Carbon isotopes of Proterozoic filamentous microfossils: SIMS analyses of ancient cyanobacteria from two disparate shallow-marine cherts

Abstract

The surface ecosystem of the Precambrian Earth was dominated by marine, planktonic and benthic phototrophic microorganisms, most prominently stromatolite-forming cyanobacteria, anoxygenic photosynthetic bacteria, and associated microbes. Although coupling the early microfossil record to physiologically definitive geochemical and isotopic signatures remains challenging, insights can be derived from isotopic studies of individual fossil microorganisms permineralized in Precambrian cherts. Here, we use correlated optical microscopy, Raman spectroscopy, and secondary ion mass spectrometry (SIMS) to link morphology with the molecular and carbon isotopic composition (δ13Corg) of individual filamentous microfossils (Eomycetopsis sp.) composed of thermally immature kerogen and preserved in two spatially and temporally distinct shallow-marine cherts of the Proterozoic Gaoyuzhuang Formation (∼1,560 Ma, northern China) and Kwagunt Formation (∼850 Ma, Arizona, USA). In both geologic units studied, the Eomycetopsis fossils yielded virtually indistinguishable δ13Corg values (–29.0 ± 2.0‰) that are comparable to values resulting from carbon fixation via the RuBisCO enzyme in the Calvin cycle of oscillatoriacean cyanobacteria, the main oxygenic phototrophs of the Proterozoic.