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951.
Mark?D.?Rowe Judith?A.?PerlingerEmail author Christopher?W.?Fairall 《Boundary-Layer Meteorology》2011,140(1):87-103
A model was developed to predict the modification with fetch in offshore flow of mixing ratio, air–water exchange flux, and
near-surface vertical gradients in mixing ratio of a scalar due to air–water exchange. The model was developed for planning
and interpretation of air–water exchange flux measurements in the coastal zone. The Lagrangian model applies a mass balance
over the internal boundary layer (IBL) using the integral depth scale approach, previously applied to development of the nocturnal
boundary layer overland. Surface fluxes and vertical profiles in the surface layer were calculated using the NOAA COARE bulk
algorithm and gas transfer model (e.g., Blomquist et al. 2006, Geophys Res Lett 33:1–4). IBL height was assumed proportional to the square root of fetch, and estimates of the IBL growth
rate coefficient, α, were obtained by three methods: (1) calibration of the model to a large dataset of air temperature and humidity modification
over Lake Ontario in 1973, (2) atmospheric soundings from the 2004 New England Air Quality Study and (3) solution of a simplified
diffusion equation and an estimate of eddy diffusivity from Monin–Obukhov similarity theory (MOST). Reasonable agreement was
obtained between the calibrated and MOST values of α for stable, neutral, and unstable conditions, and estimates of α agreed with previously published parametrizations that were valid for the stable IBL only. The parametrization of α provides estimates of IBL height, and the model estimates modification of scalar mixing ratio, fluxes, and near-surface gradients,
under conditions of coastal offshore flow (0–50 km) over a wide range in stability. 相似文献
952.
Impacts of past climate and sea level change on Everglades wetlands: placing a century of anthropogenic change into a late-Holocene context 总被引:1,自引:1,他引:0
We synthesize existing evidence on the ecological history of the Florida Everglades since its inception ??7?ka (calibrated kiloannum) and evaluate the relative impacts of sea level rise, climate variability, and human alteration of Everglades hydrology on wetland plant communities. Initial freshwater peat accumulation began between 6 and 7?ka on the platform underlying modern Florida Bay when sea level was ??6.2?m below its current position. By 5?ka, sawgrass and waterlily peats covered the area bounded by Lake Okeechobee to the north and the Florida Keys to the south. Slower rates of relative sea level rise ??3?ka stabilized the south Florida coastline and initiated transitions from freshwater to mangrove peats near the coast. Hydrologic changes in freshwater marshes also are indicated ??3?ka. During the last ??2?ka, the Everglades wetland was affected by a series of hydrologic fluctuations related to regional to global-scale fluctuations in climate and sea level. Pollen evidence indicates that regional-scale droughts lasting two to four centuries occurred ??1?ka and ??0.4?ka, altering wetland community composition and triggering development of characteristic Everglades habitats such as sawgrass ridges and tree islands. Intercalation of mangrove peats with estuarine muds ??1?ka indicates a temporary slowing or stillstand of sea level. Although sustained droughts and Holocene sea level rise played large roles in structuring the greater Everglades ecosystem, twentieth century reductions in freshwater flow, compartmentalization of the wetland, and accelerated rates of sea level rise had unprecedented impacts on oxidation and subsidence of organic soils, changes/loss of key Everglades habitats, and altered distribution of coastal vegetation. 相似文献
953.
Ray A. F. Cas Heather M. N. Wright Christopher B. Folkes Chiara Lesti Massimiliano Porreca Guido Giordano Jose G. Viramonte 《Bulletin of Volcanology》2011,73(10):1583-1609
The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%),
pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit.
The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km
from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal
settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial
valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional
units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned
elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric
is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived
(‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics
are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing
the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed
turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question
whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited
by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with
suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In
order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional
boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass
feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which
the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive
sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed
during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular
flow regime during the final stages of transport and deposition. 相似文献
954.
Tracing dissolved O2 and dissolved inorganic carbon stable isotope dynamics in the Nyack aquifer: Middle Fork Flathead River, Montana, USA 总被引:1,自引:0,他引:1
M. Garrett Smith Christopher H. Gammons F. Richard Hauer 《Geochimica et cosmochimica acta》2011,75(20):5971-5986
The geochemistry and microbiology of shallow groundwater aquifers is greatly influenced by the concentration of dissolved oxygen gas (DO); however, the mechanisms that consume DO in groundwater (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of molecular O2 (δ18O-DO), in conjunction with stable isotopes of dissolved inorganic carbon (δ13C-DIC), has potential to discriminate between the various mechanisms causing DO depletion in subsurface waters.Here we report the results of spatial and seasonal changes in δ18O-DO and δ13C-DIC at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana, USA. Over a short, well constrained flow path (∼100 m) near a main recharge zone of the floodplain, the δ18O-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and δ13C-DIC values decreased. These observations are explained by community respiration coupled with dissolution of calcite from cobbles in the aquifer matrix. When these results are compared to data from wells distributed over the entire floodplain (several km) a much less predictable relationship was observed between DO concentration and δ18O-DO. Many wells with low DO concentrations (e.g., <125 μmol L−1 or 4 mg L−1) had anomalously low δ18O-DO values (e.g., <20‰). Mass balance calculations show that approximately equal amounts of O2 may be contributed to the aquifer by diffusion from the vadose zone and by advection from the river recharge. Calculations presented here suggest that diffusion across a narrow air-water interface can contribute isotopically light δ18O-DO to the saturated zone. Possible contributions of light δ18O-DO from other processes, such as isotopic exchange and radial oxygen loss from plant roots in or near the water table, are compared and evaluated. 相似文献
955.
George D. WardlawRobert K. Nelson Christopher M. ReddyDavid L. Valentine 《Organic Geochemistry》2011,42(6):630-639
Contamination of coastal marine sediments with polycyclic aromatic hydrocarbons is pervasive, with major sources including anthropogenic activity and natural seepage. Biodegradation serves as a major hydrocarbon sink with evaporation and dissolution responsible for the removal of low boiling range compounds and photo-oxidation acting on many multi-substituted aromatic compounds. In this work, first-order rate constants for aerobic biodegradation were quantified for naphthalene (N), benzothiophene (BT) and their alkylated congeners (1-4 carbon substituents (C1-C4)) in laboratory experiments with oil laden marine sediments from a natural seep. Rate constants were used as proxies for microbial preference, which follows the order: naphthalene > C1N > C2N > C1BT > C2BT > benzothiophene > C3BT > C3N > C4BT > C4N, with some overlap. The application of comprehensive two dimensional gas chromatography further enabled separation and quantification of multiple structural isomers for C2N-C4N and C2BT-C4BT, with 7-12 isomers resolved for each C2N-C4N and 4-7 isomers resolved for each C2BT-C4BT. A strong isomeric biodegradation preference was noted within each of these compound classes, with rate constants varying as much as a factor of 2 for structural isomers of the same compound class. Each isomer was consumed to a low, but non-zero concentration, suggesting that their residual load in sediment may be proportional to the number of structural isomers originally present, in addition to the pattern and the number of alkyl substitutions. The simultaneous first-order biodegradation rates observed for 52 aromatic hydrocarbons resolved in this study, along with reference compounds such as n-alkanes, lends support to the concept of broad scale metabolic specificity during aerobic biodegradation of petroleum. 相似文献
956.
Nestor Cardozo Christopher A.-L. Jackson Paul S. Whipp 《Journal of Structural Geology》2011,33(6):1063-1078
We show the application of a simulated annealing algorithm to trishear inverse modeling. The algorithm traverses the parameter space in search for best-fit models without being trapped in local minima, and thus sampling for more possible solutions globally. Simulated annealing is a robust and efficient technique to determine the uniqueness of best-fit trishear models; the spread of possible trishear models that can fit a structure. We first apply the algorithm to a decameter-size, contractional fault-propagation fold in west-central Taiwan, for which there is an exceptional exposure of pre-growth and growth strata. Simulated annealing shows that even for this complete fold dataset with low uncertainties, there is a range of models and fault slip/uplift histories that can fit the data, with the consequent implications for the assessment of seismic hazard. We then apply the algorithm to a kilometer-size, extensional fault-propagation fold, the Hadahid monocline, Gulf of Suez Rift, Egypt. In this monocline there is only surface coverage in the footwall anticline areas and the algorithm was used to delimit the range of possible models that can fit the data and their uncertainties, thus avoiding biases in the interpretation. Simulated annealing suggests that the along-strike structural variability of the monocline can result from along-strike variability in fault slip, fault propagation to fault slip ratio and depth of fault nucleation. Both examples illustrate the benefits of searching for a possible range of models rather than a precise best-fit model when modeling fault-propagation folds. In an attempt to understand which parameters control fault development, and also how the spread of possible solutions varies with fold growth, we apply the algorithm to four sequential stages of a published, analog clay model of an extensional forced fold. The inversions of the natural examples and the analog model suggest that the spread of the possible models is a manifestation of the data uncertainties, the suitability of the trishear model, fold evolution, and rock mechanical properties. 相似文献
957.
The Stonepark Prospect is located in County Limerick, south-central Ireland. Multiple zones of Zn–Pb mineralisation have been identified at Stonepark and these are approximately 5 km west of the Pallas Green Prospect. At Stonepark, the sulphide bodies are hosted within the Waulsortian Limestone and closely resemble other Irish-type deposits. The mineralisation is composed of pyrite-marcasite, sphalerite and galena with gangue Fe-dolomite and calcite cements. A key difference at Stonepark is the presence of Chadian-aged volcanic rocks (Knockroe Volcanics) that intrude into and overlie the Waulsortian Limestone. Subsequent hydrothermal brecciation of the Waulsortian Limestone and Knockroe intrusions resulted in the formation of tabular polymict breccia bodies containing mixed carbonate and clasts of intrusive rocks. These have then been overprinted by massive sulphide mineralisation. Further syn-mineralisation brecciation has overprinted the earlier breccias. Drilling has demonstrated a spatial relationship between the volume of intrusive rocks (dykes and polymict breccias) and Zn–Pb mineralisation. This association suggests that the intrusive rocks provided a mechanism for the introduction of the mineralising fluids into the breccia bodies. This is significant as to date no large controlling fault has been identified, as is seen at other Irish-type deposits. Further work is required to understand the alteration process of the intrusive rocks and how this may relate to the mineralising process. 相似文献
958.
Julie C. Aleman Audrey Saint-Jean Bérangère Leys Christopher Carcaillet Charly Favier Laurent Bremond 《Quaternary Research》2013
So far, no phytolith extraction protocols have been tested for accuracy and repeatability. Here we aim to display a phytolith extraction method combining the strengths of two widely used protocols, supplemented with silica microspheres as exogenous markers for quantifying phytolith concentrations. Phytolith concentrations were estimated for samples from two sedimentary sequences in which numerical age–depth models make it possible to calculate phytolith influxes (phytolith numbers per cm2per yr). Analysis of replicates confirmed the statistical robustness, the repeatability and the very few biases of our extraction technique for small phytoliths, since the relationship between grass silica short cells and microspheres was kept stable. Furthermore, we demonstrated that silica microspheres are robust exogenous markers for estimating phytolith concentrations. The minimum number of items (i.e., phytoliths plus silica microspheres) that must be counted to estimate phytolith concentrations and thus influxes depends on the ratio of phytoliths to microspheres (R) and is minimized when R = 1. Nevertheless, we recommend using ratios R ≤ 1 in order to avoid having the counting process become excessively time-consuming, because microspheres are easier to identify and count than phytoliths. 相似文献
959.
Christopher S. Swezey Arthur P. Schultz Wilma Alemán González Christopher E. Bernhardt William R. Doar III Christopher P. Garrity Shannon A. Mahan John P. McGeehin 《Quaternary Research》2013
Sand hills in the Savannah River valley in Jasper County (South Carolina, USA) are interpreted as the remnants of parabolic eolian dunes composed of sand derived from the Savannah River and stabilized by vegetation under prevailing climate conditions. Optically stimulated luminescence ages reveal that most of the dunes were active ca. 40 to 19 ka ago, coincident with the last glacial maximum (LGM) through early deglaciation. Modern surface winds are not sufficient for sustained eolian sand transport. When the dunes were active, winds blew at velocities of at least 4 m/s from west to east, and some vegetation was present. The ratio of annual precipitation to potential evapotranspiration (P:PE) was less than the modern ratio of 1.23 and may have been < 0.30, caused by stronger winds (which would have resulted in greater evaporation) and/or reduced precipitation. The Savannah River dunes are part of a larger assemblage of eolian dunes that were active in the eastern United States during and immediately after the LGM, suggesting that eolian sediment behavior in this region has been controlled by regional forcing mechanisms during the Quaternary. 相似文献
960.
Natalie O. Parker Gregory H. Sambrook Smith Philip J. Ashworth James L. Best Stuart N. Lane Ian A. Lunt Christopher J. Simpson Roberte. Thomas 《Sedimentology》2013,60(3):820-839
This study uses digital elevation models and ground‐penetrating radar to quantify the relation between the surface morphodynamics and subsurface sedimentology in the sandy braided South Saskatchewan River, Canada. A unique aspect of the methodology is that both digital elevation model and ground‐penetrating radar data were collected from the same locations in 2004, 2005, 2006 and 2007, thus enabling the surface morphodynamics to be tied explicitly to the associated evolving depositional product. The occurrence of a large flood in 2005 also allowed the influence of discharge to be assessed with respect to the process–product relationship. The data demonstrate that the morphology of the study reach evolved even during modest discharges, but more extensive erosion was caused by the large flood. In addition, the study reach was dominated by compound bars before the flood, but switched to being dominated by unit bars during and after the flood. The extent to which the subsurface deposits (the ‘product’) were modified by the surface morphodynamics (the ‘process’) was quantified using the changes in radar‐facies recorded in sequential ground‐penetrating radar surveys. These surveys reveal that during the large flood there was an increase in the proportion of facies associated with bar margin accretion and larger dunes. In subsequent years, these facies became truncated and replaced with facies associated with smaller dune sets. This analysis shows that unit bars generally become truncated more laterally than vertically and, thus, they lose the high‐angle bar margin deposits and smaller scale bar‐top deposits. In general, the only fragments that remain of the unit bars are dune sets, thus making identification of the original unit barform problematic. This novel data set has implications for what may ultimately become preserved in the rock record. 相似文献