TRACING OF THE COUPLED SI AND FE CYCLE IN THE ARCHEAN OCEAN

High Si concentrations had a profound impact on the biogeochemical cycle in the Precambrian ocean, and on the formation of important Precambrian geological records (BIFs and cherts). Si isotopes are useful tracers for the Precambrian Si cycle. Our new work on a series of biological and abiologic experiments^[1,2] shows that early interpretations of Precambrian δ³⁰Si records are too simplistic in that they do not account for the very large isotopic effects of Fe-Si interactions, relative to pure Si systems.

Our experiments showed that amorphous Fe-Si gel is a likely precursor of BIFs^[1,3], potentially revising estimates of Si and Fe concentrations of Precambrian seawater. Also, our experiments showed that abiologic incorporation of Fe(II) into Fe(III)-Si gel is difficult to reach the Fe(II)-Fe(III) stoichiometry of magnetite, a common mineral found in BIFs. Microbial dissimilatory iron reduction of Fe(III)-Si gel can, however, easily produce a solid with Fe(II)-Fe(III) stoichiometry equal to magnetite, and leaves a unique, negative δ³⁰Si signature that should be preserved in quartz associated with magnetite upon phase transformation of Fe-Si gel.

Here we test experiment predictions through measurement of δ³⁰Si values in ~3.2 Ga Manzimnyama BIF, Barberton, South Africa (BARB 4 drill core). Analysis is focused on quartz that is closely associated with hematite and magnetite. Preliminary δ³⁰Si results from hematite-rich samples show a large spread (-3.2‰ and -0.5‰). δ⁵⁶Fe results from the same samples were interpreted to reflect the earliest oxygenic photosynthesis^[4]. A broad negative correlation between δ³⁰Si and δ⁵⁶Fe for closely-related quartz and Fe oxide was observed, possibly reflecting a link between the extent of Fe oxidation and production of Fe-Si gel in the ocean. A lower extent of Fe(II) oxidation (higher δ⁵⁶Fe) presumably led to formation of Fe-Si gel with limited Fe(II) incorporation, which would decrease δ³⁰Si values. It is not yet clear, however, how formation of magnetite may occur through this mechanism. Further analysis of δ³⁰Si and δ⁵⁶Fe in magnetite-rich samples is underway, and will be used to further elucidate the origin of magnetite and the Si cycle at ~3.2 Ga.

[1] Zheng et al., GCA, 2016; [2] Reddy et al., GCA, 2016; [3] Konhauser et al., Earth-Science Rev, 2017; [4] Satkoski et al., EPSL, 2015