Such analyses do not permit correlation of the isotopic values measured with the kerogen comprising individual microscopic fossils, the cellular morphology of which might be expected to provide evidence of their affinities and, thus, their metabolic capabilities. This deficiency has

been addressed by use of secondary ion mass spectrometry (SIMS), a technique permitting direct measurement of the isotopic composition of the kerogenous cell walls of individual fossils, which has been applied to Precambrian microorganisms ranging from ~850 to nearly 3,500 Ma in age (Fig. 10 ). A technique that has been used both for the isotopic analyses (House Tipifarnib concentration et al. 2000; Ueno et al. 2001a, b) and elemental mapping (Oehler et al. 2009) of such fossils, the consistency between the δ13CPDB values measured by SIMS on individual microfossils and those obtained by conventional mass spectrometry on bulk kerogens from the same rock samples demonstrates the efficacy of the technique (Fig. 10). Recently,

McKeegan et al. (2007) have used SIMS to establish the presence of 12C-rich graphitic carbon in the oldest sedimentary rocks find more now known, from Akilia Island off southwestern Greenland, the carbon isotopic composition of which (δ13CPDB-29 ± 4‰) suggests that autotrophic microbes may have existed as early as ~3,830 Ma ago. Fig. 10 Carbon isotopic values of individual

Precambrian microfossils measured by secondary ion microprobe spectrometry (SIMS) compared with those of the carbonate and total organic carbon measured in bulk samples of the same geological units. Values plotted for carbonate and total organic carbon are from Strauss and Moore (1992); for microfossils from the Bitter Springs and Gunflint Formations, from House et al. (2000); and those for microfossils from the Dresser Formation, Olopatadine from Ueno et al. (2001a) Despite such progress and the now-established paleobiological usefulness of SIMS, evidence provided by this technique does not resolve the question of the time of origin of oxygen-producing photosynthesis. As yet, the SIMS-based data are too few and too imprecise to show definitively whether the individual fossils analyzed were oxygenic or anoxygenic photoautotrophs (cf. House et al. 2000), and the results even of the most recently published such isotopic work (McKeegan et al. 2007) can only hint at the presence of autotrophs ~3,830 Ma ago since it remains to be established whether the graphite analyzed dates from the time of deposition of the metasediment in which it occurs or was formed later, during the severe metamorphism to which the Akilia rocks have been subjected.