SIMS CARBON ISOTOPE ANALYSES OF FILAMENTOUS ORGANIC-WALLED MICROFOSSILS PERMINERALIZED IN PROTEROZOIC CHERTS

Secondary ion mass spectrometry (SIMS) has become a useful tool for exploring the early evolution of microbial metabolisms and carbon-fixation pathways by analyzing the carbon isotope values (δ¹³C) of individual organic-walled Precambrian microfossils. However, significant overlap exists for a wide range of biological fractionations (e.g., 10 to 20‰) that can complicate direct interpretation of isotopic measurements for preserved microfossils. Natural (in situ) and laboratory (culture) studies of extant microorganisms have revealed differences in the maximum carbon isotope fractionations between various lineages of microbes, and it is on the basis of such maximum fractionations that δ¹³C measurements made for morphologically distinct microfossils can be correlated with carbon fixation pathways and paleometabolic interpretations. Filamentous microfossils from conical stromatolites of the Gaoyuzhuang Formation (~1,425 Ma, northeastern China) and pisolitic chert of the Kwagunt Formation (~850 Ma, Arizona) have been measured via SIMS for their δ¹³C values. The microfossils have been previously described as cyanobacteria (e.g., Eomycetopsis filiformis; Schopf et al., 1984) owing largely to their characteristic filamentous morphology and shallow-marine depositional environment. In both of these units the fossilized microbes occur in layered mats, microbial assemblages that because of their regularly layered organization have been uniformly interpreted to be photoautotrophic, an organization attributed to the spread of such microbes across surfaces in their need for light and widely assumed to evidence an aerobic environment. However, the SIMS-measured δ¹³C values of these Proterozoic microfossils are more ¹³C-depleted than those that have been observed for modern oxygenic cyanobacteria. The ¹³C-depleted values reported here most likely evidence the presence of anaerobic methane cycling and either facultative oxic-anoxic cyanobacteria (cf. Oscillatoria limnetica; Cohen et al., 1975) or anoxygenic photosynthetic bacteria preserved in stromatolites of the oxygen-deficient lower photic zone in the (oxygen-stratified) Proterozoic ocean.