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A lake sediment record from southern Thailand: monsoon variability and ecosystem changes since 18 ka by Camilla Bredberg

Date and time: Thursday, June 18 at 10.00
Place: Högbomsalen, Geohuset (link to the house plan)

Chair: Professor Alasdair Skelton
Examiners: Professors Christoph Humborg and Carl-Magnus Mörth

Several recent studies have suggested that abrupt latest Pleistocene climatic events in the northern North Atlantic have influenced the intensity of the Indian Ocean Monsoon. These cold climate events in the North Atlantic are thought to have generated a weaker Indian Ocean Monsoon with less precipitation. This teleconnection, however, remains debated. The 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 7.37 m long composite sediment sequence from Lake Nong Thale Pron in Southern Thailand (8˚10'N, 99º23'E). An age model has been established, revealing that the sequence encompasses the last deglaciation and most of the Holocene (18,438–1,291 cal yrs BP). Sediment composition, geochemical parameters and macrofossil remains in this sequence revealed three abrupt shifts:
• The first and oldest shift occurred at the transition from a late glacial stadial period with a weaker Indian Ocean Monsoon and a drier climate, to the Bølling-Allerød interstadial with a strengthening of the Indian Ocean Monsoon and a wetter climate.
• The second marked shift occurred at the transition from the Bølling-Allerød interstadial to the Younger Dryas stadial according to the adopted age model, and involves a clear change from a C4-dominated to a C3-dominated ecosystem. The change can be explained by a weakened Indian Summer Monsoon together with the deglacial 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 going to a system with a more terrester ecosystem. This supports the idea of a weaker Indian Summer 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 Ocean Monsoon, which appears to be ultimately 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.

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