Darren A. Chevis

Rare Earth Elements in Stromatolites—1. Evidence that Modern Terrestrial Stromatolites Fractionate Rare Earth Elements During Incorporation from Ambient Waters

Ancient chemical sediments may provide critical information about early microbial life and ancient environmental conditions. For example, the rare earth element (REE) content and fractionation patterns of Archean and Proterozoic banded iron formations (BIF) and other chemical sediments are thought to preserve the REE patterns of ancient seawater, and as such have been employed to investigate secular trends in seawater chemistry through geologic time. Recently it was suggested that REEs could provide evidence for distinguishing between biotic and abiotically precipitated chemical sediments. However, it is important to underscore that very little is actually known about how stromatolites and other microbialites obtain their REE concentrations and fractionation patterns, including what biological processes, if any, the REEs may record. Here, we present REE concentration and fractionation patterns for modern, lacustrine stromatolites and the ambient waters within which they form. We show that the REE patterns of the stromatolites are highly fractionated compared to the ambient waters. Specifically, the stromatolites exhibit heavy REEs (HREE) enrichments relative to upper crustal proxies (i.e., shale composites), whereas the ambient waters are substantially depleted in the HREEs. We propose that surface complexation and subsequent preferential incorporation of HREEs by organic ligands associated with bacterial cell walls, microbialite biofilms, and/or exopolymeric substances may explain the HREE enrichments of the stromatolites.