Alkenone-Specific δ2H: A New Global Core Top Dataset and Its Implications for Paleo-Salinity and -Seawater δ2H Reconstructions

Abstract:

While alkenones have traditionally been employed as a sea surface temperature (SST) proxy, recent work has shown that the hydrogen isotopic composition of the diunsaturated 37-carbon species (δ²H_C37:2) broadly tracks seawater δ²H (δ²H_H2O). Unfortunately, the development of a δ²H_C37-based paleo-δ²H_H2O or -sea surface salinity (SSS) proxy has been hampered by divergent inter-alkenone fractionation (α_C37:3-C37:2) values and the SST- or SSS-dependence of water-alkenone fractionation (α_C37-H2O). Compounding these difficulties, multiple alkenone δ²H values are typically measured in bulk (δ²H_C37) due to the analytical difficulty of separation, and most studies use laboratory-cultured algae which may record environmental conditions differently than accumulated sedimentary material. Thus, existing attempts at a robust δ²H_C37-δ²H_H2O calibration have been mixed.

To address these issues, we present core top δ²H_C37:3 and δ²H_C37:2 values from 20 globally distributed open ocean sites. We find that the systematic α_C37:3-C37:2 observed in many culture studies is not reproduced in natural samples. Furthermore, α_C37-H2O (0.772-0.824, within the ~0.74-0.84 range published in water column and culture studies) appears independent of SST, growth rate (via chlorophyll-a concentrations and ∑C37 / ∑C38), or SSS, contrary to the positive α-SSS relationship in laboratory-grown organisms. These findings have several implications for future alkenone δ²H work: first, if α_C37:3-C37:2 ≈ 1, δ²H_C37:3 and δ²H_C37:2 may be legitimately combined during analysis. Second, α_C37-H2O’s low correlation with external factors eliminates the need to estimate paleo-SSS or -growth rate while suggesting that δ²H_C37 is primarily a proxy of δ²H_H2O.

Finally, we produce a core top-based δ²H_C37-δ²H_H2O calibration showing that change in δ²H_C37 is around than 1.4‰ / ‰ δ²H_H2O, within range of the mean 1.5‰ / ‰ δ²H_H2O observed in the handful of extant down core δ²H_C37 records spanning the Last Glacial Maximum-present. These results demonstrate that a core top δ²H_C37-δ²H_H2O calibration is viable and emphasize the importance of using environmental samples to test results obtained in culture.