Clay mineral authigenesis as an example of organomineralization in Paleozoic coated grains and peloids

The formation of clay minerals in marine environments, referred to as reversed weathering, represents an important process significantly affecting seawater chemistry. In this study, we describe organomineralization represented by clay mineral authigenesis associated with remnants of carbonaceous microstructures, interpreted as extracellular polymeric substances (EPS), from Silurian, Devonian, and Carboniferous coated grains (ooids, oncoids, and cortoids) and peloids (micritized grains). Multiple lines of evidence support the authigenic origin of these clay minerals: (1) Systematic spatial relationships with organic templates demonstrated through BSE imaging and elemental mapping; (2) Consistent qualitative elemental compositions across samples of varying ages, depositional settings, and burial histories; (3) Systematic textural, morphological, and compositional differences from detrital clay minerals, including filamentous texture, diffuse irregular boundaries, and higher Mg/(Al + Mg) and Si/(Si + Al) ratios; (4) XRD identification of authigenic I/S R3 with a diagnostic 1Md polytype structure; and (5) Consistency with the well-documented association of authigenic clay minerals with microbial surfaces, as evidenced by numerous laboratory experiments.

The documentation of authigenic clays associated with microbial structures in these common constituents of carbonate rocks suggests a potentially significant impact on element sequestration at the sediment-water interface and in the pore-water environment. We conclude that when properly contextualized, authigenic clay minerals can serve as indicators of former microbial surfaces.