A Lake Sediment Record from Southern Thailand: Monsoon Variability and Ecosystem Changes Since 18 ka

Abstract:

Several recent studies have suggested that abrupt late Pleistocene climatic events in the northern North Atlantic have influenced the intensity of the Indian Monsoon. Cold climate events in the North Atlantic are thought to have generated a weaker Indian Monsoon with less precipitation. This teleconnection, however, remains debated. Knowledge of how the vegetation in the Indo Pacific Warm Pool has responded to past rainfall variations is limited since there are few well dated paleoclimate archives from the area. Here data are presented from a sediment sequence from a lake in southern Thailand. An age model has been established, revealing that the sequence encompasses the last deglaciation and most of the Holocene. Sediment composition, geochemical parameters and macrofossil remains in this sequence revealed three abrupt shifts. The first shift occurred at the transition from a late glacial stadial period with a weaker Indian Monsoon and a drier climate, to the Bølling-Allerød interstadial with a strengthening of the Indian Monsoon and a wetter climate. The second shift occurred at the transition from the Bølling-Allerød interstadial to the Younger Dryas stadial and involves a clear change from a C₄-dominated to a C₃-dominated ecosystem. The change can be explained by a weakened Indian Monsoon together with flooding of the Sunda Shelf, leading to less seasonal variability in precipitation that resulted in an advantage for C₃ plants. The third shift occurred during the Younger Dryas stadial and reflects a change to a more terrestrial ecosystem. This supports the idea of a weaker Indian Monsoon, which may have been caused by climate events in the Northern Hemisphere. These distinct shifts are considered to be linked to the intensity of the Indian Monsoon. They ultimately appear to be caused by the combined effect of temperature anomalies in the Northern Hemisphere and to changes in sea level due to flooding of the Sundaland Shelf.