The importance of microbial mats for dolomite formation in the Dohat Faishakh sabkha, Qatar

The Dohat Faishakh sabkha in Qatar is one of the rare modern environments where it is possible to study the formation of dolomite, a mineral whose origin has been long debated. In previous studies, dolomite formation in this area was considered to be the result of a penecontemporaneous replacement of aragonite, occurring in the presence of Mg-rich evaporated pore-waters. However, a re-investigation of the sabkha revealed that dolomite is not forming exclusively under the evaporitic conditions that characterize the supratidal zone, but also in microbial mats that colonize the lower intertidal zone, indicating that evaporated pore-waters are not a strict requirement for the mineralization process. Moreover, in the supratidal zone, portions of the sediment that are rich in dolomite are also relatively richer in organic material, which derives from partially degraded microbial mats buried in the sediments. Extracellular polymeric substances (EPS) that constitute microbial mats are recognized as an important component for the formation of Mg-rich carbonates. The presence of living and decaying microbial mats comprising EPS, rather than a replacement process, may be the key factor for dolomite formation in the Dohat Faishakh sabkha.Swiss National Science Foundatio

Influence of temperature, salinity and Mg:Ca ratio on microbially-mediated formation of Mg-rich carbonates by Virgibacillus strains isolated from a sabkha environment.

Studies have demonstrated that microbes facilitate the incorporation of Mg into carbonate minerals, leading to the formation of potential dolomite precursors. Most microbes that are capable of mediating Mg-rich carbonates have been isolated from evaporitic environments in which temperature and salinity are higher than those of average marine environments. However, how such physicochemical factors affect and concur with microbial activity influencing mineral precipitation remains poorly constrained. Here, we report the results of laboratory precipitation experiments using two mineral-forming Virgibacillus strains and one non-mineral-forming strain of Bacillus licheniformis, all isolated from the Dohat Faishakh sabkha in Qatar. They were grown under different combinations of temperature (20°, 30°, 40 °C), salinity (3.5, 7.5, 10 NaCl %w/v), and Mg:Ca ratios (1:1, 6:1 and 12:1). Our results show that the incorporation of Mg into the carbonate minerals is significantly affected by all of the three tested factors. With a Mg:Ca ratio of 1, no Mg-rich carbonates formed during the experiments. With a Mg:Ca ratios of 6 and 12, multivariate analysis indicates that temperature has the highest impact followed by salinity and Mg:Ca ratio. The outcome of this study suggests that warm and saline environments are particularly favourable for microbially mediated formation of Mg-rich carbonates and provides new insight for interpreting ancient dolomite formations

Anatomy of Heinrich Layer 1 and its role in the last deglaciation

X-ray fluorescence (XRF) core scanning and X-ray computed tomography data were measured every 1 mm to study the structure of Heinrich Event 1 during the last deglaciation at International Ocean Discovery Program Site U1308. Heinrich Layer 1 comprises two distinct layers of ice-rafted detritus (IRD), which are rich in detrital carbonate (DC) and poor in foraminifera. Each DC layer consists of poorly sorted, coarse-grained clasts of IRD embedded in a dense, fine-grained matrix of glacial rock flour that is partially cemented. The radiocarbon ages of foraminifera at the base of the two layers indicate a difference of 1400 14C years, suggesting that they are two distinct events, but the calendar ages depend upon assumptions made for surface reservoir ages. The double peak indicates at least two distinct stages of discharge of the ice streams that drained the Laurentide Ice Sheet through Hudson Strait during HE1 or, alternatively, the discharge of two independent ice streams containing detrital carbonate. Heinrich Event 1.1 was the larger of the two events and began at ~16.2 ka (15.5–17.1 ka) when the polar North Atlantic was already cold and Atlantic Meridional Overturning Circulation (AMOC) weakened. The younger peak (H1.2) at ~15.1 ka (14.3 to 15.9 ka) was a weaker event than H1.1 that was accompanied by minor cooling. Our results support a complex history for Heinrich Stadial 1 (HS1) with reduction in AMOC during the early part (~20–16.2 ka) possibly driven by melting of European ice sheets, whereas the Laurentide Ice Sheet assumed a greater role during the latter half (~16.2–14.7 ka)

The high-resolution map of Oxia Planum, Mars; the landing site of the ExoMars Rosalind Franklin rover mission

This 1:30,000 scale geological map describes Oxia Planum, Mars, the landing site for the ExoMars Rosalind Franklin rover mission. The map represents our current understanding of bedrock units and their relationships prior to Rosalind Franklin’s exploration of this location. The map details 15 bedrock units organised into 6 groups and 7 textural and surficial units. The bedrock units were identified using visible and near-infrared remote sensing datasets. The objectives of this map are (i) to identify where the most astrobiologically relevant rocks are likely to be found, (ii) to show where hypotheses about their geological context (within Oxia Planum and in the wider geological history of Mars) can be tested, (iii) to inform both the long-term (hundreds of metres to ∼1 km) and the short-term (tens of metres) activity planning for rover exploration, and (iv) to allow the samples analysed by the rover to be interpreted within their regional geological context

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

ISSN:0012-821XISSN:1385-013

The Triassic–Jurassic boundary in the shallow-water marine carbonates from the western Northern Calcareous Alps (Austria)

The Triassic–Jurassic (T–J) boundary coincides with one of the five biggest mass extinctions in the Phanerozoic. This event has been extensively studied in the eastern and central Northern Calcareous Alps (NCA), where the global stratotype section and point for the base of the Jurassic is outcropping. We present one of the first combined bio-, litho- and carbonate carbon isotope stratigraphic study across the T–J boundary from the western NCA, focusing on the shallow-water marine carbonate successions “Lorüns” and “Steinernes Meer”. An almost complete T–J succession with the Upper Triassic Kössen Formation, the T–J Schattwald beds and the Lower Jurassic Lorüns oolite is preserved in the Lorüns section, whereas a stratigraphic hiatus at the T–J boundary is observed in the Steinernes Meer section. The carbonate carbon isotope record of Lorüns shows the characteristic T–J carbon isotope evolution with a short initial negative carbon isotope excursion (CIE) in the lower Schattwald beds, which is separated from the following and longer main negative CIE in the Lorüns oolite by a positive excursion in the upper Schattwald beds. The synchroneity of the initial and main CIE with the last occurrence of Triassic fauna and the first appearance of Jurassic ammonites suggests placing the T–J boundary at the base of the Lorüns oolite. The Steinernes Meer section records only the main negative CIE in the Lorüns oolite due to emersion and/or no sedimentation between the Upper Triassic Kössen Formation and the Lower Jurassic Lorüns oolite, caused by regression and synsedimentary tectonics. This study provides new evidence in support the hypothesis that the latest Rhaetian decrease and the Hettangian recovery of the carbonate production coinciding with the initial and main CIE, respectively, can be interpreted as evidence of acidification of the Tethys ocean due to elevated atmospheric CO2 concentrations, probably caused by voluminous volcanic gas emissions

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

The "Dolomite Problem" has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth's history may explain important aspects of "The Dolomite Problem". Our results provide new context for the interpretation of dolomite throughout Earth's history

Source and genesis of sulphate and phosphate-sulphate minerals in a quartz-sandstone cave environment

Gypsum (CaSO42H2O), alunite (KAl3(SO4)2(OH)6), and rare phosphate–sul- phate sanjuanite Al2(PO4)(SO4)(OH) 9(H2O) and rossiantonite (Al3(PO4)(SO4) 2(OH)2(H2O)14) have recently been identified as secondary mineral deposits in different quartz-sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz-sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral depo- sits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to per- haps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteoro- logical, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz-sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sul- phate source, with potential contributions of marine non-sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H2S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sul- phates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co-occur with silica speleothems of biologi- cal origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz-sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments

Characterization of the extracellular polymeric substances (EPS) of Virgibacillus strains capable of mediating the formation of high Mg-calcite and protodolomite

The origin of dolomite -a common mineral in the geological record- is the subject of an ongoing debate. Among different hypotheses, it has been proposed that extracellular polymeric substances (EPS) excreted by microbes include organic molecules that catalyze the incorporation of Mg in the carbonate mineral. However, limited information exists on the composition of the EPS produced by Ca-Mg carbonate-forming microbes, which in turn hampers a precise understanding of their role in the mineralization mechanism. Here, we present the results of laboratory experiments in which we cultured different strains of microbes, characterized their EPS, and identified components associated with carbonate minerals with high mol% Mg. Two Virgibacillus strains known to mediate the formation of Mg-rich carbonates, as well as a strain of Bacillus licheniformis -a negative control that does not mediate mineral formation but produces EPS, were grown under different salinities and temperatures, which caused them to produce EPS with different compositions. The EPS were subsequently characterized by measuring total carbohydrate (TCHO) and total protein (TP) contents, as well as by Fouriertransform infrared spectroscopy (FTIR). At the tested conditions, we found that Mg-carbonates with a mol% Mg higher than 40% (i.e., potential dolomite precursor phases) formed exclusively in association with EPS rich in carbohydrates (TCHO > than 75% of the total mass). FTIR spectra of Ca-Mg carbonate-forming strains were distinct from those of the non-mineral-forming strain in areas associated with the protein structures responsible for the formation of hydrogels, which contribute to hydration or dehydration of ionic clusters; further differences have been observed in the regions of phosphoryl functional groups. These results provide insight on which fraction of organic molecules and specific functional groups are, among the many constituents of EPS, important for mineral nucleation and incorporation of Mg into carbonate minerals, a crucial step for the formation of dolomite in natural environments