Astrobiology Workshop, Macquarie University July 12-13 2001

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Mission to Early Earth: Sample Integrity and Geohydrology.

Thomas L. Kieft (New Mexico Institute of Mining and Technology)

The goal of NASA's proposed Mission to Early Earth (MtEE) is to improve our understanding of the emergence and early evolution of life, particularly as it relates to the environment of the early Earth. MtEE proposes to drill and core Archaean rocks to analyze them for biomarkers. For this purpose, it will be necessary to obtain pristine core material that is representative of the subsurface, avoiding possible sources of contamination. Subcoring can be used to remove potentially contaminated outer portions of the core. Tracers can be used to quantify the potential for contaminant transport into the core. Also, the unique chemical and biological signatures of the drilling fluid can be compared to those of the subcore as a means of detecting and quantifying contamination. It will also be useful to quantify the potential for transport of microorganisms into deep Archaean strata since the time of geological deposition. We have addressed similar issues in other deep subsurface environments. Porewater ages can constrain the youngest possible age of the biological community. To do this, one can characterize the regional hydrodynamics. The chloride mass balance method quantifies recharge through the overlying vadose zone. Isotopic tracer analyses can be used to date the water. These include tritium and 14C for relatively young groundwater, 36Cl and 129I for older groundwater, and possibly analyses for accumulated noble gases such as 4He and 40Ar in very old groundwater. Sensitive assays for extant, moribund, or even long-dead microbes can also be applied. These include radiotracer assays, phospholipid fatty acid (membrane lipids from live cells) and diglyceride fatty acid (membrane lipids from dead cells) assays, and molecular characterization using 16S rDNA methods. These microbiological assays could be a useful complement to chemical and physical biomarker assays.