Long-chain alkenone patterns in the Baltic sea—an ocean-freshwater transition

Abstract

Two different patterns of long-chain alkenones are found in surficial sediments of the Baltic Sea, which is the largest brackish water body on Earth. One pattern occurs in surficial sediments from the Western Baltic Sea where surface-water salinitiy is in excess of 7.7 PSU. It corresponds to the pattern produced by the marine coccolithophorid Emiliania huxleyi with a suite of C 37 di- to tetra-unsaturated methyl ketones and C 38 di- and tri-unsaturated methyl and ethyl ketones. A second pattern, resembling that found in lake sediments in lacking C 38 methyl ketones and having distinctly higher C 37:4 methyl ketone concentrations, dominates in surficial sediments of the eastern and northern Baltic Sea, where salinities are lower than 7.7 PSU. Correspondence of sea-surface temperature (SST) estimates from the U 37 K and U 37 K′ indices (using marine calibrations) with mean SST in the euphotic zone from July-August (the main haptophyte growth season) is poor. Thus, these indices are not applicable as sedimentary thermometers in surficial sediments of the Baltic Sea. The different patterns may either reflect the salinity-dependent occurrence of specific alkenone producers or changes in the alkenone biosynthesis due to physiological stress caused by salinity variations. Furthermore, advection of saline and oxygenated North Sea water may transport marine algal material characterized by a marine E. huxleyi-like alkenone pattern into the western Baltic Sea, thus covering the signature of the local alkenone producers with a Baltic Sea pattern.