Toward a 530,000-year Hydroclimate History for the Southern Half of the Australasian Monsoon

Abstract:

Speleothem ¹⁸O/¹⁶O records have revealed key aspects of past hydroclimates in the northern Australasian monsoon domain on orbital to millennial scales, but much less is known about the southern half of the monsoon system. We aim to develop a hydroclimate history for the southern Australasian monsoon based on speleothems from southwest Sulawesi and Flores, Indonesia (latitudes 5-9^oS), which extend back to ~530 kyr BP and 90 kyr BP, respectively.

To date, the ¹⁸O/¹⁶O record for Sulawesi covers glacial terminations TIV (~340 kyr BP), TIII (~245 kyr BP) and TI (~18 kyr BP). The details of each termination are different, however two important hydroclimate patterns are emerging. First, the ¹⁸O/¹⁶O record shows sharp weakening of the monsoon immediately before each termination. This surprisingly robust pattern marks a southern extension of the northern ‘weak monsoon interval’, and reinforces the idea that southward monsoon displacement is a fundamental feature of terminations. Second, monsoon intensification around Sulawesi lags the rise in atmospheric CO₂ and Antarctic temperature by several thousand years, but parallels the ¹⁸O/¹⁶O decrease in atmospheric O₂. Our finding extends that of Wang et al. (2008) and Cheng et al. (2009) who noted the influence of the low-latitude hydrological cycle on the ¹⁸O/¹⁶O of tropical transpiration, and its potential for correlating ice core and paleomonsoon records.

Further south, the 90-kyr ¹⁸O/¹⁶O record for Flores shows clear precession-scale antiphasing with China, and southerly positioning of the summer monsoon rainfall belt during Heinrich stadials. Heinrich stadials 5, 4, 2 and 1 occur during wetter intervals in Flores that accompanied relatively high southern summer insolation. Intriguingly, these events are associated with abrupt atmospheric CH₄ signals that may be due to increased Southern Hemisphere CH₄ production related to intensification of monsoon rainfall over southern tropical land areas (Rhodes et al., 2014).