Abstract The long-term variability of the Indian monsoon in the Bay of Bengal remains inconclusive due to the lack of proximal sedimentary records. To further elucidate the long-term variability of the Indian monsoon, we analyzed the paleoproductivity regime over the last 2.3 Myr at the International Ocean Discovery Program (IODP) Expedition 353 Site U1445 located near the Mahanadi Basin in the northwestern Bay of Bengal. We measured the downcore concentrations and mass accumulation rates (MARs) of biogenic opal, CaCO3, total organic carbon (TOC), and total nitrogen over the Mid-Pleistocene Transition (MPT) to identify the links between surface water marine biogenic production and the Indian summer monsoon. TOC MARs were found to reflect both surface water marine productivity and terrestrial organic matter through the measurements of sediment C/N ratios and δ13CSOM values. Nonetheless, we identified a shift in the paleoproductivity regime from a dominance of biogenic opal deposition prior to the MPT to the dominance of CaCO3 deposition following the MPT. The shift in biogenic marine productivity across the MPT was closely related to riverine discharge, which was primarily controlled by the intensity of the Indian monsoon. Our results, therefore, infer a decrease in riverine discharge to the Bay of Bengal across the MPT in response to a weakened Indian summer monsoon (and/or strengthened Indian winter monsoon). In addition, changes in the intensity of the Indian monsoon across the MPT were more closely linked to the global climate cooling rather than the gradual uplift of the Himalaya and Tibetan Plateau.

中文翻译：

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孟加拉湾西北部（IODP Site U1445）在中更新世过渡期间的古生产力转变，以响应印度夏季风的减弱

摘要 由于缺乏近端沉积记录，孟加拉湾印度季风的长期变化尚无定论。为了进一步阐明印度季风的长期变化，我们在位于孟加拉湾西北部马哈纳迪盆地附近的国际海洋发现计划 (IODP) 远征 353 站点 U1445 中分析了过去 2.3 Myr 的古生产力状况。我们测量了中更新世过渡 (MPT) 中生物蛋白石、CaCO3、总有机碳 (TOC) 和总氮的下行浓度和质量积累率 (MAR)，以确定地表水海洋生物生产与印度夏季风。通过测量沉积物 C/N 比和 δ13CSOM 值，发现 TOC MAR 可以反映地表水海洋生产力和陆地有机质。尽管如此，我们发现古生产力制度从 MPT 之前的生物蛋白石沉积的主导地位转变为 MPT 之后的 CaCO3 沉积的主导地位。跨 MPT 的生物海洋生产力的转变与河流排放密切相关，河流排放主要受印度季风强度的控制。因此，我们的结果推断，由于印度夏季风减弱（和/或印度冬季风增强），MPT 向孟加拉湾的河流流量减少。此外，