Carbon isotope ratios of tropical freshwater mollusk shells as indicators of ancient vegetation; reconstructing paleoenvironments at a Homo erectus locality in the Solo Basin, central Java, Indonesia; Geological Society of America, 2005 annual meeting

Using a variety of techniques, (grain size analysis and delta (super 13) C values of paleosols, delta (super 13) C and delta (super 18) O values of carbonate soil nodules, and freshwater mollusk shells) we have begun to reconstruct the paleoenvironments during the early to middle Pleistocene at a Homo erectus fossil locality in the Sangiran Dome located in the Solo Basin of central Java, Indonesia. H. erectus fossils occur from the upper part of the Sangiran Fm. to the middle to upper Bapang Fm. Soil samples and carbonate nodules were collected from paleosols within the Sangiran and Bapang Fms. and freshwater mollusk shells were collected from the upper Sangiran Fm. The upper Sangiran Fm. has a fossil assemblage and sediment and paleosol characteristics indicating a paludal to lacustrine depositional environment, while the Bapang Fm. is sandy braided stream deposit with multiple fining upward cycles. The Bapang Fm. yields a variety of mammalian fossils yet contains no freshwater mollusks. Freshwater mollusks and soil carbonate nodules associated with H. erectus fossils from the Solo Basin provide a direct measure of riparian vegetation approximately 1.66 to 1.5 Ma. These carbonates consistently have delta (super 13) C values that are approximately 10 to 11 per mil higher than associated plant organic matter. The first occurrence of H. erectus in the Upper Sangiran Fm. coincides with wet conditions and C4 plant carbon isotope ratios. The last occurrence of H. erectus in the Bapang Fm. coincides with dry conditions and a C3 plant carbon isotope ratios. The transition from C4 to C3 values occurs gradually within the lower portion of the Bapang Fm. after the depositional environment has changed. Modern mollusks collected from local streams have a C3 signal similar to the upper portion of the Bapang Fm. Modern conditions are controlled by monsoonal rainfall with pronounced dry and rainy seasons. The dry season (May-Oct.) occurs when the ITCZ shifts to the north and dry air masses off Australia dominate. The rainy season (Nov. to April) occurs when the ITCZ shifts southward and humid air masses off SE Asia dominate. We conclude that the transition from C4 to C3 plants in the Solo Basin occurred when the ITCZ shifted northward (perhaps during a transition from a glacial to interglacial). High resolution stable isotope data from freshwater mollusks is being collected to examine seasonal rainfall during these transitions in vegetation.