Temperature fluctuations in the Archean ocean as trigger for varve-like deposition of iron and silica minerals in banded iron formations

Banded iron formations (BIFs) are Fe- and Si-rich chemical sediments that were deposited in the oceans throughout the Archean Eon (4.0–2.5 billion years ago; Ga). Despite extensive research pertaining to their composition, mineralogy and depositional settings, the mechanism(s) leading to their characteristic layering is still a matter of debate. Recently, it has been proposed that temperature fluctuations could have acted as the unifying trigger for the precipitation and deposition of Fe(III)-mineral-rich layers mediated by phototrophic Fe(II)-oxidizing bacteria during warm periods and the deposition of silica (SiO2)-rich layers by abiotic precipitation of dissolved silica during cold periods. To verify the feasibility of such a mechanism, we conducted laboratory experiments with the marine phototrophic Fe(II)-oxidizing bacterium Rhodovulum iodosum under simulated Archean ocean conditions. The temperature was cycled between 26 °C (warm period) and 5 °C (cold period) to mimic temperature fluctuations in the past. Our results showed that R.iodosum readily oxidized Fe(II) during warm periods resulting in the formation of an orange-brown Fe(III) mineral layer, whereas it was inactive during cold periods. Conversely, silica largely stayed in solution during warm periods but was precipitated abiotically as an amorphous, gel-like layer during cold periods, enhanced by Si addition that led to Si oversaturation. Most importantly, during Fe(III) mineral precipitation, most silica stayed in solution leading to an independent precipitation of the Fe- and Si-rich layers. This is due to inhibition of silica sorption onto the biogenic Fe-minerals caused by sorption of microbially derived organic matter that contains negatively-charged carboxyl/phosphodiester groups. Analyses of precipitation rates and theoretical sedimentological considerations suggest that this process could explain the banding in BIFs on the microband level and produce sediments of similar thicknesses as found in nature.