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Project Progress

Our research on the geochemical signatures of complex life and indicators of ocean redox conditions include studies of modern organisms and environments as well as studies of the sedimentary rock record through the Proterozoic and into the Cambrian.

In our continuing study of the rock record we have shown that sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation 635Myr ago (Love et al., 2009). These biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high precision geochronology, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate. Indirectly, these results show that the ocean contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the Sturtian (713 Myr ago), the first globally extensive Neoproterozoic glacial episode. It does, however, seem that there is a convincing record of abundant steroid-producing organisms (i.e., eukaryotes), likely protists, before that time (Ventura et al., 2009).

MIT graduate student James Saenz has been developing improved methodologies for bacteriohopanoid analysis by HPLC-MS. Hopanoid distributions, along with analyses of the sqhC gene through a land-sea transect in a modern carbonate sedimentary setting, were determined from the Bahamian island of San Salvador. The results suggest that the diversity of both hopanoid producers and the hopanoids they produce is lower in open ocean environments than it is in marine margins and on land (Pearson et al., 2009). Hopanoids with a 2-methyl substituent, and biomarkers hypothesised to be produced by cyanobacteria were present in low concentrations in all samples. However, they were most abundant at the non-marine end of the transect. This raises questions about their sources in the marine geological record (Pearson et al., 2009).

Going forward, we plan to study the hypothesised role of cyanobacterial anoxygenic photosynthesis in modulating the oxygen content of the Proterozoic ocean (Johnston et al., 2009), particularly the diversity of cyanobacteria that are able to use sulfide as an electron donor for photosynthesis. This work will be tied in with associated effects on biomarker lipid composition including pigments and hopanoids. We are also studying the lipid compositions of a wide range of protists, sponges cindarians and ctenophores and evaluating them for unique compounds that may have a preservable fossil record.