Radiocarbon and Stable Isotope Evidence for Increased Ventilation of the Southwest Pacific Ocean during the Last Deglaciation

Abstract:

Changes in ocean circulation are thought to have increased CO₂ sequestration in the deep ocean during glacial periods, resulting in lower atmospheric CO₂. The return of this CO₂ to the atmosphere was likely driven by invigorated ventilation of the ocean, but the relative influence of possible mechanisms that drove past circulation changes during the deglaciation are still debated. High-resolution benthic foraminiferal δ¹³C from Cibicidoides (primarily wuellerstorfi) and tephra-tied Δ¹⁴C on mixed benthics from an eight-core depth transect (663-3836 m water depth) in the Southwest Pacific Ocean provide compelling evidence for shallow glacial stratification in the Southern Ocean. The glacial δ¹³C difference between ~660 and ~1600 m was ~1.7‰, more than double the Holocene difference of ~0.7‰. At the same time, Δ¹⁴C of waters below ~1600 m were 3-5 times more depleted than modern, confirming increased isolation of deep water from the atmosphere. During the last glacial termination better-ventilated intermediate waters appeared in a pulse beginning ~16.5 ka, indicated by rapid δ¹³C enrichment to 1100 m and confirmed by Δ¹⁴C values with vertical structure similar to modern. Intermediate ventilation in the high resolution δ¹³C was followed by slower, progressive ventilation of increasingly deeper layers between 14.5 and 10 ka, again confirmed by lower resolution Δ¹⁴C of mid-depth water. The close coincidence of rapid atmospheric CO₂ increases, shallow ventilation and documented wind-driven upwelling in the Southern Ocean suggests the influence of wind-driven ocean ventilation. In addition, gradual atmospheric CO₂ increases coincident with the onset of deep ventilation and reinitiation of vigorous North Atlantic Deep Water formation suggest that shifts in deep water density and circulation also contributed to deglacial ocean ventilation in this region. In combination, these data indicate both processes played a role in deglacial CO₂ rise.