Changes in deep Southern Ocean circulation during the onset of the last glacial period linked to Antarctic climate

Abstract:

Southern Ocean deep circulation is thought to play a critical role in climate change on glacial-interglacial timescales in terms of both heat transport and ocean-atmosphere carbon exchange. We reconstruct palaeocirculation in the Indian sector of the Southern Ocean using neodymium isotope measurements (εNd) of uncleaned foraminifera and the distribution of the sortable silt size fraction (SS) from sediments of a new core, TT1811-GGC34 (41.718°S, 80.163°W, 3167 m. water depth), bathed in Lower Circumpolar Deep Water. Our εNd and SS records extend back 120,000 years and demonstrate a tight coupling between Antarctic climate and physical circulation within the deep Southern Ocean. Finer SS and more radiogenic εNd during MIS2 than the Holocene indicate slower bottom water flow speeds and a lower proportion of North Atlantic-sourced water (NSW) in the deep Southern Ocean. During MIS2-4, εNd values comparable to previously published records from the deep Cape Basin and Indian Ocean suggest a common circulation control on εNd. Holocene-like εNd and SS values in the penultimate interglacial period, MIS5e, transition to MIS2-like values in MIS5d, suggesting a reduction in Atlantic-sourced waters during the early stages of the transition into the last glacial period. Existing εNd records from the mid-depth South Atlantic and deep equatorial Indian Ocean indicate continued inflow of NSW into the Southern Ocean during MIS5d, however. This regional pattern in εNd suggests continued – but shoaled – entrainment of NSW within the Antarctic Circumpolar Current during the early stages of the last glaciation (MIS5d). We attribute this reconfiguring of circulation to insolation-induced increases in sea-ice around Antarctica, leading to increased salinity – and therefore density – in the deep Southern Ocean, reducing the ability of NSW to mix in to LCDW at abyssal depths. We hypothesize NSW within the deep Southern Ocean was replaced by an expanded contribution of carbon-rich, Pacific-sourced water. Coupled with a more sluggish circulation, these changes account for a portion of the 40 ppm decline in atmospheric CO₂ from MIS5e-5d. Our data implicates the high latitude Southern Ocean as a key driver of glacial inception.