Postdepositional oxic degradation of alkenones: Implications for the measurement of palaeo sea surface temperatures

Free and “bound” long‐chain alkenones (C 37∶2 and C 37∶3 ) in oxidized and unoxidized sections of four organic matter‐rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of U 37 k′ . The profiles of both alkenones across the redox boundary show a preferential degradation of the C 37∶3 compared to the C 37∶2 compound. Because of the high initial U 37 k′ values and the way of calculating the U 37 k′ this degradation hardly influences the U 37 k′ profiles. However, for lower U 37 k′ values, measured selective degradation would increase U 37 k′ up to 0.17 units, equivalent to 5°C. For most of the U 37 k′ band‐width, much smaller degradation already increases U 37 k′ beyond the analytical error (0.017 units). Consequently, for interpreting the U 37 k′ record in terms of past sea surface temperatures, selective degradation needs serious consideration.