congresos y reuniones científicas
Microbial mats colonizing gravel deposits in modern and fossil coastal settings
PAN, JERÓNIMO; MAISANO, LUCÍA; CUADRADO, DIANA G.; NOFFKE, NORA K.
Congreso; XXI Congreso Geológico Argentino; 2022
Biosedimentary, laminated microbial consortia known as epibenthic microbial mats colonize sedimentary surfaces while altering their physical and chemical characteristics. In modern siliciclastic settings they are mostly associated with fine sediments. This contribution reports modern microbial mats colonizing gravel substrates (Fig 1A).The Bahía San Blas coastline (southern Buenos Aires province, northern Patagonia, Argentina) is subject to frequent storms that create high-energy hydrological processes (Espinosa and Isla, 2011), largely responsible for gravel transport and deposition (rodados Patagónicos). At the study site of Paso Seco (40°38'27"S, 62°12'55"W), a former tidal channel, there exists a depositional mosaic consisting of Upper Quaternary beach gravel deposits of Upper Pleistocene and Holocene ages (Trebino, 1987). These gravels are characteristically rounded, with pebbles and cobbles as the dominant size fractions (Martinez et al., 2009). Their roundness, one of the salient features arises from high-energy transport. The objectives of this work are (1) to describe how biofilms and cyanobacterial biomass incorporates sedimentary particles through processes such as baffling, trapping and binding leading to the development of epibenthic microbial mats on gravel; (2) to relate microbial mat accretionary growth and biostabilization with hydrodynamic conditions at the study site; and (3) to establish morphological comparisons of modern MISS with fossil analogues.Our hypothesis is that seasonal seawater availability alternating with subaereal exposure and desiccation interplay to produce mat development on gravel. After sporadic seawater flooding of the Paso Seco basin, water stagnation (for periods ~14 d and longer) allows pre-existing microbial consortia to extend over gravel substrates. This first stage is led by EPS-rich biofilms. A shallow water column facilitates the settling of mud-sized sediment particles, which become baffled by protruding filaments and trapped and bound into the organic matrix of the primordial mat. A coherent mat growing around individual pebbles may form a rim-like border as it grows centripetally, seemingly engulfing grains (Fig 1B). Further growth of a cyanobacterial-dominated mat closes in on individual pebbles. At a cm-scale, the differential colonization of individual pebbles of different sizes is a function of their relative vertical position (i.e. their degree of outcropping in relation to the mean sediment level; Fig 1C). During a subsequent period of relative quiescence, the epibenthic microbial mat levels the sedimentary surface; once established, deformation processes may produce MISS such as fold-overs (Fig 1D), which, in turn, evidence the biostabilization potential of the mat.Microbiological analyses revealed a very cohesive mat fabric of densely-packed filaments of the cyanobacterium Coleofasciculus chthonoplastes with few pennate diatoms in the uppermost 2 mm, which corresponded to an oxic layer. The fact that filamentous cyanobacteria prefer sandy sediments (Watermann et al., 1999) makes their gravel overgrowth a remarkable feature. Their success in pebble colonization lies in some of their biological features such as their motility and migration capacity; and their characteristic growth attached to surfaces. Marked seasonal differences in organic matter and chlorophyll a contents between summer and winter, point to an actively growing epibenthic mat during the season when water stagnation is more prolonged.A morphological comparison of modern microbial mats with fossil counterparts colonizing coastal gravel deposits from the 3.48 Ga-old Dresser Fm. (Pilbara craton, Western Australia; Fig 1E) provides a strong match between both types of mat structures. In that sense, this study has relevance for paleoenvironmental interpretations relating to the genesis of such structures, and their associated hydrodynamic regimes.