CARBONATE MINERALIZATION IN MODERN STROMATOLITES: RELATING MICROFABRIC TO MICROBIAL ACTIVITY

Integrated studies of microfabric and microbial activity are of key importance in understanding mineralization in stromatolites. This approach was adopted in recent studies of marine stromatolites in Exuma Cays, Bahamas. These studies indicate that growth of Exuma stromatolites result from repeated episodes of community succession. Cycling of three mat types, representing a spectrum of community development, results in precipitation and deposition of distinct mineral deposits, which form the emergent stromatolite. The three stages of mat development and associated mineral products are as follows:

1. ‘Pioneer’ community: characterized by a sparse population of motile filamentous, photosynthetic, nitrogen-fixing cyanobacteria and the copious secretion of extracellular polymeric substances. Pioneer communities trap and bind sediment, forming unlithified grain layers.

2. Mature ‘bacterial biofilm’ community: characterized by the development of a continuous surface film of exopolymer and heterotrophic aerobic and anaerobic bacteria; this film overlies moderately dense populations of filamentous cyanobacteria. Precipitation of microcrystalline calcium carbonate within the surface biofilm forms a thin micritic crust (20-50 µm thick), which extends laterally across the stromatolite surface.

3. ‘Climax’ community: characterized by an abundant population of endolithic coccoid cyanobacteria. These endolithic bacteria infest carbonate grains beneath a surface biofilm and micritic crust; a dense population of filamentous cyanobacteria is also present below the surface film. Rapid precipitation of micrite in endolithic bore holes welds grains together, forming a lithified horizon of microbored grains.

Repeated episodes of community succession, with accompanying changes in accretion style, form a laminated rock fabric, a fundamental feature of stromatolites through time. Chronologies of former surface mats are recorded in the subsurface lamination of Exuma stromatolites. This approach, which integrates micro-scale analyses of sedimentological and microbiological properties, provides a powerful tool for investigations of microbial mineralization.