AN ABIOTIC MODEL FOR STROMATOLITE MORPHOGENESIS

STROMATOLITES are laminated, accretionary structures, which are common ly regarded to have formed by the sediment-binding or precipitating ac tivities of ancient microbial mats off biofilms (composed mainly of cy anobacteria), possibly supplemented by abiotic surface precipitation(1 -4). Stromatolites are thus considered to be a proxy for early life on Earth, as the record of these structures extends back to 3.5 Gyr ago( 5). But as stromatolites only rarely contain fossil microbes, their bi ogenicity is tacitly assumed on the basis of morphological comparisons with modern, demonstrably biological, structures(6). Little is known about the physical, chemical and biological processes that controlled the growth of ancient stromatolites(4) and, with pioneering exceptions (7-9), the analysis of the inherent geometric characteristics of the s tructures has not been pursued. Here we present a morphological charac terization of ancient stromatolites that have growth surfaces with sel f-affine fractal geometry. We deduce, from both the microscopic textur es and the fractal dimension, a purely abiotic dynamical model of stro matolite surface growth that combines chemical precipitation on the gr owing interface, fallout and diffusive rearrangement of suspended sedi ment, and uncorrelated random noise. This result calls into question t he assumption that organisms-even if present-necessarily played an ess ential role in determining stromatolite morphology during times when p recipitation at the sea floor was common, such as the earlier Precambr ian.