Variations in the distributions and isotopic composition of alkenones in Black Sea particles and sediments

Concentrations, unsaturation index (U₃₇^k) and isotopic compositions of individual alkenones were measured for samples of suspended particulate organic matter (POM), sediment trap and sedimentary organic matter from the Black Sea. In general, temperatures estimated from U₃₇^k are lower than actual sea surface temperatures, indicating existing calibrations may not be appropriate for alkenones in the Black Sea. Specifically, there is a lack of agreement among U₃₇^k values calculated with and without the concentration of the C_37:4 alkenone which lead to significant differences in temperatures estimated using the available calibrations. The more unsaturated alkenones in suspended POM with high water column residence times may be degraded preferentially, resulting in the observed increase with depth of U₃₇^k values. The σ¹³C of individual alkenones in suspended particles decrease with increased unsaturation. However, there is no correlation between temperature inferred from U₃₇^k and the isotopic composition of individual alkenones. Isotopic compositions of the C_37:4 alkenone are apparently independent of more saturated isomers. Although the overall pattern of the σ¹³C of higher molecular weight alkenones (C₃₈, C₃₉) with depth in the water column is similar to that of that of the di- and tri-unsaturated C₃₇ alkenones, isotopic compositions of individual isomers range several permil. Variation in the isotopic compositions of alkenones may be due to differences either in the biological sources for or mechanisms for removal of alkenones in organic matter suspended in the waters of the Black Sea.