Untitled Document
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