Evidence for Anaerobic Photosynthesis in an Early Triassic Sea

In the present study we have undertaken the first detailed geochemical study of the redox conditions that existed in the paleowater column during the P/Tr 'Superanoxic Event'. We have utilised carbon and sulfur isotopic data, biomarker (metalloporphyrins and green sulphur bacterial biomarkers) and redox sensitive metal abundances (dithionite-extractable iron- FeD + pyrite iron- FeP)/ total Fe- FeT) of a proven petroleum source rock of a recently discovered marine Permian-Triassic (P/Tr) drill core (Hovea-3) from the onshore northern Perth Basin, Western Australia. Several biomarkers attributed ultimately to anoxygenic photosynthetic green sulfur bacteria (GSB) have been identified in the Early Triassic (Griesbachian) drill core. Similar results from the P-Tr Global Stratotype section and Point in South China have been found. Thus, waters of the southern Tethys Ocean were periodically euxinic in the photic zone during and after the mass extinction event and the pervasiveness and duration of sulfide in surface waters may have been a leading factor in the mass extinction and the protracted recovery. Stable carbon isotopic data from the bulk kerogen fraction of Hovea-3 shows an abrupt 7.5 per mil negative shift from the Upper Permian to Lower Triassic consistent with a localised palynofacies change from charcoal-wood dominated organic matter to algal-amorphous organic matter, respectively. However, 13C/12C data on higher plant and phytoplankton biomarkers show similar isotopic changes across the P/Tr transition indicating a global disruption to the carbon cycle. GSB biomarkers have also been identified in several Perth Basin crude oils. Photic zone euxinia (PZE) is usually associated with the widespread deposition of organic matter-rich sediments that constitute important source rocks for petroleum deposits that are being exploited today. With the exception of the Perth Basin, such organic matter-rich sediments are virtually absent from Upper Permian and Lower Triassic sediments globally. The onset of PZE in Hovea-3 coincides with a sharp facies change, reflecting rapid transgression. Localized surface ocean productivity must have played a key role in the deposition of a petroleum source rock at this location, although PZE was globally more widespread during the P/Tr' Superanoxic Event'.

Fig. 1 Organic and inorganic parameters from Hovea-3 borehole (Grice et al., 2005a, Science, 307, 706-709).

Personnel

K. Grice, R. Twitchett, M. Boettcher, E. Grosjean , R.E. Summons, S.C. Turgeon, W. Dunning, G.D. Love , C. Cao, J. Yugan

Funding

ARC (QEII), Max Planck Society (Munich), MIT was supported by the NASA Exobiology grant NAG5-1236.

Collaborators

Plymouth University (UK), MIT (USA), Max Planck Institute for Marine Microbiology (Germany), Oak Ridge, Laboratory (USA), Nanjing Institute of Geology and Palaeontology, Nanjing, China.