CAN SULFUR IN LACUSTRINE SEDIMENTS BE USED AS A PROXY FOR RAINFALL?

Although water balance in terrestrial settings is an important climate parameter, relatively few proxies are available for reconstructing effective moisture. Here, we investigate the possibility of using sulfur speciation as a proxy for paleo-rainfall in sediments recovered from a large paleo-lake in the Bogota Basin, Colombia and spanning the last 600,000 years. Samples were digested with a mild acidic solution to extract acid-soluble sulfur minerals. Sulfur species extracted through this procedure included monosulfides, whereas sulfur in the remaining acid-insoluble fraction was found to be bound to organo-sulfur compounds. Monosulfide/total sulfur ratios range from 0.2 to 0.9 in the studied sediments and exhibit a cyclic distribution with depth. Low (<0.4) monosulfide/total sulfur ratios are characteristic of glacial intervals, whereas interglacial intervals exhibit both low and high ratios. Because partitioning of sulfur between monosulfides and organo-sulfur compounds depends on iron availability, we interpret that elevated iron delivery to the paleo-lake occurs at relatively high precipitation rates and results in the sequestration of sulfur in monosulfide minerals. Conversely, incorporation of sulfur into organic matter occurs at low precipitation rates when iron supply is low. Time-series analysis of sulfur ratios reveals the influence of orbital parameters (i.e., obliquity and precession). Given that rainfall in the tropics is primarily associated with the passage of the Inter-tropical Convergence Zone (ITCZ), we conclude that obliquity and precession exert a significant control on the intensity or zonality of the ITCZ that ultimately affects rainfall patterns in the Colombian Andes.