Deep water [CO32-] and circulation in the south China sea over the last glacial cycle

Records of carbonate ion concentration ([CO32-]) and benthic carbon isotope (delta C-13) have profound implications for exploring deep water circulation and global carbon cycle. However, information on the glacial-interglacial deep ocean [CO32-] changes is still limited, especially in the Pacific Ocean. Here, we present new deep water [CO32-] records of the last glacial cycle, reconstructed by the B/Ca ratios of benthic foraminifera Cibicidoides wuellerstorfi from two cores in the South China Sea (SCS). The deep water [CO32-] changes of the SCS display similar trends with that of the western Pacific as well as "Pacifictype" CaCO3 (%), suggesting that the deep water carbonate chemistry of the SCS may reflect that of the Pacific. Our results support that carbonate dissolution is the primary driver for "Pacific-type" CaCO3 preservation during the late Quaternary glacial cycles. At the marine isotope stage (MIS) 5a to MIS 4 transition, deep water [CO32-] decreased with an amplitude of similar to 7 mu mol/kg in the SCS, roughly consistent with that from the Pacific cores at 2300-4300 m water depth. Considering a greater oceanic alkalinity and stronger carbonate dissolution at that time, this implies that deep Pacific carbon storage increased. The gradients of deep water [CO32-] and delta C-13 between the Pacific and the SCS reduced during extreme glacials (e.g., MIS 2, 4 and 6) relative to interglacials (e.g., MIS 1 and 5e), indicating weakened deep ocean ventilation and more sluggish Pacific deep-water circulation. This change is also confirmed by meridional delta C-13 distributions in the western Pacific for the Holocene and the last glacial maximum (LGM), and further supported by climate simulations using a complex Earth System Model. Overall, our results suggest that the increased deep Pacific carbon storage correlated to increased oceanic alkalinity and more sluggish Pacific deep-water circulation would have contributed to the atmospheric CO2 decrease during the last glacial period. (C) 2020 Elsevier Ltd. All rights reserved.