Anoxygenic phototrophs and the forgotten art of making dolomite

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Coupled measurements of δ18O and δD of hydration water and salinity of fluid inclusions in gypsum from the Messinian Yesares Member, Sorbas Basin (SE Spain)

Financial support was provided by Clare College Geological Research Fund to N.P. Evans. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement n. 339694 (Water Isotopes of Hydrated Minerals) to D.A. Hodell.We studied one cycle (Cycle 6) of gypsum-marl deposition from the Messinian Yesares Member in Sorbas Basin, Spain. The objective was to reconstruct the changing environment of deposition and its relation to astronomically-forced climate change. The δ18O and δD of gypsum hydration water (CaSO4 • 2H2O) and salinity of fluid inclusions were measured in the same samples to test if they record the composition of the mother fluid from which gypsum was precipitated. Water isotopes are highly correlated with fluid inclusion salinity suggesting the hydration water has not exchanged after formation. The relatively low water isotope values and fluid inclusion salinities indicate a significant influence of meteoric water, whereas δ34S, δ18OSO4 and 87Sr/86Sr support a dominant marine origin for the gypsum deposits. The discrepancy between water and elemental isotope signatures can be reconciled if meteoric water dissolved previously deposited marine sulfates supplying calcium and sulfate ions to the basin which maintained gypsum saturation. This recycling process accounts for the marine δ34S, δ18OSO4 and 87Sr/86Sr signatures, whereas the low δ18O and δD values of gypsum hydration water and fluid inclusion salinities reflect the influence of freshwater. The cyclic deposition of gypsum and marl in the Yesares Member has previously been interpreted to reflect changing climate related to Earth's precession cycle. We demonstrate that the δ18O, δD and salinity of the parent brine increased from low values at the base of the cycle to a maximum in the massive gypsum palisade, and decreased again to lower values in the supercones at the top of the cycle. This pattern, together with changes in mineralogy (calcite-dolomite-gypsum), is consistent with a precession-driven change in climate with wettest conditions (summer insolation maxima) associated with the base of the calcium carbonate marls and driest conditions (summer insolation minima) during formation of the gypsum palisade.Publisher PDFPeer reviewe

Mediation of carbonate minerals formation by aerobic bacterial strains isolated from Dohat Faishakh Sabkha in Qatar

Carbon dioxide (CO2) is increasingly released to the environment as a result of the extensive use of various industrial facilities. According to the Intergovernmental Panel on Climate Change Report (2015), cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond". Different technologies can be applied for capturing and storing CO2 such as sequestration and carbonate mineral storage. Biominerals are of great importance due to their huge impact on the global biogeochemical cycle. Carbonates such as limestone and dolomite are important carbon reservoirs. Calcium carbonate formation and its burial in marine sediments account for approximately 80% of total carbon removal from the Earth's surface by abiotic and biotic precipitation. The biotic precipitation of calcium carbonates, is performed by various organisms, including bacteria, and has been widely reported and discussed in the literature, while, the formation of high-magnesium calcites is extremely challenging, due to the high level of hydration of Mg2+ ions, which promote the formation of Mg-free aragonite, rather than calcite. The dynamic evaporitic systems characteristic of sabkhas are crucial for the precipitation of minerals, and a role for microorganisms in sabkhas in the process of mineralization has been proposed. In this study the Dohat Faishakh Sabkha in Qatar was investigated for evidenc of the role of aerobic bacteria in mediating the formation of high magnesium carbonates and dolomite, two minerals that commonly occur in the sabkha sediments. 29 strains of aerobic microbes isolated from the sabkha and identified by 16S rDNA sequencing as belonging to the genera Bacillus, Salinivibrio, Staphylococcus and, primarily, Virgibacillus. All strains examined caused the pH of the artificial growth medium to increase from 7 to 8.5; however, not all were capable of mediating mineral formation. Only Salinivibrio and Virgibacillus spp. isolates mediated the formation of detectable solid phases within the agar plates. Light microscopy, scanning electron microscopy energy dispersive X-ray (SEM/EDX), and X-ray diffraction (XRD) analyses indicate that the solid phase produced in the presence of these bacterial strains is MgCa(CO3)2 with a MgCO3 mol% varying from 0% to 40%. The results of these laboratory experiments suggested that, in the Dohat Faishakh Sabkha, aerobic bacteria may contribute in the formation o very high Mg calcite, a mineral that is considered the precursor of ordered dolomite.qscienc

Calibration of the oxygen and clumped isotope thermometers for (proto-)dolomite based on synthetic and natural carbonates

Dolomite is a very common carbonate mineral in ancient sediments, but is rarely found in modern environments. Because of the difficulties in precipitating dolomite in the laboratory at low temperatures, the controls on its formation are still debated after more than two centuries of research. Two important parameters to constrain the environment of dolomitization are the temperature of formation and the oxygen isotope composition of the fluid from which it precipitated. Carbonate clumped isotopes (expressed with the parameter Δ47) are increasingly becoming the method of choice to obtain this information. However, whereas many clumped isotope studies treated dolomites the same way as calcite, some recent studies observed a different phosphoric acid fractionation for Δ47 during acid digestion of dolomite compared to calcite. This causes additional uncertainties in the Δ47 temperature estimates for dolomites analyzed in different laboratories using different acid digestion temperatures. To tackle this problem we present here a (proto-)dolomite-specific Δ47-temperature calibration from 25 to 1100 °C for an acid reaction temperature of 70 °C and anchored to widely available calcite standards. For the temperature range 25 to 220 °C we obtain a linear Δ47-T relationship based on 289 individual measurements with R2 of 0.864: [Formula presented] Tin Kelvin When including two isotopically scrambled dolomites at 1100 °C, the best fit is obtained with a third order polynomial temperature relationship (R2 = 0.924): [Formula presented]. Applying a calcite Δ47-T relationship produced under identical laboratory conditions results in 3 to 16 °C colder calculated formation temperatures for dolomites (with formation temperature from 0 to 100 °C) than using the (proto-)dolomite specific calibration presented here. For the synthetic samples formed between 70 and 220 °C we also determined the temperature dependence of the oxygen isotope fractionation relative to the water. Based on the similarity between our results and two other recent studies (Vasconcelos et al., 2005 and Horita, 2014) we propose that a combination of the three datasets represents the most robust calibration for (proto-)dolomite formed in a wide temperature range from 25 to 350 °C. 103αCaMg−carbonates−Water=2.9923±0.0557×[Formula presented]−2.3592±0.4116 Because of the uncertainties in the phosphoric acid oxygen and clumped isotope fractionation for (proto-)dolomite, we promote the use of three samples that are available in large amounts as possible inter-laboratory reference material for oxygen and clumped isotope measurements. A sample of the middle Triassic San Salvatore dolomite from southern Switzerland, the NIST SRM 88b dolomite standard already reported in other Δ47 studies and a lacustrine Pliocene dolomite from La Roda (Spain). This study demonstrates the necessity to apply (proto-)dolomite specific Δ47-T relationships for accurate temperature estimates of dolomite formation, ideally done at identical acid digestion temperatures to avoid additional uncertainties introduced by acid digestion temperature corrections. In addition, the simultaneous analyses of dolomite reference material will enable a much better comparison of published dolomite clumped and oxygen isotope data amongst different laboratories

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

International audienceOn the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/ Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28-to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions

Microbial Mats from the Khor Al-Adaid Sabkha, Qatar: Morphotypes and Association with Authigenic Minerals

The sabkhas (i.e., salt flats) of Qatar are among the rare places on Earth where carbonate and sulfate minerals similar to those constituting economically important hydrocarbon reservoirs are still forming today, under the arid conditions that characterize the coastline of the country. Since the 1960's, the sabkhas of Qatar have been studied with great interest as a modern analogue for ancient sedimentary sequences (e.g., Wells, 1962; Illing & Taylor, 1995; Alsharhan & Kendall, 2003). The results of these studies provided important insights for formulating stratigraphic models of subsurface hydrocarbon reservoirs. Notable examples of gas and oil reservoirs that formed in arid, evaporitic environments include the Permo-Triassic Khuff (which is estimated to contain about 15-20% of the world's gas reserves and is of fundamental importance for the economy of Qatar), the Jurassic Arab formations, and the Triassic Kurra Chine, all of the Middle East, and the Permian Zechstein of Northern Europe. Although extremely valuable, most of these early studies were based on purely physical and chemical approaches, which may have not fully captured the complexity of the mineralization processes occurring in the sabkha environment. Indeed, research conducted in more recent years has shown that microorganisms play an important and, as yet, poorly understood role for the mineralization processes occurring in these evaporitic environments (Bontognali et al., 2010; Bontognali et al., 2012; Bontognali et al., 2014; Brauchli et al., 2015; Paulo & Dittrich, 2013; Strohmenger et al., 2011). Here we present the results of a field campaign conducted in the Khor Al-Adaid sabkha, which is located in the southeast of Qatar, in a large tidal embayment composed of two shallow inland lagoons. The main goal of the field campaign was to identify regions of the intertidal zone that are particularly rich in microbial mats, and that represent ideal sites at which to study microbe-mineral interactions. Three sites of interest have been defined.qscienc

Subsurface scientific exploration of extraterrestrial environments (MINAR 5): analogue science, technology and education in the Boulby Mine, UK

The deep subsurface of other planetary bodies is of special interest for robotic and human exploration. The subsurface provides access to planetary interior processes, thus yielding insights into planetary formation and evolution. On Mars, the subsurface might harbour the most habitable conditions. In the context of human exploration, the subsurface can provide refugia for habitation from extreme surface conditions. We describe the fifth Mine Analogue Research (MINAR 5) programme at 1 km depth in the Boulby Mine, UK in collaboration with Spaceward Bound NASA and the Kalam Centre, India, to test instruments and methods for the robotic and human exploration of deep environments on the Moon and Mars. The geological context in Permian evaporites provides an analogue to evaporitic materials on other planetary bodies such as Mars. A wide range of sample acquisition instruments (NASA drills, Small Planetary Impulse Tool (SPLIT) robotic hammer, universal sampling bags), analytical instruments (Raman spectroscopy, Close-Up Imager, Minion DNA sequencing technology, methane stable isotope analysis, biomolecule and metabolic life detection instruments) and environmental monitoring equipment (passive air particle sampler, particle detectors and environmental monitoring equipment) was deployed in an integrated campaign. Investigations included studying the geochemical signatures of chloride and sulphate evaporitic minerals, testing methods for life detection and planetary protection around human-tended operations, and investigations on the radiation environment of the deep subsurface. The MINAR analogue activity occurs in an active mine, showing how the development of space exploration technology can be used to contribute to addressing immediate Earth-based challenges. During the campaign, in collaboration with European Space Agency (ESA), MINAR was used for astronaut familiarization with future exploration tools and techniques. The campaign was used to develop primary and secondary school and primary to secondary transition curriculum materials on-site during the campaign which was focused on a classroom extra vehicular activity simulation

Sabkha dolomite as an archive for the magnesium isotope composition of seawater

Recent studies have uncovered the potential of Mg isotopes (δ26Mg) for studying past ocean chemistry, but records of such data are still scarce. Dolomite has been suggested as a promising archive for δ26Mg of seawater. However, its enigmatic formation mechanism and the difficulty in precipitating dolomite in the laboratory at surface temperatures decrease confidence in the interpretation of δ26Mg values from the rock record. To evaluate factors determining the δ26Mg of dolomite, we studied pore water and sediment from Dohat Faishakh Sabkha, Qatar—one of the rare environments where dolomite is currently forming. The δ26Mg values of the dolomite (–2.56‰ to –1.46‰) are lower than that of seawater (–0.83‰), whereas δ26Mg values of pore water (–0.71‰ to –0.14‰) are higher. The isotope fractionation accompanying dolomite formation is generally in accordance with an empirical fractionation from the literature, extrapolated to the sabkha’s temperature (–1.84‰ to –1.51‰). The results suggest that evaporated seawater is the sole source of Mg, and isotopically light dolomite is the major sink, so that the δ26Mg of the dolomite-forming pore water is equal to or greater than that of seawater. Thus, provided that the lowest δ26Mg value among several dolomite samples is used, and the formation temperature is known, similar sabkha-type dolomites can be utilized as an archive for δ26Mg values of ancient seawater. © 2020 Geological Society of AmericaISSN:0091-7613ISSN:1943-268

Nickel and its isotopes in organic-rich sediments: implications for oceanic budgets and a potential record of ancient seawater

ISSN:0012-821XISSN:1385-013