Abstract The last deglaciation was characterized by a series of millennial‐scale climate events that have been linked to deep ocean variability. While often implied in interpretations, few direct constraints exist on circulation changes at mid‐depths. Here we provide new constraints on the variability of deglacial mid‐depth circulation using combined radiocarbon and neodymium isotopes in 24 North Atlantic deep‐sea corals. Their aragonite skeletons have been dated by uranium‐series, providing absolute ages and the resolution to record centennial‐scale changes, while transects spanning the lifetime of a single coral allow subcentennial tracer reconstruction. Our results reveal that rapid fluctuations of water mass sourcing and radiocarbon affected the mid‐depth water column (1.7–2.5 km) on timescales of less than 100 years during the latter half of Heinrich Stadial 1. The neodymium isotopic variability (−14.5 to −11.0) ranges from the composition of the modern northern‐sourced waters towards more radiogenic compositions, suggesting the presence of a greater southern‐sourced component at some times. However, in detail, simple two‐component mixing between well‐ventilated northern‐sourced and radiocarbon‐depleted southern‐sourced water masses cannot explain all our data. Instead, corals from ~15.0 ka and ~15.8 ka may record variability between southern‐sourced intermediate waters and radiocarbon‐depleted northern‐sourced waters, unless there was a major shift in the neodymium isotopic composition of the northern end‐member. In order to explain the rapid shift towards the most depleted radiocarbon values at ~15.4 ka, we suggest a different mixing scenario involving either radiocarbon‐depleted deep water from the Greenland‐Iceland‐Norwegian Seas or a southern‐sourced deep water mass. Since these mid‐depth changes preceded the Bolling‐Allerod warming and were apparently unaccompanied by changes in the deep Atlantic, they may indicate an important role for the intermediate ocean in the early deglacial climate evolution.

Abstract We reconstruct mid‐Pleistocene (marine isotope stages (MISs) 13–18) deepwater hydrography at Ocean Drilling Program Site 1063 (4583 m water depth, subtropical North Atlantic) using benthic foraminiferal stable isotope records. These new records complete an ~900 kyr long stratigraphy spanning MISs 8–29 (~250–1030 Ka) when combined with previously published records from Site 1063. The results indicate a change in the circulation regime of the abyssal subtropical North Atlantic during MIS 17. Prior to MIS 17, no significant glacial or interglacial δ13C gradients are evident between Site 1063 and the deep South Atlantic. After MIS 17, interglacial intervals at Site 1063 are characterized by δ13C values that consistently approach those recorded in the deep North Atlantic. Comparing Site 1063 δ13C values to 26 additional published records throughout the entire Atlantic basin supports the idea that this δ13C increase is unique to the deep North Atlantic. After MIS 17, the basin‐wide influence of higher δ13C values suggests an increased relative flux of northern sourced bottom waters during interglacial periods. The timing of northern sourced water influence at Site 1063 is consistent with the timing of a shift in the orientation of the Arctic Front. Thus, this shift may signify a link between the northward penetration of relatively warm, saline surface waters into the Norwegian‐Greenland Seas stimulating deep convection. Our findings fit well with the model of Imbrie et al. (1993) for the importance of the Nordic heat pump in establishing strong 100 kyr cyclicity in late Pleistocene glacial cycles.

No abstract is available for this article.

Abstract Available evidence suggests that during the early Pliocene (4–5 Ma) the mean east‐west sea surface temperature (SST) gradient in the equatorial Pacific Ocean was significantly smaller than today, possibly reaching only 1–2°C. The meridional SST gradients were also substantially weaker, implying an expanded ocean warm pool in low latitudes. Subsequent global cooling led to the establishment of the stronger, modern temperature gradients. Given our understanding of the physical processes that maintain the present‐day cold tongue in the east, warm pool in the west and hence sharp temperature contrasts, determining the key factors that maintained early Pliocene climate still presents a challenge for climate theories and models. This study demonstrates how different cloud properties could provide a solution. We show that a reduction in the meridional gradient in cloud albedo can sustain reduced meridional and zonal SST gradients, an expanded warm pool and warmer thermal stratification in the ocean, and weaker Hadley and Walker circulations in the atmosphere. Having conducted a range of hypothetical modified cloud albedo experiments, we arrive at our Pliocene simulation, which shows good agreement with proxy SST data from major equatorial and coastal upwelling regions, the tropical warm pool, middle and high latitudes, and available subsurface temperature data. As suggested by the observations, the simulated Pliocene‐like climate sustains a robust El Niño‐Southern Oscillation despite the reduced mean east‐west SST gradient. Our results demonstrate that cloud albedo changes may be a critical element of Pliocene climate and that simulating the meridional SST gradient correctly is central to replicating the geographical patterns of Pliocene warmth.

Abstract The Paleocene‐Eocene Thermal Maximum (PETM), ∼55.53 million years before present, was an abrupt warming event that involved profound changes in the carbon cycle and led to major perturbations of marine and terrestrial ecosystems. The PETM was triggered by the release of a massive amount of carbon, and thus, the event provides an analog for future climate and environmental changes given the current anthropogenic CO2 emissions. Previous attempts to constrain the amount of carbon released have produced widely diverging results, between 2000 and 10,000 gigatons carbon (GtC). Here we use the UVic Earth System Climate Model in conjunction with a recently published compilation of PETM temperatures to constrain the initial atmospheric CO2 concentration as well as the total mass of carbon released during the event. Thirty‐six simulations were initialized with varying ocean alkalinity, river runoff, and ocean sediment cover. Simulating various combinations of pre‐PETM CO2 levels (840, 1680, and 2520 ppm) and total carbon releases (3000, 4500, 7000, and 10,000 GtC), we find that both the 840 ppm plus 7000 GtC and 1680 ppm plus 7000–10,000 GtC scenarios agree best with temperature reconstructions. Bottom waters outside the Arctic and North Atlantic Oceans remain well oxygenated in all of our simulations. While the recovery time and rates are highly dependent on ocean alkalinity and sediment cover, the maximum temperature anomaly, used here to constrain the amount of carbon released, is less dependent on this slow‐acting feedback.

Key Points Cloud albedo changes may be a critical element of Pliocene climate Meridional SST gradient is a key in replicating the patterns of Pliocene warmth A robust ENSO despite the reduced mean east-west SST gradient

Abstract This study aimed at reconstructing past climatic and environmental conditions of a poorly known and documented subarctic region, the Nunatsiavut (northern Labrador). A multiproxy approach was chosen, using fossil dinoflagellate cysts, diatoms and pollen from sediment cores taken into three fjords (Nachvak 59°N, Saglek 58.5°N, and Anaktalak 56.5°N). It allowed estimating terrestrial and marine influences in each fjord and documenting the recent history of human activities of the southern fjords (Saglek and Anaktalak). Fossil pollen and dinoflagellate cyst assemblages allowed depicting the climate history of the region over the last ~200–300 years. In contrast to the general warming trend observed in the arctic and subarctic Canada since the beginning of the Industrial Era, the Nunatsiavut has experienced relative climate stability over this period. Fossil pollen data show a shift of the tree limit to the south illustrating the cooling of terrestrial conditions. Our reconstructions suggest that the Labrador region has remained climatically stable over the last ~150–300 years, with just a slight cooling trend of the reconstructed sea surface temperatures, only perceptible in Saglek and Anaktalak fjords.

No abstract is available for this article.

Despite the relative climate stability of the present interglacial, it has been punctuated by several centennial time scale climatic oscillations, the latest of which are often colloquially referred to as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA). The most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and/or volcanic activity and amplified by ocean-atmosphere-sea ice feedbacks. As such, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are widely believed to have been involved in the amplification of such climatic oscillations. The Labrador Sea is a key area of deep water formation. The waters produced here contribute approximately one-third of the volume transport of the deep limb of the AMOC and drive changes in the North Atlantic surface hydrography and subpolar gyre circulation. In this study, we present multi-proxy reconstructions from a high-resolution marine sediment core located south of Greenland that suggest an increase in the influence of polar waters reaching the Labrador Sea close to MCA-LIA transition. Changes in freshwater forcing may have reduced the formation of Labrador Sea Water and contributed towards the onset of the LIA cooling.

We present a high-resolution record of calcium carbonate preservation alongside the distribution pattern of planktic foraminifera from the Fram Strait. The record covers the marine isotope stage (MIS) 2, 28.8 to 15.4 kyr, including the Last Glacial Maximum (LGM) and the early deglaciation in multidecadal temporal resolution. The investigation is based on the distribution patterns of planktic foraminifera, stable isotopes, mean shell weight of Neogloboquadrina pachyderma, the degree of fragmentation of planktic shells, CaCO<sub>3</sub> content, and geochemical and sedimentological data. The dissolution proxies indicate long-lasting periods of markedly reduced preservation of calcium carbonate at ~28.8–27.2, ~24.1–23.2, and ~16.6–15.4 kyr in addition to a number of short-lasting periods (< 300 year) during the LGM. These periods are accompanied by a simultaneous and significant reduction in specimens of subpolar planktic foraminifera indicating a southward expansion of Arctic surface water masses. Prolonged periods with high abundances of subpolar foraminiferal species and good preservation of calcium carbonate are attributed to increased influence of Atlantic water masses, which appears to persist throughout most of the MIS 2. Comparison to Holocene (MIS 1) shell weight records shows that the preservation during MIS 2 was overall better, even during the events of dissolution. This was probably a reflection of the low concentration of atmospheric CO<sub>2</sub> at the time.