EVIDENCE FOR GAMMACERANE AS AN INDICATOR OF WATER COLUMN STRATIFICATION

A new route for the formation of gammacerane from tetrahymanol is prop osed; in addition to dehydration and hydrogenation, sulphurisation and early C-S cleavage are shown to be important in the pathway of format ion, especially in marine sediments. Evidence is twofold. First, relat ively large amounts of the gammacerane skeleton are sequestered in S-r ich macromolecular aggregates formed by natural sulphurisation of func tionalised lipids. Selective cleavage of polysulphide linkages with Me Li/Mel led to formation of 3-methylthiogammacerane, indicating that th e gammacerane skeleton is primarily bound via sulphur at position 3, c onsistent with the idea that tetrahymanol (or the corresponding ketone ) is the precursor for gammacerane. Second, upon mild artificial matur ation of two sediments using hydrous pyrolysis, gammacerane is release d from S-rich macromolecular aggregates by cleavage of the relatively weak C-S bonds. The stable carbon isotopic compositions of gammacerane and lipids derived from primary producers and green sulphur bacteria in both the Miocene Gessoso-solfifera and Upper Jurassic Allgau Format ions indicate that gammacerane is derived from bacterivorous ciliates which were partially feeding on green sulphur bacteria. This demonstra tes that anaerobic ciliates living at or below the chemocline are impo rtant sources for gammacerane, consistent with the fact that ciliates only biosynthesize tetrahymanol if their diet is deprived of sterols. This leads to the conclusion that gammacerane is an indicator for wate r column stratification, which solves two current enigmas in gammacera ne geochemistry. Firstly, it explains why gammacerane is often found i n sediments deposited under hypersaline conditions but is not necessar ily restricted to this type of deposits. Secondly, it explains why lac ustrine deposits may contain abundant gammacerane since most lakes in the temperate climatic zones are stratified during summer.