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 (δ2HC37) have been shown to reflect salinity in culture studies and have been proposed as a tool to reconstruct sea surface salinity in the geological record. The correlation between δ2HC37 - salinity in culture is prominently caused by the relationship between δ2HH2O and salinity, as well as the increase in fractionation factor α with increasing salinity. The δ2HC37 - salinity relationship in the natural environment is poorly understood. Here, surface sediments from a variety of environments that cover a wide range of salinities were analyzed to constrain the environmental relationship between salinity and hydrogen isotopes of alkenones. δ2HC37 correlates significantly (R2 =0.55, p < 0.0001, n = 95) with annual mean salinity, but interestingly, the biological hydrogen isotope fractionation (αC37) 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 αC37 and salinity for marine surface sediments might be best explained by a mixing of multiple alkenone-producing species that fractionate in distinct ways contributing to the sedimentary alkenone signal. Nevertheless, sedimentary δ2HC37 ratios still correlate with salinity and δ2HH2O, suggesting that δ2HC37 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.
