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: A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.

Description: Highlights • Sea-ice cover limits the accumulation of both coccoliths and alkenones in sediments. • Calcite dissolution in shelf sediments may explain the accumulation of alkenones in the presence of few or no coccoliths. • Non-calcifying haptophytes most likely produce alkenones in nearshore environments. Abstract We determined the abundances and concentrations of coccoliths and alkenones in 66 surface sediment samples from the northwest North Atlantic to evaluate the role that surface ocean temperature, salinity, sea-ice cover, and productivity have on the regional distribution of these two biological remains produced by haptophytes in the photic zone. In areas with sea-ice cover of more than 1 month per year, coccolith and alkenone concentrations in sediments are extremely low to nil. Elsewhere, the distribution of coccolith taxa generally displays strong relationships to water temperature, salinity, and productivity. Coccolithus pelagicus is associated with low summer sea-surface temperatures (〈8°C) and relatively high summer sea-surface salinities (〉33.5), whereas Helicosphaera carteri seems to follow the path of the North Atlantic Drift. The distribution of Emiliania huxleyi, the dominant alkenone producer, is not strongly correlated with that of alkenones. Calcite dissolution in shelf sediments could explain the occurrence of alkenones in the absence of coccoliths but alkenone production by non-calcifying haptophytes seems to also exert some control on alkenone concentrations in surface sediments, thus blurring alkenone abundance links to coccolithophorid production and their relative preservation.

Description: The geological record shows that abrupt changes in the Earth system can occur on timescales short enough to challenge the capacity of human societies to adapt to environmental pressures. In many cases, abrupt changes arise from slow changes in one component of the Earth system that eventually pass a critical threshold, or tipping point, after which impacts cascade through coupled climate–ecological–social systems. The chance of detecting abrupt changes and tipping points increases with the length of observations. The geological record provides the only long-term information we have on the conditions and processes that can drive physical, ecological and social systems into new states or organizational structures that may be irreversible within human time frames. Here, we use well-documented abrupt changes of the past 30 kyr to illustrate how their impacts cascade through the Earth system. We review useful indicators of upcoming abrupt changes, or early warning signals, and provide a perspective on the contributions of palaeoclimate science to the understanding of abrupt changes in the Earth system.

Description: Highlights • Porewater calcite dissolution may have occurred during the deglacial interval. • There is a significant decoupling of coccolith and alkenone concentrations in core 004. • Non-calcifying haptophytes most likely produced the alkenones in the glacial interval. Abstract The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008–029-004 PC (61.46°N and 58.04°W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate microfossils (9.5 ± 3.9 × 105 coccoliths g−1 and 2,860 ± 580 planktonic foraminifers g−1) than sediments from the deglacial and postglacial intervals (up to 3.1 × 108 coccoliths g−1 and 2.9 × 104 foraminifers g−1). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bølling-Allerød of the late glacial interval. Samples from the Bølling-Allerød and the deglacial interval also display a slightly elevated B/P index (〉0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bølling-Allerød and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bølling-Allerød and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected UK37-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical UK37-based temperature calibrations.

Description: For the interpretation of proxy reconstructions it is necessary to understand the recent signals of different sea-ice proxies from various regions. We present 260 new sediment surface samples collected in the (sub-) Arctic Oceans that were analysed for specific sea-ice (IP~25~) and open-water phytoplankton biomarkers (brassicasterol, dinosterol, HBI III). This new biomarker dataset was combined with 615 previously published biomarker surface samples into a pan-Arctic database. This comprehensive dataset is compared to satellite-based sea-ice concentrations and other environmental parameters to contribute to a better understanding of biomarker distributions as a basis for sea-ice reconstructions on a (sub-) Arctic scale.

Description: Core PS2757-6 was retrieved with a multicorer (MUC) on the Lomonosov Ridge next to the Russian continental shelf during the Polarstern expedition ARK-XI/1 in 1995. This expedition had the principal aim to study material fluxes and ecological connections between the Arctic Deep Basins and the adjacent Siberian shelf seas, the northern Laptev Sea and the northeastern Kara Sea. In 2018 and 2019, this core had samples analyzed by MC-ICPMS at the Geotop-UQAM laboratories for uranium-series isotopes with the aim to establish radiochronological age constraints for the coring location.

Description: Core PS2757-8 was taken with a Kastenlot corer (KAL) on the Lomonosov Ridge next to the Russian continental shelf during the Polarstern expedition ARK-XI/1 in 1995. This expedition had the principal aim to study material fluxes and ecological connections between the Arctic Deep Basins and the adjacent Siberian shelf seas, the northern Laptev Sea and the northeastern Kara Sea. In 2018 and 2019, the core had samples analyzed by MC-ICPMS at the Geotop-UQAM laboratories for uranium-series isotopes with the aim to establish radiochronological age constraints for the coring locations.

Description: Fossil fish otoliths were collected in surface sediments (0 to 1 cm) from the Lomonosov Ridge, Arctic Ocean, recovered with a giant box corer during the R/V Polarstern Expedition PS87 of 2014. Carbon isotopic measurements of 22 specimens were performed in 2022, at the 14C laboratory of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany, with a MICADAS-mini carbon dating system. Results show a wide range of 14C-ages from ~44 to 0.4 ka, illustrating the mixing of late Quaternary fossils at the sediment surface in low sedimentation rate sites of the Arctic Ocean. Significant δ13C-values differences could be identified between pelagic and benthic fish, in accordance with their distinct habitats.

Description: The 51.58 m long marine sediment core MD99-2220 was raised from the Laurentian Channel in the St. Lawrence Estuary during the IMAGES-V (International Marine Past Global Change Study) oceanographic expedition, in July 1999, on board the Marion Dufresne II, using the Calypso piston corer. It provided very high-resolution records of middle to late Holocene paleoceanography changes (e.g., de Vernal et al., 2011) and a reference paleomagnetic record (St-Onge et al., 2003). After splitting, the sediment core has been subsampled at 1-cm interval and analyzed at the Geotop-UQAM Research Center. The volume of wet sediment subsamples was measured by water displacement. Subsamples were weighed, then air-dried and weighed again. Percentages of organic carbon and calcium carbonate were measured by Coulometer. Samples for geochemical analysis were dried at 60°C and then ground to fine powder using an agate mortar. Total carbon and nitrogen measurements were performed on ground aliquots, using a Carlo-Erba™ elemental analyzer. Another aliquot was then acidified twice with HCl (1 M) to remove carbonates, washed, dried, and divided into two portions for stable isotope analysis and determination of residual carbon content, considered to represent the organic carbon (OC) fraction. OC and N contents are expressed in dry weight percent of total sediment. Uncertainties, as determined from replicate measurements of standard substances, are estimated to average ±5% relative for OC and N contents. Aliquots for isotopic analysis were combusted in a quartz tube for 1 h at 850°C in the presence of purified cupric oxide wire and high-quality granular copper (e.g., Macko, 1981). The gases were then analyzed on a VG-PRISM™ instrument. Isotope data of OC and N are reported in δ ‰ values, after usual corrections (Craig, 1965), and with reference to V-PDB (Coplen, 1996), and atmospheric N, respectively. Uncertainties were lower than ±0.1‰, as determined from routine replicate measurements of standards. Whenever present, tests of the benthic foraminifera Globobulimina auriculata were hand-picked from the 125–250 μm fraction for δ18O and δ13C analyses. CO2 was extracted at 90°C using an ISOCARB™ device on-line with a VG-PRISM™ mass spectrometer. All measurements were made using an in-house standard marble calibrated against the Carrara marble and other current standard materials of the International Atomic Energy Agency of Vienna. Results were converted to the VPDB scale (Coplen 1996) after usual corrections (Craig 1957). The overall analytical reproducibility, as determined from replicate measurements on the in-house standard material, was routinely better than ±0.05‰ (±σ) for both δ13C and δ18O. Mollusk shell fragments were handpicked for AMS 14C dating at the Isotrace Radiocarbon Laboratory of the University of Toronto.