Organic matter preservation in the carbonate matrix of a recent microbial mat – Is there a ‘mat seal effect’?

Abstract

Phototrophic mats (microbial mats with a phototrophic top layer) are complex systems in terms of microbial diversity, biogeochemical cycles and organic matter (OM) turnover. It has been proposed that these mats were a predominant life form in Proterozoic shallow water settings, prior to the emergence of bioturbating organisms in the Ediacaran–Cambrian transition. For most of the Precambrian, microbial mats were not only quantitative important carbon fixing systems, but also influenced the transfer and transformation of OM before it entered the geosphere. The profound alteration of compound inventories during transit through microbial mats, implying substantial consequences for OM preservation in the Proterozoic, was recently proposed as a “mat-seal effect” [Pawlowska et al. (2012) Geology 41, 103–106]. To obtain a better understanding of the early diagenetic fate of primary produced OM in microbial mats, we studied a recent calcifying mat from a hypersaline lake in Kiritimati, which showed in the deeper mat layers a maximum 14Ccarbonate age of ∼1500years. We particularly focused on OM entrapped in the carbonate matrix, because of the better potential of such biomineral-encapsulated OM to reach the geosphere before degradation (and remineralization). Our data indicate that selective preservation is important in phototrophic mats. While a diagenetic transformation of lipid fatty acids (FAs) was evident, their fatty acyl-derived hydrocarbon moieties were not introduced into protokerogen, which was instead mainly comprised of cyanobacterial and/or algal biomacromolecules. Our data support the proposed major impact of the “mat-seal effect” on OM turnover and preservation; i.e. the suppression of biosignatures derived from the upper mat layers, while signals of heterotrophic microbes thriving in deeper mat layers become preferentially preserved (e.g. high hopane/sterane ratios). This mechanism may have broad consequences for the interpretation of biomarkers from Proterozoic shelf environments, because biosignatures of phototrophic mat dwellers as well as planktonic signals may have become heavily biased by the production and turnover of OM in microbial mat systems.