Comment on “Early Archean biogeochemical iron cycling and nutrient availability: New insights from a 3.5 Ga land-sea transition” by Clark M. Johnson, Xin-Yuan Zheng, Tara Djokic, Martin J. Van Kranendonk, Andrew D. Czaja, Eric E. Roden, Brian L. Beard, 2022, Earth-Science Reviews, https://doi.org/10.1016/j.earscirev.2022.103992

In their recent paper entitled "Early Archean biogeochemical iron cycling and nutrient availability: New insights from a 3.5 Ga land-sea transition", Johnson et al. (2022) use the Fe isotope and trace-element compositions of ~3.5 Ga jaspilites from the North Pole Dome area of the Pilbara Craton, northwestern Australia, to investigate links between the ancient Fe biogeochemical cycle, continental exposure and nutrient (P) availability. The geochemical arguments are based on the underlying assumption that the jaspilites contain primary hematite that was initially deposited as Si-ferrihydrite precipitates following Fe(II)(aq) oxidation by photoferrotrophs. In support of a primary origin for the iron oxide, the authors present observations from standard light petrography showing the presence of fine-grained hematite dispersed amongst the chert. Here, we question whether the fine-grained hematite is the sole iron precipitate, and indeed, whether it is primary. These concerns arise from work on nearby and slightly younger jaspilites (3.46 Ga Marble Bar Chert Member), in which SEM and TEM studies revealed the presence of fine-grained particles (typically <1 µm) of greenalite and siderite, as well as evidence for pervasive secondary oxidation. Although fine-grained hematite is readily identified using light microscopy, tiny Fe(II)-rich particles can easily be overlooked. For this reason, we believe that the Dresser jaspilites, which closely resemble the nearby Marble Bar jaspilites, may also contain Fe(II)-rich precipitates. In addition, given the widespread evidence for secondary oxidation in the Marble Bar jaspilites, there remains a distinct possibility that the fine-grained hematite in the Dresser jaspilites is not primary. The precipitation of Fe(II)-rich phases, rather than ferrihydrite, would undermine interpretations for Fe biogeochemical cycling centred on photoferrotrophy. We suggest that the conclusions of Johnson et al. (2022) should be treated with caution until the jaspilites have been examined by high-spatial resolution electron imaging and analysis.