Abstract For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (δ 13 C=−9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (δ 13 C=−1.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (δ 18 O=1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (δ 18 O=4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 °C, whereas it is 32 °C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.

Abstract Integrated Ocean Drilling Program (IODP) Expedition 310 (Tahiti Sea Level) offered an opportunity to study the geomicrobiology of a reef framework. Offshore drilling was conducted on the coastal reefs of Tahiti (French Polynesia) at 22 sites in water depths of up to 117 m. Up to 80% of the retrieved core material comprises authigenic grey microbial carbonates with laminated or thrombolitic morphologies, which are associated with corals. Microbialites infilled the cavities during reef development and stabilized the coral reef framework. Rock-surface analyses were performed to track ongoing microbial activity in biofilms that could represent a modern counterpart of the processes at the origin of the formation of fossil microbialites. Significant concentrations of adenosine 5′-triphosphate, indicative of the presence of living microorganisms, were detected at relatively shallow depths, 0–6 m below the seafloor. Exoenzyme activities confirmed the presence of an active metabolizing microbiota forming biofilms in reef cavities. Onshore investigations of the recovered microbes and biofilms completed our picture that the rapid postglacial formation of carbonate microbialites was mediated by the activity of anaerobic microbes, such as sulphate-reducing bacteria and iron-respiring organisms, stimulated by the highly productive reef environment.

Abstract A mineralogical, geochemical, and rock-magnetic investigation of sediments deposited during the last 300 years in Lake Greifen, a hard-water lake with moderate sulfate concentrations (<250 μmol/L) and seasonal anoxia, shows that both authigenic single-domain biogenic magnetite and multidomain detrital titanomagnetite were preserved within the bioturbated marls deposited prior to the onset of anthropogenically induced eutrophication. Subsequently, in response to a gradual change from oxic to dysaerobic to anoxic bottom-waters, the deposition of organic carbon-rich varved sediments occurred and the degree of magnetite preservation decreased as altered diagenetic conditions resulted in the rapid dissolution and sulfidization of the biogenic and detrital magnetite. The occurrence of both biogenically produced magnetite and detrital titano-magnetite within the upper 4 cm of sediment indicates that (1) biogenic magnetite may be produced within the near surface organic carbon-rich sediments, probably on an annual basis when the overlying waters are oxygenated, and (2) detrital magnetite is continuously deposited. Changes in magnetic properties below this zone of surface magnetite production and microscopic examination of corroded fine-grained biogenic magnetite extracted from this interval indicate the rapid destruction of the most recently produced (or deposited) magnetite. Our findings demonstrate that (1) lacustrine sedimentary magnetic properties may reflect redox conditions, which are in the case of Lake Greifen determined by productivity, and (2) rapid destruction and sulfidization of fine-grained and coarse-grained magnetite can occur in lacustrine systems that are characterized by high productivity, low available lake-water sulfate, low concentrations of dissolved sulfide, and rapid sediment accumulation rates. These findings differ from marine studies in which magnetite dissolution and sulfidization is postulated to occur in systems characterized by high productivity, high concentrations of dissolved sulfide, and low sediment accumulation rates. Based on our observations, we propose that microbially mediated processes are contributing, either directly or indirectly, not only to the authigenesis of magnetite in the Lake Greifen sediments but also to its destruction.