EPIBENTHIC MICROBIAL MATS COLONIZE GRAVEL IN MODERN AND FOSSIL SILICICLASTIC COASTAL SETTINGS

JERÓNIMO PAN; LUCIA MAISANO; CUADRADO, DIANA G.; NORA K. NOFFKE

Congreso; XXI Congreso Geológico Argentino; 2022

Biosedimentary, laminated microbial consortia known as epibenthic microbial mats colonize sedimentary surfaces while altering their physical characteristics. In modern siliciclastic settings they aremostly associated with sandy sediments. This contribution reports modern microbial mats colonizing gravelsubstrates (Fig. 1A).The Bahía San Blas coastline (southern Buenos Aires province, northern Patagonia, Argentina) wassubject to frequent storms that create high-energy hydrological processes (Espinosa and Isla 2011), largelyresponsible for gravel transport and deposition known as Rodados Patagónicos. At the study site of PasoSeco (40º38'27"S, 62º12'55"W), a former tidal channel, there exists a depositional mosaic consisting of UpperQuaternary beach gravel deposits of Upper Pleistocene and Holocene ages (Trebino 1987). These gravels arecharacteristically 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 workare: (1) to describe how biofilms and cyanobacterial biomass incorporates sedimentary particles throughprocesses such as baffling, trapping and binding leading to the development of epibenthic microbial mats ongravel; (2) to relate microbial mat accretionary growth and biostabilization with hydrodynamic conditions atthe study site; and (3) to establish morphological comparisons of modern microbially induced sedimentarystructures (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 frequent seawater flooding of the Paso Secobasin, water stagnation (for periods ~14 days and longer) allows pre-existing microbial consortia to extendover gravel substrates. This first stage is led by extracellular polymeric substances -EPS-rich biofilms. Ashallow water column facilitates the settling of mud-sized sediment particles, which become baffled byprotruding filaments, and trapped and bound into the organic matrix of the primordial mat. A coherent matgrowing around individual pebbles may form a rim-like border as it grows centripetally, seemingly engulfinggrains (Fig. 1A). Further growth of a cyanobacterial-dominated mat closes in on individual pebbles. At acm-scale, the initial colonization of individual pebbles of different sizes by biofilm is a function of theirrelative vertical position (i.e. their degree of outcropping in relation to the mean sediment level; Fig. 1B).The epibenthic microbial mat levels the sedimentary surface, and, once established, deformation processesmay produce MISS such as fold-overs (Fig. 1C), which, in turn, evidence the biostabilization potential of the mat. Interestingly, the biofilm colonizing pebbles has similar characteristics than the one developedon sandy substrate.Microbiological analyses revealed a very cohesive mat fabric of densely-packed filaments of thecyanobacterium Coleofasciculus chthonoplastes with few pennate diatoms in the uppermost 2 mm, whichcorresponded to an oxic layer. The fact that filamentous cyanobacteria prefer sandy sediments (Watermannet al. 1999) makes their gravel overgrowth a remarkable feature. Their success in pebble colonization liesin some of their biological features such as their motility and migration capacity; and their characteristicgrowth attached to surfaces. Marked seasonal differences in organic matter and chlorophyll a contentsbetween seasons, point to an actively growing epibenthic mat during winter, when water stagnation ismore prolonged.A morphological comparison of modern microbial mats with fossil counterparts colonizing coastalgravel deposits from the 3.48 Ga-old Dresser Formation (Pilbara craton, Western Australia) provides a strongmatch 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.