Print this pageAn Abiotic Model for Stromatolite Morphogenesis
GROTZINGER, J. and D. ROTHMAN
Stromatolites are regarded as an important proxy for early life on earth, and their record extends back to 3.5 Gyr ago. However, because stromatolites only rarely contain fossil microbes, their biogenicity is tacitly assumed on the basis of morphologic comparisons with modern, demonstrably biologic structures. As a result, little is known about the physical, chemical, and biological processes that controlled the growth of ancient stromatolites. The growth morphology of ca. 1.8 Ga stromatolites from the Cowles Lake Fm., Canada, was analyzed and shown to have a fractal dimension D approx. 2.5, similar to the morphology of theoretical models for abiotic surface growth. A dynamical model of stromatolite surface growth composed of chemical precipitation on the growing interface, fallout and diffusive rearrangement of suspended sediment, and uncorrelated random noise is consistent with this statistical measure of stromatolite geometry. This result demonstrates that the morphology of at least some, and perhaps many stromatolites may be accounted for exclusively by abiotic mechanisms. Although microbes may have played a role in morphogenesis, it is no longer clear what unique influence they may have provided. As the textural evidence for precipitation in stromatolitic growth (particularly early Precambrian) continues to increase it seems prudent to remain cautious in the assumption of biogenicity without direct evidence supplied by the presence of fossilized microbes that can be shown to have influenced morphogenesis. In attempting to demonstrate through rigorous analysis that morphology may uniquely reflect biology, the null hypotheses- that morphology is the result of physical and chemical processes- cannot be falsified in the case studied here and probably for many other stromatolites.