We present a continuous record of the Asian monsoon over the last 16 ka from δ18O measurements of stalagmite calcite. Over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230Th dates. δ18O and therefore Asian monsoon intensity generally follows changes in insolation, although changes in δ18O are generally accommodated in abrupt shifts in contrast to smoothly varying insolation, indicating that threshold effects may be important. δ18O decreased dramatically (∼3‰) at the start of the Holocene (∼11.5 ka) and remained low for ∼6 ka. Four positive δ18O events centered at 11225±97 yr BP (1.05‰), 10880±117 yr BP (1.15‰), 9165±75 yr BP (1.4‰), and a double event centered at 8260±64 yr BP (1.1‰) and 8080±74 yr BP (1.0‰) punctuated this period of high monsoon intensity. All four events correlate within error with climate changes in Greenland ice cores. Thus, the relationship between the Asian monsoon and the North Atlantic observed during the glacial period appears to continue into the early Holocene. In addition, three of the four events correlate within error with outburst events from Lake Agassiz. The decline of monsoon intensity in the mid-late Holocene is characterized by an abrupt positive shift in δ18O which occurs at 3550±59 yr BP (1.1‰ in ∼100 yr). In addition, the Holocene is punctuated by numerous centennial- and multi-decadal-scale events (amplitudes 0.5 to 1‰) up to half the amplitude of the glacial interstadial events seen in the last glacial period. Thus, Holocene centennial- and multi-decadal-scale monsoon variability is significant, although not as large as glacial millennial-scale variability. The monsoon shows a strong connection with northern South American hydrological changes related by changes in ITCZ position. Spectral analysis of the δ18O record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing. However, there are numerous other significant peaks including peaks at El Niño frequencies (observed for high-resolution portions of the record between 8110 and 8250 yr) which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling Holocene monsoon climate. In addition, for this high-resolution portion, we observe a distinctive biennial oscillation of the Asian monsoon, which has been associated with the Tropospheric Biennial Oscillation (TBO).
Bibliographical noteFunding Information:
We are grateful to G. Comer and W. Broecker for their generous support of our work and Broecker's suggestion of possible links to outflow events. Financial support for this research was funded by NSF grants ESH0214041 and MRI0116395; a Gary Comer Science and Education Foundation Grant (CC8); the National Science Foundation of China grants 40328005 and 40231008; and grants from the Ministry of Land and Resources and the Ministry of Science and Technology of China.
- Asian monsoon
- Inductively coupled plasma