δ18O and δ13C of Cyprideis torosa from coastal lakes: Modern systematics and down-core interpretation

Abstract

Stable isotope analyses of ostracod shells are a commonly-used proxy for palaeoenvironmental reconstruction. Although the fundamental controls on isotope composition of ostracod shells are well understood and, in some instances, quantifiable, the paleoclimatic and palaeoenvironmental interpretation of records from lake sediments depends strongly on the characteristics of individual lakes including the climatic setting, depth, volume, hydrology, aquatic vegetation and catchment properties. This is particularly important for coastal lakes where physio-chemical variations may occur on diurnal timescales. Here, we combine variations in δ 18 O water , δ 18 O ostracod and δ 13 C ostracod , hourly water temperature, and Mg/Ca ostracod inferred water temperatures (constraining calcification temperature) to improve palaeoenvironmental interpretation and provide insights into lake carbon cycle. The dataset improves understanding of complex coastal lake site systematics and downcore interpretation of stable isotopes from C. torosa , a geographically widespread brackish water ostracod. The δ 18 O ostracod values show a complex relationship with temperature and suggest, in most circumstances, that δ 18 O water is the dominant control on δ 18 O ostracod . During times of fresher water, δ 13 C ostracod increases, suggesting increasing aquatic productivity. Above a certain δ 18 O water threshold however, aquatic productivity begins to decline. The interpretation of δ 13 C ostracod in some coastal lakes, may therefore be dependent on understanding of the range of expected δ 18 O water . Due to short-term (diurnal to seasonal) variations that cause large ranges in δ 18 O water and δ 18 O ostracod , stable isotope analyses of C. torosa should be: (1) undertaken on multiple single shells (2) where carapaces are preserved, paired with trace-element/Ca analyses on the same individual; and (3) undertaken alongside a study of the modern lake system. • In most circumstances, δ 18 O water is the dominant control on δ 18 O ostracod . • δ 13 C ostracod is likely controlled by productivity and the oxidation of terrestrial organic matter. • Samples that show a relationship with temperature have higher δ 18 O and a decreasing trend with δ 13 C. • Only some valves in a palaeo-dataset with mixed seasonal signals may relate to palaeo-temperature.