Bolide impact–driven ocean redox stratification and microbial Fe cycling as a trigger for Neoarchean BIF deposition

Bolide impact events represent an important connection between solar system dynamics and the evolution of Earth’s early atmosphere and biosphere. Preserved carbonate platforms offer important records of these processes, as these successions have the ability to capture changes in sea water chemistry and biological processes. The Archean represents a critical time interval in Earth’s geobiological evolution when large scale bolide impacts occurred frequently. The Neoarchean in particular covers a time period in which major changes occurred in ocean and atmospheric composition, crustal evolution and the biosphere, potentially contributing to the rise in atmospheric oxygen during the great oxidation event (GOE).

The ~2.63 Ga Carawine Dolomite (Hamersley Group, Pilbara Craton, Western Australia) preserves an extensive carbonate platform succession, including a bolide impact event expressed as an impact spherule bearing megabreccia (SBMB)¹. Notably, stratigraphically above the impact layer, we find the shallow-water expression of the first banded iron formation (BIF) of the Hamersley Group (recorded in the Marra Mamba Iron Formation) within the Carawine Dolomite. In this study, we aim to reconstruct a detailed stratigraphic framework representing a conformable transect from deep to shallow-water facies, including the SBMB and BIF, and a chemostratigraphy using major-, trace- and rare earth element (+ yttrium, REY) concentrations and inorganic/organic stable carbon isotope pairs.  

Seven stratigraphic sections were measured in the field, covering >400 meters of stratigraphy. One shallow and one deep-water section were sampled in high resolution (every 0.5 to 1 m) for geochemistry. The shallow-water section comprises a shallow subtidal facies containing tented microbialites and aragonite (pseudomorph) crystal fans; a storm facies consisting of oncolytic peloidal grainstone; and sub- to peritidal facies assemblage consisting of shallow water sedimentary features including thrombolites, ooids and exposure surfaces. REY-patterns are consistent with a shallow marine environment with episodic mixing of river water. The absence of negative Eu-anomalies indicates only minor contribution of hydrothermal fluids to the REY-budget. The storm facies shows elevated lithophile element concentrations and flat, “shale-like” REY-patterns, which probably reflects contamination with siliciclastic material during storm events. Additionally, the presence of local true negative Ce-anomalies may suggest local oxygen accumulation sufficient to oxidize Ce.  δ¹³C_org values in the shallow facies are typical for photosynthetic biomass (-30.4 to -21.8 ‰), possibly oxygenic photosynthesis given the negative Ce-anomalies. REY-patterns of the deep-water facies are consistent with typical Archean marine patterns and possess a positive La- and Eu-anomaly, superchondritic Y/Ho ratio and HREE/LREE enrichment.

Ce-anomalies are absent below the impact layer, but a notable shift to positive anomalies is observed stratigraphically above and persists after BIF deposition. We suggest the onset of a redox stratified ocean state post impact event, which initiated a manganese and iron shuttle. This oxygenation of shallow water settings may have been driven by an increase in nutrient availability (for example via upwelling due ocean mixing or enhanced continental erosion of volcanic products due to post impact uplift/volcanism) and increased productivity².

Manganese and iron were then oxidized, and elements including Ce were scavenged by Mn-oxide minerals, in the upper water column. After sinking, and transport across the redoxcline, these elements were released and consequently enriched in the anoxic water column, where Mn-oxides were reductively dissolved. Positive Ce-anomalies consequently imparted to deeper water carbonate sediments, recording the operation of this redox-driven Mn shuttle³.

In our partially stratified ocean model, BIF deposition was driven by a biological Fe pump⁴. In this model, Fe(II) to Fe(III) oxidation occurs above the redoxcline, while Fe(III) is subsequently microbially reduced to Fe(II) below the redoxcline via dissimilatory iron reduction (DIR). This Fe(II) is readily bioavailable for photoferrotrophy, which re-oxidizes iron to Fe(III). We propose that impact related Fe(II) oversupply driven by increased terrestrial and/or hydrothermal input⁵, and Fe-oxidation rates, exceeded the rates of DIR, resulting in BIF precipitation.

Collectively, our results suggest that bolide impact related perturbations may have been sufficient to drive changes in ocean chemistry (increased iron concentrations and other nutrients) and in microbial activity (increased Fe cycling), and potentially led to ocean redox restructuring and served as a trigger for BIF deposition in the Neoarchean.  Further research focussing on Nd and Fe isotopes in the BIF interval may reveal the source of iron and the role of photoferrotrophy. Additionally, expanded geochemical analysis of stratigraphic sections, including carbonate and shale units associated with the impact layer, could further resolve the extent, duration, and intensity of redox stratification.

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2. Drabon, A. H. Knoll, D. R. Lowe, S. M. Bernasconi, A. R. Brenner and D. A. Mucciarone, Proceedings of the National Academy of Sciences 2024 Vol. 121 Issue 44

3. R. Warke, H. Strauss and S. Schröder, Precambrian Research 2020 Vol. 344 Pages 105767

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