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Project Progress

Evidence for life on the early Earth more than 3 billion years ago comes mainly from four criteria: (1) putative microfossils (Schopf, 1993); (2) ?light? carbon isotope ratios in sedimentary organic matter (Mojzsis et al., 1996); (3) ?heavy? carbon isotope ratios in marine limestones (Schidlowski, 2001); and (4) conical sedimentary structures known as stromatolites (Hofmann et al., 1999). Each of these criteria is being challenged and is in need of further verification (Kerr, 2002).

The chemical composition of the earliest and best candidate microfossils has been established to be carbonaceous for the first time by means of laser-Raman spectral imaging at a microscopic scale (Schopf et al., 2002). It will now be important to measure in-situ carbon isotope compositions using the ion microprobe technique pioneered at UCLA by Mojzsis et al. (1996) and House et al. (2000).

Although the source and biological significance of graphite particles within the 3.8-3.9 billion-year-old metamorphic rocks of southwest Greenland remain contentious, the sedimentary origin of the host rocks on Akilia island has been unequivocally demonstrated by detailed field mapping (Manning et al., 2001). This contradicts the conclusion of Fedo and Whitehouse (2002), who postulated a metamorphosed igneous origin for the host quartzites. The 3.83 billion-year age of the metamorphosed sediments is well established by new zircon dates on cross-cutting granites (Mojzsis and Harrison, 2002).

Complex conical layering in a hundred-kilometer-wide sedimentary unit known as the Strelley Pool Chert (Lowe, 1983) has been considered evidence for the earliest known microbial communities (Hofmann et al., 1999). These 3.5 billion-year-old stromatolites, seen in cross-section or as naturally weathered three-dimensional features, are striking features of the Western Australian landscape. As such, they are obvious targets for Rovers on Mars. However, Grotzinger and Rothman (1996) argued that abiogenic processes could be responsible for equally complex stromatolites.

Building on Grotzinger?s and Rothman?s work, UCLA physicist Per Jögi is developing quantitative methods to understand the evolution of complex surfaces in three dimensions (technically, 2+1 dimensions). Frequency spectra obtained from vertical slices through successive modeled surfaces are compared with spectra obtained from natural outcrops. Complex conical structures can be constructed in this way, but only by introducing properties, such as correlated noise (think of honking in a traffic jam), that require the spatial communication of information as might be carried out by microorganisms (Jögi and Runnegar, in preparation).

Until recently, it was thought that the sulfur isotope composition of early Archean seawater, and any sulfide derived from it by reduction of seawater sulfate, was close to the Earth?s mantle value of 0?. Working on intimately associated sulfate and sulfide minerals from the 3.5 billion-year-old North Pole area of Western Australia, Shen et al. (2001) concluded that the broad spread of sulfur isotope values they obtained is evidence for the early appearance of bacterial sulfate reduction. However, measurements made at UCLA on similar samples suggest a different interpretation (Runnegar et al., submitted). By measuring the four stable sulfur isotopes (³²S, ³³S, ³⁴S, ³⁶S) using an ion microprobe, UCLA geochemists were able to show that sedimentary sulfides could not have been made from seawater sulfate. The extra dimension provided by the ion microprobe measurements implicates atmospheric processes in Archean sulfur cycling (Farquhar et al., 2000, 2001), an effect that could not be observed by Shen et al. using only the two common isotopes of sulfur (³²S and ³⁴S).