The Role of Biofilms in the Sedimentology of Actively Forming Gypsum Deposits at Guerrero Negro, Mexico

Abstract

Actively forming gypsum deposits at the Guerrero Negro sabkha and saltern system provided habitats for stratified, pigmented microbial communities that exhibited significant morphological and phylogenetic diversity. These deposits ranged from meter-thick gypsum crusts forming in saltern seawater concentration ponds to columnar microbial mats with internally crystallized gypsum granules developing in natural anchialine pools. Gypsum-depositing environments were categorized as forming precipitation surfaces, biofilm-supported surfaces, and clastic surfaces. Each surface type was described in terms of depositional environment, microbial diversity, mineralogy, and sedimentary fabrics. Precipitation surfaces developed in high-salinity subaqueous environments where rates of precipitation outpaced the accumulation of clastic, organic, and/or biofilm layers. These surfaces hosted endolithic biofilms comprised predominantly of oxygenic and anoxygenic phototrophs, sulfate-reducing bacteria, and bacteria from the phylum Bacteroidetes. Biofilm-supported deposits developed in lower-salinity subaqueous environments where light and low water-column turbulence supported dense benthic microbial communities comprised mainly of oxygenic phototrophs. In these settings, gypsum granules precipitated in the extracellular polymeric substance (EPS) matrix as individual granules exhibiting distinctive morphologies. Clastic surfaces developed in sabkha mudflats that included gypsum, carbonate, and siliclastic particles with thin gypsum/biofilm components. Clastic surfaces were influenced by subsurface brine sheets and capillary evaporation and precipitated subsedimentary gypsum discs in deeper regions. Biofilms appeared to influence both chemical and physical sedimentary processes in the various subaqueous and subaerially exposed environments studied. Biofilm interaction with chemical sedimentary processes included dissolution and granularization of precipitation surfaces, formation of gypsum crystals with equant and distorted habits, and precipitation of trace carbonate and oxide phases. Fine-scale wrinkle structures visible in clastic surfaces of sabkha environments offered evidence of the biofilm's role in physical sedimentary processes. These findings are highly relevant to astrobiology because they expand and refine the known characteristics of gypsum deposits, including their biological components.

Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

“…Thompson and Ferris () have shown that in high‐sulfate environments as pH approaches 7.9, gypsum becomes unstable and dissolution is favored. Dissolution of solid gypsum under similar conditions has been documented in other environments (Douglas, ; Wright & Kirkham, ; Wright & Oren, ), as well as in the Guerrero Negro gypsum crust mats (Vogel et al, , ). While high sulfate reduction rates often facilitate the deposition of CaCO 3 , only trace amounts of carbonate are observed in these crusts (Vogel et al, ).…”

“…Previously, we reported on the distribution and diversity of the microbes within the distinct colorful laminations of a gypsarenite mat (Jahnke et al, ). Gypsum crusts displaying various degrees of induration are present throughout Pond 9, and they host laminated microbial mats similar to the one studied here (Vogel et al, ). The gypsarenite mat studied here is a highly diverse ecosystem composed of multiple trophic levels and reaction networks through which carbon flows (Figure ).…”

Biological influences on modern sulfates: Textures and composition of gypsum deposits from Guerrero Negro, Baja California Sur, Mexico

“…Dissolution surfaces in gypsum have been recognized by previous workers in both modern and ancient deposits (Vai and Lucchi, 1977;Douglas, 2004). Vogel et al (2009) discuss potential mechanisms by which the biofilm EPS may influence precipitation and dissolution of gypsum but further study is necessary to resolve the biofilm's specific role in the formation of irregular surface textures. The unique association of irregular textures with biofilms in samples from several different depositional settings however demonstrates a potential utility of these features in supporting biofacies interpretations.…”

“…At depths of 1 cm or greater, pigmented biofilms occupy interstitial spaces and defect sites. Biofilms occur as discrete orange, green purple and brown layers within the crust (see Vogel et al, 2009). The inner regions of the crust have dissolution zones where euhedral crystals verged into uncemented granular residues associated with thicker biofilms.…”

Enterolithic folds in evaporites as microbially induced sedimentary structures: New model of formation and interpretation in the geological record

“…The biogenic (microbial) origin of this dolomite is characterized by the dumb-bell shape of the hollow cores of the crystals, being the hollows produced by dissolution in relation to bacterial activity (Barbieri et al, 2006). Filamentous bacteria remains (Schopf et al, 2012;Saint Martin & Saint Martin, 2018) and coccoids (Vogel et al, 2009) are also common features indicating microbial activity. Moreover, the lenticular habit of the gypsum crystals is determined by the presence of organic molecules that minimize growth parallel to the crystallographic c-axis (Aref, 1998).…”