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.

中文翻译：

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末次冰期循环中南海深水[CO32−]与环流

摘要 碳酸根离子浓度 ([CO32−]) 和底栖碳同位素 (δ13C) 的记录对探索深水循环和全球碳循环具有深远的意义。然而，关于冰川-间冰期深海 [CO32−] 变化的信息仍然有限，尤其是在太平洋。在这里，我们展示了上次冰期循环的新深水 [CO32−] 记录，通过来自南海 (SCS) 两个核心的底栖有孔虫 Cibicidoides wuellerstorfi 的 B/Ca 比率重建。南海深水[CO32−]变化与西太平洋以及“太平洋型”CaCO3（%）变化趋势相似，表明南海深水碳酸盐化学可能反映了太平洋。我们的结果支持碳酸盐溶解是晚第四纪冰期循环中“太平洋型”CaCO3 保存的主要驱动因素。在海洋同位素阶段 (MIS) 5a 到 MIS 4 过渡时，SCS 中深水 [CO32−] 以~7 μmol/kg 的幅度下降，与太平洋核心在 2300-4300 m 水深的幅度大致一致。考虑到当时更大的海洋碱度和更强的碳酸盐溶解，这意味着深太平洋碳储量增加。相对于间冰期（如 MIS 1 和 5e），在极端冰期（如 MIS 2、4 和 6）期间太平洋和 SCS 之间的深水 [CO32−] 和 δ13C 梯度降低，表明深海通风减弱等太平洋深水环流缓慢。这种变化也得到了西太平洋全新世和末次冰期最大值 (LGM) 经向 δ13C 分布的证实，并得到了使用复杂地球系统模型的气候模拟的进一步支持。总体而言，我们的研究结果表明，太平洋深部碳储存量的增加与海洋碱度的增加和太平洋深水环流更加缓慢有关，这将导致上一次冰期大气 CO2 减少。