We thank Nutman and Friend (2008) for their interest in our work on the enigmatic Akilia rocks (McKeegan et al., 2007) and for affording us an opportunity to further discuss our results. Contrary to the impression given by Nutman and Friend, the purpose of our study was neither to explicitly seek biogenic remnants nor to quantify the frequency of occurrence of apatite-hosted graphitic inclusions in Akilia sample G91–26. Our intent was also not to elucidate the complex geologic history of the Akilia supracrustals (interested readers should consult Manning et al. ). Rather, our more modest goal was to demonstrate the occurrence of apatite-hosted graphitic inclusions in this rock and to document the morphology, molecular state, and isotopic composition of one such specimen.
Our work was motivated by Lepland et al. (2005) who reported that they had been unable to confirm the presence of graphite in Akilia apatite, a result echoed in a widely circulated opinion piece (Moorbath, 2005). Such contributions cast doubt on the reliability of the carbon isotope data obtained in the University of California–Los Angeles secondary ion mass spectrometry (SIMS) lab, where Mojzsis et al. (1996) carried out the original study, suggesting, for example, that “it remains to be established what objects were analyzed” (Lepland et al., 2005, p. 78). A measure of uncertainty was justified: the original study did not include high-resolution images of the targets analyzed, and the isotopic measurements themselves consumed the subnanogram carbon spots uncovered by successive sputtering of the Cs+ primary ion beam. To resolve such uncertainty, we employed instrumentation that was not available to Mojzsis and colleagues in 1995. Raman imagery enabled us to make a high-resolution three-dimensional map of a quartz-enclosed, apatite-hosted graphite inclusion and to characterize this three-mineral complex in situ, prior to ion micro-probe analysis. Isotopically light carbonaceous (graphitic) inclusions demon strably occur in apatite in this rock. The implications of this finding may be debatable, but the existence of graphitic inclusions in apatite grains of the Akilia sample G91–26 is not.
In their commentary, Nutman and Friend place emphasis on the fact that the Akilia apatite grains are of metamorphic origin, rather than being syngenetic with rock formation. That both the apatite and their graphitic inclusions are metamorphic minerals is not in doubt, but as our work was intended to address the existence of such inclusions, not their geologic history, we saw no need to cite Nutman and Friend (2006) regarding rare-earth element (REE) distributions. In fact, the relevance of an exchange of REE between apatite and a garnet-equilibrated reservoir to the central issue of whether graphite retains an isotope composition of any pre-diagenetic material is not clear. Nevertheless, we agree that the petrogenesis of the graphitic inclusions and the timing of their emplacement are important unresolved issues. Regarding the origin of such carbonaceous matter, Nutman and Friend acknowledge that “recrystallization of … biogenic material is ... one possibility,” a possibility, it should be noted, that is consistent with the measured carbon isotopic composition.
Nutman and Friend also raise uncertainty about the accuracy of the isotopic data, suggesting that they may result from the presence of “a mixed-carbon source” or “carbonate-bearing apatites that are 13C-depleted.” This concern, based on measurements (Nutman and Friend, 2006) of infrared spectra of seven large (100 μm) apatites separated from G91–26 which showed that up to 0.5 wt% carbonate may substitute for phosphate, is misguided. No evidence of carbonate was detected in the Raman spectrum of the specific apatite grain analyzed. But even if we assume that carbonate was present at 20× the level suggested by Nutman and Friend (2006), it would make no quantitative difference on the carbon isotopic composition measured by SIMS (regardless of its isotopic composition). The reason is well known among SIMS practitioners: under Cs bombardment, the yield of negative carbon ions is ~100× higher from reduced carbon-bearing compounds than from carbonate (for C2−, which we utilized, this factor is close to 2000). Thus, because our analyses were accomplished in situ by focusing the Cs beam onto the specific inclusion imaged by the Raman confocal microscope, any carbon signal coming from beam overlap onto the apatite would have been negligible. Even given the more subtle issue of “matrix effects” on instrumental mass bias, a problem well investigated for SIMS measurements of carbon isotopes both in geologic (McKeegan et al., 1985) and mineral-hosted biologic samples (House et al., 2000; Sangély et al., 2005), we are confident that the isotopic results presented are accurate to the precision stated in our paper.
Our paper firmly establishes the occurrence of isotopically light graphitic carbon included in apatite grains of ~3,830 Ma rocks at Akilia Island. These data are consistent with the existence of life at that remote time in geologic history, but as we state in the paper, “we do not claim [them to be] unequivocal evidence of early life” (McKeegain et al., 2007, p. 593).