In this multiproxy study, we used new isotopic data on planktonic foraminifera to highlight the strong instability that characterized surface conditions in the Iceland Basin during Marine Isotope Stage 11 (MIS 11). We produced new oxygen isotope data on the planktonic species Neogloboquadrina incompta and Turborotalita quinqueloba, foraminifera-bound nitrogen isotope data on N. incompta, and calcareous nannofossil data at coring site IODP Site U1314. The multiproxy record displays two distinct upper ocean regimes: a relatively stable pre-climate optimum and an unstable post-climate optimum with high amplitude variations in nutrient utilization and seasonality proxies, and strong enrichment in oxygen isotopes suggesting colder and/or saltier upper waters. The latter regime was concomitant with a resurgence in ice-rafted debris. Interestingly, this surface instability is not observed in cores from sites affected by the North Atlantic Current. Moreover, deep water ventilation is reconstructed to decrease throughout the eastern North Atlantic, while remaining rather constant in the Labrador Sea. The evidence presented here indicates that deep-water formation was unstable throughout MIS 11, and that peak periods of deep-water formation varied across high latitude North Atlantic basins, depending on the prevailing surface conditions in each region. These findings suggest that reconstructing deep-water formation and ventilation require a comprehensive approach that accounts for the interconnectivity between different components of the overturning circulation system.

Push-coring is used to collect Holocene-aged coral sub-fossils, but its applications can be limited by underlying reef geology. Here, we report on a new approach of coring inundated coastal karst formations—i.e., coral holes—to determine the historical context of coral reefs in Guam. Three cores were extracted and processed to recover coral fragments as well as abundances of the photosymbiotic foraminifera Baculogypsina sphaerulata, a proxy for water clarity. Coral fragments, with the oldest dating to a median calibrated age of 221 cal. BP (1729 CE), revealed greater than 100 years of consistent accumulation and composition, suggesting a period of relative stability for macrobenthos. However, we documented a drop in B. sphaerulata abundance from approximately 100 years cal. BP, suggesting a period of environmental decline in Guam. Our results provide the first multi-centennial record of coral assemblages from Guam and provide a proof of concept for future historical investigations.

Abstract Reconstructing intermediate and bottom‐water temperature in the Arctic Ocean is key for understanding paleoclimatic phenomena, such as the region's interactions with warm Atlantic waters, stratification, and sea‐ice dynamics. However, benthic proxy archives are sparse throughout the Arctic circle compared to lower latitudes. Trace‐element ratios (E/Ca) derived from ostracodes, a group of bivalved microscopic crustaceans, have shown promise in this regard. Samples for E/Ca measurements typically require rigorous cleaning prior to analysis, and signs of contamination are routinely monitored through the presence of other trace elements such as Al, Fe, and Mn, which are associated with suspected sources of overprinting. However, there has not yet been an intra‐valve investigation of all of these trace elements, which may hinder our ability to effectively identify geochemical overprinting. Here, we present several elemental concentration and E/Ca ratio measurements in two ostracode genera, Krithe and Polycope, extracted from Chukchi Sea sediment samples. We further investigate the intra‐valve distribution of elements within single shells of adult and juvenile specimens using electron probe microanalysis (EPMA). Our findings suggest that brushing and bleach treatments may not be effective for completely eliminating clays from the edges of valves, which can bias paleoclimatologically relevant trace‐element proxies such as Mg/Ca ratios, particularly in the case of incomplete or small samples with low amounts of calcite material. In addition, we report the first trace‐element data from the genus Polycope, which shows potential as a new Arctic paleotemperature archive.

The Atlantic meridional overturning circulation (AMOC) is a critical element of Earth's climate system and it is currently weakening. While this weakening is frequently explained by freshwater-driven disruptions to deep-water formation, uncertainties about the impacts of prolonged freshening limit our capacity to predict its future state. For example, during the warm and unusually long marine isotope stage (MIS) 11 interglacial, ∼424 to 374 ka, several lines of evidence suggest that a strong AMOC persisted concomitant with fresher-than-present conditions in the Nordic Seas, challenging our current understanding of deep-water formation. Here, we present new foraminifer-bound nitrogen isotope data along with multiple additional geochemical reconstructions of upper-ocean hydrography in the Nordic Seas during this anomalous interval. Our data suggest that a weak summer stratification was driven by the prolonged upper-ocean accumulation of freshwater beginning at the onset of the climatic optimum, ∼410 to 407 ka, which could have helped precondition the region for deep-water formation. A box model constrained by paleo-proxy data additionally suggests that the density gradient between the subpolar North Atlantic and Nordic Seas was favorable for the onset of deep-water formation in the Nordic Seas during the climatic optimum. It is thus likely that the Nordic Seas became a locus of deep-water formation around this time. Enhanced northern-hemisphere heating driven by deep-water formation in the Nordic Seas may have been important for delaying glacial conditions, thereby driving the extended warming characteristic of MIS 11. Such findings may also be relevant for near-future changes under a relatively fresher high-latitude North Atlantic.

The impacts of climate change on north Atlantic nutrient chemistry remain poorly understood, as there exist a multitude of rapidly changing biological and physical drivers of nutrient conditions throughout the region. Here, we present nitrogen isotope measurements derived from a six‐hundred‐year‐old crustose coralline alga (δ15Nalgal) to elucidate historical and contemporary trends in nitrate utilization and circulation patterns along the Labrador Shelf. Prior to the early 1900s, we argue that intervals during which utilization approached completion were controlled by reduced nitrate advection linked to an increased proportion of nitrate‐poor polar waters and subdued Atlantic influence, as expected from concurrent negative modes of the Atlantic multidecadal oscillation. While nitrate conditions should have recovered in recent years, our record suggests that high utilization persisted since ∼1870, which we also attribute to reduced Atlantic advection, likely associated with the twentieth‐century anthropogenic weakening of the Atlantic meridional overturning circulation. These results highlight the role of ongoing climate‐induced circulation changes in modulating nutrient distributions throughout the subpolar north Atlantic, which may have implications for other environmental phenomena such as fisheries and oceanic carbon storage.

The Labrador Sea region has experienced unprecedented changes in sea-ice extent, primary productivity and ocean circulation over the last century. While these changes should have also intuitively had consequences for nutrient availability in the region, modern oceanographic observations are limited to the very recent past and thus hinder our ability to appropriately document such changes. Here, we present nitrogen (N) isotope data derived from a six-hundred-year-old crustose coralline alga, Clathromorphum compactum , to investigate past and present drivers of ocean nutrient chemistry along the Labrador Shelf. Our record reveals five phases during which N was nearly completely consumed, with the most recent and anomalously-long phase corresponding to the onset and duration of the industrial era. We attribute this to reduced nitrate input, which would be expected from the ongoing weakening and migration of the Labrador Current. By contrast, we argue that the briefer historical phases of nearly-complete consumption were modulated by oceanographic conditions favorable for simultaneous phytoplankton growth and reduced N input. Due to the potential effects of marine nutrient distributions on socioeconomically-important fisheries and oceanic carbon uptake, such biogeochemical changes in the North Atlantic may have additional implications for numerous environmental challenges facing us in the twenty-first century. Our study thus adds to the growing body of literature demonstrating the intense sensitivity of the high-latitude North Atlantic to modern warming.

Surface waters in the Nordic Seas were colder and fresher throughout the marine isotope stage (MIS) 11 interglacial compared to present day. This has been previously attributed to the continuous delivery of freshwater sourced from large ice structures characteristic of the preceding glacial interval, MIS 12. While it is conventionally believed that high-latitude surface freshening can trigger a reduction of the Atlantic Meridional Overturning Circulation (AMOC), multiple lines of evidence suggest a vigorous AMOC despite elevated freshwater forcing in the Nordic Seas. Here, we review and reanalyze evidence for sea surface properties throughout the Nordic Seas and North Atlantic. We find that surface waters in the Nordic Seas experienced an unusually variable inception of interglacial temperature conditions with multiple high-magnitude cold excursions. While cold events in the North Atlantic were frequently associated with in situ meltwater deposition as reconstructed by ice-rafted debris, this proxy was virtually uncorrelated with cold events in the Nordic Seas. Additionally, stable nitrogen analysis revealed variable levels of nutrient utilization in the Nordic Seas' surface layer throughout MIS 11. This may reflect a dynamic structure of the upper ocean concomitant with an intermittent rate of freshwater delivery. Based on this combination of evidence, we suggest that the colder and fresher surface layer in the Nordic Seas was supplied from higher latitudes, rather than from locally-sourced iceberg meltwater as is characteristic of North Atlantic forcing. Pairing proxy-based evidence with recent numerical simulations further decouples surface freshening in the Nordic Seas and Greenland meltwater input, discrediting Greenland as a source of freshwater to this region during the later phase of MIS 11. Because the origin of freshwater has implications for its rate of delivery, our study might help to explain the active AMOC despite surface freshening during MIS 11 and should be recognized when considering this interglacial as an analog for near-future climate change scenarios.

Deep-water formation in the high latitudes partially controls the strength of the Atlantic Meridional Overturning Circulation (AMOC). While recent research has illustrated a reduction in ocean convection likely attributable to enhanced freshwater fluxes in this region, elevated surface freshening of the Nordic Seas occurred in tangent with a strong AMOC during the marine isotope stage (MIS) 11 interglacial, 424,000 – 374,000 ka. It was previously hypothesized that the prolonged introduction of freshwater resulted in a thicker mixed layer relative to other quaternary interglacials, which is supported by bulk nitrogen reconstructions of nutrient utilization. Here, we further investigate the phasing of nutrient utilization during MIS 11 using foraminifera-bound stable nitrogen isotope ratios, which are better protected from diagenetic alteration and thus allow for higher-confidence interpretation. Our record faithfully tracks the global AMOC trend as reconstructed from benthic carbon data, where lower levels of nutrient utilization occur in phase with stronger AMOC conditions. While this may reflect the enhanced advection of nutrient-rich North Atlantic waters into the Nordic Seas, reconstructions of sea surface temperature and salinity suggest that the influence of southern-sourced waters was delayed relative to the observed AMOC and nutrient trends. We therefore hypothesize that the coupling of nutrient utilization and AMOC strength reflects the physical conditions of a deeper mixed layer, allowing for an increased mixing of nutrients from below.