GENESIS OF DONGPING GOLD-TELLURIDE DEPOSIT BASED ON GEOCHEMICAL CHARACTERISTICS OF FLUIDS, 40Ar/39Ar DATING, STABLE AND RADIOGENIC ISOTOPES (NORTH CHINA)

The Dongping goldfield is located within the Shuiquangou alkaline complex of the western Yanshan Mountains of Hebei Province, on the northern margin of the North China Craton. It is one of the largest gold deposits in China, with a planned gold production of 2.57 tons annually over a lifespan of 12 years. The Dongping gold deposit is enriched in the elements Au, Te, Ag, Pb, Bi, Sb and As. Most of the gold is present in the telluride minerals calaverite (43% Au, 38% Ag) and petzite (23% Au, 46% Ag). Gold mineralization is hosted mainly by K-feldspar-quartz stockworks, veins and disseminated sulfides. The deposit contains three ore types that are distinguished by their mineral associations: vein quartz gold, telluride gold, and disseminated gold. The paragenesis of the ores exhibits three distinct hydrothermal stages, of which the second one was the main ore-enrichment stage. The ore-related 40Ar/39Ar ages determined on K-feldspar samples indicate two episodes of gold mineralization, at 154.89 ± 0.70 Ma and 176.93 ± 4.66 Ma. The gold mineralization ages thus postdate the Devonian-age granite intrusion but overlap possible JurassicCretaceous magmatic activity. The homogenization temperatures of the majority of inclusions range from 120 to 240°C and from 240 to 400°C. The inclusions in quartz veins are CO2-rich and characterized by low salinity (average 6.0–8.8 wt% NaCl eq.). The laser Raman spectrum of the inclusions shows that the fluid compositions are dominantly waterrich but also contain CO2. The hydrogen isotope compositions (δ2H) of the fluid inclusions range from ‒100.3 to ‒66.5 ‰, and the calculated oxygen isotope compositions (δ18O) for source fluids range from ‒0.3 to +6.9 ‰ "Standard Mean Ocean Water" (SMOW). These values indicate that the ore-forming fluid came from a deep magmatic hydrothermal system, with involvement of meteoric water and possibly water affected by organic matter. The sulphur isotope compositions (δ34S) of pyrite are mainly from ‒0.3 to ‒13.6 ‰ Vienna Cañon Diablo Troilite (VCDT), suggesting homogeneity of sulphur in the magmatic source with subsequent fractionation under relatively oxidizing conditions in ore-bearing quartz veins. The relationship of δ2HH2O to 87Sr/86Sr indicates that the fluid inclusions and host granitoid rocks were influenced by mixing of magmatic and meteoric waters. Key words: Chongli county; Dongping deposit; 40Ar/39Ar dating; stable and radiogenic isotopes; ore-forming fluids; gold deposit; North Chin

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte-Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability.National Science Foundation (U.S.) (Grant EAR-1338810)National Science Foundation (U.S.) (Grant EAR-1455258

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte–Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability

Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction

Copyright © 2018 The Authors. Explaining the ∼5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (δ238U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our δ238U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO43- concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geobiology 15 (2017): 211–224, doi:10.1111/gbi.12222.Records of the Ediacaran carbon cycle (635 to 541 million years ago) include the Shuram excursion (SE), the largest negative carbonate-carbon isotope excursion in Earth history (down to -12 ‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13C of seawater dissolved inorganic carbon (DIC). Here, we present carbonate and organic carbon isotope (δ13Ccarb and δ13Corg) records from the Ediacaran Doushantuo Formation along a proximal-to-distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34S of carbonate-associated sulfate (CAS) and increased 87Sr/86Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near-surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13C presented in this study.We thank the National Key Basic Research Program of China (Grant 2013CB955704) and the State Key R&D project of China (Grant 2016YFA060104) as well as the NSF-ELT program and the NASA Astrobiology Institute (TWL) for funding

Enhanced Continental Weathering as a Trigger for the End‐Devonian Hangenberg Crisis

The Hangenberg Crisis coincided with a large decline of biodiversity and widespread anoxia in the end-Devonian ocean. Previous research attributed marine anoxia to the spread of deeply-rooted plants and/or increased volcanism on the continents, but crucial links have not been thoroughly explored. Herein, we propose enhanced weathering as a key trigger, as evidenced by a negative shift (∼8‰) in lithium isotopes and a coupled response in carbon isotopes of marine carbonates in South China. Our findings imply that rapid weathering of crustal rocks increased nutrient delivery to the ocean, as indicated by an increase in the carbonate-associated phosphate levels, contributing to oceanic eutrophication. In the absence of massive volcanic emissions and intense orogeny, the cause of enhanced continental weathering was likely the expansion of the terrestrial rhizosphere, highlighting the potential for land plant evolution to initiate weathering changes of sufficient severity to trigger a major bio/environmental crisis in the Earth system

Environmental influences on the stable carbon isotopic composition of Devonian and Early Carboniferous land plants

Systematic analysis of the stable carbon isotopic composition of fossil land plants (δ13Cp) has the potential to offer new insights regarding paleoclimate variation and plant-environment interactions in early terrestrial ecosystems. δ13Cp was measured for 190 fossil plant specimens belonging to 10 genera of Early to Late Devonian age (Archaeopteris, Drepanophycus, Haskinsia, Leclercqia, Pertica, Psilophyton, Rhacophyton, Sawdonia, Tetraxylopteris, and Wattieza) and 2 genera of Early Carboniferous age (Genselia and Rhodeopteridium) collected from sites located mainly in the Appalachian Basin (22–30°S paleolatitude). For the full carbon-isotopic dataset (n = 309), δ13Cp ranges from −20.3‰ to −30.5‰ with a mean of −25.5‰, similar to values for modern C3 land plants. In addition to a secular trend, δ13Cp exhibits both intra- and intergeneric variation. Intrageneric variation is expressed as a small (mean 0.45‰) 13C-enrichment of leaves and spines relative to stems that may reflect differential compound-specific compositions. Intergeneric variation is expressed as a much larger (to ~5‰) spread in the mean δ13Cp values of coeval plant genera that was probably controlled by taxon-specific habitat preferences and local environmental humidity. Among Early Devonian taxa, Sawdonia yielded the most 13C-depleted values (−27.1 ± 1.7‰), reflecting lower water-use efficiency that was probably related to growth in wetter habitats, and Leclercqia, Haskinsia, and Psilophyton yielded the most 13C-enriched values (−23.0 ± 1.6‰, −22.3 ± 1.3‰, and −24.8 ± 1.6‰, respectively), reflecting higher water-use efficiency probably related to growth in drier habitats

Environmental influences on the stable carbon isotopic composition of Devonian and Early Carboniferous land plants

Systematic analysis of the stable carbon isotopic composition of fossil land plants (δ13Cp) has the potential to offer new insights regarding paleoclimate variation and plant-environment interactions in early terrestrial ecosystems. δ13Cp was measured for 190 fossil plant specimens belonging to 10 genera of Early to Late Devonian age (Archaeopteris, Drepanophycus, Haskinsia, Leclercqia, Pertica, Psilophyton, Rhacophyton, Sawdonia, Tetraxylopteris, and Wattieza) and 2 genera of Early Carboniferous age (Genselia and Rhodeopteridium) collected from sites located mainly in the Appalachian Basin (22–30°S paleolatitude). For the full carbon-isotopic dataset (n=309), δ13Cp ranges from −20.3‰ to −30.5‰ with a mean of −25.5‰, similar to values for modern C3 land plants. In addition to a secular trend, δ13Cp exhibits both intra- and intergeneric variation. Intrageneric variation is expressed as a small (mean 0.45‰) 13C-enrichment of leaves and spines relative to stems that may reflect differential compound-specific compositions. Intergeneric variation is expressed as a much larger (to ~5‰) spread in the mean δ13Cp values of coeval plant genera that was probably controlled by taxon-specific habitat preferences and local environmental humidity. Among Early Devonian taxa, Sawdonia yielded the most 13C-depleted values (−27.1 ± 1.7‰), reflecting lower water-use efficiency that was probably related to growth in wetter habitats, and Leclercqia, Haskinsia, and Psilophyton yielded the most 13C-enriched values (−23.0 ± 1.6‰, −22.3 ± 1.3‰, and −24.8 ± 1.6‰, respectively), reflecting higher water-use efficiency probably related to growth in drier habitats

Global warming and mass extinctions associated with large igneous province volcanism

The coincidence of large igneous province (LIP) eruptions with at least three, if not all of the “Big Five” biotic crises of the Phanerozoic implies that volcanism is a key driver of mass extinctions. Many LIP-induced extinction scenarios invoke global warming, caused primarily (but not exclusively) by greenhouse gases emitted at the site of LIP emplacement and by contact metamorphism of carbon-rich host rocks. Here we explore a) the climate-changing products of volcanism including sulfur dioxide (SO2), carbon dioxide (CO2) and methane (CH4) from eruptions, contact metamorphism, and melting (dissociation) of gas hydrates; b) their deadly effects, including marine anoxia and thermal stress; c) increasingly sophisticated paleotemperature proxies (e.g. δ18O of shell material) through case studies of the best-known LIP-warming-extinction nexi; and d) global warming through the lens of the putative “Anthropocene” extinction