Holocene Shifts in Southeast Asian Hydrology Recorded in Speleothems from Laos
Abstract
Rainfall in the tropical Indo-Pacific region is controlled by a suite of complex ocean-atmospheric phenomena, such as the Intertropical Convergence Zone (ITCZ), the Asian monsoon system, and the El Niño Southern Oscillation. Whilst the evolution of the East Asian Summer Monsoon (EASM) through the Holocene has been well documented from proxy records in China, very few high resolution paleo records exist from farther south in Mainland Southeast Asia. Consequently, additional records are needed to obtain a comprehensive picture of the spatial variability of the EASM system through the Holocene. Here we present the first high-resolution Holocene record of southern EASM evolution compiled from multiple speleothems collected from Tham Mai Cave in northern Laos (20.75 N, 102.65 E). The stable isotope record is tied to a chronology constructed from a total of 9 U-Th dates, which indicate the record extends from 4 to 10.1 thousand years before the present (ka). The δ 18O record displays an abrupt decrease in values at 9.2 ka, followed by a monotonic increase through the mid to late Holocene. We interpret these patterns to reflect a sudden increase in EASM intensity and/or rainfall amount during the early Holocene, followed by a general weakening through the remainder of the record. This decreasing trend in speleothem δ 18O is in strong agreement with the records in China, thus suggesting that the 'southern half' of the EASM system was also controlled by Northern Hemisphere summer insolation. In addition, we find positive δ 18O excursions around 8.2 ka and 9.2 ka, indicative of a sudden weakening of the monsoon coincident with North Atlantic cooling. This finding, in line with previous results from the region, suggests that abrupt temperature shifts in the high northern latitudes controlled the latitudinal position and/or strength of the boreal summer ITCZ. Given the potential convoluting influence of moisture source region, transport history, and other factors on speleothem δ 18O in this region, however, we utilize speleothem δ 13C as an additional proxy that should primarily reflect local hydrology. δ 13C results also indicate dry conditions around 8.2 and 9.2 ka, but suggest an increasing rainfall trend towards the middle Holocene, suggesting that the δ 18O values may be influenced by changes in moisture source region and/or upstream rainout and not simply monsoon strength or rainfall amount.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP41A1991Y
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1833 HYDROLOGY / Hydroclimatology;
- 3344 ATMOSPHERIC PROCESSES / Paleoclimatology;
- 4958 PALEOCEANOGRAPHY / Speleothems