CONTROLS ON ALKENONE UNSATURATION RATIOS ALONG THE SALINITY GRADIENT BETWEEN THE OPEN OCEAN AND THE BALTIC SEA

Abstract

Alkenone unsaturation ratios of sedimentary lipids are used as a geochemical proxy for sea surface temperatures, and interest is growing in their potential as indicators of different water masses and possibly of salinity. We analyzed the abundance of unsaturated C37 to C38 ketones in lipid extracts of 57 surface sediment (0–1 cm) samples along a salinity gradient from 8 to 33 psu in the transition from the Skagerrak to the Baltic Sea (NW Europe). In addition to surface sediments, we analyzed alkenones in suspended particulate matter at 13 stations—over a gradient in salinity from 25 to 33 psu—during a bloom of the coccolithophore Emiliania huxleyi. Alkenones were detected in all samples (suspended matter and sediment) with variable contributions of the tetra-unsaturated C37 alkenone compound (%C37:4; range from 2 to 10% of total C37 alkenone content). Comparing the alkenone unsaturation index (U37K′) and %C37:4 data to climatological sea surface temperature and sea surface salinity data sets revealed that SST estimated from U37K′ of saline end members (samples from the Skagerrak) is in the general range of modern SST during bloom periods of haptophytes. At salinities below ∼30 psu %C37:4 increases to above 5% and the unsaturation ratios cease to be related to climatological annual or seasonal sea surface temperatures. On the other hand, the %C37:4 appears to be inversely and significantly correlated to salinity: Highest C37:4 proportions in the inner Baltic Sea are caused by an unidentified organism, but in the transition area at salinities down to 10 psu, the producer apparently is E. huxleyi. The suspended matter data together with those from the water column support the hypothesis of changing biosynthesis of alkenones under salt stress by the coccolithophore E. huxleyi, but constrain the maximum of %C37:4 attributable to salt stress to 10% of all C37 alkenones.