Evidence for Deglacial Venting of CO2 from the deep Southeast Indian Ocean using Planktic and Benthic Stable Isotopes

Abstract:

Changes in Southern Ocean (SO) circulation and wind-driven upwelling played a critical role in ventilating the deep ocean during the last deglaciation and are often invoked to explain the deglacial rise of atmospheric carbon dioxide (CO₂) concentrations. Paleo-reconstructions examining these mechanisms have largely focused on the Atlantic and Pacific sectors of the SO. The Indian sector, by contrast, remains understudied, precluding a comprehensive and coherent view of SO dynamics and ocean-atmosphere carbon exchange as Earth’s climate transitioned out of the last ice age. Here, we present new foraminiferal stable isotope records (δ¹⁸O and δ¹³C) from two sediment cores recently recovered from the southeast Indian Ocean as part of the Coring to Reconstruct Ocean Circulation and Carbon Dioxide Across 2 Seas (CROCCA-2S) expedition: core TT1811-50GGC (38.334°S, 77.715°E; 1118 m) and core TT1811-34GGC (41.718°S, 80.163°E, 3167 m). Planktic (Globigerina bulloides, Globigerina inflata and Globrotalia tuncatulinoides) and benthic foraminifera (Cibicidoides kullenbergi and Cibicidoides lobatulus) were used to track the δ¹³C composition of surface, subsurface, and deep ocean DIC across the deglacial period. G. bulloides δ¹³C indicates a large and abrupt excursion to more negative surface ocean values (by ~1.5‰) at the initiation of deglacial warming and glacial ice retreat (evidenced by the δ¹⁸O record). The magnitude and timing of this δ¹³C excursion is reflected in the subsurface-dwelling G. inflata but not in the deeper-dwelling G. truncatulinoides record. At the same time, benthic δ¹³C values in 34GGC (3167 m) climbed from a glacial low of -0.60‰ to 0.40‰ by the end of the glacial termination. In contrast, benthic δ¹³C values in 50GGC (1118 m) remain flat throughout the deglaciation. We suggest that the δ¹³C-depleted signal in the surface and shallow subsurface, combined with the progressive increase in deep ocean δ¹³C, reflects the upwelling of a ¹²C-rich DIC pool from the deep ocean at the beginning of the deglaciation. Additionally, the low variability in δ¹³C of G. truncatulinoides and benthic foraminifera from 1118 m depth suggest well ventilated mode waters throughout the deglacial period. Taken together, our new data provide evidence of a deglacial SO ventilation signal within the subpolar Indian Ocean.