IT'S NOT JUST RUST: BACTERIAL BIOFILMS, AUTHIGENIC ALUMINOSILICATE CLAYS, AND LEAF FOSSILIZATION

Most fossil leaves are preserved as adpressions, a form of preservation spanning a continuum from organic-poor impressions to well-preserved organic compressions. Some of the most enigmatic adpressions are known as leaf molds, which retain fine morphological details (e.g. veins, stomata impressions) despite being preserved in coarse sandstones – a widespread phenomenon attributed to a fine-grained mineral coating on the fossil surface. Previous taphonomic studies have demonstrated the importance of microbial biofilms in promoting mineral precipitation in this and other contexts, and some authors have hypothesized that authigenic iron-oxides are responsible for the preservation of leaf morphology in coarse sediments. However, the iron-oxide hypothesis is not sufficient to explain the variation of quality observed in leaf adpression fossils more broadly. Here, we test an alternative hypothesis — that authigenic aluminosilicate clays are the minerals responsible for leaf mold preservation.

Using electron dispersive X-ray spectroscopy (EDS), we analyzed thin sections through fossil leaves from five localities that differ in age (Cretaceous through Oligocene) and depositional environments (fluvial, lacustrine, and floodplain). Elemental point spectra taken directly from the leaf–sediment interface reveal that cation-enriched aluminosilicate clays separate the leaf fossils from the host matrix. Additional EDS spectra taken from biofilms from both the surface of a fossil leaf and from modern oak leaves decaying experimentally in freshwater demonstrate an aluminosilicate composition, consistent with the results of the thin section analyses, and establish a biofilm-mediated, authigenic origin. Informed by results, we present a novel taphonomic model of leaf adpression formation, the ‘Biofilm-Clay Template’ model’ in which microbially mediated authigenic clay authigenesis is the first step in leaf adpression preservation.