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991.
Stephanie A. Ewing Wenbo Yang Greg Michalski Brian W. Stewart Ronald Amundson 《Geochimica et cosmochimica acta》2008,72(4):1096-1110
We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R2 = 0.78) and negatively correlated with sulfate δ44/40CaE values (R2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events. 相似文献
992.
Karem Azmy Brian Kendall Robert A. Creaser Larry Heaman Tolentino F. de Oliveira 《Precambrian Research》2008,164(3-4):160-172
The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along more than 300 km in the external zone of the Brasilia Fold Belt of the São Francisco Basin in east central Brazil. The sequence is about 4.8 km thick and contains a preserved glaciomarine diamictite unit (containing dropstone) at the top and a lower diamictite unit at the bottom. Previous C- and Sr-isotope profiles suggested the correlation of the upper diamictite unit with the “Sturtian” glacial event (ca. 750–643 Ma). However, new Re–Os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 ± 46 and 1100 ± 77 Ma. U–Pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 ± 15 and 1000 ± 25 Ma, respectively. Based on the Re–Os and U–Pb ages, the best age estimate of the Vazante Group is constrained to be 1000–1100 Ma and thus the two diamictite units are not correlative with the Sturtian glaciation(s) but most likely are records of glacial events that occurred during the late Mesoproterozoic. 相似文献
993.
The chemical composition of surface water in the photic zone of the Precambrian ocean is almost exclusively known from studies of stromatolitic carbonates, while banded iron formations (IFs) have provided information on the composition of deeper waters. Here we discuss the trace element and Nd isotope geochemistry of very shallow-water IF from the Pongola Supergroup, South Africa, to gain a better understanding of solute sources to Mesoarchean shallow coastal seawater. The Pongola Supergroup formed on the stable margin of the Kaapvaal craton ∼2.9 Ga ago and contains banded iron formations (IFs) that represent the oldest documented Superior-type iron formations. The IFs are near-shore, pure chemical sediments, and shale-normalized rare earth and yttrium distributions (REYSN) exhibit positive LaSN, GdSN, and YSN anomalies, which are typical features of marine waters throughout the Archean and Proterozoic. The marine origin of these samples is further supported by super-chondritic Y/Ho ratios (average Y/Ho = 42). Relative to older Isua IFs (3.7 Ga) from Greenland, and younger Kuruman IFs (2.5 Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and appear to record variations in solute fluxes related to sea level rise and fall. Sm-Nd isotopes were used to identify potential sediment and solute sources within pongola shales and IFs. The ?Nd(t) for Pongola shales ranges from −2.7 to −4.2, and ?Nd(t) values for the coeval iron-formation samples (range −1.9 to −4.3) are generally indistinguishable from those of the shales, although two IF samples display ?Nd(t) as low as −8.1 and −10.9. The similarity in Nd isotope signatures between the shale and iron-formation suggests that mantle-derived REY were not a significant Nd source within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples indicates that high-T hydrothermal input did contribute to their REY signature. Isotopic mass balance calculations indicate that most (?72%) of the Nd in these seawater precipitates was derived from continental sources. If previous models of Fe-Nd distributions in Archean IFs are applied, then the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally considered. 相似文献
994.
Mineralogy and origin of rhizoliths on the margins of saline,alkaline Lake Bogoria,Kenya Rift Valley
Richard Alastair Owen Richard Bernhart Owen Robin W. Renaut Jennifer J. Scott Brian Jones Gail M. Ashley 《Sedimentary Geology》2008,203(1-2):143-163
A wide range of rhizoliths occurs around the margins of Lake Bogoria, Kenya. These include root casts, moulds, tubules, rhizocretions, and permineralised root systems. These rhizoliths are variably composed of opaline silica, calcite, zeolites (mainly analcime), fluorite, and possibly fluorapatite, either alone or in combinations. Some rhizoliths are infilled moulds with detrital silicate grains. Most rhizoliths are in situ, showing both vertical and horizontal orientations. Reworked rhizoliths have been concentrated locally to form dense rhizolites.Hot-spring fluids, concentrated by evapotranspiration and capillary evaporation, have provided most of the silica for the permineralisation of the plant tissues. Precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses. Calcite rhizoliths formed following evaporative concentration, evapotranspiration, and (or) CO2 degassing of Ca-bearing runoff water that infiltrated the sediment, or by mixing of runoff with saline, alkaline groundwater. Fluorite precipitated in areas where mixing of hot-spring and meteoric waters occurred, or possibly where hot-spring fluids came into contact with pre-existing calcite. Zeolitic rhizoliths formed during a prolonged period of aridity, when capillary rise and evaporative pumping brought saline, alkaline waters into contact with detrital silicate minerals around roots. 相似文献
995.
Ran Liu Amy L. Wolfe David A. Dzombak Colin P. Horwitz Brian W. Stewart Rosemary C. Capo 《Applied Geochemistry》2008
The dissolution of pyrite is of interest in the formation of acid mine drainage and is a complex electrochemical process. Being able to measure the rate of dissolution of particular pyrite samples under particular conditions is important for describing and predicting rates of AMD generation. Electrochemical techniques offer the promise of performing such measurements rapidly and with small samples. The oxidation of pyrite and the reduction of Fe3+ ions and/or O2 half reactions involved in the pyrite dissolution process were investigated by cyclic voltammetry and steady-state voltammetry using three pyrite materials formed in both sedimentary and hydrothermal environments. For each sample, two kinds of pyrite working electrodes (conventional constructed compact solid electrode, and carbon paste electrode constructed from fine-grained pyrite particles) were employed. Results indicated that for both the hydrothermal and sedimentary pyrite samples the oxidation and reduction half reactions involved in dissolution were governed by charge transfer processes, suggesting that hydrothermal and sedimentary pyrites obey the same dissolution mechanism despite their different formation mechanisms. In addition, the results showed that it is feasible to use a C paste electrode constructed from fine-grained or powdered pyrite to study the pyrite dissolution process electrochemically and to derive approximate rate expressions from the electrochemical data. 相似文献
996.
During the Mitch Hurricane event (October 1998), severe floods occurred in the village of La Trinidad (Departamento de Estelí,
NW Nicaragua), which spreads at the margin of La Trinidad river. As a consequence, the need for hazard assessment and land
use planning to reduce the effects of these natural processes arose. Nicaragua is a developing country, which means that there
is a scarcity of good quality data on which to base these hazard assessments (i.e., lack of detailed topographic maps, lack
of meteorological and discharge data series). Therefore, the main objective of the present work was to generate a flood hazard
map of La Trinidad by means of a simple method, with a resulting map easy to understand and to use by the municipality for
land use planning. There is no topographic map of the area at a more detailed scale than 1:50,000. So the main document that
supports all the data and on which the final hazard map was based is the orthophotograph at 1:5,000 scale (generated from
vertical aerial photographs taken in 2000). The method used was based on classical interpretation of vertical aerial photographs
(pre Mitch and a post Mitch event), detailed field work, inquiries among the population and analysis of the main pattern of
storms occurring in the area. All these data allowed the reconstruction of different extensions and water levels corresponding
to events of different frequency and magnitude, and the qualitative association of them to three hazard levels by means of
energy and frequency. The use of orthophotographs of 1:5,000 proved to be very useful both for the development of the work
and for the presentation of the final map, because they are very easily understandable for people not trained in the interpretation
of topographic maps. 相似文献
997.
Brian Mills Dan Unrau Carla Parkinson Brenda Jones Jennifer Yessis Kelsey Spring Laurel Pentelow 《Natural Hazards》2008,47(2):157-183
This article summarizes research completed to assess the risk of lightning-related injuries and fatalities in Canada. Although
lightning mortality has declined significantly over the past century, it remains a common meteorological hazard that regularly
kills and injures. Based on an analysis of media reports, vital statistics, hospital admission and emergency room visit records,
and fire loss data, the authors estimate that on average about 9–10 lightning-related deaths and 92–164 injuries occur each
year in Canada. The distribution of casualties reflects current provincial population and cloud-to-ground lightning densities.
Consistent with similar studies in other developed nations, most lightning-related fatalities and injuries in Canada occur
during the June-August summer season, coincident with peak lightning, and during the Thursday-Saturday period, most likely
related to higher rates of participation in outdoor activities. The majority of victims are male, less than 46 years old,
and engaged in outdoor recreational activities when injured or killed in a lightning incident. Media reports used in the study
were found to underestimate both lightning mortality (36%) and morbidity (20–600%).
相似文献
Brian MillsEmail: |
998.
Comparison of dissolution under oxic acid drainage conditions for eight sedimentary and hydrothermal pyrite samples 总被引:1,自引:0,他引:1
Ran Liu Amy L. Wolfe David A. Dzombak Brian W. Stewart Rosemary C. Capo 《Environmental Geology》2008,56(1):171-182
The abiotic oxidative dissolution behaviors of eight natural pyrite samples, five sedimentary and three hydrothermal, from
various geological environments were compared under oxic conditions at pH 3 and 6 in a highly controlled batch reactor dissolution
system. The three sedimentary pyrite samples associated with coal had greater specific surface areas and also exhibited greater
apparent dissolution rates and extent than the other two sedimentary and three hydrothermal samples under both pH conditions.
However, after normalizing for surface area, the dissolution rate constants for the different pyrite samples were similar;
the greatest difference was between the two non-coal sedimentary pyrite samples. Pyrite morphology and the presence of trace
metals could contribute to the differences in dissolution behavior as reflected in the normalized dissolution rates. The sulfur:iron
ratio observed in the aqueous solution at pH 3 increased with time, but was always less than 2.0 (predicted from the stoichiometry
of dissolution) for all the pyrite samples during the 24-h experimental duration. This can be explained by the disproportionation
dissociation of thiosulfate, an initial product of pyrite dissolution, to elemental sulfur and sulfate which does not occur
in a 1:1 ratio. The results of this work indicate the importance of extracting and using the specific pyrite(s) relevant to
particular mining areas in order to understand pyrite dissolution rates and the influence of environmental conditions on those
rates. 相似文献
999.
In this paper the development of a large-scale gravitational deformation involving the eastern lateral moraine of the Athabasca
Glacier in Jasper National Park, Alberta, Canada, is described. Interpretation and analysis of sequential aerial photographs
indicates that a 540-m-wide segment of the eastern lateral moraine began to deform in the early 1950s; however, significant
movement only began in the late 1960s. Since then, the moraine has undergone progressive gravitational deformation leading
to a network of fractures, bulging, and the development of a large gap in the moraine crest. Geographic information system
analysis of topographic changes between 1967 and 2006 indicates that the displaced volume of the moraine is approximately
9.0 × 105 m3. In the last 39 years, the moraine crest has displaced 55 m (1.4 m yr−1) down towards the glacier. The development of slope instability is linked to a combination of debuttressing from recent glacier
recession, deformation of the moraine, as well as the movement of a large, mobile, debris-mantled slope impinging the upslope
margin of the lateral moraine. This case study illustrates the importance of glacial conditioning and local geomorphological
factors in creating conditions for large-scale moraine instability in recently deglacierized alpine basins. 相似文献
1000.
Brian Fry 《Estuaries and Coasts》2008,31(4):776-789
Previous studies of Louisiana estuaries have indicated a central role of Spartina alterniflora marshes in supporting production of the commercially important brown shrimp, Farfantepenaeus aztecus. Brown shrimp are an estuarine-dependent species and spend one to three springtime months in estuaries as small juveniles, with highest shrimp densities found at marsh edges. Later estuarine and offshore production of brown shrimp is correlated both with marsh area and with abundance of smaller juveniles found in unvegetated open bays near marshes. This paper investigates the idea that open bays are an additional important nursery habitat for Louisiana brown shrimp, with bays possibly supporting the bulk of shrimp populations even while shrimp densities expressed on a square meter basis are lower in the bays. To assay possible differences in shrimp abundances and residency in marsh ponds vs. adjacent open bays, springtime field work was conducted in 2004–2006 near the Louisiana University Marine Consortium Laboratory at Cocodrie, Louisiana. Seine surveys showed similar-sized shrimp were present in marsh ponds (<20 m in diameter) and an adjacent open bay (<1 m deep, 2 km in diameter) and that shrimp were twice as dense in the marsh ponds. Natural C, N, and S isotope tags provided distinctive labeling of shrimp from marsh ponds versus bays; shrimp residency appeared high in both areas with <10% of shrimp present as immigrants from other areas. Widely spaced collections from several Louisiana bay systems and also Galveston Bay, Texas showed that the S isotope tags provided the most general tags for marsh origins, with low S isotope values of 1–9‰ in shrimp muscle tissue consistently indicating marsh origins. Importance of marshes for brown shrimp production across Terrebonne and Barataria Bays, Louisiana was evaluated with S isotopes using spring 2005 collections. Results showed that marshes supported about 1/3 of total shrimp production; 2/3 of Louisiana brown estuarine shrimp production may depend on the three to four times more extensive open bays. Given these results, coastal restoration efforts in Louisiana might focus on measures such as barrier island conservation and restoration that protect both bays and marshes, rather than focusing on measures that specifically target marshes and neglect open bays. 相似文献