Middle to late holocene palaeo environmental shifts based on sediment records from the Pookode and Vellayani lakes Kerala South India

Abstract

Fresh water bodies such as lakes, and ponds are sinks that preserve signatures of geochemical variations derived principally from the surrounding rock weathering and soils. Sediment cores retrieved from the lakes reveal continuous sediment deposition, however only on application of radiometric dating methods, the chronology, rate and duration of sediment deposition is inferred and many times inversion of age is noted. Pookode Lake (elliptical in shape) and Vellayani Lake (linear in shape) are two natural fresh water basins situated in two different geomorphic and rainfall regimes in the Kerala state, tropical Southern India. Pookode Lake is situated at an altitude of ~770 m.a.s.l. and the Vellayani Lake, Kerala, lies in the coastal plain and it covers nearly 7.5 sq. km. In this study several lake bed samples and two sediment cores from both the lakes were collected to understand the source of sediments and to reconstruct the palaeoenvironmental shifts. Radiocarbon dates were obtained on the organic carbon rich sediments for the Pookode lake and the dates range in age from the mid Holocene to Recent (6240 to 565 yrs BP) while the radiocarbon dates obtained from the Vellayani lake core range in age from 3025 to 1210 yrs BP revealing two post bomb inversion of ages. In this study phytolith and pollen record supported by continuous sedimentation is the point of interpretation since the mid Holocene for palaeoclimatic changes. The data revealed that during the overall dry phase of the mid Holocene period (6240 yrs BP) to present, very wet intervals of short duration were caused by intense SWM. This caused the water levels to rise and the lake margin to expand over the surrounding low lying region characterised by lateritic soils. The lake margin expanded in response to the short wetter periods and contracted during the long dry phases. CaO/MgO ratios and chemical weathering intensity (CWI) values of Vellayani lake sediment core indicate overall dry conditions since 3000 years BP (late Holocene). newline newline newline