24-NORCHOLESTANES AS AGE-SENSITIVE MOLECULAR FOSSILS

Biomarkers, molecular fossils, are organic compounds in Holocene to Pr ecambrian sedimentary deposits that can be related to specific chemica l compounds produced in the biosphere. We demonstrate here that 24-nor cholestane biomarkers, i.e., C26 steranes (saturated hydrocarbons havi ng a steroid skeleton), can be useful to constrain the age and paleola titude of geologic samples. The biological precursors of 24-norcholest anes remain unclear, but samples from more than 100 basins provide evi dence that 24-norcholestanes may be derived directly or indirectly fro m diatoms, Ratios of 24-norcholestanes to 27-norcholestanes show an in itial increase above background in Jurassic oils, but they increase dr amatically in Cretaceous oils, coincident with diatom evolution. The h ighest ratios are found in oils and rock extracts from Oligocene or yo unger marine siliceous source rocks in which the sources were deposite d at paleolatitudes greater than 30 degrees N.