Tidal boundary conditions in SEAWAT

SEAWAT, a U.S. Geological Survey groundwater flow and transport code, is increasingly used to model the effects of tidal motion on coastal aquifers. Different options are available to simulate tidal boundaries but no guidelines exist nor have comparisons been made to identify the most effective approach. We test seven methods to simulate a sloping beach and a tidal flat. The ocean is represented in one of the three ways: directly using a high hydraulic conductivity (high-K) zone and indirect simulation via specified head boundaries using either the General Head Boundary (GHB) or the new Periodic Boundary Condition (PBC) package. All beach models simulate similar water fluxes across the upland boundary and across the sediment-water interface although the ratio of intertidal to subtidal flow is different at low tide. Simulating a seepage face results in larger intertidal fluxes and influences near-shore heads and salinity. Major differences in flow occur in the tidal flat simulations. Because SEAWAT does not simulate unsaturated flow the water table only rises via flow through the saturated zone. This results in delayed propagation of the rising tidal signal inland. Inundation of the tidal flat is delayed as is flow into the aquifer across the flat. This is severe in the high-K and PBC models but mild in the GHB models. Results indicate that any of the tidal boundary options are fine if the ocean-aquifer interface is steep. However, as the slope of that interface decreases, the high-K and PBC approaches perform poorly and the GHB boundary is preferable.     

Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium () substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, δ¹⁵N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a δ¹⁵N-Cu relationship that does not match the δ¹⁵N-Cu correlation found in low-strain rocks. This δ¹⁵N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH₃)₂²⁺, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same δ¹⁵N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system.We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10¹¹ g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ∼80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ∼9.6 × 10¹¹ g/yr. Assuming a constant flux of subducted N over the Earth’s history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.     

About the effect of relative humidity on the indentation response of Meuse/Haute�CMarne argillite

The unsaturated poromechanical behavior of Callovo-Oxfordian argillite (also referred to as Meuse/Haute–Marne argillite) is analyzed by means of indentation tests at different hygrometries. Numerical simulations carried out with a finite element code are then used to simulate flat punch indentation tests at different controlled hygrometries. The numerical response F(h) is compared to experimental data in order to estimate the drained Young’s modulus E and the internal friction angle \Upphi\Upphi (at high confinement) by means of a back analysis. A linear decrease in the drained Young’s modulus and quasi-constant values of the internal friction angle are observed when the relative humidity ranges. Some predictions of the model are also presented to quantify the activation of hydraulic phenomena. Indeed, the maximal variation of the saturation and porosity relative to the initial state of the specimen are lower than 10% and 8%, respectively.     

Freshwater seepages and ephemeral macroalgae proliferation in an intertidal bay: II. Effect on benthic biomass and metabolism

Intertidal soft-sediments biomass and metabolism are naturally heterogeneous in time and space at different scales. Particular perturbations such as freshwater seepages and seasonal proliferation of ephemeral macroalgae can intermittently and/or locally create additional variability in these systems. Since the impacts of such environmental stresses on natural processes are not well understood, the hypothesis that they would affect the functioning of the benthic system was tested. An intertidal bay whose structure and functioning has been previously described and where a carbon budget has been calculated, was chosen. The results showed that the metabolism of the intertidal sediments was greatly impacted by the above perturbations. Freshwater seepage increased meiofauna and microalgae biomasses and enhanced the total benthic metabolism (increasing community respiration and gross primary production until 4 and 2 fold respectively) without altering its seasonal trend. Ephemeral macroalgae proliferation had a more important effect on the total benthic metabolism, increasing community respiration and gross primary production 8 and 12 fold respectively and leading to a change in the seasonal trend.     

Cross-Spectra Over the Sea from Observations and Mesoscale Modelling

Cospectra and quadrature spectra are calculated for six pairs of tall offshore measurement masts near the Horns Rev I wind farm in the Danish North Sea and the Nysted wind farm in the Baltic sea. The mast-pairs are separated from one another by horizontal distances of 2.13–12.4 km. Cospectra and quadrature spectra for the two sites are classified in terms of the angle between the mean wind direction and the line connecting each pair of masts. The frequency axes of the spectra are normalized to remove the effect of mean wind speed and separation distance. Results indicate a larger contribution to the quadrature spectrum for flow from the sea than for flow from the land, and the patterns in the spectra are clearer and better defined for Horns Rev I (which has a long uninterrupted sea-fetch from the west) than for Nysted (which is surrounded by a more complicated coastline). The analysis is replicated based on 3-month simulations using the weather research and forecasting (WRF) numerical model with a horizontal grid spacing of 2 km. For the sea-fetch directions, good agreement in spectral properties between the model and observations is found. Analytical expressions based on the properties of the cross-correlation function and an exponentially decaying coherence function are fitted to the normalized cospectra and quadrature spectra. The expressions are shown to be a good fit to the spectra calculated from the WRF simulations and to the observed spectra for directions with a long sea-fetch, which suggests that to a good approximation, the average cospectra and quadrature spectra over the sea can be written as functions of frequency, mean wind speed, separation distance and the angle between the wind direction and the orientation of the masts.     

Transport of Snow by the Wind: A Comparison Between Observations in Ad��lie Land, Antarctica, and Simulations Made with the Regional Climate Model MAR

For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Adélie Land, Antarctica. A regional climate model featuring a coupled atmosphere/blowing snow/snowpack model is forced laterally by meteorological re-analyses. The vertical grid spacing was 2 m from 2 to 20 m above the surface and the horizontal grid spacing was 5?km. The simulation was validated by comparing the occurrence of blowing snow events and other meteorological parameters at two automatic weather stations. The Nash test allowed us to compute efficiencies of the simulation. The regional climate model simulated the observed wind speed with a positive efficiency (0.69). Wind speeds higher than 12 m s ^?1 were underestimated. Positive efficiency of the simulated wind speed was a prerequisite for validating the blowing snow model. Temperatures were simulated with a slightly negative efficiency (?0.16) due to overestimation of the amplitude of the diurnal cycle during one week, probably because the cloud cover was underestimated at that location during the period concerned. Snowfall events were correctly simulated by our model, as confirmed by field reports. Because observations suggested that our instrument (an acoustic sounder) tends to overestimate the blowing snow flux, data were not sufficiently accurate to allow the complete validation of snow drift values. However, the simulation of blowing snow occurrence was in good agreement with the observations made during the first 20 days of January 2010, despite the fact that the blowing snow flux may be underestimated by the regional climate model during pure blowing snow events. We found that blowing snow occurs in Adélie Land only when the 30-min wind speed value at 2 m a.g.l. is >10 m s ^?1. The validation for the last 10 days of January 2010 was less satisfactory because of complications introduced by surface melting and refreezing.     

Tropical shrimp trawl fisheries: Fishers’ knowledge of and attitudes about a doomed fishery

Tropical shrimp trawl fisheries are unsustainable, and similar sets of management measures are used globally to address the direct and indirect costs of their practices. Yet little is known about shrimp fishers’ perceptions, despite the clear importance of human behaviour in determining the success of fisheries management. This article presents the results of interviews with industrial shrimp trawl fishers from the southern Gulf of California, Mexico, and reveals fishers’ knowledge and attitudes that should be considered when developing management plans for industrial shrimp trawl fisheries. Fishers were asked to comment on problems facing the fishery, management options to address the issues, and the future of the fishery in general. The interviews also elicited new knowledge on effort and valuable components of bycatch, useful to the management process. Among the problems facing the Gulf of California fishery, fishers tended to identify those generated externally—fluctuations in shrimp populations, increases in fishing effort, decreases in shrimp prices and increasing overheads—and thus distance themselves from responsibility for management options. The successes of any mitigation measures for the fishery are likely to depend on proper enforcement and reliable governance, as our study indicates. Should strong enforcement be put in place, then trawl free areas seem to be the most pragmatic way to alleviate problems associated with the fishery; our effort data point to areas that might have greatest acceptance among fishers. A reduction in capacity would clearly complement marine zoning for trawl free areas. In the long run, however, it may be economic extinction of the fishery that reduces pressure on the marine ecosystem.     

Some Aspects of Diet and Foraging Behavior of Octopus dofleini Wülker, 1910 in its Northernmost Range

Abstract. Diet and foraging behavior of Octopus dofleini in Prince William Sound and Port Graham, Alaska, were studied from collections of den litter, measurements of octopuses and estimates of live prey abundance. Based on 193 den collections from depths of - 31 to + 1.3 m, a diet of hard-bodied prey could be identified. The five most common litter species made up 80% of the litter remains, with nearly 30 species of various taxa malung up the rest. The proportions of major hard-bodied prey species in each litter pile significantly differed with depth of den, associated substrata, presence of cobble and geographical location but not with the presence of boulders or outcrop, the density of kelp or season of litter collection. Octopus weight was not significantly correlated with the size of prey remains although O. dofleini may take a wide range of prey sizes once it reaches 2.5 kg. O. dofleini was more likely to be found at sites with crabs, although no litter species was significantly preferred over its estimated abundance, and one species was significantly avoided. These results suggest that O. dofleini includes suitable bivalve and crab species in its diet in relation to live abundance, while at the same time exercising an element of selectivity, as some species were actively avoided. Comparison of litter remains from the northern-most range of O. dofleini with those published for British Columbia show that diet breadth was similar, but the identity of major hard-bodied prey species was very different for Alaskan octopuses.     

Modulation of Intraseasonal (25–70 Day) Processes by theSuperimposed ENSO Cycle Across the Pacific Basin

Summary  Although variability is a fundamental aspect of the climate system, the interaction of different time scales of variability remains difficult to assess and verbalize. Two well-documented examples of tropical variability on different time scales are the El Ni?o/Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). Using the fifteen-year ECMWF Re-analysis (ERA) data archive and outgoing longwave radiation (OLR) data, some of the ENSO modulations of the intraseasonal MJO are examined. During El Ni˜o years, the MJO convection is able to propagate farther eastward into the central Pacific than it typically does during normal years. The corresponding intraseasonal circulation anomalies are similarly translated farther east. However, not all changes are this straight forward; for example, MJO convection is found to occur within westerly wind bursts during the Normal years, but somewhat ahead of the winds during El Ni˜o. ENSO-induced changes to the large-scale upper-tropospheric state provide different environments for outflow from MJO convection across the North Pacific. During the El Ni?o years, the eastward shift of the local Hadley circulation means that MJO convection must propagate farther east to reach an environment in which its meridional outflow can produce an appropriate Rossby wave source for the extension of the east Asian jet and subsequent midlatitude height falls. Received December 1, 1998