Long-chain alkenones in Baltic Sea surface sediments: New insights

C37 alkenones produced by certain haptophytes of the Isochrysidales are valuable sedimentary biomarkers used to estimate sea surface temperature (SST) in the open ocean. However, in coastal seas the role of salinity gradients on alkenone producing species and SST estimates is poorly known. Alkenones were analyzed in surface sediments from the marine Skagerrak region and the entire brackish Baltic Sea. Three types of alkenone distribution patterns were identified: type A distribution, which resembles the distribution in Emiliania huxleyi, type B distribution, which is similar to Ruttnera lamellosa, Isochrysis galbana and Pseudoisochrysis paradoxa distributions, although these haptophytes are absent from the Baltic Sea, and type C distribution, which is also found in worldwide lake sediments. These types of distribution are apparent in the percentage of C37:4 alkenone (%C37:4), which is significantly negatively correlated to sea surface salinity (SSS). The distribution of alkenones very likely results from distinct alkenone-producing haptophytes, whose spatial distribution is ultimately related to SSS, as supported by the hydrogen isotope fractionation (α) between alkenones and water. U37K and U37K″ correlate more significantly than U37K′ with both SST and SSS, probably due to the superimposed effect of changing alkenone-producing species. The application of U37K and U37K″ as SST proxies results in unrealistic Holocene temperature records for the southern Baltic Sea, but %C37:4 as a SSS proxy reveals reasonable salinity changes. Interestingly, a C32 hopanoic acid was found abundantly as the methyl ester in the ketone fractions and may represent a marker for (cyano)bacterial biomass in the Baltic Sea.