Biotic and abiotic controls on the morphological and textural development of modern microbialites at Lago Sarmiento, Chile. [electronic resource]
Keywords: Geological and Environmental Sciences.

Microbialites are sedimentary deposits that develop under the influence of an associated surficial microbial community and have been the dominant reef-builders throughout the Precambrian. Their external shapes, internal textures, and petrography provide much of what is known about early Earth's environments and biota. With the Cambrian diversification and the rise of metazoans and modern corrals, microbialite reefs have almost entirely vanished from the oceans with the exception of few unusual coastal environments (e.g. Shark Bay, Australia). Although in geological terms lacustrine environments are generally very short-lived, their quasi-isolated emergence and frequently extreme and unusual physicochemistry has provided niches for microbialites to thrive in until present day. Lago Sarmiento in Southern Chile is such a lake in which microbialites have been developing throughout most of the Holocene. A drop in lake level by 9 m over the last two millennia has resulted in the exposure of extensive microbialite reefs along the 70 km long shoreline, providing the opportunity to complement and compare data on the actively growing subaqueous microbialites with those stranded onshore. The thesis research presented here has focused on the three main goals: (1) Identification of the factors controlling the macroscopic development of microbialites at Lago Sarmiento, such as their spatial arrangement and individual shapes. (2) Identification of the factors controlling the microscopic development of microbialites at Lago Sarmiento, such as the relationship between the framework texture and the associated microbial community. (3) The development of a computer model able to simulate photosynthetic microbialite growth for testing the interdependent relationships between the morphological development of microbialites, the properties of the associated microbial community and the environmental boundary conditions. In summary the study of the Lago Sarmiento microbialites has shown that their macroscopic development is largely controlled by environmental factors such as shoreline slope, water depth, sediment input, and water agitation. At the meso- to micro-scale the textural development of the Lago Sarmiento microbialites is linked to the properties of the surficial microbial community such as growth rate, mat thickness, rate of in-situ mineral precipitation and cementation. These findings are supported by computer simulations showing that mesoscopic microbialite textures are mainly controlled by the growth properties of the surficial mat and only to a lesser degree affected by the amount of sediment input. At the macro-scale the simulation results support the field observations as well, demonstrating how environmental factors such as sediment input, water agitation and clarity control the large-scale development of microbialites. These results lend insight into the early evolution of life on Earth and provide a working hypothesis for the origin of stromatolitic and thrombolitic microbialite textures, suggesting that the appearance of thrombolites reflects evolutionary advancements of microbial mats at that time. An understanding of how biological processes influence the shape and texture of sedimentary deposits in general can be used as a tool for the search for life beyond Earth.