June 11, 2010

A High-Resolution Asian Monsoon Record From 16.2 To 7.3 Thousand Years Before Present

Research at the School of Geographical Sciences, Southwest University (SWU) in Chongqing, China-Research, has demonstrated that the record of the Asian Summer Monsoon (ASM) covers the last deglaciation and the early Holocene (from 16.2 to 7.3 ka BP), with an average oxygen isotope resolution of 9 years (issue 53, May 2010 of SCIENCE CHINA Earth Sciences).

Understanding the factors responsible for past climatic changes is a key to understand future climate change. Such climatic changes include abrupt events that take place over time scales of centuries to decades. Evidence of these rapid changes is present in the geologic record as distinctive signatures in marine sediments, ice cores, loess, lake sediments, tree rings, and cave sediments. For example, recent studies have used cave stalagmite records to study monsoon history. Nevertheless, well-dated, continuous, and high-resolution records are not common.

In this work, a high-resolution (9-year intervals, on average) record of the ASM was established covering the time range from 16.2 to 7.3 ka BP. Generally speaking, a high-resolution record is important to identify the mechanisms responsible for changes in the monsoonal climate. Dr. Yang reported a record based on 33 U/Th dates and 1020 oxygen isotopes from stalagmite Y1 from Yamen Cave (107°54"²E, 25°29"²N), Guizhou Province, China. This region is currently influenced by both the East Asian and the Southwest Indian monsoons.

The main millennial-scale deglacial events first identified in Greenland (Greenland Interstadial Events: GIS 1e through GIS 1a), and later in China, are clearly present in the stalagmite Y1 record. Analogous to earlier work, these are referred to as Chinese Interstadials (CIS): CIS A.1e to CIS A.1a. The onset of these events in Y1 ÃŽ´18O records are nominally dated at: 14750±50, 14100±60, 13870±80, 13370±80, and 12990±80 a BP. The end of the CIS A.1a or the beginning of the Younger Dryas (YD) event is nominally at 12850±50 a BP, and the end of the YD dates are 11500±40 a BP. The ÃŽ´18O values shift to about 3"° during the transition into the Bølling-Allerød (BA, the onset of CIS A.1e) and at the end of the YD. Comparisons of Y1 to previously published early Holocene records show no significant phase differences. Thus, the East Asian and the Indian monsoons do not appear to have been out of phase during this interval. The Y1 record confirms earlier work suggesting that solar insolation and North Atlantic climate both affect the Asian Monsoon.

"This paper not only enriches the high-resolution record of this period for paleo-climatic reconstruction, but based on this time-scale for discussion of the characteristics of climate changes, it also has important scientific significance" said one journal reviewer. "Stalagmite Y1 record has higher time resolution than others in the study region, which can be more subtle to the discussion of scientific issues, such as the phase differences of the Asian Monsoon in the past" said another reviewer.


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