Oxic Fe(III) reduction could have generated Fe(II) in the photic zone of Precambrian seawater

Abstract

Many marine Precambrian iron formations (IF) record deep anoxic seawater enriched in Fe(II) (i.e. ferruginous) overlain by mildly oxygenated surface water. This is reflected by iron-rich sediments forming in deep basins, and relatively iron-poor sediments forming in shallow, sunlit waters. Such an iron gradient is often interpreted as a redox interface where dissolved Fe(II) was oxidized and precipitated as Fe(III)-bearing minerals. As such, sedimentary iron enrichments are proxy to the progressive oxidation of the oceans through geological time. However, this interpretation is founded on the assumption that Fe(II) could not persist within an oxygenated water column. Here, we cultivated cyanobacteria in an illuminated column supplied with Fe(II)-rich seawater medium in a laboratory-scale analog of a continental margin supporting IF deposition. We first observed Fe(II) oxidation with oxygen, then biologically-mediated reduction of Fe(III) (oxyhydr)oxides, which maintained a pool of Fe(II) in the presence of oxygen. Such steady-state iron redox cycling may have maintained dissolved, and hence mobile Fe(II) in oxygenated seawater above ferruginous deep basins such as those inferred for many Precambrian IF.

Depositional Setting of Archean BIFs from Congo: New Insight into Under-Investigated Occurrences

“…Moreover, several negative Ce anomalies are reported from the IAB and CAB (Mayoko and Zanaga BIF samples). They are neither related to negative La and Y excursions nor to low Y/Ho values, suggesting oxic, open-basin depositional conditions (e.g., MO1160-2 from the Mayoko BIF unit, or ZA6210-1 and ZA6210-3 from the Zanaga unit; Figure 8), as recently observed in several Precambrian metasedimentary units [52][53][54].…”

“…Meanwhile, redox conditions may have constrained the relative depth of sample deposition, suggesting that samples indicating oxic conditions would have been deposited in the photic zone, whereas others would be from deeper, anoxic areas [54].…”

The biogeochemistry of ferruginous lakes and past ferruginous oceans

“…Further explorations of this oxic iron cycle and could be explored as indicators of oxygenic photosynthesis (e.g. Swanner et al, 2018).…”

Iron isotope transformations in the meromictic Lake Cadagno

“…As iron(III) oxy and hydroxy compounds sink through the chemocline, dissimilatory iron(III) reduction and dissolution reactions are also likely to occur (Dos Santos Afonso and Johnson et al, 2004;Lovley, 1991;Swanner et al, 2018). Dissimilatory iron(III) reduction and dissolution tends to occur when the supply of organic matter is high and can be facilitated directly and indirectly by sulfur reducing bacteria Lovley, 1991;Lovley et al, 1993;Weber et al, 2006) and anaerobic heterotrophic bacteria (Berg et al, 2016).…”