Accelerated relative sea-level rise and rapid coastal erosion:: testing a causal relationship for the Louisiana barrier islands

The role of relative sea-level rise as a cause for the rapid erosion of Louisiana's barrier island coast is investigated through a numerical implementation of a modified Bruun rule that accounts for the low percentage of sand-sized sediment in the eroding Louisiana shoreface. Shore-normal profiles from 150 km of coastline west of the Mississippi delta are derived from bathymetric surveys conducted during the 1880s, 1930s and 1980s. An RMS difference criterion is employed to test whether an equilibrium profile form is maintained between survey years. Only about half the studied profiles meet the equilibrium criterion; this represents a significant limitation on the potential applicability of the Bruun rule. The profiles meeting the equilibrium criterion, along with measured rates of relative sea-level rise, are used to hindcast shoreline retreat rates at 37 locations within the study area. Modeled and observed shoreline retreat rates show no significant correlation. Thus, in terms of the Bruun approach, relative sea-level rise has no power for hindcasting (and presumably forecasting) rates of coastal erosion for the Louisiana barrier islands.     

Contact Partial Melting of Granitic Country Rock, Melt Segregation, and Re-injection as Dikes into Ferrar Dolerite Sills, McMurdo Dry Valleys, Antarctica

Numerous, interconnected, granitic dikes (<30 cm in widthand hundeds of meters in length) cut Ferrar dolerite sills ofthe McMurdo Dry Valleys, Antarctica. The source of the graniticdikes is partial melting of granitic country rock, which tookplace in the crust at a depth of about 2–3 km adjacentto contacts with dolerite sills. Sustained flow of doleriticmagma through the sill generated a partial melting front thatpropagated into the granitic country rock. Granitic partialmelts segregated and collected at the contact in a melt-rich,nearly crystal-free reservoir adjacent to the initial doleritechilled margin. This dolerite chilled margin was subsequentlyfractured open in the fashion of a trapdoor by the graniticmelt, evacuating the reservoir to form an extensive complexof granitic dikes within the dolerite sills. At the time ofdike injection the dolerite was nearly solidified. Unusuallycomplete exposures allow the full physical and chemical processesof partial melting, segregation, and dike formation to be examinedin great detail. The compositions of the granitic dikes andthe textures of partially melted granitic wall rock suggestthat partial melting was characterized by disequilibrium mineraldissolution of dominantly quartz and alkali feldspar ratherthan by equilibrium melting. It is also unlikely that meltingoccurred under water-saturated conditions. The protolith granitecontains only 7 vol.% biotite and estimated contact temperaturesof 900–950°C suggest that melting was possible ina dry system. Granite partial melting, under closed conditions,extended tens of meters away from the dolerite sill, yet meltsegregation occurred only over less than one-half a meter fromthe dolerite chilled margin where the degree of partial meltingwas of the order of 50 vol.%. This segregation distance is consistentwith calculated length scales expected in a compaction-drivenprocess. We suggest that the driving force for compaction wasdifferential stress generated by a combination of volume expansionas a result of granite partial melting, contraction during doleritesolidification, and relaxation of the overpressure driving doleriteemplacement. On a purely chemical basis, the extent of meltsegregation necessary under fractional and batch melting tomatch the Rb concentrations between melt and parent rock isa maximum of 48 and 83 vol.% melt, respectively. KEY WORDS: Antarctica; dike injection; disequilibrium; granite partial melting; silicic melt segregation     

On melting of the subducted oceanic crust: Effects of subduction induced mantle flow

Based on petrological and geochemical arguments, it is possible that arc magma is derived from subducted oceanic crust. In this paper, regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth (i.e. at depths of about 100 km) by a dynamic mantle. Calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab. This current continuously feeds hot mantle material into the corner and onto the slab surface. A high temperature thermal environment can be maintained in the vicinity of the wedge corner, immediately beneath the over-riding plate. Our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner. Additional geological processes such as reasonable amounts of shear heating and minor dehydration (which will lower the local melting temperature) will further increase the probability of melting a cold subducted oceanic crust at shallow depth.     

Baseline levels of benzo(a)pyrene in southern California mussels

Marine mussels accumulate the carcinogen benzo(a)pyrene from contaminated environments. Baseline studies in California indicate that levels of the carcinogen in mussels are at or near zero, except in areas of human activity. This finding runs counter to previous suggestions that benzo(a)pyrene is widely distributed in marine organisms.     

Influence of landscape restoration and degradation on storm surge and waves in southern Louisiana

The purpose of this investigation was to examine storm surge and wave reduction benefits of different environmental restoration features (marsh restoration and barrier island changes), as well as the impact of future wetland degradation on local surge and wave conditions. Storm surge simulations of two representative hurricanes were performed using the ADCIRC storm surge model with the inclusion of radiation stress gradients from the STWAVE nearshore wave model. Coupled model simulations were made for a number of landscape configurations that involved both restored and degraded wetland features. The impact of barrier island condition on hurricane surge and waves was also evaluated. Effects of landscape features were represented by changes in elevation and frictional resistance. Restoration and degradation of marsh resulted in decreases (for restoration cases) and increases (for degradation cases) in both surge and waves. The magnitude of change was correlated with the magnitude of the horizontal extent and elevation changes in the marsh. In general, the wave change patterns are consistent with the water level changes. Deflation of the Chandeleur Islands (barrier island chain) resulted in slightly increased surge. Results suggest that coastal marsh does have surge and wave reduction potential. Results also indicate that the impact of the landscape features is amplified in areas where there are levee “pockets.” Barrier islands and coastal ridges reduce wave heights, even if in a degraded condition and thus can reduce wave energy in wetland areas, protecting them from erosion.     

Estimating gambling venue catchments for impact assessment using a calibrated gravity model

Gambling using electronic gaming machines (EGMs) has emerged as a significant public health issue. While social impact assessments are required prior to the granting of new gaming machine licenses in Australia, there are a few established techniques for estimating the spatial distribution of a venue’s clientele. To this end, we calibrated a Huff model of gambling venue catchments based on a geocoded postal survey (n = 7040). We investigated the impact of different venue attractiveness measures, distance measures, distance decay functions, levels of spatial aggregation and venue types on model fit and results. We then compared model estimates for different behavioural subgroups. Our calibrated spatial model is a significant improvement on previously published models, increasing R² from 0.23 to 0.64. Venue catchments differ radically in size and intensity. As different population subgroups are attracted to different venues, there is no single best index of venue attractiveness applicable to all subpopulations. The calibrated Huff model represents a useful regulatory tool for predicting the extent and composition of gambling venue catchments. It may assist in decision-making with regard to new license applications and evaluating the impact of health interventions such as mandated reductions in EGM numbers. Our calibrated parameters may be used to improve model accuracy in other jurisdictions.     

Bidirectional Reflectance Spectroscopy: 5. The Coherent Backscatter Opposition Effect and Anisotropic Scattering

A model published previously by the author that describes light scattering from particulate media is modified to include several improvements: (1) a better approximation to the Ambartsumian-Chandrasekhar H-functions that is especially important for particles with single scattering albedos close to 1.00, (2) increased accuracy for anisotropically scattering particles, and (3) incorporation of coherent backscattering. The goal of the original model of being analytic and mathematically tractable is preserved. No new parameters are introduced by the first and second modifications; however, the third unavoidably adds two new parameters: the amplitude and width of the coherent backscatter opposition effect. Several examples are given in which the results of calculations using the original and new models are compared with exact numerical computations.It is shown that a medium composed of complex particles that are large compared to the wavelength can have a coherent backscatter opposition effect (CBOE) that is broad enough to be readily observable. The CBOE multiplies the entire reflectance and not just the multiply scattered component, so that a low-albedo medium, such as lunar regolith, can have a strong CBOE.     

Reduced rates of primary production in the Columbia River Estuary following the eruption of Mt Saint Helens on 18 May 1980

The eruption of Mt Saint Helens on 18 May 1980 resulted in a massive increase in suspended particulate material in the Columbia River Estuary, producing a substantial increase in light attenuation. Since photosynthesis in the estuarine water column is partly controlled by the depth of light penetration in the water, photosynthesis was reduced by about 75% during the period of increased turbidity. It took about five weeks for the estuary to clear. Although primary production within the estuarine water column was greatly diminished during this period, the flux of particulate carbon through the estuary was high, and the total production we estimate to have been lost amounted to only about 2% of the total particulate carbon flux just after the eruption. That the high levels of turbidity in the estuary did not induce a severe fall in the phytoplankton population is evidence that phytoplankton biomass concentrations in the Columbia River Estuary are mostly a function of import from the Columbia River, rather than a function of in situ production.     

Amplification of vertically propagating SH waves by multiple layers of Gibson soils

Using the Haskell–Thomson transfer matrix approach, an analytical solution is obtained for SH wave amplification by multiple layers of Gibson soils (i.e. viscoelastic layers with linearly varying shear moduli). Amplification spectra for typical soil and basement rock properties are calculated. A comparison of the Gibson soil response with that obtained for homogeneous soil models shows generally stronger amplifications associated with the Gibson soil.