Traces of Bacterial Contribution to Calcite Weathering Detected by Statistical Characterizations of Surface Microtopography

There is a crucial need to identify reliable imprints of microbe-mineral interactions to quantify the contribution of microorganisms to chemical weathering and detect traces of life in the geological record. Yet conventional methods based on qualitative descriptions of supposedly bioinduced etching features have often proven equivocal. Here, calcite dissolution experiments were carried out at various solution compositions, in the presence or absence of cyanobacterial biofilms. Nanoscale chemical and crystallographic characterizations failed to detect any distinctive biogenicity feature. Conversely, high-elevation regions at the calcite surface were detected through statistical characterizations of the microtopography, which made microbially weathered surfaces quantitatively distinguishable from their abiotic counterparts. Interestingly, the high-elevation regions that formed beneath clusters of microbial cells are at odds with the etching features that resemble cell morphologies and are usually sought as bioweathering markers. Atomic-scale stochastic simulations of the dissolution process suggested that these regions resulted from a local increase in fluid saturation state at the biofilm-mineral contact, which led to a localized reduction in dissolution rates. Overall, this study offers a new avenue for the nondestructive identification of bioweathering signatures in natural settings.