nai

Project Progress

The Archean Biosignatures project is not so new to the JSC team. We have done work on the Waroonga formation and now we have extended that work in to the Pilbara of Western Australia. We can apply what we learn from Earth’s history to other worlds in our solar system. We can do this comparison through astromaterials, such as Martian meteorites. Astromaterials come to us largely devoid of context, making their interpretation both difficult and complex. To develop effective biospheric models we need to understand the context, at a stratigraphic and planetary level, in which life evolved and was preserved. Given our knowledge of the terrestrial planets, it is reasonable to assume that regardless of size their early histories will have many parallels. In this context Archean Earth should provide us with a reasonable analogue for early Mars and other terrestrial planets. To this end we have an ongoing program at the JSC Astrobiology Institute analysing rocks from the Achean and Early Paleoproterozoic succession from the Pilbara of Western Australia. The Pilbara Craton is unique in that it includes part of an Archaean protocontinent consisting of granitoid complexes that were emplaced into and overlain by a 3.51-2.94 Ga volcanigenic carapace, the Pilbara Supergroup (Van Kranendonk, 2000). The craton is in turn overlain by a series of younger basins that form a time series recording Earth history from c.2.8 Ga to c.1.9 Ga. In spite of their age the rocks of the Pilbara are only gently deformed and metamorphosed, for the most part, to lower greenschist facies.

Our work to date on the hydrothermal systems preserved in the Archean Pilbara succession suggests that abiotic hydrothermal processes dominate sedimentation, making it very difficult to isolate biotic signatures. The zones around hydrothermal vents in particular are swamped by silica and carbonate sedimentation, which is frequently disrupted by later hydrothermal events forcing their way, sometimes explosively, to the sea floor. During our 2002 field season to the Pilbara we initiated a reconnaissance program to search for signs of microbial mats in zones of quieter sedimentation away from vents. Promising results have come from fine-grained clastic sediments in the Paddy Market Formation. The formation consists of thinly laminated shales and sandstone — probably turbiditic in origin (Van Kranendonk, 2000). We have been able to identify structures comparable with wrinkle mats and also concretionary structures suggesting activity of sulfur reducing life forms. We will now expand this program both in outcrop, where the mat structures are more obvious because of weather, and also in core samples, where the primary geochemistry is more likely to have been preserved. We have access to large numbers of deep cores through local mineral exploration companies (Sipa Resources and Newexco, both in Perth) and will carry out fieldwork in a joint program with the Geological Survey of Western Australia.