AN ISOTOPIC BIOGEOCHEMICAL STUDY OF NEOPROTEROZOIC AND EARLY CAMBRIANSEDIMENTS FROM THE CENTRALIAN SUPERBASIN, AUSTRALIA

Organic matter from Neoproterozoic and Early Cambrian sediments of the Amadeus and Officer basins of the Centralian Superbasin, Australia, h as been studied for biomarker distributions and the carbon isotopic co mpositions of kerogen and individual hydrocarbons. These sediments rep resent both shallow and deep water marine facies in the older sections and marine and saline lacustrine carbonate deposits in the Cambrian. Hydrocarbon biomarker patterns were found to be quite consistent with the known sedimentary environments and provide valuable insights into the biogeochemical changes which accompanied the transition from a mic robially-dominated ocean to the early stages of metazoan radiation. In particular, carbon isotopic data for n-alkyl and isoprenoid lipids pr esented here, and in earlier studies, showed a reversal in carbon isot opic ordering between the Proterozoic and Phanerozoic. By comparison w ith the delta(13)C of kerogen, n-alkyl lipids from deep-water Proteroz oic sediments were enriched in C-13 and appear to be derived mainly fr om heterotrophs whilst open marine Phanerozoic counterparts are C-13 d epleted and evidently derived mainly from autotrophs. Data from the sa mples studied here are consistent with a model invoking a change in th e redox structure of the ocean, possibly aided by the innovation of fa ecal pellets. Copyright (C) 1997 Elsevier Science Ltd.