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991.
Noble gas tracing of groundwater/coalbed methane interaction in the San Juan Basin, USA 总被引:1,自引:0,他引:1
Zheng Zhou Chris J. Ballentine Martin Schoell 《Geochimica et cosmochimica acta》2005,69(23):5413-5428
The San Juan Basin natural gas field, located in northwestern New Mexico and southwestern Colorado in the USA, is a case-type coalbed methane system. Groundwater is thought to play a key role in both biogenic methane generation and the CO2 sequestration potential of coalbed systems. We show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas. We collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control. Stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin, and the underpressured low producing region as having a significant admix of thermogenic coal gas. 3He/4He ratios increase from 0.0836Ra at the basin margin to 0.318Ra towards the center, indicating a clear but small mantle He signature in all gases. Coherent fractionation of water-derived 20Ne/36Ar and crustal 4He/40Ar* are explained by a simple Rayleigh fractionation model of open system groundwater degassing. Low 20Ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater-associated gas by desorbed coalbed methane. This Rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well. The quantified water volumes in both underpressured and overpressured zones range from 1.7 × 103 m3 to 4.2 × 105 m3, with no clear distinction between over- and underpressured production zones. These results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds. There is no requirement of continuous groundwater flow for renewing the microbes or nutrient components. We furthermore observe strong mass related isotopic fractionation of 20Ne/22Ne and 38Ar/36Ar isotopic ratios. This can be explained by a noble gas concentration gradient in the groundwater during gas production, which causes diffusive partial re-equilibration of the noble gas isotopes. It is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air-derived noble gases can occur when such concentration gradients are established during gas production. Excess air-derived Xe and Kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if Xe and Kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form CH4. 相似文献
992.
Rapid development of shrimp farming may lead to unrecognized and undesirable changes of land cover/land use patterns in coastal areas. Of special concern is the loss of mangrove forest in coastal areas such as Quang Ninh, Vietnam, which is adjacent to the World Heritage-listed Ha Long Bay. Understanding the status and changes of land cover/land use for coastal shrimp farms and mangrove forests can support environmental protection and decision-making for sustainable development in coastal areas. Within this context, this paper uses the 1999/2001 Landsat ETM+ and the 2008 ALOS AVNIR-2 imagery to investigate the contraction and expansion of shrimp farms and mangrove forests in coastal areas of Ha Long and Mong Cai, which now have a high concentration of intensive and semi-intensive shrimp farms. Images were separately analyzed and classified before using post-classification comparisons to detect land cover/land use changes in the study area. The results of this study found that the area of mangrove forest has been reduced by an estimated 927.5 ha in Ha Long and 1,144.4 ha in Mong Cai, while shrimp farming areas increased by an estimated 1,195.9 and 1,702.5 ha, respectively, over the same period. The majority of shrimp farms in Mong Cai were established at the expense of mangrove forest (49.4 %) while shrimp farms in Ha Long were mainly constructed on areas previously occupied by bare ground (46.5 %) and a significant proportion also replaced mangroves (23.9 %). The remarkable rate of mangrove loss and shrimp farming expansion detected in this study, over a relatively short time scale indicate that greater awareness of environmental impacts of shrimp farm expansion is required if this industry is to be sustainable, the important estuarine and coastal marine ecosystems are to be protected over the long term, and the capturing and storing of carbon in mangrove systems are to be enhanced for global climate change mitigation and for use as carbon offsets. 相似文献
993.
Shucheng Xie Chris J Nott Darrel Maddy Richard P Evershed 《Geochimica et cosmochimica acta》2004,68(13):2849-2862
A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ13C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. 210Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ13C values of individual hydrocarbons, especially the C23n-alkane, a major compound in certain Sphagnum spp. The δD values of the C29 and C33n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C23n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ13C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced. 相似文献
994.
Chris L. Hayward 《Geoarchaeology》2004,19(5):383-405
The pre‐8th century B.C. palaeotopography of the center of Corinth, southern Greece, was reconstructed using a combination of geological and geomorphological observation and archaeological evidence. Preserved areas of calcrete and overlying calcareous soil, which represent surviving remnants lying close to the pre‐8th century B.C. land surface, were mapped. The geology of the site was also mapped. The topography of Corinth in pre‐8th century B.C. times was radically different from that observed today. Most topographic modification occurred as a result of quarrying from a line of Pleistocene oolitic sand dunes for construction‐stone during antiquity. The reconstruction presented differs in key respects from previously published palaeotopographies, which did not include geological evidence. The results of the new reconstruction illustrate the importance of combining geological, geomorphological, and archaeological evidence and have implications for the urban development of Corinth in post‐8th century B.C. times. © 2004 Wiley Periodicals, Inc. 相似文献
995.
Most studies using GRACE (Gravity Recovery and Climate Experiment) data for examining water storage anomalies have rich hydrogeological databases. Here, GRACE data are analyzed for southern Mali, Africa, a region with sparse hydrogeological data. GRACE data (2002?C2008) did not overlap with observed groundwater-level data (1982?C2002). Terrestrial water storage from GRACE was corrected for soil moisture using the Global Land Data Assimilation System (GLDAS) model to obtain monthly groundwater storage anomalies and annual net recharge. Historical storage anomalies and net recharge were determined using the water-table fluctuation method for available observation wells. Average annual net recharge averaged 149.1?mm (or 16.4% of annual rainfall) and 149.7?mm (14.8%) from historical water level and GRACE data, respectively. Monthly storage anomaly lows and peaks were observed in May and September, respectively, but have a shift in peak to November using the corrected GRACE data, suggesting that the GLDAS model may poorly predict the timing of soil-water storage in this region. Notwithstanding problems with the GLDAS model, the soil moisture-corrected GRACE data accurately predict the relative timing and magnitude of groundwater-storage changes, suggesting that GRACE data are valuable for identifying long-term regional changes in groundwater storage in areas with sparse hydrogeological data. 相似文献
996.
997.
Michael Wiedenbeck Robert B. Trumbull Martin Rosner Adrian Boyce John H. Fournelle Ian A. Franchi Ralf Halama Chris Harris Jack H. Lacey Horst Marschall Anette Meixner Andreas Pack Philip A.E. Pogge von Strandmann Michael J. Spicuzza John W. Valley Franziska D.H. Wilke 《Geostandards and Geoanalytical Research》2021,45(1):97-119
Three tourmaline reference materials sourced from the Harvard Mineralogical and Geological Museum (schorl 112566, dravite 108796 and elbaite 98144), which are already widely used for the calibration of in situ boron isotope measurements, are characterised here for their oxygen and lithium isotope compositions. Homogeneity tests by secondary ion mass spectrometry (SIMS) showed that at sub‐nanogram test portion masses, their 18O/16O and 7Li/6Li isotope ratios are constant within ± 0.27‰ and ± 2.2‰ (1s), respectively. The lithium mass fractions of the three materials vary over three orders of magnitude. SIMS homogeneity tests showed variations in 7Li/28Si between 8% and 14% (1s), which provides a measure of the heterogeneity of the Li contents in these three materials. Here, we provide recommended values for δ18O, Δ’17O and δ7Li for the three Harvard tourmaline reference materials based on results from bulk mineral analyses from multiple, independent laboratories using laser‐ and stepwise fluorination gas mass spectrometry (for O), and solution multi‐collector inductively coupled plasma‐mass spectroscopy (for Li). These bulk data also allow us to assess the degree of inter‐laboratory bias that might be present in such data sets. This work also re‐evaluates the major element chemical composition of the materials by electron probe microanalysis and investigates these presence of a chemical matrix effect on SIMS instrumental mass fractionation with regard to δ18O determinations, which was found to be < 1.6‰ between these three materials. The final table presented here provides a summary of the isotope ratio values that we have determined for these three materials. Depending on their starting mass, either 128 or 512 splits have been produced of each material, assuring their availability for many years into the future. 相似文献
998.
Tom Bradwell David Small Derek Fabel Chris D. Clark Richard C. Chiverrell Margot H. Saher Dayton Dove S. Louise Callard Matthew J. Burke Steven G. Moreton Alicia Medialdea Mark D. Bateman David H. Roberts Nicholas R. Golledge Andrew Finlayson Sally Morgan Colm Ó Cofaigh 《第四纪科学杂志》2021,36(5):681-722
The offshore sector around Shetland remains one of the least well-studied parts of the former British–Irish Ice Sheet with several long-standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice-Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice-sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi-proxy suite of new isotopic age assessments, including 32 cosmogenic-nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector-wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP , this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km2, accompanied by several ice-margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re-growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km2 to ca. 15 000 km2 between 19 and 18 ka BP , stabilizing at ca. 2000 km2 by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP , and may have involved one or more subsidiary ice centres on now-submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine-influenced Shetland Ice Cap. 相似文献
999.
Jimena Samper-Villarreal Peter J. Mumby Megan I. Saunders Chris Roelfsema Catherine E. Lovelock 《Estuaries and Coasts》2018,41(6):1732-1743
Blue carbon initiatives require accurate monitoring of carbon stocks. We examined sources of variability in seagrass organic carbon (Corg) stocks, contrasting spatial with short temporal scales. Seagrass morphology and sediment Corg stocks were measured from biomass and shallow sediment cores collected in Moreton Bay, Australia. Samples were collected between 2012 and 2013, from a total of 77 sites that spanned a gradient of water turbidity. Environmental measures of water quality between 2000 and 2013 revealed strong seasonal fluctuations from summer to winter, yet seagrass biomass exhibited no temporal variation. There was no temporal variability in Corg stocks, other than below ground biomass stocks were slightly higher in June 2013. Seagrass locations were grouped into riverine, coastal, and seagrass loss locations and short temporal variability of Corg stocks was analysed within these categories to provide clearer insights into temporal patterns. Above ground Corg stocks were similar between coastal and riverine meadows. Below ground Corg stocks were highest in coastal meadows, followed by riverine meadows. Sediment Corg stocks within riverine meadows were much higher than at coastal meadows and areas of seagrass loss, with no difference in sediment Corg stocks between these last two categories. Riverine seagrass meadows, of higher turbidity, had greater total Corg stocks than meadows in offshore areas irrespective of time. We suggest that Corg stock assessment should prioritise sampling over spatial gradients, but repeated monitoring over short time scales is less likely to be warranted if environmental conditions remain stable. 相似文献
1000.
Tyrone?O.?RooneyEmail author William?K.?Hart Chris?M.?Hall Dereje?Ayalew Mark?S.?Ghiorso Paulo?Hidalgo Gezahegn?Yirgu 《Contributions to Mineralogy and Petrology》2012,164(3):407-426
The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit. 相似文献