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Abstract. Free and "bound" long-chain alkenones (C37:2 and C37:3 ) in oxidized and unoxidized sections of four organic matter-rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 95 lB and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of U3k'7 . The profiles of both alkenones across the redox boundary show a preferential degradation of the C37:3 compared to the C37:2 compound. Because of the high initial U3k'7 values and the way of calculating the U3k'7 this degradation hardly influences the U3k'7 profiles. However, for lower U3k'7 values, measured selective degradation would increase U3k'7 up to 0.17 units, equivalent to 5øC. For most of the U3k'7 band-width, much smaller degradation already increases U3k'7 beyond the analytical error (0.017 units). Consequently, for interpreting the U3k'7 record in terms of past sea surface temperatures, selective degradation needs serious consideration.

Acyclic and cyclic biphytanes derived from the membrane ether lipids of archaea were found in water column particulate and sedimentary organic matter from several oxic and anoxic marine environments. Compoundspecific isotope analyses of the carbon skeletons suggest that planktonic archaea utilize an isotopically heavy carbon source such as algal carbohydrates and proteins or dissolved bicarbonate. Due to their high preservation potential, these lipids provide a fossil record of planktonic archaea and suggest that they have thrived in marine environments for more than 50 million years.

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