NOAA/WDS Paleoclimatology - Madagascar Speleothem Stable Isotope Data from 13200-11280 year BP

As prominent abrupt climatic events during the last deglaciation and the early Holocene, the Younger Dryas (YD) and the 8.2 ka events have been intensely discussed to reveal the relationship between their phases and intensities, and their underlying mechanisms based on massive marine and terrestrial archives. However, the related paleoclimate records with sufficient resolution and/or precise age constraints from the Southern Hemisphere, especially East Africa, are relatively sparse, hindering our comprehensive understanding about the phases of these two events. Here, we provide a precisely dated record of an aragonite-calcite stalagmite covering 11.3–13.5 ka BP from northwest Madagascar to unravel the arid conditions during the YD, in contrast to the pluvial conditions in the 8.2 ka event that has been evidenced before. Changes in austral summer precipitation related to the Intertropical Convergence Zone (ITCZ) have always been interpreted to be the primary means of controlling regional rainfall amounts and thus the d18O variations in stalagmite. However, ITCZ’s meridional migration alone is not enough to interpret the opposite hydroclimatic conditions during the YD and the 8.2 ka events in northwest Madagascar. The variation in convection intensity within the ITCZ combined with the rainfall dipole mode in East Africa, and the redistribution of the duration of the ITCZ’s presence at different latitudes might be responsible for this phenomenon. In addition, sea surface temperature could play a nonnegligible role.

作为末次冰消期与早全新世期间的典型突变气候事件，新仙女木事件（Younger Dryas, YD）和8.2 ka事件一直是学界研究的热点：研究者依托海量海洋与陆地沉积记录，深入探讨了二者的发生阶段、强度特征及其内在驱动机制。然而，南半球（尤其是东非地区）具备足够分辨率与精确定年约束的相关古气候记录相对匮乏，这极大阻碍了我们对这两类事件发生阶段的全面认知。
本研究针对马达加斯加西北部一处覆盖11.3~13.5 ka BP的文石-方解石混合质石笋，提供了精确的定年记录，以此解析新仙女木事件期间的干旱环境——这与此前已被证实的8.2 ka事件期间的多雨环境形成鲜明对比。
长期以来，学界普遍认为，与热带辐合带（Intertropical Convergence Zone, ITCZ）相关的南半球夏季降水变化，是控制区域降水量及石笋δ¹⁸O同位素组成变化的核心因素。但仅依靠热带辐合带的经向迁移，无法解释马达加斯加西北部新仙女木事件与8.2 ka事件期间截然相反的水文气候特征。本研究推测，热带辐合带内部对流强度的变化、东非地区降水偶极子模态，以及热带辐合带在不同纬度停留时长的重新分配，或是造成这一现象的关键原因。此外，海表温度也可能发挥了不可忽视的作用。