Insolation-triggered Eurasian Ice Sheets collapse initiates the Last Termination

Insolation-triggered Eurasian Ice Sheets collapse initiates the Last Termination | 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 Article Insolation-triggered Eurasian Ice Sheets collapse initiates the Last Termination Yanguang Liu, Wu Dong, Siqi Li, Jón Eiríksson, Esther Guðmundsdóttir, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5588827/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The collapse of Northern Hemisphere ice sheets has been deemed as a trigger for the chain of positive climate feedback during glacial terminations in Quaternary. Increasing boreal summer insolation is considered the ultimate driver of their collapse, however, the initiating mechanisms remain elusive. Here we report an unambiguous warming trend in the southern Nordic Seas, which coincides with the initial phase of Northern Hemisphere summer insolation increase at the end of the Last Glacial Maximum. A subsequent phase of surface cooling is observed, closely corresponding to the massive freshwater discharge attributed to the meltwater pulse at 19ka BP. Our reconstructions demonstrate that the initial collapse of Northern Hemisphere ice sheets during the Last Termination occurred in Eurasian Ice Sheet, driven by a chain of oceanic responses to the insolation increase. Specifically, increasing boreal insolation induced a northward migration of the mid-latitude Westerlies under a positive NAO phase, promoting poleward oceanic heat transport and hence subsequent warming in Nordic Seas, thereby accelerating the ablation of the marine-based Eurasian Ice Shelves. This led to a catastrophic release of icebergs into Nordic Seas, eventually triggering a series of ocean circulation feedbacks that further promoted the deglaciation. Earth and environmental sciences/Climate sciences/Palaeoceanography Earth and environmental sciences/Climate sciences/Palaeoclimate Earth and environmental sciences/Climate sciences/Biogeochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Quaternary glacial cycles have been closely linked to the changes of Earth's orbital parameters 1 , 2 , characterized by a long cooling period ending with a relatively short warming interval marked by significant reduction of ice sheets, a “sawtooth” shape variation consistent with boreal summer insolation changes 3 , 4 . Identifying the processes that triggered these rapid collapses of ice sheets, also known as the "glacial terminations", is essential for elucidating the mechanisms of glacial cycles 5 . A commonly discussed hypothesis attributed the terminations to the peak of surface air temperature induced by the insolation maxima 6 , leading to a rapid thinning and basal-sliding of the oversized continental ice 6 , 7 . The following extensive release of freshwater disrupted interhemispheric ocean circulations, ultimately initiating an interglacial climate across the planet 6 , 8 . However, precisely dated ice-rafting events in the high latitude North Atlantic 9 – 11 indicate that the culmination of boreal iceberg discharge occurred thousands of years prior to the marked increase in Greenland ice core temperatures 12 during the Last Termination (Fig. 2 A, B). Moreover, the initial reduction of Atlantic Meridional Overturning Circulation (AMOC) also preceded the episodic retreat of the dominated Laurentide Ice Sheet 9 , 13 , 14 (LIS). Numerical models suggest that ocean temperature warming can cause the grounding line retreat of marine ice shelves, destabilizing the overall stability of the continental ice sheets 15 , 16 . This implies that the coupled ocean-icesheet system may exhibit more prompt and earlier feedbacks to insolation change than the atmospheric warming and internal oscillations of ice sheets at the onset of Last Termination 5 , 17 , 18 , and the oceanic processes could potentially serve as a key trigger for the collapse of the northern ice sheets 9 , 13 . To comprehend the factors that triggered the Last Termination, it is crucial to determine the extent to which the decay of northern ice sheets is systematically linked to the environmental conditions of the surrounding oceans, and how such an indirect solar-driving mechanism can produce the sensitive oceanic response observed at the end of a glaciation cycle. In high latitudes, the effects of insolation are considered to be potentially amplified in sensitive ocean regions and efficiently transmitted to the margins of ice sheets through thermohaline circulation 19 , 20 . In addition, the disparity in solar heating between the tropics and poles (latitudinal insolation gradient) can regulate the poleward-oceanic heat and moisture transport by modulating the latitudinal temperature gradient and mid-latitude wind fields, potentially influencing the dynamics of ice sheets 21 , 22 . During the last glacial maximum (LGM), the Eurasian Ice Sheet complex (EIS) attained a peak ice volume responsible for around 24 m of eustatic sea-level reduction 23 , 24 , forming an extensive interface with the Nordic Seas and the North Atlantic 25 , 26 . The southern edge of EIS is situated downstream of the North Atlantic Current (NAC), which constitutes an integral part of the upper branch of the AMOC, thereby potentially undergoing substantial impacts from the temperature fluctuations induced by the Atlantic warm water 27 . Since the Nordic Seas are the main gateway of the iceberg trajectory associated with the EIS, and large marine-based sectors of the glacial EIS extended all the way to its continental shelf edge 28 , 29 , the sedimentary records of the Nordic Sea are ideal for studying the interaction between ocean dynamics and the ice sheet instabilities. In the present study, we combined multi-biological proxy data from the southern Nordic Seas to reconstruct the evolution of upper ocean water masses under the orbital forcing. Processes responsible for inflow Atlantic Water (AW) oscillations are discussed with aid of transient climate simulations to better understand the role of the fluctuations of northward oceanic heat transport in triggering the early instabilities of EIS during the transition period of Last Termination. Furthermore, we assessed the impacts of intermittent iceberg discharges from the EIS on the advanced slowdown of AMOC prior to Heinrich Stadial 1 (HS1). Results and discussion Geological setting, chronology, and geochemical proxies During the 5th China Arctic Research Expedition in August 2012, a gravity core (ARC5-IS-1B, 65°36′N, 8°59′W, here after IS-1B) was retrieved in the north of the Iceland-Faroe Ridge at 819 m water depth (Fig. 1 ). The core location lies beneath the path of the modern Faroe Current, the main branch of the warm AW inflow and an integral component of the upper branch of the AMOC 30 , suggesting that IS-1B is a sensitive recorder of the past ocean dynamics in temperature and salinity 31 . The chronostratigraphic framework of core IS-1B was established by eight accelerator mass spectrometry radiocarbon (AMS 14 C) ages spanning the past ~ 30 ka. We measured the Mg/Ca ratio of dominantly polar water-dwelling planktonic foraminifera Neogloboquadrina pachyderma sinistral ( N. pachyderma (s)) as a proxy for the temperature of surface cold water mass (SST cwm , Supplementary Materials). The positive correlations between salinity and the stable oxygen isotopic composition of sea water (δ 18 O sw ) are widely accepted across different oceans 32 – 34 , so that the ambient δ 18 O sw information was extracted from the N. pachyderma (s) to calculate the global ice-volume-effect-removed oxygen isotopic composition (δ 18 O sw−local ), allowing us to reconstruct the salinity in the surface cold water mass (SSS cwm ). To further monitor the stability of surface cold water mass and the sensitivity of upper ocean structure in response to the oscillation in northward heat flux, we measured the Mg/Ca temperatures of individual foraminiferal shells within each sampling layer, combined with their standard deviation, to study the degree of temperature fluctuations at the sampling interval resolution. Surface cold water mass warming in later stage of LGM In the Nordic Seas, the interaction between inflowing warm AW and fresh meltwater through convection is closely linked to regional climate and sea ice behavior 40 . More recent reconstructions and modeling simulations have indicated that the AW never ceased to flow into the Nordic Seas during the glacial period, and the warm surface inflow subducted to the subsurface and intermediate area below the thick halocline and sea ice 40 , 48 , 49 . Our new SST cwm record from the core IS-1B show relatively low values over most of the LGM. However, an intriguing temperature increase of 2.72℃ after 21.1 ka BP marks a change from the previous interval characterized by weak fluctuations near the freezing point, supporting the notion of upper ocean warming in the southern Nordic Seas during the late stage of the LGM (Fig. 2 F). At the same time, an SSS cwm increase, which was greater than the impact of global ice volume forcing, revealed that the saltier surface cold water mass may have reduced the surface salinity gradient and stability of the halocline (Fig. 2 D). Another salient feature is the synchronous upward trend of the Mg/Ca temperature standard deviation in individual shells (Fig. 2 E). This drastic increase in the degree of dispersion of temperature distribution indicates that temperature inside the water mass varied frequently over a wide range. Compared to the growth of average temperature, the frequent variability of individual shell temperature signals most probably indicates instability of the cold water mass. This instability and the warming trend could be further associated with a thinning halocline caused by local invasion of subducted warm AW, which could ultimately lead to the vertical shift of AW crossing the halocline (Fig. 5 B). Further comparisons of proxy records from adjacent stations in the Nordic Seas and North Atlantic, alongside TraCE simulation results, yield two key observations. First, the general trend of temperature increase at the end of LGM is visible in surface North Atlantic 42 , the eastern Nordic Seas margin close to the Fennoscandian Ice Sheet 38 (FIS) and the temperature profiles simulated by the TraCE model (Fig. 3 J, K and Fig. 4 A). This warming trend, although significant, was not as large as other pronounced amplitudes in the last deglacial period and Holocene. Nonetheless, the surface warming trend reported in dinocyst records 35 (Fig. 3 L) farther downstream of the Atlantic Water pathway also reveals that the northern boundary of such a short-term warming event may have reached the Svalbard Islands and affected the entire Nordic Seas from south to north. Secondly, evidence of prominent warm intermediate water appears in the benthic records from the southeastern Nordic Seas, suggesting a continuous inflow of AW throughout most of the LGM 40 (Fig. 5 A). The correlation with the consistent surface warming in the subpolar North Atlantic is likely reflect a significant accumulation of ocean heat during the later LGM 42 , which further accelerates the northward heat transport and contributes to the buildup of a subsurface heat reservoir in the Nordic Seas. Although the larger benthic-atmosphere (B-A) 14 C offset revealed that the Norwegian Sea was poorly ventilated during much of the late glacial and deglacial period 37 , 41 , the reversed decrease in ventilation age is presumably considered as a brief resumption of upper ocean convection around 20.5 ka BP (Fig. 3 F). The synchronous onset of SST cwm increase observed in our records when the ventilation in Faroe–Shetland Channel and central Norwegian Sea transitory was enhanced 37 , 41 (Fig. 3 E, F), provides evidence from another angle for a causal role of subsurface AW upwelling in strengthening the surface mixing and warming. In particular, the substantial heat emission from AW could facilitate the sea-air exchange, causing a more active sea ice and a further-retreated ice margin in the glacial Nordic Seas 64 . Thus, the potential contribution of the continuous heat transport from AW and this millennial-scale warming to the altered climate should not be ignored. Ocean response sensitive to the insolation changes From a broader perspective, the observed changes in surface and subsurface Nordic Seas hydrography during the late stage of LGM could have been a local feedback mechanism to global scale climate driving forces. Our proxy data of temperature standard deviations (Fig. 2 E) indicate that the starting point of surface warming may be earlier than previous deductions from the studies in the Norwegian Sea and the area south of Iceland 38 , 65 , 66 . It is important to note that the timing of our increasing temperature standard deviation and the SST cwm warming peak, constrained by two radiocarbon dates at 22.7 ka BP and 18.6 ka BP, respectively, are systematically reliable and independent of the age model uncertainties. These suggest that the instability of surface cold water, as a result of the weakened halocline when the subsurface AW transformed into an active mode, may begin earlier at ~ 22 ka BP, even though the total average surface temperature varied slightly in the interval of 22–21 ka BP. Moreover, the timing of the AW upwelling is concurrent with the significant recovery of 65°N caloric summer insolation at around 22 ka BP 54 , 67 (Fig. 3 A), which may reveal a close link between solar forcing and the heat accumulation in the AW layer of Nordic Seas. Furthermore, the concurring initial weakening of latitudinal winter-insolation-gradient between 60°N and 30°N 22 , 54 and the subsequent basin-wide surface warming trend (Fig. 4 A) suggest the latitudinal variations in external insolation forcing are accompanied by positive regional oceanic feedback processes. During the cold conditions, the apparent instantaneous oceanic reaction to the growing solar insolation in the surface Nordic Seas can be shown in the reduced sea ice cover and lower ice-albedo feedback, as well as the in situ warming surface water affecting the hydrographic conditions 68 – 70 . However, the model simulation results revealed that the energy of local solar insolation are insufficient to drive large-scale surface warming associated with the ice sheet instabilities and major climate transitions in the north Hemisphere 5 , 46 , 71 , 72 . Our correlations between ocean feedback and solar forcing are most likely tied to the orbitally forced modulations of atmospheric-oceanic variability and the poleward flux of latent/sensible heat. Indeed, the solar influence on climate of the magnitude and consistency implied by our evidence could not only have been confined to the Nordic Seas. As demonstrated in late Pleistocene and Holocene, the weaker winter latitudinal temperature gradient, first order forced by the decreasing winter latitudinal insolation gradient, has been found to cause a positive North Atlantic Oscillation (NAO + ) state, promoting poleward shifts in the position of the westerly flow 21 , 22 . Similarly, model simulation results of TraCE experiments 8 , 73 have indicated that isolated orbital forcing can lead to an increase in the intensity of 850 hPa wind south of Iceland since 22 ka BP (Fig. 4 B; Supplementary Materials), which is also consistent with the trend of NAO + phase. This enhanced wind stress in the cyclone direction can present a pronounced east-west extension of subpolar gyre 74 (SPG), resulting a stronger AMOC and NAC with higher latitudinal heat flux transport into the North-East Atlantic and the Norwegian Seas 75 – 77 . Meanwhile, the TraCE results revealed that the growing influence of insolation also strengthened the winter surface winds in the Nordic Seas at the later stage of the LGM 8 , 73 (Fig. 4 B). This implies the enhanced surface winds may have further intensified the vertical mixing and facilitated the upwelling of the subsurface AW 78 . We thus infer that the heat accumulation in the AW layer and the observed surface warming in Nordic Seas are mainly triggered by the atmospheric-oceanic circulation process underlying the orbital forcing mechanism. Upper ocean water mass activities as a trigger for the termination of EIS and the deglacial climate The reduction of the LIS and EIS from maximum volume and the acceleration occurring during HS1 represent a rapid transition in recurring ice sheet cycles and the termination of last glaciation climate 5 , 20 , 79 . The onset of the deglacial epoch in the Nordic Seas is interpreted to have a chilled surface conditions at around 19 ka BP in the wake of the anomalous warming event in the late LGM, as evidenced by our N. pachyderma (s) Mg/Ca temperature record and the enhanced IRD concentration in core IS-1B (Fig. 3 I, M). This is in line with the abrupt rise in sea level and the melt water pulse recognized at 19 ka BP (19ka-MWP) 56 , 80 . Paleoglaciological studies describe a significant ice margin retreat of the British-Irish Ice Sheet (BIIS) and FIS accompanied by the disintegration of the North Sea Ice Bridge and an enormous meltwater outburst into the Nordic Sea and North Atlantic during this period 39 , 81 , 82 . The separation of the FIS and BIIS resulted in the collapse of an ice-dammed lake in the North Sea, finally leading to a subsequent catastrophic fresh water release which contributed to the 19ka-MWP event and the elevated IRD flux in Nordic Seas 82 – 84 . Meanwhile, Rørvik et al., argued that the main IRD maxima and plumate deposition records in the Norwegian shelf post-date periods with increased adjacent sea temperature may indicate that upper ocean warming was probably an important external forcing factor on the fast-flowing paleo-ice streams and meltwater release from the EIS 38 , 39 (Fig. 3 H). Based on the close correspondence between the anomalously warm conditions and meltwater activities recorded in our study region and both margins of the FIS 38 , 39 and the BIIS 42 , we presume that the extensive upper ocean warming in Nordic Seas and the strengthened ocean heat stored near the grounding line continuously eroded the ice shelves, further constituting a warm moisture source triggering the collapse of the southern EIS and massive iceberg/meltwater output at the beginning of the Last Termination (Fig. 5 B). Thereafter, evidence from the B-A 14 C offset reconstruction at 18.8 ka BP led to the interpretation that the enhanced freshwater flux hampered the Nordic Seas surface convection and deep-water ventilation 37 (Fig. 3 F). At this point, the proxy records of AMOC strength show an early decline in the Rockall Trough 14 (Fig. 3 D), which reflects the initiation of ocean circulation reduction in high latitude prior to HS1. However, the classic AMOC signals 61 , 62 of mid latitude Atlantic remain invigorated in 19ka BP, and its successive decrease trend came hundreds of years after that of the north. We hypothesized that the freshwater pulses causing these sequential slowdowns of AMOC at different latitudes are all originated from the EIS, and the delay of mid latitude can be interpreted as a two-stage process of the meltwater release in Nordic Seas. As soon as the rapid cooling in the Nordic Sea, due to the fast cooling at 19ka BP, the occurrence of thick and perennial sea ice limited the southward transport of massive icebergs collapse from the EIS into mid latitude Atlantic (Fig. 5 C). This is in line with the previous sea ice reconstructions from extremely low PIP25 level of south Norwegian Sea proposing an expansive perennial sea ice sealed the Nordic Seas and icebergs 63 (Fig. 3 G). As the gradual rise of boreal summer insolation, the secondary release of sea ice and icebergs occurred over the next thousand years, leading to the meltwater intrusions in mid latitude and the substantial collapse of North Atlantic Deep Water (NADW) formation. In addition, this also indicates that the initial weakening of the AMOC during the last deglaciation may have been caused by a precursor event involving European freshwater discharge, rather than the HS1 event attributed to the instabilities of the LIS 9 , coinciding with the identification of additional European-origin IRD flux predating the LIS surges in the Heinrich layer 85 , 86 . Physically, the meltwater discharged through the Nordic Seas would have led to a direct isolation of AW and the formation of a stronger halocline, potentially resulting in the redistribution of heat in the upper layer and a basin-wide subsurface warming in the Atlantic 9 , 13 , 48 , 87 . Several model simulations have indicated that AMOC reduction configures a feedback mechanism with subsurface ocean warming, which even affects a depth of approximately 2,500 m due to continued downward mixing of heat in the Atlantic basin 13 , 49 . This subsurface temperature anomaly can propagate on advective timescales toward the Labrador Sea 13 . Enhanced heat transfer would accelerate the basal melt below the Labrador ice shelf, substantially increasing the calving rate of the LIS 13 , 87 . Accordingly, the sudden loss of buttress is considered to have resulted in large ice shelf disintegration corresponding to the first pronounced peak of the IRD flux and freshwater discharge, marking the prologue of HS1 and deglaciation climate 15 . Overall, both solar insolation, thermal transformation and ocean-ice sheet interactions were clearly interlocked in the millennia preceding HS1. This explains why it is critical to highlight the role of upper ocean activities in the initiation of Last Termination. Cheng et al. suggested that the promoted ventilation of ocean CO 2 reinforced interhemispheric teleconnections and augmented global warming during the ice-sheet terminations 4 . We also consider the rising atmospheric CO 2 to be a key factor to complete the deglaciation. However, the timing of obvious increase in atmospheric CO 2 concentration, as revealed in ice cores 88 , 89 , is later than the early collapse of EIS, suggesting that CO 2 may have less impact on the boreal ice-sheet process at the onset of Last Termination. As summarized in our proxy records, we propose that the anomalous warming in the surface Nordic Seas amplified the effect of boreal summer insolation, contributing to the meltwater surge from the EIS at the beginning of the Last Termination. This signifies an earlier and more prompt feedback of the ocean heat conveyor system to the solar activity in initiating the collapse of northern ice sheets. Subsequently, a two-stage freshwater release through the Nordic Seas shifted North Atlantic to a state with weak AMOC and NADW formation, further inducing large-scale subsurface warming and triggering the destabilization of the LIS 9 , 13 . Our findings expand the climatological evolutionary chain of the driving mechanism for the Last Termination of Northern hemisphere ice sheets to an earlier period (the late stage of the LGM), and support the implication that the sensitive oceanic responses to enhanced solar insolation are primary trigger of the climate transition in the Last Termination. Methods Age models Radiocarbon ( 14 C) analyses were performed at the Beta Analytic Laboratory, Florida, USA, on the dominant foraminifera N. pachyderma (s), using accelerator mass spectrometry (AMS) to establish the chronostratigraphic framework. We picked N. pachyderma (s) up to a weight of ~ 12 mg with the single size fraction between 250–350 µm from eight representative layers for accurate AMS measurement. To establish the calendar age-depth model, radiocarbon dates were calibrated using the R package Bacon (v2.3.9.1) 90 and CALIB 7.04 software 91 , both of which used the Marine13 dataset 92 . Reservoir ages of 800 and 400 years were applied during the LGM and from the last deglacial period to the Holocene, respectively 93 , 94 . Table 1 AMS 14 C dating data and the calibrated calendar age of core IS-1B Station Laboratory code Depth/cm Species AMS 14 C age/a Calendar age (1σ calibrated)/a IS-1B Beta-426129 5–7 N.pachyderma (sin.) 1560 ± 30 1120 ± 6 IS-1B Beta-426130 17–18 N.pachyderma (sin.) 4350 ± 30 4472 ± 18 IS-1B Beta-426131 30–31 N.pachyderma (sin.) 6780 ± 30 7303 ± 31 IS-1B Beta-497030 39–40 N.pachyderma (sin.) 8550 ± 30 9186 ± 40 IS-1B Beta-426133 70–72 N.pachyderma (sin.) 15760 ± 50 18636 ± 71 IS-1B Beta-497031 80–82 N.pachyderma (sin.) 18710 ± 60 21705 ± 81 IS-1B Beta-426134 92–96 N.pachyderma (sin.) 22620 ± 90 26024 ± 94 IS-1B Beta-426135 125–130 N.pachyderma (sin.) 28010 ± 140 31152 ± 128 The Bacon age model was constructed with an accumulation rate distribution width of 1.5, a default student-t distribution, and a section thickness of 2 cm. The age-depth model calibrated with CALIB 7.04 was established based on linear interpolation and extrapolation of eight age control points (Table 1 ). The results show that there is little difference between these two age models. Therefore, we follow the Bacon age-depth model in this research to reconstruct the Bayesian accumulation histories of core IS-1B (Fig. 6 ). Oxygen isotope Stable oxygen isotope analysis of planktonic foraminifera (δ 18 O c ) was carried out by 20–25 specimens of N. pachyderma (s) on mass-spectrometer (MAT253 Plus) combined with the carbonate preparation device (Kiel Ⅲ) in Laboratory of Pilot National Laboratory for Marine Science and Technology, Qingdao, China. Data are reported in per-mil notation (‰) relative to the Vienna Pee Dee Belemnite (VPDB) scale and referenced to the NBS18 standard (δ 18 O VPDB = − 23.0 ± 0.1‰). The analytical standard deviation indicated by replicate measurements was less than 0.07‰. Mg/Ca temperatures of N. pachyderma (s) and salinity of surface cold water mass For the Mg/Ca ratio measurements, 12 specimens of N. pachyderma (s) (intact, no contamination, ~ 300 µm size fraction) were continuously picked from each layer of the sediment core at 2 cm intervals. The foraminiferal samples were first ultrasonically washed in deionized water and methanol to remove clay, followed by oxidative cleaning by immersion in hydrogen peroxide to remove organic material 95 . The Mg/Ca ratios were measured using a Thermo Scientific RQ ICP-MS coupled to an Geolas ArF excimer laser at the Key Laboratory of Marine Geology and Metallogeny at the First Institute of Oceanography, Qingdao, China. An N 2 gas stream was used to transport the ablation products to the RQ ICP-MS. The RQ ICP-MS instrument response was calibrated using SRM NIST610, 612, and 614 glasses and corrected for variations in the ablation yield and instrument drift by normalizing to the measured internal standard elements ( 88 Sr and 55 Mn) intensities during each mass spectrometer cycle 96 , 97 . To isolate only the banding patterns in our data, ICPMSDataCal12.2 98,99 software was applied to the RQ ICP-MS signals to remove low- and high- frequency noise. The results indicate that 3–5 s contamination signals enriched in Fe and Mn on the outermost of shell surface should be removed hence 10–30s continuous and stable signals can be obtained depending on the shell wall thickness. The Mg/Ca ratios for each sample of N. pachyderma (s) were determined from the ratios of the average polished signal intensity in the ablation profile 100 . Surface cold water mass temperatures (SST cwm ) were estimated from the average Mg/Ca ratios of 12 individual tests of N. pachyderma (s) in each sediment layer, based on different N. pachyderma (s) specific transfer equations 101 – 103 and multispecies transfer Eq. 1 04 . The comparision between the instrumental temperature at calcification depth and the calculated N. pachyderma (s) Mg/Ca temperature from the IS-1B core-top suggested that equation of Nurnberg 102 (Eq. 1) established with N. pachyderma (s) from high northern latitudes best matched our downcore samples (Supplementary Materials). Mg/Ca = 0.55 exp (0.099×T) (1) The ambient seawater oxygen isotope (δ 18 O sw ) was calculated from SST cwm and foraminifera shells δ 18 O c using the equation of Nyland et al. 105 (Eq. 2), modified after Shackleton 106 and O’Neil et al. 107 with calibration (Eq. 3) to convert the calcite value from the VPDB to the Vienna Standard Mean Ocean Water scale 108 (δ 18 O sw, V−SMOW ). δ 18 O sw = δ 18 O c - \(\:\left[\frac{4.38-\sqrt{{4.38}^{2}-0.4\times\:\left(16.9-T\right)}}{0.2}\right]\) (2) δ 18 O sw, V−SMOW =(δ 18 O sw +0.2‰)/0.998 (3) In addition, the positive correlations between salinity and δ 18 O sw, V−SMOW are widely accepted over different oceans 32 – 34 so that the global ice volume and sea level effect were removed (δ 18 O sw−local ) to further reconstruct the salinity in the surface cold water mass (SSS cwm ) using Eqs. 4109,110 and 5108. δ 18 O sw−local= δ 18 O sw, V−SMOW -0.0083‰×SL (4) SSS cwm =(δ 18 O sw−local +12.17)/0.36 (5) Where SL is sea level changes come from the curve of Lambeck et al 23 . LA-ICP-MS test conducted on N. pachyderma (s) There was a significant difference in the shell diameter and thickness of N. pachyderma (s) in different sea areas. Therefore, we tried different laser intensities, pulse rates and ablation scopes to obtain steady and clear signals of N. pachyderma (s) in core IS-1B. Experiments ultimately showed that the 60 µm diameter circular spots and laser pulse rate of 4 Hz at a fluence of 3 J/cm 2 coupled well with N. pachyderma (s) with 10–30 s persistent signals. Considering the growth cycle of different chambers in N. pachyderma (s), we performed laser ablation on the four largest chambers in a single shell to compare the Mg/Ca between chambers (Fig. 7 B). Eight shells from different layers reveal that the Mg/Ca temperature difference between four different chambers in single shell are less than 0.47 ℃, which is much less than the temperature amplitudes observed in our time series and the summer-winter temperature gap of ambient water. We conclude that the growth cycle effects on the production of N. pachyderma (s). was minor with respect to the reconstructed SST cwm . Therefore, the last chamber with a bulky and thick shell wall was selected for ablation (Fig. 7 A). During ablation, we noticed that the initial signals showed abnormal signal peaks for Mg, Fe and Mn (Fig. 8 ). Because the signals reflect the polluted outermost layer of the shells caused by post-depositional diagenesis, we abandoned these signal zones for a more stable and accurate reconstruction of the element ratio. TraCE Experiments To understand the role of large-scale ocean-atmosphere circulations in the evolution of the Nordic Seas and Eurasian ice sheets during this period, we carried out the TraCE simulations 73 , 111 to elucidate the remote North Atlantic forcing, which was relevant to the change of solar insolation. The TraCE transient climate experiments were performed in the Community Climate System Model ver. 3 112–114 (CCSM3), a global coupled atmosphere-ocean-sea ice-land general circulation model (AOGCM) that has a latitude–longitude resolution of ∼3.75° in the atmosphere and ∼3° in the ocean and includes a dynamic global vegetation module. The prescribed forcings and boundary conditions for the full-forcing simulation included orbitally forced insolation changes, increasing atmospheric concentrations of the long-lived greenhouse gases, and retreating ice sheets and associated meltwater release to the oceans 73 , 111 . Meanwhile, the transient sensitivity experiments adopt a factorized forcing scheme to allow isolating each of these forcings separately 115 . Therefore, the isolated insolation forcing experiments were selected to simulate the changes in winter 850 hPa wind velocity during the warming peak (20-22ka BP), aiming to examine the effect of increasing solar insolation on atmospheric circulation over the north Atlantic. We also compared the winter surface wind velocity and sea temperatures of all-forcing simulation from 20-22ka BP to study the ocean process related to this warming trend. All the changes were analyzed with Student’s t test, with confidence levels greater than 95%. Declarations Competing interests The authors declare no competing interests. Author contributions D.W., Y.L. and X.Z. designed the study; Y.L. sampled the core; W.D. constructed the age model, prepared and analyzed δ 18 O data, performed the LA-ICP-MS test and drafted the original manuscript; S.L. and X.Z. contributed to the model simulation results; D.F. contributed substantially to all aspects; J.E., E.R.G, Y.R and N.H. were responsible for the evaluation and quality control. All authors contributed to the interpretation and the final manuscript. Acknowledgements This research was supported by the National Key Research and Development Program of China (Grant No. 2023YFF0804600, 2023YFF0805200) and the National Natural Science Foundation of China (Grant No. 42076079, 41676053). We thank the crew of icebreaker Xuelong for assistance in retrieving the sediment core. We thank Yan Zhang (First Institute of Oceanography, MNR) for her technical assistance of LA-ICP-MS. 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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-5588827","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":397442106,"identity":"d7809492-cd57-4841-8e68-ae4b129f0607","order_by":0,"name":"Yanguang Liu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9UlEQVRIiWNgGAWjYDADPmYGxgcVBqRoYWNmYDY4Q5oWIJI4Q4xKg+NnD7/mqblj18bOY1ZxoMCGgV/6+AWGnzvwaDmTl2bNc+xZchszj9mNAwZpDJJ9OQWMvXjsMziQY2bMw3Y4mQ2o5fYHg8NAQ3gSmBnb8Gg5/wao5R9ES8EBorTcyDF+zNt22A6khQGihf0AXi2SN96YMc7tO5zAxsxWLAH0C49kDw/DwV48WvjO5xh/ePPtsD0//+GNHw78sZHj52F/+OAnHi0KB4DRAaQTG6ACPEBkcAC3BgYG+QYG5g9A2h5JjP0BPh2jYBSMglEw8gAA6DlOCapAe0sAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-4524-2930","institution":"First Institute of Oceanography, MNR, China","correspondingAuthor":true,"prefix":"","firstName":"Yanguang","middleName":"","lastName":"Liu","suffix":""},{"id":397442107,"identity":"966aabf3-2a44-4e25-be10-e22c9ab51a01","order_by":1,"name":"Wu Dong","email":"","orcid":"https://orcid.org/0009-0000-3511-9499","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Wu","middleName":"","lastName":"Dong","suffix":""},{"id":397442108,"identity":"a638a6ad-effc-45bd-b92e-f34f5200e901","order_by":2,"name":"Siqi Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Siqi","middleName":"","lastName":"Li","suffix":""},{"id":397442109,"identity":"62c3d63a-1a71-49ee-986e-d46687b5698e","order_by":3,"name":"Jón Eiríksson","email":"","orcid":"","institution":"University of Iceland","correspondingAuthor":false,"prefix":"","firstName":"Jón","middleName":"","lastName":"Eiríksson","suffix":""},{"id":397442110,"identity":"e41bc370-5ad7-407a-8589-13a1e78286f7","order_by":4,"name":"Esther Guðmundsdóttir","email":"","orcid":"","institution":"University of Iceland","correspondingAuthor":false,"prefix":"","firstName":"Esther","middleName":"","lastName":"Guðmundsdóttir","suffix":""},{"id":397442111,"identity":"836fb232-7f79-4e15-9484-993728336ebf","order_by":5,"name":"Yair Rosenthal","email":"","orcid":"","institution":"Rutgers the State University","correspondingAuthor":false,"prefix":"","firstName":"Yair","middleName":"","lastName":"Rosenthal","suffix":""},{"id":397442112,"identity":"084d8042-b3ee-4985-82dc-fbaefe683c5a","order_by":6,"name":"Ningjing Hu","email":"","orcid":"","institution":"First Institute of Oceanography, Ministry of Natural Resources, Qingdao, China","correspondingAuthor":false,"prefix":"","firstName":"Ningjing","middleName":"","lastName":"Hu","suffix":""},{"id":397442113,"identity":"62776754-10ee-4b91-a8a4-3fef8c8a8471","order_by":7,"name":"Dejiang Fan","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Dejiang","middleName":"","lastName":"Fan","suffix":""},{"id":397442114,"identity":"76d50f1e-361a-46dd-8ca1-a4da0a9d0dce","order_by":8,"name":"Xu Zhang","email":"","orcid":"https://orcid.org/0000-0003-1833-9689","institution":"British Antarctic Survey","correspondingAuthor":false,"prefix":"","firstName":"Xu","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2024-12-05 17:40:44","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5588827/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5588827/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":73074571,"identity":"883cbbc6-9222-4376-80f7-f7f1db1ba4cf","added_by":"auto","created_at":"2025-01-06 13:09:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":762454,"visible":true,"origin":"","legend":"\u003cp\u003eLocation map of studied cores and the pattern of modern surface ocean circulation in the study area. \u0026nbsp;Yellow star: location of core site IS-1B (this study); white squares: location of reference core sites, SV-0435 (74°57′N, 13°54′E; 1839 m water depth), PS124336,37 (69°22′N, 6°32′W; 2710 m water depth), MD99-229438,39 (68°18′N, 10°30′E, 1122 m water depth), JM11-F1-19PC40,41 (62°49′N, 3°52′W, 1179 m water depth), MD01-246142 (51°45′N, 12°55′W, 1153 water depth), DAPC214 (58°58′N, 9°36′E, 1709 m water depth), GeoB18530-19 (42°50′N, 49°14′W, 1888 m water depth) and black dot: North Greenland Ice Core Project43,44 (NGRIP; 75°50′N, 42°17′W). Red arrows indicate modern warm inflow of Nordic Seas: NAC (North Atlantic Current), FC (Faroe Current), IC (Inminger Current) and WSC (Western Spitsbergen Current); dark blue arrows indicate modern cold outflow of Nordic Seas and Labrador Sea: EGC (East Greenland Current), EIC (East Iceland Current) and LC (Labrador Sea Current); solid white line indicate SPG (Subpolar Gyre); white dashed line indicate Arctic Front and Polar Front36,45 . The area shaded in white indicate the approximate extent of ice sheets during the LGM46 and the area shaded in black indicate the ice-rafted detritus (IRD) belt in the North Atlantic47.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/28a7a18f3559b6c04eeb4407.png"},{"id":73074561,"identity":"55a893de-ae25-407c-aa44-11effac07928","added_by":"auto","created_at":"2025-01-06 13:09:01","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":204929,"visible":true,"origin":"","legend":"\u003cp\u003eDowncore variation of multiple proxies in core IS-1B sediment in comparison to the NGRIP and North Atlantic proxy records over the last 30 ka. A. NGRIP stable oxygen isotopic composition (δ18O) and temperature from ice core43,44,50. Grey dots indicate the δ18O and the black line indicates the temperature. B. Ca/Sr from core GeoB18530-1, reflecting detrital carbonate IRD layers9. The elevated Ca/Sr ratios are attributed to the high content of detrital carbonate IRD originating from Paleozoic limestone and dolostone in Hudson Bay and Hudson Strait51. C. IRD (\u0026gt;125μm) content from core IS-1B. D. Surface cold water mass salinities (SSScwm) calibrated from the Mg/Ca temperature and δ18O of the N. pachyderma (s) from core IS-1B; the blue triangle on the Y axis indicates the modern salinity (34.77PSU, World Ocean Atlas 201852) of core-top N. pachyderma (s) calcification depth. E. Standard deviation of shell Mg/Ca temperature in each layer of core IS-1B, the blue line is a 3-point running average of the gray line data. F. Surface cold water mass temperatures (SSTcwm) calculated from the Mg/Ca of N. pachyderma (s) from core IS-1B; red dots indicate the Mg/Ca temperature of each shell; the black triangle on the Y axis indicates the modern temperature (3.26℃, World Ocean Atlas 201853) of core-top N. pachyderma (s) calcification depth. G. δ18O values of N. pachyderma (s) from core IS-1B. The white triangles on the X axis indicate the calibrated radiocarbon age control points (AMS 14C ages). The shaded blue bars denote cold stages: Younger Dryas (YD), Heinrich Stadial (HS), and Last Glacial Maximum (LGM).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/7fa09947a9053195aeceb0c6.png"},{"id":73074568,"identity":"c0816ff5-cede-40d1-ae33-6264cb18de26","added_by":"auto","created_at":"2025-01-06 13:09:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":140598,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of proxy records between core IS-1B and representative cores in north Atlantic and Nordic Seas. A. Summer solstice insolation at 65° N1. B. Winter (December) insolation gradient between 30-60°N54.C. Relative sea level indicators reconstructed from Barbados55, Bonaparte Gulf56,57, Tahiti58,59 and Sunda Shelf60. D. The Pa/Th ratio from core GGC5661 and ODP106362 from the Bermuda Rise are showed in dark blue. Time series from 0 to 19 ka BP derived from the core GGC56 and the time series from 20 to 27 ka BP derived from core ODP1063. The Pa/Th ratio from core DAPC214 from the Rockall Trough is showed in light blue. E. Benthic foraminifera-atmosphere 14C age offsets from core JM-F1-19PC41, reflecting the strength of ocean ventilation. F. Benthic foraminifera-atmosphere 14C age offsets from core PS124337. G. PBIP25 (the calculated ratio of IP25 to brassicasterol) signals of sediment core JM11-FI-19PC63. H. IRD (\u0026gt;1 mm) contents from core MD99-229439. I. IRD (\u0026gt;125 μm) contents from core IS-1B (this study). J. Surface temperatures calculated by transfer functions on planktic foraminifera census from core MD99-224938. K. Mg/Ca based surface sea temperatures measured on G. bulloides from core MD01-2461 in northeast Atlantic42. L. August surface sea temperatures derived from dinocyst assemblages from core SV-04 in the shelf margin of Spitsbergen35. M. Surface cold water mass temperatures (SSTcwm) calculated from the Mg/Ca of N. pachyderma (s) in this study. This warming trend since 21.1 ka BP is statistically significant at the 95% level (using Student’s t test).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/1dd0c251325a1120934be40c.png"},{"id":73074576,"identity":"39b8104a-8dfd-4e57-845b-12bed5cb4e5b","added_by":"auto","created_at":"2025-01-06 13:09:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":400671,"visible":true,"origin":"","legend":"\u003cp\u003eTraCE simulated oceanic and atmospheric changes in North Atlantic and Nordic Seas. A. Zonal mean annual temperature anomaly (20-22 ka BP). B. Winter anomaly of 850 hPa wind (vector) and surface wind (shaded color) simulated by orbital forcing only (20-22 ka BP).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/423d8edd58bc2c6dad3adca2.png"},{"id":73074572,"identity":"cbaf78c2-c2fa-4ad0-a872-f000ff07e6d9","added_by":"auto","created_at":"2025-01-06 13:09:04","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":236122,"visible":true,"origin":"","legend":"\u003cp\u003eCartoon of surface water masses evolution of Nordic Seas\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/042aa6b876e1a524112b4c2c.png"},{"id":73074583,"identity":"645e4e1e-cb07-40d1-b513-b0247f741fec","added_by":"auto","created_at":"2025-01-06 13:09:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":72955,"visible":true,"origin":"","legend":"\u003cp\u003eAge-depth models of core IS-1B established by different ways\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/4f7fe89692e14724b0567bd6.png"},{"id":73074562,"identity":"0a896e0c-fe7c-4738-bef6-08b2d1b5b2d9","added_by":"auto","created_at":"2025-01-06 13:09:02","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":229979,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron microscope image of laser-ablated \u003cem\u003eN. pachyderma\u003c/em\u003e (s) shells\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/8c0a5be375c01312b7b74791.png"},{"id":73075130,"identity":"6fe08c98-4dcc-4365-b24d-853ca19b63a9","added_by":"auto","created_at":"2025-01-06 13:17:06","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":198465,"visible":true,"origin":"","legend":"\u003cp\u003eTypical profile of LA-ICP-MS signals\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/1e9d375495b0a1a37afe935a.png"},{"id":79464515,"identity":"24519744-7043-41d4-8921-dd0f8870ff0d","added_by":"auto","created_at":"2025-03-28 18:34:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3460308,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/67831add-ca31-4cf4-92f0-9ff376820341.pdf"},{"id":73076121,"identity":"b556f610-d721-4147-a534-354d6345fa3e","added_by":"auto","created_at":"2025-01-06 13:25:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":484357,"visible":true,"origin":"","legend":"Supplementary Materials","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-5588827/v1/75b0543fa4279989d75d455b.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Insolation-triggered Eurasian Ice Sheets collapse initiates the Last Termination","fulltext":[{"header":"Introduction","content":"\u003cp\u003eQuaternary glacial cycles have been closely linked to the changes of Earth's orbital parameters\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, characterized by a long cooling period ending with a relatively short warming interval marked by significant reduction of ice sheets, a \u0026ldquo;sawtooth\u0026rdquo; shape variation consistent with boreal summer insolation changes\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Identifying the processes that triggered these rapid collapses of ice sheets, also known as the \"glacial terminations\", is essential for elucidating the mechanisms of glacial cycles\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. A commonly discussed hypothesis attributed the terminations to the peak of surface air temperature induced by the insolation maxima\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, leading to a rapid thinning and basal-sliding of the oversized continental ice\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. The following extensive release of freshwater disrupted interhemispheric ocean circulations, ultimately initiating an interglacial climate across the planet\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. However, precisely dated ice-rafting events in the high latitude North Atlantic\u003csup\u003e\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e indicate that the culmination of boreal iceberg discharge occurred thousands of years prior to the marked increase in Greenland ice core temperatures\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e during the Last Termination (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA, B). Moreover, the initial reduction of Atlantic Meridional Overturning Circulation (AMOC) also preceded the episodic retreat of the dominated Laurentide Ice Sheet\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e (LIS). Numerical models suggest that ocean temperature warming can cause the grounding line retreat of marine ice shelves, destabilizing the overall stability of the continental ice sheets\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. This implies that the coupled ocean-icesheet system may exhibit more prompt and earlier feedbacks to insolation change than the atmospheric warming and internal oscillations of ice sheets at the onset of Last Termination\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, and the oceanic processes could potentially serve as a key trigger for the collapse of the northern ice sheets\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. To comprehend the factors that triggered the Last Termination, it is crucial to determine the extent to which the decay of northern ice sheets is systematically linked to the environmental conditions of the surrounding oceans, and how such an indirect solar-driving mechanism can produce the sensitive oceanic response observed at the end of a glaciation cycle.\u003c/p\u003e \u003cp\u003eIn high latitudes, the effects of insolation are considered to be potentially amplified in sensitive ocean regions and efficiently transmitted to the margins of ice sheets through thermohaline circulation\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. In addition, the disparity in solar heating between the tropics and poles (latitudinal insolation gradient) can regulate the poleward-oceanic heat and moisture transport by modulating the latitudinal temperature gradient and mid-latitude wind fields, potentially influencing the dynamics of ice sheets\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. During the last glacial maximum (LGM), the Eurasian Ice Sheet complex (EIS) attained a peak ice volume responsible for around 24 m of eustatic sea-level reduction\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, forming an extensive interface with the Nordic Seas and the North Atlantic\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The southern edge of EIS is situated downstream of the North Atlantic Current (NAC), which constitutes an integral part of the upper branch of the AMOC, thereby potentially undergoing substantial impacts from the temperature fluctuations induced by the Atlantic warm water\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Since the Nordic Seas are the main gateway of the iceberg trajectory associated with the EIS, and large marine-based sectors of the glacial EIS extended all the way to its continental shelf edge\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, the sedimentary records of the Nordic Sea are ideal for studying the interaction between ocean dynamics and the ice sheet instabilities. In the present study, we combined multi-biological proxy data from the southern Nordic Seas to reconstruct the evolution of upper ocean water masses under the orbital forcing. Processes responsible for inflow Atlantic Water (AW) oscillations are discussed with aid of transient climate simulations to better understand the role of the fluctuations of northward oceanic heat transport in triggering the early instabilities of EIS during the transition period of Last Termination. Furthermore, we assessed the impacts of intermittent iceberg discharges from the EIS on the advanced slowdown of AMOC prior to Heinrich Stadial 1 (HS1).\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eGeological setting, chronology, and geochemical proxies\u003c/h2\u003e \u003cp\u003eDuring the 5th China Arctic Research Expedition in August 2012, a gravity core (ARC5-IS-1B, 65\u0026deg;36\u0026prime;N, 8\u0026deg;59\u0026prime;W, here after IS-1B) was retrieved in the north of the Iceland-Faroe Ridge at 819 m water depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The core location lies beneath the path of the modern Faroe Current, the main branch of the warm AW inflow and an integral component of the upper branch of the AMOC\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, suggesting that IS-1B is a sensitive recorder of the past ocean dynamics in temperature and salinity\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. The chronostratigraphic framework of core IS-1B was established by eight accelerator mass spectrometry radiocarbon (AMS \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC) ages spanning the past ~\u0026thinsp;30 ka. We measured the Mg/Ca ratio of dominantly polar water-dwelling planktonic foraminifera \u003cem\u003eNeogloboquadrina pachyderma\u003c/em\u003e sinistral (\u003cem\u003eN. pachyderma\u003c/em\u003e (s)) as a proxy for the temperature of surface cold water mass (SST\u003csub\u003ecwm\u003c/sub\u003e, Supplementary Materials). The positive correlations between salinity and the stable oxygen isotopic composition of sea water (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw\u003c/sub\u003e) are widely accepted across different oceans\u003csup\u003e\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, so that the ambient δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw\u003c/sub\u003e information was extracted from the \u003cem\u003eN. pachyderma\u003c/em\u003e (s) to calculate the global ice-volume-effect-removed oxygen isotopic composition (δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u0026minus;local\u003c/sub\u003e), allowing us to reconstruct the salinity in the surface cold water mass (SSS\u003csub\u003ecwm\u003c/sub\u003e). To further monitor the stability of surface cold water mass and the sensitivity of upper ocean structure in response to the oscillation in northward heat flux, we measured the Mg/Ca temperatures of individual foraminiferal shells within each sampling layer, combined with their standard deviation, to study the degree of temperature fluctuations at the sampling interval resolution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSurface cold water mass warming in later stage of LGM\u003c/h3\u003e\n\u003cp\u003eIn the Nordic Seas, the interaction between inflowing warm AW and fresh meltwater through convection is closely linked to regional climate and sea ice behavior\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. More recent reconstructions and modeling simulations have indicated that the AW never ceased to flow into the Nordic Seas during the glacial period, and the warm surface inflow subducted to the subsurface and intermediate area below the thick halocline and sea ice\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. Our new SST\u003csub\u003ecwm\u003c/sub\u003e record from the core IS-1B show relatively low values over most of the LGM. However, an intriguing temperature increase of 2.72℃ after 21.1 ka BP marks a change from the previous interval characterized by weak fluctuations near the freezing point, supporting the notion of upper ocean warming in the southern Nordic Seas during the late stage of the LGM (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF). At the same time, an SSS\u003csub\u003ecwm\u003c/sub\u003e increase, which was greater than the impact of global ice volume forcing, revealed that the saltier surface cold water mass may have reduced the surface salinity gradient and stability of the halocline (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAnother salient feature is the synchronous upward trend of the Mg/Ca temperature standard deviation in individual shells (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE). This drastic increase in the degree of dispersion of temperature distribution indicates that temperature inside the water mass varied frequently over a wide range. Compared to the growth of average temperature, the frequent variability of individual shell temperature signals most probably indicates instability of the cold water mass. This instability and the warming trend could be further associated with a thinning halocline caused by local invasion of subducted warm AW, which could ultimately lead to the vertical shift of AW crossing the halocline (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). Further comparisons of proxy records from adjacent stations in the Nordic Seas and North Atlantic, alongside TraCE simulation results, yield two key observations. First, the general trend of temperature increase at the end of LGM is visible in surface North Atlantic\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, the eastern Nordic Seas margin close to the Fennoscandian Ice Sheet\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e (FIS) and the temperature profiles simulated by the TraCE model (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eJ, K and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA). This warming trend, although significant, was not as large as other pronounced amplitudes in the last deglacial period and Holocene. Nonetheless, the surface warming trend reported in dinocyst records\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eL) farther downstream of the Atlantic Water pathway also reveals that the northern boundary of such a short-term warming event may have reached the Svalbard Islands and affected the entire Nordic Seas from south to north.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSecondly, evidence of prominent warm intermediate water appears in the benthic records from the southeastern Nordic Seas, suggesting a continuous inflow of AW throughout most of the LGM\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). The correlation with the consistent surface warming in the subpolar North Atlantic is likely reflect a significant accumulation of ocean heat during the later LGM\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, which further accelerates the northward heat transport and contributes to the buildup of a subsurface heat reservoir in the Nordic Seas. Although the larger benthic-atmosphere (B-A) \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC offset revealed that the Norwegian Sea was poorly ventilated during much of the late glacial and deglacial period\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, the reversed decrease in ventilation age is presumably considered as a brief resumption of upper ocean convection around 20.5 ka BP (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eF). The synchronous onset of SST\u003csub\u003ecwm\u003c/sub\u003e increase observed in our records when the ventilation in Faroe\u0026ndash;Shetland Channel and central Norwegian Sea transitory was enhanced\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eE, F), provides evidence from another angle for a causal role of subsurface AW upwelling in strengthening the surface mixing and warming. In particular, the substantial heat emission from AW could facilitate the sea-air exchange, causing a more active sea ice and a further-retreated ice margin in the glacial Nordic Seas\u003csup\u003e\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e. Thus, the potential contribution of the continuous heat transport from AW and this millennial-scale warming to the altered climate should not be ignored.\u003c/p\u003e\n\u003ch3\u003eOcean response sensitive to the insolation changes\u003c/h3\u003e\n\u003cp\u003eFrom a broader perspective, the observed changes in surface and subsurface Nordic Seas hydrography during the late stage of LGM could have been a local feedback mechanism to global scale climate driving forces. Our proxy data of temperature standard deviations (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE) indicate that the starting point of surface warming may be earlier than previous deductions from the studies in the Norwegian Sea and the area south of Iceland\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e,\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e. It is important to note that the timing of our increasing temperature standard deviation and the SST\u003csub\u003ecwm\u003c/sub\u003e warming peak, constrained by two radiocarbon dates at 22.7 ka BP and 18.6 ka BP, respectively, are systematically reliable and independent of the age model uncertainties. These suggest that the instability of surface cold water, as a result of the weakened halocline when the subsurface AW transformed into an active mode, may begin earlier at ~\u0026thinsp;22 ka BP, even though the total average surface temperature varied slightly in the interval of 22\u0026ndash;21 ka BP.\u003c/p\u003e \u003cp\u003eMoreover, the timing of the AW upwelling is concurrent with the significant recovery of 65\u0026deg;N caloric summer insolation at around 22 ka BP\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e,\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA), which may reveal a close link between solar forcing and the heat accumulation in the AW layer of Nordic Seas. Furthermore, the concurring initial weakening of latitudinal winter-insolation-gradient between 60\u0026deg;N and 30\u0026deg;N\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e and the subsequent basin-wide surface warming trend (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA) suggest the latitudinal variations in external insolation forcing are accompanied by positive regional oceanic feedback processes.\u003c/p\u003e \u003cp\u003eDuring the cold conditions, the apparent instantaneous oceanic reaction to the growing solar insolation in the surface Nordic Seas can be shown in the reduced sea ice cover and lower ice-albedo feedback, as well as the in situ warming surface water affecting the hydrographic conditions\u003csup\u003e\u003cspan additionalcitationids=\"CR69\" citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e. However, the model simulation results revealed that the energy of local solar insolation are insufficient to drive large-scale surface warming associated with the ice sheet instabilities and major climate transitions in the north Hemisphere\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e,\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e. Our correlations between ocean feedback and solar forcing are most likely tied to the orbitally forced modulations of atmospheric-oceanic variability and the poleward flux of latent/sensible heat. Indeed, the solar influence on climate of the magnitude and consistency implied by our evidence could not only have been confined to the Nordic Seas. As demonstrated in late Pleistocene and Holocene, the weaker winter latitudinal temperature gradient, first order forced by the decreasing winter latitudinal insolation gradient, has been found to cause a positive North Atlantic Oscillation (NAO\u003csup\u003e+\u003c/sup\u003e) state, promoting poleward shifts in the position of the westerly flow\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Similarly, model simulation results of TraCE experiments\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e\u003c/sup\u003e have indicated that isolated orbital forcing can lead to an increase in the intensity of 850 hPa wind south of Iceland since 22 ka BP (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB; Supplementary Materials), which is also consistent with the trend of NAO\u003csup\u003e+\u003c/sup\u003e phase. This enhanced wind stress in the cyclone direction can present a pronounced east-west extension of subpolar gyre\u003csup\u003e\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e\u003c/sup\u003e (SPG), resulting a stronger AMOC and NAC with higher latitudinal heat flux transport into the North-East Atlantic and the Norwegian Seas\u003csup\u003e\u003cspan additionalcitationids=\"CR76\" citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e\u003c/sup\u003e. Meanwhile, the TraCE results revealed that the growing influence of insolation also strengthened the winter surface winds in the Nordic Seas at the later stage of the LGM\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB). This implies the enhanced surface winds may have further intensified the vertical mixing and facilitated the upwelling of the subsurface AW\u003csup\u003e\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e\u003c/sup\u003e. We thus infer that the heat accumulation in the AW layer and the observed surface warming in Nordic Seas are mainly triggered by the atmospheric-oceanic circulation process underlying the orbital forcing mechanism.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"1\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eUpper ocean water mass activities as a trigger for the termination of EIS and the deglacial climate\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe reduction of the LIS and EIS from maximum volume and the acceleration occurring during HS1 represent a rapid transition in recurring ice sheet cycles and the termination of last glaciation climate\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e\u003c/sup\u003e. The onset of the deglacial epoch in the Nordic Seas is interpreted to have a chilled surface conditions at around 19 ka BP in the wake of the anomalous warming event in the late LGM, as evidenced by our \u003cem\u003eN. pachyderma\u003c/em\u003e (s) Mg/Ca temperature record and the enhanced IRD concentration in core IS-1B (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eI, M). This is in line with the abrupt rise in sea level and the melt water pulse recognized at 19 ka BP (19ka-MWP)\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e,\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e\u003c/sup\u003e. Paleoglaciological studies describe a significant ice margin retreat of the British-Irish Ice Sheet (BIIS) and FIS accompanied by the disintegration of the North Sea Ice Bridge and an enormous meltwater outburst into the Nordic Sea and North Atlantic during this period\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e,\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e\u003c/sup\u003e. The separation of the FIS and BIIS resulted in the collapse of an ice-dammed lake in the North Sea, finally leading to a subsequent catastrophic fresh water release which contributed to the 19ka-MWP event and the elevated IRD flux in Nordic Seas\u003csup\u003e\u003cspan additionalcitationids=\"CR83\" citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e84\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMeanwhile, R\u0026oslash;rvik et al., argued that the main IRD maxima and plumate deposition records in the Norwegian shelf post-date periods with increased adjacent sea temperature may indicate that upper ocean warming was probably an important external forcing factor on the fast-flowing paleo-ice streams and meltwater release from the EIS\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eH). Based on the close correspondence between the anomalously warm conditions and meltwater activities recorded in our study region and both margins of the FIS\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e and the BIIS\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, we presume that the extensive upper ocean warming in Nordic Seas and the strengthened ocean heat stored near the grounding line continuously eroded the ice shelves, further constituting a warm moisture source triggering the collapse of the southern EIS and massive iceberg/meltwater output at the beginning of the Last Termination (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB).\u003c/p\u003e \u003cp\u003eThereafter, evidence from the B-A \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC offset reconstruction at 18.8 ka BP led to the interpretation that the enhanced freshwater flux hampered the Nordic Seas surface convection and deep-water ventilation\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eF). At this point, the proxy records of AMOC strength show an early decline in the Rockall Trough\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD), which reflects the initiation of ocean circulation reduction in high latitude prior to HS1. However, the classic AMOC signals\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e,\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e of mid latitude Atlantic remain invigorated in 19ka BP, and its successive decrease trend came hundreds of years after that of the north. We hypothesized that the freshwater pulses causing these sequential slowdowns of AMOC at different latitudes are all originated from the EIS, and the delay of mid latitude can be interpreted as a two-stage process of the meltwater release in Nordic Seas. As soon as the rapid cooling in the Nordic Sea, due to the fast cooling at 19ka BP, the occurrence of thick and perennial sea ice limited the southward transport of massive icebergs collapse from the EIS into mid latitude Atlantic (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC). This is in line with the previous sea ice reconstructions from extremely low PIP25 level of south Norwegian Sea proposing an expansive perennial sea ice sealed the Nordic Seas and icebergs\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eG). As the gradual rise of boreal summer insolation, the secondary release of sea ice and icebergs occurred over the next thousand years, leading to the meltwater intrusions in mid latitude and the substantial collapse of North Atlantic Deep Water (NADW) formation. In addition, this also indicates that the initial weakening of the AMOC during the last deglaciation may have been caused by a precursor event involving European freshwater discharge, rather than the HS1 event attributed to the instabilities of the LIS\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, coinciding with the identification of additional European-origin IRD flux predating the LIS surges in the Heinrich layer\u003csup\u003e\u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e85\u003c/span\u003e,\u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e86\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003ePhysically, the meltwater discharged through the Nordic Seas would have led to a direct isolation of AW and the formation of a stronger halocline, potentially resulting in the redistribution of heat in the upper layer and a basin-wide subsurface warming in the Atlantic\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e\u003c/sup\u003e. Several model simulations have indicated that AMOC reduction configures a feedback mechanism with subsurface ocean warming, which even affects a depth of approximately 2,500 m due to continued downward mixing of heat in the Atlantic basin\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. This subsurface temperature anomaly can propagate on advective timescales toward the Labrador Sea\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Enhanced heat transfer would accelerate the basal melt below the Labrador ice shelf, substantially increasing the calving rate of the LIS\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e\u003c/sup\u003e. Accordingly, the sudden loss of buttress is considered to have resulted in large ice shelf disintegration corresponding to the first pronounced peak of the IRD flux and freshwater discharge, marking the prologue of HS1 and deglaciation climate\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOverall, both solar insolation, thermal transformation and ocean-ice sheet interactions were clearly interlocked in the millennia preceding HS1. This explains why it is critical to highlight the role of upper ocean activities in the initiation of Last Termination. Cheng et al. suggested that the promoted ventilation of ocean CO\u003csub\u003e2\u003c/sub\u003e reinforced interhemispheric teleconnections and augmented global warming during the ice-sheet terminations\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. We also consider the rising atmospheric CO\u003csub\u003e2\u003c/sub\u003e to be a key factor to complete the deglaciation. However, the timing of obvious increase in atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration, as revealed in ice cores\u003csup\u003e\u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e88\u003c/span\u003e,\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e, is later than the early collapse of EIS, suggesting that CO\u003csub\u003e2\u003c/sub\u003e may have less impact on the boreal ice-sheet process at the onset of Last Termination. As summarized in our proxy records, we propose that the anomalous warming in the surface Nordic Seas amplified the effect of boreal summer insolation, contributing to the meltwater surge from the EIS at the beginning of the Last Termination. This signifies an earlier and more prompt feedback of the ocean heat conveyor system to the solar activity in initiating the collapse of northern ice sheets. Subsequently, a two-stage freshwater release through the Nordic Seas shifted North Atlantic to a state with weak AMOC and NADW formation, further inducing large-scale subsurface warming and triggering the destabilization of the LIS\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Our findings expand the climatological evolutionary chain of the driving mechanism for the Last Termination of Northern hemisphere ice sheets to an earlier period (the late stage of the LGM), and support the implication that the sensitive oceanic responses to enhanced solar insolation are primary trigger of the climate transition in the Last Termination.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eAge models\u003c/h2\u003e \u003cp\u003eRadiocarbon (\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC) analyses were performed at the Beta Analytic Laboratory, Florida, USA, on the dominant foraminifera \u003cem\u003eN. pachyderma\u003c/em\u003e (s), using accelerator mass spectrometry (AMS) to establish the chronostratigraphic framework. We picked \u003cem\u003eN. pachyderma\u003c/em\u003e (s) up to a weight of ~\u0026thinsp;12 mg with the single size fraction between 250\u0026ndash;350 \u0026micro;m from eight representative layers for accurate AMS measurement.\u003c/p\u003e \u003cp\u003eTo establish the calendar age-depth model, radiocarbon dates were calibrated using the R package Bacon (v2.3.9.1)\u003csup\u003e\u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e90\u003c/span\u003e\u003c/sup\u003e and CALIB 7.04 software\u003csup\u003e\u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e91\u003c/span\u003e\u003c/sup\u003e, both of which used the Marine13 dataset\u003csup\u003e\u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e92\u003c/span\u003e\u003c/sup\u003e. Reservoir ages of 800 and 400 years were applied during the LGM and from the last deglacial period to the Holocene, respectively\u003csup\u003e\u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e93\u003c/span\u003e,\u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e94\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAMS\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC dating data and the calibrated calendar age of core IS-1B\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLaboratory code\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDepth/cm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAMS\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003eC age/a\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCalendar age (1σ calibrated)/a\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u0026ndash;7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e1560\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e1120\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17\u0026ndash;18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e4350\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e4472\u0026thinsp;\u0026plusmn;\u0026thinsp;18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426131\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u0026ndash;31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e6780\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e7303\u0026thinsp;\u0026plusmn;\u0026thinsp;31\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-497030\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e39\u0026ndash;40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e8550\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e9186\u0026thinsp;\u0026plusmn;\u0026thinsp;40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426133\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e70\u0026ndash;72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e15760\u0026thinsp;\u0026plusmn;\u0026thinsp;50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e18636\u0026thinsp;\u0026plusmn;\u0026thinsp;71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-497031\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u0026ndash;82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e18710\u0026thinsp;\u0026plusmn;\u0026thinsp;60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e21705\u0026thinsp;\u0026plusmn;\u0026thinsp;81\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426134\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e92\u0026ndash;96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e22620\u0026thinsp;\u0026plusmn;\u0026thinsp;90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e26024\u0026thinsp;\u0026plusmn;\u0026thinsp;94\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIS-1B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBeta-426135\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e125\u0026ndash;130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eN.pachyderma\u003c/em\u003e (sin.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e28010\u0026thinsp;\u0026plusmn;\u0026thinsp;140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e31152\u0026thinsp;\u0026plusmn;\u0026thinsp;128\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe Bacon age model was constructed with an accumulation rate distribution width of 1.5, a default student-t distribution, and a section thickness of 2 cm. The age-depth model calibrated with CALIB 7.04 was established based on linear interpolation and extrapolation of eight age control points (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The results show that there is little difference between these two age models. Therefore, we follow the Bacon age-depth model in this research to reconstruct the Bayesian accumulation histories of core IS-1B (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eOxygen isotope\u003c/h2\u003e \u003cp\u003eStable oxygen isotope analysis of planktonic foraminifera (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003ec\u003c/sub\u003e) was carried out by 20\u0026ndash;25 specimens of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) on mass-spectrometer (MAT253 Plus) combined with the carbonate preparation device (Kiel Ⅲ) in Laboratory of Pilot National Laboratory for Marine Science and Technology, Qingdao, China. Data are reported in per-mil notation (\u0026permil;) relative to the Vienna Pee Dee Belemnite (VPDB) scale and referenced to the NBS18 standard (δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eVPDB\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;23.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u0026permil;). The analytical standard deviation indicated by replicate measurements was less than 0.07\u0026permil;.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMg/Ca temperatures of\u003c/b\u003e \u003cb\u003eN. pachyderma\u003c/b\u003e \u003cb\u003e(s) and salinity of surface cold water mass\u003c/b\u003e\u003c/p\u003e \u003cp\u003eFor the Mg/Ca ratio measurements, 12 specimens of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) (intact, no contamination, ~\u0026thinsp;300 \u0026micro;m size fraction) were continuously picked from each layer of the sediment core at 2 cm intervals. The foraminiferal samples were first ultrasonically washed in deionized water and methanol to remove clay, followed by oxidative cleaning by immersion in hydrogen peroxide to remove organic material\u003csup\u003e\u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e95\u003c/span\u003e\u003c/sup\u003e. The Mg/Ca ratios were measured using a Thermo Scientific RQ ICP-MS coupled to an Geolas ArF excimer laser at the Key Laboratory of Marine Geology and Metallogeny at the First Institute of Oceanography, Qingdao, China. An N\u003csub\u003e2\u003c/sub\u003e gas stream was used to transport the ablation products to the RQ ICP-MS. The RQ ICP-MS instrument response was calibrated using SRM NIST610, 612, and 614 glasses and corrected for variations in the ablation yield and instrument drift by normalizing to the measured internal standard elements (\u003csup\u003e\u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e88\u003c/span\u003e\u003c/sup\u003eSr and \u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003eMn) intensities during each mass spectrometer cycle\u003csup\u003e\u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e96\u003c/span\u003e,\u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e97\u003c/span\u003e\u003c/sup\u003e. To isolate only the banding patterns in our data, ICPMSDataCal12.2\u003csup\u003e98,99\u003c/sup\u003e software was applied to the RQ ICP-MS signals to remove low- and high- frequency noise. The results indicate that 3\u0026ndash;5 s contamination signals enriched in Fe and Mn on the outermost of shell surface should be removed hence 10\u0026ndash;30s continuous and stable signals can be obtained depending on the shell wall thickness. The Mg/Ca ratios for each sample of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) were determined from the ratios of the average polished signal intensity in the ablation profile\u003csup\u003e\u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e100\u003c/span\u003e\u003c/sup\u003e. Surface cold water mass temperatures (SST\u003csub\u003ecwm\u003c/sub\u003e) were estimated from the average Mg/Ca ratios of 12 individual tests of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) in each sediment layer, based on different \u003cem\u003eN. pachyderma\u003c/em\u003e (s) specific transfer equations\u003csup\u003e\u003cspan additionalcitationids=\"CR102\" citationid=\"CR101\" class=\"CitationRef\"\u003e101\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR103\" class=\"CitationRef\"\u003e103\u003c/span\u003e\u003c/sup\u003e and multispecies transfer Eq.\u0026nbsp;1\u003csup\u003e04\u003c/sup\u003e. The comparision between the instrumental temperature at calcification depth and the calculated \u003cem\u003eN. pachyderma\u003c/em\u003e (s) Mg/Ca temperature from the IS-1B core-top suggested that equation of Nurnberg\u003csup\u003e\u003cspan citationid=\"CR102\" class=\"CitationRef\"\u003e102\u003c/span\u003e\u003c/sup\u003e (Eq.\u0026nbsp;1) established with \u003cem\u003eN. pachyderma\u003c/em\u003e (s) from high northern latitudes best matched our downcore samples (Supplementary Materials).\u003c/p\u003e \u003cp\u003eMg/Ca\u0026thinsp;=\u0026thinsp;0.55 exp (0.099\u0026times;T) (1)\u003c/p\u003e \u003cp\u003eThe ambient seawater oxygen isotope (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw\u003c/sub\u003e) was calculated from SST\u003csub\u003ecwm\u003c/sub\u003e and foraminifera shells δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003ec\u003c/sub\u003e using the equation of Nyland et al.\u003csup\u003e\u003cspan citationid=\"CR105\" class=\"CitationRef\"\u003e105\u003c/span\u003e\u003c/sup\u003e (Eq.\u0026nbsp;2), modified after Shackleton\u003csup\u003e\u003cspan citationid=\"CR106\" class=\"CitationRef\"\u003e106\u003c/span\u003e\u003c/sup\u003e and O\u0026rsquo;Neil et al.\u003csup\u003e\u003cspan citationid=\"CR107\" class=\"CitationRef\"\u003e107\u003c/span\u003e\u003c/sup\u003e with calibration (Eq.\u0026nbsp;3) to convert the calcite value from the VPDB to the Vienna Standard Mean Ocean Water scale\u003csup\u003e\u003cspan citationid=\"CR108\" class=\"CitationRef\"\u003e108\u003c/span\u003e\u003c/sup\u003e (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw, V\u0026minus;SMOW\u003c/sub\u003e).\u003c/p\u003e \u003cp\u003eδ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003ec\u003c/sub\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left[\\frac{4.38-\\sqrt{{4.38}^{2}-0.4\\times\\:\\left(16.9-T\\right)}}{0.2}\\right]\\)\u003c/span\u003e\u003c/span\u003e (2)\u003c/p\u003e \u003cp\u003eδ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw, V\u0026minus;SMOW\u003c/sub\u003e=(δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u003c/sub\u003e+0.2\u0026permil;)/0.998 (3)\u003c/p\u003e \u003cp\u003eIn addition, the positive correlations between salinity and δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003esw, V\u0026minus;SMOW\u003c/sub\u003e are widely accepted over different oceans\u003csup\u003e\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e so that the global ice volume and sea level effect were removed (δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u0026minus;local\u003c/sub\u003e) to further reconstruct the salinity in the surface cold water mass (SSS\u003csub\u003ecwm\u003c/sub\u003e) using Eqs.\u0026nbsp;4109,110 and 5108.\u003c/p\u003e \u003cp\u003eδ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u0026minus;local=\u003c/sub\u003e δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw, V\u0026minus;SMOW\u003c/sub\u003e-0.0083\u0026permil;\u0026times;SL (4)\u003c/p\u003e \u003cp\u003eSSS\u003csub\u003ecwm\u003c/sub\u003e=(δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003esw\u0026minus;local\u003c/sub\u003e +12.17)/0.36 (5)\u003c/p\u003e \u003cp\u003eWhere SL is sea level changes come from the curve of Lambeck et al\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cb\u003eLA-ICP-MS test conducted on\u003c/b\u003e \u003cb\u003eN. pachyderma\u003c/b\u003e \u003cb\u003e(s)\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThere was a significant difference in the shell diameter and thickness of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) in different sea areas. Therefore, we tried different laser intensities, pulse rates and ablation scopes to obtain steady and clear signals of \u003cem\u003eN. pachyderma\u003c/em\u003e (s) in core IS-1B. Experiments ultimately showed that the 60 \u0026micro;m diameter circular spots and laser pulse rate of 4 Hz at a fluence of 3 J/cm\u003csup\u003e2\u003c/sup\u003e coupled well with \u003cem\u003eN. pachyderma\u003c/em\u003e (s) with 10\u0026ndash;30 s persistent signals.\u003c/p\u003e \u003cp\u003eConsidering the growth cycle of different chambers in \u003cem\u003eN. pachyderma\u003c/em\u003e (s), we performed laser ablation on the four largest chambers in a single shell to compare the Mg/Ca between chambers (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eB). Eight shells from different layers reveal that the Mg/Ca temperature difference between four different chambers in single shell are less than 0.47 ℃, which is much less than the temperature amplitudes observed in our time series and the summer-winter temperature gap of ambient water. We conclude that the growth cycle effects on the production of \u003cem\u003eN. pachyderma\u003c/em\u003e (s). was minor with respect to the reconstructed SST\u003csub\u003ecwm\u003c/sub\u003e. Therefore, the last chamber with a bulky and thick shell wall was selected for ablation (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eA).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDuring ablation, we noticed that the initial signals showed abnormal signal peaks for Mg, Fe and Mn (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Because the signals reflect the polluted outermost layer of the shells caused by post-depositional diagenesis, we abandoned these signal zones for a more stable and accurate reconstruction of the element ratio.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eTraCE Experiments\u003c/h3\u003e\n\u003cp\u003eTo understand the role of large-scale ocean-atmosphere circulations in the evolution of the Nordic Seas and Eurasian ice sheets during this period, we carried out the TraCE simulations\u003csup\u003e\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e,\u003cspan citationid=\"CR111\" class=\"CitationRef\"\u003e111\u003c/span\u003e\u003c/sup\u003e to elucidate the remote North Atlantic forcing, which was relevant to the change of solar insolation. The TraCE transient climate experiments were performed in the Community Climate System Model ver. 3\u003csup\u003e112\u0026ndash;114\u003c/sup\u003e (CCSM3), a global coupled atmosphere-ocean-sea ice-land general circulation model (AOGCM) that has a latitude\u0026ndash;longitude resolution of \u0026sim;3.75\u0026deg; in the atmosphere and \u0026sim;3\u0026deg; in the ocean and includes a dynamic global vegetation module. The prescribed forcings and boundary conditions for the full-forcing simulation included orbitally forced insolation changes, increasing atmospheric concentrations of the long-lived greenhouse gases, and retreating ice sheets and associated meltwater release to the oceans\u003csup\u003e\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e,\u003cspan citationid=\"CR111\" class=\"CitationRef\"\u003e111\u003c/span\u003e\u003c/sup\u003e. Meanwhile, the transient sensitivity experiments adopt a factorized forcing scheme to allow isolating each of these forcings separately\u003csup\u003e\u003cspan citationid=\"CR115\" class=\"CitationRef\"\u003e115\u003c/span\u003e\u003c/sup\u003e. Therefore, the isolated insolation forcing experiments were selected to simulate the changes in winter 850 hPa wind velocity during the warming peak (20-22ka BP), aiming to examine the effect of increasing solar insolation on atmospheric circulation over the north Atlantic. We also compared the winter surface wind velocity and sea temperatures of all-forcing simulation from 20-22ka BP to study the ocean process related to this warming trend. All the changes were analyzed with Student\u0026rsquo;s t test, with confidence levels greater than 95%.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eD.W., Y.L. and X.Z. designed the study; Y.L. sampled the core; W.D. constructed the age model, prepared and analyzed δ\u003csup\u003e18\u003c/sup\u003eO data, performed the LA-ICP-MS test and drafted the original manuscript; S.L. and X.Z. contributed to the model simulation results; D.F. contributed substantially to all aspects; J.E., E.R.G, Y.R and N.H. were responsible for the evaluation and quality control. All authors contributed to the interpretation and the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis research was supported by the National Key Research and Development Program of China (Grant No. 2023YFF0804600, 2023YFF0805200) and the National Natural Science Foundation of China (Grant No. 42076079, 41676053). We thank the crew of icebreaker Xuelong for assistance in retrieving the sediment core. We thank Yan Zhang (First Institute of Oceanography, MNR) for her technical assistance of LA-ICP-MS. We thank Yi Zhong (Southern University of Science and Technology) for reading the early version of the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBerger, A. \u0026amp; Loutre, M. F. Insolation values for the climate of the last 10 million years. \u003cem\u003eQuat Sci Rev\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, 297\u0026ndash;317 (1991).\u003c/li\u003e\n\u003cli\u003eHays, J. 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The Low-Resolution CCSM3. \u003cem\u003eJ Clim\u003c/em\u003e \u003cstrong\u003e19\u003c/strong\u003e, 2545\u0026ndash;2566 (2006).\u003c/li\u003e\n\u003cli\u003eHe, C. \u003cem\u003eet al.\u003c/em\u003e Hydroclimate footprint of pan-Asian monsoon water isotope during the last deglaciation. \u003cem\u003eSci Adv\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 1\u0026ndash;11 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5588827/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5588827/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe collapse of Northern Hemisphere ice sheets has been deemed as a trigger for the chain of positive climate feedback during glacial terminations in Quaternary. Increasing boreal summer insolation is considered the ultimate driver of their collapse, however, the initiating mechanisms remain elusive. Here we report an unambiguous warming trend in the southern Nordic Seas, which coincides with the initial phase of Northern Hemisphere summer insolation increase at the end of the Last Glacial Maximum. A subsequent phase of surface cooling is observed, closely corresponding to the massive freshwater discharge attributed to the meltwater pulse at 19ka BP. Our reconstructions demonstrate that the initial collapse of Northern Hemisphere ice sheets during the Last Termination occurred in Eurasian Ice Sheet, driven by a chain of oceanic responses to the insolation increase. Specifically, increasing boreal insolation induced a northward migration of the mid-latitude Westerlies under a positive NAO phase, promoting poleward oceanic heat transport and hence subsequent warming in Nordic Seas, thereby accelerating the ablation of the marine-based Eurasian Ice Shelves. This led to a catastrophic release of icebergs into Nordic Seas, eventually triggering a series of ocean circulation feedbacks that further promoted the deglaciation.\u003c/p\u003e","manuscriptTitle":"Insolation-triggered Eurasian Ice Sheets collapse initiates the Last Termination","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-06 13:08:42","doi":"10.21203/rs.3.rs-5588827/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"3edd8af9-049c-42c6-aa38-09b22989d216","owner":[],"postedDate":"January 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":42345724,"name":"Earth and environmental sciences/Climate sciences/Palaeoceanography"},{"id":42345725,"name":"Earth and environmental sciences/Climate sciences/Palaeoclimate"},{"id":42345726,"name":"Earth and environmental sciences/Climate sciences/Biogeochemistry"}],"tags":[],"updatedAt":"2026-04-30T16:15:40+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-06 13:08:42","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5588827","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5588827","identity":"rs-5588827","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}