Abundant caves are found in the Iron Quadrangle/Southern Espinhaço of Brazil, which are hosted by hematite-rich banded iron formation (BIF) and a goethite rich caprock, known as canga. The BIF and canga are resistant to weathering, mostly due to the relatively low solubility of the hematite goethite (K_sp ≥ 10^-40), making cave formation unexpected. Our overarching hypothesis for cave formation in these poorly soluble rocks is that Fe(III) bacteria reduce Fe(III) (hydr)oxides, yielding dissolved Fe(II) that can be exported groundwater flow. We have found communities of microorganisms beneath the cave walls (referred to as sub muros), where they are isolated from O₂ that would otherwise inhibit Fe(III) reduction. These are capable of extensive canga-Fe(III) reduction (100 mmol/L Fe(II) accumulation in batch incubations). Examination of microbial communities in batch incubations, and enrichment cultures indicate that and Fe(III) reduction by the sub murosmicroorganisms is driven predominantly by Firmicutes, including Desulfosporosinus and Clostridia. In column incubations, porewater was semi-continuously circulated through canga inoculated with sub muros microbial communities, and canga-Fe(III) reduction was enhanced by water flow and advective removal of Fe(II). These results indicate that Fe(III) reduction in the cave systems and surrounding subsurface rocks are enhanced by the export of biogenic Fe(II) by water that circulates behind the cave walls. Similarly, up to 12 mmol/L Fe(II) accumulated in batch incubations with BIF-Fe(III), and the biogenic Fe(II) was resistant to oxidation when O₂was introduced to the incubation headspace. Our results indicate that robust Fe(III) reducing microbial activities, export of biogenic dissolved Fe(II) by groundwater flow, and resistance of biogenic Fe(II) to oxidation could all enhance cave formation in the Iron Quadrangle/Southern Espinhaço. These results also indicate that the weathering of the Fe(III)-rich rocks of this region could be far more extensive than previously believed.

John Senko

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