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11.
Gregory D. Lazear 《Hydrogeology Journal》2006,14(8):1582-1598
The Tongue Creek watershed lies on the south flank of Grand Mesa in western Colorado, USA and is a site with 1.5 km of topographic relief, heat flow of 100 mW/m2, thermal conductivity of 3.3 W m–1 °C–1, hydraulic conductivity of 10-8 m/s, a water table that closely follows surface topography, and groundwater temperatures 3–15°C above mean surface temperatures. These data suggest that convective heat transport by groundwater flow has modified the thermal regime of the site. Steady state three-dimensional numerical simulations of heat flow, groundwater flow, and convective transport were used to model these thermal and hydrological data. The simulations provided estimates for the scale of hydraulic conductivity and bedrock base flow discharge within the watershed. The numerical models show that (1) complex three-dimensional flow systems develop with a range of scales from tens of meters to tens of kilometers; (2) mapped springs are frequently found at locations where contours of hydraulic head indicate strong vertical flow at the water table, and; (3) the distribution of groundwater temperatures in water wells as a function of surface elevation is predicted by the model. 相似文献
12.
13.
Drought in Bulgaria is analyzed from the multiple viewpoints of statistical occurrence, spatial patterns, and synoptic conditions.
A new index of drought, the SD (spatial-dryness) index, characterizes drought by both intensity and spatial extent. The occurrence of the SD index is analyzed using global
gridded data sets. Examination of transitional probabilities of multiple months and years with drought occurrence suggests
persistence is sufficiently frequent to be important for climate-related environmental planning. Finally, it is shown that
specific seasonal synoptic patterns are associated with wet and dry conditions in Bulgaria.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
Seagrass meadows are often cited as important nursery areas for newly settled red drum even though many estuaries, such as Galveston Bay, Texas, support large numbers of red drum and have limited seagrass cover, suggesting the use of alternate nursery areas. We examined patterns of habitat use for newly settled red drum at six sampling areas in Galveston Bay; two areas had seagrass beds and four areas had no seagrass. We measured densities in different habitat types using epibenthic sleds and enclosure samplers. Peak recruitment of young red drum to the estuary occurred during September through December. Highest densities of new settlers were found in seagrass meadows (primarilyHalodule wrightii), but when seagrass was absent, the highest densities of red drum occurred along theSpartina alterniflora marsh edge interface. Densities were relatively low on nonvegetated bottom away from the marsh edge. We also examined density patterns in other habitat types at selected sampling areas and found no red drum within marsh vegetation away from the marsh edge interface (5 and 10 m into the marsh interior). Oyster reefCrassostrea virginica was sampled using lift nets, and we found no red drum using this habitat, although adjacent seagrass and marsh interface habitats were used. Even though red drum densities in marsh edge were low relative to seagrass, the large areal extent of marshes in the bay complex probably makes marsh edge the most important nursery habitat for red drum in Galveston Bay. 相似文献
15.
16.
Gregory K. Druschel Robert J. Hamers George W. Luther III Jillian F. Banfield 《Aquatic Geochemistry》2003,9(2):145-164
The oxidation kinetics of trithionate (S3O62-
) and tetrathionate (S4O
6
2-
) with hydroxyl radicals (OH*) have been investigated in systems analogous to acid mine drainage (AMD) environments. The discovery of hydroxyl radical
(OH*) formation on pyrite surfaces (Borda et al., 2003) suggests hydroxyl radicals may affect the oxidation kinetics of intermediate
sulfur species such as tetrathionate. Cyclic voltammetry experiments in acidic solutions indicate that the reaction of S4O
6
2-
with OH* goes through an unknown intermediate, tentatively assigned as S3O
4
n-
. An outer-sphere electron transfer mechanism for the reaction of S4O
6
2-
with OH* to form S3O
4
n-
is proposed based on experimental results. Oxidation rates for trithionate and tetrathionate in the presence of Fenton's
reagent (which forms hydroxyl radicals) are too fast to be directly measured using UV-Vis spectrophotometry, electrochemical,
or stop-flow spectrophotometry methods. Competitive reaction kinetics within the context of the Haber—Weiss mechanism suggests
that the rate constant for the oxidation of trithionate and tetrathionate with OH* is in excess of 108 M-1 sec-1. 相似文献
17.
18.
Deformation of subducted oceanic lithosphere 总被引:2,自引:0,他引:2
19.
I. J. Richards Theodore C. Labotka Robert T. Gregory 《Contributions to Mineralogy and Petrology》1996,123(2):202-221
Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events
including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement
of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion
of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions
that can be divided into two groups, one with positive δ13C values from+3.1 to +1.4 ‰ (PDB) and high δ18O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ13C values from –3.3 to –3.6‰ and low δ18O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent,
clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ18O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite
marbles typically have low δ18O values from +2.0 to +9.3‰. The δ18O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and
+14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ18O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions
because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and
quartz overgrowths in dolomite marbles, low δD values of micas, negative δ13C values and low δ18O values of calcite and dolomite, and depleted δ18O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the
Tertiary. Partial oxygen isotopic exchange between calcite and low 18O meteoric fluids lowered the δ18O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite
fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration.
Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in 18O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the
quartz as a result of its dissolution by meteoric fluids. The negative δ13C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic
data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western
Nevada in Tertiary time.
Received: 1 February 1994/Accepted: 12 September 1995 相似文献
20.
Comments on “Evidence for global runoff increase related to climate warming” by Labat et al. 总被引:1,自引:0,他引:1
We have examined the evidence presented by Labat et al. and found that (1) their claims for a 4% increase in global runoff arising from a 1 °C increase in air temperature and (2) that their article provides the “first experimental data-based evidence demonstrating the link between the global warming and the intensification of the global hydrological cycle” are not supported by the data presented. Our conclusions are based on the facts that (1) their discharge records exhibit non-climatic influences and trends, (2) their work cannot refute previous studies finding no relation between air temperature and runoff, (3) their conclusions cannot explain relations before 1925, and (4) the statistical significance of their results hinges on a single data point that exerts undue influence on the slope of the regression line. We argue that Labat et al. have not provided sufficient evidence to support their claim for having detected increases in global runoff resulting from climate warming. 相似文献