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Abstract
Abstract. Ostracods are common lacustrine calcitic microfossils. Their faunal assemblage and morphological characteristics are important ecological proxies, and their valves are archives of geochemical information related to palaeoclimatic and palaeohydrological changes. In an attempt to assess ostracod ecology (taxonomic diversity and valve morphology) combined with valve geochemistry (δ18O and δ13C) as palaeosalinity indicators, we analysed sedimentary material from the International Continental Scientific Drilling Program (ICDP) Ahlat Ridge site from a terminal and alkaline lake, Lake Van (Turkey), covering the last 150 kyr. Despite a low species diversity, the ostracod faunal assemblage reacted sensitively to changes in the concentration of total dissolved salts in their aquatic environment. Limnocythere inopinata is present throughout the studied interval, while Limnocythere sp. A is restricted to the Last Glacial period and related to increased lake water salinity and alkalinity. The presence of species belonging to the genus Candona is limited to periods of lower salinity. Valves of Limnocytherinae species (incl. L. inopinata) display nodes (hollow protrusions) during intervals of increased salinity. Both the number of noded valves and the number of nodes per valve appear to increase with rising salinity, suggesting that node formation is related to hydrological changes (salinity and/or alkalinity). In contrast to Lake Van's bulk δ18O record, the δ18O values of ostracod valves do record relative changes of the lake volume, with lower values during high lake level periods. The δ13C values of different species reflect ostracod habitat preferences (i.e. infaunal vs. epifaunal) but are less sensitive to hydrological changes. However, combined with other proxies, decreasing Holocene δ13C values may indicate a freshening of the lake water compared to the low lake level during the Last Glacial period. The Lake Van example underscores the significance and value of coupling ostracod ecology and valve geochemistry in palaeoenvironmental studies of endorheic lake basins.
Highlights
The water volume of closed lakes in arid and semi-arid regions reacts sensitively to climatically induced changes in the ratio of meteoric precipitation to evaporation (P / E)
Our results suggest that noding of L. inopinata and other Limnocytherinae species is controlled by ambient water salinity and perhaps alkalinity
Taking advantage of the legacy of the International Continental Scientific Drilling Program (ICDP) PALEOVAN project – the well-documented hydrological evolution of Lake Van – this study examined the applicability of ostracods as multi-proxy archives for relative changes of the past lake water chemistry, with an emphasis on salinity reconstructions
Summary
The water volume of closed lakes (endorheic basins) in arid and semi-arid regions reacts sensitively to climatically induced changes in the ratio of meteoric precipitation to evaporation (P / E). Dana and Lenz, 1986; Aladin and Potts, 1992), and (pore)water salinity is a conservative tracer of past climate fluctuations (Tomonaga et al, 2017). Porewater salinity profiles have a relatively low resolution and record only major changes at millennial timescales. Palaeoecological (e.g. faunal assemblage, phenological and morphological variation) and geochemical (e.g. biomarker, carbonate trace elemental and isotope geochemistry) salinity proxies offer higher temporal resolution but are, by definition, indirect indicators. Calcitic valves of ostracods (benthic, bivalve Crustacea) are established biological proxies to infer past environmental conditions from species ecological tolerance, morphometry and/or valve geochemistry. Numerous previous studies exemplified the application of ostracod valves in tracking past hydrological changes
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