Description: At the southern tip of Africa, the Agulhas Current reflects back into the Indian Ocean causing so-called "Agulhas rings" to spin off and release relatively warm and saline water into the South Atlantic Ocean. Previous reconstructions of the dynamics of the Agulhas Current, based on paleo-sea surface temperature and sea surface salinity proxies, inferred that Agulhas leakage from the Indian Ocean to the South Atlantic was reduced during glacial stages as a consequence of shifted wind fields and a northwards migration of the subtropical front. Subsequently, this might have led to a buildup of warm saline water in the southern Indian Ocean. To investigate this latter hypothesis, we reconstructed sea surface salinity changes using alkenone dD, and paleo-sea surface temperature using TEXH 86 and UK'37, from two sediment cores (MD02-2594, MD96-2080) located in the Agulhas leakage area during Termination I and II. Both UK'37 and TEXH 86 temperature reconstructions indicate an abrupt warming during the glacial terminations, while a shift to more negative dDalkenone values of approximately 14 per mil during glacial Termination I and II is also observed. Approximately half of the isotopic shift can be attributed to the change in global ice volume, while the residual isotopic shift is attributed to changes in salinity, suggesting relatively high salinities at the core sites during glacials, with subsequent freshening during glacial terminations. Approximate estimations suggest that dDalkenone represents a salinity change of ca. 1.7-1.9 during Termination I and Termination II. These estimations are in good agreement with the proposed changes in salinity derived from previously reported combined planktonic Foraminifera d18O values and Mg/Ca-based temperature reconstructions. Our results confirm that the dD of alkenones is a potentially suitable tool to reconstruct salinity changes independent of planktonic Foraminifera d18O.

Description: The Baltic Sea has had a complex salinity history since the last deglaciation. Here we show how distributions of alkenones and their dD values varied with past fluctuations in salinity in the Baltic Sea over the Holocene by examining a Holocene record (11.2-0.1 cal kyr BP) from the Arkona Basin. Major changes in the alkenone distribution, i.e., changes in the fractional abundance of the C37:4 Me alkenone, the C38:2 Et alkenone and a C36:2 Me alkenone, the latter which has not been reported in the Baltic Sea previously, correlated with known changes in salinity. Both alkenone distributions and hydrogen isotopic composition suggest a shift in haptophyte species composition from lacustrine to brackish type haptophytes around 7.7-7.2 cal kyr BP, corresponding with a salinity change that occurred when the connection between the basin and the North Sea was re-established. A similar salinity change occurred in the Black Sea. Previously published alkenone distributions and their d-values from the Black Sea were used to reconstruct Holocene changes in surface water salinity and, hence, it was shown that the unusual C36:2 alkenone dominates the alkenone distribution at salinities of 2-8 ppt (g/kg). This information was used to corroborate the interpretations made about salinity changes from the data presented for the Baltic Sea. Low and variable salinity waters in the Baltic Sea over the Holocene have led to a variable alkenone producing haptophyte community composition, including low salinity adapted species, hindering the use of the unsaturation ratios of long-chain alkenones for sea surface temperature reconstruction. However, these alkenone based indices are potentially useful for studying variations in salinity, regionally as well as in the past.

Description: 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.