EVOLUTION OF STROMATOLITE COHESIVE STRENGTH FROM THE ARCHEAN TO THE MODERN

Stromatolites and fossil microbial mats form an important historical record of interactions between microorganisms and their environments since the Paleoarchean, but this record has been difficult to interpret and quantify. Recent work suggests that the physical cohesive strength of stromatolitic laminations reflects both the environment in which the stromatolite grows and biological properties of the constructing community, and that maximum lamination strengths have varied throughout geologic time. Initial observations suggested that an increase in cohesion occurred at ~2.73 Ga (Tice, 2011). We extend the stromatolite strength record by estimating cohesions for a new suite of stromatolitic samples from the Neoproterozoic and Phanerozoic.

Preliminary results suggest the initial increase in stromatolite cohesive strength during the Neoarchean was maintained through the Neoproterozoic. While variable, cohesion during the Phanerozoic was intermediate between pre-2.73 Ga and 2.73-0.54 Ga values. Cohesions estimated for a range of modern actively growing mats and stromatolites (excluded from the Phanerozoic fossil suite) are generally comparable to those of the relatively weak pre-2.73 Ga examples.

Changes in mat cohesion may be due to changes in mat productivity, or extracellular polymeric substance production. We note that Phanerozoic microbial mats are typically confined to extreme environments by competition with macrophytes and grazing metazoa. These environments may limit productivity below levels that would have been achievable in the Proterozoic, and they are also often low-energy, suggesting that mat-constructing organisms would not face selective pressure to develop high physical cohesion. Thus, long-term changes in community productivity and organism behavior seem more likely to explain recent trends in stromatolite strength than changes in environmental factors. The Neoarchean increase in stromatolite strength may have been related to increased productivity associated with the evolution of oxygenic photosynthesis, but behavioral and environmental changes cannot yet be ruled out. Future work will test for correlated changes in microbialite composition as an independent proxy for metabolic or environmental evolution.