QSR：Deep water [CO3^2−] and circulation in the south China sea over the last glacial cycle

Sui Wan^abc，Zhimin Jian^a，Xun Gong^d，Haowen Dang^a，Jiawang Wu^a，Peijun Qiao^a

a State Key Laboratory of Marine Geology, Tongji University, Shanghai, 200092, China

b CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, China

c Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Science, Guangzhou, 510301, China

d Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bussestr. 24, 27570, Bremerhaven, Germany

Abstract

Records of carbonate ion concentration ([CO₃²⁻]) and benthic carbon isotope (δ¹³C) 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 “Pacific-type” 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 ∼7 μmol/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 δ¹³C 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 δ¹³C 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.  

Full Article：https://www.sciencedirect.com/science/article/pii/S0277379120304613