Reconstructing hydrologic changes in Indonesia by deuterium isotope compositions of plant waxes

The Indonesian region plays an essential role as a center of atmospheric heat and moisture source of the Earth's climate system and is known for its characteristic abundant rainfall throughout the year. Here, we examine hydrological variability during the Late Quaternary in Indonesia from a NW-SE transect from Sumatra to Flores based on compound-specific plant leaf wax isotope measurements. The five investigated coring sites are located on the continental shelf in the Indian Ocean, spanning the last 20 ka. The transect covers a pronounced gradient in climate and vegetation. Sumatra is located in an area with sufficient rainfall throughout the year, whereas in eastward direction rainfall decreases and a dry season exist. Compound specific analyses of n-alkanes of plant leaf waxes (D/H and δ13C) give insights into continental hydrologic changes and the distribution of C3 and C4 vegetation. We compared data from 4 time intervals that characterize the last glacial maximum (20 ka), the Heinrich event 1 (15 ka) and the Holocene with maximum in summer insolation (8 ka) and minimum in summer insolation at 30°N (3 ka). Our results show that estimated D/H values of precipitation, i.e., by vegetation-correction of compound-specific D/H values of n-alkanes, in the Holocene (3 and 8 ka) coincide with modern averaged D/H values of rain water. In contrast, during glacial times the results deviate significantly to lighter values, which would indicate higher rainfall intensity due to the amount-effect. The largest deviations to lighter values are found in the Java region, whereas Sumatra and Flores shows only slightly changes. Previous studies and the δ13C values of the n-alkanes, however, suggest that climate in Indonesia, especially in the eastern part, was drier during the last glacial, including Heinrich 1, and C4 grasses were more abundant, respectively. This discrepancy must be explained by other effects on the isotopic composition of rainfall that occurred in the Indonesian region. Ongoing work is dedicated towards an accurate understanding of present and past hydrological processes in Indonesia and will be presented at the meeting.