Description: High latitude ocean-atmosphere CO2 dynamics are considered important in glacial-interglacial climate, with deep-ocean carbon burial via the biological pump being highly variable through the Quaternary. During the Mid-Pleistocene Transition (MPT) shift to 100 kyr glacials, it has been suggested that elevated atmospheric-driven iron fertilisation and increased efficiency of the biological pump in the Southern Ocean was key in lowering atmospheric pCO2 and facilitating rapid land ice accumulation. Growing evidence suggests carbon cycling in the subarctic Pacific Ocean played a key role in late Quaternary glacials, although this has not yet been assessed during the MPT. Here, the silicon isotope composition of diatoms (δ30Sidiatom) from the high productivity upwelling region in the Bering Sea is used to assess the role of the subarctic Pacific biological pump in the MPT. Results show the “900 kyr event” was characterised by low silicic acid but high nitrate utilisation, coincident with the dominance of diatom resting spores. This indicates the region became nitrate- and light-limited, rather than iron-limited, due to the development of thick pack ice and expansion of glacial North Pacific Intermediate Water (GNPIW) which suppressed nutrient upwelling. We posit that iron fertilisation from sea ice expansion, coupled with the preferential preservation and higher cellular carbon content of diatom resting spores, increased regional carbon export and contributed to lower atmospheric pCO2. Remnant iron and remineralised silicic acid also likely propagated into the lower subarctic Pacific Ocean through GNPIW, aiding regionally high productivity once upwelling/vertical mixing was restored during deglaciations.

Description: Recent intensification of wind-driven upwelling of warm upper circumpolar deep water (UCDW) has been linked to accelerated melting of West Antarctic ice shelves and glaciers. To better assess the long term relationship between UCDWupwelling and the stability of theWest Antarctic Ice Sheet, we present a multi-proxy reconstruction of surface and bottom water conditions in Marguerite Bay, West Antarctic Peninsula (WAP), through the Holocene. A combination of sedimentological, diatom and foraminiferal records are, for the first time, presented together to infer a decline in UCDW influence within Marguerite Bay through the early to mid Holocene and the dominance of cyclic forcing in the late Holocene. Extensive glacial melt, limited sea ice and enhanced primary productivity between 9.7 and 7.0 ka BP is considered to be most consistent with persistent incursions of UCDW through Marguerite Trough. From 7.0 ka BP sea ice seasons increased and productivity decreased, suggesting that UCDW influence within Marguerite Bay waned, coincident with the equatorward migration of the Southern Hemisphere Westerly Winds (SWW). UCDW influence continued through the mid Holocene, and by 4.2 ka BP lengthy sea ice seasons persisted within Marguerite Bay. Intermittent melting and reforming of this sea ice within the late Holocene may be indicative of episodic incursions of UCDW into Marguerite Bay during this period. The cyclical changes in the oceanography within Marguerite Bay during the late Holocene is consistent with enhanced sensitively to ENSO forcing as opposed to the SWW-forcing that appears to have dominated the early to mid Holocene. Current measurements of the oceanography of the WAP continental shelf suggest that the system has now returned to the early Holocene-like oceanographic configuration reported here, which in both cases has been associated with rapid deglaciation.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) incursions onto the West Antarctic continental shelf cause melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet retreat today. Here we present a multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the Holocene epoch (from 11.7 thousand years ago to the present). The chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector from at least 10,400 years ago until 7,500 years ago - when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream - and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models.

Description: The dataset compiles sea-ice biomarkers (IP25, HBI II, HBI III, Triene E, Brassicasterol, Campesterol, Cholesterol, β- sitosterol) together with benthic and planktonic foraminiferal geochemistry (U/Ca, Mn/Ca, U/Mn, Mg/Ca) and benthic foraminiferal assemblage counts from Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea. The dataset covers the time range from 7.6-42 ka.

Description: The dataset compiles sea-ice biomarkers (IP25, HBI II, HBI III, Triene E, Brassicasterol, Campesterol, Cholesterol, β- sitosterol) together with benthic and planktonic foraminiferal geochemistry (U/Ca, Mn/Ca, U/Mn, Mg/Ca) and benthic foraminiferal assemblage counts from Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea. The dataset covers the time range from 7.6-42 ka.