Microbially generated ferruginous crusts: A potential biosignature of continental input in coastal settings

Thin ferruginous sandy crusts are common on top of sandstone beds in the Early Permian post‐glacial deposits of the Paraná Basin in southern Brazil. These crusts usually preserve wrinkle structures, suggesting that they might be a product of microbial mediation. Iron‐synthesizing bacteria use iron for essential metabolic activities and precipitate iron minerals as a by‐product. Bacteria could also indirectly mediate iron minerals precipitation through the excretion of extracellular polymeric substances, which act as a glue, enabling iron adhesion and nucleation. Samples of ferruginous sandy crusts from different sandstone beds of the Rio Bonito Formation were investigated through microtextural high‐resolution, mineralogical and geochemical methods to validate this hypothesis. Lumpy and filamentous morphologies and globular goethite minerals are abundant in these samples and were herein interpreted as bacterial products. The presence of kerogen reinforces the evidence of bacteria–substrate interaction and suggests the development of biofilms/microbial mats on the top of these beds. Quiescence is needed to develop and preserve microbial mats and low sedimentation rates are required to form ironstones. All these aspects allowed interpreting the studied ferruginous sandy crusts as representing depositional hiatus in estuarine settings impacted by periods of significant continental input. The occurrence of these crusts in coastal deposits may be a biosignature of continental contribution, potentially driven by climate warming.