Sedimentary organic carbon burial in marine oxic sediments modulated by anticyclonic eddy

Sedimentary organic carbon burial in marine oxic sediments modulated by anticyclonic eddy | 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 Sedimentary organic carbon burial in marine oxic sediments modulated by anticyclonic eddy Rui Bao, Gang Xu, Xiaoyong Duan, Yangli Che, Tongya Liu, Zhiyou Jing, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5022150/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Anticyclonic eddies (AEs) are a common feature of ocean circulation and play a significant role in influencing the rates of carbon fixation and export. However, the mechanism underlying organic carbon (OC) export and sedimentation modulated by AEs are poorly deciphered. Here, we utilized in-situ observations of dissolved oxygen, chlorophyll, pH, and turbidity in the water column and experimental analyses of molecular biomarkers, OC, and iron trioxide in sediments from the East China Sea, to unravel the processes driving OC sedimentation modulated by AE. Our findings reveal that a significant amount of OC is preserved in oxic sediments, influenced by the presence of an anticyclonic eddy. We suggest that the eddy promotes the accumulation of OC along its periphery, and transports OC downward under the pycnocline. The combination of OC with iron trioxide, facilitated by oxidation, impedes the mineralization of OC in sediments. The accumulation of OC in oxic sediment, modulated by anticyclonic eddies, has significant implications for OC burial in mid-latitude oceans on millennial timescales. Earth and environmental sciences/Ocean sciences Earth and environmental sciences/Hydrology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Mesoscale eddies, including cyclonic and anticyclonic eddies (AEs), are a common feature of ocean circulation 1 . These eddies play a crucial role in regulating the supply of nutrients to the sunlit layers of the ocean, thereby influencing the rates of carbon fixation and export 2 . It was previously believed that AEs suppress productivity and phytoplankton biomass due to the downward displacement of isopycnals within them 3,4 , recent studies suggested that AEs can actually enhance biomass and nutrient fluxes through eddy-induced mixing, potentially increasing marine organic carbon (OC) production 2,5,6,7 . The prevalence of phytoplankton within AEs 8,9 and subduction facilitated by AEs may significantly contribute to OC export in the ocean 10,11 . Furthermore, AEs, being more energetic and nonlinear compared to surrounding currents, have the ability to trap and transport mass, physical tracers, and biogeochemical properties from their surroundings to their edges 12,13,14 , thus may affecting the accumulation of OC in sediments. However, the mechanism underlying OC export and sedimentation modulated by AEs are poorly deciphered. Mesoscale eddies in open oceans exhibit a wide range of spatial (tens to hundreds of kilometers) and temporal (weeks to months) scales，and most of them are in the process of dynamic migration 1 , posing challenges to studying carbon export and sedimentation mechanisms driven by these eddies. Conversely, some mesoscale eddies on continental shelves, such as those in the eastern marginal seas of China 15,16,17 , are relatively stationary, making them ideal regions for investigating AEs-driven OC accumulation in marine sediments. The East China Sea (ECS) is a typical passive marginal sea characterized by a shallow and broad epicontinental shelf. It is also one of the most active areas for OC transport and sedimentation 18 . Approximately 3.21× 10 12 g OC is buried annually in the inner-shelf of the ECS 19 . Recent report indicated the presence of an AE on the ECS shelf 17 . This AE remains stationary and has a life cycle of around 26 weeks, occurring from May to October each year 17 , creating favorable conditions for studying OC burial mechanism in sediment modulated by AEs. Furthermore, there is a hot spot of OC burial in the ECS inner-shelf 19 , coinciding with the location of the AE. Therefore, it is important to investigate the impact of the AE on OC sedimentation in the ECS. Here, we conducted a thorough investigation using a range of methods and analyses in summer. This included collecting sedimentation rates at 19 sites, determining the OC and Fe 2 O 3 content in surface sediments at 519 stations, analyzing molecular biomarkers in surface sediments from 127 sites, and performing in-situ observations of vertical dissolved oxygen, chlorophyll content, pH, and turbidity of the water mass. Additionally, we examined the OC content and molecular biomarkers in one sediment core (Figure 1). We employed the organic geochemical data to gain insights into the composition, sources, and degradation of OC in the sediments in the area modulated by the AE. Correlation analysis was performed between Fe 2 O 3 and OC to understand the preservation mechanism of OC. In-situ observations were used to uncover the progress of OC burial influenced by AE. Results And Discussion OC burial in oxidizing environment within AE The dissolved oxygen concentration in the bottom water is approximately 5.7 mg/L, higher than the concentration outside the AE, where it is 4.9 mg/L (Figure 2a-b). The water mass within AE shows an oxic environment, with higher oxidation of bottom water mass within AE compared to the non-AE region. In the sediment core within AE (Figure S1), the ratio of pristane (Pr) to phytane (Ph), a reliable indicator of redox conditions 20,21 , ranges from 1.9 to 3.6, indicating an oxic environment in the subseafloor sediments. In addition, the spatial distribution of the Pr/Ph ratios in surface sediments shows higher values (>0.8) predominantly within the AE, and lower values (<0.6) around the AE (Figure 3a). This also indicates a spatially distinct oxic depositional environment within the AE compared to the exterior. The redox potential (Eh) in the study area also confirms the oxic nature of sediments within the AE (Figure S2). Figure 3a shows the spatial thickness distribution of the surface mixed layer, which serves as an indicator of the hydrodynamic intensity 17,22 . The coupling between the surface mixed layer and the Pr/Ph ratio within AE highlights the pronounced downward dynamic feature of the AE 17 (Figure 3a). Isopycnals within the AE deepen towards its center 3,23 , leading to the intensification of fronts within the eddying flow field and the generation of downwelling 11,24 . This downwelling promotes the intrusion of oxygen-rich surface water 11,25,26 , resulting in an oxidizing environment within the AE. The sediments in our study area modulated by the AE contain OC content of up to 0.7% even more. It is now recognized that the previously perceived barren desert of the AE-dominated environment is actually a fertile land of OC 2 . The OC in the AE-modulated region is characterized by high levels C 15 +C 17 +C 19 n-alkanes (Figure 3b), which are derived from marine phytoplankton 27 . This indicates that the OC in this area mainly originates from marine sources. In contrast, OC with high levels of C 27 +C 29 +C 31 n-alkanes, originating from terrestrial plants 28 , is found in the coastal area rather than the AE-modulated area (Figure 3c). The degradation index of OC, known as CPI 25–33 19 , in surface sediments shows that the OC buried in the AE- dominated area mainly contains fresh marine OC that has not undergone complete degradation (Figure 3d). Our findings suggest that fresh marine OC can accumulate within an oxic depositional setting. The burial fluxes of OC in the AE-dominated deposit are relatively high, with an average flux of approximately 47 g C m −2 yr −1 (Figure 4a). This value is three times higher than the average flux observed in the ECS shelf 19 . Furthermore, it has been observed that the OC content in the AE-dominated deposit can stabilize at a background level of approximately 0.6% (Figure S1), indicating that the OC accumulation in oxic sediments, modulated by AE, remains relatively stable and significant in the subseafloor sediment. These findings challenge the conventional notion that OC accumulation is limited to reducing environments 29 , as we show that even in oxic sediments, significant amounts of OC accumulate. Export of OC under pycnocline driven by AE At an in-situ observation station located in a normal area without the influence of AE (Figure 1), we found that the water column from the top to the bottom is stratified, with distinct layers including the mixed layer, pycnocline (where salinity, temperature, or density changes most rapidly with depth 30 ), and a homogeneous layer (Figure 2b). Two subsurface chlorophyll maximum layers (SCMLs) which influenced by interactions between the ecosystem and hydrodynamics in stratified waters 31,32 are typically observed in the mixed layer and near the upper boundary of the pycnocline (Figure 2b). While the deepest SCML can play a crucial role in the downward export of OC 33,34 , there is a rapid decrease in chlorophyll content below the deepest SCML and a notable changes in dissolved oxygen, pH value, and minimal changes in water turbidity within pycnocline (Figure 2b)，suggesting that the majority of phytoplankton (marine)-derived OC within the pycnocline undergoes mineralization. Pycnocline in non-AE region thus acts as a barrier layer hindering the downward transport of OC 35 . While the biological carbon pump is a well-known mechanism for exporting marine OC, we argue that the prevalent pycnocline in the ocean significantly impede the efficiency of OC downward export. In contrast, we find more water layers in the water column within the AE, including a mixed layer, pycnocline, mixed layer, and homogeneous layer (Figure 2a). Additionally, three SCMLs within AE are observed, located in the mixed layer, the upper boundary of the pycnocline, and mixed layer from top to bottom (Figure 2a). Notably, a SCML is observed below the pycnocline within AE, which distinguishes it from the non-AE region (Figure 2a and 2b). Since the AE exhibits elevated energy levels and nonlinearity, enabling it to capture and transport materials from its surrounding to its periphery 12,13 , the predatory behavior of the AE leads to the accumulation of a significant amount of phytoplankton along its periphery (Figure 4b). For instance, on the right side of the AE, there are two distinct spiral-shaped bands with high chlorophyll-a concentrations extend outward (Figure 4b). Similar bands are also present on the left side of the AE 36 . These bands likely act as the main pathways for the influx of materials from the surrounding to the periphery of the AE, resembling tentacles for the AE to feed on. Once phytoplankton accumulates along the AE's periphery, it is transported downwards under the pycnocline 11 . Therefore, the phytoplankton (marine)-derived OC can be rapidly transported downwards under the action of downwelling, significantly reducing the possibility of phytoplankton (marine)-derived OC being consumed by predators 29 . In our case, the SCML located below the pycnocline within AE is evident. We think that the SCML located below the pycnocline driven by the AE could significantly enhance its ability to transport OC downwards. Below the SCML located below the pycnocline within AE, we observed a gradual decrease in chlorophyll concentration, accompanied by a rapid increase in water turbidity, and minimal changes in dissolved oxygen and pH (Figure 2a). These observations suggest that the export of phytoplankton (marine)-OC driven by the AE under the SCML is efficient. The content of particulate OC and suspended sediment in the water column can be reflected by turbidity value 37 . As this in-situ observation station is located far from the shore (Figure 1), changes in turbidity primarily reflect variations in particulate OC content. Therefore, the elevated turbidity values in the water column mainly reflect an increase in particulate OC. In the South China Sea, a flux of particulate OC within AE is 1.6-fold higher compared to non-eddy region 10 . Furthermore, our study reveals a positive correlation between the OC content in sediments and the intensity of downwelling (Figure 3a and 4a), suggesting that the downwelling enhances the subduction of OC under the pycnocline driven by the AE, providing an efficient pathway for OC accumulation in the sediments. Although, the export of OC under the pycnocline driven by the AE is enhanced, theoretically, the oxidizing environment within AE is not conducive to the preservation of OC 29,38 . The abnormal enrichment of OC in oxidizing environment may reflect its strong antioxidant capacity. Previous research suggested that iron oxide in oxic sediment plays a crucial role in protecting OC 39 , as the association of Fe 2 O 3 with OC hinders its mineralization in marine environments 40,41 . Our results show that that the Fe 2 O 3 content in surface sediments within AE-influenced area exceeds 0.7% (Figure 4c). Since the AE exhibits elevated energy levels and nonlinearity, enabling it to capture and transport element Fe from its surrounding to its internal 12,13 . Furthermore, the interior of AE is an oxidizing environment, leading to the enrichment of Fe 2 O 3 . In our study, a clear spatial correlation is observed between Fe 2 O 3 and OC content in surface sediments (Figure 4a and 4c). This is further supported by a strong positive correlation between the OC and Fe 2 O 3 content in surface sediments (Figure 4d). Higher Fe 2 O 3 levels increase the affinity for particulate OC, and the presence of abundant dissolved oxygen promotes the oxidation of sinking OC, resulting in OC with potent antioxidant properties. To sum up, the efficient downward transport and sedimentation of OC, modulated by AEs, can be viewed as an enhanced biological carbon pump (Figure 5). Implication of OC accumulations driven by AE Numerous mesoscale AEs have been identified in the mid-latitude oceans, characterized by long lifetimes and the ability to propagate over extensive distances 1 . The concentration of surface chlorophyll is the highest in the mid-latitude oceans, particularly between 30° and 60°, excluding the equatorial region 42 . This coupling between AEs and surface chlorophyll concentration creates favorable conditions for the export of OC in mid-latitude oceans, as supported by the global chlorophyll export flux 43 . For instance, in the southern mid-latitude oceans, the regions particularly between 30° S and 50° S 44 play a substantial role in the oceanic carbon sink 45,46 . In the northern mid-latitude oceans, such as the North Atlantic, AEs also promote the export of particulate OC, contributing up to half of the total export of particulate OC 11 . The export of driven OC in the mid-latitude deep-sea water 47,48 , due to the high kinetic energy of AE that may influence deeper water depths even 4000 meters or more 49 , likely influencing the global long-term carbon cycle. Additionally, mid-latitude oceans, especially within circulation areas with active mesoscale AEs, represent significant reservoirs of OC in oxic sediments. The South Atlantic Ocean with active mesoscale AEs, such as those between 40° and 60°, exhibits OC accumulation rates ranging from 0.05~0.1 g C m −2 yr −1 , the highest in the entire South Atlantic Ocean excluding coastal areas 50 . Therefore, the majority of AEs are dynamic 1 , and their widespread distribution significantly might contributes to the total burial of OC in the mid-latitude oceans. On million-year timescales, around 1.6 × 10 19 g of OC may be buried in oxic sediment 51 , which represents a previously overlooked carbon reservoir with significant importance in carbon cycles and deep-sea life 52,53,54 . However, the dynamic mechanisms underlying the formation and maintenance of the OC reservoir in oxic sediment have not been deeply deciphered. Our findings provide a potential perspective for the formation of these OC reservoirs in oxic sediments. AEs not only facilitate the accumulation of OC in oxic sediments, but also promote the fusion of OC with iron oxides protecting against remineralization. Furthermore, AEs can prevent acidification and hypoxia in the bottom water (Figure 2a). This unique carbon sequestration mechanism also offers insights into the potential use of artificial AEs to enhance the burial of OC in the inner shelf. Methods Sample Collection In this study, a total of 519 surface sediment samples were collected in the ECS shelf in the summer of 2013 for OC and Fe 2 O 3 analysis. In addition, 127 samples were used for biomarker analysis. The samples were immediately frozen at -20°C until organic chemistry analysis. The surface sediment samples represent the top 1 cm layer at the center of the sampling box. The study utilized MODIS images of chlorophyll-a on 20 October 2012, as reported by reference 55 . Data on surface mixed layer thickness and sedimentation rates were obtained from reference 56 . The Pr/Ph values in sediment core Sc01 were sourced from reference 17 , while the OC concentration data in sediment core Sc01 were provided by reference 57 . Geochemical Analysis The freeze-dried and homogenized samples were analyzed for OC. Approximately 500 mg of sediment was treated with 4 N HCl at room temperature for 12 hours to remove calcium carbonate, followed by four rinses with deionized water. The decarbonated samples were then freeze-dried and placed into tin capsules. OC determination was performed using a Euro EA 3000 Elemental Analyzer, with a standard deviation of ±0.02% dry weight (n = 6). The concentration of Fe 2 O 3 in the samples was determined using an X-ray fluorescence spectrometer (model Philips PW4400), following the method described by reference 58 . The accuracy of the element analyses was verified by replicating the analysis of certified national standard reference materials (GBW07343, GBW07344, and GBW07334). The analytical error for Fe 2 O 3 was found to be -1.26%. Freeze-dried biomarker samples were finely ground to a size smaller than 200 μm, then subjected to Soxhlet-extracted for 24 h using dichloromethane (DCM) as the solvent. The resulting extracts were divided into four fractions based on the different polarities of their components through flash column chromatography packed with activated silica gel (particle size 40–63 μm) under a nitrogen pressure of 1.5 bars or less. The saturated hydrocarbon fraction was eluted with n-hexane, the aromatic hydrocarbon fraction with a mixture of n-hexane and DCM (9:1 v/v), the ketones/esters fraction with DCM, and the heterocompounds with methanol. The so-called “saturated hydrocarbon fraction” comprises both saturated hydrocarbons and mono-unsaturated hydrocarbons. Solvents were then removed from each fraction using a stream of nitrogen. The nonpolar lipid fractions were dissolved in hexane and analyzed using gas chromatography-mass spectrometer (GC-MS; Agilent 7890A/5975C) with an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm). Helium was used as the carrier gas at a flow of 1.0 ml/min, 1 μl of sample was injected at 280 °C. The temperature program for the nonpolar fractions, which include n-alkanes and isoprenoids, started at 60 °C, held for 1 minute, followed by a 5 °C/min increase to 310 °C, where it was held for 20 minutes. The electron ionization source was set at 230 °C and 70 eV, with scanning conducted between mass-to-charge ratio (m/z) ranges of 50-600 Daltons. Individual components were identified by comparing retention times with literature and NIST mass spectrometry library. The corresponding indices were quantified by the integration the peak areas of the total ion chromatogram. A procedural blank was included in the sequence to account for any background interferences, which were found to be insignificant. The carbon preference index (CPI) was calculated following the method described by reference 19 . In-situ measurement of Eh The Eh values of seven surface sediment samples were measured in-situ using the pHS-3C precision acidity meter during the summer of 2019. Prior to measurement, the electrode was immersed in distilled water for 24 hours, preheated for 30 minutes, and calibrated with standard buffer solution. The instrument displayed results updated every second, and the measured Eh values stabilized within 2 minutes, providing an accurate representation of the surface sediment Eh state. Once stabilized, the Eh measurement value of the sediment was recorded. The instrument's measuring accuracy was ±0.01. In-situ observations of the water mass The vertical profiles of salinity, pH, dissolved oxygen, turbidity, and chlorophyll concentration were measured in-situ using the Seabird SBE 19 plus V2 high-precision temperature, salinity, and depth profiler. The equipment was deployed into the water using a winch, and the temperature was recorded for 2 minutes while controlling the descent speed at 0.2 m/s. The Conductivity sensor has a measurement range of 0-9 S/m, an accuracy of 0.0005 S/m, and a resolution of 0.00005 S/m. The chlorophyll sensor has a measurement range of 0-125 μg/L, a sensitivity of 0.02 μg/L, and operates at an excitation/emission wavelength of 470/692 nm. Declarations Acknowledgements We thank Dr. Neal Blair for his constructive suggestions. This study was financially supported by the National Natural Science Foundation of China (Grant Nos. 42125602, 42176091, 92058207, and 42076037), Ocean Negative Carbon Emissions Program, the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2020MD069 and ZR2021JQ12), the Fundamental Research Funds for the Central Universities (Grant No. 2020042010), the Taishan Young Scholars (tsqn202103030), and the China Geological Survey (Grant Nos. GZH201200506, DD20230071, and DD20230409). Author Contributions G. X. and X. D. collected the data. G. X. and R. B. analyzed the data. G. X., R. B.co-wrote the manuscript. All authors revised and contributed the manuscript. Competing financial interests The authors declare no competing financial interests. References Chelton, D.B., Schlax, M.G. & Samelson, R.M. Global observations of nonlinear mesoscale eddies. Prog. Oceanogr. 91, 167–216 (2011). Dufois, F., Hardman-Mountford, N. J., Greenwood, J., Richardson, A. 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Zhang, X., et al. Sedimentary signals of the upwelling along the Zhejiang coast, China. Estuar. Coast. Shelf Sci. 232,106396 (2020). Chen, L., et al. Historical changes in organic matter input to the muddy sediments along the Zhejiang-Fujian Coast, China over the past 160 years. Org. Geochem. 111, 13–25 (2017). Xia, N., Zhang, Q., Yao, D. & Li, G.H. Geochemistry analysis of marine sediments using fused glass disc by X-ray fluorescence spectrometry. Chin. J. Oceanol. Limnol. 26, 475–479 (2008). Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryfigures.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5022150","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":350184720,"identity":"02087de1-324b-4bdd-843e-d548a16a54ac","order_by":0,"name":"Rui Bao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtklEQVRIiWNgGAWjYBACAwhpA+HxkKAljWQtDIdJ0GLOfvjYZ56C83IGxw8wPnjbxiBvTkiLZU9a8mweg9vGBmcSmA3ntjEY7mwg5LADOcbMQC2JG24wsEnztjEkGBwgpOX8G5CWcyAt7L+J03IDbMsBsC3MRGmxnPEsmXGOQbKx5JnEZsk55yQMNxDSYs6ffJjhzR87Ob7jhw9+eFNmI0/QFiTA2AAkJIhXPwpGwSgYBaMANwAAvyg6WQizTVwAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-8159-5269","institution":"Ocean University of China","correspondingAuthor":true,"prefix":"","firstName":"Rui","middleName":"","lastName":"Bao","suffix":""},{"id":350184721,"identity":"aaf6d719-b3bc-4ab1-9f28-000a83925eef","order_by":1,"name":"Gang Xu","email":"","orcid":"","institution":"Institute of Oceanology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Gang","middleName":"","lastName":"Xu","suffix":""},{"id":350184722,"identity":"8973a282-df60-4677-bf0c-281e38579b75","order_by":2,"name":"Xiaoyong Duan","email":"","orcid":"","institution":"Qingdao Institute of Marine Geology, China","correspondingAuthor":false,"prefix":"","firstName":"Xiaoyong","middleName":"","lastName":"Duan","suffix":""},{"id":350184723,"identity":"b939dcdc-4a7f-4a37-8834-386f5329a76f","order_by":3,"name":"Yangli Che","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Yangli","middleName":"","lastName":"Che","suffix":""},{"id":350184724,"identity":"e0e0a7b0-6dd4-4b00-915a-69a36fc58fea","order_by":4,"name":"Tongya Liu","email":"","orcid":"https://orcid.org/0000-0001-6945-1393","institution":"Second Institute of Oceanography, Ministry of Natural Resources","correspondingAuthor":false,"prefix":"","firstName":"Tongya","middleName":"","lastName":"Liu","suffix":""},{"id":350184725,"identity":"d66dbb48-b267-485b-8684-5263f58c8936","order_by":5,"name":"Zhiyou Jing","email":"","orcid":"","institution":"State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Zhiyou","middleName":"","lastName":"Jing","suffix":""},{"id":350184726,"identity":"6410d53d-6cdd-4c01-87c7-53e7cfd76f98","order_by":6,"name":"Shiming Wan","email":"","orcid":"","institution":"Key Laboratory of Marine Geology and Environment","correspondingAuthor":false,"prefix":"","firstName":"Shiming","middleName":"","lastName":"Wan","suffix":""},{"id":350184727,"identity":"da70c199-f791-48d1-9db3-2e8b797f0031","order_by":7,"name":"Bin Chen","email":"","orcid":"","institution":"Qingdao Institute of Marine Geology, China","correspondingAuthor":false,"prefix":"","firstName":"Bin","middleName":"","lastName":"Chen","suffix":""},{"id":350184728,"identity":"124d5035-f8a5-44a9-b3de-3d59ba399d9a","order_by":8,"name":"Jian Liu","email":"","orcid":"","institution":"Qingdao Institute of Marine Geology, China","correspondingAuthor":false,"prefix":"","firstName":"Jian","middleName":"","lastName":"Liu","suffix":""},{"id":350184729,"identity":"084bd544-d68e-4e66-9aaa-c4b61a740cdd","order_by":9,"name":"J. Paul Liu","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"J.","middleName":"Paul","lastName":"Liu","suffix":""},{"id":350184730,"identity":"07465073-81dd-4bbb-96ee-8318914d9ffa","order_by":10,"name":"Ping Yin","email":"","orcid":"","institution":"Qingdao Institute of Marine Geology, China","correspondingAuthor":false,"prefix":"","firstName":"Ping","middleName":"","lastName":"Yin","suffix":""}],"badges":[],"createdAt":"2024-09-03 05:50:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5022150/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5022150/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":64160934,"identity":"22405e94-03d0-4953-b999-b23647bbfcd2","added_by":"auto","created_at":"2024-09-09 07:34:49","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":830323,"visible":true,"origin":"","legend":"\u003cp\u003eResearch stations and local dynamic. Including sampling stations, \u003cem\u003ein-situ\u003c/em\u003e observation stations, and the anticyclonic eddy (AE) with a lifetime of approximately 26 weeks in the northwestern East China Sea. The inset map shows the global geographical location of the study area. The AE is modified after reference\u003csup\u003e17\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/5d4296fbd127ba92017d6edb.png"},{"id":64160935,"identity":"fe3802b0-2406-4a44-bd73-24cf0e2c234e","added_by":"auto","created_at":"2024-09-09 07:34:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":297053,"visible":true,"origin":"","legend":"\u003cp\u003eVertical profiles of hydrological parameters. \u003cstrong\u003ea\u003c/strong\u003e \u003cem\u003eIn-situ\u003c/em\u003e measurement of dissolved oxygen (mg/L), chlorophyll (mg/m\u003csup\u003e3\u003c/sup\u003e), pH, turbidity (NTU), and salinity (psu) at station of IO1 located within anticyclonic eddy. \u003cstrong\u003eb\u003c/strong\u003e \u003cem\u003eIn-situ\u003c/em\u003e observations of dissolved oxygen (mg/L), chlorophyll (mg/m3), pH, turbidity (NTU), and salinity (psu) at station of IO2 located outside anticyclonic eddy. SCML = subsurface chlorophyll maximum layer; DO = dissolved oxygen.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/f8b739bf29bdf93f34722d26.png"},{"id":64161474,"identity":"7f68060a-b7bf-461c-82ff-dc0f71402023","added_by":"auto","created_at":"2024-09-09 07:50:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":923498,"visible":true,"origin":"","legend":"\u003cp\u003eSpatial distribution of ratio of pristine to phytane, and molecular biomarkers in surface sediments form the northwestern East China Sea shelf. \u003cstrong\u003ea\u003c/strong\u003e spatial distribution of ratio of pristine to phytane (Pr/Ph) in surface sediments. \u003cstrong\u003eb\u003c/strong\u003e spatial distributions of C\u003csub\u003e15\u003c/sub\u003e+C\u003csub\u003e17\u003c/sub\u003e+C\u003csub\u003e19\u003c/sub\u003e \u003cem\u003en\u003c/em\u003e-alkanes concentration. \u003cstrong\u003ec\u003c/strong\u003e spatial distributions of C\u003csub\u003e27\u003c/sub\u003e+C\u003csub\u003e29\u003c/sub\u003e+C\u003csub\u003e31\u003c/sub\u003e \u003cem\u003en\u003c/em\u003e-alkanes concentration. \u003cstrong\u003ed\u003c/strong\u003e spatial distributions of the high molecular weight carbon preference index (CPI\u003csub\u003e25-33\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/0523d8bffd60b56b0b8be03f.png"},{"id":64161263,"identity":"fed6c76c-cbe8-4b2d-a89c-7b113f4d74dc","added_by":"auto","created_at":"2024-09-09 07:42:49","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":433508,"visible":true,"origin":"","legend":"\u003cp\u003eSpatial distributions of deposition fluxes of organic carbon, Chlorophyll-a, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content, and the linear relationship between organic carbon and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. \u003cstrong\u003ea\u003c/strong\u003e spatial distributions of sedimentary organic carbon and deposition fluxes of organic carbon (unit: g C m\u003csup\u003e−2\u003c/sup\u003e yr\u003csup\u003e−1\u003c/sup\u003e). \u003cstrong\u003eb\u003c/strong\u003e chlorophyll-a (in mg/m\u003csup\u003e3\u003c/sup\u003e) on 20\u003csup\u003eth\u003c/sup\u003e October 2012 (based on reference\u003csup\u003e55\u003c/sup\u003e). \u003cstrong\u003ec\u003c/strong\u003e spatial distributions of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in surface sediments. \u003cstrong\u003ed\u003c/strong\u003e the linear relationship between organic carbon and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in surface sediment in the northwestern East China Sea shelf. White patchy areas in Figure 4b indicate cloud cover or bad algorithm retrieval from the original satellite images.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/78d4b695e35b26895221dd2f.png"},{"id":64160939,"identity":"a511dc1f-dfac-4d4a-ad91-1dc079d8452a","added_by":"auto","created_at":"2024-09-09 07:34:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":889231,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual diagram of biological carbon pump. \u003cstrong\u003ea\u003c/strong\u003enatural biological carbon pump (BCP). \u003cstrong\u003eb\u003c/strong\u003eenhanced biological carbon pump modulated by anticyclonic eddy. DO = dissolved oxygen.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/51b695234af0e348a541cb5a.png"},{"id":67233943,"identity":"49ccc4f1-8e39-4b7c-a985-47209059b05a","added_by":"auto","created_at":"2024-10-22 17:28:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4035742,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/f490ebfe-a84d-4d19-a81e-8c734692f468.pdf"},{"id":64160937,"identity":"fb1febde-f26c-47d7-8186-6901d5117f2e","added_by":"auto","created_at":"2024-09-09 07:34:49","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1632986,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Supplementaryfigures.docx","url":"https://assets-eu.researchsquare.com/files/rs-5022150/v1/ef9092d4a1a2902b7cba0b3f.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Sedimentary organic carbon burial in marine oxic sediments modulated by anticyclonic eddy","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMesoscale eddies, including cyclonic and anticyclonic eddies (AEs), are a common feature of ocean circulation\u003csup\u003e1\u003c/sup\u003e.\u0026nbsp;These eddies play a crucial role in regulating the supply of nutrients to the sunlit layers of the ocean, thereby influencing the rates of carbon fixation and export\u003csup\u003e2\u003c/sup\u003e. It was previously believed that AEs suppress productivity and phytoplankton biomass due to the downward displacement of isopycnals within them\u003csup\u003e3,4\u003c/sup\u003e, recent studies suggested that AEs can actually enhance biomass and nutrient fluxes through eddy-induced mixing, potentially increasing marine organic carbon (OC) production\u003csup\u003e2,5,6,7\u003c/sup\u003e. The prevalence of phytoplankton within AEs\u003csup\u003e8,9\u003c/sup\u003e and subduction facilitated by AEs may significantly contribute to OC export in the ocean\u003csup\u003e10,11\u003c/sup\u003e. Furthermore, AEs, being more energetic and nonlinear compared to surrounding currents, have the ability to trap and transport mass, physical tracers, and biogeochemical properties from their surroundings to their edges\u003csup\u003e12,13,14\u003c/sup\u003e, thus may affecting the accumulation of OC in sediments. However, the mechanism underlying OC export and sedimentation modulated by AEs are poorly deciphered.\u003c/p\u003e\n\u003cp\u003eMesoscale eddies in open oceans exhibit a wide range of spatial (tens to hundreds of kilometers) and temporal (weeks to months) scales，and most of them are in the process of dynamic migration\u003csup\u003e1\u003c/sup\u003e,\u0026nbsp;posing challenges to studying carbon export and sedimentation mechanisms driven by these eddies.\u0026nbsp;Conversely, some mesoscale eddies on continental shelves, such as those in the\u0026nbsp;eastern marginal seas of China\u003csup\u003e15,16,17\u003c/sup\u003e, are relatively stationary, making them ideal regions for investigating AEs-driven OC accumulation in marine sediments.\u0026nbsp;The East China Sea (ECS) is a typical passive marginal sea characterized by a shallow and broad epicontinental shelf. It is also one of the most active areas for OC transport and sedimentation\u003csup\u003e18\u003c/sup\u003e. Approximately 3.21\u0026times; 10\u003csup\u003e12\u003c/sup\u003e g OC is buried annually in the inner-shelf of the ECS\u003csup\u003e19\u003c/sup\u003e. Recent report indicated the presence of an AE on the ECS shelf \u003csup\u003e17\u003c/sup\u003e. This AE remains stationary and has a life cycle of around 26 weeks, occurring from May to October each year\u003csup\u003e17\u003c/sup\u003e, creating favorable conditions for studying OC burial mechanism in sediment modulated by AEs. Furthermore, there is a hot spot of OC burial in the ECS inner-shelf\u003csup\u003e19\u003c/sup\u003e, coinciding with the location of the AE. Therefore, it is important to investigate the impact of the AE on OC sedimentation in the ECS.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHere,\u0026nbsp;we conducted a thorough investigation using a range of methods and analyses in summer. This included collecting sedimentation rates at 19 sites, determining the OC and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in surface sediments at 519 stations, analyzing molecular biomarkers in surface sediments from 127 sites, and performing \u003cem\u003ein-situ\u003c/em\u003e observations of vertical dissolved oxygen, chlorophyll content, pH, and turbidity of the water mass. Additionally, we examined the OC content and molecular biomarkers in one sediment core (Figure 1). We employed the organic geochemical data to gain insights into the composition, sources, and degradation of OC in the sediments in the area modulated by the AE. Correlation analysis was performed between Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and OC to\u0026nbsp;understand the preservation mechanism of\u0026nbsp;OC. \u003cem\u003eIn-situ\u003c/em\u003e observations were used to uncover the progress of OC burial influenced by AE.\u0026nbsp;\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003e\u003cstrong\u003eOC burial in oxidizing environment within AE\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe dissolved oxygen concentration in the bottom water is approximately 5.7 mg/L, higher than the concentration outside the AE, where it is 4.9 mg/L (Figure 2a-b). The water mass within AE shows an oxic environment, with higher oxidation of bottom water mass within AE compared to the non-AE region.\u0026nbsp;In the sediment core within AE (Figure S1), the ratio of pristane (Pr) to phytane (Ph), a reliable indicator of redox conditions\u003csup\u003e20,21\u003c/sup\u003e, ranges from 1.9 to 3.6, indicating an oxic environment in the subseafloor sediments.\u0026nbsp;In addition, the spatial distribution of the Pr/Ph ratios in surface sediments shows higher values (\u0026gt;0.8) predominantly within the AE, and lower values (\u0026lt;0.6) around the AE (Figure 3a). This also indicates a spatially distinct oxic depositional environment within the AE compared to the exterior. The redox potential (Eh) in the study area also confirms the oxic nature of sediments within the AE (Figure S2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 3a\u0026nbsp;shows the spatial thickness distribution of the surface mixed layer, which serves as an indicator of the hydrodynamic intensity\u003csup\u003e17,22\u003c/sup\u003e. The coupling between the surface mixed layer and the Pr/Ph ratio within AE highlights the pronounced downward dynamic feature of the AE\u003csup\u003e17\u003c/sup\u003e (Figure 3a). Isopycnals within the AE deepen towards its center\u003csup\u003e3,23\u003c/sup\u003e, leading to the intensification of fronts within the eddying flow field and the generation of downwelling\u003csup\u003e11,24\u003c/sup\u003e. This downwelling promotes the intrusion of oxygen-rich surface water\u003csup\u003e11,25,26\u003c/sup\u003e, resulting in an oxidizing environment within the AE.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe sediments in our study area modulated by the AE contain OC content of up to 0.7% even more. It is now recognized that the previously perceived barren desert of the AE-dominated environment is actually a fertile land of OC\u003csup\u003e2\u003c/sup\u003e. The OC in the AE-modulated region is characterized by high levels C\u003csub\u003e15\u003c/sub\u003e+C\u003csub\u003e17\u003c/sub\u003e+C\u003csub\u003e19\u003c/sub\u003e n-alkanes (Figure 3b), which are derived from marine phytoplankton\u003csup\u003e27\u003c/sup\u003e.\u0026nbsp;This indicates that the OC in this area mainly originates from marine sources. In contrast, OC with high levels of C\u003csub\u003e27\u003c/sub\u003e+C\u003csub\u003e29\u003c/sub\u003e+C\u003csub\u003e31\u003c/sub\u003e n-alkanes, originating from terrestrial plants\u003csup\u003e28\u003c/sup\u003e, is found in the coastal area rather than the AE-modulated area (Figure 3c). The degradation index of OC, known as CPI\u003csub\u003e25\u0026ndash;33\u003c/sub\u003e\u003csup\u003e19\u003c/sup\u003e, in surface sediments shows that the OC buried in the AE- dominated area mainly contains fresh marine OC that has not undergone complete degradation (Figure 3d). Our findings suggest that fresh marine OC can accumulate within an oxic depositional setting.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe burial fluxes of OC in the AE-dominated deposit are relatively high, with an average flux of approximately 47 g C m\u003csup\u003e\u0026minus;2\u003c/sup\u003e yr\u003csup\u003e\u0026minus;1\u0026nbsp;\u003c/sup\u003e(Figure 4a). This value is three times higher than the average flux observed in the ECS shelf \u003csup\u003e19\u003c/sup\u003e. Furthermore, it has been observed that the OC content in the AE-dominated deposit can stabilize at a background level of approximately 0.6% (Figure S1), indicating that the OC accumulation in oxic sediments, modulated by AE, remains relatively stable and significant in the subseafloor sediment. These findings challenge the conventional notion that OC accumulation is limited to reducing environments\u003csup\u003e29\u003c/sup\u003e, as we show that even in oxic sediments, significant amounts of OC accumulate.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eExport of OC under pycnocline driven by AE\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAt an \u003cem\u003ein-situ\u0026nbsp;\u003c/em\u003eobservation station located in a normal area without the influence of AE (Figure 1), we found that the water column from the top to the bottom is stratified, with distinct layers including the mixed layer, pycnocline (where salinity, temperature, or density changes most rapidly with depth\u003csup\u003e30\u003c/sup\u003e), and a homogeneous layer (Figure 2b).\u0026nbsp;Two subsurface chlorophyll maximum layers (SCMLs) which influenced by interactions between the ecosystem and hydrodynamics in stratified waters\u003csup\u003e31,32\u003c/sup\u003e are typically observed in the mixed layer and near the upper boundary of the pycnocline (Figure 2b). While the deepest SCML can play a crucial role in the downward export of OC\u003csup\u003e33,34\u003c/sup\u003e, there is a\u0026nbsp;rapid decrease in chlorophyll content below the deepest SCML and a notable changes in dissolved oxygen, pH value, and\u0026nbsp;minimal changes in water turbidity within pycnocline (Figure 2b)，suggesting that the majority of phytoplankton (marine)-derived OC within the pycnocline undergoes mineralization. Pycnocline in non-AE region thus acts as a barrier layer hindering the downward transport of OC\u003csup\u003e35\u003c/sup\u003e. While the biological carbon pump is a well-known mechanism for exporting marine OC, we argue that the prevalent pycnocline in the ocean significantly impede the efficiency of OC downward export.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn contrast, we find more water layers in the water column within the AE, including a mixed layer, pycnocline, mixed layer, and homogeneous layer (Figure 2a). Additionally, three SCMLs within AE are observed, located in the mixed layer, the upper boundary of the pycnocline, and mixed layer from top to bottom (Figure 2a). Notably, a SCML is observed below the pycnocline within AE, which distinguishes it from the non-AE region (Figure 2a and 2b).\u0026nbsp;Since the AE exhibits elevated energy levels and nonlinearity, enabling it to capture and transport materials from its surrounding to its periphery\u003csup\u003e12,13\u003c/sup\u003e,\u0026nbsp;the predatory behavior of the AE leads to the accumulation of a significant amount of phytoplankton along its periphery (Figure 4b). For instance, on the right side of the AE, there are two distinct spiral-shaped bands with high chlorophyll-a concentrations extend outward (Figure 4b). Similar bands are also present on the left side of the AE\u003csup\u003e36\u003c/sup\u003e. These bands likely act as the main pathways for the influx of materials from the surrounding to the periphery of the AE, resembling tentacles for the AE to feed on. Once phytoplankton accumulates along the AE\u0026apos;s periphery,\u0026nbsp;it is transported downwards under the pycnocline\u003csup\u003e11\u003c/sup\u003e. Therefore, the phytoplankton (marine)-derived OC can be rapidly transported downwards under the action of downwelling, significantly reducing the possibility of phytoplankton (marine)-derived OC being consumed by predators\u003csup\u003e29\u003c/sup\u003e. In our case, the SCML located below the pycnocline within AE is evident. We think that the SCML located below the pycnocline driven by\u0026nbsp;the AE\u0026nbsp;could significantly enhance its ability to transport OC downwards.\u003c/p\u003e\n\u003cp\u003eBelow the SCML located below the pycnocline within AE, we observed a gradual decrease in chlorophyll concentration, accompanied by a rapid increase in water turbidity, and minimal changes in dissolved oxygen and pH (Figure 2a).\u0026nbsp;These observations\u0026nbsp;suggest that the export of phytoplankton (marine)-OC driven by the AE under the SCML is efficient. The content of particulate OC and suspended sediment in the water column can be reflected by turbidity value\u003csup\u003e37\u003c/sup\u003e. As this \u003cem\u003ein-situ\u0026nbsp;\u003c/em\u003eobservation station is located far from the shore (Figure 1), changes in turbidity primarily reflect variations in particulate OC content. Therefore, the elevated turbidity values in the water column mainly reflect an increase in particulate OC. In the South China Sea,\u0026nbsp;a flux of particulate OC within AE is 1.6-fold higher compared to non-eddy region\u003csup\u003e10\u003c/sup\u003e. Furthermore, our study reveals a positive correlation between the OC content in sediments and the intensity of downwelling (Figure 3a and 4a), suggesting that the downwelling enhances the subduction of OC under the pycnocline driven by the AE, providing an efficient pathway for OC accumulation in the sediments.\u003c/p\u003e\n\u003cp\u003eAlthough, the export of OC under the pycnocline driven by the AE is enhanced, theoretically, the oxidizing environment within AE is not conducive to the preservation of OC\u003csup\u003e29,38\u003c/sup\u003e. The abnormal enrichment of OC in oxidizing environment may reflect its strong antioxidant capacity. Previous research suggested that iron\u0026nbsp;oxide in oxic sediment plays a crucial role in protecting OC\u003csup\u003e39\u003c/sup\u003e, as the association of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with OC hinders its mineralization in marine environments\u003csup\u003e40,41\u003c/sup\u003e. Our results show that that the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in surface sediments within AE-influenced area exceeds 0.7% (Figure 4c). Since the AE exhibits elevated energy levels and nonlinearity, enabling it to capture and transport element Fe from its surrounding to its internal\u003csup\u003e12,13\u003c/sup\u003e. Furthermore, the interior of AE is an oxidizing environment, leading to the enrichment of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. In our study, a clear spatial correlation is observed between Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and OC content in surface sediments (Figure 4a and 4c). This is further supported by a strong positive correlation between the OC and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in surface sediments (Figure 4d). Higher Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e levels increase the affinity for particulate OC, and the presence of abundant dissolved oxygen promotes the oxidation of sinking OC, resulting in OC with potent antioxidant properties. To sum up, the efficient downward transport and sedimentation of OC, modulated by AEs, can be viewed as an enhanced biological carbon pump (Figure 5).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eImplication of OC accumulations driven by AE\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNumerous mesoscale AEs have been identified in the mid-latitude oceans, characterized by long lifetimes and the ability to propagate over extensive distances\u003csup\u003e1\u003c/sup\u003e. The concentration of surface chlorophyll is the highest in the mid-latitude oceans, particularly between 30\u0026deg; and 60\u0026deg;, excluding the equatorial region\u003csup\u003e42\u003c/sup\u003e. This coupling between AEs and surface chlorophyll concentration creates favorable conditions for the export of OC in mid-latitude oceans, as supported by the global chlorophyll export flux\u003csup\u003e43\u003c/sup\u003e. For instance, in the southern mid-latitude oceans, the regions particularly between 30\u0026deg; S and 50\u0026deg; S\u003csup\u003e44\u003c/sup\u003e play a substantial role in the oceanic carbon sink\u003csup\u003e45,46\u003c/sup\u003e. In the northern mid-latitude oceans, such as the North Atlantic, AEs also promote the export of particulate OC, contributing up to half of the total export of particulate OC\u003csup\u003e11\u003c/sup\u003e. The export of driven OC in the mid-latitude deep-sea water\u003csup\u003e47,48\u003c/sup\u003e, due to the high kinetic energy of AE that may influence deeper water depths even 4000 meters or more\u003csup\u003e49\u003c/sup\u003e, likely influencing the global long-term carbon cycle. Additionally, mid-latitude oceans, especially within circulation areas with active mesoscale AEs, represent significant reservoirs of OC in oxic sediments. The South Atlantic Ocean with active mesoscale AEs, such as those between 40\u0026deg; and 60\u0026deg;, exhibits OC accumulation rates ranging from 0.05~0.1 g C m\u003csup\u003e\u0026minus;2\u003c/sup\u003e yr\u003csup\u003e\u0026minus;1\u003c/sup\u003e, the highest in the entire South Atlantic Ocean excluding coastal areas\u003csup\u003e50\u003c/sup\u003e. Therefore, the majority of AEs are dynamic\u003csup\u003e1\u003c/sup\u003e, and their widespread distribution significantly might contributes to the total burial of OC in the mid-latitude oceans.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOn million-year timescales, around 1.6 \u0026times; 10\u003csup\u003e19\u003c/sup\u003e g of OC may be buried in oxic sediment\u003csup\u003e51\u003c/sup\u003e, which represents a previously overlooked carbon reservoir with significant importance in carbon cycles and deep-sea life\u003csup\u003e52,53,54\u003c/sup\u003e. However, the dynamic mechanisms underlying the formation and maintenance of the OC reservoir in oxic sediment have not been deeply deciphered. Our findings provide a potential perspective for the formation of these OC reservoirs in oxic sediments. AEs not only facilitate the accumulation of OC in oxic sediments, but also promote the fusion of OC with iron oxides protecting against remineralization. Furthermore, AEs can prevent acidification and hypoxia in the bottom water (Figure 2a). This unique carbon sequestration mechanism also offers insights into the potential use of artificial AEs to enhance the burial of OC in the inner shelf.\u0026nbsp;\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eSample Collection\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this study, a total of 519 surface sediment samples were collected in the ECS shelf in the summer of 2013 for OC and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e analysis. In addition, 127 samples were used for biomarker analysis. The samples were immediately frozen at -20\u0026deg;C until organic chemistry analysis. The surface sediment samples represent the top 1 cm layer at the center of the sampling box.\u003c/p\u003e\n\u003cp\u003eThe study utilized MODIS images of chlorophyll-a on 20 October 2012, as reported by reference\u003csup\u003e55\u003c/sup\u003e. Data on surface mixed layer thickness and sedimentation rates were obtained from reference\u003csup\u003e56\u003c/sup\u003e. The Pr/Ph values in sediment core Sc01 were sourced from reference\u003csup\u003e17\u003c/sup\u003e, while the OC concentration data in sediment core Sc01 were provided by reference\u003csup\u003e57\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGeochemical Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe\u0026nbsp;freeze-dried and homogenized samples were analyzed for\u0026nbsp;OC.\u0026nbsp;Approximately 500 mg of sediment was treated with 4 N HCl at room temperature for 12 hours to remove calcium carbonate, followed by four rinses with deionized water. The decarbonated samples were then freeze-dried and placed into tin capsules.\u0026nbsp;OC\u0026nbsp;determination was performed using a Euro EA 3000 Elemental Analyzer, with a standard deviation of \u0026plusmn;0.02% dry weight (n = 6).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe concentration of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in the samples was determined using an X-ray fluorescence spectrometer (model Philips PW4400), following the method described by reference\u003csup\u003e58\u003c/sup\u003e. The accuracy of the element analyses was verified by replicating the analysis of certified national standard reference materials (GBW07343, GBW07344, and GBW07334). The analytical error for Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was found to be -1.26%.\u003c/p\u003e\n\u003cp\u003eFreeze-dried biomarker samples were finely ground to a size smaller than 200 \u0026mu;m, then subjected to Soxhlet-extracted for 24 h using dichloromethane (DCM) as the solvent. The resulting extracts were divided into four fractions based on the different polarities of their components through flash column chromatography packed with activated silica gel (particle size 40\u0026ndash;63 \u0026mu;m) under a nitrogen pressure of 1.5 bars or less. The saturated hydrocarbon fraction was eluted with n-hexane, the aromatic hydrocarbon fraction with a mixture of n-hexane and DCM (9:1 v/v), the ketones/esters fraction with DCM, and the heterocompounds with methanol. The so-called \u0026ldquo;saturated hydrocarbon fraction\u0026rdquo; comprises both saturated hydrocarbons and mono-unsaturated hydrocarbons. Solvents were then removed from each fraction using a stream of nitrogen. The nonpolar lipid fractions were dissolved in hexane and analyzed using gas chromatography-mass spectrometer (GC-MS; Agilent 7890A/5975C) with an HP-5 capillary column (30 m \u0026times; 0.25 mm \u0026times; 0.25 \u0026mu;m). Helium was used as the carrier gas at a flow of 1.0 ml/min, 1 \u0026mu;l of sample was injected at 280 \u0026deg;C. The temperature program for the nonpolar fractions, which include n-alkanes and isoprenoids, started at 60 \u0026deg;C, held for 1 minute, followed by a 5 \u0026deg;C/min increase to 310 \u0026deg;C, where it was held for 20 minutes. The electron ionization source was set at 230 \u0026deg;C and 70 eV, with scanning conducted between mass-to-charge ratio (m/z) ranges of 50-600 Daltons. Individual components were identified by comparing retention times with literature and NIST mass spectrometry library. The corresponding indices were quantified by the integration the peak areas of the total ion chromatogram.\u0026nbsp;A procedural blank was included in the sequence to account for any background interferences, which were found to be insignificant. The carbon preference index (CPI) was calculated following the method described by\u0026nbsp;reference\u003csup\u003e19\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eIn-situ\u0026nbsp;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003emeasurement of Eh\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Eh values of seven surface sediment samples were measured \u003cem\u003ein-situ\u003c/em\u003e using the pHS-3C precision acidity meter during the summer of 2019. Prior to measurement, the electrode was immersed in distilled water for 24 hours, preheated for 30 minutes, and calibrated with standard buffer solution. The instrument displayed results updated every second, and the measured Eh values stabilized within 2 minutes, providing an accurate representation of the surface sediment Eh state. Once stabilized, the Eh measurement value of the sediment was recorded. The instrument\u0026apos;s measuring accuracy was \u0026plusmn;0.01.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eIn-situ\u0026nbsp;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003eobservations of the water mass\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe vertical profiles of salinity, pH, dissolved oxygen, turbidity, and chlorophyll concentration were measured \u003cem\u003ein-situ\u003c/em\u003e using the Seabird SBE 19 plus V2 high-precision temperature, salinity, and depth profiler. The equipment was deployed into the water using a winch, and the temperature was recorded for 2 minutes while controlling the descent speed at 0.2 m/s. The Conductivity sensor has a measurement range of 0-9 S/m, an accuracy of 0.0005 S/m, and a resolution of 0.00005 S/m. The chlorophyll sensor has a measurement range of 0-125 \u0026mu;g/L, a sensitivity of 0.02 \u0026mu;g/L, and operates at an excitation/emission wavelength of 470/692 nm.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Dr. Neal Blair for his constructive suggestions. This study was financially supported by the National Natural Science Foundation of China (Grant Nos. 42125602, 42176091, 92058207, and 42076037), Ocean Negative Carbon Emissions Program, the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2020MD069 and ZR2021JQ12), the Fundamental Research Funds for the Central Universities (Grant No. 2020042010), the Taishan Young Scholars (tsqn202103030), and the China Geological Survey (Grant Nos. GZH201200506, DD20230071, and DD20230409).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eG. X. and X. D. collected the data. G. X. and R. B. analyzed the data. G. X., R. B.co-wrote the manuscript. All authors revised and contributed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting financial interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eChelton, D.B., Schlax, M.G. \u0026amp; Samelson, R.M. Global observations of nonlinear mesoscale eddies. Prog. Oceanogr. 91, 167\u0026ndash;216 (2011).\u003c/li\u003e\n \u003cli\u003eDufois, F., Hardman-Mountford, N. J., Greenwood, J., Richardson, A. J., Feng, M. \u0026amp; Matear, R.J. Anticyclonic eddies are more productive than cyclonic eddies in subtropical gyres because of winter mixing. Sci. 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Limnol. 26, 475\u0026ndash;479 (2008).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5022150/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5022150/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Anticyclonic eddies (AEs) are a common feature of ocean circulation and play a significant role in influencing the rates of carbon fixation and export. However, the mechanism underlying organic carbon (OC) export and sedimentation modulated by AEs are poorly deciphered. Here, we utilized in-situ observations of dissolved oxygen, chlorophyll, pH, and turbidity in the water column and experimental analyses of molecular biomarkers, OC, and iron trioxide in sediments from the East China Sea, to unravel the processes driving OC sedimentation modulated by AE. Our findings reveal that a significant amount of OC is preserved in oxic sediments, influenced by the presence of an anticyclonic eddy. We suggest that the eddy promotes the accumulation of OC along its periphery, and transports OC downward under the pycnocline. The combination of OC with iron trioxide, facilitated by oxidation, impedes the mineralization of OC in sediments. The accumulation of OC in oxic sediment, modulated by anticyclonic eddies, has significant implications for OC burial in mid-latitude oceans on millennial timescales.","manuscriptTitle":"Sedimentary organic carbon burial in marine oxic sediments modulated by anticyclonic eddy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-09 07:34:44","doi":"10.21203/rs.3.rs-5022150/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5e42cff0-bb3b-401e-ab25-f0138c86734c","owner":[],"postedDate":"September 9th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":37177686,"name":"Earth and environmental sciences/Ocean sciences"},{"id":37177687,"name":"Earth and environmental sciences/Hydrology"}],"tags":[],"updatedAt":"2024-10-22T17:20:22+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-09 07:34:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5022150","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5022150","identity":"rs-5022150","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}