Controls on alkenone unsaturation ratios along the salinity gradient between the open ocean and the Baltic Sea

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 C-37 to C-38 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 C-37 alkenone compound (%C-37:4 range from 2 to 10% of total C-37 alkenone content). Comparing the alkenone unsaturation index (U-37(K)) and %C-37:4; data to climatological sea 37 surface temperature and sea surface salinity data sets revealed that SST estimated from U-37(K) of saline end 37 members (samples from the Skagerrak) is in the general range of modern SST during bloom periods of haptophytes. At salinities below similar to 30 psu %C-37:4; 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 %C-37:4 appears to be inversely and significantly correlated to salinity: Highest C37:1 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 %C-37:4 attributable to salt stress to 10% of all C-37 alkenones. Copyright (c) 2005 Elsevier Ltd.