundances in core SN\u003csub\u003e2\u003c/sub\u003e ranged between 5.47 x 10\u003csup\u003e6\u003c/sup\u003e valves/g and 448.05 x 10\u003csup\u003e6\u003c/sup\u003e valves/g of dry sediment (mean of 157.44 x 10\u003csup\u003e6\u003c/sup\u003e valves/g) for the studied time interval of 39.21\u0026ndash;7.86 ka (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). Diatom abundances were highest during early Marine Isotope Stage - MIS 2 (29.5\u0026ndash;23 ka, mean of 253.61 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, range 120.97 x 10\u003csup\u003e6\u003c/sup\u003e \u0026minus;\u0026thinsp;448.05 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, n\u0026thinsp;=\u0026thinsp;23), lowest during deglacial to Holocene period (18-7.86 ka, mean of 64.69 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, range 5.47 x 10\u003csup\u003e6\u003c/sup\u003e \u0026minus;\u0026thinsp;153.79 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, n\u0026thinsp;=\u0026thinsp;17), and intermediate values were recorded during late MIS 3 (39.21\u0026ndash;29.5 ka, mean of 146.46 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, range 104.99 x 10\u003csup\u003e6\u003c/sup\u003e \u0026minus;\u0026thinsp;232.57 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, n\u0026thinsp;=\u0026thinsp;26) and during the Last Glacial Maximum \u0026ndash; LGM (23\u0026ndash;18 ka, mean of 118.62 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, range 43.74 x 10\u003csup\u003e6\u003c/sup\u003e \u0026minus;\u0026thinsp;154.49 x 10\u003csup\u003e6\u003c/sup\u003e valves/g, n\u0026thinsp;=\u0026thinsp;9) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). One-way ANOVA tests confirm this variability and are graphically shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The millennial-scale variability of diatom abundances was highest during the deglacial to Holocene period. Conversely, early MIS 2 and the LGM periods, although represented with much lesser variability of diatom abundances compared to the deglacial and Holocene periods, were still higher than late MIS 3. These observations suggest heterogeneity in diatom abundances among the different glacial periods, such as late MIS 3, early MIS 2, and the LGM.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Diatom Community compositions\u003c/h2\u003e\u003cp\u003eThe diatom community in the core SN\u003csub\u003e2\u003c/sub\u003e is composed of a total of 31 species/species groups. To identify the major diatom species that co-varied through time and contributed to the diatom productivity, the principal component analysis was applied, which yielded six major species, namely, \u003cem\u003eFragilariopsis kerguelensis\u003c/em\u003e, \u003cem\u003eThalassiosira lentiginosa\u003c/em\u003e, \u003cem\u003eThalassiosira antarctica\u003c/em\u003e, \u003cem\u003eEucampia antarctica\u003c/em\u003e, \u003cem\u003eChaetoceros\u003c/em\u003e resting spores, and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e. The initial three principal components (PC1, PC2, and PC3) contributed to \u0026gt;\u0026thinsp;90% of the data variance. PC1 is represented by 69.71% variance and comprises \u003cem\u003eF. kerguelensis\u003c/em\u003e with the highest positive factor loadings, whereas \u003cem\u003eT. lentiginosa\u003c/em\u003e, \u003cem\u003eE. antarctica\u003c/em\u003e, and \u003cem\u003eThalassiosira antarctica\u003c/em\u003e have significant negative factor loadings. PC2 is represented with 14.15% variance, including \u003cem\u003eThalassiosira antarctica\u003c/em\u003e with positive factor loadings, whereas \u003cem\u003eT. lentiginosa\u003c/em\u003e and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e have significant negative factor loadings. PC3 is represented with 6.73% variance and included \u003cem\u003eThalassiosira antarctica\u003c/em\u003e and \u003cem\u003eT. lentiginosa\u003c/em\u003e with positive factor loadings, while \u003cem\u003eChaetoceros\u003c/em\u003e resting spores and \u003cem\u003eEucampia antarctica\u003c/em\u003e had significant negative factor loadings. PC1 mimics the pattern of total diatom abundances in core SN\u003csub\u003e2\u003c/sub\u003e, indicating the substantial contribution of \u003cem\u003eF. kerguelensis\u003c/em\u003e, \u003cem\u003eT. lentiginosa\u003c/em\u003e, \u003cem\u003eE. antarctica\u003c/em\u003e, and \u003cem\u003eThalassiosira antarctica.\u003c/em\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Diatom species\u0026rsquo; absolute abundances and diatom groups\u003c/h2\u003e\u003cp\u003eBased on the PCA, the absolute abundances of the six significant diatom species that mainly co-varied through time and ultimately superimposed on the diatom productivity are presented. In core SN\u003csub\u003e2\u003c/sub\u003e, both \u003cem\u003eF. kerguelensis\u003c/em\u003e and \u003cem\u003eT. lentiginosa\u003c/em\u003e absolute abundances mimic the pattern of total diatom abundances and were highest during early MIS 2, lowest during the deglacial to Holocene period, and were intermediate during late MIS 3 and the LGM (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). The absolute abundances of \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e also showed a similar pattern to that of \u003cem\u003eF. kerguelensis\u003c/em\u003e and \u003cem\u003eT. lentiginosa\u003c/em\u003e, except for the LGM, where its abundances remained high, like early MIS 2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eH). The other three species, namely, \u003cem\u003eThalassiosira antarctica\u003c/em\u003e, \u003cem\u003eEucampia antarctica\u003c/em\u003e, and \u003cem\u003eChaetoceros\u003c/em\u003e resting spores\u0026rsquo; absolute abundances showed inverse correlation with \u003cem\u003eF. kerguelensis\u003c/em\u003e, \u003cem\u003eT. lentiginosa\u003c/em\u003e, \u003cem\u003eand Thalassiothrix antarctica\u003c/em\u003e, as their abundances were higher during late MIS 3, whereas lower during early MIS 2, LGM, and deglacial to Holocene (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eG) .\u003c/p\u003e\u003cp\u003eAmong the above-noted species, \u003cem\u003eF. kerguelensis\u003c/em\u003e, \u003cem\u003eT. lentiginosa\u003c/em\u003e, and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e are the permanent open ocean zone (POOZ) group diatoms. In contrast, \u003cem\u003eThalassiosira antarctica\u003c/em\u003e, \u003cem\u003eEucampia antarctica\u003c/em\u003e, and \u003cem\u003eChaetoceros\u003c/em\u003e resting spores are water stratification group diatoms. Both POOZ and water stratification diatom groups showed strong inverse correlation from late MIS 3 to the Holocene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eIn the core SN\u003csub\u003e2\u003c/sub\u003e from the Polar Front Zone (PFZ) of the Indian sector of the Southern Ocean (SO), the total diatom abundances were highest during early MIS 2 and conversely lowest during the deglacial-Holocene periods, with lower intermediate values during late MIS 3 and the LGM periods. Such patterns of diatom productivity in core SN\u003csub\u003e2\u003c/sub\u003e are confirmed with the first principal component PC1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA) and one-way ANOVA tests (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA). If high diatom productivity during the glacial periods results from the northward APF migration that supplied increased nutrients through upwelling (Nair et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Ghadi et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) or through increased dust-bearing iron (Shukla et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Nair et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), then increased diatom productivity should have been found throughout the glacial period of MIS 2\u0026ndash;3 in core SN\u003csub\u003e2\u003c/sub\u003e. However, the magnitude of the diatom productivity is much larger during early MIS 2 compared to the lower intermediate diatom productivity found during late MIS 3 and the LGM periods. The heterogeneous diatom productivity during the glacial periods may have been related to non-synchronous northward APF migration or variable dust-bearing iron availability. We, therefore, explore the possible reasons for the heterogeneous diatom productivity patterns found in the PFZ core SN\u003csub\u003e2\u003c/sub\u003e from late MIS 3 to LGM.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e4.1 Correlation of diatom Groups from Polar Front Zone (Core SN\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e) and Antarctic Zone (Core PS2606-6): POOZ diatoms vs water stratification group diatoms\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe total diatom abundances in core SN\u003csub\u003e2\u003c/sub\u003e co-varied with the POOZ group diatoms from late MIS 3 to the deglacial-Holocene periods (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). Conversely, POOZ group diatoms were inversely correlated with water stratification group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). An inverse correlation of both groups of diatoms from core SN\u003csub\u003e2\u003c/sub\u003e is also found when comparing with the Antarctic zone sediment core PS2606-6 from the Conrad Rise at 53\u0026deg;S (Jacot des Combes et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) over the studied period (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC). Consequently, the POOZ group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e were higher during early MIS 2, when core PS2606-6 showed lower values (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). Conversely, the POOZ group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e were lower during late MIS 3 and the deglacial-Holocene periods, when core PS2606-6 showed higher values. Interestingly, both cores showed almost comparable POOZ group diatoms during the LGM (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). The water stratification group diatoms are also inversely correlated between the two cores, with higher abundances during late MIS 3, the LGM, and deglacial-Holocene periods in core SN\u003csub\u003e2\u003c/sub\u003e, when core PS2606-6 showed lower abundances (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC). Conversely, lower abundances were found during early MIS 2 in core SN\u003csub\u003e2\u003c/sub\u003e, when core PS2606-6 showed higher abundances (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC) Thus, it seems that the abundances of the POOZ and water stratification group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e from late MIS 3 to the Holocene wax and wane through the northward APF migration due to the variable oceanographic conditions and the related nutrient availability, which are needed to be explained.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Correlation of diatom productivity with nutrient availability\u003c/h2\u003e\u003cp\u003eThe studies from the SAZ of the Atlantic and Pacific sectors of the SO suggested an imperative role of dust-bearing iron-availability, which resulted in increased productivity during the glacial periods (Anderson et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Jaccard et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Mart\u0026iacute;nez-Garcia et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2011\u003c/span\u003e, 2014). However, a muted response of export production in the Indian SAZ was observed despite high iron fluxes through the dust from the Kerguelen Plateau (Thole et al., 2019). The Indian SAZ similarly showed low diatom productivity during the glacial periods of MIS 2\u0026ndash;3, which was related to frontal shifts and silica availability (Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe high diatom productivity of core SN\u003csub\u003e2\u003c/sub\u003e during the early MIS 2 period, when the POOZ group diatoms (\u003cem\u003eF. kerguelensis\u003c/em\u003e, \u003cem\u003eT. lentiginosa\u003c/em\u003e, and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e) were high, reveals a strong correlation with high fluxes of dust (Lambert et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and iron (Thole et al., 2019). The higher δ\u003csup\u003e15\u003c/sup\u003eN values (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and lower values of δ\u003csup\u003e30\u003c/sup\u003eSi (Dumont et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) hint towards the high nitrate utilization in presence of iron-repleted conditions, which could have resulted in high diatom productivity (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eD). Conversely, low diatom productivity during the deglacial-Holocene periods, when the POOZ group diatoms were low, is in concordance with the lower δ\u003csup\u003e15\u003c/sup\u003eN values (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and higher values of δ\u003csup\u003e30\u003c/sup\u003eSi (Dumont et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), when both dust and iron fluxes were low (Lambert et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Thole et al., 2019) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). However, late MIS 3 and the LGM periods showed lower intermediate diatom productivity in core SN\u003csub\u003e2\u003c/sub\u003e, when the POOZ group diatoms were low despite higher fluxes of dust and iron, and δ\u003csup\u003e15\u003c/sup\u003eN (Lambert et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Thole et al., 2019; Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The higher surface nitrate consumption during late MIS 3 and the LGM suggests stratified waters (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) that might have promoted higher abundances of water stratification group diatoms than the POOZ group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e (Francois et al., 1997; Sigman et al., \u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The δ\u003csup\u003e30\u003c/sup\u003eSi records from the Indian SAZ (Beucher et al., 2007; Dumont et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) showed lower silicic acid availability during the glacial period and therefore suggest the dependency of diatoms on higher nitrate availability, when iron-repleted conditions prevailed (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Nonetheless, higher nitrate utilization during the glacial periods compared to the deglacial and Holocene periods is evident from both the Antarctic zone and SAZ of the SO (Shemesh et al., \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Crosta et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and suggests increased stratification conditions with reduced upwelling (Francois et al., 1997; Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Although the opal fluxes in the Indian SAZ were higher during late MIS 3 to the LGM, suggesting greater export production linked to the high dust-bearing iron availability, the organic carbon fluxes were contrarily low, especially during MIS 3 and the LGM (Thole et al., 2019). Therefore, our data of lower intermediate diatom productivity patterns during late MIS 3 and the LGM coincide with the lower export production in the Indian SAZ (Thole et al., 2019).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e4.3 Diatom productivity and frontal migration\u003c/h2\u003e\u003cp\u003eA sediment core PS2606-6 at 53\u0026deg;S from the POOZ of the Conrad Rise showed SST between 0\u0026deg;C and 2\u0026deg;C from late MIS 3 to LGM, whereas SST between 2\u0026deg;C and 5\u0026deg;C occurred during the deglacial-Holocene periods (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA) (Xiao et al., \u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Civel-Mazens et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The cooler SSTs of 3\u0026deg;C during late MIS 3 to LGM compared to the deglacial-Holocene periods could have resulted from the northward migration of the Southern Antarctic Circumpolar Current Front (SACCF \u0026ndash; Ghadi et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The subsequent migration of the APF could potentially lead to the cooler SSTs of 3\u0026ndash;5\u0026deg;C in core DCR-1PC (at 46\u0026deg;S from the Del Cano Rise) during late MIS 3 to early MIS 2, compared to 5\u0026ndash;8\u0026deg;C SSTs during the deglacial-Holocene periods (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA) (Crosta et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Shukla et al., \u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Although low-resolution, the SST range 5 to 8\u0026deg;C in core DCR-1PC during the LGM does not show the northward migration of the APF. Based on the SST data, although the APF migration was evident during late MIS 3 and early MIS 2 (Crosta et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), the diatom productivity was lower during both periods in core DCR-1PC, and this was attributed to the ineffective APF migration and silica supply (Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Such lower productivity is also mirrored by the lower abundances of the pelagic SO diatom species found in the Agulhas Plateau (Romero et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). These contrasting evidence for the northward positioning of the APF during the glacial periods prompted us to present an APF index based on the cumulative relative abundances of \u003cem\u003eF. kerguelensis, T. lentiginosa\u003c/em\u003e, and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e, as these three species are the permanent open ocean zone diatom species (Crosta et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Armand et al., 2008), and can help track the past APF position (Cortese and Gersonde, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Shukla et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nair et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) at the location of PFZ for core SN\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe APF index applied here, including the POOZ group diatoms (\u003cem\u003eF. kerguelensis\u003c/em\u003e, \u003cem\u003eT. lentiginosa\u003c/em\u003e, and \u003cem\u003eThalassiothrix antarctica\u003c/em\u003e), was \u0026gt;\u0026thinsp;70% in core SN\u003csub\u003e2\u003c/sub\u003e is found only during early MIS 2, when the APF index in core PS2606-6 was \u0026lt;\u0026thinsp;70% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB), suggesting the northward APF migration, when southern nutrient-rich Antarctic zone surface water allowed greater diatom productivity in core SN\u003csub\u003e2\u003c/sub\u003e by leaving nutrient-deficient and thereby lower diatom productivity in core PS2606-6. However, the APF index during late MIS 3 is \u0026lt;\u0026thinsp;70% in core SN\u003csub\u003e2\u003c/sub\u003e, while\u0026thinsp;\u0026gt;\u0026thinsp;70% in core PS2606-6, suggesting no northward frontal migration (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB). The APF index was also \u0026lt;\u0026thinsp;70% during the deglacial to Holocene periods in core SN\u003csub\u003e2\u003c/sub\u003e, while\u0026thinsp;\u0026gt;\u0026thinsp;70% in core PS2606-6, suggesting no effective southward frontal migration. The LGM is an exception when the APF index in both cores shows nearly comparable values and is \u0026lt;\u0026thinsp;70% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB), and suggests no effective frontal migration in the Indian sector of the SO. Therefore, our data suggests that early MIS 2 was only characterized by the efficient northward APF migration, and late MIS 3 and the LGM were characterized by inefficient APF migration at 47\u0026deg;S in the vicinity of Crozet and Kerguelen islands. Our results agree with Wu et al. (\u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), who suggested the weakening of the ACC during the glacial periods of MIS 2\u0026ndash;4. The inferences drawn for the LGM also corroborate those of Sime et al. (\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and Ai et al. (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), who suggested more southerly positions of the frontal systems during the LGM, which might have resulted in the lower diatom productivity found in core SN\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e4.4 Diatoms and Palaeoceanographic Conditions in the Indian Polar Front Zone\u003c/h2\u003e\u003cp\u003eThe higher abundances of water stratification group diatoms when abundances of the POOZ group diatoms decreased during late MIS 3 and the LGM in our data suggest more stratified conditions during both these periods. Conversely, increased abundances of the POOZ group diatoms in core PS2606-6 during late MIS 3 and the LGM could have resulted from the increased surface nutrients through upwelling (Anderson et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Shukla et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Xiao et al., \u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Gottschalk et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Sigman et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Shukla et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These observations suggest that there might be no efficient northward APF migration during late MIS 3 and the LGM, which led to decreased POOZ group diatoms in the PFZ core SN\u003csub\u003e2\u003c/sub\u003e and, conversely, increased POOZ diatoms south of the APF in core PS2606-6. The increased abundances of water stratification group diatoms in core SN\u003csub\u003e2\u003c/sub\u003e during late MIS 3 and the LGM suggest increased stratification conditions. Thus, the absence of ample nutrients in surface waters from the deep waters through upwelling during late MIS 3 and the LGM could have resulted in decreased POOZ group diatoms and overall lower diatom productivity in core SN\u003csub\u003e2\u003c/sub\u003e (Gottschalk et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Sigman et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The relationship of upwelling in the Indian sector of the SO is related to the intense interaction of the ACC with local bathymetry, based on which upwelling hotspots have been identified (Tamsitt et al., \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The east of the Kerguelen Plateau is proposed as an upwelling hotspot, with higher mean particle transport. Conversely, west of the Kerguelen Plateau receives lesser mean particle transport and is therefore characterized by reduced upwelling (Tamsitt et al., \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The core SN\u003csub\u003e2\u003c/sub\u003e from the west of the Kerguelen Plateau, where less interaction of the ACC with local bathymetry may have resulted in less surface nutrient availability and consequently lower diatom productivity. Further, the high opal fluxes during late MIS 3 and the LGM in the SAZ core located at the east of the Kerguelen Island (Thole et al., 2019) could be attributed to the contribution of iron-rich shelf waters from the Heard Island (53\u0026deg;S) into the surface and subsurface waters of east of the Kerguelen Island (Park et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) and related to the intense upwelling (Tamsitt et al., \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). However, an opposite scenario persists west of the Kerguelen Plateau, where reduced upwelling conditions prevail (Park et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Therefore, the observed lower diatom productivity in core SN\u003csub\u003e2\u003c/sub\u003e during late MIS 3 and the LGM could be attributed to the lack of nutrient-rich deep waters through reduced upwelling in the west of the Kerguelen Islands (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Civel-Mazens et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe latitudinal shifting of the ACC in the SO is proposed to be more northward during the glacial periods and conversely\u0026thinsp;~\u0026thinsp;6 degrees southward during the interglacial periods due to the strength and latitude range of the SO upwelling (Ai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Although the latitudinal shifts of the ACC were concluded not to be strongly correlated during the MIS 2\u0026ndash;4 due to iron fertilization in the SAZ, the ACC was found to be shifted\u0026thinsp;~\u0026thinsp;2 degrees northward during MIS 2\u0026ndash;3, while 0 to 4 degrees southward during the LGM (Ai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Thus, there is a possibility that the location of the northern flank of the APF was at 49\u0026deg;S during late MIS3 and the LGM, currently located at 51\u0026deg;S. As a result, the absence of the APF at the location of core SN\u003csub\u003e2\u003c/sub\u003e of 47\u0026deg;S during late MIS 3 and the LGM might have resulted in lower diatom productivity, as evidenced through decreased POOZ group diatoms and conversely increased water stratification group diatoms. A recent study from the Pacific sector of the SO similarly observed the absence of the APF during the LGM based on the lack of open ocean diatom species (Oliva et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Therefore, we assume that the shifting patterns of ACC and SO upwelling somehow controlled the oceanographic conditions during the late MIS 3 and the LGM, and indeed the northward migration of the APF, which was probably ineffective during both these periods. A southward shift of SHWW has been recorded in the SO during the LGM (Toggweiler et al., \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Kohfeld et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), through which the PFZ waters were possibly pushed into the Antarctic zone, where nutrient-poor waters led to the low diatom productivity due to strong stratification (Francois et al., 1997; Sigman et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), and decreased wind-driven upwelling (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Civel-Mazens et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). A similar condition could have prevailed during late MIS 3, when obliquity-driven SHWW were either more poleward or less mixing of the surface and deep waters (reduced upwelling) could have resulted in lower diatom productivity (Ai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Thus, based on the modern data (Blain et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Pollard et al., \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Robinson et al., \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and down-core studies (Ai et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Civel-Mazens et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), we propose that the lower diatom productivity during late MIS 3 and the LGM in the PFZ of the Indian sector of the SO was possibly controlled by the co-limitation of deep-water iron and macro-nutrient (mainly silica) in the absence of efficient northward APF migration and intense upwelling.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e4.5 Implications for glacial-interglacial atmospheric CO\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e\u003cp\u003eA 5\u0026ndash;10 ppmv decrease in atmospheric CO\u003csub\u003e2\u003c/sub\u003e during 40\u0026thinsp;\u0026minus;\u0026thinsp;18 ka (Bereiter et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) has been proposed due to the strengthening of the biological pump in the SO (Kohfeld and Chase, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Compared with the atmospheric CO\u003csub\u003e2\u003c/sub\u003e records, our new diatom productivity data show coherence with lower diatom productivity during late MIS 3 when atmospheric CO\u003csub\u003e2\u003c/sub\u003e increased by an average of ~\u0026thinsp;10 ppmv compared to the early MIS 2 (Bereiter et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), having a larger magnitude of the diatom productivity (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eF). Nonetheless, both the early MIS 2 and the LGM showed comparable values of atmospheric CO\u003csub\u003e2\u003c/sub\u003e, and each of these periods showed heterogeneous diatom productivity in the PFZ of the SO. Conversely, lower diatom productivity during the deglacial-Holocene periods coincides with the higher atmospheric CO\u003csub\u003e2\u003c/sub\u003e records and agrees with the higher deglacial-Holocene diatom productivity in the Antarctic zone of the SO, when ACC shifts southward (Anderson et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe lower diatom productivity during late MIS 3 and the LGM could be attributed to the more stratified conditions evidenced through the higher abundances of water stratification group diatoms and lower abundances of the POOZ group diatoms. Such stratified surface ocean conditions during late MIS 3 and the LGM, although preventing the release of deep carbon into the atmosphere through upwelling (Anderson et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Sigman et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), might have resulted in the lower POOZ diatom species, especially \u003cem\u003eF. kerguelensis\u003c/em\u003e, a major species of carbon sequestration in the PFZ (Riguel-Hernandez et al., 2015). Thus, the supply of deeper nutrients to the surface through upwelling somewhere counteracts the SO as increased nutrients strengthen the biological carbon pump, while deeper nutrients also carry carbon-rich deep waters (Toggweiler and Samuels, \u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e1995\u003c/span\u003e) that eventually raise the atmospheric CO\u003csub\u003e2\u003c/sub\u003e (Anderson et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Therefore, the reduced upwelling and more stratified conditions may have helped lower atmospheric CO\u003csub\u003e2\u003c/sub\u003e during glacial periods in the Indian sector of the SO (Ronge et al., \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Our diatom productivity data and inferred oceanographic conditions provide essential insights into diatoms\u0026rsquo; role in controlling the biological carbon pump. Moreover, the new diatom productivity data suggest the role of the ACC-driven frontal migration and availability of micro (iron) and macro-nutrient (silica) in shaping the diatom productivity in the PFZ of the SO and therefore show a strong linkage with atmospheric CO\u003csub\u003e2\u003c/sub\u003e (Ai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Gottschalk et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eWe have presented a high-resolution diatom productivity record from the Indian PFZ of the SO, spanning the last 40 ka. The total diatom abundances and diatom community compositions were used to assess diatom productivity and reconstruct the palaeoceanographic conditions in the Indian sector of the SO. Our results demonstrate that the diatom productivity was highest during early MIS 2. In contrast, it was lowest during the deglacial-Holocene periods, with lower intermediate values during late MIS 3 and the LGM periods. The abundances of the POOZ group diatoms covary with the total diatom abundances and show an inverse correlation with the water stratification group diatoms. The higher abundances of POOZ group diatoms during early MIS 2 indicate the effective northward migration of the APF, due to which southern nutrient-rich Antarctic zone waters and stronger SO upwelling might have resulted in high diatom productivity. In contrast, lower intermediate diatom productivity during late MIS 3 and the LGM reflects a less efficient northward migration of the APF, when increased water stratification and reduced upwelling might have limited the supply of deep-water nutrients to the surface. Overall, our results demonstrate that the diatom productivity in the PFZ showed weaker correlation with the fluxes of dust and iron.\u003c/p\u003e\u003cp\u003eThe observed heterogeneous glacial diatom productivity highlights the variable oceanographic conditions that prevailed in the Indian sector of the SO, where deep-water nutrients (mainly iron and silica) controlled the diatom productivity. Thus, the ACC-driven frontal migration in the Indian sector of the SO and the availability of deep-water iron and silica through the upwelling possibly mediated the diatom productivity. The SO upwelling can be counter-intuitive, in that on one hand, deep-water iron and silica boost the diatom productivity and sequester the atmospheric carbon; on the other hand, deep-water carbon raises the atmospheric carbon dioxide. Therefore, our diatom productivity data provide an essential insight for the strengthening/weakening of the biological carbon pump during glacial-interglacial periods.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of Competing Interest\u003c/h2\u003e\u003cp\u003eThere is no known competing interest.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eSupplementary File A\u003c/h2\u003e\u003cp\u003eThe First four principal components (PC1, PC2, PC3, and PC4) are provided for all diatom species of sediment core SN\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThis research was funded by Anusandhan National Research Foundation (ANRF), New Delhi, Core Research Grant (ANRF Project No. CRG/2023/003120). The Director, BSIP, is thankfully acknowledged for the necessary laboratory and infrastructural facilities. We also sincerely thank the Director, NCPOR, for all the encouragement. The research presented in this manuscript is part of AN's ongoing Ph.D. research work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAbelmann A, Gersonde R, Cortese G, Kuhn G, Smetacek V (2006) Extensive phytoplankton blooms in the Atlantic sector of the glacial Southern Ocean. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1029/2005pa001199\u003c/span\u003e\u003cspan address=\"10.1029/2005pa001199\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. 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Palaeogeogr Palaeoclimatol Palaeoecol 129:213\u0026ndash;250\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003e\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/s0031-0182(96)00130-7\u003c/span\u003e\u003cspan address=\"10.1016/s0031-0182(96)00130-7\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Birbal Sahni Institute of Palaeosciences, Lucknow, India","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Antarctic Circumpolar Current, Frontal Migration, Upwelling, Diatom Community Composition, Principal Component Analysis, Heterogeneous glacial diatom productivity","lastPublishedDoi":"10.21203/rs.3.rs-7470794/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7470794/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHigh diatom blooms characterize the Crozet and Kerguelen Plateaus of the Southern Ocean (SO) due to the natural iron fertilization despite the co-limitation of iron and light in the Polar Front Zone (PFZ). However, information on glacial-interglacial diatom productivity and associated biological carbon pump from this region is sparse. We present a diatom productivity record to decipher the palaeoceanographic changes for the past 40 ka using sediment core SN\u003csub\u003e2\u003c/sub\u003e (47\u0026deg;S and 57\u0026deg;30\u0026rsquo;E) amidst the Crozet and Kerguelen Plateau. Our results show the highest diatom productivity during early Marine Isotope Stage (MIS) 2 (29.5\u0026ndash;23 ka), lowest during the deglacial-Holocene periods (18\u0026thinsp;\u0026minus;\u0026thinsp;7.8 ka), with lower intermediate diatom productivity during late MIS 3 (40-29.5 ka) and the Last Glacial Maximum (LGM, 23\u0026thinsp;\u0026minus;\u0026thinsp;18 ka). The abundances of the permanent open ocean zone (POOZ) group diatoms covaried with diatom productivity and showed an inverse correlation with water stratification group diatoms. The patterns of diatom productivity and the POOZ group diatoms do not strongly correlate with the fluxes of dust and iron. Based on the inverse correlation between the diatom groups from the PFZ (core SN\u003csub\u003e2\u003c/sub\u003e) and the Antarctic zone, we suggest that higher diatom productivity during early MIS 2 could be due to the availability of nutrient-rich southern waters through SO upwelling as a result of the northward Antarctic Polar Front (APF) migration. Conversely, the lower intermediate diatom productivity during late MIS 3, the LGM, and the deglacial-Holocene periods could have resulted from the unavailability of southern nutrient-rich waters due to inefficient APF migration and weaker SO upwelling. We propose that the Antarctic Circumpolar Current-driven APF migration and the intensity of upwelling possibly resulted in heterogeneous diatom productivity in the Indian PFZ of the SO. Consequently, the availability of deep-water nutrients (iron and silicate) might have controlled diatom productivity and was responsible for the strengthening/weakening of the biological carbon pump.\u003c/p\u003e","manuscriptTitle":"Deep-water nutrients mediated glacial-interglacial diatom productivity in the Indian Polar Front Zone of the Southern Ocean","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-28 13:12:44","doi":"10.21203/rs.3.rs-7470794/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c6a9e0ae-99de-4801-8eca-eb851b609868","owner":[],"postedDate":"August 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":53788400,"name":"Geology"},{"id":53788401,"name":"Oceanography"},{"id":53788402,"name":"Paleoecology"},{"id":53788403,"name":"Climatology"}],"tags":[],"updatedAt":"2025-08-28T13:12:44+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-28 13:12:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7470794","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7470794","identity":"rs-7470794","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}