Sedimentological and carbonate data evidence for lake level variations during the past 3700 years from a southern Indian lake

Abstract

Over the years, several proxies have been developed to reconstruct rainfall variability. However, most rely on indirect approaches to provide qualitative paleorainfall estimate. In an attempt to obtain a more direct measure of paleorainfall, Shankar et al. (2006) explored the rock magnetic properties of lake sediments from Thimmannanayakanakere (TK) in tropical southern India. They proposed the use of magnetic susceptibility as a proxy for rainfall in the tropics. Warrier and Shankar (2009) provided geochemical evidence in support of this proposition. Here, sedimentological and carbonate data is provided as further evidence to bolster Shankar et al.'s (2006) proposition.High (low) values of χlf indicate high (low) rainfall in the region of TK during the past 3700years. Particle size variations suggest that the sand % was high (low) during arid (humid) periods, when the TK lake level was low (high). Hence, a negative correlation is documented between sand % and χlf along with other rock magnetic parameters. HIRM (an indicator of magnetically “hard” minerals like haematite and goethite) is suggestive of a relatively arid climate; the high (low) HIRM values in TK sediments indicate arid (humid) conditions. For this reason, sand % is positively correlated with HIRM. By contrast, fine silt and clay contents are low during low-rainfall periods and vice versa. Thus, both fine silt and clay contents are positively correlated with χlf and other rock magnetic parameters, but negatively correlated with HIRM. Magnetic minerals reside principally in the fine silt fraction of TK sediments as evidenced from the positive correlation between fine silt content and magnetic susceptibility. Carbonate content too is indicative of paleorainfall conditions, being high (low) during arid (humid) climatic conditions. Based on the χlf, sand % and carbonate % data, we have inferred lake level variations in TK during the past 3700years.