Microbial mats are layered consortia of microorganisms colonizing surface sediments that alter their physical and chemical characteristics. The northern Patagonia coastline (Argentina) includes gravel deposits (termed rodados Patagónicos) accumulated during the Pleistocene and Holocene by high-energy hydrodynamic processes. In this area, surface sediments in a relict tidal channel (Paso Seco; 40° 38′ 27″ S, 62° 12′ 55″ W) are extensively colonized by microbial mats, appearing to overgrow exposed gravel deposits. To date, such substrates have not been reported as suitable for the development of microbial mats. The objectives of this paper are: 1) to describe the mechanisms of microbial baffling, trapping, and binding of sedimentary particles, and biostabilization that enable epibenthic microbial mats to develop on gravel substrates, 2) to relate microbial mat growth to a variety of hydrodynamic conditions, and 3) to describe resulting microbially induced sedimentary structures (MISS). Our hypothesis is that the alternation of episodic seawater flooding, stagnation, and draining with subsequent subaerial exposure and desiccation are the controlling factors for mat development on gravel. Once stagnant, mud-size sediment particles settle from suspension. At the same time, an initial biofilm may become established on the bottom, using the fine-grained material as substrate. Subsequently introduced particles are baffled, trapped, and bound into the developing biofilm matrix. During the Austral winter comparatively higher values for chlorophyll a and organic matter point towards increased growth of the microbial mat during this season. With increasing coherence, the developing microbial mat may encroach onto individual pebbles, ultimately engulfing them. Eventually, a mature, epibenthic microbial mat levels the sedimentary surface. Hydrodynamic reworking during flooding produces MISS such as mat chips and flipped-over mats.

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