Carbon Isotope Fractionation

Carbon isotope fractionation is the only technique that provides evidence for life in 3.8-3.9-Gyr-old rocks (Mojzsis et al., 1996; Rosing, 1999; Schidlowski, 1988), because these ancient rocks have been so heavily metamorphosed that fossil evidence could not have survived to the present day. Graphite in the 3.8-Gyr-old highly metamorphosed sedimentary rocks (metasediments) of Western Greenland was found to have an isotopic signature of ¿13C « — 28%o, seemingly indicative of biology early in the Earth's history (Schidlowski, 1988). Mojzsis et al. (1996) claimed that graphite particles were found within a banded iron formation and in apatite grains in the oldest, Akilia, metasediments, providing a reasonable geological context for the proposed biosignature.5 In this picture, the graphite would have formed from carbonaceous material originally derived from microbial life.

However, some have since argued against the biogenic nature of the graphite (Fedo and Whitehouse, 2002; van Zuilen et al., 2002). A ¿13C value of —10 to —12 in graphite can be obtained when the mineral siderite (FeCO3) is thermally decomposed at temperatures above 450°C (van Zuilen et al., 2002) according to 6FeCO3^2Fe3O4 + 5CO2 + C. The resulting magnetite (Fe3O4) and istopically light graphite (C) remain in the rock while the CO2 escapes. Van Zuilen et al. (2002) also suggest that recent organic material possibly emplaced by percolating ground water may also play a role in the especially light graphite values. The original claims for life reported ¿13C « —28%o (Mojzsis et al., 1996; Schidlowski, 1988), but more recent analysis of the graphite particles with a larger number of samples has found the signature to be more varied, ranging from ¿13C of — 18%o to +2%o (Ueno et al., 2002). This could be in agreement with the abiogenic fractionation process just described. The observed carbonates would then not be sedimentary, but rather metasomatic in origin (van Zuilen et al., 2002). That is, they would result from high-temperature fluids interacting with iron-rich material, rather than from iron precipitation and sedimentation, as is often associated with microbial communities. Worse is the claim that the apatite crystals do not, in fact, contain graphite inclusions (Lepland et al., 2005). Moorbath (2005) concludes a brief review by saying that there is only one site left in the Isua region where an argument for an isotopic biosignature older than 3.7 Gyr remains possible (Rosing and Frei, 2004).

It is therefore at least possible that the short-time windows (<108 years) for the origin of life once posited to be bounded by the last ocean-vaporizing

5 Banded iron formations are sedimentary rocks rich in oxidized iron that may therefore be indicative of the onset of oxygenic photosynthesis. Apatite, a form of calcium phosphate, is a mineral often found in association with microbial activity.

impact and the earliest (—3.8-3.9 Gyr ago) evidence for life on the Earth will need to be relaxed by hundreds of millions of years. If the origin of life occurred closer to 3.5 Gyr ago than to 3.9 Gyr ago, exogenous input of organics from comets would be substantially less at the time of life's origin. Here we continue to use 4.0 Gyr ago as our time of comparison; moving this date to 3.5 Gyr ago would reduce our estimates of exogenous input by about an order of magnitude.

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