Zan, BowenMou, ChuanlongLash, Gary G.Yan, Jiaxin...
15页
查看更多>>摘要：The sulfate-methane transition zone (SMTZ) serves as an important geochemical transition especially during early diagenesis. Organoclastic sulfate reduction (OSR), sulfate-driven anaerobic oxidation of methane (SD-AOM), fermentation, and methanogenesis, the dominant diagenetic processes near the SMTZ, play key roles in marine sulfur and carbon cycling. However, the paleo-SMTZ has rarely been identified in the geologic record because of equivocal evidence. The presence of barite-calcite-pyrite assemblages (abundant nodules and pyritic laminae) concentrated over a 1-m thick interval of the Silurian Longmaxi Formation, Yangtze Block, South China, provide a unique opportunity to investigate possible vertical movements of the paleo-SMTZ in ancient sediments. The inferred paleo-SMTZ comprises nodules made up of barite (BaSO4), calcium carbonate (CaCO3), and pyrite (FeS2), and pyritic laminae. Strong S-34 enrichment of barite (50 to 62.1 parts per thousand VCDT) and modestly C-13-depleted calcite (-11.4 parts per thousand to -5.3 VPDB) of the nodules associated with positive delta S-34 values of pyritic laminae (4 to 6.2 parts per thousand VCDT) suggest precipitation of these minerals sourced by mixed carbon and sulfur sources close to and/or within the paleo-SMTZ. Nodules from center to edge display mineral zonation that may reflect the effects of depth fluctuations of the paleo-SMTZ. Initial (stage 1) barite mineralization in nodule centers appears to have occurred within the base of the sulfate reduction zone (SRZ). By the time if stage 2 mineralization, the stratigraphic locus of precipitation had migrated closer to highly alkaline SMTZ resulting in barite dissolution and calcium carbonate precipitation. Higher proportions of calcium carbonate exhibiting some enrichment of C-12 associated with cubic pyrite crystals in nodule centers are consistent with the occurrence of SD-AOM within the SMTZ. Stage 3 acicular barite mineralization along nodule edges likely took place near at the base of SRZ immediately above the SMTZ, whereas the replacement of barite by calcite during stage 4 mineralization occurred again within the SMTZ. Vertical movement of paleo-SMTZs suggested by the occurrences of barite-calcite-pyrite assemblages in the Longmaxi Formation across Yangtze Block, are likely the consequence of the complex interplay of varying sedimentation rate, organic matter quality, methane flux, and perhaps seawater sulfate concentration.
查看更多>>摘要：Glycerol dialkyl glycerol tetraethers (GDGTs) have received considerable attention because they are useful indicators for paleoenvironment, organic matter source, and microbial community. In this study, a new simple method was developed for the analysis of GDGTs by using direct large-volume injection (LVI)-liquid chromatography (LC)-atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) to reduce the usage of sediment or seawater suspended particulate matter (SPM) samples. The parameters of accelerated solvent extraction and LVI of target GDGTs were optimized. Under optimal conditions, the LVI approach (500 mu L injection) coupled with LC-APCI-MS could determine various GDGTs even with 0.02-0.10 g of sediment samples or SPM samples collected from 0.2-1.0 L of marginal sea water or 10-20 L of open ocean seawater with satisfactory method precision (relative standard deviations <= 10.2%) and linearity (R-2 >= 0.991). Compared with the classical method, the LVI-LC-APCI-MS required significantly reduced amount of sediment or SPM sample. Finally, the proposed method was applied to determine GDGTs in real sediment samples collected from hydrothermal vent sites at the Southwest Indian Ridges. Four groups of GDGTs, namely, isoprenoid GDGTs (isoGDGTs), branched GDGTs, H-GDGTs and OH-GDGTs, and archaeol, glycerol dialkanol diethers (GDDs) and glycerol monoalkanediol diethers (GMDs) were detected with only 0.02 g of sediment sample collected from Longqi and Tianzuo hydrothermal fields. The organic matter in the sediment sample from Tianzuo was significantly affected by hydrothermal activity. In addition, isoGDGTs in seawater SPM from the open area of Eastern Indian Ocean were investigated for the first time. Six common isoGDGTs were all detected in surface and subsurface SPM samples (filtering 10 L of seawater), which indicated that the LVI approach could noticeably reduce the workload of collecting seawater SPM on board and could be used to investigate conveniently the distribution of GDGTs in water column especially in open oceans and deep seas.
查看更多>>摘要：Abiotic H-2 produced in the Precambrian lithospheric crust is a key substrate at the base of the metabolic chain of chemosynthetic and photosynthesis-independent microbial communities, significant to our understanding of life on early Earth and other planets. H-2 cycling processes are also relevant to recent hydrogen exploration efforts and engineered subsurface environments such as radioactive waste disposal sites. In the lithospheric crust, H-2 is produced through water-rock reactions (serpentinisation) and radiolysis; the latter directly linked to He through radioelement decay (U, Th). The Witwatersrand Basin in South Africa is an ideal place to study the radiolytic production pathway in particular, because of the low abundance of ultramafic and mafic minerals and therefore low potential for serpentinisation reactions. Gas samples and gas flow rate data (n = 12) were collected from the surface of exploration boreholes tapping the Witwatersrand and Ventersdorp Supergroups. The samples were predominantly composed of CH4 (65-99%), N-2 (3-27%), He (0.1-15%), and trace amounts of C2+ hydrocarbons. Notably, H-2 in these samples was below detection limit, despite the presence of He - providing a critical indicator of processes removing H-2 from the system. Using a Bayesian modelling approach, we test the hypothesis that the observed fluids are generated in-situ, driven by radioelement decay and subsequent microbial methanogenesis, and controlled by porosity of the host rock. The observed data is consistent with this hypothesis, and can be accounted for by a variation in porosity between 0.3 and 2.2% (typical values to Precambrian basement) across the different sampling sites. These He-rich hydrocarbon gases observed at the surface originate from a hydrogeological system that is porosity-constrained and isolated from externally-sourced fluids. Radioelement decay is the primary process driving the generation of H-2 and therefore energy production in this subsurface system, utilised by hydrogenotrophic methanogens at the base of the deep carbon cycle. Microbial utilisation is the key mechanism for H-2 consumptions and, conversely, preservation, suggesting that conditions favourable to commercial H-2 discoveries are likely constrained to hypersaline environments where microbial activity is inhibited. The model results under the proposed hypothesis (consistent N-2/H-2 ratio between different boreholes) raises the possibility that N-2, which often co-occurs with He-rich deep fluids, is also produced through radiolysis, and future work is needed to fully evaluate this hypothesis.
查看更多>>摘要：Recent studies suggest that pelagic sediments can enrich rare-earth elements (REE) acting as a significant reservoir for the global REE budget as well as a potential resource for future exploitation. Although Ca-phosphate (e.g., bioapatite fossils) and Fe-Mn (oxyhydr)oxides (e.g., micronodule) have been considered important REE carriers in deep-sea sediments, the proportion of REE held by each mineral phase remains enigmatic. Here, we have investigated the sediments from two promising REE-rich prospective areas: the Tiki Basin in the Southeast Pacific (TKB) and the Central Indian Ocean Basin (CIOB). The mineral grains including bioapatite fossils and FeMn micronodules have been inspected individually by in-situ microscale analytical methods. Correspondently, the REE bound to Ca-phosphate and Fe-Mn (oxyhydr)oxides have been sequentially extracted and quantified. The crucial role of Ca-phosphate is substantiated by sequential leaching which reveals its dominance in hosting similar to 69.3-89.4% of total REE. The Fe-Mn (oxyhydr)oxides carry similar to 8.2% to 22.0% of REE in bulk sediments, but they account for similar to 70.0-80.5% of Ce owing to their preferential adsorption of Ce over the other REE. Surface sediment on modern seafloor can accumulate high REE contents resulting from the REE scavenging by the host phases within the range of sediment-seawater interface. Differences between TKB and CIOB samples indicate that the REE enrichment in the deep-sea environment may be controlled by multiple factors including the productivity of overlying seawater (e.g., phosphorus flux), water depth relative to carbonate compensation depth (CCD), sedimentation rate, redox condition, and hydrothermal vent input (e.g., Fe-Mn precipitations).
查看更多>>摘要：Fission-track annealing models aim to extrapolate laboratory annealing kinetics to the geological timescale for application to geological studies. Model trends empirically capture the mechanisms of track length reduction. To facilitate the interpretation of the fission-track annealing trends, a formalism, based on quantities already in use for the study of physicochemical processes, is developed and allows for the calculation of rate constants, Arrhenius activation energies, and transitivity functions for the fission-track annealing models. These quantities are then obtained for the parallel Arrhenius, parallel curvilinear, fanning Arrhenius, and fanning curvilinear models, and fitted with Durango apatite data. Parallel models showed to be consistent with a single activation energy mechanism and a reaction-order model of order approximate to - 4. However, the fanning curvilinear model is the one that results in better fits laboratory data and predictions in better agreement with geological evidence. Fanning models seem to describe a more complex picture, with concurrent recombination mechanisms presenting activation energies varying with time and temperature, and the reaction-order model seems not to be the most appropriate. It is apparent from the transitivity analysis that the dominant mechanisms described by the fanning models are classical (not quantum) energy barrier transitions.
Ernst, David M.Schier, KatharinaGarbe-Schoenberg, DieterBau, Michael...
12页
查看更多>>摘要：Germanium (Ge) and silicon (Si) are an element pair which shows rather coherent behaviour in many geochemical systems. In aqueous systems and in the critical zone, however, Ge and Si may fractionate from each other, leading to slight differences between the Ge/Si ratios of continental crust, river water and seawater. We here show that even stronger fractionation of the Ge-Si couple occurs within seawater during the formation of hydrogenetic ferromanganese (Fe-Mn) crusts and nodules. We determined Ge/Si ratios of hydrogenetic Fe-Mn crusts from the Pacific Ocean following a new analytical approach using laser-ablation ICP-MS on nanoparticulate tablets prepared from the Fe-Mn crust material. Data quality was monitored and demonstrated by repetitive measurements of Fe-Mn nodule reference material (RM) NOD-P1 from the Pacific Ocean (Ge/Si-wt: 16.8 x 10(-6) g/g; Ge/Si-mol: 6.47 x 10(-6) mol/mol), which is the only RM for which high-quality Ge-Si reference data are available. We also report Ge/Si ratios for Fe-Mn nodule RMs NOD-Al from the Atlantic Ocean (Ge/Si-wt: 24.9 x 10(-6); Ge/Si-mol: 9.6 x 10(-6)) and for JMn-1 from the Pacific Ocean (Ge/Si-wt: 17.9 x 10(-6); Ge/Si-mol: 6.9 x 10(-6)). The Ge/Si-wt ratios of the hydrogenetic Fe-Mn crusts from the Pacific Ocean range from 13.2 x 10(-6) to 33.5 x 10(-6) (Ge/Si-mol: 5.1 x 10(-6) to 13 x 10(-6)). The Ge/Si ratios of both Fe-Mn crusts and nodules are significantly higher than those of ambient seawater (ca. Ge/Si-wt : 1.7 x 10(-6); Ge/Si-mol: 0.7 x 10(-6)). Their Ge/Si ratios are also well-above those of potential (alumino)silicate detritus. Hence, these high Ge/Si ratios reveal fractionation of Ge and Si during scavenging from seawater by the iron (oxy)hydroxide component in Fe-Mn crusts, and do not result from the presence of detrital material in the chemical precipitate. Their high Ge/Si ratios relative to seawater also suggest that despite higher complexation constants with organic ligands for Ge than Si, dissolved organic ligands do not control marine Ge-Si fractionation via preferential retention of Ge in seawater.
查看更多>>摘要：Oil seepage is widespread in the Gulf of Mexico, which also applies to the Bay of Campeche in the southern Gulf. The ascension of crude oil and oil-derived hydrocarbons poses a challenge to most seep-dwelling organisms. On the other hand, short- and long-chain hydrocarbons have been shown to be potentially degraded by a range of heterotrophic sulfate-reducing bacteria. Here we present lipid biomarker and carbon isotope data from authigenic carbonates from the Campeche Knolls in the southern Gulf of Mexico and use methane-seep carbonates from the Makran convergent margin (southern Pakistan and Iran) for comparison. The Campeche Knolls and Makran were selected for such comparison as endmember types for oil versus methane seepage in an attempt to identify molecular signatures of oil seepage. The Campeche carbonates yielded on average higher delta C-13 values (-31.3 parts per thousand to -21.9 parts per thousand) than the Makran methane-seep carbonates (-54.3 parts per thousand to -39.8 parts per thousand) and revealed a particularly high content of ether lipids. The Campeche carbonates revealed a greater variety of alkyl chains in nonisoprenoidal dialkyl glycerol diether lipids (DAGE) and a higher relative proportion of long alkyl chains (DAGEs>C-36) compared with the Makran methane-seep carbonates. Bacterial biomarkers showed an on average distinctively heavier delta C-13 signature than at the studied methane seeps, indicating bacterial oxidation of oil-derived hydrocarbons. Mixing calculations indicate that up to 73% of the local sulfate reduction is coupled to the oxidation of non-methane hydrocarbons at the oil seeps, contributing as much as 40% to carbonate precipitation. These data imply that oil-seep carbonates can be discriminated from methane-seep carbonates by their respective biomarker inventories in modern and, in cases of moderate to good biomarker preservation, ancient environments, setting the stage for the reconstruction of the influence of fluid composition on the compositions of chemosynthesis-based communities at seeps.
Hiett, Coleman D.Newell, Dennis L.Jessup, Micah J.Grambling, Tyler A....
14页
查看更多>>摘要：Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO2 and N-2) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (He-3/He-4) and gas abundance data. delta O-18 and delta H-2 values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. delta N-18 values of N-2 gas range from -0.5 to +4.4 parts per thousand (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. delta C-13 values of dissolved inorganic carbon (DIC) and CO2 gas from bubbling springs are used to model initial delta C-13 values before degassing, with resulting values ranging from -13.6 to -0.3 parts per thousand (vs VPDB) and an average value of -6.9 parts per thousand. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO2 sources with endmember delta C-13 values of -7 parts per thousand and -14 parts per thousand which, in light of relationships with He-3/He-4 ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO2 is migrating with mantle helium. Metamorphic CO2 and N-2 are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO2 flux of similar to 3 x 10(8) mol yr(-1) (34 t d(-1)) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO2 emissions from Peruvian thermal springs may approach 2 x 10(9) mol yr(-1) (203 t d(-1)), or similar to 0.2% of global emissions from subaerial volcanism. Regional CO2 emissions may be 10 to 100 times greater when considering CO2 diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism.
NSTL
Elsevier