EVIDENCE OF MICROBIAL BIOTURBATION AND ITS UNIQUE RECORDS IN CLASTIC SEDIMENTS

Microbes have long been thought as physically passive elements in the sedimentary record, at most “trapping and binding” particles that fall onto them. Here we show evidence that communities of filamentous cyanobacteria can physically construct structures from the sediments they live on. Experimental cultures of Leptolyngbya sp. originally isolated from conical mats in Yellowstone hot springs were observed to socially coordinate themselves into locally-dense, regularly-spaced millimeter-to-centimeter-scale conical shapes. Movement of groups of bacteria generated forces sufficient to roll individual sand grains over the sediment surface and even up slopes, resulting in millimeter-scale mounds where higher conical shapes could further develop. These activities effectively sorted loose sand grains based on density, size and roundness because larger, lighter and more rounded grains were relatively easier to roll. In addition, these communities followed a diel cycle where aggregation of organisms during the morning was followed by partial disaggregation of cone communities in the evening, enhancing sediment sorting. High resolution x-ray fluorescence scans were employed to map compositionally distinctive minerals on thin slabs of a 3.22 Ga microbial mat sandstone deposit (Moodies Group, Barberton Greenstone Belt, South Africa). Sand grains were found sorted based on size, roundness and density in a pattern indicating net motion of sand grains toward and up conical slopes, confirming the hypothesis that centimetre-scale cones in this Paleoarchean sandstone were built by similar processes to those observed in experiments. This research thus suggests a novel process by which microbes can modify sediments resulting in specific measurable biosignatures in granular sedimentary rocks.