The Role of Microbial Mats in the Preservation of Bird Footprints: A Case Study from the Mesotidal Bahia Blanca Estuary (Argentina)

Field studies in a siliciclastic mesotidal flat in Bahia Blanca Estuary, Argentina, reveal the presence of extensive areas with microbial mats, covering the upper intertidal and lower supratidal areas. Study of recent environments with microbial mats has increased considerably in recent years, not only because of their unique sedimentologic and ecologic characteristics but also because they provide important implications for the understanding of fossil environments. The main purpose of this research was to evaluate the role of microbial mats in the preservation of biogenic structures.We recorded the distribution of recent biogenic structures all over the siliciclastic tidal flat, focusing the analysis on the preservation of bird tracks. Several footprints were selected and photographed; we recorded the morphologic modifications they experienced over the course of 10 months. This study revealed that most of the footprints showed resistance to tide and wind erosion and also to the heavy rains and storms that affected the tidal flat. This resistance is clearly associated with the presence of the microbial mats, which are known to biostabilize the sediment. In addition, microscopic analysis of the tidal-flat sediment revealed the presence of zeolites, indicating early cementation, which may have favored the consolidation of the footprints. Mat thickness also affected the morphology of the footprints; in areas with thick microbial mats overlying water-saturated sands, the tracks were deeply impressed and did not show fine details. On the contrary, in zones with thin microbial mats overlying relatively stiff muds, the traces were shallowand preserved details such as skin impressions and skid marks. Both types of footprints were affected by mat growth, although in the shallow traces the modification was faster and the fine details were progressively obliterated. This study yields valuable insight into the relationship between microbial mats and the morphology of the footprints and provides key information for the analysis of fossil tracks in equivalent paleoenvironments.