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

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 delta O-18(water), delta O-18(ostracod) and delta C-13(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 delta O-18(ostracod) values show a complex relationship with temperature and suggest, in most circumstances, that delta O-18(water) is the dominant control on delta O-18(ostracod) . During times of fresher water, delta C-13(ostracod) increases, suggesting increasing aquatic productivity. Above a certain delta O-18(water) threshold however, aquatic productivity begins to decline. The interpretation of delta C-13(ostracod) in some coastal lakes, may therefore be dependent on understanding of the range of expected delta O-18(water). Due to short-term (diurnal to seasonal) variations that cause large ranges in delta O-18(water) and delta O-18(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.