RARE EARTH ELEMENTS IN STROMATOLITES: WINDOWS INTO EARLY LIFE AND CLIMATE?

The geochemistry of sediments is commonly employed to investigate past climates. Furthermore, it is generally agreed that ancient chemical sediments preserve evidence of early Archean life and that careful geochemical analysis of such rocks can provide robust proxies for investigating secular trends in ocean chemistry through geologic time. Of particular interest are ancient chemical sediments that contain stromatolitic bedforms, which may provide critical information about early microbial life and the Archean climate. For example, shale-normalized rare earth element (REE) patterns of ancient chemical sediment have been cited as evidence for their formation as biochemical precipitates (e.g., microbialites) within shallow marine environments, and some researchers argue that the REE contents and shale-normalized REE fractionation patterns of ancient chemical sediments can be used to distinguish between biotic and abiotically precipitated chemical sediments. Specifically, a 250-fold enrichment in the REE content of laminated carbonate strata compared to adjacent chert layers from Archean rocks in Western Australia has been cited as evidence of microbial precipitation of these ancient rocks. 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. Claims that REEs can be used to discriminate between abiotic vs. biotic origins of ancient chemical sediments are entirely based on solid-liquid partitioning coefficients. Although an important first step, partitioning coefficients provide no mechanistic understanding of REE uptake and incorporation into microbialites, and certainly tell us nothing about biological vs. abiological processes that may be important during the uptake and incorporation of REEs in ancient chemical sediments. Here, we present the REE content for different fractions (i.e., carbonate, biological materials) of modern, lacustrine stromatolites from Cuatro Ciénegas in Coahuila, México, along with the REE concentrations of the ambient waters in which the stromatolites are living. Comparisons are drawn between the REE patterns of the waters from Cuatro Ciénegas and the REE content of the stromatolites.