Stable hydrogen and carbon isotopes of C37 alkenones during Termination I and II

At the southern tip of Africa, the Agulhas Current reflects back into the Indian Ocean causing so-called "Agulhas rings" to spin off and release relatively warm and saline water into the South Atlantic Ocean. Previous reconstructions of the dynamics of the Agulhas Current, based on paleo-sea surface temperature and sea surface salinity proxies, inferred that Agulhas leakage from the Indian Ocean to the South Atlantic was reduced during glacial stages as a consequence of shifted wind fields and a northwards migration of the subtropical front. Subsequently, this might have led to a buildup of warm saline water in the southern Indian Ocean. To investigate this latter hypothesis, we reconstructed sea surface salinity changes using alkenone dD, and paleo-sea surface temperature using TEXH 86 and UK'37, from two sediment cores (MD02-2594, MD96-2080) located in the Agulhas leakage area during Termination I and II. Both UK'37 and TEXH 86 temperature reconstructions indicate an abrupt warming during the glacial terminations, while a shift to more negative dDalkenone values of approximately 14 per mil during glacial Termination I and II is also observed. Approximately half of the isotopic shift can be attributed to the change in global ice volume, while the residual isotopic shift is attributed to changes in salinity, suggesting relatively high salinities at the core sites during glacials, with subsequent freshening during glacial terminations. Approximate estimations suggest that dDalkenone represents a salinity change of ca. 1.7-1.9 during Termination I and Termination II. These estimations are in good agreement with the proposed changes in salinity derived from previously reported combined planktonic Foraminifera d18O values and Mg/Ca-based temperature reconstructions. Our results confirm that the dD of alkenones is a potentially suitable tool to reconstruct salinity changes independent of planktonic Foraminifera d18O.在非洲南端，阿古拉斯海流（Agulhas Current）折返印度洋，进而脱离形成所谓的阿古拉斯环（Agulhas rings），并将相对温暖且高盐的海水注入南大西洋。此前基于古海表温度（paleo-sea surface temperature）与海表盐度代用指标（sea surface salinity proxies）的阿古拉斯海流动态重建研究推断，冰期（glacial stages）阶段印度洋向南大西洋的阿古拉斯输运（Agulhas leakage）因风场改变与副热带锋面（subtropical front）北移而减弱。这一过程随后可能导致南印度洋出现暖盐水体堆积。为验证这一后续假说，我们针对阿古拉斯输运区域内的两个沉积物岩芯（MD02-2594、MD96-2080），分别利用烯酮类氢同位素比值（alkenone dD）重建海表盐度变化，并用TEXH 86与UK'37指标重建古海表温度，分析时段覆盖冰消终止期I与II。UK'37与TEXH 86的温度重建结果均显示，冰消终止期内出现突发性增温；同时观测到，在冰消终止期I与II期间，烯酮类氢同位素比值（dDalkenone）向约14‰的负值方向偏移。约一半的同位素偏移可归因于全球冰量变化，剩余的同位素偏移则由盐度变化导致，这表明冰期时段岩芯站位的盐度相对较高，而在冰消终止期发生了后续的盐度淡化过程。近似估算结果显示，在冰消终止期I与II期间，烯酮类氢同位素比值对应的盐度变化幅度约为1.7~1.9。该估算结果与此前基于浮游有孔虫（planktonic Foraminifera）氧同位素比值（d18O）与镁钙比（Mg/Ca）温度重建结果所推导的盐度变化趋势高度吻合。本研究结果证实，烯酮类氢同位素比值可作为独立于浮游有孔虫氧同位素比值的海表盐度重建工具，具备潜在应用价值。