An identity card for sabkha dolomite

Abstract

The Dohat Faishakh sabkha in Qatar was among the first modern environments studied to understand low-temperature dolomite formation in association with gypsum and other evaporites. Since the 1960s, research conducted in this sabkha has significantly influenced geological models that remain widely used today, helping in the interpretation of sedimentary sequences that dominated certain periods of Earth's history. Here, we present results of an investigation of the dolomite occurring in this sabkha using techniques more advanced than those available during the initial pioneering studies. By integrating our new results with previously published data, we establish an ‘identity card’ for this sabkha dolomite and the environment it forms. The dolomite exhibits a rhombohedral morphology, contains 50.8 mol% Mg, and has an ordering degree of 0.25 (poorly ordered). Isotopic values are approximately: δ¹³C = 5.0‰, δ¹⁸O = 4.1‰ and δ²⁶Mg = −2.6‰ to −1.5‰ and Δ₄₇ = 0.611‰. Annual temperature data indicate an average of 32.2°C in the subsurface intervals with the highest dolomite content. The associated pore water has an Mg/Ca ratio of 156, a salinity roughly nine times that of sea water and a pH of 6.9. Sediment total organic carbon is ~2%. Microbial diversity in the dolomite-bearing layers is dominated by Euryarchaeota—an extremophilic, opportunistic and metabolically versatile archaeal phylum. Together, these data provide a reference for identifying sabkha-type dolomites in the geological record, calibrating paleoclimatic proxies and interpreting biomarker signals that may be recorded in ancient dolomites.