Untitled Document

Speleothems from Flores, Indonesia: tropical archives of climate change

L. K. Ayliffe1, M. K. Gagan1, J-x. Zhao2, R. N. Drysdale3, J. Hellstrom4, W. S. Hantoro5, H. Scott-Gagan1, J. Cowley1, G. K. Smith6, N. Anderson7, 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.
6 Spinnaker Ridge Way, Belmont 2280, NSW, Australia.
7 3 Garrard St, Spence 2614, ACT, Australia.

Very few precisely-dated paleorecords of climate from the tropics currently exist, despite the fact the tropics play a critical role in driving the Earth's large-scale atmospheric circulation by the export of heat and moisture to higher latitudes. Here we present some initial δ18O results for the past 25ka from tropical speleothems from the island of Flores, Indonesia, which is a focal point for our ARC Discovery research (Gagan et al. 2006). The island of Flores, 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 (Sturman and Tapper 1996), is sensitive to past climate change. 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).

Two stalagmites from Liang Luar Cave (8°32'S, 120°27'E) were collected in July 2006 ~500m from the cave entrance. The δ18O results of these two specimens are shown in Figure 1 together with other climate proxy records covering the past 30ka. When compared to the speleothem δ18O records from nearby Borneo (4°N, 114°E) the Flores speleothems display a somewhat different response in rainfall δ18O during the past 25ka, Figures 1 and 2. This is perhaps not surprising given the present day differences in climatology between the two sites. Borneo lies under the ITCZ year-round and exhibits little seasonality in rainfall (Cobb et al. 2007), while Flores has a distinct wet and dry season determined by the annual migration of the ITCZ.

Last Glacial Maximum (LGM) (19-23ka) δ18O values of the Borneo speleothems are 1.3±0.3‰ greater than modern values (Partin et al. 2007) while Flores speleothems are only 0.9±0.3‰ greater than modern values during the LGM. The modern-LGM δ18O differences of the Flores speleothems are less than what would be anticipated from global ice volume changes (+1‰) and ~2-3.5°C lower regional temperatures (+0.4‰ to +0.7‰). This suggests that rainfall δ18O values were lower during the LGM at the Flores site in contrast to the neighboring Borneo site. Changes in eustatic sea levels during the LGM would have increased the continentality of Flores which probably resulted in decreased rainfall d18 values (Rozanski et al. 1993) at this time. Local differences in proximity to exposed continental shelves between Flores and Borneo may explain why LGM rainfall δ18O at Borneo was not affected to the same extent as Flores by lowered sea levels.

The response of the Flores and Borneo speleothem δ18O records appear anticorrelated during the deglacial (17-10ka), Fig. 2. Peaks(troughs) in the Borneo record correspond with troughs(peaks) in the Flores speleothem record within error of the chronological errors (Partin et al. 2007). Furthermore most of these features appear to be synchronous with known climate excursions during this time interval, namely that of: Heinrich Event 1 (H1); Antartic Cold Reversal (ACR) and the Younger Dryas (YD), seen in ice core and Chinese speleothem records Fig. 1,2. Modeling results of Zhou and Delworth (2005) predict that the ITCZ migrated south in the Pacific ocean, the Walker circulation moved eastward and that the east Asian monsoon intensity weakened during H1. Negative rainfall anomalies in parts of the S/W Pacific, including Borneo, were predicted outcomes of these coupled model simulations. Although predictions for changes in rainfall are less certain for eastern Indonesia (incl. Flores) during H1, slight increases in rainfall are suggested at the less than 95% level of confidence by Zhou and Delworth (2005). If the ITCZ did move south during H1 to be located more directly over eastern Indonesia throughout the year, then this could explain the negative δ18O excursion seen in the Flores record (indicating higher rainfall) at the same time that Borneo was experiencing rainfall diminishment. The antiphase response in the Flores and Borneo records during the ACR (Flores: dryer, Borneo: wetter), YD (Flores: wetter, Borneo: dryer) and at ~11.4-11.8ka (Flores: dryer, Borneo: wetter) might also be accounted for by similar oscillations in the mean position of the ITCZ.

The last major difference between the Flores and Borneo speleothem records is observed at ~5ka when the Borneo record displays a decrease in rainfall δ18O values (increased rainfall) while the Flores 18O record shows no change, Fig. 1. Partin et al. (2007) attribute this 18O decrease to either enhanced warm pool convection from an insolation-driven increased tropical Pacific zonal SST gradient, or changes in the position of the ITCZ. The fact that the Flores speleothem record does not show the same 18O increase as the Borneo record at this time suggests that intensification of the Walker circulation is unlikely to be the principal cause, as if it were the case, similar increases in Flores rainfall δ18O might also be expected at 5ka.
Traditionally global climate models have struggled to robustly predict past climate changes in the tropics. It is hoped that additional palaeo-rainfall δ18O records, such as those presented here, will contribute significantly to improving the skill of future generations of GCM's.

Figure 1. Speleothem δ18O records from Flores (8°32'S, 120°27'E) (LR06-B3 (blue) & LR06-C6 (red)) compared with speleothem δ18O records from Borneo (4°N, 114°E) and S/E China (Dongge: 25° 17'N, 108° 5'E, Hulu: 32° 30'N, 119° 10'E) and ice core δ18O records from The North Greenland Ice core Project (NGRIP) and the European Project for Ice Coring in Antarctica (EPICA).
Figure 2. Speleothem δ18O record from Flores (LR06-C6 (red)) and Borneo (blue).

 


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