Valve chemistry of Limnocythere inopinata (Ostracoda) in a cold arid environment — Implications for paleolimnological interpretation

Abstract

The geochemical fingerprint of biogenic calcite is largely governed by the physical environment and water chemistry at the place and time of calcification, which is in turn related to both the ecology and phenology of the target study species. We present data on the valve chemistry (δ 18O, δ 13C, Mg/Ca, Sr/Ca) of living Limnocythere inopinata, a common and eurytopic Holarctic ostracode species, and on simultaneously collected water chemistry data from 15 water bodies in western Mongolia. These selected lake and pond study sites provide a fair representation of the regional trend in solute evolution towards pronounced calcium depletion, which is further characterized by good correlation between Mg/Ca water and total dissolved solids. Ostracode valve chemistry analysis showed that the Sr/Ca ratio (Sr/Ca valve) correlates better with Sr/Ca water than Mg/Ca valve does with Mg/Ca water, perhaps because Sr/Ca is less affected by calcification rate and temperature. The difference in δ 18O between the collected valves and a hypothetical ostracode calcite formed during mid-summer reveals an overall coherent trend of evaporative enrichment during summer in the sampled perennial water bodies, whereas smaller and/or ephemeral water bodies appear to be stronger impacted by stochastic rain events. Among-lake variation in δ 13C likely reflects the regional gradient in aquatic productivity, and the uniform pattern of relatively small mean offset between the δ 13C of ostracode valves and of surface-water dissolved inorganic carbon suggests epibenthic molting behavior in a well-mixed littoral habitat. The observed patterns in water and valve chemistry further suggest that females of L. inopinata, in this particular climatic setting, molt to adult in early summer. This information on calcification time and environment contributes to more rigorous paleo-environmental interpretation of lake-sediment records of this common species in central Asia. Our results also suggest potential for quantitative salinity reconstruction based on ostracode Mg/Ca valve in western Mongolian lakes, however this needs to be constrained by insight in the local hydrology and hydrochemistry of the reconstruction site.