MARINE MICROBIAL PLANKTON CONTRIBUTIONS TO THE TRACE ELEMENT COMPOSITION OF PRECAMBRIAN BANDED IRON FORMATION (Invited Presentation)

Banded iron formations (BIFs) are economically important sedimentary deposits in Earth’s Precambrian rock record, consisting of alternating iron-rich and silicate/carbonate layers. It is widely accepted that most, if not all, of the iron in BIF precipitated initially as a ferric oxyhydroxide in the marine water column through the metabolisms of either anoxygenic phototrophic bacteria (photoferrotrophs) or cyanobacteria. Using chemical analyses from BIF units of the 2.48 Ga Dales Gorge Member of the Hamersley Group in Western Australia as an example, it was suggested that with turnover times comparable to those seen in modern ecosystems, the same phytoplankton populations required to form BIF could have supplied the entirety of trace elements found in this iron-rich deposit (1). Further, spurred by the similarities between BIF and plankton trace element stoichiometries, it was suggested that much of the trace element inventory preserved in BIF was at some point biologically assimilated in the water column, released from degrading biomass at the seafloor and in the sediment pile, and ultimately fixed in the iron-rich sediment in approximately stoichiometric proportions. However, this view is complicated by two recent experimental studies which demonstrated that in the presence of dissolved silica, different photoferrotroph cell surfaces repel ferric oxyhydroxides to different extents (2-3). Therefore, in silica-rich Precambrian seawater, this repulsion would separate biomass from ferric iron and would lead to large-scale deposition of BIFs lean in organic matter. Simultaneously, excess biomass not deposited with BIF would have deposited in coastal sediments to form organic-rich shales. In an attempt to resolve these apparent contradictions, current studies are being aimed at testing a range of photoferrotroph and cyanobacteria strains to better ascertain how different cell surface reactivities (i.e., ligand composition, presence or absence of extracellular polysaccharides) influence cell-iron oxyhydroxide aggregation and sedimentation.

(1) Konhauser et al., 2018. Geological Society of America Bulletin, 130:941-951.

(2) Thompson et al., 2019. Science Advances, 5:2869, 9 pp.

(3) Schad et al., 2019. Geochimica et Cosmochimica Acta, 265:386-412.