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, is 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 by petrography (microbially induced sedimentary structures interpreted as cyanobacterial mats, and spheroidal dolomite with empty bacterial voids), and geochemical data (elemental concentrations enrichment in Fe²⁺ and Mn²⁺, up to 1388 and 605 ppm, respectively; supported by δ⁵⁶Fe values of between −0.16 and 0.44‰). Collectively, these data suggest that the studied dolomite is a primary precipitate that records the geochemistry of the saline depositional environment influenced by microbial activity. The Vilobí Gypsum Unit is here dated to 16.05 Ma and 16.30 Ma (Burdigalian) by correlating the ⁸⁷Sr/⁸⁶Sr values of dolomite and shell of the oysters (shells between 0.70870 and 0.70872) with the global Sr curve. Moreover, dolomite records marine (porewater) values (C-O-Fe-Sr) that can be used to infer the depositional environment and geochemical conditions at the time of deposition. Results of non-traditional isotopes and trace metal concentrations support using dolomite formed in evaporitic environments as an archive of paleoenvironmental conditions.