Deglacial CO2 release and ventilation in the Indian Ocean sector attributed to a Southern Ocean deep gateway effect

Abstract:

The Southern Ocean exerts a profound control on ocean mixing and water mass exchange between the major oceanic basins, and is thought to play a critical role in glacial-interglacial circulation changes and ocean-atmosphere carbon exchange. Shoaling of north Atlantic-sourced waters (NSW) during the Last Glacial Maximum (LGM) contributed to the observed reduction of glacial atmospheric CO₂, in part due to the resulting reconfiguration of Southern Ocean circulation caused by the deep gateway effect. This effect is produced by the absence of a net zonal circumpolar pressure gradient above the Drake Passage sill that prevents net meridional geostrophic transport of intermediate depth NSW across the Antarctic Circumpolar Current; an effect that closed this gateway to shoaled NSW during the LGM. A closed gateway led to the expansion of a lower circulation cell with an increased component of Pacific and Indian Deep Water higher in remineralized carbon, thereby increasing the oceanic carbon store. We present here data from two Southeast Indian Ocean cores; TT1811-34GGC (41.718°S, 80.163°W, 3167 m. water depth) bathed in Circumpolar Deep Water (CDW) and TT1811-50GGC (38.334°S, 77.715°W, 1118 m.) bathed in Antarctic Intermediate Water (AAIW). In our deep core, Holocene–LGM δ¹³C differences of ~ 1‰ based on Cibicidoides spp. indicate increased respired CO₂ in the LGM, while neodymium isotopes (_εNd) measured on uncleaned planktic foraminifera, decreased from -6.5 in the LGM to -8.6 in the Holocene, recording significantly reduced NSW. Sortable silt (SS) records from both cores suggest reduced flow speeds throughout the water column while AAIW d¹³C records no change. CDW δ¹³C increased early in Heinrich Stadial 1, indicating early CDW ventilation, whereas SS and εNd shift later in the deglaciation, suggesting the return of NSW and reinvigorated deep water flow occurred later in the deglaciation. This sequence is consistent with the idea that shoaled NSW was unable to escape the Atlantic and contribute to deep flow in the Southern Indian Ocean until/after the Antarctic Cold Reversal, similar to observations from the Southern Pacific. From these data, we infer that Southern Ocean dynamics, rather than Atlantic overturning circulation, controlled the timing of early deglaciation and CO₂ release from the Southern Hemisphere.