Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats

Abstract

Carbonaceous cherts in Proterozoic carbonate sequences provide an exceptionally clear record of early microbial life, but one that is significantly biased with respect to the range of environments inhabited by contemporary organisms. Many of the best preserved Proterozoic microfossil assemblages come from microbial mats and organicrich muds that accumulated in protected coastal areas where a combination of high productivity, limited water circulation, and, often, hypersalinity limited post mortem degradation. The close distributional relationship between early diagenetic silica and organic matter can be explained in terms of a model developed by Leo and Barghoorn for the silicification of wood. Three factors appear to control the distribution of early diagenetic chert in Proterozoic sequences: sediment permeability, availability of silica in ground water solution, and locally high concentrations of organic matter in near-surface sediments. Of these, organic content appears to impose the major environmental bias. In terms of their excellent preservation, geochemistry of formation, and limited environmental coverage, Phanerozoic silicified peats bear comparison with their Proterozoic counterparts. Swamp dwellers may be the plants most likely to be preserved exceptionally well, but they may also be the plants least likely to give rise to new populations that will become ecologically widespread and evolutionarily important in subsequent periods. Allochthonous elements in permineralized peats may be unusually important to palaeobotany because they combine the exceptional preservation conferred by peat permineralization with ecological representation of floodplain and upland evolutionary cradles rather than swampland museums.