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Past glacial cycle monsoon variation recorded in speleothems from Flores, Indonesia

L. K. Ayliffe1, M. K. Gagan1, M. T. McCulloch1, G. E. Mortimer1, J-x. Zhao2, J. Hellstrom4, R. N. Drysdale3, S. Lewis1, W. S. Hantoro5, B. W. Suwargadi5


1 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia.
2 Radiogenic Isotope Laboratory, Centre for Microscopy and Microanalysis, University of Queensland, St. Lucia, Queensland 4072, Australia.
3 Environmental and Climate Change Research Group, University of Newcastle, Callaghan, NSW 2308, Australia.
4 School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia.
5  Research and Development Center for Geotechnology, Indonesian Institute of Sciences, Jalan Cisitsu No. 21/154 D, Bandung 40135, Indonesia.

Figure 1. Speleothem δ18O records from Flores (8°32'S, 120°27'E) (stalagmites (blue) & flowstones (black)) compared with summer insolation at 8.5°S (red). δ18O values are not corrected for the effects of changes in global ice volume or regional temperatures. Ice volume changes would amount to corrections of ~+1‰ (at 20ka) and ~+0.6‰ (at 50k) to speleothem calcite δ18O values. Temperature reductions of ~2-3.5°C (such as was likely experienced during the LGM at the site) would also result in further corrections of +0.4‰ to +0.7‰ calcite δ18O values.


Tropical speleothems are ideal archives of changes in past rainfall as they can be dated precisely with the U-Th technique and their δ18O values can be interpreted in terms of rainfall intensity as tropical rainfall δ18O values are inversely proportional to rainfall amount (Dansgaard, 1964). Here we present some preliminary δ18O results over the past ~90ka from speleothems from the island of Flores, Indonesia. Located at the southern-most extent of the Intertropical Convergence Zone (ITCZ) in the Austral summer and just within the current southern boundary of the Western Pacfic Warm Pool (WPWP) Flores is ideally situated to record changes in each of these major climate systems.

Several stalagmites and flowstones were collected from deep within Liang Luar Cave (8°32'S, 120°27'E) in 2006 and 2007. Oxygen isotope results of two stalagmites and two flowstone cores are shown in Figure 1. As can be seen good agreement is observed between the oxygen isotopes of the faster growing stalagmites and the much slower growing flowstones where they overlap in age. This suggests that speleothem calcite is being deposited under conditions of near isotopic equilibrium in this cave environment.

Although much work still remains to firmly establish the chronology of the >50ka flowstone record, some interesting trends are emerging from the younger portion of our speleothem record. In contrary to what might be expected, monsoon intensity appears not markedly reduced during past cool phases such as the LGM and stage 3 compared to the Holocene. Rather at times of lowered sea level, monsoon intensity appears more directly coupled to southern hemisphere insolation changes compared to times of high sea level, Fig. 1. This effect could be the result of increases in the degree of continentality experienced at times of low sea level, and consequent greater impact of insolation changes on monsoon strength.

 


Dansgaard W. (1964) Stable isotopes in precipitation. Tellus 16:436-468
Sturman A, Tapper N. (1996) The Weather and Climate of Australia and New Zealand. Oxford University Press: Oxford