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81.
Rainfall, slopewash (the erosion of soil particles), surface runoff and fine-litter transport steepland sites in the Luquillo Experimental Forest, Puerto Rico (18° 20’ N, 65° 45’ W) were measured from 1991 to 1995. Hillslopes underlain by (1) Cretaceous tuffaceous sandstone and silstone in subtropical rain (tanonuco) forest with vegetation recovering from Hurricane Hugo (1989), and (2) Tertiary quartz diorite in subtropical lower mantone wet (colorado and dwarf) forest with undisturbed forest canopy were compared to recent landslide scars. Monthly surface runoff on these very steep hillslopes (24° to 43°) was only 0·2 to 0·5 per cent of monthly rainfall. Slopewash was higher in sandy loam soils whose parent material is quartz diorite (averaging 46 g m−2 a−1) than in silty clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9 g m−2 a−1. Annual slopewash of 100 to 349 g m−2 on the surfaces of two recent, small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m−2 a−1 by the end of the study. The mean annual mass of fine litter (mainly leaves and twigs) transported downslope at the forested sites ranged from 5 to 8 g m−2 and was lower at the tabonuco forest site, where post-Hurricane Hugo recovery is still in progress. Mean annual fine-litter transport was 2·5 g m−2 on the two landslide scars. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
82.
The Plio-Pleistocene wedge off mid Norway is characterized by a pronounced increase in sonic velocities versus depth. At its base, a velocity inversion of 0.55 km s−1 exists with respect to the underlying unit. We propose that the inversion may be a result of the stage margin evolution, in which both the pre-depositional physiography of the margin and the depositional history of the wedge itself have been key elements. The glaciation of the uplifted mainland and the cyclic advancement of glaciers over the shelf led to the formation of the Plio-Pleistocene prograding megasequence. Underlying sequences may have been overpressured due to the rapid deposition of the wedge and because shales at the base of the wedge, deposited in a distal deltaic environment, represented a seal for water drainage. Overpressure provides sediment instability, and is an attractive mechanism to explain intra-sequence deformation and partly lateral mass movements. 相似文献
83.
W. R. Boynton J. D. Hagy J. C. Cornwell W. M. Kemp S. M. Greene M. S. Owens J. E. Baker R. K. Larsen 《Estuaries and Coasts》2008,31(4):623-651
Multi-year nitrogen (N) and phosphorus (P) budgets were developed for the Patuxent River estuary, a seasonally stratified
and moderately eutrophic tributary of Chesapeake Bay. Major inputs (point, diffuse, septic, and direct atmospheric) were measured
for 13 years during which, large reductions in P and then lesser reductions in N-loading occurred due to wastewater treatment
plant improvements. Internal nutrient losses (denitrification and long-term burial of particulate N and P) were measured in
tidal marshes and sub-tidal sediments throughout the estuary as were nutrient storage in the water column, sediments, and
biota. Nutrient transport between the oligohaline and mesohaline zones and between the Patuxent and Chesapeake Bay was estimated
using a salt and water balance model. Several major nutrient recycling terms were directly and indirectly evaluated and compared
to new N and P inputs on seasonal and annual time-scales. Major findings included: (1) average terrestrial and atmospheric
inputs of N and P were very close to the sum of internal losses plus export, suggesting that dominant processes are captured
in these budgets; (2) both N and P export were a small fraction (13% and 28%, respectively) of inputs, about half of that
expected for N based on water residence times, and almost all exported N and P were in organic forms; (3) the tidal marsh-oligohaline
estuary, which by area comprised ~27% of the full estuarine system, removed about 46% and 74% of total annual upland N and
P inputs, respectively; (4) recycled N and P were much larger sources of inorganic nutrients than new inputs during warm seasons
and were similar in magnitude even during cold seasons; (5) there was clear evidence that major estuarine processes responded
rapidly to inter-annual nutrient input variations; (6) historical nutrient input data and nutrient budget data from drought
periods indicated that diffuse nutrient sources were dominant and that N loads need to be reduced by about 50% to restore
water quality conditions to pre-eutrophic levels. 相似文献
84.
Søren Jessen Flemming Larsen Dieke Postma Pham Hung Viet Nguyen Thi Ha Pham Quy Nhan Dang Duc Nhan Mai Thanh Duc Nguyen Thi Minh Hue Trieu Duc Huy Tran Thi Luu Dang Hoang Ha Rasmus Jakobsen 《Applied Geochemistry》2008
To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta’s Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe2+, H2S, NH4, CH4, δ18O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15–20 km wide and 50–60 m deep Holocene incised valley. The NE part of the transect consists of 60–120 m of Pleistocene yellowish alluvial deposits underneath 10–30 m of estuarine clay overlain by a 10–20 m veneer of Holocene sediments. The distribution of δ18O-values (range −12.2‰ to −6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe2+ (0.03–2.0 mM), Mn (0.7–320 μM), SO4 (<2.1 μM–0.75 mM), H2S (<0.1–7.0 μM), NH4 (0.03–4.4 mM), and CH4 (0.08–14.5 mM). Generally, higher concentrations of NH4 and CH4 and low concentrations of SO4 were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013–11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH4, CH4 and SO4. Low concentrations of As (?0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe2+ and H2S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19–65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO4 was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration. 相似文献
85.
Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska 总被引:1,自引:0,他引:1
Finney Benjamin; Turner Simon; Hawkesworth Chris; Larsen Jessica; Nye Chris; George Rhiannon; Bindeman Ilya; Eichelberger John 《Journal of Petrology》2008,49(5):857-884
Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km3) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H2O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO2 with 0·8–3·3wt % K2O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (18O 4·4–4·9,87Sr/ 86Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(230Th/ 238U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc 相似文献
86.
R. I. McLean J. K. Summers C. R. Olsen S. L. Domotor I. L. Larsen H. Wilson 《Estuaries and Coasts》1991,14(2):148-156
The Susquehanna River is the major contributor to sediment loadings in the Chesapeake Bay. Because many environmental contaminants are associated with suspended particulates, the degree of particle retention within the reservoirs of the lower Susquehanna River is an important consideration in evaluating contaminant loadings to the Chesapeake Bay. Profiles of weapons-test Cs-137, nuclear power plant-related Cs-134 and Cs-137, and naturally-derived Pb-120 were used to estimate rates of sediment accretion in the conowingo Reservioir, an impoundment of the Susquehanna River along the Maryland-Pennsylvania border. Net accretion rates ranged from about 2 cm yr?1 downstream of a nuclear power plant cooling discharge to a high of about 7 cm yr?1 at the mount of an incoming creek. Slight, but consistent, increases in the annual rate of accretion since the creation of the reservoir in 1928 are apparent. The current net average annual sediment load reatined by the reservoir is estimated to be 0.4×106 to 1.5 × 106 metric tons yr?1. The retained sediment load represents about 8–23% of the long-time average sediment input to the reservoir. 相似文献
87.
88.
Mixed-ethnicity partnerships are becoming increasingly common in Australia and other countries of high immigration. Formal and de facto marriages involving partners from different ethnic backgrounds are key indicators of decreasing social distance between groups. Yet mixed-ethnicity couples have received scant attention from Australian geographers. We use customised data from the 2011 Australian census to analyse the nation-wide distribution of several types of mixed-ethnicity couples. We focus on couples comprised of an Anglo-European (ethnic majority) partner, and a partner from a ‘visible’ ethnic minority group. Our analyses explore the residential geographies of mixed-ethnicity couples vis-à-vis ‘co-ethnic couples’ (where partners share the same ethnicity). We find that mixed-ethnicity couples are more widely dispersed across Australian cities and regions than comparative co-ethnic couples. However, each type of mixed-ethnicity couple has its own unique residential pattern: there are multiple geographies of mixed-ethnicity couples in Australia. These distinctive patterns reflect the migration and settlement histories of the couples’ constituent ethnic groups, but also hold great potential to shift seemingly entrenched ethnic residential geographies in the present and future. 相似文献
89.
90.
A wide variety of substances, including known or potential pollutants, become associated with particles in coastal marine environments. This association may result from: (1) ion exchange, precipitation, or hydrophobic interactions with the particle surface, (2) co-precipitation with iron and manganese hydrous oxide coatings, (3) complexation with organic substances bound or aggregated with particles, (4) incorporation into mineral lattices, organisms or fecal material, or (5) flocculation of colloidal organic and inorganic matter during river and sea water mixing. The transport, accumulation and fate of such chemically-reactive pollutants in coastal marine environments are governed to a great extent by particle dynamics. Understanding of the mechanisms and processes affecting pollutant-particle associations and dynamics allows the prediction of the fate of a wide variety of pollutants in estuarine, coastal and marine environments. 相似文献