ALBERT

Description: The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

Description: Sea-level rise resulting from the instability of polar continental ice sheets represents a major socioeconomic hazard arising from anthropogenic warming, but the response of the largest component of Earth’s cryosphere, the East Antarctic Ice Sheet (EAIS), to global warming is poorly understood. Here we present a detailed record of North Atlantic deep-ocean temperature, global sea-level, and ice-volume change for ∼2.75 to 2.4 Ma ago, when atmospheric partial pressure of carbon dioxide (pCO2) ranged from present-day (〉400 parts per million volume, ppmv) to preindustrial (

Description: The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

Description: Deepwater circulation significantly changed during the last deglaciation from a shallow to a deep-reaching overturning cell. This change went along with a drawdown of isotopically light waters into the abyss and a deep ocean warming that changed deep ocean stratification from a salinity-to a temperature-controlled mode. Yet, the exact mechanisms causing these changes are still unknown. Furthermore, the long-standing idea of a complete shutdown of North Atlantic deepwater formation during Heinrich Stadial 1 (HS1) (17.5–14.6 kyr BP) remains prevalent. Here, we present a new compilation of benthic δ13C and δ18O data from the North Atlantic at high temporal resolution with consistent age models, established as part of the international PAGES working group OC3, to investigate deepwater properties in the North Atlantic. The extensive compilation, which includes 105 sediment cores, reveals different water masses during HS1. A water mass with heavy δ13C and δ18O signature occupies the Iceland Basin, whereas between 20 and 50°N, a distinct tongue of 18O depleted, 13C enriched water reaches down to 4000 m water depths. The heavy δ13C signature indicates active deepwater formation in the North Atlantic during HS1. Differences in its δ18O signature indicate either different sources or an alteration of the deepwater on its southward pathway. Based on these results, we discuss concepts of deepwater formation in the North Atlantic that help to explain the deglacial change from a salinity-driven to a temperature-driven circulation mode. Highlights • Spatial analyses of benthic δ13C and δ18O data from OC3 Atlantic compilation for HS1. • Heavy δ13C, light δ18O waters migrated into deep western North Atlantic basin during HS1. • Active deepwater formation between 30 and 60°N in the North Atlantic during HS1. • New concepts for transport of isotopically light δ18O into deep ocean. • Major contribution of North Atlantic waters to deglacial deep ocean stratification changes.

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: Deepwater circulation significantly changed during the last deglaciation from a shallow to a deep- reaching overturning cell. This change went along with a drawdown of isotopically light waters into the abyss and a deep ocean warming that changed deep ocean stratification from a salinity-to a temperature-controlled mode. Yet, the exact mechanisms causing these changes are still unknown. Furthermore, the long-standing idea of a complete shutdown of North Atlantic deepwater formation during Heinrich Stadial 1 (HS1) (17.5e14.6 kyr BP) remains prevalent. Here, we present a new compi- lation of benthic d13C and d18O data from the North Atlantic at high temporal resolution with consistent age models, established as part of the international PAGES working group OC3, to investigate deepwater properties in the North Atlantic. The extensive compilation, which includes 105 sediment cores, reveals different water masses during HS1. A water mass with heavy d13C and d18O signature occupies the Iceland Basin, whereas between 20 and 50� N, a distinct tongue of 18O depleted, 13C enriched water reaches down to 4000 m water depths. The heavy d13C signature indicates active deepwater formation in the North Atlantic during HS1. Differences in its d18O signature indicate either different sources or an alteration of the deepwater on its southward pathway. Based on these results, we discuss concepts of deepwater formation in the North Atlantic that help to explain the deglacial change from a salinity-driven to a temperature-driven circulation mode.

Description: We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (〈10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.

Description: For trace element analyses, single specimens of C. wuellerstorfi have been analyzed using femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP131 MS) at the Geochemistry Laboratory at the Max Planck Institute for Chemistry. Na, Mg, and Sr of 142 specimens of C. wuellerstorfi have been measured at high-resolution on (up to) each of the final five chambers to eliminate possible inaccuracies that are likely to be caused by varying sample size. The fs-LA-ICP-MS analyses were performed on 25-μm diameter spots on each chamber with a pulse repetition rate of 15 Hz at low fluence (0.1-0.3 J/cm2). Calibration was performed with the microanalytical synthetic reference material MACS‐3 for carbonate and NIST612 for silica. Average values of all chambers analyzed in the same specimen were used for further analyses. Since foraminifers from different regions show particular offsets in the Mg/Ca composition of their shell, depending on region specific temperature ranges of ambient seawater, region-specific formulas are used. We have applied the formula Mg/Ca=0.82e^0.19*BWT of Tisserand et al. (2013) from samples collected from similar geographic situations to avoid offsets. The formula has been applied to calculate temperature variability of each analyzed chamber. The average value obtained from chamber-to-chamber temperature variability of each specimen has been applied as average temperature data of the entire test.