A MULTI-PROXY RECORD OF CLIMATE VARIABILITY AND ANTHROPOGENIC ACTIVITY FROM MAAR LAKE SEDIMENTS IN EAST JAVA, INDONESIA

Understanding rainfall variability in Java, Indonesia can provide important insights into the interactions between globally significant climate systems such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the East Asian monsoon system. Ranu Logung, a maar lake in East Java, Indonesia, is an ideal lake for high-resolution paleoclimate reconstruction because of its high sedimentation rates (up to 1.5 cm/yr), sensitivity to changes in hydrology, and laminated sediments. Past and present climate variations in Java trigger food insecurity in one of the most densely inhabited regions on Earth, yet these human populations have also significantly altered the Javan landscape during the past millennium. In order to use data from lake sediments to infer past climate variability, and to differentiate that from impacts of anthropogenic activity, it is essential to understand how changes in anthropogenic activity and climate control in-lake processes and how they are reflected in the sediment record. Trends in the δ¹³C and δ¹⁵N of organic matter preserved in the sediments of Ranu Logung compared with sediment compositional data provide valuable insights into the response of in-lake processes to changes in regional climate and anthropogenic activity over the past ~1400 years. Calcium carbonate concentrations exhibit four pronounced increases that are concurrent with significant enrichments in the δ¹³C of organic matter, providing evidence for four multidecadal to centennial-scale droughts ca. 1030, 1550, 1825, and 1997 CE. These droughts are superimposed on a long-term trend toward more depleted δ¹³C of organic matter, which suggests a gradual moistening trend in Eastern Java during the past millennium consistent with evidence for the southward migration of the Intertropical Convergence Zone (Zhang et al., 2008, Science 322: 940). Pronounced isotopic enrichment in δ¹⁵N is concurrent with abrupt increases in Ti abundance, increased magnetic susceptibility, the appearance of siderite, and decreased C:N ratios and biogenic opal abundance around 1860 CE, indicating changes in the lacustrine ecosystem in response to land clearance and increased human activity within the lake catchment.