Cyanobacteria as potential biomarkers of hydrological changes in the Late Quaternary sediments of South Kerala Sedimentary Basin, India

摘要

Cyanobacteria tolerate extreme hydrological regimes, and their mucilaginous filaments account for binding and preservation of delicate structures. As the hydrological set-up is controlled by rainfall, they serve as biomarkers to distinguish palaeoenvironmental scenario and ecological shifts ranging from Late Pleistocene to Early Holocene (40 ka-2.5 ka) in the studied boreholes of the South Kerala Basin (Ayiramthengu, Pan-avally and West Kallada). The accrued cyanobacteria microflora is unique, as it has different morphotypes of Rivularia sp. and Gloeotrichia sp. at varying levels in the sediment profiles, probably reflecting the signatures of hydrological modifications associated with the development of wetland system. The response of cyanobacteria is similar in Panavally and Ayiramthengu from Early to Late Holocene with short signals of pauses probably relating to aberrations in precipitation. However, the Late Pleistocene and Late Holocene periods show a poor record of cyanobacteria reflecting decline in rainfall and prevalence of long spell of dry phase. From Early Holocene to Middle Holocene, the turnover of cyanobacteria has been prolific, which may be due to the Holocene Optimum rainfall. Relatively larger size and better carbonised and smaller size and lighter coloured morphotypes are correlated with such hydrological modifications as a result of high and low rainfall respectively during the Early Holocene and Late Holocene. As the cyanobacteria have the potential for adaptation and stabilization of numerous specialized eco- and morphotypes in time and space, they provided additional palaeodata as good biomarkers of climatic change.