A 430 kyr record of ice-sheet dynamics and organic-carbon burial in the central Eurasian Arctic Ocean

Domain: earth_science
Published: 2025-04-23
Editorial outcome: minor_revision

Abstract
Late Quaternary central Arctic Ocean paleoclimatic records going beyond the last climate cycle, are still very rare. Here, we present data from a well-dated deep-sea sediment core that allow for the first time to reconstruct in detail the interrelationship between ice-sheet dynamics and organic-carbon (OC) burial in the Eurasian Basin during the last 430 kyr, and to correlate marine and terrestrial records of the Eurasian Ice Sheet (EIS) history. Using a multi-proxy approach, we demonstrate that within pulses of EIS advance and retreat during glacial to subsequent deglacial times, erosion of ancient (petrogenic) OC-rich sedimentary rocks and soils deliver sediments, further supplied onto the shelf and beyond. Down-slope and long-distance transport of the fine-grained OC-rich suspension by turbidity and contour currents cause elevated burial of terrestrial OC and anoxic sedimentary conditions in the deep Eurasian Basin that allow preservation of labile algae-type OC.

Introduction
Within the ongoing debate of recent climate change, the remote Arctic Ocean with its strong seasonal forcing and variability in runoff, sea ice formation, and sunlight, became a major focus during the last few decades as the high latitudes mirror the global warming trend strongly amplified1,2. Due to complex feedback processes (collectively known as “polar amplification”), the Arctic is both a contributor of climate change and a region that will be most affected by global warming. That means, the Arctic Ocean and surrounding areas are (in real time) and were (over historic and geologic time scales) subject to rapid and dramatic change. On geological timescales, the glacial history of the Arctic Ocean characterized by the repeated waxing and waning of circum-Arctic continental ice sheets (Fig. 1 showing the ice-sheet extent during MIS 6), the growth and disintegration of ice shelves, and related changes in ocean circulation patterns and sea-ice cover3–8, have to be taken into account as well. There is, however, still an ongoing and partly controversial debate about the timing and extent of maximum glaciations and related interrelationships with changes in climate and environment on land and in the ocean. A key problem in this debate about the history and significance of the Arctic cryosphere within the climate system, that has not been fully resolved yet, is the accurate dating of the Quaternary sediment sequences, a major challenge for the entire Arctic research community for several decades.

When talking about the (global) climate system and its driving processes, the organic carbon (OC) cycle plays a key role, i.e., production on land and in the oceans, storage/burial in the ocean and sediments, diagenesis/degradation, and release of greenhouse gases to the atmosphere9. Concerning the modern Arctic Ocean OC cycle, i.e., OC origin and burial in the sediments, there is a clear predominance of terrestrially-derived OC, and huge amounts of terrestrial OC (TerrOC) are currently buried in circum-Arctic shelf sediments, soils and peat deposits and stored frozen in permafrost soils and ice complex deposits10–12. As a result of increased surface warming in Arctic regions, a widespread decay of permafrost, collapse and erosion of coastal ice complex deposits, and the decomposition of hydrates could lead to the release of large quantities of CH4 (and CO2) to the atmosphere as well as an enhanced mobilization and export of ancient, previously frozen soil-derived OC13–16.

Looking at geological timescales, the last deglacial atmospheric CO2 increase may be explained by the release and subsequent oxidation of large amounts of ancient TerrOC17,18. On the other hand, during glacial climate cooling and times of extended circum-Arctic ice sheets and lowered sea-level, huge amounts of OC-rich soils, sediments and rocks might be eroded and transported by glacigenic processes across the shelf edge, and buried in deep ocean sediments, thus acting as an important geological CO2 sink and storage (cf., refs. 19, 20). Well-dated detailed records dealing with such glacigenic and oceanographic processes, sediment discharge, and rates of OC burial in the central Arctic Ocean during late Quaternary glacial stages, however, are quite limited and mainly restricted to the last climate cycle.

Here, we have utilized a sediment core recovered from the Amundsen Basin in 3600m water depth at the eastern flank of the Gakkel Ridge during Polarstern Expedition PS115/2 in 2018 (ref. 21), representing four glacial/interglacial cycles of the last 430 kyr. Our age model, a fundamental prerequisite for any kind of paleoclimatic reconstruction, is based on records of the inclination and relative paleointensity (RPI) of the Earth’s magnetic field approved by rock magnetic data, 230Th and 231Pa excess (230Th_ex and 231Pa_ex) records, 230Th_ex maxima, and AMS14C ages. Using organic-geochemical proxies, we have identified prominent well-defined sections with strongly elevated concentration of ancient (petrogenic) predominantly terrestrial OC, coinciding with glacial time intervals of extended ice sheets and the subsequent terminations/deglacials. Mineral assemblages related to the complex geology of the surrounding hinterland (Fig. 1), in combination with sediment structures and textures are used to identify the provenance and transport pathways of these OC-rich sediments. Our multi-proxy approach allowed for the first time a detailed reconstruction of the OC burial history and its relationship to the Eurasian ice sheet (EIS) history, sediment and meltwater discharge, and depositional environment during the last about 430 kyr. Based on the preservation of labile algae-type OC under anoxic sedimentary conditions, we demonstrate that even during strong glacial intervals there must have been at least occasionally open-water (polynya-type) conditions along the Eurasian continental margin in front of the ice sheet with marine and sea-ice algae productivity. Furthermore, we show that the glacial rates of predominantly TerrOC burial are three to six times higher than those determined in Holocene Arctic deep-sea sediments representing interglacial conditions, suggesting that Arctic deep-sea sediments (and especially those in the Eurasian Basin) were an important sink for TerrOC during glacial intervals. Finally, the prominent dolomite-rich pink layer (“Pink-White Layer 2”) dated to about 282 ka, represents a key marker horizon to be used for ocean-wide correlation and dating of Arctic sediment cores. In summary, we are confident that Core PS115/2-2 is a key core for reconstructing Arctic paleo-climate conditions that is of major interest for the broader Arctic scientific community and beyond (in this context see also Supplementary Information Nos. 1–7 for additional important data).

Results
Sediment characteristics and lithostratigraphic framework
Polarstern Expedition PS115/2 was carried out in 2018 in the area of the southern Lomonosov Ridge and the Amundsen Basin towards the Laptev Sea continental margin (see Supplementary Information No. 1). The expedition’s program was mainly devoted (1) to finalize the exact drilling locations for the IODP Expedition 377 “Arctic Ocean Paleoceanography” (scheduled for August/September 2022 but unfortunately canceled in February 2022 due to political reasons) and (2) to reconstruct in detail the short- and long-term changes in circum-Arctic ice sheets, sea-ice cover, and surface- and deep-water characteristics21. In order to reach the latter goals, a Hydrosweep and Parasound survey was carried-out to select optimum locations for obtaining long undisturbed sediment cores (Supplementary Fig. 1). At the eastern flank of the Gakkel Ridge representing well-stratified pelagic sediments (Supplementary Fig. 1b, c), a complete and undisturbed composite sedimentary sequence was recovered using a kastenlot corer (Supplementary Fig. 2; for details see “Methods” below). This 764 cm long sequence of Core PS115/2-2 is characterized by two main lithologies: (1) brown to dark brown and light olive brown, partly bioturbated silty clays with alternations of thin silty clay/silt/sandy layers of varying brownish/grayish colors, and (2) very prominent distinctive intervals of dark gray silty clays (Supplementary Figs. 2 and 3)21. Such dark gray intervals that are enriched in OC (see below for details) have also been found in several sediment cores from the area across the southern Lomonosov Ridge (Supplementary Fig. 4) and along the continental margin of the Laptev and East Siberian seas and assigned to glacial intervals21–25. In most of these sediment cores, only one or two intervals with dark gray sediments have been recovered in the lowermost part of the sedimentary sections, and often the issue of accurately dating these cores has not been resolved yet (Supplementary Fig. 4)7,25,26. Furthermore, in cores from the NW-Svalbard, Eurasian and Siberian continental margins located more proximal to the glacial ice sheets, the dark gray sediments often represent coarser-grained ice-rafted debris (IRD)-rich or even diamicton facies, related to extensive glacial activities at times of extended ice sheets on the shelf and beyond22,24,27,28. In contrast, Core PS115/2-2 contains seven intervals of dark gray very fine-grained (no IRD) silty clays that are characterized by a sharp base and display prominent internal fining-upward structures, interpreted as distal turbidites and/or deposition from contour currents (Supplementary Fig. 3). Such a lithostratigraphic sequence with a series of “cyclic” events represents significant recurrent changes of the depositional environment including sedimentary processes, detrital sediment input, and OC burial rates, i.e., changes that are highly probable related to glacial/interglacial climate variability.

Before any paleoenvironmental reconstruction can be done, however, the lithostratigraphic framework has first to be transferred into a reliable chronostratigraphic framework—still a major challenge for many decades when working on Arctic Ocean sedimentary sections representing time intervals beyond the use of AMS14C dating29–32. Just based on shipboard data and the assumption that the dark gray intervals represent glacial intervals21,23, a preliminary age model had been proposed, suggesting that the sedimentary sequence of Core PS115/2-2 might represent the last about 500 kyr (ref. 21). However, robust chronostratigraphic tie points as established in our study here, were missing at that time.

Establishing an accurate chronology—a challenging task in Arctic paleoclimate research
Whereas dating techniques such as oxygen isotope stratigraphy, magnetostratigraphy, and biostratigraphy are routine tools in studies from most world ocean areas, several problems are obvious for Arctic Ocean sediments resulting in difficulties in establishing accurate age-depth relationships in the existing sediment cores. Poor preservation of calcareous and biosiliceous microfossil faunas and floras in Arctic cores often precludes application of conventional biostratigraphic and isotopic dating techniques. Paleomagnetic studies carried out on marine sediments from various regions of the Arctic Ocean, led to quite different age models and related sedimentation rates because the changes between reverse and normal polarities may be interpreted differently, i.e., as long-term reversals of the geomagnetic field, short-term geomagnetic excursions and/or a result of diagenetic overprint in the initial natural remanent magnetization (NRM)33–39. Because of these uncertainties in interpretation of magnetic inclination changes it is crucial to provide independent age control to guide the interpretation of the paleomagnetic polarity pattern.

Our age model for Core PS115/2-2 is based on a multi-proxy approach (Fig. 2; Supplementary Tables 1 and 2). We have used magnetostratigraphic proxies, i.e., inclination changes and RPI variations of the geomagnetic field, combined with rock magnetic measurements to assess a possible diagenetic overprint on the paleomagnetic signal. As independent age proxies, extinction ages of 230Th and 231Pa excesses (230Th_ex and 231Pa_ex) and further 230Th_ex maxima31,40–42 as well as AMS14C ages have been determined.

The inclination record of Core PS115/2-2 is dominated by normal polarity (88% of the samples), interrupted by a series of prominent short intervals of reversed polarity (Fig. 2a). The RPI record of Core PS115/2-2 displays distinct cyclic variations with the prominent RPI minima coinciding with the geomagnetic excursions (Fig. 2a). When looking at the RPI and inclination records, however, it is obvious that in several instances RPI maxima coincide with the dark gray intervals (representing OC-rich, probably anoxic conditions; see below), and that the paleomagnetic reversals coincide mostly with beige-brownish (oxic) sediments (Fig. 2a).

Before using the inclination reversals and the RPI values as indicative for stratigraphy/chronology, diagenetic effects/overprint have to be excluded. Thus, to obtain information on the magnetic mineral composition and the degree of eventual oxidation, we carried out temperature-dependent measurements of magnetic remanence on selected samples characterized by both normal and reversed polarity and different OC-rich/OC-poor lithologies (Fig. 2a). The results and interpretation of these measurements are outlined in detail in the Supplementary Information No. 2 and Supplementary Figs. 5–7. They allow the conclusion that there is no systematic difference in the magnetic properties between normal and reverse samples, and therefore that there is no obvious effect on the magnetostratigraphy for core PS115-2-2. Thus, our inclination record can be considered a reliable record of the Earth’s magnetic field, reflecting intervals of normal and reverse polarities.

As an independent age control for supporting or rejecting the option of short-term geomagnetic excursions that have caused the negative inclinations, 230Th_ex and 231Pa_ex have been determined (see “Methods” for details). Both 230Th_ex (with a half-lifetime of 75,690 years) and 231Pa_ex (with a half-life time of 32,760 years) are particle-reactive radionuclides produced uniformly in the ocean by alpha decay of soluble 234U and 235U, respectively, and provide useful information for dating deep-sea sediments43. Assuming as a first approximation that 230Th_ex and 231Pa_ex remain detectable for four to five half-lives (around 300 kyr for 230Th_ex and 140 kyr for 231Pa_ex)31, this age limit is found at core depths of about 590 cm and 300 cm, respectively (Fig. 2). We use here an approach of using the last confirmed presence of water column-derived excess. This approach is complementary to the 230Th_ex and 231Pa_ex extinction age of Hillaire-Marcel et al. (ref. 31), but it relies on the last positive identification of 230Th_ex or 231Pa_ex rather than confirming its continuous absence, which might have other causes than decay only44,45 and is more complex26,42. The sequence of significantly positive 231Pa_ex measurements in a strictly oxic part of the sediment around 300 cm depth (Fig. 2a), marks this point as clearly post-MIS 6. Together with the distribution of 230Th_ex, these radiometric age constraints support our chronostratigraphic framework that the prominent negative inclinations at 573 and 293 cm represent the Portuguese Orphan and Blake geomagnetic excursions, respectively (Figs. 2 and 3).

Taking together the 230Th_ex and 231Pa_ex data in combination with the paleomagnetic records we use 290 ka (MIS 9a) and 120 ka (MIS 5) for the last confirmed presence of water-column derived 230Th_ex and 231Pa_ex, respectively (Supplementary Fig. 8). Intervals with elevated 230Th_ex values are correlated with interglacials (late MIS 9, MIS 7, MIS 5, MIS 3 and MIS 1). For MIS 5, MIS 3, and MIS 1, this correlation is also in line with the occurrence of planktic foraminifers (Fig. 2). The intercalated glacial (stadial) intervals, on the other hand, display minimum 230Th_ex values <0, which may be related to uranium mobility around oxygen-deficient parts of the sediment. Based on this 230Th_ex record, the MIS 9/8, MIS 8/7 and MIS 6/5 boundaries are at about 544 cm, 500 cm and about 325-284 cm core depth, respectively, and the upper 150 cm are of MIS 3 to MIS 1 age (Fig. 2 and Supplementary Fig. 8). This stratigraphic age/depth assignment is very similar to that determined in a 230Th_ex record of Core PS2757 recovered close-by from southern Lomonosov Ridge (Supplementary Fig. 8f; for core location see Fig. 1)26, further supporting our chronostratigraphic approach.

For the upper 180 cm, additional support for our age model comes from AMS14C measurements. Unfortunately, sufficient numbers of foraminifers for dating are only present in the upper five centimeters. Thus, only three AMS14C datings of planktic foraminifers are available that give ages between 0.2 and 3.2 ka (Fig. 3, Supplementary Table 1). These ages are calendar years before the present (for calculation see methods and Supplementary Table 1). Due to this lack of biogenic carbonate, we also used bulk acid-insoluble OC (AIOC) residues for AMS14C dating. The interpretation of such AMS14C dates, however, is more complicate as the samples may contain both fresh marine OC but also fresh and “old” (dead) reworked terrestrial OC46–48. Especially in Arctic Ocean sediments, the terrigenous OC fraction is mainly predominant10. Thus, the determined AIOC AMS14C ages may be several thousands of years older than the true age of deposition in the marine environment, i.e., the ages should represent the maximum ages of the marine sediments. This is in line with the AIOC-based AMS14C dates from Core PS115/2-2, indicating increasing ages with increasing depth, an offset of about 8 kyr between the AIOC and the foraminifera-based AMS14C dates at 3 cm core depth, and a maximum uncalibrated AIOC age of about 43 ka at 146 cm core depth (Fig. 3, Supplementary Table 1). Furthermore—and most important—these AIOC AMS14C dates strongly support that the prominent youngest negative inclinations at about 120–140 cm and 74–79 cm represent the Laschamp (41 ka) and Mono Lake (32 ka) geomagnetic excursions38 (Fig. 3).

Our RPI record is compared with the global RPI/isotope stack “PISO-1500” that is based on coupled RPI and oxygen isotope records and developed as stratigraphic template for correlating and dating sedimentary records for the last 1.5 Myr49 (for the last 450 kyr of the stack see Fig. 2b). Both RPI records show a quite similar pattern, and alignment of the records is possible. Based on this correlation, RPI tie points for the establishment of our age model have been identified (Figs. 2 and 3). For an alternative interpretation of the RPI record we refer to the Supplementary Information No. 3 and Supplementary Fig. 8.

Considering our independent proxy datasets we are quite confident that in our sediment core (study area), the negative magnetic inclinations represent short-term geomagnetic excursions within the normal Brunhes Chron and can be used as geochronological tie-points. We assigned the negative inclinations of Core PS115/2-2 to the Bermuda (409 ka), Portuguese Orphan (286 ka), Pringle Falls (212 ka), Iceland Basin (188 ka), Blake (114 ka), Laschamp (41 ka) and Mono Lake (32 ka) excursions (Fig. 2a)38. Neither the shallow inclinations at about 99–100 cm core depth nor the negative inclinations at about 550 and 594 cm below and above our Portuguese Orphan excursion, respectively, have been attributed to any excursion discussed in the literature. In conclusion, we are able to establish an accurate age model (“Age Model A”; Fig. 3 and Supplementary Fig. 8) that is based on seven geomagnetic excursion ages, 17 RPI tie-points, the two 230Th_ex and 231Pa_ex extinction ages, two 230Th_ex maxima as well as AMS14C dates for the uppermost part of the core (Figs. 2 and 3; Supplementary Tables 1 and 2). Plots of the magnetic inclination, RPI, and 230Th_ex records vs. age are shown in Supplementary Fig. 8. According to this age model, the sedimentary sequence of Core PS115/2-2 represents four glacial/interglacial cycles (MIS 11 to Modern), i.e., the last 430 kyr. During this time span, sedimentation rates vary between 0.5 and 5 cm/kyr (Fig. 3c), reflecting distinct changes in climate-controlled environmental conditions.

Glacial to interglacial variability in organic carbon sources and burial
Based on our age model and the comparison with the global benthic oxygen isotope stack50, the prominent dark gray intervals characterized by elevated TOC values of 1–2% correlate with cold climate stages (Fig. 4a, h; Supplementary Table 3). In detail, OC-6, OC-5, and OC-3 intervals coincide with full glacial intervals (MIS 10, MIS 8, and MIS 6, respectively) and the subsequent terminations/deglacials. OC-4, a minor event with a TOC content of about 1%, falls into the early phase of MIS 6 (or a colder phase in the late MIS 7 if “Age Model B” is applied; see Supplementary Fig. 8), and OC-2 and OC-1 fall into late MIS 5 (MIS 5b?) and MIS 4/3. The age of the oldest OC-7 interval probably falls into the uppermost MIS 12/MIS 11 transition. The thicknesses of the OC-rich intervals are quite different. Whereas OC-6, OC-5, OC-3, and OC-2 have thicknesses between 36 and 68 cm, OC-4 and OC-1 (and OC-7?) have thicknesses of only 10–20 cm (Supplementary Table 3). Furthermore, it is important to note that during MIS 2, i.e., the Last Glacial Maximum (LGM), the sediments do not show increased TOC contents (Fig. 4h).

The brownish-beige sediments representing the intercalated interglacials, on the other hand, are characterized by low TOC values of 0.2–0.5% (Fig. 4h), i.e., they are quite similar to those determined in surface sediments from the central Arctic Ocean representing the modern interglacial conditions10. In terms of absolute OC burial, the glacial OC accumulation rates reaching about 15–30 mg cm−2 kyr−1 are about three to six times higher than those determined for interglacial time intervals (<5 mg cm−2 kyr−1) (Fig. 4f).

The elevated TOC contents in all seven OC intervals are predominantly of terrigenous origin (TerrOC) as reflected in the high C/N ratios of 10–16 (or >20 when corrected for inorganic nitrogen; see “Methods”) (Fig. 4g; Supplementary Fig. 9), maxima in terrestrial biomarkers (campesterol, ß-sitosterol and long-chain n-alkanes; Supplementary Table 4) as well as the Rock-Eval pyrolysis data (Fig. 5). Furthermore, the Rock-Eval Tpeak (Tmax) data allow to classify the TerrOC in terms of its maturity stage (Fig. 5d; see “Methods” for further details). The upper about 40 cm section with Tmax values of 300–330°C is characterized by immature TerrOC, although a contribution of some more mature (“older”) OC is obvious from the bi-/polymodal signature of the pyrolysis temperature plots (Fig. 5e). Below 40 cm, two main OC types can be distinguished (Fig. 5): first, the brown-beige, OC-poor sediments that mainly contain highly degraded refractive/oxidized (“post-mature”) OC with very low HC yield generated during pyrolysis (Fig. 5c); no Tpeak value was reached before the end of pyrolysis at 650°C. Second, the dark gray OC-rich intervals with maximum HC yield generated during pyrolysis (Fig. 5c) that display a unimodal HC signature with distinct Tmax values of 435–440°C, indicating mature (i.e., ancient/petrogenic) TerrOC (Figs. 4i and 5e). The presence of ancient/petrogenic carbon is further supported by the presence of sand-sized black particles, identified as coal in SEM photographs and EDAX analysis (Supplementary Fig. 9). These black particles are predominant in the >63 µm fraction but of minor significance when looking at the total abundance as the >63 µm fraction content in all OC intervals is <0.1% (Fig. 6b). However, the continental-derived coal might have been ground by glacigenic processes and mainly occurring in the silt/clay fraction.

Although TerrOC is clearly predominant in the OC-rich intervals, low but significant concentrations of more labile biomarkers dinosterol, brassicasterol and short-chain n-alkanes (indicative for open-water phytoplankton productivity), and specific highly-branched isoprenoids (IP25 and HBI-II indicative for sea-ice algae productivity) are preserved in the glacial sediments as well (Fig. 4c, d and Supplementary Table 4). On the other hand, such labile algae OC is absent or only present in very minor concentrations in the beige-brown sediments representing interglacial conditions. This difference in OC composition points to fundamentally different depositional environments during glacial and interglacial times (see “Discussion” for further details).

Detrital sediment provenance, transport processes, and ice-sheet history
Remaining questions dealing with the input of detrital TerrOC-rich sediments are (i) from where this material did come from, (ii) what were the main transport pathways, and (iii) how these processes are related to ice-sheet dynamics. Due to the complex geology of the circum-Arctic Ocean hinterland specific mineral assemblages determined in Arctic marine sediment cores may allow for identifying source areas of the detrital sediments (Fig. 1)51–56. For example, elevated dolomite concentrations are indicative for a North Canadian source region as well as Svalbard, and high concentrations of amphiboles (in combination with high illite values) are indicative for Siberia and the adjacent East Siberian Sea (Fig. 1). Eurasian source areas are characterized by input of sediments with high concentrations of clinopyroxene, siderite, smectite and kaolinite (Fig. 1). Having this in mind and looking at the records of mineralogical tracers determined in Core PS115/2-2, it is obvious that the dark gray sediments of the OC-1 to OC-7 intervals display significant maxima in (clino)pyroxene, smectite, kaolinite and especially siderite (Fig. 6d–h), but obviously no dolomite (Supplementary Fig. 11a). The maxima in relative siderite concentration mirror the maxima in inorganic carbon content (Fig. 6e), suggesting that siderite is the predominant carbonate mineral (with only a few short-term events where calcite and dolomite, respectively, are the predominant carbonate minerals; Supplementary Fig. 11a). Most probable source areas of these sediments are Paleozoic and Mesozoic sedimentary rocks (clay-, mud and sandstones, carbonates, especially siderite) characterized by partly elevated contents of ancient/petrogenic OC, plant fragments and coals, and cropping out on Taimyr Peninsula and the West Siberian Basin (Supplementary Fig. 12)57–61.

The OC-1 to OC-7 intervals correlating with glacials/cold intervals, were formed during times when the area around Taimyr Peninsula, Severnaya Zemlya, and the Kara Sea was covered by the extended EIS (Figs. 1 and 7a). With pulses of ice-sheet advance and retreat during glacial to deglacial times, ice-sheet erosion of sub-glacial OC-rich sedimentary rocks and soils in the Taimyr-Severnaya Zemlya-Kara Sea area delivered sediments that were “pushed” onto the shelf and further across the shelf edge towards the Eurasian Basin, accompanied by subglacial outburst of meltwater and suspensions62. For the EIS advance during MIS 4/3, these glacigenic processes are clearly reflected in diamictons, debris flows, and terminal moraines mapped in Parasound profiles off Severnaya Zemlya and Taimyr, as well as sediment Core PS2782 recovered in the Schokalsky Channel (Supplementary Fig. 13)27,63. Parts of the sediments may have also been delivered onto the subaerial Laptev Shelf and transported fluvially towards the shelf edge and beyond. Further down-slope sediment transport occurred via turbidity currents, sediments’ rain-out from nepheloid layers and/or hyperpycnal flows, with coarse-grained material deposited more proximal whereas the fine-grained material may become available for a long-distance transport (Fig. 7)64,65. Such direct transport via turbidity currents is reflected in the sediments of the more coarse-grained turbidites in the more proximal Core PS2471 and the more fine-grained sediments in the distal Core PS115/2-2, both characterized by elevated TerrOC contents, high C/N ratios, and high kaolinite contents as well as minima in 230Th_ex concentrations that can be correlated perfectly between both cores (Supplementary Fig. 14). The internal sedimentary structures of the glacial OC-rich intervals as well as grain-size proxies such as the Zr/Rb ratio also point to current-controlled sedimentation at Core PS115/2-2 (Supplementary Fig. 15).

Methods
Basic shipboard and shore-based surveys and analyses
During Polarstern Expedition PS115/2, Core PS115/2-2-2 (81.2441°N, 123.4839°E; 3669m water depth) and Core PS115/2-2-1 (81.2441°N, 123.4799°E; 3665m water depth) representing near-surface sediments, were recovered at the same location by means of a Kastenlot (KAL) corer (764 cm recovery) and a box corer (46 cm recovery), respectively21. The KAL corer is a giant gravity corer with a rectangular cross section of 30×30 cm, and it has a penetration weight of 3.5 t and a core box segment size 30×30×575 cm (Supplementary Fig. 2). The length of the KAL corer used was 11.5 m (= two connected boxes) plus 30 cm for the core catcher. The great advantage of the KAL is a wall-thickness of the barrel of only 2 mm. Because of the great cross-sectional area (900 cm2) and the small thickness of the barrels, the quality of the cores was generally excellent, and recovery equals penetration depth.

After opening the kastenlot boxes, the sediment surface was cleaned and photographs were taken, followed by sampling the sediment with large-sized plastic boxes of 1 m in length and 10×10 cm cross section, taking sediment slabs for x-radiographs, and ship-board sampling of discrete samples (Supplementary Fig. 2b). The sedimentary sections stored in these boxes were then used for visual core description, further photographs, multi-sensor core logging, and XRF scanning. A total of five sets of sub-cores are available from one kastenlot core, to be used for archiving and any kind of further detailed shore-based studies, including one complete set of sub-core sections stored deep-frozen for later biomarker studies (Supplementary Fig. 2b).

As the uppermost 11 cm of the Kastenlot Core PS115/2-2-2 was lost during the handling on deck (Supplementary Fig. 3), the top of the studied sedimentary sequence is represented by the near-surface sediment section of Core PS115/2-2-1. Geochemical data (e.g., CaCO3 and TOC contents) and elemental ratios from XRF scanning (e.g., Mn/Al) produced shore-based in the AWI labs, allow a perfect correlation between both cores. Based on this correlation, the upper 34 cm of Core PS115/2-2-1 and the 34–760 cm section of Core PS115/2-2 were spliced together, resulting in the 760 cm thick complete and undisturbed composite section of Core PS115/2-2 (Supplementary Fig. 3).

For our study, a detailed continuous sampling of both cores PS115/2-2-1 and PS115/2-2 was carried out in the AWI sediment lab, using 10 ml syringes (PS115/2-2-1: 22 samples; PS115/2-2-2: 376 samples) and 6.2 cm3 plastic cubes (PS115/2-2-1: 19 samples; PS115/2-2-2: 323 samples) (Supplementary Fig. 2d). For the spliced composite sedimentary sequence of PS115/2-2, this results in totals of 381 and 328 samples, respectively. The syringes samples were freeze-dried, ground, and used for organic geochemical, radiogenic isotope, and XRD analyses, the plastic cubes were used for paleo- and rock magnetic measurements (see below for details). In addition, 144 large-sized samples of about 100 ml volume were taken every 5 cm, freeze-dried, weighed, and then sieved using a 63 µm sieve. The fraction >63 µm has been used for qualitative coarse-fraction analysis (e.g., determination of occurrences of foraminifers, coal fragments, detrital minerals, etc.).

XRF scanning
The AWI Avaatech XRF (X-Ray Fluorescence) Core Scanner was used for rapid and non-destructive determination of the chemical composition of sediment sections of cores PS115/2-2-1 and PS115/2-2. Split core surfaces were scanned during a 10 kV, 30 kV, and 50 kV run, in order to obtain reliable intensities (counts per second) of specific major elements by analysing the spectra with for Arctic sediments adapted WinAxial™ models. In this paper, we only used Al, Mn, Fe, Rb, and Zr data, plotted and interpreted as elemental ratios Mn/Al, Fe/Al, and Zr/Rb. A more extended dataset is available at https://doi.org/10.1594/PANGAEA.975853. For documentation and analytical details, we refer to the Avaatech Manual (https://epic.awi.de/id/eprint/37355/4/XRFCore-Scanner_user-manualV2.pdf).

AMS14C dating
AMS14C datings were carried-out with the Accelerator Mass Spectrometry MICADAS system at the Alfred Wegener Institute in Bremerhaven. Three samples of the planktic foraminifer (Neogloboquadrina pachyderma sinistral) were taken from the box corer PS115/2-2-1 (Supplementary Table 1a). Using the radiocarbon calibration program CALIB 8.20 and the Marine20 calibration curve with a global mean reservoir age (R = 550 years), and a local marine reservoir correction (delta R) value of 345±60 years, calendar ages were calculated. In addition, 17 samples of bulk AIOC residues were selected from upper 180 cm section for AMS14C dating due to the lack of biogenic carbonate (Supplementary Table 1b). As the exact composition of the AIOC (i.e., marine vs. terrestrial OC, fresh vs. reworked OC) is not known, reservoir corrections have to be taken with caution. Thus, uncalibrated 14C ages of the AIOC are shown in Fig. 3 and calibrated ages (to be used cautiously) are only included in Supplementary Table 1. For further details including references see Supplementary Table 1.

Magnetic properties and magnetostratigraphy
Paleo- and rock magnetic measurements were conducted at the paleomagnetic laboratory at the Faculty of Geosciences, University of Bremen. Paleomagnetic directions and intensities of NRM, anhysteretic remanent magnetization (ARM), generated in a peak alternating field of 100 mT and a biasing DC field of 50 μT, as well as isothermal remanent magnetization (IRM), imparted by applying 23 steps in DC fields from 0 to 700 mT using the internal pulse coil, were measured on an automated superconducting rock magnetometer (model 2G Enterprises 755HR)87. Low-temperature measurements were conducted at the University of Bremen using a Quantum Design MPMS XL-7. RPIs of the Earth’s magnetic field were calculated using the so-called “slope-method” or pseudo Thellier method88. RPI was computed as the slope of the regression line of NRM intensities plotted versus the intensities of ARM and IRM for AF demagnetization levels 20 to 40 mT and 20 to 70 mT, respectively. Both RPI curves, RPI_arm and RPI_irm, show almost similar variations (Supplementary Fig. 5g, h) and appear to be robust. The maximum angular deviation89 varies between 0.3 and 12.5° with a mean of 3.5° (Supplementary Fig. 5b), which, in the context of Quaternary marine sediments from the Arctic Ocean, supports the reliability of RPI data of Core PS115/2-2 for chronostratigraphic applications.

Radioisotope analyses (230Th, 231Pa, U isotopes)
The radioisotopes 230Th, 232Th, 231Pa, 234U, and 235U (238U) were analysed in several batches over more than 2 years, with a slight change in separation methods during this period. All samples were digested in a microwave-assisted pressurized digestion using a CEM Mars Xpress® system equipped with an evaporation accessory. About 75 mg of sediment was digested together with isotopic spikes 229Th, 233Pa, and 236U in a mixture of 0.5 mL HF suprapur®, 2 mL distilled concentrated HCl, and 3 mL distilled concentrated HNO3. Each batch comprised the certified reference materials Irish Sea 385 (for radioisotopes)90 and MESS-4 (for multielement analyses; provided by the National Research Council Canada), as well as procedural blanks.

Most samples were measured for Th and U isotopes using a previously published method47 using only TRU resin in a prepFAST system (Elemental Scientific). Later in the procedure, the method was expanded to include Pa. Differing from the previous method, 50 µL of a 50 g/L iron solution was added to the full digestion, precipitated as Fe(OH)3, reduced in volume and taken up in and taken up in 2 mL 10 M HCl after repeated evaporation steps. This solution was passed over a BioRad AG1 X8 2 mL column in the prepFAST system, eluting first Th with 9 M HCl, then Pa with 9 M HCL+0.13 M HF, then Fe with 5 M HNO3 (fraction discarded), then U with 0.01 M HCl (collected together with the Th fraction). Later, the Pa fraction was taken up again in 9 M HCl after repeated evaporation steps to eliminate HF. This solution was then passed another time over the prepFAST system equipped with a 2 mL BioRad AG1 X8 column to further separate Pa and Th, avoiding tailing and interferences from 232Th on the 231Pa and 233Pa isotopes. The combined Th+U fractions were passed over the prepFAST system with TRU resin like the previous batches, separating Th from U. All fractions were finally converted to 1 M HNO3 and analyzed on an Element 2 sector-field ICP-MS equipped with a Jet interface. U isotopes were analyzed using a cyclonic spray chamber, thorium isotopes and Pa isotopes using an Aridus II system for best sensitivities (typical sensitivity ca. 10–20 million cps/ppb U). U and Pa isotopes were analysed in low resolution (R=300), Th isotopes in a special intermediate resolution of R=2000, offering flat peaks for precise isotope ratios together with an improved abundance sensitivity compared to R=300. 238U was calculated from 235U in order to avoid the intense ion beam of 238U on the detector. For the certified reference material IAEA-385, we found concentrations of 1.810+−0.037 dpm/g 238U (n=31), 1.792+−0.051 dpm/g 230Th (n=32) and 0.1079+−0.0072 dpm/g 231Pa (n=18). The certified values of IAEA-385 are 1.74+−0.06 dpm/g for 238U and 1.89+−0.108 for 230Th (ref. 90).

230Th_ex was calculated as the activity of 230Th in excess over the activity of its progenitor 234U, in line with previous publications on U-series data in Arctic sediments31,41,91. We note that several sections of the core show 230Th_ex values below zero, and we also note signs of oxygen deficient conditions that led to the enrichment of authigenic uranium and possibly a secondary mobility of uranium in parts of the core as some oxygen is supplied from neighboring sections. 234U/238U values far in excess of modern open ocean values (activity ratio 1.14) are seen in some deep parts of the sediment core. Our age constraints therefore rely on parts of the record where excess 230Th and 231Pa are seen in the absence of uranium enrichments or secondary mobility.

Organic-geochemical bulk parameters
For the measurement of bulk parameters by means of elemental analysis and Rock-Eval pyrolysis, freeze-dried and homogenized sediments were used. Total organic carbon (TOC) contents were measured by Carbon-Sulfur Analyser (CS-125, Leco) after removing carbonate with hydrochloric acid. Total carbon (TC) and total nitrogen (TN) contents were determined by Carbon-Nitrogen-Sulfur Analyser (Elementar III, Vario). Inorganic (carbonate) carbon (IC) was calculated as IC = TC − TOC. The C/N ratios used as a first-order proxy for estimating the marine (C/N = 5–8) and terrestrial (C/N >> 10) proportions of the OC (see ref. 12 and references therein), were calculated as “TOC/TN” ratio. As the TN values in sediments from the Laptev Sea continental margin and adjacent deep sea as well as the sediments from Core PS115/2-2 may also contain significant amount of inorganic nitrogen, the C/N ratios of Core PS115/2-2 are certainly minimum values, i.e., when corrected for inorganic nitrogen the maxima of 14–16 would increase to 25 and more (Supplementary Fig. 9)10. Based on dry bulk density values and linear sedimentation rates, bulk sediment accumulation rates and, by considering TOC values, bulk TOC accumulation rates were calculated. For calculation procedure, we refer to https://doi.org/10.1594/PANGAEA.975805.

Rock-Eval pyrolysis was performed on bulk ground sediment samples using a Rock-Eval 6 analyzer92–95 to determine (1) the amount of hydrocarbons (HC) present and generated by pyrolytic degradation of the OC during heating up to 650°C, (2) the amount of carbon dioxide (CO2) generated from the decomposing OC during heating up to 390°C, and (3) the temperature of maximum pyrolysis yield (Tmax). The measured temperature of maximum (peak) generation of HCs during pyrolysis represents “Tpeak”, the corrected value after apparatus vs. standard calibration represents “Tmax” (Tmax = Tpeak – 36°; see Fig. 5). The HC and CO2 yields were normalized to OC and expressed as hydrogen index (HI) in mg HC/g OC and oxygen index (OI) in mg CO2/g OC, respectively. Plotted in a HI vs. OI (“van Krevelen-type”) diagram, this allows a classification of the OC types. In immature sediments, HI values of <100 mg HC/g OC and high OI values of >>100 mg CO2/g OC are generally typical of terrigenous and/or more refractive/oxidized organic matter (kerogen type III and IV), whereas HI values of 300–600 mg HC/g TOC are typical of marine OC (kerogen types I and II) (Fig. 5). Extremely high OI values for bulk sediments (>>300 mg CO2/g OC), i.e., values also measured in samples from Core PS115/2-2 (Fig. 5a), might be an artifact due to the cracking at low temperature of carbonate minerals (especially siderite) that interferes with the determination of OC-derived CO2 (cf., refs. 94, 95). As proxy for the OC maturity, i.e., to identify ancient (petrogenic) OC, the Tmax values were used (Fig. 5b–e). Tmax values <435°C are indicative for immature OC, whereas mature (ancient/petrogenic) OC has Tmax values between about 435 and 475°C. In combination with HI values, Tmax values may give further information about the composition as well as the maturity of the organic matter (cf., Fig. 5b).

Biomarker analyses
For biomarker analysis, 5g of freeze-dried and homogenized sediments were used for extraction by ultrasonication with dichloromethane/methanol (DCM/MeOH, 2:1 v/v). The internal standards 7-hexylnonadecane (7-HND, 0.076 μg), 9‐octylheptadec‐8‐ene (9‐OHD, 0.1 μg), 5α-androstan-3β-ol (androstanol, 10.8 μg), and squalane (3.2 μg) were added prior to the extraction. The extracts were concentrated and separated into hydrocarbon and sterol fraction by open silica gel column chromatography using 5 ml n-hexane and 9 ml ethyl acetate/n‐hexane, respectively. Furthermore, the sterol fraction was derivatized with 200 μl bis-trimethylsilyl-trifluoracet-amid (BSTFA) (60°C, 2h). Highly branched isoprenoids (HBIs) were analysed by gas chromatography/mass spectrometry (GC/MS) (Agilent 7890 GC-Agilent 5977A), whereas n-alkanes, pristane, and phytane were analyses by GC (Agilent 6890 GC). Sterols and C33 dialkylglycerol diether lipids (DGD, consisting of two C15 alkyl chains) were analyses by GC/MS (Agilent 6850 GC-Agilent 5975A). Compound identification was based on comparison of GC retention times with reference compounds and published mass spectra for HBIs, sterols, and DGDs. IP25, HBIII, and HBIIII(Z) were quantified based on their molecular ions m/z 350, m/z 348, and m/z 346, respectively, in relation to the fragment ion m/z 266 of the internal standard (7-HND). The ratio of the related response factors is nearly 1:1:1. Brassicasterol (24-methylcholesta-5,22E-dien-3β-ol), campesterol (24-methylcholest-5-en-3β-ol), sitosterol (24-ethylcholest-5-en-3β-ol), dinosterol (4a-23,24-trimethyl-5a-cholest-22E-en-3β-ol), and DGDs were quantified as trimethylsilyl ethers. The molecular ions m/z 470, m/z 472, m/z 486, and m/z 500 for the sterols, and the fragment ion m/z 130 for DGDs were used in relation to the molecular ion m/z 348 of the internal standard androstanol. External calibration curves and specific response factors were applied for balancing the different responses of molecular ions of the analytes and the molecular/fragment ions of the internal standards. For quantification of n-alkanes, pristane, and phytane, squalane was used as internal standard. The sum of short-chain n-alkanes was calculated using C17 and C19 compounds, whereas the sum of long-chain n-alkanes was calculated using C27, C29, and C31 n-alkanes. The carbon preference index (CPI) was calculated based on the concentrations of the long-chain n-alkanes C24 to C34, using the following equation:

CPI = 0.5 * ((C25+C27+C29+C31+C33) / (C24+C26+C28+C30+C32) + (C25+C27+C29+C31+C33) / (C26+C28+C30+C32+C34))

For analytical details and further references see refs. 96, 97 and Supplementary Fig. 9. All biomarker data are summarized in Supplementary Table 4.

XRD analyses of bulk sediments
Dried bulk samples were ground to a fine powder (<20 µm particle size) and prepared with the Philips back loading system. A standard deviation of ±5% can be considered as a general guideline for mineral groups with >20% clay fraction. In addition, the determination of well crystallized minerals like quartz, calcite, or aragonite can be done with better standard deviations (±1–3%)98.

The samples were measured on a Bruker D8 Discover diffractometer equipped with a Cu-tube (kα 1.541, 40 kV, 40 mA), a fixed divergence slit of ¼° 2θ, a 90 samples changer, a monochromatisation via energy discrimimation on the highest resolution Linxeye detector system. The measurements have been done as a continuous scan from 3 to 65° 2θ, with a calculated step size of 0.016° 2θ. Mineral identification has been done by means of the Philips software X’Pert High Score™ Version 1.2 (ref. 99), and identification of sheet silicates have been done with the freely available Apple MacIntosh X-ray diffraction interpretation software MacDiff 4.25 (ref. 100). This was followed by peak identification and determination of intensity ratios101. The relative contents of smectite and kaolinite as well as the carbonate minerals siderite, dolomite, calcite, and aragonite are expressed as ratio of the XRD single mineral peak intensity vs. sum of total analyzed intensities (see Fig. 6 and Supplementary Fig. 11). The entire list of minerals determined by XRD (including quartz, feldspars, heavy minerals etc.) that are used to calculate the sum of total analyzed intensities are available at https://doi.org/10.1594/PANGAEA.975865.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study of one deep sea sediment core from the Arctic Ocean off Siberia is a very nice study of sediment flux variability associated glacial/interglacial ice sheet forcing. The chronology is well developed and consistent among the different dating techniques. I think this is a very nice and well developed paper.", "reviewer_2": "Manuscript NCOMMS-24-59102\nTitle: A 430 kyr record of ice-sheet dynamics, sediment discharge, and organic-carbon burial in the central Eurasian Arctic\nOcean\nAuthors: Ruediger Stein, Thomas Frederichs, Kirsten Fahl, Walter Geibert, Jens Matthiessen, Frank Niessen, Christoph\nVogt, Cynthia Sassenroth, Evgenia Bazhenova\nThe manuscript reports about the interrelationship between ice-sheet dynamics and Organic Carbon (mainly of continental\norigin) burial in the central Eurasian Basin during the last 430 kyr.\nThe work is based on a very large (impressive), multi-proxy dataset collected on two complementary sediment records (1\nKastens core and 1 box core), and the data were compared with previous studies conducted on other cores in the area.\nThe multi-proxy approach allowed for a very good and solid stratigraphic reconstruction of the new studied cores and\ncorrelation with previously studied cores.\nThe work outlines the huge delivery of organic matter during glacial stages and glacial terminations and present strong\nevidence of possible anoxic conditions during deposition.\nI have two main concerns about the manuscript for which I ask a moderate revision:\n1) The method used for the AMS radiocarbon data calibration.\nThere are not information about the software that was used for the radiocarbon calibration (Calib software?). The global\nmean reservoir should be already considered by the software, whereas an additional local reservoir effect should be\nconsidered by a DeltaR +/- error, that is not mentioned by the authors. Also, why the authors considered to use the 2-sigma\nvalues whereas the most used are the 1-sigma? Alternatively, the authors may want to consider the use of the Median\nProbability given by the (Calib) calibration software. In any case, it would be better to report all the calibration information on\nTable 1 of the Supplementary (1- and 2-sigma and the Median Probability)\n2) The interpretation about the origin of the pink layers and, therefore, about the paleo ice sheet dynamic and the\npalaeoceanographic structure of the paleo ocean currents.\nHave the authors considered the work of Rasmussen and Thomsen from 2013 (Pink marine sediments reveal rapid ice melt\nand Arctic meltwater drain during Dansgaard–Oeschger warmings, Nature Comm.) which discusses the presence of pink\nlayers in the sedimentary sequence recovered from the north-western margin of Svalbard?\nIn Rasmussen and Thomsen (2013) the pink layers derive from erosion of the Devonian Red Beds composed mainly of\nconglomerates and sandstones that outcrop in the North-central area of Spitsbergen. The geology of Svalbard also indicates\nthe presence of carbonates (including dolomite) of Carboniferous age in the same area (Piepjohn, K. and Dallmann, W.K.,\n2014, Stratigraphy of the uppermost Old Red Sandstone of Svalbard\n(Mimerdalen Subgroup), Polar Research 2014, 33, 19998; Manby, G. and Lyberis, N., 1992, Tectonic evolution of the\nDevonian Basin of northern Svalbard. Norsk Geologisk Tidsskrift, Vol. 72, pp. 7-19, JSSN 0029-196X).\nPerhaps the authors may consider and discuss an alternative origin of the pink layers recovered in the studied record, which\nconsiders a more local origin: deposition of fine-grained sediments suspended in meltwater plumes released into\nWoodfjorden and Widgefjorden (north of Spitsbergen) during the retreat of the paleo-ice streams that eroded the Devonian\nRed Stones and Carboniferous carbonates (including dolomite) outcropping in the nearby inland.\nMETHODS\nRadiocarbon calibration (Table 1 of supplementary), which software was used for the calibration? Calib? The global mean\nreservoir should be already considered by the software, whereas an additional local reservoir effect should be considered by\na DeltaR +/- error.\nWhy the authors considered the 2-sigma values when the most common is to consider the 1-sigma? Alternatively, the\nauthors may want to consider the Median Probability given by the calibration software (Calib)\nXRD analysis, the method describes the use of the entire sample (bulk XRD analysis), however the percentages of the clay\nmineral kaolinite is used for the discussion of the provenance of the sediment. Did you extract the kaolinite content from the\nbulk sediment analysis, or did you also perform analysis on the clay fraction alone? If the latter, please describe the sample\npreparation, and report also the information about the other clay minerals.\nSUPPLEMENTARY\nTables 1 to 4 looks like a snapshoot of an xls spreadsheet and they are not easy to read, please make a proper table.\nTable 2 Suggested some changes in the caption\nFIGURES\nFigure 1, The table reported in figure 1 is too small\nFigure 2, The core location in figure 2a is difficult to see, please change colour of the marker\nFigure 3, Nice hand-made core logs, however the details are difficult to see (also possibly too many). Please digitalize the\ncore logs\nFigure 10, Legend difficult to read\nFigure 14, A core log legend and a simple map are missing\nAdditional comments are reported in the attached document.\nBest regards, Renata G Lucchi", "reviewer_3": "The authors found that terrOC deposition increased during glacial (stadial) periods in the deep Eurasian Basin. These terrOC was transported by ice sheet advance (\"pushed\") across the shelf edge, then by mass-wasting processes to the\ndown slope area. In addition, these OC is glacially eroded from subglacial sedimentary rocks and soils in the Taimyr-\nSevernaya Zemlya-Kara Sea area. Glacial OC burial rates are 3-6 times higher than interglacial times and the anoxic\ncondition improved preservation of more labile algae-type OC. Therefore, they argued that Arctic deep-sea sediments were\nan important OC (and CO2) sink during glacial periods. The proposal is interesting and the records show clear patterns;\nincreased values in the dark layers and decreased values in the brown layers for all records. However, there are some\nissues should be solved.\n1. chronology\nChronology is one of hot issues in the Arctic Ocean; lithostratigraphic chronology vs magnetic property based chronology\n(e.g., RPI and/or magnetic reversals). 5 pages of 16 pages of main text are about justification and how the age model was\nset. As a result, more attention is likely to be drawn to the chronological part rather than the key findings of this study. Why\nthe proportion of chronological part is significant in this manuscript is related to the current age constraining issue. In this\nsense, I cautiously suggest that this manuscript might be more suitable for more specific journals rather than Nature Comm.\nwhich deals with broad interests rather than local/regional ones.\n2. glacial erosion of subglacial sedimentary rocks and soils in the Taimyr-Severnaya Zemlya-Kara Sea areas?\nHigh C/N ratios and high Tmax support that OC is reworked and have terrestrial characteristics. Organic-geochemical and\nmineralogical proxies showed significant maxima in (Clino)pyroxene, smectite, kaolinite and siderite in high OC dark\nintervals originated from the above region. Subglacial sedimentary rocks and soils in that area are the source for high OC-\nrich dark layer. Sedimentary rocks mean (semi)consolidated form? After glacial erosion, chips or fragments of mudstone are\nexpected to be found rather than mud form. However, very fine grain size are characteristics for the dark layers.\nAncient (patriotic) OC has lower Tmax than relatively recently deposed brown layer sediments' OC? Is there any evidence\nfor ancient OC? Such as microfossils, or some specific minerals found in the far past? In the Ross Sea, continental slope\nsediment study showed increased biogenic opal, TOC, and C/N (Ha et al., 2022) and a deep-basin sediment study also\nshowed increased diatoms (reworked) in laminated mud during glacial periods (Bollen et al., 2022). They also interpreted\nthat they are transported by turbidity current as ice sheet advanced across the continental shelf edge. But, Ha et al. (2022)\nsuggested that the sediments deposited in the slope during glacial periods were originally deposited on the shelf during\nformer interglacials.\nAccording to the authors, the dark layer sediments are a result of contour and turbidity currents (upward fining supports this).\nThis means, they are laterally transported and likely sedimentation rates are high. However, marine and sea ice algae\nimmediately increased as if they were already in the sediment. The strong correlation between TOC and all other proxies\nsuggests that they were transported together rather than due to better preservation under anoxic conditions.\nIf authors want to argue that increases in marine and sea ice algae are due to surface biological activity under glacial\npolynas conditions, how much OC is derived from these glacial surface biological productions compared to terrOC should be\ncalculated. This is important because authors also argued that the Arctic Ocean played a role as a sink for CO2.\n3. OC-3,-5,and -6 covers full glaciation to deglaciation phases. However, there is no difference between glacial part and\ndeglacial part in the records although more subglacial melt water is likely expected during deglacial (retreating phase). How\nabout outburst of meltwater and suspension changes according to ice sheet phases should be considered.\n4. Although authors argued 3-6 times more OC burial occurred in the Arctic Ocean during glacial periods, 0.015-\n0.03g/cm2/kyr is not outstandingly high, but is relatively low compared to other regions. Thus, the increased OC burial during\nglacial periods are difficult to be thought as a major significance in the Earth's climate system.\nMore importantly during glacial periods continental margin deep sea areas receives huge amount of reworked sediment\ninput from continental shelf regions as a result of sea level drop. In the Antarctic Ocean, Ross Sea and Amundsen Sea\n(+West Antarctic Peninsula) also showed ice sheet dynamics associated OC burial during glacial periods (high OC, C/N in\ndark laminated mud with higher sedimentation rates than this study). Thus, the clear peaks of OC accompanying with distinct\nother proxies in the Arctic Ocean during glacial periods may not be a significant phenomenon in a global point of view.\nIn summary, the clear cyclicity of lithology and OC, C/N, and all other biomarkers is interesting. However, why OC burial in\nthe Arctic Ocean is important is not well convinced. In addition, its impact seems to be far more exaggerated, particularly\nglobal sink for OC and CO2.\nBollen et al. 2022. Pleistocene oceanographic variability in the Ross Sea: A multiproxy approach to age model development\nand paleoenvironmental analyses. Global and Planetary Changes 216.\nHa et al. 2022. Glaciomarine sediment deposition on the continental slope and rise of the central Ross Sea since the Last\nGlacial Maximum. Marine Geology 445.\nHillenbrand et al., 2021. New insights from multi-proxy data from the West Antarctic continental rise: implications for dating\nand interpreting Late Quaternary palaeoenvironmental records. 257."}, "author_response": "Bremen, 20.12.2024\nProf. Dr. Ruediger Stein\nFaculty of Geosciences\nUniversity of Bremen\nKlagenfurter Str. 2-4\nD-28359 Bremen, Germany\nSubj.: Nature Communications manuscript NCOMMS-24-59102A\n\" A 430 kyr record of ice-sheet dynamics, sediment discharge, and organic-carbon burial\nin the central Eurasian Arctic Ocean \"\nPoint-by-point response to the referees' comments\nFollowing the numerous suggestions/comments/corrections given by the reviewers as well as the editor, we\nhave carried-out a substantial revision of the manuscript. Furthermore, the Supplementary Information has\nbeen re-structured and divided into more logical “subchapters” (Supplementary Information No.1 to No.7).\nAll our changes/corrections done in the manuscript text are highlighted by yellow background color and\nexplained in the point-by-point response below.\n(1) Comments of Reviewer #1 (in italics) and our/Authors’ response\nReviewer 1: “This study of one deep sea sediment core from the Arctic Ocean off Siberia is a very nice\nstudy of sediment flux variability associated glacial/interglacial ice sheet forcing. The chronology is well\ndeveloped and consistent among the different dating techniques. I think this is a very nice and well-\ndeveloped paper.”\nAuthors: Thanks for this very positive statement.\n(2) Comments of Reviewer #2 (in italics) and our/Authors’ response\nReviewer 2: “The work is based on a very large (impressive), multi-proxy dataset ….., and the data were\ncompared with previous studies conducted on other cores in the area. The multi-proxy approach allowed\nfor a very good and solid stratigraphic reconstruction of the new studied cores and correlation with\npreviously studied cores. The work outlines the huge delivery of organic matter during glacial stages and\nglacial terminations and present strong evidence of possible anoxic conditions during deposition.\nI have two main concerns about the manuscript for which I ask a moderate revision ….”\nAuthors: Thank you very much for this very positive general statement highlighting the multi-proxy key\napproach with its main results – despite two main concerns we will comment on in the following.\nReviewer 2: Concern (1) “The method used for the AMS radiocarbon data calibration. ….. no\ninformation about the software that was used for the radiocarbon calibration (Calib software?). … local\nreservoir effect should be considered by a DeltaR +/- error, that is not mentioned by the authors. …. why\nthe authors considered to use the 2-sigma values whereas the most used are the 1-sigma? Alternatively,\nthe authors may want to consider the use of the Median Probability given by the (Calib) calibration\nsoftware. In any case, it would be better to report all the calibration information on Table 1 of the\nSupplementary (1- and 2-sigma and the Median Probability) ….”\nAuthors: We are very grateful for this important comment, and we have to apologize that we did not give\nthis information in the first version of the manuscript. Following Reviewer 2, all the missing information\nabout the method used for the AMS radiocarbon data calibration, has been outlined under “Methods” (line\n548-556) and, especially, in the legend of Supplementary Table 1 (i.e., Calib 8.2 software, Marine 20\ncalibration, local reservoir correction, 1- and 2-sigma, Median Probability etc. as well as related\nreferences).\nReviewer 2: Concern (2) “The interpretation about the origin of the pink layers. …..\n….. Rasmussen and Thomsen (2013) …. discuss the presence of pink layers in the sedimentary sequence\nrecovered from the north-western margin of Svalbard……Geology of Svalbard also indicates the …..\npresence of carbonates (including dolomite) of Carboniferous age in the same area (Piepjohn and\nDallmann 2014; Manby and Lyberis 1992) ….. authors may consider and discuss an alternative origin of\nthe pink layers ….. which considers a more local origin: deposition of fine-grained sediments suspended\nin meltwater plumes …. during the retreat of the paleo-ice streams that eroded the Devonian Red Stones\nand Carboniferous carbonates (including dolomite) outcropping in the nearby inland.”\nAuthors: Thank you for this comment related to the origin of the pink layers. Reviewer 2 is right that\n(Carboniferous) carbonate rocks (including dolomites) are also cropping out on Northern Svalbard (we\nrefer to Vogt et al. 2001 and have added Manby & Lyberis 1992 as suggested by Reviewer 2). We also\nagree that glacial erosion on Svalbard and input of reworked carbonates (dolomites) across the Svalbard\nshelf edge, followed by lateral transport of fine fraction within the intermediate (Atlantic) water mass\nalong the northern Svalbard continental margin, is a possible process for sediment transport from Svalbard\ntowards the east (as described and discussed in Vogt et al., 2001 and also Rasmussen & Thomsen 2013).\nHowever, this interpretation is not realistic for our situation. Here, we are referring to a single but very\nprominent “pink-layer event” characterized by very coarse-grained dolomite-rich sediments, i.e., a\nsediment fraction for which a long-distance transport in suspension can be excluded. Instead, discharge as\nice-rafted debris via icebergs with a North Canadian source is the only realistic explanation and also\nsupported by literature data (for details and references see revised manuscript line 466-486).\nReviewer 2: XRD analysis, the method describes the use of the entire sample (bulk XRD analysis),\nhowever, the percentages of the clay mineral kaolinite is used for the discussion of the provenance of the\nsediment. Did you extract the kaolinite content from the bulk sediment analysis, or did you also perform\nanalysis on the clay fraction alone?\nAuthors: Thank you for this comment. We have to apologize that we did not outline our XRD approach\nand XRD records shown in relevant figures clearly enough. In our study, XRD analysis was carried out\nusing bulk ground sediments. Thus, relative mineral contents are expressed as ratio of the XRD single\nmineral peak intensity vs. sum of total analyzed intensities (see Fig. 6 and Supplementary Figs. 14b and\n15). In Supplementary Fig. 14, relative kaolinite contents determined in bulk sediments from Core\nPS115/2-2 (Supplementary Fig. 14b) are compared with published kaolinite contents determined in clay\nfraction samples from near-by Core PS2471-4 (Supplementary Fig. 14a). Thus, the latter kaolinite\ncontents are expressed as percentage values (see “Methods” line 704-709 and Supplementary Fig. 14 for\ndetails and references).\nReviewer 2: Supplementary tables 1 to 4 looks like a snapshoot of an xls spreadsheet and they are not\neasy to read, please make a proper table.\nAuthors: Supplementary figures are shown in a more proper way. Furthermore, the tables will also be\nsubmitted separately as excel tables/spreadsheets.\nReviewer 2: Comments to Supplementary figures\nFigure 1, The table reported in figure 1 is too small\nAuthors: Figure has been deleted.\nFigure 2, The core location in figure 2a is difficult to see, please change color of the marker\nAuthors: Size and color of core locations have been changed (new Suppl. Fig. 1a).\nFigure 3, Nice hand-made core logs, however the details are difficult to see (also possibly too many).\nPlease digitalize the core logs\nAuthors: Core logs have been digitalized.\nFigure 10, Legend difficult to read\nAuthors: Legend in the figure has been changed to make more readable (new Suppl. Fig. 12).\nFigure 14, A core log legend and a simple map are missing\nAuthors: Core log legend has been added (new Suppl. Fig. 16); for core locations we refer to Fig. 1.\nReviewer 2 marked additional smaller comments in the pdf attached to the review that we have\nconsidered/corrected in the revised version of the manuscript:\nLines 303-306: Please rephrase between lines 305-307 to make more clear that the dark-gray intervals are\nthe second type of OC-rich intervals indicated in line 302\nAuthors: Done - Line 296-300\nLine 302: coal to continental-derived coal\nAuthors: Done - Line 306\nLine 355: correct “end morains”\nAuthors: Done (changed to “terminal moraines” - Line 347\nLine 410: … measured on vertical transects CTD? Give details\nAuthors: Done – “…using a CTD equipped with a fluoro- and turbidimeter and an underwater camera\nsystem…” (Line 405-406)\nLine 412: ….triggered by Barents Sea Bottom Water, may expand and penetrate into deeper waters as\nwell as into the sediment changed to “ …. – please correct.\nAuthors: Done - Line 408-409\n(3) Comments of Reviewer #3 (in italics) and our/Authors’ response\nReviewer 3: “The authors found that terrOC deposition increased during glacial (stadial) periods in the\ndeep Eurasian Basin. These terrOC was transported by ice sheet advance (\"pushed\") across the shelf\nedge, then by mass-wasting processes to the down slope area. In addition, these OC is glacially eroded\nfrom subglacial sedimentary rocks and soils in the Taimyr-Severnaya Zemlya-Kara Sea area. Glacial OC\nburial rates are 3-6 times higher than interglacial times and the anoxic condition improved preservation\nof more labile algae-type OC. Therefore, they argued that Arctic deep-sea sediments were an important\nOC (and CO2) sink during glacial periods. The proposal is interesting and the records show clear patterns;\nincreased values in the dark layers and decreased values in the brown layers for all records. However,\nthere are some issues should be solved.”\nAuthors: Thank you very much for this positive general statement – despite several concerns we will\ncomment on in the following.\nReviewer 3: Chronology is one of hot issues in the Arctic Ocean; lithostratigraphic chronology vs. magnetic property-based chronology (e.g., RPI and/or magnetic reversals). 5 pages of 16 pages of main\ntext are about justification and how the age model was set. As a result, more attention is likely to be\ndrawn to the chronological part rather than the key findings of this study. Why the proportion of\nchronological part is significant in this manuscript is related to the current age constraining issue. In this\nsense, I cautiously suggest that this manuscript might be more suitable for more specific journals rather\nthan Nature Comm. which deals with broad interests rather than local/regional ones.\nAuthors: We totally agree with Reviewer 3 that “Chronology is one of hot issues in the Arctic Ocean” or,\nas described in our introduction (line 48-51) “A key problem in this debate about the history and\nsignificance of the Arctic cryosphere within the climate system, that has not been fully resolved yet, is the\naccurate dating of the Quaternary sediment sequences, a major challenge for the entire Arctic research\ncommunity for several decades.” Core PS115/2-2 represents an undisturbed complete sedimentary\nsequence representing the last about 500 kyr, i.e., the last four glacial interglacial cycles (based on our age\nmodel). Such a well-dated 500 ka record was not available from this part of the central Arctic Ocean\nbefore our study. It allows for the first time detailed reconstructions of the history (timing) and dynamics\nof the Eurasian ice sheet history as well as the related OC burial history, i.e., a topic that is of overall\ninterest also for a broad scientific community dealing with climate reconstructions in general. However,\nfor this type of detailed climate reconstructions that would allow correlations with other Arctic, mid-/low-\nlatitude and/or global climate records, a robust and data-supported age model is a fundamental\nprerequisite. Thus, a major part of our manuscript is devoted to this important topic. We realized that this\nsubchapter became quite long and - based on the reviewer’s comment - we shorten the text and shifted\nparts as well as one figure into the supplement. However, we do not agree with the Reviewer’s opinion\nthat because of focusing that much on the age model our study is not of broad interest (i.e., not suitable for\na Nature Communications-type journal) but only is of local/regional interest. In contrast, we are quite\nconfident that - because of the well-established and convincing age model - Core PS115/2-2 might\nbecome a key core also for future studies on reconstructing paleo-climate conditions that is of major\ninterest for the entire Arctic scientific community and beyond.\nReviewer 3: Glacial erosion of subglacial sedimentary rocks and soils in the Taimyr-Severnaya\nZemlya-Kara Sea areas. ….. Subglacial sedimentary rocks and soils in that area are the source for high\nOC-rich dark layer. Sedimentary rocks mean (semi)consolidated form? After glacial erosion, chips or\nfragments of mudstone are expected to be found rather than mud form. However, very fine grain size are\ncharacteristics for the dark layers.\nAuthors: As outlined in the text (line 376-383), we proposed that “During times of an extended EIS and\nlowered sea-level, huge amounts of OC-rich soils, sediments and rocks have been eroded and transported\nby glacigenic processes onto the shelf and across the shelf edge, followed by down-slope by turbidity\ncurrents and/or lateral current-induced transport of the fine-grained suspension. That means, the fine-\ngrained OC-rich proportion of the sediments may have fed the eastward flowing contour currents along\nthe Kara-Laptev-Sea continental margin transporting the suspension towards Site PS115/2-2 where the\nfinal OC burial occurred”. Processes of glacial erosion certainly resulted in very different grain-size\nfractions from very coarse-grained to very fine-grained particles (including chips and fragments of\nmudstone but also mud-/clay-sized material as typical for diamicton deposits). However, the coarser-\ngrained particles accumulated more proximal whereas only the fine-grained mud was transported over\nlong distances (e.g., in contour currents) to our core location (see also text line 353-362 and\nSupplementary Fig. 14).\nReviewer 3: Ancient (patriotic) OC has lower Tmax than relatively recently deposed brown layer\nsediments' OC?\nAuthors: Thanks for this comment related to the Rock Eval data, especially the Tmax values and the\ninterpretation. The Tmax values shown in the original Figure 6 were misleading, and we have replotted\nthe Rock Eval data in the new/revised Figure 5. For the beige-brown OC-poor sediments, extremely low\nhydrocarbon yields (S2 peak) were measured (Fig. 5c) and no Tpeak value was reached before the end of\nthe pyrolysis at 650°C and thus no Tmax could be calculated (Fig. 5d and 5e). On the other hand, for the\ndark gray OC-rich intervals, high hydrocarbon yields (clear and prominent S2 peaks) were measured (Fig.\n5c and 5e) and Tmax values of 435-440°C determined (indicating mature OC). Thus, the OC deposited in\nthe brownish intervals are interpreted as highly degraded and/or oxidized (see Fig. 5).\nReviewer 3: Is there any evidence for ancient OC? Such as microfossils, or some specific minerals found\nin the far past? In the Ross Sea, continental slope sediment study showed increased biogenic opal, TOC,\nand C/N (Ha et al., 2022) and a deep-basin sediment study also showed increased diatoms (reworked)\nin laminated mud during glacial periods (Bollen et al., 2022). They also interpreted that they are\ntransported by turbidity current as ice sheet advanced across the continental shelf edge. But, Ha et al.\n(2022) suggested that the sediments deposited in the slope during glacial periods were originally\ndeposited on the shelf during former interglacials.\nAuthors: Unfortunately, we could not find any microfossils to be used for age determination. Our\nargument for ancient OC is coming (1) from the Rock Eval data (Tmax 435-440°C) and the occurrence of\ncoal fragments and (2) the contents of specific minerals (siderite, kaolinite, smectite, pyroxene etc.) – all\ndata supporting that most probable source areas of these sediments are Paleozoic and Mesozoic to\nCenozoic sedimentary rocks (clay-, mud and sandstones and carbonates/siderite) characterized by partly\nelevated contents of ancient/petrogenic OC, plant fragments and coals, and cropping out on Taimyr\nPeninsula and the West Siberian Basin (see line 335-345). Processes of ice-sheet dynamics, glacial\nerosion, and sediment transport/deposition described for the Ross Sea continental margin (Bollen et al.\n2022; Ha et al., 2022) have been considered/discussed in the revised manuscript (Line 399-403).\nReviewer 3: According to the authors, the dark layer sediments are a result of contour and turbidity\ncurrents (upward fining supports this). This means, they are laterally transported and likely sedimentation\nrates are high. However, marine and sea ice algae immediately increased as if they were already in the\nsediment. The strong correlation between TOC and all other proxies suggests that they were transported\ntogether rather than due to better preservation under anoxic conditions.\nAuthors: Thanks for this comment. Yes, we also agree that the terrestrial as well as the marine/sea ice\nalgae OC were mainly transported by the same process (e.g., long-distance transport by turbidity and/or\ncontour currents), as all the different OC proxies are strongly correlated. However, the original source of\nthe terrestrial and algae marine/sea ice algae OC must be different (i.e., ancient terrestrial OC and - due to\nthe biomarker proxies – more recent/Quaternary marine/sea ice algae OC). The presence of marine/sea ice\nalgae OC point to open-water and ice-edge conditions. As even today, i.e., during interglacial conditions,\nprimary productivity is very low and almost no marine OC is preserved in the interglacial sediments of the\ncentral Arctic Ocean, the chance to produce sediment with significant amounts of labile marine/sea-ice\nalgae OC, is even worse during glacial times. That means, because increased primary productivity in the\nglacial central Arctic Ocean characterized by more extended sea-ice conditions is less probable, it seems\nto be more realistic that the elevated algae-type (marine) OC concentrations are related to lateral transport\noriginated from the marginal ice zones (e.g., polynyas) that are known to occur in front of the ice sheet\nduring glacial times. However, under normal oxic conditions, the labile marine/sea-ice algae OC cannot be\npreserved in the sedimentary record. Preservation of this type of labile OC is only possible under anoxic\nsedimentary conditions at the core location, probably caused by sediment-laden hyperpycnal meltwater\ndischarge and increased TerrOC burial (see manuscript line 426-430). This interpretation is exactly what\nour biomarker and 234U records tell us (Fig. 4c-4e and Supplementary Fig. 9c-9f; manuscript line 446-\n458).\nFurthermore, the presence of the algae-type OC at Core PS115/2-2 is most important for the overall ice-\nsheet reconstruction, a finding we have highlighted in the revised manuscript as one of the key results of\nour study. Its presence clearly demonstrates that even during strong glacial intervals there must have been\nat least occasionally open-water (polynya-type) conditions along the Eurasian continental margin with\nmarine and sea-ice algae productivity in front of the ice sheet, indicating that not all parts of the Arctic\nOcean were permanently covered by ice shelves throughout major glaciations during the last 450 kyr (line\n458-464).\nReviewer 3: If authors want to argue that increases in marine and sea ice algae are due to surface\nbiological activity under glacial polynas conditions, how much OC is derived from these glacial surface\nbiological productions compared to terrOC should be calculated. This is important because authors also\nargued that the Arctic Ocean played a role as a sink for CO2.\nAuthors: To give more exact estimates of the terrestrial and marine/sea ice algae OC is difficult (e.g.,\nStein and Macdonald 2004). Based on the Rock Eval data and kerogen microscopy data determined in OC\nof Cenozoic Arctic Ocean sediments, only rough first-order estimates are possible (e.g., Boucsein et al.,\n2002; Stein et al., 2006). From these data, only <10% of the OC is derived from marine/sea ice algae (Fig.\n5 and legend line 1098-1101).\nReviewer 3: OC-3,-5,and -6 covers full glaciation to deglaciation phases. However, there is no difference\nbetween glacial part and deglacial part in the records although more subglacial melt water is likely\nexpected during deglacial (retreating phase). How about outburst of meltwater and suspension changes\naccording to ice sheet phases should be considered.\nAuthors: We are in line with this comment. That means, pulses of suspension-loaded meltwater and\nsediment may have occurred during the EIS glacial advance and/or deglaciation. Sediments delivered by\nturbidity currents or density plumes through fluvial and glacial processes, might have been deflected and\ndeposited under the influence of contour currents (line 399-402).\nReviewer 3: Although authors argued 3-6 times more OC burial occurred in the Arctic Ocean during\nglacial periods, 0.015-0.03g/cm2/kyr is not outstandingly high, but is relatively low compared to other\nregions. Thus, the increased OC burial during glacial periods are difficult to be thought as a major\nsignificance in the Earth's climate system. More importantly during glacial periods continental margin\ndeep sea areas receives huge amount of reworked sediment input from continental shelf regions as a result\nof sea level drop. In the Antarctic Ocean, Ross Sea and Amundsen Sea (+West Antarctic Peninsula) also\nshowed ice sheet dynamics associated OC burial during glacial periods (high OC, C/N in dark laminated\nmud with higher sedimentation rates than this study). Thus, the clear peaks of OC accompanying with distinct\nother proxies in the Arctic Ocean during glacial periods may not be a significant phenomenon in a global\npoint of view.\nAuthors: Thanks for this important comment. We agree that although the glacial OC burial rates at our\ncore site (i.e., the central Arctic Ocean) are 3-6 times higher than those determined for interglacial time\nintervals, these increased burial rates are still low in comparison to data from other continental margin\nareas. Nevertheless, thick turbidite deposits were deposited in glacial sediments of the central Arctic\nOcean basins that are significant for budget calculations (although this is quite challenging due to missing\nage control (see line 411-422). Because most of the OC that accumulated in the Arctic deep-sea basins, is\npredominantly terrestrial (and ancient) OC, we weaken our too strong statement about the significance of\nthe increased OC burial in central Arctic Ocean deep-sea sediments for the global OC and CO2 budget\n(and by this for the Earth’s climate system). This aspect, however, was not the main focus of our paper.\nReviewer 3: Suggestions for additional references.\nBollen et al. 2022. Pleistocene oceanographic variability in the Ross Sea: A multiproxy approach to\nage model development and paleoenvironmental analyses. Global and Planetary Changes 216.\nHa et al. 2022. Glaciomarine sediment deposition on the continental slope and rise of the central Ross Sea\nsince the Last Glacial Maximum. Marine Geology 445.\nHillenbrand et al., 2021. New insights from multi-proxy data from the West Antarctic continental rise:\nimplications for dating and interpreting Late Quaternary palaeoenvironmental records. 257.\nAuthors: Thanks for suggestions for further references. We have referred to Bollen et al. (2022) and Ha et\nal. (2022) in our discussion (line 396-402)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "Manuscript NCOMMS-24-59102\nTitle: A 430 kyr record of ice-sheet dynamics, sediment discharge, and organic-carbon burial in the central Eurasian Arctic\nOcean\nAuthors: Ruediger Stein, Thomas Frederichs, Kirsten Fahl, Walter Geibert, Jens Matthiessen, Frank Niessen, Christoph\nVogt, Cynthia Sassenroth, Evgenia Bazhenova\nThe manuscript reports about the interrelationship between ice-sheet dynamics and Organic Carbon (mainly of continental\norigin) burial in the central Eurasian Basin during the last 430 kyr.\nI have already reviewed this manuscript at the first round, and I can confirm this is a very interesting (thought impressive)\nwork that identified key sites for stratigraphic, paleocenographic and paleoclimatic reconstructions in the central Arctic but\nalso dealt with OC burial at high-latitudes in deep-sea sediments during glacial stages / glacial terminations having a strong\nimpact on the global OC and CO2 budget in Earth’s climate system. If I may, I would like to suggest a couple of articles in\nsupport of their discussion: Lucchi et al., 2013 that reports about high concentration of OC buried in glacigenic diamicton on\nthe western margin of Svalbard (Arctic) and Lucchi et al., 2007 that defined the concept of “glacial contourites” for barren, not\nbioturbated conturitic sedimentation deposited during glacial stages on the Pacific Margin of the Antarctic Peninsula (West\nAntarctica) as consequence of suppressed biologic activity induced by oxygen reduced/anoxic conditions during glacial\nperiods.\nThe authors made a meticulous revision of the manuscript taking in consideration all the suggestion and concerns\nhighlighted by the former reviewers. I made some additional, very minor, suggestions to the reviewed manuscript that are\nreported in the attached pdf file.\nI consider the manuscript ready for publishing\nBest regards\nRenata G. Lucchi", "reviewer_4": "My role in this review is to comment upon authors responses/actions to previous reviewer #3 comments on the original\nmanuscript, after this manuscripts´ revision. This is done on attached file.\n\n======================================================================\nATTACHED REVIEWER REPORT — IMPORTANT\nThe text below is a complete reviewer report submitted as a PDF\nattachment. It appears after the Creative Commons license block.\nAssign it to whichever reviewer wrote 'see pdf' or had no inline text.\n======================================================================\n\nComment on the paper \"A 430 kyr record of ice-sheet dynamics, sediment discharge, and\norganic-carbon burial in the central Eurasian Arctic Ocean\" (Nature Communications\nmanuscript NCOMMS-24-59102A)\nI have been asked to give my thoughts on the authors' responses to reviewer #3's comments\n(reviewer #3 unavailable for the task) on the above mentioned paper by Stein et al., submitted to\nNature Communications, and was also welcomed to comment on the authors' responses to reviewer\n#2's comments and comment on the paper in the general as well, if I would like.\nI have read the paper and my overall impression is that it is a very well written such, with a wealth of\ndata giving insights into the glacial history for the European Ice Sheet in its eastern extreme over the\npast 430 kyr, as deduced from just one sediment core, just outside of the shelf break from the Kara\nSea to the Arctic Basin. I am especially delighted with the author´s concern to link the marine data to\nthe terrestrial field data (as being a “land” geologist” myself), which is so often NOT done in marine-\nbased works. And finding such a nice fit with that data (Taimyr Peninsula) at least for MIS 2 to MIS 6.\nReading through reviewer #3 comments to the original manuscript (which I have not read), and how\nthe authors have responded to those comments, I give my comments to authors responses to those\ncomments below.\nReviewer 3: Chronology is one of hot issues in the Arctic Ocean; lithostratigraphic chronology vs. \nmagnetic property-based chronology (e.g., RPI and/or magnetic reversals). 5 pages of 16 pages of\nmain text are about justification and how the age model was set. As a result, more attention is likely to be\ndrawn to the chronological part rather than the key findings of this study. Why the proportion of\nchronological part is significant in this manuscript is related to the current age constraining issue. In\nthis sense, I cautiously suggest that this manuscript might be more suitable for more specific journals\nrather than Nature Comm. which deals with broad interests rather than local/regional ones.\nMy comment: First, I would like to not agree with reviewer #3 on “I cautiously suggest that this\nmanuscript might be more suitable for more specific journals rather than Nature Comm.”. I find the\npaper highly suited for a broader public, as it is “one-of-a-kind” marine core that they have retrieved.\nI think authors response to the reviewer says this well.\nReviewer 3: Glacial erosion of subglacial sedimentary rocks and soils in the Taimyr-Severnaya\nZemlya-Kara Sea areas. ….. Subglacial sedimentary rocks and soils in that area are the source for high\nOC-rich dark layer. Sedimentary rocks mean (semi)consolidated form? After glacial erosion, chips or\nfragments of mudstone are expected to be found rather than mud form. However, very fine grain\nsizes are characteristics for the dark layers.\nMy comment: Find the authors give a solid explanation to reviewers´ concern.\nReviewer 3: Ancient (patriotic) OC has lower Tmax than relatively recently deposed brown layer\nsediments' OC?\nMy comment: Authors have replotted misleading values in figure and solved the question.\nReviewer 3: Is there any evidence for ancient OC? Such as microfossils, or some specific minerals\nfound in the far past? In the Ross Sea, continental slope sediment study showed increased biogenic\nopal, TOC, and C/N (Ha et al., 2022) and a deep-basin sediment study also showed increased diatoms\n(reworked) in laminated mud during glacial periods (Bollen et al., 2022). They also interpreted that\nthey are transported by turbidity current as ice sheet advanced across the continental shelf edge. But,\nHa et al. (2022) suggested that the sediments deposited in the slope during glacial periods were\noriginally deposited on the shelf during former interglacials.\nMy comment: The authors have expanded on this question in the revised manuscript in a\nsatisfactorily way.\nReviewer 3: According to the authors, the dark layer sediments are a result of contour and turbidity\ncurrents (upward fining supports this). This means, they are laterally transported and likely\nsedimentation rates are high. However, marine and sea ice algae immediately increased as if they\nwere already in the sediment. The strong correlation between TOC and all other proxies suggests that\nthey were transported together rather than due to better preservation under anoxic conditions.\nMy comment: The authors give a long explanation why they do not agree with the reviewers\nconcerns about the occurrence of marine algae abundance. I find that the authors’ rebuttal on this\nissue is quite convincing. They have highlighted the issue in the revised manuscript.\nReviewer 3: If authors want to argue that increases in marine and sea ice algae are due to surface\nbiological activity under glacial polynyas conditions, how much OC is derived from these glacial\nsurface biological productions compared to terrOC should be calculated. This is important because\nauthors also argued that the Arctic Ocean played a role as a sink for CO2.\nMy comment: Authors conclude that such %-age division between the two sources is difficult to\ncalculate, but explain that their studies suggest less than 10% of the OC is derived from marine/sea\nice algae, which now is shown.\nReviewer 3: OC-3,-5, and -6 covers full glaciation to deglaciation phases. However, there is no\ndifference between glacial part and deglacial part in the records although more subglacial melt water\nis likely expected during deglacial (retreating phase). How about outburst of meltwater and\nsuspension changes according to ice sheet phases should be considered.\nMy comment: The authors have added text on this issue in revised manuscript.\nReviewer 3: Although authors argued 3-6 times more OC burial occurred in the Arctic Ocean during\nglacial periods, 0.015-0.03g/cm2/kyr is not outstandingly high, but is relatively low compared to\nother regions. Thus, the increased OC burials during glacial periods are difficult to be thought as a\nmajor significance in the Earth's climate system. More importantly during glacial periods continental\nmargin deep sea areas receives huge amount of reworked sediment input from continental shelf\nregions as a result of sea level drop. In the Antarctic Ocean, Ross Sea and Amundsen Sea (+West\nAntarctic Peninsula) also showed ice sheet dynamics associated OC burial during glacial periods (high\nOC, C/N in dark laminated mud with higher sedimentation rates than this study). Thus, the clear\npeaks of OC accompanying with distinct other proxies in the Arctic Ocean during glacial periods may\nnot be a significant phenomenon in a global point of view.\nMy comment: The authors has addressed this issue by weakening their before to strong statements\non the significance in the revised manuscript, which I think is OK.\nIn addition, I have found a few minor errors in the manuscript:\nReference #8; publication date should be 2019 (not 2029)\nSupplementary Fig. 2. A mismatch between lettering of figure frames and caption text. Text for frame\n(c) in caption text refers to frame (b) in figure. Text for frame (b) in caption text refers to frame (c) in\nfigure.\nI find the authors responses/actions to reviewer #2 comments totally adequate.\nMy overall evaluation is to recommend publication of this very nice paper."}, "author_response": "Bremen, 05.03.2025\nProf. Dr. Ruediger Stein\nFaculty of Geosciences\nUniversity of Bremen\nKlagenfurter Str. 2-4\nD-28359 Bremen, Germany\nSubj.: Nature Communications manuscript NCOMMS-24-59102A\nPoint-by-point response to the referees' comments\nFollowing the suggestions/comments given by reviewers 2 and 4, we have carried-out a final revision of the\nlast version of the manuscript. All our changes/corrections done in the manuscript text are highlighted by\nyellow background color.\n(1) Comments of Reviewer #2 (in italics) and our/Authors’ response\nReviewer 2: “I have already reviewed this manuscript at the first round, and I can confirm this is a very\ninteresting (thought impressive) work that identified key sites for stratigraphic, paleocenographic and\npaleoclimatic reconstructions in the central Arctic but also dealt with OC burial at high-latitudes in deep-\nsea sediments during glacial stages/glacial terminations having a strong impact on the global OC and\nCO2 budget in Earth’s climate system. If I may, I would like to suggest a couple of articles in support of\ntheir discussion: Lucchi et al., 2013 that reports about high concentration of OC buried in glacigenic\ndiamicton on the western margin of Svalbard (Arctic) and Lucchi et al., 2007 that defined the concept of\n‘glacial contourites’……”\nAuthors: We have included the references Lucchi et al. (2013) and Lucchi & Rebesco (2007) in our\ndiscussion.\nReviewer 2: “……I made some additional, very minor, suggestions to the reviewed manuscript that are\nreported in the attached pdf file.”\nAuthors: We have considered all the (very minor) suggestions/corrections made by Reviewer 2 in lines 99-\n100, 133, 188, 247, 388, 393, 396, 419 and 426.\nReviewer 2: “……The authors made a meticulous revision of the manuscript taking in consideration all the\nsuggestion……… I consider the manuscript ready for publishing.“\nAuthors: Thank you very much for this very positive final statement.\n(2) Comments of Reviewer #4 (in italics) and our/Authors’ response\nReviewer 4: “I have been asked to give thoughts on the authors' responses to reviewer #3's comments\n(reviewer #3 unavailable for the task), and was also welcomed to comment on the authors' responses to\nreviewer #2's comments, and to comment on the paper in the general as well, if I would like”.\nAuthors: Reviewer 4 went through Reviewer#3’s comments and our responses very carefully, and was\nfully in line with all our responses. Furthermore, Reviewer 4 also found our responses/actions to reviewer\n#2’s comments totally adequate. Thank you very much for these very positive statements! Thus, we have\nnothing to add here.\nReviewer 4: “My overall evaluation is to recommend publication of this very nice paper.”\nAuthors: Thank you very much for this final statement!"}]

Decision letter