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Abstract. We report the results of amino acid racemization (AAR) analyses of aspartic acid (Asp)and glutamic acid (Glu) in the planktic Neogloboquadrina pachyderma, and the benthic Cibicidoides wuellerstorfi, foraminifera species collected from sediment cores from the Arctic Ocean. The cores were retrieved at various deep-sea sites of the Arctic, which cover a large geographical area from the Greenland and Iceland seas (GIS) to the Alpha and Lomonosov ridges in the central Arctic Ocean. Age models for the investigated sediments were developed by multiple dating and correlation techniques, including oxygen isotope stratigraphy, magnetostratigraphy, biostratigraphy, lithostratigraphy, and cyclostratigraphy. The extent of racemization (D/L values) was determined on 95 samples (1028 subsamples) and shows a progressive increase downcore for both foraminifera species. Differences in the rates of racemization between the species were established by analysing specimens of both species from the same stratigraphic levels (n=21). Aspartic acid (Asp) and glutamic acid (Glu) racemize on average 16 ± 2 % and 23 ± 3 % faster, respectively, in C. wuellerstorfi than in N. pachyderma. The D/L values increase with sample age in nearly all cases, with a trend that follows a simple power function. Scatter around least-squares regression fits are larger for samples from the central Arctic Ocean than for those from the Nordic Seas. Calibrating the rate of racemization in C. wuellerstorfi using independently dated samples from the Greenland and Iceland seas for the past 400 ka enables estimation of sample ages from the central Arctic Ocean, where bottom water temperatures are presently relatively similar. The resulting ages are older than expected when considering the existing age models for the central Arctic Ocean cores. These results confirm that the differences are not due to taxonomic effects on AAR and further warrant a critical evaluation of existing Arctic Ocean age models. A better understanding of temperature histories at the investigated sites, and other environmental factors that may influence racemization rates in central Arctic Ocean sediments, is also needed.

 

The oxygen isotopic composition of benthic foraminifera (d18Ob) is widely used to date and correlate marine sediment sequences. However, d18Ob has found comparatively little use in the Arctic Ocean due both to uncertainty in Arctic marine sediment chronology and the lack of resemblance between Arctic and open ocean d18Ob records. We address this issue by combining Arctic d18Ob records (Cronin et al., 2019) with benthic ostracode Mg/Ca-BWT reconstructions (Cronin et al., 2017) to create a composite record of the history of seawater d18O in the intermediate-to-deep Arctic Ocean over the last 600 kyr. Seawater d18O and its uncertainty was calculated using PSU Solver (Thirumalai et al., 2016). 

Abstract. The oxygen isotopic composition of benthic foraminiferal tests (δ18Ob) is one of the pre-eminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ18Ob applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ18Ob variability compared to open-ocean records. Here we evaluate the controls on Arctic δ18Ob by using ostracode Mg/Ca paleothermometry to generate a composite record of the δ18O of seawater (δ18Osw) from 12 sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) that covers the last 600 kyr based on biostratigraphy and orbitally tuned age models. Results show that Arctic δ18Ob was generally higher than open-ocean δ18Ob during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial–interglacial Arctic δ18Ob range resulted in part from the opposing effect of temperature, with intermediate to deep Arctic warming during glacials counteracting the whole-ocean δ18Osw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ18Ob, we find that the intermediate to deep Arctic experienced large (≥1 ‰) variations in local δ18Osw, with generally higher local δ18Osw during interglacials and lower δ18Osw during glacials. Both the magnitude and timing of low local δ18Osw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ18Osw in the intermediate to deep Arctic Ocean during glaciations reflected weaker upper-ocean stratification and more efficient transport of low-δ18Osw Arctic surface waters to depth by mixing and/or brine rejection. 

Abstract. The northern sector of the Greenland Ice Sheet is considered to beparticularly susceptible to ice mass loss arising from increased glacierdischarge in the coming decades. However, the past extent and dynamics ofoutlet glaciers in this region, and hence their vulnerability to climatechange, are poorly documented. In the summer of 2019, the Swedish icebreakerOden entered the previously unchartered waters of Sherard Osborn Fjord, whereRyder Glacier drains approximately 2 % of Greenland's ice sheet into theLincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier andits ice tongue by combining radiocarbon dating with sedimentary faciesanalyses along a 45 km transect of marine sediment cores collected betweenthe modern ice tongue margin and the mouth of the fjord. The resultsillustrate that Ryder Glacier retreated from a grounded position at thefjord mouth during the Early Holocene (> 10.7±0.4 ka cal BP) and receded more than 120 km to the end of Sherard Osborn Fjord by theMiddle Holocene (6.3±0.3 ka cal BP), likely becoming completelyland-based. A re-advance of Ryder Glacier occurred in the Late Holocene,becoming marine-based around 3.9±0.4 ka cal BP. An ice tongue,similar in extent to its current position was established in the LateHolocene (between 3.6±0.4 and 2.9±0.4 ka cal BP) andextended to its maximum historical position near the fjord mouth around 0.9±0.3 ka cal BP. Laminated, clast-poor sediments were deposited duringthe entire retreat and regrowth phases, suggesting the persistence of an icetongue that only collapsed when the glacier retreated behind a prominenttopographic high at the landward end of the fjord. Sherard Osborn Fjordnarrows inland, is constrained by steep-sided cliffs, contains a number ofbathymetric pinning points that also shield the modern ice tongue andgrounding zone from warm Atlantic waters, and has a shallowing inlandsub-ice topography. These features are conducive to glacier stability andcan explain the persistence of Ryder's ice tongue while the glacier remainedmarine-based. However, the physiography of the fjord did not halt thedramatic retreat of Ryder Glacier under the relatively mild changes inclimate forcing during the Holocene. Presently, Ryder Glacier is groundedmore than 40 km seaward of its inferred position during the Middle Holocene,highlighting the potential for substantial retreat in response to ongoingclimate change. 

Abstract. Amino acid racemization (AAR) geochronology is a powerful tool for datingQuaternary marine sediments across the globe, yet its application to ArcticOcean sediments has been limited. Anomalous rates of AAR in foraminiferafrom the central Arctic were reported in previously published studies,indicating that either the rate of racemization is higher in this area, orinaccurate age models were used to constrain the sediment ages. This studyinvestigates racemization rates in foraminifera from three well-datedsediment cores taken from the Yermak Plateau during the 2015 TRANSSIZ (TRansitions in the Arctic Seasonal Sea Ice Zone) expedition on RV Polarstern. D and L isomers of the amino acids asparticacid (Asp) and glutamic acid (Glu) were separated in samples of theplanktic foraminifer Neogloboquadrina pachyderma and the benthic species Cassidulina neoteretis to quantify the extent ofracemization. In total, 241 subsamples were analysed, extending back tomarine oxygen isotope stage (MIS) 7. Two previously published powerfunctions, which relate the extent of racemization of Asp and Glu inforaminifera to sample age are revisited, and a comparison is made betweenthe ages predicted by these calibrated age equations and independentgeochronological constraints available for the cores. Our analyses reveal anexcellent match between ages predicted by a global compilation ofracemization rates for N. pachyderma and confirm that a proposed Arctic-specificcalibration curve is not applicable at the Yermak Plateau. These resultsgenerally support the rates of AAR determined for other cold bottom watersites and further highlight the anomalous nature of the purportedly highrate of racemization indicated by previous analyses of central Arcticsediments. 

Marine Isotope Stage 11 from ~424 to 374 ka experienced peak interglacial warmth and highest global sea level ~410–400 ka. MIS 11 has received extensive study on the causes of its long duration and warmer than Holocene climate, which is anomalous in the last half million years. However, a major geographic gap in MIS 11 proxy records exists in the Arctic Ocean where fragmentary evidence exists for a seasonally sea ice‐free summers and high sea‐surface temperatures (SST; ~8–10 °C near the Mendeleev Ridge). We investigated MIS 11 in the western and central Arctic Ocean using 12 piston cores and several shorter cores using proxies for surface productivity (microfossil density), bottom water temperature (magnesium/calcium ratios), the proportion of Arctic Ocean Deep Water versus Arctic Intermediate Water (key ostracode species), sea ice (epipelagic sea ice dwelling ostracode abundance), and SST (planktic foraminifers). We produced a new benthic foraminiferal δ¹⁸O curve, which signifies changes in global ice volume, Arctic Ocean bottom temperature, and perhaps local oceanographic changes. Results indicate that peak warmth occurred in the Amerasian Basin during the middle of MIS 11 roughly from 410 to 400 ka. SST were as high as 8–10 °C for peak interglacial warmth, and sea ice was absent in summers. Evidence also exists for abrupt suborbital events punctuating the MIS 12‐MIS 11‐MIS 10 interval. These fluctuations in productivity, bottom water temperature, and deep and intermediate water masses (Arctic Ocean Deep Water and Arctic Intermediate Water) may represent Heinrich‐like events possibly involving extensive ice shelves extending off Laurentide and Fennoscandian Ice Sheets bordering the Arctic.