Sabkha and salina dolomite preserves the biogeochemical conditions of its depositional paleoenvironment

Abstract

Precipitation of Mg-carbonate minerals, ranging from Mg-calcite to Ca-rich dolomite, are typically associated with microbial activity, and commonly takes place in saline environments along with other evaporitic minerals. The geochemistry of such evaporitic minerals can contribute to the interpretation of the environmental conditions and those (bio)geochemical processes occurring at the time of deposition. We use a multiproxy approach that combines stratigraphic, petrographic, mineralogical and geochemical data, such as Fe and Sr isotope values, to describe the environmental and biogeochemical conditions under which dolomite precipitated in a sabkha/salina environment using the Miocene Vilobí Gypsum Unit (Penedès Basin, NE Spain) as a case study. The lithofacies of Vilobí Gypsum Unit are interpreted to indicate sedimentary environments alternating between sabkha and coastal salina. The δ13C and δ18O values of Vilobí dolomites (from −6.3 to −0.4‰ and from 0.2 to 2.3‰, respectively) compare well with microbially influenced dolomite from modern sabkha and coastal environments. The dolomite 87Sr/86Sr ratios (0.70816 to 0.70874) show significant variations and indicate that the coastal sabkha/salina was strongly influenced by seawater. Microbial activity is evidenced in petrographic (microbially induced sedimentary structures, MISS, interpreted as cyanobacterial mats; spheroidal dolomite with empty bacterial voids), and geochemical features (elemental concentrations enrichment in F…