Phytoplankton contributions to the trace element composition of Precambrian banded iron formation

Abstract

Banded iron formations (BIFs) are prominent sedimentary deposits in Earth’s Precambrian rock record, consisting of alternating iron-rich (hematite, magnetite and siderite) and silicate/carbonate (quartz, clay-like minerals, dolomite and ankerite) layers. On the basis of chemical analyses from BIF units of the 2.48 Ga Dales Gorge Member of the Hamersley Group in Western Australia, it was previously suggested that most, if not all, of the iron in BIF could have been oxidized by anoxygenic phototrophic bacteria (photoferrotrophs) at cell densities considerably less than those found in modern Fe-rich aqueous environments (Konhauser et al., 2002). However, oxygen-producing phytoplankton may have also been capable of supplying the necessary oxidizing power. Here, we revisit the question of the anoxygenic and oxygenic phytoplankton populations necessary to account for BIF deposition and quantify the amount of selected trace elements (P, Mn, Co, Ni, Cu, Zn, Mo, Cd) that could have been associated with their biomass. Using an expanded geochemical dataset for the Dales Gorge Member as an example, we find 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. Further, spurred by the similarities between BIF and anoxygenic phytoplankton trace element stoichiometries, we suggest that much of the trace element inventory preserved in the BIF was at some point biologically assimilated in the water column, released from degrading photoferrotrophic biomass at the seafloor and in the sediment pile, and ultimately fixed in the iron-rich sediment in approximately stoichiometric proportions by near quantitative adsorption to ferrihydrite. Our observations suggest that, as today, phytoplankton and the recycling of their biomass exerted significant control over the trace element composition of ancient seawater and sediment.