The role of biostabilization in the preservation of biogenic structures in an estuarine setting

BOURNOD, CONSTANZA N.; CUADRADO, DIANA G.; CARMONA, NOELIA B.; PONCE, JUAN JOSE; PAN, JERÓNIMO

Punta del Este

Congreso; XV COLACMAR; 2013

ALICMAR

Previous studies in tidal flats of the Bahía Blanca estuary (Argentina) revealed that bird footprints were capable of resisting tide and wind erosion, heavy rains and storms due to the presence of microbial mats. The main purpose of this study was to quantify the biostabilization effect of microbial mats on tracked surfaces and analyze the implications for footprint preservation. In order to determine the resistance to erosion of mat-covered sediments, the erosion threshold was measured with a Cohesive Strength Meter KIV (CSM). This portable device was employed in three stations (S1, S2, and S3) located in the lower supratidal zone. Additionally, sediment samples were collected for grain size, organic matter and moisture content determinations. CSM measurements, expressed as Stagnation Pressure (SP), did not show evidence of significant differences between sampling sites. However, there were statistically-significant differences between dates (ANOVA, F (4, 33) = 6.34, p < 0.001), with mean SP ranging from 220-10,368 N.m-2. In general, SP variations fitted well to the organic matter/moisture content of sediments, with particular higher values in August. Moreover, cross-cutting thin sections of a recently-preserved track were observed under a petro-calcographic microscope. The microscopic analysis revealed the presence of internal overtracks, recording normal- and storm-sedimentation. The ongoing microbial growth atop each deposit developed a typical mat-microfabric with ?oriented grains? and authigenic minerals (e.g. framboidal pyrite). Several diatoms and filamentous cyanobacteria in the organic matrix were recognized. In conclusion, CSM-derived results provided an estimate of the relatively higher stability of this tidal flat and its response capacity to erosional agents. In that sense, footprints preservation strongly depended on microbial binding and associated processes such as mineral precipitation and overgrowth. The presence of overtracks supplies a framework for the understanding of the preservation story of footprints in this setting, and a valuable insight for paleoenvironmental interpretations (i.e. sedimentary regime).