Formation of Large Native Sulfur Deposits Does Not Require Molecular Oxygen

Large native (i.e., elemental) sulfur deposits can be part of caprock assemblages found on top of or in lateral position to salt diapirs and as stratabound mineralization in gypsum and anhydrite lithologies. Native sulfur is formed when hydrocarbons come in contact with sulfate minerals in presence of liquid water. The prevailing model for native sulfur formation in such settings is that sulfide produced by sulfate-reducing bacteria is oxidized to zero-valent sulfur in presence of molecular oxygen (O2). Although possible, such a scenario is problematic because: (1) exposure to oxygen would drastically decrease growth of microbial sulfate-reducing organisms, thereby slowing down sulfide production; (2) on geologic timescales, excess supply with oxygen would convert sulfide into sulfate rather than native sulfur; and (3) to produce large native sulfur deposits, enormous amounts of oxygenated water would need to be brought in close proximity to environments in which ample hydrocarbon supply sustains sulfate reduction. However, sulfur stable isotope data from native sulfur deposits emplaced at a stage after the formation of the host rocks indicate that the sulfur was formed in a setting with little solute exchange with the ambient environment and little supply of dissolved oxygen. We deduce that there must be a process for the formation of native sulfur in absence of an external oxidant for sulfide. We hypothesize that in systems with little solute exchange, sulfate-reducing organisms, possibly in cooperation with other anaerobic microbial partners, drive the formation of native sulfur deposits. In order to cope with sulfide stress, microbes may shift from harmful sulfide production to non-hazardous native sulfur production. We propose four possible mechanisms as a means to form native sulfur: (1) a modified sulfate reduction process that produces sulfur compounds with an intermediate oxidation state, (2) coupling of sulfide oxidation to methanogenesis that utilizes methylated compounds, acetate or carbon dioxide, (3) ammonium oxidation coupled to sulfate reduction, and (4) sulfur comproportionation of sulfate and sulfide. We show these reactions are thermodynamically favorable and especially useful in environments with multiple stressors, such as salt and dissolved sulfide, and provide evidence that microbial species functioning in such environments produce native sulfur. Integrating these insights, we argue that microbes may form large native sulfur deposits in absence of light and external oxidants such as O2, nitrate, and metal oxides. The existence of such a process would not only explain enigmatic occurrences of native sulfur in the geologic record, but also provide an explanation for cryptic sulfur and carbon cycling beneath the seabed

Geological Problems with Microbiological Solutions: Deciphering the Authigenesis of Calcite, Dolomite, and Native Sulfur in Salty Environments

Microbial activity is known to impact the formation and alteration of many different rock types. For carbonate caprock (CCR), a lithology found on salt diapirs, it is generally accepted that microbial activity drives the precipitation of carbonate minerals, forming limestone and/or dolomite and native (elemental) sulfur. It appears that there are two types of CCR: 1) limestone associated with native sulfur (S0) and 2) limestone associated with dolomite. The mechanics of CCR formation are poorly understood. For example, it is unclear why native sulfur and dolomite are rarely found in the same CCR assemblage, and why either are formed at all. Filling these gaps in knowledge is important for several reasons. From the perspective of oil and gas exploration, CCR serves as archives of the temperature and fluid history at salt diapirs, can act as reservoirs or conduits for hydrocarbons, can pose drilling hazards or result in dry holes if not identified or misidentified as a different lithology, and finally, may hold critical information on microbial hydrocarbon degradation coupled to the production of sulfide – a process called oil souring that poses major health hazards and is a significant cost to the industry due to corrosion of drilling equipment. From the perspective of basic science, CCR is a unique natural laboratory in which microbial processes took place over long durations in the subsurface with hostile properties. These environments are characterized by elevated pressure and temperature, high salinities, extreme scarceness of oxidants, and are prone to accumulation of sulfide and carbon dioxide. Such conditions are extremely difficult to achieve and maintain under laboratory conditions. As archive of such processes, CCR opens a window into microbial activity in Earth’s subsurface, as well as Earth surface processes in a distant past, and may provide critical clues to the understanding and discovery of life beyond Earth. For my Ph.D., I studied lithological and geochemical signatures of different CCR types to elucidate the geologic setting and dynamics and associated microbial metabolic pathways responsible for the genesis of the different mineral assemblages. I also developed a tool to identify yet undiscovered microbial sulfur transformations in culturing experiments. Central to these activities was the testing of two hypotheses: 1) CCR formation is intimately tied to abiotic and/or microbial sulfur transformations, and 2) native sulfur associated with CCR can be produced by microbial activity in the absence of molecular oxygen (O2), an oxidant that is usually inferred to play a critical role in the genesis of large native sulfur deposits. The dissertation comprises two chapters that tackle the second hypothesis (Chapters 2 and 3), a chapter that investigates an example that may challenge the first hypothesis (Chapter 4), and a chapter that describes the development of an isotope tracer to identify yet undiscovered microbial sulfur transformations in culturing experiments (Chapter 5). Following an outlook on future research (Chapter 6), the appendix includes a manuscript that provides a general overview on the current state of CCR research. As second author I collaboratively developed with my advisor Dr. Brunner, who is lead author, the concept for the paper, contributed to the literature research that went into this review, compiled data, drafted figures, wrote parts of the manuscript, and coordinated the communication with the total of 23 authors in the writing and revision of the manuscript. At the point of the submission of this dissertation, the overview paper and Chapter 2 have been published (Brunner et al., 2019; Labrado et al., 2019), and Chapter 3 is in preparation for submission to Geochimica et Cosmochimica Acta

New Classification of Caprock Associated with Salt Diapirs

Caprock assemblages associated with salt bodies typically consist of a vertically zoned sequence in ascending order: anhydrite directly above the salt body, a transitional gypsum zone, and occasionally a complex zone of limestone and/or dolomite. Caprock forms when the upper part of a rising diapir is exposed to a crossflow of NaCl-undersaturated water, causing halite to dissolve and the less soluble components, largely anhydrite and to a lesser extent gypsum, to accrete via underplating to the base of the previously formed caprock. If hydrocarbons are present, the CaSO4 minerals are replaced by carbonate minerals in a process mediated by sulfate-reducing bacteria. Utilizing new facies mapping and petrographic analysis of outcropping caprock from three different salt basins, Paradox Basin and Gulf Coast Region, USA and Flinders Ranges, South Australia, we recognize a wider variety of fabrics and mineralogies. We propose a new classification based on fabric types in order to facilitate a discussion and interpretation of caprock lithologies in an organized and effective manner. The development of a comprehensive classification is the first step toward deciphering the complex diagenetic processes involved in caprock formation. Understanding the genetic history of caprock fabrics will allow for better identification and prediction of the distribution of caprock mineralogies and fabrics. We introduce the term “capstone” to be used when discussing a particular caprock lithology, as disambiguation for the term “caprock” that is used for discussing the entire rock body. Capstone classification is based on the recognition of four distinct megascopic fabric types: 1) massive: consisting of a homogeneous mineralogy and texture; 2) porphyritic: comprising two distinct crystal sizes; 3) layered, with microlaminated, laminated, and banded as subdivisions based on layer size; and 4) brecciated, which is subdivided based on the degree of separation between capstone clasts, which may be closely, loosely, or spatially independent and subdivided into crackle breccia, mosaic breccia and disorganized breccia, respectively. The classification scheme presented here is chiefly descriptive and directs attention to specific diagenetic fabric properties that may be significant to deciphering a paragenetic history, thereby unlocking an archive of the fluid history at salt diapirs

The tectonic significance of the Early Cretaceous forearc-metamorphic assemblage in south-central Alaska based on detrital zircon U-Pb dating of sedimentary protoliths

A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc-forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U-Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older mélange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to Earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose new terminology for the Knik River terrane. We propose the terms: (1) “Carpenter Creek metamorphic complex” for the Early Cretaceous “metamorphic subterrane”; (2) “western Chugach trondhjemite suite” for the Early Cretaceous forearc plutons within the belt; (3) “Friday Creek assemblage” for a transitional mélange unit that contains blocks of the Carpenter Creek complex in a chert-argillite matrix; and (4) “Knik River metamorphic complex” in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc). The correlation of the Carpenter Creek metamorphic complex with the Chugach mélange indicates that the trace of the Border Ranges fault lies ~1–5 km north of the map trace shown on geologic maps, although like other segments of the Border Ranges fault, this boundary is blurred by local complexities within the Border Ranges fault system. Ductile deformation of the mélange is sufficiently intense that few vestiges of its original mélange fabric exist, suggesting the scarcity of rocks described as mélange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Rapid Shifts in Soil Nutrients and Decomposition Enzyme Activity in Early Succession Following Forest Fire

While past research has studied forest succession on decadal timescales, ecosystem responses to rapid shifts in nutrient dynamics within the first months to years of succession after fire (e.g., carbon (C) burn-off, a pulse in inorganic nitrogen (N), accumulation of organic matter, etc.) have been less well documented. This work reveals how rapid shifts in nutrient availability associated with fire disturbance may drive changes in soil enzyme activity on short timescales in forest secondary succession. In this study, we evaluate soil chemistry and decomposition extracellular enzyme activity (EEA) across time to determine whether rapid shifts in nutrient availability (1–29 months after fire) might control microbial enzyme activity. We found that, with advancing succession, soil nutrients correlate with C-targeting β-1,4-glucosidase (BG) EEA four months after the fire, and with N-targeting β-1,4-N-acetylglucosaminidase (NAG) EEA at 29 months after the fire, indicating shifting nutrient limitation and decomposition dynamics. We also observed increases in BG:NAG ratios over 29 months in these recently burned soils, suggesting relative increases in microbial activity around C-cycling and C-acquisition. These successional dynamics were unique from seasonal changes we observed in unburned, forested reference soils. Our work demonstrates how EEA may shift even within the first months to years of ecosystem succession alongside common patterns of post-fire nutrient availability. Thus, this work emphasizes that nutrient dynamics in the earliest stages of forest secondary succession are important for understanding rates of C and N cycling and ecosystem development

Rapid Shifts in Soil Nutrients and Decomposition Enzyme Activity in Early Succession Following Forest Fire

While past research has studied forest succession on decadal timescales, ecosystem responses to rapid shifts in nutrient dynamics within the first months to years of succession after fire (e.g., carbon (C) burn-off, a pulse in inorganic nitrogen (N), accumulation of organic matter, etc.) have been less well documented. This work reveals how rapid shifts in nutrient availability associated with fire disturbance may drive changes in soil enzyme activity on short timescales in forest secondary succession. In this study, we evaluate soil chemistry and decomposition extracellular enzyme activity (EEA) across time to determine whether rapid shifts in nutrient availability (1–29 months after fire) might control microbial enzyme activity. We found that, with advancing succession, soil nutrients correlate with C-targeting β-1,4-glucosidase (BG) EEA four months after the fire, and with N-targeting β-1,4-N-acetylglucosaminidase (NAG) EEA at 29 months after the fire, indicating shifting nutrient limitation and decomposition dynamics. We also observed increases in BG:NAG ratios over 29 months in these recently burned soils, suggesting relative increases in microbial activity around C-cycling and C-acquisition. These successional dynamics were unique from seasonal changes we observed in unburned, forested reference soils. Our work demonstrates how EEA may shift even within the first months to years of ecosystem succession alongside common patterns of post-fire nutrient availability. Thus, this work emphasizes that nutrient dynamics in the earliest stages of forest secondary succession are important for understanding rates of C and N cycling and ecosystem development

Two modes of gypsum replacement by carbonate and native sulfur in the Lorca Basin, SE Spain

International audienceOrganoclastic sulfate reduction and bacterial sulfide oxidation have been suggested to explain the formation of authigenic carbonate and native sulfur replacing gypsum in the Lorca Basin, Spain. To gain more insight into the nature of this replacement, two types of sulfur-bearing carbonate (laminated and brecciated) from the late Miocene Lorca Basin were studied. Petrographic observations revealed that a sulfur-bearing laminated carbonate consists of clay-rich and dolomite-rich laminae with carbonate and native sulfur pseudomorphs after gypsum. Positive δ 18 O carbonate values in the laminae (δ 18 O = 2.6‰) and lipid biomarkers of halophilic archaea (e.g., extended archaeol) suggest formation under hypersaline conditions. Bacterial sulfate reduction, evidenced by biomarkers such as iso -C 15 , iso -C 16 , and iso -C 17 fatty acids, produced hydrogen sulfide inducing the abiotic formation of organic sulfur compounds. Gypsum in the laminated carbonate likely dissolved due to undersaturation as evidenced by a low content of carbonate-associated sulfate (3,668 ppm) and 34 S-enriched native sulfur (δ 34 S = 22.4‰), reflecting sulfate limitation. Such 34 S-enrichment implies limited fluid flow, which probably restricted the supply of molecular oxygen required for native sulfur formation through oxidation of hydrogen sulfide. Alternatively, sulfate-reducing bacteria may have mediated native sulfur formation directly as a stress response to environmental conditions. The formation of sulfur-bearing calcite in brecciated carbonates is due to post-depositional alteration. Negative δ 18 O values of the calcite (δ 18 O = −1.5‰) and a tenfold decrease in carbonate-associated sulfate content (752 ppm) suggest gypsum dissolution and subsequent calcite precipitation from meteoric water. Relatively 34 S-depleted native sulfur (δ 34 S = 13.1‰) leaves it ambiguous whether meteoric water influx could have supplied sufficient molecular oxygen for oxidation of hydrogen sulfide. In case of the brecciated carbonate, methanogenesis, anaerobic oxidation of methane, and bacterial sulfate reduction apparently mediated the formation of secondary minerals as indicated by 13 C-depleted lipid biomarkers representative for the respective metabolisms. This study reveals that the conditions and timing of gypsum replacement are variable–taking place 1) during or shortly after gypsum deposition or 2) significantly after sedimentation–and suggests that methanogens in addition to anaerobic methanotrophic archaea and sulfate-reducing bacteria may be involved in the mineral-forming processes in the sedimentary subsurface

Secondary Electrons as an Energy Source for Life

13 páginas.-- 3 figuras.-- 3 tablas.-- 69 referenciasLife on Earth is found in a wide range of environments as long as the basic requirements of a liquid solvent, a nutrient source, and free energy are met. Previous hypotheses have speculated how extraterrestrial microbial life may function, among them that particle radiation might power living cells indirectly through radiolytic products. On Earth, so-called electrophilic organisms can harness electron flow from an extracellular cathode to build biomolecules. Here, we describe two hypothetical mechanisms, termed “direct electrophy” and “indirect electrophy” or “fluorosynthesis,” by which organisms could harness extracellular free electrons to synthesize organic matter, thus expanding the ensemble of potential habitats in which extraterrestrial organisms might be found in the Solar System and beyond. The first mechanism involves the direct flow of secondary electrons from particle radiation to a microbial cell to power the organism. The second involves the indirect utilization of impinging secondary electrons and a fluorescing molecule, either biotic or abiotic in origin, to drive photosynthesis. Both mechanisms involve the attenuation of an incoming particle's energy to create low-energy secondary electrons. The validity of the hypotheses is assessed through simple calculations showing the biomass density attainable from the energy supplied. Also discussed are potential survival strategies that could be used by organisms living in possible habitats with a plentiful supply of secondary electrons, such as near the surface of an icy moon. While we acknowledge that the only definitive test for the hypothesis is to collect specimens, we also describe experiments or terrestrial observations that could support or nullify the hypotheses.Peer reviewe