Sun, RuiyangGrasby, Stephen E.Shen, JunYin, Runsheng...
6页
查看更多>>摘要：Late Sturtian deglaciation, resulting in development of oxic conditions and deposition of banded iron formations (BIFs), was a pivotal period in evolution of global ocean chemistry. The driving forces behind these Neo-proterozoic BIFs and climatic/oceanic changes remain controversial, however. Here we use mercury (Hg) to elucidate the cause(s) of these dramatic changes to earth biogeochemical cycles. In the Guandong section, South China, we observe anomalously low Hg concentrations and Hg to total organic carbon (Hg/TOC) ratios in the Late Sturtian BIFs, suggesting a period of limited continental volcanism during the Late Sturtian deglaciation. We hypothesize that instead of a major LIP event, long-lasting and subdued submarine volcanism slowly increased atmospheric CO2 to trigger the greenhouse climate that caused deglaciation. Negative shifts of Delta Hg-199 in the studied section suggest that deglaciation was accompanied by atmospheric mercury depletion events (AMDEs). Extremely low TOC levels in the studied section may suggest low oceanic productivity, and this may support a non-biological cause of oceanic oxygenation during the Late Sturtian glaciation, through the release of photo-chemically produced H2O2 in melting glaciers.
Meyers, Zachary P.Rademacher, Laura K.Frisbee, Marty D.Warix, Sara R....
24页
查看更多>>摘要：The seminal studies of Feth et al. (1964) and Garrels and Mackenzie (1967) describe the chemical weathering processes controlling the geochemistry of spring waters in the Sierra Nevada (CA) and provide a framework for understanding geochemical weathering processes at local groundwater scales (i.e., short distances between recharge and subsequent groundwater discharge). Here, we extend these concepts to investigate the factors controlling geochemical evolution at the intermediate, mountain-block scale (i.e., increased flowpath length, circulation depth, and rock-water interaction). We accomplish this by applying a multi-tracer approach to mountain-front springs emerging along the Sierra Nevada frontal fault zone in Owens Valley, CA. These springs emerge at a significantly lower elevation (1100-2000 mamsl) than the eastern Sierra crest (4000+ mamsl) and provide a window into the hydrogeological and hydrochemical processes occurring within the mountain block from high-elevation mountain-block recharge to low-elevation mountain-front discharge, a recognized knowledge gap in hydrogeology. We delineate approximate spring contributing areas and identify geologic units likely sourcing springflow using stable isotopes of water and dissolved noble gases. We then classify four major geochemical groups after identifying the likely geologic units sourcing springflow and subsequent analysis and modeling of spring geochemistry. Our results lead to three main conclusions: 1) geochemical evolution within the mountain block from high elevation mountain block weathering to mountain front discharge follows power-law weathering relationships and becomes increasingly dependent on the dissolution of disseminated calcite present in plutonic rocks with increasing flowpath length, 2) geologic heterogeneity (i.e., differences in plutonic compositions and the presence/absence of Paleozoic metasedimentary roof pendants) exerts a dominant control on geochemical evolution with increased flowpath length, and 3) within geochemical groups, simple metrics like TDS or mole transfers from inverse geochemical models scale with physical and isotopic indicators of groundwater circulation and flowpath length.
查看更多>>摘要：New Zealand kauri trees are an important dendrochonologic resource in the southern hemisphere due to their long lifespans and the correlation of ring widths to El Nino - Southern Oscillation (ENSO)-events. This study represents the first chronology-constrained dendrochemical investigation of New Zealand kauri. Strontium isotope (Sr-87/Sr-86) and trace element analyses are combined to determine the potential for provenance (spatial) and environmental (temporal) dendrochemical investigations utilizing the geochemistry of kauri wood. Soil Sr isotopic measurements match 80% of the corresponding analyses from local trees and two of the four study sites have kauri isotopic compositions tightly clustered and distinctive, supportive of provenance investigations. The two sites which record inter-tree isotopic variability, a combination of variables, including geologic, atmospheric (dust and sea spray), and anthropogenic (land-use), all likely influence the isotopic and trace element variations. Discrete annual events or fluctuating conditions are difficult to observe from the isotopic tree ring record, reflecting the potential mobility of Sr within tree rings. However, the preservation of long term geochemical trends suggest processes such as climate change or anthropogenic soil modification may still be preserved in the kauri record.
Wilde, Simon A.Sun, Ming-DaoXu, Yi-GangGazel, Esteban...
14页
查看更多>>摘要：Primitive magmas with distinct geochemical signatures that record mantle heterogeneity are obscured in continental intraplate volcanic settings owing to crustal interaction. Here, we report new geochronological and geochemical data for two adjacent Late Neogene primitive intraplate volcanoes in Northeast China: the Great and Lesser Wangqing River twin volcanoes (GWR/LWR). Our results suggest that the GWR and LWR volcanics are controlled by olivine crystal sorting and that crustal contamination is negligible, which implies these twin volcanoes provide a rare window into mantle components. The LWR tephrite is marked by moderate enrichment of light rare earth elements (REEs) with significant positive Sr anomalies. It is also enriched in radiogenic Sr-Nd-Hf-Pb isotopes but exhibits Os-187/Os-188 values that are comparable to those of the primitive upper mantle. The GWR volcanics (picrite, basanite and tephrite) are characterised by a steeper REE pattern and a lack of Sr anomalies, while being isotopically more radiogenic. Combined with olivine chemistry, which reveals a hybrid source lithology, we propose that the LWR and GWR sampled recycled lower and upper oceanic crust components, respectively. Both components have high Sr-87/Sr-86 and low Pb-206/Pb-204 ratios, akin to an enriched mantle I (EM1) source, indicating that the EM1 components can be formed by recycling oceanic crust with limited sediment.
查看更多>>摘要：The black shale in the Lower Cambrian Niutitang Formation in Southwest China is anomalously enriched in sedimentary exhalative Ni-and Mo-sulfides. The occurrence of large fluctuation of ocean redox conditions and thriving of benthic animals highlight the uniqueness of the stratum and its link to the Cambrian Explosion. Here, we report our observation of the high-resolution structures of the Ni-and Mo-extremely enriched sulfidic spheroids and measurements of their Ni-and Mo-isotopic compositions. Mo-sulfides appear as clusters of oval spheroids disseminated with pyrite framboids inside; differently, Ni-sulfides fill fractures or appear as coronas of the Mo-sulfidic spheroids. These observations indicate that the Mo-sulfide and the Ni-sulfide are mineralized by different processes. The Ni isotopic compositions of the spheroids and the groundmass fall on the mixing line between seawater (low [Ni]/[Mo] and heavy Ni isotopes) and hydrothermal fluids (high [Ni]/[Mo] and light Ni isotopes), confirming the hydrothermal-fluid-addition of Ni to the deposit. In contrast, the Mo isotopic compositions of the spheroids and the groundmass consistently exhibit their seawater-source. Along with our analyses of biomarkers and X-ray absorption fine structure of Mo, we deduce that Mo in the seawater was reduced and deposited as sulfidic spheroids, while Ni had an additional source from hydrothermal fluids that enhanced the biogeochemical cycles during their depositions.
查看更多>>摘要：Schwertmannite (Sch) possessing a typical "hedgehog" morphology is ubiquitously found in acid mine drainage (AMD) through biological oxidation of Fe2+ by Acidithiobacillus ferrooxidans (Acidithiobacillus ferrooxidans). However, little information is available on the effect of A. ferrooxidans on the morphology evolution of Sch during its formation. In this study, it was found that Fe2+ and A. ferrooxidans (with the presence of Fe2+) could promote the "hedgehog" structure formation of Sch in acid aerobic environment, respectively. Aerobic systems led to the increase in needle-like goethite on the surface of ellipsoidal aggregates with comparison to anaerobic systems. Hydroxyl radical (center dot OH) produced only in the aerobic systems facilitated the formation of Fe3+. Meanwhile, A. ferrooxidans, containing mainly tryptophan, could significantly change the interfacial potential and favor the electron transfer from Fe2+ to Sch. Moreover, it also could coordinate with Fe (Fe2+, Fe3+). Thus, a possible mechanism for developing a typical "hedgehog" morphology of biogenic Sch was that Fe3+ from center dot OH oxidization of Fe2+ and/or released from Sch would chelate with tryptophan to form Fe3+-tryptophan complex, and further induce the formation of needle-like goethite. However, during the crystal growth of goethite, tryptophan would be released to solution. These findings have important implications for comprehensively understanding the role of microorganism on mineral formation in AMD environment.
NSTL
Elsevier