Direct evidence for eoarchean iron metabolism?

Metasedimentary rocks from Isua, West Greenland (> 3,700 million years old) contain carbonaceous compounds, compatible with a biogenic origin (Hassenkam, Andersson, Dalby, Mackenzie, & Rosing, 2017; Ohtomo, Kakegawa, Ishida, Nagase, & Rosing, 2014; Rosing, 1999). The metamorphic mineral assemblage with garnet and quartz intergrowths contains layers of carbonaceous inclusions contiguous with carbon-rich sedimentary beds in the host rock. Previous studies (Hassenkam et al., 2017; Ohtomo et al., 2014; Rosing, 1999) on Isua rocks focused on testing the biogenic origin of the carbonaceous material, but here we searched for evidence which could provide new insights into the nature of the life that generated this carbonaceous material. We studied material trapped in inclusions armoured within quartz grains inside garnet porphyroblasts by non-destructive ptychographic X-ray nanotomography (PXCT). The 3D electron density maps generated by PXCT were correlated with maps from X-ray fluorescence tomography and micro-Raman spectroscopy. We found that the material trapped inside inclusions in the quartz grains consist of disordered carbon material encasing domains of iron-rich carbonaceous material. These results corroborate earlier claims (Hassenkam et al., 2017; Ohtomo et al., 2014; Rosing, 1999) for biogenic origins and are compatible with relics of metamorphosed biological material originally containing high iron/carbon ratios, comparable to ratios found in most extant organisms. These iron-rich domains represent the oldest evidence for organic iron complexes in the geologic record and are consistent with Fe-isotopic evidence for metabolic iron fractionation in > 3,700 Ma Isua banded iron formation (Czaja et al., 2013; Whitehouse & Fedo, 2007).