Hydrologic Controls on Modern Microbialite Formation

Abstract:

Modern microbialites occur in a variety of shallow aquatic environments and are often spatially co-located with zones of groundwater discharge. Here we show that feedbacks between groundwater inflow to the aquatic environment and microbial productivity result in the self-organization of modern thrombolitic microbialites. Groundwater springs emerge directly through thrombolites in Lake Clifton, Western Australia, a site with large salinity and temperature contrasts between groundwater and surface water. The thrombolites are highly permeable compared to nearby sandy sediments and spring flow occurs through an interconnected network of conduits within the thrombolite structure. Spring flow reduces osmotic stress and increases nutrient fluxes for microbial mats covering the thrombolites, which otherwise form in a hypersaline and oligotrophic environment. Microbial mats lithify into a permeable structure through which spring flow can be maintained as additional material accretes and the thrombolites grow upward. Thus favorable growth conditions are sustained for microbial communities. Microbialites are some of the oldest organosedimentary structures evidencing life on Earth. If this model of thrombolite formation applies broadly, then fossil thrombolites are perhaps the earliest examples of microbial communities facilitating their survival through the creation of a solid structure.