Dinosteranes (4,23,24-trimethylsteranes) and other biological markers in dinoflagellate-rich marine sediments of Rhaetian age

Alkane biological marker distributions and concentrations have been determined in 18 samples from a section of Rhaetian age from the Bristol Trough and a quantitative micropalaeontological analysis carried out. Taking into account previous geological investigations of the sequence, three 'end-member' depositional settings have been recognized from changes in total organic carbon content, the biological markers and the micropalaeontological composition. These depositional environments are attributed as follows. The first, which occurs in the lower part of the section, corresponds to deposition in a supratidal/sabkha-type setting with the major organic input being of allochthonous origin. The second, also in the lower part of the section, indicates the occurrence of periods of marine incursion, with enhanced salinity resulting from evaporation. In the upper part of the section marginal marine conditions are apparent, with higher organic carbon, biological marker and amorphous organic matter contents at the top, suggesting the establishment of more stable conditions of oxygen depletion. Some sediments in the sequence are notable for containing an abundance of cysts of Rhaetogonyaulax rhaetica, one of the oldest marine dinoflagellates. Comparison of microfossil abundances with hydrocarbon abundances extends the circumstantial evidence for the Dinophyceae being the major biological source of dinosteranes (4,23,24-trimethylcholestanes) and some other 4-methyl steranes in marine sediments. The presence of dinosteranes and their 24-ethyl counterparts extends the stratigraphic range of occurrence of such components back to the Rhaetian. C sub(31) methyl steranes in the sediments from the second depositional setting are tentatively assigned a 4,22,23,24-tetramethyl-sterane skeleton and a dinoflagellate origin.