Climate models and paleo-reconstructions suggest that alterations in the Atlantic Meridional Overturning Circulation (AMOC) are not only indicators but also drivers of climate changes. Therefore, the AMOC is considered a critical tipping element within Earth’s climate system. Many lines of evidence indicate that the last glacial termination was characterised by large swings in AMOC strength, yet proxy evidence remains ambiguous about centennial-scale fluctuations during the Holocene. Inconsistencies persist regarding the timing, spatial pattern, and intensity of North Atlantic deep-water production. This study evaluates the variability of the AMOC during the Holocene based on several marine sediment cores covering the North Atlantic in high temporal resolution. For this, we exploit the 231Pa/230Th proxy, which indicates the bottom water advection strength. Additionally, past particle fluxes were reconstructed to determine a possible influence of particle composition and particle rain rate on the 231Pa/230Th signal. This study thus aims to extend existing paleo-circulation reconstructions of the AMOC from the last deglacial period with more recent analyses. Five new high-resolution 231Pa/230Th down-core records from different oceanographic settings and water depths in the North Atlantic consistently exhibit low variability throughout the entire Holocene. The 231Pa/230Th records generally display deviations of ± 10% from their respective Holocene mean. A generalised additive model (GAM) was fitted to the timeseries to detect mean North Atlantic trends within the different Holocene-normalised datasets. This model exhibits a virtually constant 231Pa/230Th level throughout the Holocene, interrupted by two time periods of slightly increased ratios, indicative of a weaker AMOC. The first time period is within the timeframe of the 8.2 ka event, characterised by a sudden cold spell across parts of the Northern Hemisphere. During this interval, four of the five timeseries show slightly elevated 231Pa/230Th ratios, although two records within this period hold a reduced sampling resolution. This limited temporal resolution and the shortness of the event make it challenging to decidedly conclude on the magnitude of the AMOC weakening during this time. The second period of higher 231Pa/230Th coincides with the 4.2 ka event and is only evident from the ODP 1063 data (Bermuda Rise). However, these higher 231Pa/230Th ratios can be explained by increased bottom scavenging of 231Pa presumably caused by benthic storms, induced by the transfer of eddy kinetic energy from the surface to the deep ocean. Consequently, atmospheric forcing during the 4.2 ka event seems to be a more plausible explanation than a paleoceanographic cause for the observed higher 231Pa/230Th. In conclusion, our study suggests that deep ocean circulation in the North Atlantic did not exhibit high variability on sub-millennial time scales, but has remained relatively stable throughout the Holocene.

Earth system models and paleo-reconstructions indicate that shifts in Atlantic Meridional Overturning Circulation (AMOC) strength profoundly impact global climate. While the last glacial termination experienced large AMOC variations, evidence of AMOC changes during the Holocene are poorly constrained. Here we present a Holocene AMOC reconstruction by quantifying mean bottom water advection strength in the deep North Atlantic. For this, we estimated volumetric flow rates from sedimentary 231Pa/230Th records with millennial resolution using the Bern3D model. We found that while during the Early Holocene the AMOC recovered from its weak deglacial state, it experienced a weakening between 9.2 to 8 ka BP, coinciding with North Atlantic meltwater pulses. From 6.5 ka BP onward, the AMOC strength stabilized, reaching its pre-industrial state around ~18 Sv. Hence, according to future projections, anthropogenic climate change may result in an AMOC slowdown unprecedented for most of the ongoing Holocene interglacial.

Species assemblage composition of marine microfossils offers the possibility to investigate ecological and climatological change on time scales inaccessible using conventional observations. Planktonic foraminifera - calcareous zooplankton - have an excellent fossil record and are used extensively in palaeoecology and palaeoceanography. During the Last Glacial Maximum (LGM; 19,000 - 23,000 years ago), the climate was in a radically different state. This period is therefore a key target to investigate climate and biodiversity under different conditions than today. Studying LGM climate and ecosystems indeed has a long history, yet the most recent global synthesis of planktonic foraminifera assemblage composition is now nearly two decades old. Here we present the ForCenS-LGM dataset with 2,365 species assemblage samples collected using standardised methods and with harmonised taxonomy. The data originate from marine sediments from 664 sites and present a more than 50% increase in coverage compared to previous work. The taxonomy is compatible with the most recent global core top dataset, enabling direct investigation of temporal changes in foraminifera biogeography and facilitating seawater temperature reconstructions.

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.

Although the global environmental impact of Laurentide Ice-Sheet destabilizations on glacial climate during Heinrich Events is well-documented, the mechanism driving these ice-sheet instabilities remains elusive. Here we report foraminifera-based subsurface (~150 m water depth) ocean temperature and salinity reconstructions from a sediment core collected in the western subpolar North Atlantic, showing a consistent pattern of rapid subsurface ocean warming preceding the transition into each Heinrich Event identified in the same core of the last 27,000 years. These results provide the first solid evidence for the massive accumulation of ocean heat near the critical depth to trigger melting of marine-terminating portions of the Laurentide Ice Sheet around Labrador Sea followed by Heinrich Events. The repeated build-up of a subsurface heat reservoir in the subpolar Atlantic closely corresponds to times of weakened Atlantic Meridional Overturning Circulation, indicating a precursor role of ocean circulation changes for initiating abrupt ice-sheet instabilities during Heinrich Events. We infer that a weaker ocean circulation in future may result in accelerated interior-ocean warming of the subpolar Atlantic, which could be critical for the stability of modern, marine-terminating Arctic glaciers and the freshwater budget of the North Atlantic.

Deglacial dissolved oxygen concentrations were semiquantitatively estimated for intermediate and deep waters in the western Bering Sea using the benthic foraminiferal-based transfer function developed by Tetard et al. (2017) , Tetard et al. (2021a) . Benthic foraminiferal assemblages were analyzed from two sediment cores, SO201-2-85KL (963 m below sea level (mbsl), the intermediate-water core) and SO201-2-77KL (2,163 mbsl, the deep-water core), collected from the Shirshov Ridge in the western Bering Sea. Intermediate waters were characterized by an oxygen content of ∼2.0 ml L −1 or more during the Last Glacial Maximum (LGM)–Heinrich 1 (H1), around 0.15 ml L −1 during the middle Bølling/Allerød (B/A)–Early Holocene (EH), and a slight increase in [O 2 ] (∼0.20 ml L −1 ) at the beginning of the Younger Dryas (YD) mbsl. Deep-water oxygen concentrations ranged from 0.9 to 2.5 ml L −1 during the LGM–H1, hovered around 0.08 ml L −1 at the onset of B/A, and were within the 0.30–0.85 ml L −1 range from the middle B/A to the first half of YD and the 1.0–1.7 ml L −1 range from the middle to late Holocene. The [O 2 ] variations remind the δ 18 O NGRIP record thereby providing evidence for a link between the Bering Sea oxygenation at intermediate depths and the deglacial North Atlantic climate. Changes in the deep-water oxygen concentrations mostly resemble the deglacial dynamics of the Southern Ocean upwelling intensity which is supposed to be closely coupled with the Antarctic climate variability. This coherence suggests that deglacial deep-water [O 2 ] variations were primarily controlled by changes in the circulation of southern-sourced waters. Nevertheless, the signal from the south at the deeper site might be amplified by the Northern Hemisphere climate warming via an increase in sea-surface bioproductivity during the B/A and EH. A semi-enclosed position of the Bering Sea and sea-level oscillations might significantly contribute to the magnitude of oxygenation changes in the study area during the last deglaciation. Interregional correlation of different proxy data from a wide range of water depths indicates that deglacial oxygenation changes were more pronounced in the Bering and Okhotsk marginal seas than along the open-ocean continental margin and abyssal settings of the North Pacific.

Increased carbon sequestration in the ocean subsurface is commonly assumed to have been one of the main causes responsible for lower glacial atmospheric CO2 concentrations. Remineralized carbon must have been stored away from the atmosphere for thousands of years, yet the water mass structure accommodating such increased carbon storage continues to be debated. Here, we present new sediment-derived bottom-water neodymium isotope records that allow fingerprinting of water masses and provide a more complete picture of the Atlantic Meridional Overturning Circulation geometry during the Last Glacial Maximum. These results suggest that the vertical and meridional structure of the Atlantic water mass distribution only experienced minor changes since the last ice age. In particular, we find no compelling evidence supporting glacial southern-sourced water substantially expanding to shallower depths and farther into the Northern Hemisphere than today, which had been previously inferred from stable carbon isotope (δ13C) reconstructions. We argue that depleted δ13C values observed in the deep Northwest Atlantic do not necessarily indicate the presence of southern-sourced water. Instead, these values may represent a northern-sourced water mass with lower than modern preformed δ13C values that were further modified downstream by increased sequestration of remineralized carbon, facilitated by a more sluggish glacial deep circulation, corroborating previous evidence.

The alkenone unsaturation index (U-37(k)) as proxy for sea surface temperature (SST) is an important tool in paleoclimatology for reconstructing past ocean temperature variability. Typically, Uk(37)(k) recorded in marine surface sediments shows a linear correlation with modern mean annual SST. However, in high-latitude oceanic regions, such as the subpolar Pacific, U-37(k)-based SSTs do overestimate the mean annual temperature by up to 6 degrees C, potentially leading to obscured paleoclimatic information drawn from stratigraphic U-37(k)-records. The reason for this "warm bias" is still not well understood. Here, we present a compilation of 97 sediment surface samples from Multicores collected in the Bering Sea, the Okhotsk Sea and the North Pacific to evaluate the alkenone-temperature proxy against observational data from the North Pacific. Sediment surface samples were analysed for alkenones and the derived U-37(k)-indices converted to water temperatures using different calibration equations established in the literature. U-37(k)-based SSTs were then compared to instrumental SST data, as well as modern alkenone flux data from sediment traps in the North Pacific. Our results confirm that most U-37(k)-based SSTs from the subpolar Pacific are 2-6 degrees C too warm compared to instrumental mean annual SSTs for calibrations applied. However, with an uncertainty at the level of +/- 1.5 degrees C or less reconstructed SSTs fit quite well to modern autumn temperatures north of the Subarctic Front (SAF), when maximum export flux of alkenones to the seafloor is indicated by sediment trap data. South of the SAF, reconstructed SSTs largely mimic the modern mean annual SST signal with an uncertainty of +/- 1.5 degrees C or less, which is likely due to the attenuation of seasonality and longer growth season of coccolithophorids according to sediment trap data. Our study further demonstrates that U-37(k), when seasonality in alkenone production and export are known and considered, is able to provide reasonable estimates of SSTs in modern high-latitude ocean settings. We conduct a case study using available alkenone time-series derived from a sediment core collected from the south-western Okhotsk Sea to better understand the potential effect of seasonality in alkenone production on stratigraphic U-37(k)-record in the subpolar Pacific. The case study from the Okhotsk Sea indicates that even a small shift in seasonality may lead to strongly biased SSTs with broader regional implications for paleoclimate reconstructions in high-latitude ocean settings. (C) 2020 The Author(s). Published by Elsevier Ltd.

We present high-resolution multi-proxy records from a marine sediment core (SO201-2-85KL) from the western Bering Sea to assess orbital- and millennial-scale paleoceanographic conditions during two last glacial intervals, including both terminations. Based on changes in foraminiferal assemblages, grain-size content and previously published TOC and δ13C records, we reconstruct variations in sea-surface biological productivity, intermediate-water oxygenation and sea-ice conditions during the last 180 kyr. Our data demonstrate remarkable differences between the penultimate (MIS 6) and last (MIS 4-2) glacial. Relatively high sea surface bioproductivity and reduced sea-ice cover are reconstructed for the penultimate glacial interval, whereas low bioproductivity and expanded sea-ice cover appear to be typical for the last glacial. Millennial-scale changes in intermediate water ventilation are inferred from faunal records for the middle part of the penultimate glacial. High-amplitude environmental variability during the penultimate glacial time in the Bering Sea resembles the well-known Dansgaard-Oeschger oscillations, and roughly corresponds to similar rapid climatic fluctuations found in North Atlantic records. The Termination II and I intervals display a similar succession of high-bioproductivity events, being more pronounced during the penultimate glacial-interglacial transition, probably due to the different orbital configuration. During the late phase of Termination II, two short intervals, characterized by high sea surface bioproductivity and low oxygen content of bottom waters, resemble the Bølling and Allerød warmings, whereas an episode with low bioproductivity occurs in between, similar to the Older Dryas. Our results provide support for a close circumpolar coupling between high-latitude environments on millennial timescales at least since the penultimate glacial.

The eastern equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low-latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient-rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial subthermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (delta C-13) record of subthermocline waters derived from deep-dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published delta C-13 records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern-sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial subthermocline waters is recognized during glacial terminations and marks changes to modern-like conditions in nutrient concentrations and biological productivity in the EEP.

We provide high-resolution foraminiferal stable carbon isotope (delta C-13) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extratropical and tropical North Pacific during the past 60 kyr. We measured the delta C-13 composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58 degrees 52.52' N, 170 degrees 41.45' E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the delta C-13 of subthermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00 degrees 01.31' N, 82 degrees 27.76' W; 2921m water depth). Apparent similarities in the long-term evolution of delta C-13 between GNPIW, intermediate waters in the eastern tropical North Pacific and subthermocline water masses in the EEP suggest the expansion of relatively delta C-13-depleted, nutrient-enriched, and northern sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.

The modern North Pacific plays a critical role in marine biogeochemical cycles, as an oceanic sink of CO2 and by bearing some of the most productive and least oxygenated waters of the World Ocean. The capacity to sequester CO2 is limited by efficient nutrient supply to the mixed layer, particularly from deeper water masses in the Pacific's subarctic and marginal seas. The region is in addition only weakly ventilated by North Pacific Intermediate Water (NPIW), which receives its characteristics from Okhotsk Sea Intermediate Water (OSIW). Here, we present reconstructions of intermediate water ventilation and productivity variations in the Okhotsk Sea that cover the last glacial termination between eight and 18 ka, based on a set of high-resolution sediment cores from sites directly downstream of OSIW formation. In a multi-proxy approach, we use total organic carbon (TOC), chlorin, biogenic opal, and CaCO3 concentrations as indicators for biological productivity. C/N ratios and XRF scanning-derived elemental ratios (Si/K and Fe/K), as well as chlorophycean algae counts document changes in Amur freshwater and sediment discharge that condition the OSIW. Stable carbon isotopes of epi- and shallow endobenthic foraminifera, in combination with 14C analyses of benthic and planktic foraminifera imply decreases in OSIW oxygenation during deglacial warm phases from c. 14.7 to 13 ka (Bølling-Allerød) and c. 11.4 to 9 ka (Preboreal). No concomitant decreases in Okhotsk Sea benthic-planktic ventilation ages are observed, in contrast to nearby, but southerly locations on the Japan continental margin. We attribute Okhotsk Sea mid-depth oxygenation decreases in times of enhanced organic matter supply to maxima in remineralization within OSIW, in line with multi-proxy evidence for maxima in primary productivity and supply of organic matter. Sedimentary C/N and Fe/K ratios indicate more effective entrainment of nutrients into OSIW and thus an increased nutrient load of OSIW during deglacial warm periods. Correlation of palynological and sedimentological evidence from our sites with hinterland reference records suggests that millennial-scale changes in OSIW oxygen and nutrient concentrations were largely influenced by fluvial freshwater runoff maxima from the Amur, caused by a deglacial northeastward propagation of the East Asian Summer Monsoon that increased precipitation and temperatures, in conjunction with melting of permafrost in the Amur catchment area. We suggest that OSIW ventilation minima and the high lateral supply of nutrients and organic matter during the Allerød and Preboreal are mechanistically linked to concurrent maxima in nutrient utilization and biological productivity in the subpolar Northwest Pacific. In this scenario, increased export of nutrients from the Okhotsk Sea during deglacial warm phases supported subarctic Pacific shifts from generally Fe-limiting conditions to transient nutrient-replete regimes through enhanced advection of mid-depth nutrient- and Fe-rich OSIW into the upper ocean. This mechanism may have moderated the role of the subarctic Pacific in the deglacial CO2 rise on millennial timescales by combining the upwelling of old carbon-rich waters with a transient delivery of mid-depth-derived bio-available Fe and silicate.

Little is known about the climate evolution on the Kamchatka Peninsula during the last glacial–interglacial transition as existing climate records do not reach beyond 12 ka BP. In this study, a summer temperature record for the past 20 kyr is presented. Branched glycerol dialkyl glycerol tetraethers, terrigenous biomarkers suitable for continental air temperature reconstructions, were analyzed in a sediment core from the western continental margin off Kamchatka in the marginal northwest Pacific (NW Pacific). The record suggests that summer temperatures on Kamchatka during the Last Glacial Maximum (LGM) equaled modern temperatures. We suggest that strong southerly winds associated with a pronounced North Pacific High pressure system over the subarctic NW Pacific accounted for the warm conditions. A comparison with an Earth system model reveals discrepancies between model and proxy-based reconstructions for the LGM temperature and atmospheric circulation in the NW Pacific realm. The deglacial temperature development is characterized by abrupt millennial-scale temperature oscillations. The Bølling–Allerød warm phase and the Younger Dryas cold spell are pronounced events, suggesting a connection to North Atlantic climate variability.

Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP25, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP25 data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bolling-Allerod and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes.

It has been proposed that North Pacific sea surface temperature (SST) evolution was intimately linked to North Atlantic climate oscillations during the last glacial-interglacial transition. However, during the early deglaciation and the Last Glacial Maximum, the SST development in the subarctic northwest Pacific and the Bering Sea is poorly constrained as most existing deglacial SST records are based on alkenone paleothermometry, which is limited prior to 15ka B.P. in the subarctic North Pacific realm. By applying the TEX86L temperature proxy we obtain glacial-Holocene-SST records for the marginal northwest Pacific and the Western Bering Sea. Our TEX86L-based records and existing alkenone data suggest that during the past 15.5ka, SSTs in the northwest Pacific and the Western Bering Sea closely followed millennial-scale climate fluctuations known from Greenland ice cores, indicating rapid atmospheric teleconnections with abrupt climate changes in the North Atlantic. Our SST reconstructions indicate that in the Western Bering Sea SSTs drop significantly during Heinrich Stadial 1 (HS1), similar to the known North Atlantic climate history. In contrast, progressively rising SST in the northwest Pacific is different to the North Atlantic climate development during HS1. Similarities between the northwest Pacific SST and climate records from the Gulf of Alaska point to a stronger influence of Alaskan Stream waters connecting the eastern and western basin of the North Pacific during this time. During the Holocene, dissimilar climate trends point to reduced influence of the Alaskan Stream in the northwest Pacific.