Stratigraphic control of flow and transport characteristics

Ground water flow and travel time are dependent on stratigraphic architecture, which is governed by competing processes that control the spatial and temporal distribution of accommodation and sediment supply. Accommodation is the amount of space in which sediment may accumulate as defined by the difference between the energy gradient and the topographic surface. The temporal and spatial distribution of accommodation is affected by processes that change the distribution of energy (e.g., sea level or subsidence). Fluvial stratigraphic units, generated by FLUVSIM (a stratigraphic simulator based on accommodation and sediment supply), with varying magnitudes and causes of accommodation, were incorporated into a hydraulic regime using MODFLOW (a ground water flow simulator), and particles were tracked using MODPATH (a particle-tracking algorithm). These experiments illustrate that the dominant type of accommodation process influences the degree of continuity of stratigraphic units and thus affects ground water flow and transport. When the hydraulic gradient is parallel to the axis of the fluvial system in the depositional environment, shorter travel times occur in low-total accommodation environments and longer travel times in high-total accommodation environments. Given the same total accommodation, travel times are longer when sea-level change is the dominant process than those in systems dominated by subsidence.     

Large-Eddy Simulation Of The Stably Stratified Planetary Boundary Layer

In this work, we study the characteristics of a stably stratifiedatmospheric boundary layer using large-eddy simulation (LES).In order to simulate the stable planetary boundary layer, wedeveloped a modified version of the two-part subgrid-scalemodel of Sullivan et al. This improved version of themodel is used to simulate a highly cooled yet fairly windy stableboundary layer with a surface heat flux of(W)_o = -0.05 m K s^-1and a geostrophic wind speed of U_g = 15 m s^-1.Flow visualization and evaluation of the turbulencestatistics from this case reveal the development ofa continuously turbulent boundary layer with small-scalestructures. The stability of the boundary layercoupled with the presence of a strong capping inversionresults in the development of a dominant gravity wave atthe top of the stable boundary layer that appears to be relatedto the most unstable wave predicted by the Taylor–Goldsteinequation. As a result of the decay of turbulence aloft,a strong-low level jet forms above the boundary layer.The time dependent behaviour of the jet is compared with Blackadar'sinertial oscillation analysis.     

The impact of yellow shore crabs,<Emphasis Type="Italic">Hemigrapsus oregonensis</Emphasis>, on Early Benthic Phase dungeness crabs,<Emphasis Type="Italic">Cancer magister</Emphasis>, in intertidal oyster shell mitigation habitat

Dungeness crab (Cancer magister) megalopae recruit to northeastern Pacific coastal estuaries, and settle into intertidal and subtidal habitats where they molt into Early Benthic Phase (EBP) crabs, and are dependent on epibenthic structure for shelter from predation. Given the importance of shell refuge to their post-settlement ontogeny, the U.S. Army Corps of Engineers began constructing intertidal plots of oyster shell in the Grays Harbor estuary, Washington, to enhance recruitment and mitigate losses of subtidalC. magister entrained and killed during extensive dredging efforts. When shell habitat was newly constructed, settlement and survival ofC. magister were high, and expectations for the mitigation project were met. During the first several years (1992-1997), plots greater than 1 yr postconstruction were colonized by yellow shore crabs,Hemigrapsus oregonensis (often≥75 crabs m⁻², and abundance of EBPC. magister was reduced to nearly zero. While some predation of settling megalopae byH. oregonensis does occur, the relationship between these species is characterized by density-dependent competitive interactions. Laboratory observations of competition for shell habitat indicate thatH. oregonensis are dominant over EBPC. magister and can evictC. magister from refuge spaces. Field experiments show that high densities of the former cause, the latter to emigrate from shell, and suggest detection and avoidance of areas with high densities ofH. oregonensis by settlingC. magister megalopae. More recently (1998-2001), abundance ofH. oregonensis has declined dramatically within plots of oyster shell, apparently due to recruitment failure, and patterns ofC. magister abundance and production have returned to levels consistent with original expectations of the mitigation project. Both intraspecific and interspecific competition for space are significant factors effecting population, regulation ofC. magister when they are strongly dependent on refuge from predation. Efficacy of constructed oyster shell plots as a mitigation tool may hinge on the population dynamics of a species previously considered of little consequence to the target species.     

A fluid inclusion study of vug minerals in dawsonite-altered phonolite sills,Montreal, Quebec: implications for HFSE mobility

Ordovician limestones in the Francon quarr on the island of Montreal, Quebec, are host to three sills of Cretaceous age composed of phonolite that has been extensively altered to dawsonite. An interesting feature of the sills is the presence of abundant vugs containing a wide variety of minerals, including several in which one or more high field strength elements (Zr, Hf, Nb, Ti) is a major component. The most important of these latter minerals is weloganite, a rare strontium zirconium hydrous carbonate, first identified in the Francon, quarry. Four types of inclusions have been recognized in vug minerals: aqueous, aqueous-carbonic, carbonic and solidbearing. Aqueous inclusions homogenize at temperatures mainly between 70° and 170°C and between 230° and 390°C. The homogenization temperatures of primary inclusions cluster around 350°C. Aqueous inclusions and the aqueous phase in aqueous-carbonic inclusions have salinities ranging between 10 and 24 eq.wt.% NaCl equivalent. Primary aqueous-carbonic inclusions have low XCO₂ (<0.03), whereas secondary aqueous-carbonic inclusions can have high XCO₂ (>0.7); carbonic inclusions are all secondary. Nahcolite, dawsonite and weloganite occur as daughter minerals or trapped solids. Nahcolite and possibly natron or mirabilite appear to form in frozen inclusions. Analyses of fluid inclusion decrepitates detected high concentrations of Na, Cl, Al, S, and C. The extraordinarily high concentration of Al in the fluid (possibly exceeding 1 wt.%) suggests a pH of approximately 10. Pressure and temperature conditions, estimated from stratigraphic reconstruction and the isochores of primary aqueous fluid inclusions, were 450 bar and 360 to 400°C, respectively. The relatively high temperatures and compositions of primary fluid inclusions suggest that vug filling was the result of mineral precipitation from an orthomagmatic fluid. A model is proposed in which a partially crystallized phonolite melt started exsolving a homogeneous low XCO₂ fluid immediately prior to or after intrusion. Sodium, aluminium, chlorine, fluorine, sulphur and HFSE elements such as Zr, Hf, Nb and Ti were partitioned into the hydrous phase, in the case of Zr, possibly to a concentration of 300 ppm. The near horizontal orientation of the sills and the chilled margins, produced by quenching of the magma, created a tight seal that inhibited escape of the fluids. As a result, the phonolite stewed in its own juices long after crystallization, giving rise to widespread replacement of primary igneous minerals by dawsonite, and precipitation of this and other minerals in vugs. Once the sills had colled to temperatures between 200 and 300°C, the aqueous fluid exsolved a high CO₂ fluid which was trapped as the secondary three-phase type II and type III inclusions. Decreasing temperature is considered to have been the principal control of mineralization, although in the case of the lower temperature minerals, decreased bicarbonate or carbonate ion activity, and a lower dielectric constant, as a result of CO₂ exsolution, may have played a role in the deposition of HFSE-bearing minerals.     

Factors influencing the magnitude of phytoplankton primary production in a high-energy surf zone

The main factors influencing phytoplankton primary production in the surf zone of the Sundays River Beach, Algoa Bay have been characterized. These factors include cell concentration, chlorophyll concentration, irradiance, temperature and salinity. Good relationships have been obtained between cell concentration, chlorophyll concentration and primary production. The P-I curves showed dependence on temperature with a linear regression between temperature and I_k values. Light saturation was shown to occur between 300 and 510 μmol m⁻² s⁻¹ at normal field temperatures. T_max and T_min were found to be 34°C and 0°C, respectively; P_max was 25°C. Salinity had a marked effect on primary production with S_max occurring at 60 ppt and an extrapolated S_min at 0 ppt. P_max was found to occur at 30 ppt.