Stable isotope and trace element ratios of bulk sediment and planktic foraminifera in ODP Hole 121-758A and IODP Hole 353-U1443A

The early Paleogene represents the most recent interval in Earth's history characterized by global greenhouse warmth on multi-million year timescales, yet our understanding of long-term climate and carbon cycle evolution in the low latitudes, and in particular the Indian Ocean, remains very poorly constrained. Here we present the first long-term sub-eccentricity-resolution stable isotope (δ13C and δ18O) and trace element (Mg/Ca and B/Ca) records spanning the late Paleocene–early Eocene (~58–53 Ma) from a surface–deep transect of the northern Indian Ocean, resolving late Paleocene 405-kyr paced cyclicity and a portion of the PETM recovery. Our new records reveal a long-term warming of ~4–5°C at all depths in the water column, with absolute surface ocean temperatures and magnitudes of warming comparable to the low latitude Atlantic and Pacific. As a result of warming, we observe a long-term increase in δ18Osw of the mixed layer, implying an increase in net evaporation. We also observe a collapse in the temperature gradient between mixed layer- and thermocline-dwelling species, potentially due to either the development of a more homogeneous water column with a thicker mixed layer, or depth migration of the Morozovella in response to warming. Synchronous warming at both low and high latitudes, along with decreasing B/Ca ratios in planktic foraminifera implying falling ocean pH and/or increasing dissolved inorganic carbon, suggest that global climate was forced by rising atmospheric CO2 concentrations during this time.

早古近纪是地球历史中最近一段以百万年级别全球温室变暖为特征的时期，但目前学界对低纬度地区，尤其是印度洋的长期气候与碳循环演化的认知仍极为匮乏。本文首次发布了北印度洋表层-深层断面获取的、覆盖晚古新世-早始新世（约5800万至5300万年前）的长期次偏心率分辨率稳定同位素（δ¹³C与δ¹⁸O）及微量元素（Mg/Ca与B/Ca）记录，解析了晚古新世时期40.5万年轨道偏心率驱动的旋回性信号，以及古新世-始新世极热事件（PETM）的部分恢复阶段。新的同位素与微量元素记录显示，海水水柱各深度层均存在约4~5℃的长期升温，绝对表层海水温度与升温幅度与低纬度大西洋、太平洋的观测结果相当。受此次长期变暖影响，混合层海水的δ¹⁸Osw（海水氧同位素组成）呈现长期升高趋势，指示区域净蒸发作用增强。同时研究团队观测到混合层栖息类群与温跃层栖息类群之间的温度梯度消失，这一现象可能源于两方面原因：一是混合层增厚导致水柱均一性增强，二是磨拟抱球虫属（Morozovella）因海水变暖发生深度迁移。高低纬度同步变暖，加之浮游有孔虫壳体中B/Ca比值下降（指示海水pH降低与/或溶解无机碳含量升高），表明该时期全球气候演化受大气二氧化碳浓度持续升高所驱动。