ALBERT

Description: Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (〉8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bölling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell.

Description: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low- and mid-latitude upwelling cells started to decline by more than 2-4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO2 as recorded in ice cores and, with a slight delay of more than 1000 years, to a large-scale gradual CO2 depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ13C by 0.4–0.5‰. The decrease in new production also matches a clear 13C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high-latitude insolation, i.e., to orbital forcing as primary cause.

Description: Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (〉8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bölling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell.

Description: Highlights • North Atlantic records indicate an intense Heinrich stadial (HS) during Termination V. • The HS probably resulted from extreme glacial conditions during MIS12. • The HS curtailed AMOC, shaping TV via meridional heat transport anomalies. • The rate of CO2 release during the HS of TV was lower than during HS1. • North Atlantic overturning during MIS11 was enhanced with respect to the Holocene. Abstract The role of millennial scale climate variability in supplementing the astronomical forcing of glacial–interglacial transitions remains a major unresolved question. Here we compare the occurrence and character of “terminal” ice rafting events in both the North and South Atlantic during the last deglaciation (Termination I, TI) and during the transition between Marine Isotope Stages (MIS) 12 and 11 (or Termination V, TV). We show that TV experienced a massive terminal ice rafting event in the North Atlantic that was more intense and longer lasting than Heinrich event 1 (H1) of the last deglaciation. This massive ice rafting event was linked to cold stadial conditions and reduced deep water formation in the North Atlantic, in parallel with warming at high southern latitudes, similar to the bipolar seesaw pattern exhibited during H1 over the last deglaciation. We propose that the particular intensity and duration of the TV ice rafting event resulted from the especially large volume of Northern Hemisphere ice sheets during MIS12. In turn, the unusually long duration and large amplitude of TV likely resulted from the exceptionally prolonged collapse of the AMOC during the TV Heinrich stadial, and from a subsequent transient AMOC “overshoot” with respect to later MIS11 interglacial circulation. Furthermore, we suggest that the intense Heinrich stadial of TV contributed to the deglaciation primarily via meridional heat transport anomalies that would have enhanced the incipient warming arising from relatively weak insolation forcing, and only secondarily via CO2 release.

Description: The role of millennial scale climate variability in supplementing the astronomical forcing of glacial-interglacial transitions remains a major unresolved question. Here we compare the occurrence and character of "terminal" ice rafting events in both the North and South Atlantic during the last deglaciation (Termination I, TI) and during the transition between Marine Isotope Stages (MIS) 12 and 11 (or Termination V. TV). We show that TV experienced a massive terminal ice rafting event in the North Atlantic that was more intense and longer lasting than Heinrich event 1 (H1) of the last deglaciation. This massive ice rafting event was linked to cold stadial conditions and reduced deep water formation in the North Atlantic, in parallel with warming at high southern latitudes, similar to the bipolar seesaw pattern exhibited during H1 over the last deglaciation. We propose that the particular intensity and duration of the TV ice rafting event resulted from the especially large volume of Northern Hemisphere ice sheets during MIS12. In turn, the unusually long duration and large amplitude of TV likely resulted from the exceptionally prolonged collapse of the AMOC during the TV Heinrich stadia], and from a subsequent transient AMOC "overshoot" with respect to later MIS11 interglacial circulation. Furthermore, we suggest that the intense Heinrich stadial of TV contributed to the deglaciation primarily via meridional heat transport anomalies that would have enhanced the incipient warming arising from relatively weak insolation forcing, and only secondarily via CO2 release