Australia: The Land Where Time Began

A biography of the Australian continent 

Holocene Changes in Australian-Indonesian Monsoon Rainfall - Stalagmite Evidence from Trace element & Stable Isotope Ratios

The authors¹ have studied trace element and stable isotope ratios from an active stalagmite, LR06-B1, that was recovered from Liang Luar Cave, Flores Island, eastern Indonesia to establish the location in the austral summer of the inter-tropical convergence zone and the variability of the Australian-Indonesian and its variability during the austral summer in the Holocene. The stalagmite was found to have began growing 12,640 years ago, based on uranium-series dating, with hiatuses at 8,560-6,420 and 3,670-2,780 years ago. Throughout the record the Mg/Ca ratios in the stalagmite, and the Sr/Ca ratios were found to correlate significantly with one another and with δ¹⁸O and δ¹³C, suggesting prior calcite precipitation dominates the Mg/Ca and Sr/Ca ratios, a process that involves degassing in the vadose zone in periods of low recharge that cause calcite deposition and a disproportionate loss of Ca²⁺ ions, relative to Mg²⁺ and Sr²⁺, 'upstream' of the stalagmite. Recharge of the overlying aquifer appears to have controlled the degree of initial disequilibrium of the ²³⁴U/²³⁸U. A generally drier Early Holocene, that coincided with lower sea levels and lower insolation in the Southern Hemisphere summer, is implied by the uranium isotope activity ratios, ²³⁴U/²³⁸U, as well as the values of Mg/Ca, Sr/Ca and δ¹⁸O.

According to the authors¹ when speleothem δ¹⁸O time series from Flores are compared with those from Borneo shows that through much of the Holocene they varied in unison. A difference between the Flores record and that from Borneo is that in Borneo there is a significant decrease in the δ¹⁸O record at about 6,000-4,000 years ago that is not present in the record from Flores. It is suggested by the authors¹ that this may be related to a change in the circulation of the Australian-Indonesian summer monsoon as a response to a protracted positive phase of the Indian Ocean Dipole. The result of this scenario would be stronger upwelling off western Indonesia, based on effects operating at the present, with summer convective activity over Flores being reduced and subsequently the intertropical convergence zone would shift to the north.

See Collapse of Prehistoric Aboriginal Society in Northwestern Australia Triggered by an ENSO Mega-Drought

Sources & Further reading

1. Griffiths, Michael L., Russell N. Drysdale, Michael K. Gagan, Silvia Frisia, Jian-xin Zhao, Linda K. Ayliffe, Wahyoe S. Hantoro, et al. "Evidence for Holocene Changes in Australian–Indonesian Monsoon Rainfall from Stalagmite Trace Element and Stable Isotope Ratios." Earth and Planetary Science Letters 292, no. 1–2 (3/15/ 2010): 27-38.