Labile barite contents and dissolved barium concentrations on Blake Ridge: New perspectives on barium cycling above gas hydrate systems   总被引：1，自引：0，他引：1  

G.T. Snyder  G.R. Dickens  D.G. Castellini 《Journal of Geochemical Exploration》2007,95(1-3):48

Blake Ridge hosts an extensive gas hydrate system where escaping CH₄ is consumed through anaerobic oxidation of methane (AOM) at a sulfate–methane transition (SMT) in shallow sediment. Previous geochemical work on ridge crest sediment has documented Ba fronts above the SMT, and has suggested that these horizons can be used to constrain the evolution of the SMT and AOM over time. We expand on this concept and further test it by determining the labile Ba contents of sediment and the dissolved Ba²⁺ concentrations of pore waters at four ODP sites on Blake Ridge (on the crest at Sites 994, 995 and 997, and on the southern flank at Site 1059). Labile Ba contents are fairly low at all four sites (0.44 and 1.32 mmol/kg), except within 3 m above the SMT at Sites 994, 995 and 997, where they typically exceed 1.24 mmol/kg and can reach 11.3 mmol/kg. These Ba fronts have a diagenetic origin, and SEM analyses show them to be composed of microcrystalline barite. Site 1059 lacks a prominent Ba front. The lowest labile Ba contents generally underlie the Ba fronts and correlate to the base of the SMT. Dissolved Ba²⁺ concentrations are low (< 1–4 μM) from the seafloor to within 2 m above the main Ba front. Below this depth, they rapidly increase at Sites 994, 995, and 1059, reaching peak concentrations (to 57 μM) at the base of the SMT. By contrast, a rapid rise in dissolved Ba²⁺ is not observed at Site 997. Dissolved Ba²⁺ concentrations are only moderately high (10–25 μM) below the SMT at all four sites. Collectively, this information supports a diagenetic model where barite passing into the SMT dissolves, and some of the dissolved Ba²⁺ then migrates up to form an authigenic barite peak. The contrasting signatures at the different sites indicate non-steady-state differences in the overall process. The size of the peaks on the crest of Blake Ridge necessitates that the recycling of Ba across the SMT has been operating at the current sub-bottom depths for > 100 kyr. Thus, CH₄ escaping through the AOM has likely been fairly constant over this time. It is possible that the SMT is currently rising toward the seafloor at Site 1059.  相似文献   

Effects of uncertainty in rock-physics models on reservoir parameter estimation using seismic amplitude variation with angle and controlled-source electromagnetics data     

Jinsong Chen  Thomas A. Dickens 《Geophysical Prospecting》2009,57(1):61-74

This paper investigates the effects of uncertainty in rock-physics models on reservoir parameter estimation using seismic amplitude variation with angle and controlled-source electromagnetics data. The reservoir parameters are related to electrical resistivity by the Poupon model and to elastic moduli and density by the Xu-White model. To handle uncertainty in the rock-physics models, we consider their outputs to be random functions with modes or means given by the predictions of those rock-physics models and we consider the parameters of the rock-physics models to be random variables defined by specified probability distributions. Using a Bayesian framework and Markov Chain Monte Carlo sampling methods, we are able to obtain estimates of reservoir parameters and information on the uncertainty in the estimation. The developed method is applied to a synthetic case study based on a layered reservoir model and the results show that uncertainty in both rock-physics models and in their parameters may have significant effects on reservoir parameter estimation. When the biases in rock-physics models and in their associated parameters are unknown, conventional joint inversion approaches, which consider rock-physics models as deterministic functions and the model parameters as fixed values, may produce misleading results. The developed stochastic method in this study provides an integrated approach for quantifying how uncertainty and biases in rock-physics models and in their associated parameters affect the estimates of reservoir parameters and therefore is a more robust method for reservoir parameter estimation.  相似文献   

The HNC/HCN Ratio in Comets     

Irvine  W. M.  Dickens  J. E.  Lovell  A. J.  Schloerb  F. P.  Senay  M.  Bergin  E. A.  Jewitt  D.  Matthews  H. E. 《Earth, Moon, and Planets》1997,78(1-3):29-35

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4−3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced insignificant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Hco+ in the Coma of Comet Hale–Bopp     

Lovell  A. J.  Schloerb  F. P.  Bergin  E. A.  Dickens  J. E.  Devries  C. H.  Senay  M. C.  Irvine  W. M. 《Earth, Moon, and Planets》1997,77(3):253-258

Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO⁺ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO⁺ emission require a low abundance of HCO⁺ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (T_e). To set constraints on the formation of HCO⁺ in the coma, as well as the location and magnitude of the transition to higher T_e, the data are compared with the results of ion-molecule chemistry models. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

National Center for Geographic Information and Analysis     

Nicholas M. Short Jr  Leonard Dickens 《International journal of geographical information science》2013,27(1):47-52

Abstract

Abstract. The growth of the remote sensing field has caused and explosion in the size and complexity of Earth satellite image databases. The explosion of data is already challenging current satellite-based GIS databases. New technologies for information management will be required in the future to maintain these large geographically-oriented systems created by projects such as NASA's Earth Observing System (EOS). This paper presents a technology adapted from the planning/scheduling field in Artificial Intelligence (AI) that promises to automate and facilitate the process of creating and storing satellite images and their associated data products.  相似文献   

THE KNUD RASMUSSEN MEMORIAL EXPEDITION Introduction     

dr. phil. N. KINGO JACOBSEN  CHR. BAY  B. FREDSKILD  B. GRøNNOW  B. HOLM JAKOBSEN  M. MELDGAARD 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):29-44

Jacobsen, N. Kingo, et.al. Report of activities at Tugtuligssuaq, Melville Bugt, 1978–79. Geografisk Tidsskrift 80: 29–44. Copenhagen, June 1980.

A joint cooperative Greenland/Danish expedition to the Melville Bugt area, Tugtuligssuaq/Kap Seddon to throw light on the Eskimos' immigration routes to Greenland seen in relation to climatic fluctuations. Studies of geography, botany, zoology, archaeology, and ethnohistory have been carried out.  相似文献   

Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland   总被引：7，自引：3，他引：7  

NICHOLSON  HUGH; CONDOMINES  MICHEL; FITTON  J. GODFREY; FALLICK  ANTHONY E.; GR?NVOLD  KARL; ROGERS  GRAEME 《Journal of Petrology》1991,32(5):1005-1020

The Krafla volcanic system consists of a central volcano andassociated fissure swarm in the NE axial rift zone of Iceland.Lavas spanning the whole of Krafla's exposed volcanic history(estimated to be 0-> 300 ka) have been analysed and rangein composition from olivine tholeiite to rhyolite. Major-elementcompositions suggest that fractional crystallization exertsthe main control over the differentiation process. However,K₂O and the very incompatible trace elements, Rb, Th, and U,are all enriched beyond the extent expected by closed-systemfractional crystallization. Fractionation coupled with periodicreplenishment and tapping of the reservoir is unlikely to beresponsible for this enrichment, despite the geophysical evidencesuggesting a large number of inflations and deflations of ashallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authorsthat crustal assimilation is operating on a local scale beneathKrafla. A model is suggested, fitting both the Th- and O-isotopicdata, which involves the partial melting and incorporation ofa hydrothermally altered wall-rock contaminant during fractionalcrystallization (i. e., AFC processes). This process of partialmelting is likely to enhance the most highly incompatible elementconcentrations (e. g., increasing Rb/Zr) more than expectedby closed-system fractional crystallization.  相似文献   

A protocol standard for heterogeneous telescope networks     

A. Allan  F. Hessman  K. Bischoff  M. Burgdorf  B. Cavanagh  D. Christian  N. Clay  R. Dickens  F. Economou  M. Fadavi  S. Fraser  T. Granzer  S. Grosvenor  T. Jenness  A. Koratkar  M. Lehner  C. Mottram  T. Naylor  E. Saunders  N. Solomos  I. Steele  G. Tuparev  T. Vestrand  R. White  S. Yost 《Astronomische Nachrichten》2006,327(8):744-750

The scientific need for a standard protocol permitting the exchange of generic observing services is rapidly escalating as more observatories adopt service observing as a standard operating mode and as more remote or robotic telescopes are brought on‐line. To respond to this need, we present the results of the first interoperability workshop for Heterogeneous Telescope Networks (HTN) held in Exeter. We present a draft protocol, designed to be independent of the specific instrumentation and software that controls the remote and/or robotic telescopes, allowing these telescopes to appear to the user with a unified interface despite any underlying architectural differences. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Reconstructing past fire regimes: methods,applications, and relevance to fire management and conservation     

Marco Conedera  Willy Tinner  Christophe Neff  Manfred Meurer  Angela F. Dickens  Patrik Krebs 《Quaternary Science Reviews》2009,28(5-6):555-576

Biomass burning and resulting fire regimes are major drivers of vegetation changes and of ecosystem dynamics. Understanding past fire dynamics and their relationship to these factors is thus a key factor in preserving and managing present biodiversity and ecosystem functions. Unfortunately, our understanding of the disturbance dynamics of past fires is incomplete, and many open questions exist relevant to these concepts and the related methods. In this paper we describe the present status of the fire-regime concept, discuss the notion of the fire continuum and related proxies, and review the most important existing approaches for reconstructing fire history at centennial to millennial scales. We conclude with a short discussion of selected directions for future research that may lead to a better understanding of past fire-regime dynamics. In particular, we suggest that emphasis should be laid on (1) discriminating natural from anthropogenic fire-regime types, (2) improving combined analysis of fire and vegetation reconstructions to study long-term fire ecology, and (3) overcoming problems in defining temporal and spatial scales of reference, which would allow better use of past records to gain important insights for landscape, fire and forest management.  相似文献   

Detection of interstellar ethylene oxide (c-C2H4O)     

Dickens JE  Irvine WM  Ohishi M  Ikeda M  Ishikawa S  Nummelin A  Hjalmarson A 《The Astrophysical journal》1997,489(2):753-757

We report the identification of 10 transitions that support the detection of the small cyclic molecule ethylene oxide (c-C2H4O) in Sgr B2N. Although one of these transitions is severely blended, so that an accurate intensity and line width could not be determined, and two other lines are only marginally detected, we have done Gaussian fits to the remaining seven lines and have performed a rotation diagram analysis. Our results indicate a rotation temperature T(rot) = 18 K and a molecular column density N(c-C2H4O) = 3.3 x 10(14) cm-2, corresponding to a fractional abundance relative to molecular hydrogen of order 6 x 10(-11). This is a factor of more than 200 higher than the abundance for this molecule suggested by the "new standard" chemistry model of Lee, Bettens, & Herbst. This result suggests that grain chemistry might play an effective role in the production of c-C2H4O. No transitions of this molecule were detected in either Sgr B2M or Sgr B2NW.  相似文献