Constraining the application of hydrogen isotopic composition of alkenones as a salinity proxy using marine surface sediments

Sea surface salinity is an essential environmental parameter necessary to understand past changes in global climate. However, reconstructing absolute salinity of the surface ocean with high enough accuracy and precision remains a complicated task. Hydrogen isotope ratios of long-chain alkenones (δ²H_C37) have been shown to reflect salinity in culture studies and have been proposed as a tool to reconstruct sea surface salinity in the geologic record. The correlation between δ²H_C37 – salinity in culture is prominently caused by the relationship between δ²H_H2O and salinity, as well as the increase in fractionation factor α with increasing salinity. The δ²H_C37 – salinity relationship in the natural environment is poorly understood. Here, surface sediments from a variety of environments covering a wide range of salinities were analyzed to constrain the environmental relationship between salinity and hydrogen isotopes of alkenones. δ²H_C37 correlates significantly (r = 0.75, p < 0.0001) with annual mean salinity. Interestingly, the biological hydrogen isotope fractionation (α_C37) seems independent of salinity. These findings are different from what has previously been observed in culture experiments, but align with other environmental datasets and suggest that the salinity effect on biological hydrogen isotope fractionation observed in culture is not apparent in sediments. The absence of a correlation between α_C37 and salinity for marine surface sediments might be best explained by a mixing of multiple alkenone-producing species contributing to the sedimentary alkenone signal that fractionate in distinct ways. Nevertheless, sedimentary δ²H_C37 ratios still correlate with salinity and δ²H_H2O, suggesting that δ²H_C37 ratios are useful for paleosalinity reconstructions. Our surface sediment calibration presented here can be used when different species contribute to the sedimentary alkenone pool and substantial changes in salinity are expected