Seasonal dynamics of modern ostracod stable isotope (δ18O, δ13C) compositions in a large tropical lake (Lago Enriquillo, Dominican Republic)

The stable oxygen (O) and carbon (C) isotope composition of bulk carbonate in sediments is widely used for paleoenvironmental reconstructions. This approach, however, does often not consider seasonal variability in environmental conditions. Although, this can be overcome by the consideration of the specific composition of ostracod shells which have a relatively short and variable life history, this approach has not been consequently applied in the past. Only few studies investigated the potential of ostracodes in high-resolution (e.g. seasonal) paleoenvironmental reconstructions, based on their stable isotope composition, so far.

Seasonal meteorological conditions in tropical areas such as the Caribbean region are broadly divided into dry and rainy seasons with often profound effects on hydrological and ecological conditions. How this hydrological seasonality is archived by ostracode stable isotopes (δ¹⁸O, δ¹³C) is largely still unclear. The present study uses variations in lake water isotopes (δ¹⁸O, δ²H) and δ¹³C_DIC together with the hydrochemical composition (major and trace elements) of the hyperhaline Lago Enriquillo. Water samples and living ostracodes were taken during March and September (i.e. dry and rainy season) in 2022. The C and O isotopic composition of single ostracode valves of different ostracode species (Cyprideis similis, C. edentata, Perissocytheridea cribrosa, Thalassocypria cf. sarbui) were analysed. These species provide differences in their temporal-spatial distribution but are generally restricted to the upper 8 m water depth of the lake. Both types of life cycles (permanent and seasonally restricted) are shown by the ostracodes species.

Questions that are addressed in this study include: Does the water isotopic composition reflect significant differences between the dry and the rainy season? What are the sources and sinks of water for the lake? Do ostracode δ¹⁸O and δ¹³C values reflect the composition of water and dissolved DIC in the lake? How does ostracode ecology (i.e. habitat preferences, life cycle) affects their isotope signatures?

Our results show a low intra-annual variation in δ¹⁸O and pronounced local variability of δ¹³C_DIC values of the lake water. Ostracode δ¹⁸O and δ¹³C signatures reflect the lake water composition. The individual species display differences in their isotopic composition and variation ranges. δ¹⁸O results agree with the low intra-annual variation of the lake water and display a pronounced gradient with increasing values towards areas with reduced influence of inflows. δ¹³C values of ostracodes, on the other hand, show strong local differences between and even heterogeneity within samples sites.

Results imply that the lake water was buffered against seasonal hydrological variations during the sampling period, but reveals large spatial variations associated with e.g. strong contrast between inflows and the lake reservoir. The deduction of paleoenvironmental conditions of Lago Enriquillo based on stable isotopes of fossil ostracodes requires therefore consideration of multiple species. The ostracode proxy information then may help to deduce past changes in the lake water cycle.