Description: The impact of the ongoing anthropogenic warming on the Arctic Ocean sea ice is ascertained and closely monitored. However, its long-term fate remains an open question as its natural variability on centennial to millennial timescales is not well documented. Here, we use marine sedimentary records to reconstruct Arctic sea-ice fluctuations. Cores collected along the Lomonosov Ridge that extends across the Arctic Ocean from northern Greenland to the Laptev Sea were radiocarbon dated and analyzed for their micropaleontological and palynological contents, both bearing information on the past sea-ice cover. Results demonstrate that multiyear pack ice remained a robust feature of the western and central Lomonosov Ridge and that perennial sea ice remained present throughout the present interglacial, even during the climate optimum of the middle Holocene that globally peaked ∼6,500 y ago. In contradistinction, the southeastern Lomonosov Ridge area experienced seasonally sea-ice-free conditions, at least, sporadically, until about 4,000 y ago. They were marked by relatively high phytoplanktonic productivity and organic carbon fluxes at the seafloor resulting in low biogenic carbonate preservation. These results point to contrasted west–east surface ocean conditions in the Arctic Ocean, not unlike those of the Arctic dipole linked to the recent loss of Arctic sea ice. Hence, our data suggest that seasonally ice-free conditions in the southeastern Arctic Ocean with a dominant Arctic dipolar pattern, may be a recurrent feature under “warm world” climate.

Description: To reconstruct the climatic and paleoceanographic variability offshore Northeast Greenland during the last ~10 ka with multidecadal resolution, sediment core PS93/025 from the outermost North-East Greenland continental shelf (80.5°N) was studied by a variety of micropaleontological, sedimentological and isotopic methods. High foraminiferal fluxes, together with high proportions of ice-rafted debris and high Ca/Fe ratios, indicate a maximum in bioproductivity until ~8 ka related to a low sea-ice coverage. Sortable silt values, planktic foraminifer associations, and stable isotope data of planktic and benthic foraminifers suggest a strong westward advection of relatively warm Atlantic Water by the Return Atlantic Current during this time, with a noticeable bottom current activity. This advection may have been facilitated by a greater water depth at our site, resulting from postglacial isostatic depression. For the following mid-Holocene interval (ca. 8–5 ka), isotope data, lower foraminiferal fluxes and a shift in grain size maxima point to a lasting but successively decreasing Atlantic Water inflow, a weakening productivity, and a growing sea-ice coverage which is also revealed by the P III IP 25 index. A final stage in the environmental development was reached at ~5 ka with the establishment of pre-industrial conditions. Low Ca/Fe ratios, low foraminiferal fluxes, low sortable silt values and the sea-ice indicating P III IP 25 index point to a limited productivity and a weak Atlantic Water inflow by the Return Atlantic Current to our research area, as well as a higher and/or seasonally more extended sea-ice coverage during the Late Holocene. Two intervals with somewhat enhanced Atlantic Water advection around 2.0 and 1.0 ka are indicated by slightly increased foraminiferal fluxes and the reoccurrence of subpolar foraminifers. These intervals may correlate with the Roman Warm Period and the Medieval Climate Anomaly, as defined in the North Atlantic region.

Description: Highlights • Highly variable sea-ice conditions off eastern North Greenland during the Holocene. • The mid to late Holocene is characterized by seasonal to marginal sea-ice conditions. • Seasonal formation of the Northeast-Water (NEW) Polynya during the last 1 ka. Understanding the processes controlling the natural variability of sea ice in the Arctic, one of the most dynamic components of the climate system, can help to constrain the effects of future climate change in this highly sensitive area. For the first time, a high-resolution biomarker study was carried out to reconstruct past sea-ice variability off eastern North Greenland. This area is strongly influenced by cold surface waters and drift ice transported via the East Greenland Current, meltwater pulses from the outlet glaciers of the Northeast Greenland Ice Stream and the build-up of landfast ice. The well-dated Holocene sedimentary section of Core PS93/025 provides insights into variations of the sea-ice conditions (regional and local sea-ice signal), oceanic and atmospheric circulation and the biotic response to these changes. These biomarker records show a reduced to variable sea-ice cover during the early Holocene between 10.2 and 9.3 ka, followed by a steady increase in sea-ice conditions during the mid Holocene. During the last 5–6 ka, sea-ice conditions remained more stable representing a seasonal to marginal sea-ice situation. Based on our biomarker records, stable sea-ice edge conditions, with a fully developed polynya situation occurred since the last 1 ka.

Description: Highlights • Neogloboquadrina pachyderma morphotypes can have different isotope compositions. • Isotopic offsets between morphotypes depend on environmental background conditions. • Thin-shelled Neogloboquadrina pachyderma are good recorders of near-surface salinity. • A strong freshwater event off NE Greenland at 12.7 ka may be related to deglaciation. Abstract We report on stable oxygen and carbon isotope data obtained on two different morphotypes of polar planktic foraminifers, i.e., fully encrusted and minor encrusted Neogloboquadrina pachyderma, from a sediment core taken on the NE Greenland continental margin. These morphotypes are supposed to live at different water depths of the upper water column in the area which today is strongly stratified, with a low-saline, cold-water layer at the surface. The paired isotopic data sets inform on temporal variations of past water salinity and temperature in the preferred water depth ranges of the investigated morphotypes and allow conclusions on the stratification of the upper water column. The radiocarbon-dated sediment core covers the time interval from 21 to 4 cal-ka, but the early part of the deglacial interval (18.5–12.7 cal-ka) is not represented, probably due to intense erosion by bottom currents. In sediments from the late last glacial maximum, oxygen isotope differences between thin-shelled and thick-shelled N. pachyderma are low and point at a weaker stratification with less freshwater than today near the surface. The carbon isotopes indicate a dense, perennial sea ice cover, very limited bioproduction, and the presence of a subsurface Atlantic Water layer. In the late deglaciation until ∼10.3 cal-ka, the stable isotope values of both analyzed morphotypes are considerably lower, with significantly stronger amplitudes in the record of thin-shelled specimens than later on. The high-amplitude record stems from a laminated sediment sequence whose older part was deposited within just a few decades. The data are evidence of a strong freshwater event in the research area that probably started before 12.7 ka and may have reduced sea surface salinities by 4–5 practical salinity units. As freshwater sources we discuss both the disintegration of NE Greenland shelf ice and export from the Arctic Ocean interior. The event may have contributed to the weakening of the Atlantic meridional overturning circulation during the Younger Dryas cold event. For the early and mid-Holocene (10-4 cal-ka), the isotope data suggest a structure of the upper water column similar to today, with a well-developed halocline separating low-saline near-surface waters from the underlying Atlantic Water layer. A seasonally disintegrated sea ice cover allowed for a considerable planktic bioproduction.

Description: We present a global atlas of downcore foraminiferal oxygen and carbon isotope ratios available at https://doi.org/10.1594/PANGAEA.936747 (Mulitza et al., 2021a). The database contains 2106 published and previously unpublished stable isotope downcore records with 361 949 stable isotope values of various planktic and benthic species of Foraminifera from 1265 sediment cores. Age constraints are provided by 6153 uncalibrated radiocarbon ages from 598 (47 %) of the cores. Each stable isotope and radiocarbon series is provided in a separate netCDF file containing fundamental metadata as attributes. The data set can be managed and explored with the free software tool PaleoDataView. The atlas will provide important data for paleoceanographic analyses and compilations, site surveys, or for teaching marine stratigraphy. The database can be updated with new records as they are generated, providing a live ongoing resource into the future.

Description: The Nordic Seas are the primary location where the warm waters of the North Atlantic Current densify to form North Atlantic Deep Water, which plays a key part in the modern Atlantic Meridional Overturning Circulation. The formation of dense water in the Nordic Seas and Arctic Ocean and resulting ocean circulation changes were probably driven by and contributed to the regional and global climate of the last glacial maximum (LGM). Here we map the source and degree of mixing of deep water in the Nordic Seas and through the Arctic Gateway (Yermak Plateau) over the past 35 thousand years using neodymium isotopes (εNd) measured on authigenic phases in deep-sea sediments with a high spatial and temporal resolution. We find that a large-scale reorganization of deep-water formation in the Nordic Seas took place between the LGM (23–18 thousand years ago) and the rapid climate shift that accompanied the subsequent deglaciation (18–10 thousand years ago). We show that homogeneous εNd signatures across a wide range of sites support LGM deep-water formation in the Nordic Seas. In contrast, during the deglaciation, disparate and spatially variable εNd values are observed leading to the conclusion that deep-water formation may have been reduced during this time.

Description: Changes in ocean circulation are considered a major driver of centennial‐to‐millennial scale climate variability during the last deglaciation. Using four sediment records from the Nordic Seas, we studied radiocarbon ventilation ages in subsurface and bottom waters to reconstruct past variations in watermass overturning. Planktic foraminiferal ages show significant spatial variability over most of the studied period. These differences suggest that the ventilation of the shallower subsurface waters is strongly influenced by local conditions such as sea‐ice and meltwater input, changes in mixed‐layer depth, and/or variable contributions of water masses with different 14C signatures. Despite covering a significant water depth range, the benthic foraminiferal records show common long‐term patterns, with generally weaker ventilation during stadials and stronger during interstadials. The Greenland Sea record differs the most from the other records, which can be explained by the greater depth and the geographical distance of this site. The benthic records reflect regional shifts in deep convection and suggest that the deep Nordic Seas have been generally bathed by a single, though changing, deep‐water mass analogous to the present‐day Greenland Sea Deep Water. Since significant offsets in ventilation ages are yielded by different taxonomic or ecological groups of benthic foraminifera, the use of uniform material seems a prerequisite to reconstruct bottom water ventilation histories.

Description: Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures. Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.