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571.
George W. LutherIII Aubin Thibault de Chanvalon Véronique E. Oldham Emily R. Estes Bradley M. Tebo Andrew S. Madison 《Aquatic Geochemistry》2018,24(4):257-277
Manganese oxides, typically similar to δ-MnO2, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O2, as the reaction of [Mn(H2O)6]2+ with O2 alone is not thermodynamically favorable below pH of ~?9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λmax?=?623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO2 with ammonia (NH3). Reactions are unfavorable for NH4+ and sulfide with oxidized Fe and Mn solids, and NH3 with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO2. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO2 solutions (λmax?~?370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (<?0.2 µm) have not been detected as Mn oxides are quantitatively trapped on 0.2-µm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO2, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty e g * conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO2 reduction using laboratory and field data for the reaction of MnO2 with hydrogen sulfide and other reductants. 相似文献
572.
Analysis and Estimation of Natural Processes with Nonhomogeneous Spatial Variation Using Secondary Information 总被引:1,自引:0,他引:1
Natural processes encountered in mining, hydrogeologic, environmental, etc. applications usually are poorly known because of scarcity of data over the area of interest. Therefore, stochastic estimation techniques are the tool of choice for a careful accounting of the heterogeneity and uncertainty involved. Within such a framework, a better utilization of all available data concerning the process of interest and all other natural processes related to it, is of primary importance. Because many natural processes show complicated spatial trends, the hypothesis of spatial homogeneity cannot be invoked always, and the more general theory of intrinsic spatial random fields should be employed. Efficient use of secondary information in terms of the intrinsic model requires that suitable permissibility criteria for the generalized covariances and cross-covariances are satisfied. A set of permissibility criteria are presented for the situation of two intrinsic random fields. These criteria are more general and comprehensive than the ones currently available in the geostatistical literature. A constrained least-square technique is implemented for the inference of the generalized covariance and cross-covariance parameters, and a synthetic example is used to illustrate the methodology. The numerical results show that the use of secondary information can lead to significant reductions in the estimation errors. 相似文献
573.
Colloidal-sized material from the Patuxent River estuary, Maryland, was analyzed for more than 30 elements by instrumental neutron activation analysis. Sample data from stations ranging in salinity from 0.04 g/kg to 12 g/kg indicate that the winter colloidal material is dominated upstream by poorly crystallized clay minerals and iron oxide, but above a salinity of 10 g/kg, it consists primarily of carbonaceous material. Winter colloidal material decreases non-conservatively seaward from 29 to 0.6 mg/l. The summer colloidal material, however, is carbon-rich throughout the estuary and the amount of colloidal material in the upper water column does not change appreciably with increasing salinity. Many trace elements (Cs, Hf, Rb, Se, Th, V and the REE) covary with Al and probably are associated with the clay component. Other elements (As, Ba, Sb, Se, U and Zn) are considerably enriched relative to Al and may covary with the organic component. The results suggest that the freshwater winter colloidal system is a product of continental weathering processes, whereas the summer colloidal material is derived primarily from estuarine biological processes. 相似文献
574.
Dimitris?S.?KostopoulosEmail author Sevket?Sen George?D.?Koufos 《International Journal of Earth Sciences》2003,92(5):779-794
The late Miocene mammalian record in Samos Island, Greece, is extremely important for the Eurasian Neogene mammalian history and chronology. However, due to the mixed nature of old fossil collections and controversies on the stratigraphic position of fossil quarries, great confusion has arisen concerning the recognition of distinct faunal assemblages, their age(s) and biostratigraphic significance. This paper presents the magnetostratigraphy of the late Miocene continental deposits of the fossiliferous Mytilinii Basin, Samos Island, Greece. Old and new sites are stratigraphically controlled with accuracy and correlated with each other. The magnetostratigraphy of seven individual sections, covering the entire Mytilinii Formation, provides good correlation with the Geomagnetic Polarity Time Scale (GPTS). These results, as well as the relocation and precise litho- and magnetostratigraphic correlation of the old and new mammal localities, combined with new and reviewed paleontological data and thorough studying of the numerous radiometric dates, allowed us to define five biostratigraphic horizons. Their correlation with, and implications to the European Neogene mammal chronology ages and zones make up the following discussion and provides a modified determination of the middle Turolian (MN 12) boundaries.Abbreviations GPTS Geomagnetic Polarity Time Scale - IPGP Institute de Physic du Globe de Paris - MGL Musée Cantonal de Géologie, Lausanne - MNHA Museum of Natural History of the Aegean, Samos - BMNH Natural History Museum of London - Q quarries - Fm Formation 相似文献
575.
576.
P. Sargent Bray Claudia M. Jones Stewart J. Fallon Jochen J. Brocks Simon C. George 《Quaternary Research》2012,77(1):104-109
Assigning accurate dates to hypersaline sediments opens important terrestrial records of local and regional paleoecologies and paleoclimatology. However, as of yet no conventional method of dating hypersaline systems has been widely adopted. Biomarker, mineralogical, and radiocarbon analyses of sediments and organic extracts from a shallow (13 cm) core from a hypersaline playa, Lake Tyrrell, southeastern Australia, produce a coherent age-depth curve beginning with modern microbial mats and extending to ~ 7500 cal yr BP. These analyses are furthermore used to identify and constrain the timing of the most recent change in hydrological regime at Lake Tyrrell, a shift from a clay deposit to the precipitation of evaporitic sands occurring at some time between ~ 4500 and 7000 yr. These analyses show the potential for widespread dating of hypersaline systems integrating the biomarker approach, reinforce the value of the radiocarbon content of biomarkers in understanding the flow of carbon in modern ecologies, and validate the temporal dimension of data provided by biomarkers when dating late Quaternary sediments. 相似文献
577.
578.
CHEN Xuanhua WANG Xiaofeng George GEHRELS YANG Yi QIN Hong CHEN Zhengle YANG Feng CHEN Bailin LI Xuezhi 《《地质学报》英文版》2004,78(2):515-523
Abstract This paper discusses the relationships between granitic magmatism and gold mineralization and the exhumation history of the Dapinggou gold deposit in northern Altun, NW China based on the geochronological data, including zircon U‐Pb ages, Rb‐Sr isochron age and 40Ar‐39Ar dating and MDD modeling data. The main granitic magmatism age in this area is attained from the ID TIMS U‐Pb geochronology of zircons from the Kuoshibulak granite, the biggest granite in the northern Altun area, which gives a concordant age of 443±5 Ma in the Late Ordovician. Zircon ID TIMS U‐Pb geochronology of the West Dapinggou biotite granite west of the Dapinggou gold deposit gives concordant ages around 485±10 Ma, representing the early stage of Ordovician magmatism. The Rb‐Sr isochron age (487±21 Ma) of 6 quartz inclusion samples from quartz veins in this gold deposit is very close to that of the West Dapinggou granite. MDD modeling of step heating 40Ar‐39Ar data of K‐feldspar from the same West Dapinggou biotite granite gives a rapid cooling history from 300°C to 150°C during 200–185 Ma. According to the age data and the geological setting of this area, we conclude that the Dapinggou gold deposit was formed at the early stage of the Early Paleozoic granitic magmatism in northern Altun, and exhumed in the Early Jurassic due to the normal faulting of the Lapeiquan detachment. The Early Paleozoic magmatism may provide heat source and produce geological fluids, which are very important for gold mineralization. Exhumation in the Mesozoic caused the uplift of the deposit towards the ground surface. 相似文献
579.
The Middle to Upper Ordovician foreland succession of the Ottawa Embayment in central Canada is divided into nine transgressive‐regressive sequences that defines net deepening of a platform succession over ~15 m.y. from peritidal to outer ramp settings, then a return to peritidal conditions over ~3 m.y. related to basin filling by orogen‐derived siliciclastics. With a backdrop of net eustatic rise through the Middle to Late Ordovician, there are several different expressions of structural influence on sequence development in the embayment. During the Middle Ordovician (Darriwilian), foreland‐basin initiation was marked by regional onlap with abundant synsedimentary deformation across a faulted trailing‐margin platform interior; subsequent craton‐interior uplift resulted in voluminous influx of siliciclastics contemporary with local structurally influenced local channelization; then, a formation of a platform‐interior shale basin defines continued intrabasin tectonism. During the Late Ordovician (Sandbian, early Katian), structural influence was superimposed on sea‐level rise as indicated by renewed local development of a platform‐interior shale basin; differential subsidence and thickness variation of platform carbonate successions; abrupt deepening across shallow‐water shoal facies; and, micrograben development coincident with foreland‐platform drowning. These stratigraphic patterns are far‐field expressions of distal orogen development amplified in the platform interior through basement reactivation along an inherited buried Precambrian fault system. Comparison of Upper Ordovician (Sandbian‐lower Katian) sequence stratigraphy in the Ottawa Embayment with eustatic frameworks defined for the Appalachian Basin reveals greater regional variation associated with Sandbian sequences compared to regional commonality in base level through the early Katian. 相似文献
580.