Abstract

Changes in composition during the transition from sediment to rock are usually attributed to long, complicated histories and atmospheric influences, while the contribution of benthic mat-building cyanobacteria is not typically considered. Here the goal is to understand the influence of cyanobacterial mats on mineral weathering in postdepositional settings of sandy, shallow subaquatic environments. Laboratory incubation experiments were done using ilmenite sands and ilmenite-enriched quartz sands colonized by cyanobacterial mats for five months at three temperatures: 25°C and 37°C, representative of postdepositional weathering regimes, and 70°C corresponding to early diagenesis. As a comparative control to represent abiotic processes, ilmenite sands and ilmenite-enriched quartz sands were also subjected to the same conditions without cyanobacterial colonization. The precipitation of minerals on cyanobacterial cells and extracellular polymeric substances (EPS) as well as the phase changes in natural ilmenites (FeTiO₃) were documented to determine if cyanobacteria influence mineral reaction pathways. The precipitates, ilmenite grains, and permineralized cells were analyzed using complementary techniques of scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro-Raman spectroscopy. The results of this study show that a variety of pure and mixed mineral phases precipitate under postdepositional conditions (T ≤ 70°C) in wet, sandy environments with or without cyanobacteria. Akaganeite, anatase, ankerite, lepidocrocite, gibbsite, kaolinite, and natrojarosite formed exclusively in the samples incubated with cyanobacteria. In the samples incubated with cyanobacteria, more mineral phases formed at 37°C, suggesting that cyanobacteria play a greater role in weathering than in early diagenesis. Sulfate phases that formed in the presence of cyanobacteria differed in chemical composition from the abiotic precipitates as Na, Al, Mg, and Si were incorporated into the structures of newly formed biotic phases. Understanding the possible fate of these precursor mineral phases will help redefine geochemical biosignatures that can be used for the detection of ancient microbial life in sedimentary rocks on Earth as well as for future missions exploring life on other planets.