Consolidation and erosion of deposited cohesive sediments in Northern Chesapeake Bay,USA

Deposits of dredged cohesive sediments were monitored for changes in volume, bulk characteristics, and susceptibility to resuspension and erosion at disposal sites in Chesapeake Bay. There is a 23–48% volume reduction during the first six months, with correspondingly greater changes over longer time periods. A bulk density increase from 1.15 to 1.3 g/cm³ due to dewatering and compaction accounts for the majority of the volume change. Tidal current induced resuspension is a minor process. The observed suspended sediment load can be accounted for by erosion of only a fraction of a millimeter of sediment on each tidal cycle.     

Groundwater transport of crater-lake brine at Poa´s Volcano, Costa Rica

Poa´s Volcano is an active stratovolcano in Costa Rica that has a lake in its active crater. The crater lake has high temperatures (50–90 °C), high acidity (pH ≈ 0.0), and a high dissolved-solids content (100 g/kg). The volcano has numerous freshwater springs on its flanks, but a few on the northwestern flank are highly acidic (pH = 1.6–2.5) and have high dissolved-solids concentrations (2–22 g/kg). This study analyzes the regional groundwater system at Poa´s and demonstrates the likelihood that the water discharging from the acidic springs in the Rio Agrio watershed originates at the acidic crater lake. Both heat and solute transport are analyzed on a regional scale through numerical simulations using the HST3D finite-difference model, which solves the coupled equations for fluid flow, heat transport, and solute transport. The code allows fluid viscosity and density to be functions of both temperature and solute concentration. The simulations use estimates for recharge to the mountain and a range of values and various distributions of permeability and porosity. Several sensitivity analyses are performed to test how the uncertainty in many of the model parameters affects the simulation results. These uncertainties yield an estimated range of travel times from the crater lake to the Rio Agrio springs of 1–30 years, which is in close agreement with the results of tritium analyses of the springs. Calculated groundwater fluxes into and out of the crater lake are both about several hundred kg/s. These fluxes must be accounted for in water budgets of the crater lake.     

An annual fossil record of production,planktivory and piscivory during whole-lake manipulations

Annual and subannual paleolimnological records of pigments and zooplankton were used to analyze three whole-lake manipulations. The relative abundance of cladoceran remains in recent seasonal laminae was significantly correlated with the relative abundance of species in the plankton (r ²=0.59,P<0.001). Comparison of food-web structure after piscivore introduction showed that there are multiple outcomes of predation and that final food-web structure depended on the strength of interaction between piscivorous and planktivorous fish. Intense predation on cyprinids (Phoxinus eos, P. neogaeus, Umbra limi) by largemouth bass (Micropterus salmoides) allowed large herbivores (Daphnia pulex) and invertebrate predators (Chaoborus punctipennis) to dominate. Analysis of fossil invertebrate morphology suggested that small grazers (Bosmina longirostris, Diaphanosoma birgei) were eliminated by invertebrate predators. Under moderate predation by rainbow trout (Oncorhynchus mykiss), cyprinids remained, only intermediate-size herbivores (D. rosea) increased in abundance, andBosmina persisted. In contrast to food-web manipulations, increased algal abundance resulting from watershed disturbance (road construction) did not alter the species composition or size-structure of fossil Cladocera.     

The influence of episodic events on transport of striped bass eggs to the estuarine turbidity maximum nursery area

The estuarine turbidity maximum (ETM) is an important nursery area for anadromous fish where early-life stages can be retained in high prey concentrations and favorable salinities. Episodic freshwater flow and wind events could influence the transport of striped bass (Morone saxatilis) eggs to the ETM. This hypothesis was evaluated with regression analysis of observational data and with a coupled biological-physical model of a semi-idealized upper Chesapeake Bay driven by observed wind and freshwater flow. A particle-tracking model was constructed within a numerical circulation model (Princeton Ocean Model) to simulate the transport of fish eggs in a 3-dimensional flow field. Particles with the sinking speed of striped bass eggs were released up-estuary of the salt front in both 2-d event-scale and 60-d seasonal-scale scenarios. In event scenarios, egg-like particles with observed specific gravities (densities) of striped bass eggs were transported to the optimum ETM nursery area after 2 d, the striped bass egg-stage duration. Wind events and pulses in river discharge decreased the number of egg-like particles transported to the ETM area by 20.9% and 13.2%, respectively, compared to nonevent conditions. In seasonal scenarios, particle delivery to the ETM depended upon the timing of the release of egg-like particles. The number of particles transported to the ETM area decreased when particles were released before and during wind and river pulse events. Particle delivery to the ETM area was enhanced when the salt front was moving up-estuary after river pulse events and as base river flow receded over the spawning season. Model results suggest that the timing of striped bass spawning in relation to pulsed events may have a negative (before or during events) or positive (after river flow events) effect on egg transport. Spawning after river flow events may promote early-stage survival by taking advantage of improved transport, enhanced turbidity refuge, and elevated prey production that may occur after river pulse events. In multiple regression analysis of observed data, mean spring freshwater flow rates and the number of pulsed freshwater flow events during the striped bass spawning season explained 71% of the variability in striped bass juvenile abundance in upper Chesapeake Bay from 1986 to 2002. Positive parameter estimates for these effects support the hypothesis that pulsed freshwater flow events, coupled with spawning after the events, may enhance striped bass early-stage survival. Results suggest that episodic events may have an important role in controlling fish recruitment.     

Analysis of a vertical dipole tracer test in highly fractured rock

The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only -25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 microm) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (approximately 50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters.     

Sm-Nd dating of Seve eclogites,Norrbotten, Sweden — Evidence for early Caledonian (505 Ma) subduction

The Seve Nappes consist of long-transported thrust sheets belonging to the Upper Allochthon within the Scandinavian Caledonides. Eclogites from two different megalenses within the Seve Nappe Complex in Norrbotten (northern Sweden) have been dated with the Sm-Nd method. The two eclogite-bearing megalenses have been subjected to different pressure and temperature conditions during the high-pressure metamorphism. Garnet + omphacite + whole-rock from TS2 within the Tsäkkok Lens give an age of 505±18 Ma (2), I=0.512231±0.000024, MSWD=0.10. Garnet and whole-rock from TS5 and whole-rock TS3 also fall on the above isochron. The composite isochron gives an age of 503±18 Ma (2). These results are interpreted to date the peak of the high-pressure metamorphism (500–630° C and 12–15 kbars) for the Tsäkkok Lens. The eclogites in the Vaimok Lens (at Grapesvarre) were subjected to higher pressure and temperature conditions and more extensive reequilibration during the early cooling stages. Retrograde breakdown-reactions accompanied by retrograde zoning of relict garnet seems to be associated with disturbance of the Sm-Nd isotopic systems. In contrast, a sample with unzoned garnet and well preserved high-pressure mineralogy from Grapesvarre gives a Sm-Nd garnet + omphacite age of 503±14 Ma (2), I = 0.512010±0.000038 (2). The ages for the Seve eclogites are significantly older than the Sm-Nd eclogite dates from the Western Gneiss Region of Norway (WGR), suggesting the existence of at least two eclogite-forming events in the Caledonide Orogen. The younger event has been related to the main continent-continent collision stage of the Caledonian Orogeny, while the older event that led to the production of the Norrbotten eclogites must have taken place several hundred kilometres to the north and in a different tectonic setting more oceanward to the WGR. It appears that the older event (ca. 505 Ma) was restricted to the subduction of dyke-intruded sedimentary cover rocks which are thought to represent the rifted edge of the Baltic continent.     

Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±högbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region.

The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone (P>8–10 kbar) to the sillimanite–cordierite zone (P<5 kbar) at T750 °C, with maximum recorded temperatures of 800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±högbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks.

Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.     

Wild horses and the political ecology of nature restoration in the Missouri Ozarks

From 1990 to 1996, the National Park Service and residents living near the Ozark National Scenic Riverways in south-central Missouri clashed over the federal agency’s intention to remove 25-30 wild horses from the protected area. The struggle was carried out in various legal and legislative arenas, the media, and in community protests and meetings. The dispute ended only with Congressional approval of the 1996 Omnibus Parks and Public Lands Management Act, which included an amendment ordering an end to any removal efforts.This article focuses on the contested social constructions of the horses themselves. To government scientists and managers, the animals represented a feral and exotic species with no legitimate place in agency-mandated ecosystem management and restoration scenarios. To many local members of the Missouri Wild Horse League, which contested the removal, the horses had critical historical and cultural importance as icons of regional identity, history and personal experience, and as core symbols of communities increasingly politically and economically marginalized.Local disputes with environmental groups and agencies concerned with Ozark ecosystem preservation and restoration have become more pronounced and numerous over the past two decades. This article approaches citizen opposition to environmental agendas not as an anti-environmental movement, but as a contemporary effort of marginalized groups to identify sources of economic, political, and social loss, and symbols of local identity and power. The wild horse issue reveals wider structural divides, and thus speaks to the question of which social groups shall have the power to impose their visions of the landscape and political economy.