Depositional Conditions of Cretaceous Ironstones Deposit in the Chulym-Yenisey Basin (Western Siberia)

This study reconstructs the depositional conditions of ironstones within the Chulym-Yenisey basin and assesses the iron source. The detrital minerals of the studied deposits include quartz and feldspar. The authigenic minerals are goethite, siderite, aragonite, dolomite, calcite, apatite, barite, and pyrite. The clay components include minerals of the chlorite group (possible chamosite), nontronite, kaolinite, illite, and beidellite. Local bacterial sulfate reduction led to the formation of pyrite framboids in siltstone layers. The subsequent diagenetic iron reduction promoted the formation of chamosite from siderite. The goethite precipitation occurred in an oxidic aqueous environment. The Cretaceous continental sediments of the Ilek and Kia Formations of the Chulym-Yenisei depression consist of fine- and medium-grained, cross-stratified, poorly sorted litho-feldspatho-quartzose sandstones of fluvial channel origin alternating with bluish-gray siltstones and ironstones of floodplain–lacustrine–bog origin. Thin layers of iron-bearing rocks within siltstones formed in meromictic waters. The changes in geochemical proxies demonstrate fluctuations of paleoenvironmental conditions within the Cretaceous sequence. Siltstones and sandstones formed under humid and arid conditions, respectively. The primary iron source for sediments of the Chulym-Yenisey depression was determined as volcanogenic and igneous rocks of the Altai-Sayan mountainous region.

Catchment-scale metal retention revealed from natural bacterial sulfate reduction (BSR)

<p>Acid mine drainage (AMD) threaten ecosystems world-wide and research on biological remediation techniques are increasing. One of them is bacterial sulfate reduction (BSR) that immobilizes the aqueous sulfate and through coprecipitation removes dissolved metals from the more bioavailable phase. Although BSR has previously only been investigated at the local wastewater treatment scale (e.g. for constructed wetlands), it is unknown to which extent they contribute to contaminant attenuation at larger scales (e.g. a hydrological basin). We developed a new method to trace the activity of BSR within an AMD-impacted catchment using sulfur isotopes (δ<sup>34</sup>S) and found that they naturally reduce 30% of the riverine sulfate and metal concentrations, with a spread from 10 to 50% reduction within the catchment. These results are based on surface water field measurements from our test site in northern Sweden combined in a mass-balance mixing model where we explicitly addressed the isotopic fractionation from bacterial activity. This innovative mapping of catchment-scale biogeochemical natural attenuation provides important clues to strategically target remediation measures, e.g. potential in-situ enhancement of the BSR activity. In combination with stable water isotopes we hope to refine this method to further identify BSR hot spots within the catchment and to extend its application to other sites, e.g. the Khibiny mining region, Russia.</p>

Biogenic native Sulfur linked to the Neogene deposits from the Riffian Corridors (Northern Morocco): preliminary study and characterization

<p>Elemental sulfur in sedimentary rocks is commonly associated with evaporites and authigenic carbonates. The genesis of this evaporite hosted native sulfur has been traditionally considered as a result of bacterial sulfate reduction under specific geological and paleogeographic conditions. Some biogenic sulfur occurrences are found in the Mediterranean area associated with the Neogene formations (e.g. Hellin, Lorca, Teruel, Sicily). They are described as interbedded layers in large evaporitic sequences or as sulfur nodules enclosed in secondary gypsum or carbonate deposits. Quite similar geological settings are present in Northern Morocco where several sediment-hosted sulfur showings were noted. However, these potential sulfur occurrences in the Pre-Rif and post-nappe Neogene basins have not been studied and still basically unknown. This work aims to explore these occurrences and assess their potentials using preliminary field, mineralogical and geochemical data. Several potential areas were identified at the Tortono-Messinian formations of Oued Amlil, Arbaa Taourirt, Taghzout Tassa, and Boudinar basins. They show favorable settings composed mainly of gypsiferous marls, carbonate, and organic matter-rich black sediments. In terms of sulfur contents, preliminary XRD data confirmed the presence of elemental sulfur and geochemical analyses show total sulfur content reaching 18.5 wt.%. However, further fieldwork combined with advanced mineralogical and isotopic geochemistry is still necessary for this area to try understanding their paragenesis in comparison with other similar Mediterranean occurrences.</p>

Sr, S, and O Isotope Compositions of Evaporites in the Lanping–Simao Basin, China

Evaporites are widely distributed within continental “red beds” in the Lanping–Simao Basin, west Yunnan, China. Sr (Strontium), S (Sulfur), and O (Oxygen) isotope compositions have been measured on 54 sulfate or/and sulfate-bearing samples collected from Lanping, Nuodeng, Jinggu, Mengyejing, Baozang throughout the Lanping–Simao Basin. The 87Sr/86Sr ratios of all samples (0.708081 to 0.710049) are higher than those of contemporaneous seawater, indicating a significant continental contribution to the drainage basin. Sulfates in the Lanping Basin have higher 87Sr/86Sr ratios (0.709406 to 0.710049) than those (0.708081 to 0.709548) in the Simao Basin. Nevertheless, the δ34S values of gypsums (13.4‰ to 17.6‰) in Lanping and Baozang fall within the range of Cretaceous seawater. Gypsums from a single section in Baozang have trends of decreasing δ34S values and increasing 87Sr/86Sr ratios from base to top, indicating continental input played an increasingly significant role with the evaporation of brines. High δ34S values (20.5‰ to 20.7‰) of celestites in Lanping are probably caused by bacterial sulfate reduction (BSR) process in which 34S were enriched in residual sulfates and/or recycling of Triassic evaporites. The reduced δ34S values of gypsums (9.5‰ to 10.4‰) in Nuodeng could have been caused by oxidation of sulfides weathered from Jinding Pb-Zn deposit. The complex O isotope compositions indicate that sulfates in the Lanping–Simao Basin had undergone sulfate reduction, re-oxidation, reservoir effects, etc. In conclusion, the formation of continental evaporites was likely derived from seawater due to marine transgression during the Cretaceous period. Meanwhile, non-marine inflows have contributed to the basin significantly.

Sources, trends, and fate of methane in shallow aquifers of Alberta, Canada

<p>Due to concerns regarding potential impacts of the development of natural gas from unconventional hydrocarbon resources on groundwater systems in North America and elsewhere, it has been crucial to improve methods of Environmental Baseline Assessment (EBA). Any subsequent deviations from the EBA could indicate migration of natural gas into the monitored groundwater systems. In collaboration with Alberta Environment and Parks, over 800 groundwater samples have been collected from dedicated monitoring wells since 2006 resulting in an extensive high-quality database of aqueous and gaseous geochemical and isotopic compositions. Because methane is the main component of natural gas, it had been the principal target of our groundwater studies. Our objectives were a) to assess the occurrence of methane in groundwater throughout the province of Alberta (Canada), b) to use isotope techniques to track the predominant sources of methane, c) to use a combination of chemical and multi-isotopic techniques and models to assess the fate of methane in groundwater, and d) to use probability for predicting the presence of methane in groundwater based on hydrogeochemical parameters in regions where no gas data exist.</p><p>Methane was found to be ubiquitous in groundwater samples throughout the province of Alberta with concentrations varying from 2.9 10<sup>-4</sup> to >2.4 mmol/l. The highest methane concentrations were found in Na-HCO<sub>3</sub> and Na-Cl water-types where the sulfate concentrations were <1 mmol/l. Analyses of the isotopic compositions of sulfate, dissolved inorganic carbon (DIC) and methane revealed that in some groundwater systems bacterial sulfate reduction occurred (δ<sup>34</sup>S<sub>SO4</sub> >+10‰ associated with lowest sulfate concentrations) and evidence for methane oxidation was also detected (highest δ<sup>13</sup>C<sub>CH4</sub> values > ‑55‰ associated with lowest methane concentrations). Moreover, some δ<sup>13</sup>C<sub>DIC</sub> values were as high as +13.8‰ associated with the highest methane concentrations. A geochemical and multi-isotope model using long-term monitoring data was developed and revealed two different sources of methane: 1) microbial methane resulting from in-situ methanogenesis within the aquifer for a subset of the samples; 2) migration of microbial methane into aquifers characterized by various redox conditions, followed by methane oxidation potentially coupled with bacterial sulfate reduction within sulfate-rich zones causing a pseudo-thermogenic carbon isotopic fingerprint for the remaining methane. So far, no evidence of unambiguously thermogenic methane in the groundwater samples collected from dedicated monitoring wells has been found. Efforts to assess the probability of regional occurrence of methane in groundwater systems in Alberta have then focused on a model for methane prediction model based on logistic regression (LR) for regions of Alberta where no gas data exist. Using basic hydrogeochemical parameters such as occurrence of electron donors, well depth and total dissolved solids of groundwater, the LR approach shows excellent performance metrics e.g. model sensitivity, specificity >80% regarding the prediction of methane occurrence in groundwater of Alberta.</p>