A late quaternary ostracod record from the Tso Kar basin (North India) with a note on the distribution of recent species

Abstract

The fossil ostracod associations from a radiocarbon dated sediment core (15.3–0 cal kyr) of the high altitude (4,527 m a.s.l.) hyperhaline Tso Kar lake in North India reveal changes in ostracod species abundances and composition. These document the process of lake formation and ongoing desiccation during the latest quaternary and broadly confirm the results of previous geochemical, geomorphological and pollen analysis. The most striking feature of the core record is a period of freshwater conditions between 9.0 and 6.5 cal kyr BP, as calculated by means of an ostracod-based conductivity transfer function. This early- to mid-Holocene interval of the core correlates with similar ostracod assemblages (Cytherissa lacustris—Candona candida—Eucypris afghanistanensis) of an outcrop section at the basin margin, about 98 m above the modern lake level and marks the highest lake level under the influence of an intensified Indian Summer Monsoon. After 6.5 cal kyr BP, Limnocythere inopinata is the sole representative of the ostracod fauna in the sediment core, which suggests rising salinity conditions most probably due to lake shallowing. From 3.2 cal kyr BP to present, the near absence of ostracods in the core Tk106 is most likely a consequence of salinisation of the lake towards the hyperhaline conditions that are realised at present. Although the modern morphology and physico-chemical properties of Tso Kar lake are in contrast to the past lake conditions, most of the ostracod species, except for Limnocythere mirabilis found in the sediment core, are also identified from surface sediment and outcrop samples. In contrast, Heterocypris salina and Eucypris dulcifons are widespread in the shallow surface waters with a total dissolved solids content ranging from 0.6 to 15.0 g L−1, but are excluded from the core record and outcrop strata. At a basinal scale, the lake shrinkage and segregation into the Tso Kar and hydrologically open freshwater lake Startsapuk Tso have forced diversification of ostracod taxa, probably as an effect of the emergence of new ecological niches under enhanced “environmental pressure”.