Variability of alkenone producers recorded in brackish lacustrine sediments of Lake Nakaumi, western Japan

Long-chain alkenones, haptophyte-derived biomarkers, is a powerful tool for reconstructing sea surface temperature. Despite increasing reports of non-marine occurrence of alkenones, application of alkenone proxies in the settings are still challenging. Because the temperature calibrations of the alkenone unsaturation indices (U^K₃₇, U^K'₃₇) vary depending on the taxonomic differences of the alkenone producers, identification of the producer is a key to apply the calibrations for the coastal/lacustrine settings. The phylogeny of the alkenone producers is largely divided into three family-level groups, namely the Groups I - III (Theroux et al., 2010). In Japanese lakes, the Group I alkenone producer is identified in Lake Toyoni (Hokkaido) and Ichi-no-Megata (northeastern Japan) (Longo et al. 2018). Here we report the third example of lacustrine alkenone in Japan, which revealed from the analysis of ca. 1.8 m sediment core taken from Lake Nakaumi, western Japan. The L. Nakaumi is a coastal brackish lake and lagoonal in origin. Its water changed oligohaline to polyhaline by intrusion of seawater from Japan Sea. evaluated the source species of alkenones in Lake Nakaumi from the composition of alkenone and related compounds, and water temperature were preliminary reconstructed by selecting applicable calibrations.

The long-chain alkenones were detected in all samples from the 1.8 m core taken from near the center of L. Nakaumi (17NK-3 core). The alkenone compositions are clearly different between the samples above and below the horizons around 20 cm depth. corresponding to the horizon in about 1960s. Below 20 cm depth , the di-unsaturated alkenones such as C37:2 alkenones are dominant, and the U^K'₃₇ are higher. Meanwhile, above 20 cm depth, the C37:3 alkenones are dominant and C37:4 and C40 alkenones are relatively abundant, resulting contrastingly lower U^K'₃₇ and U^K₃₇ values. By comparing with chemotaxonomic features of known alkenone producers as reported previously , we estimate that the main alkenone producers in sediments below and above the horizons in about 1960s are Group III species (including marine producers: e.g. Emiliania huxleyi and Gephyrocapsa oceanica ) and Group II species (coastal and lacustrine producers: e.g. Ruttnera lamellosa and Isochrysis garbana), respectively.