Differential reproductive strategies of two bivalves in the Dutch Wadden Sea

Cerastoderma edule and Mya arenaria are two common bivalve species in European waters. Longevity and maximum size are much greater in the latter species. Because comparison of species life-history strategies states that a long life span (i.e. high annual survival) generally goes with lower fecundity, we hypothesise that reproductive output would be lower in M. arenaria than in C. edule. In the present paper, we studied the reproductive strategies of these two species in an intertidal and a subtidal area of the western Dutch Wadden Sea, by following seasonal changes in absolute and relative weights of somatic and gonadal tissues in these bivalves. Starting of spawning was similar in the two species, around May, except for intertidal M. arenaria, which initiated spawning in August. Individual energy investment in reproduction was similar for the two species but, unlike M. arenaria, C. edule spawned completely, releasing all energy of gonadal mass in the form of gametes. Mya arenaria used the gonad not only for reproduction but also for storage. In the intertidal area, we found a trade-off between longevity and reproduction, i.e. maximum reproductive output (expressed as a proportion of body mass) was higher in C. edule than in M. arenaria. However, since body size is larger and life span longer in M. arenaria than in C. edule, mean lifetime reproductive output per individual must be higher in the first than in the latter. Based on the differences in reproductive strategies of these two species, we hypothesise that the negative effects of warming climate on bivalve population dynamics in the Wadden Sea will be stronger in C. edule than in M. arenaria.     

The new wave overtopping database for coastal structures

Significant effort has been made to generate a homogeneous database on wave overtopping consisting of more than 10,000 irregular wave overtopping tests from more than 160 independent projects or test series, each described by means of 31 parameters. Many coastal structures, including dikes, rubble mound breakwaters, berm breakwaters, caisson structures and combinations have been considered and have been schematised for inclusion in the database. All these overtopping tests are represented by over 300,000 numbers in the database.     

Influence of discrete step size on wind setup component of storm surge

Open coast storm surge water levels consist of wind setup due to wind shear at the water surface; a wave setup component caused by wind induced waves transferring momentum to the water column; an atmospheric pressure head component due to the atmospheric pressure deficit over the spatial extent of the storm system; a Coriolis forced setup or setdown component due to the effects of the rotation of the earth acting on the wind driven alongshore current at the coast; a possible seiche component due to resonance effects initiated by moving wind system, and, if astronomical tides are present, an astronomical tide component (although the tide is typically considered to be a forced astronomical event and not really a direct part of the external wind-driven meteorological component of storm surge). Typically the most important component of a storm surge is the wind setup component, especially on the U.S. East Coast and the Gulf of Mexico shorelines. In many approaches to storm surge modeling, a constant depth approximation is invoked over a limited step size in the computational domain. The use of a constant depth approximation has received little attention in the literature although can be very important to the resulting magnitude of the computed storm surge. The importance of discrete step size to the wind setup storm surge component is considered herein with a simple case computation of the wind setup component on a linear slope offshore profile. The present study findings show that the constant depth approximation to wind setup storm surge estimation is biased on the low side (except in extremely shallow water depths) and can provide large errors if discrete step size is not sufficiently resolved. Guidance has been provided on the error that one might encounter for various step sizes on different slopes.     

Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane

Oscillatory zoning in low δ¹⁸O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (X_Adr = 0.13–0.36). The δ¹⁸O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X_Adr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ¹⁸O indicate that a high Fe³⁺/Al, high δ¹⁸O fluid mixed with a lower Fe³⁺/Al and δ¹⁸O fluid. The high δ¹⁸O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ¹⁸O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.     

Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and Pressure

Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027–1427 °C, confining pressures of 2.5–3.5 GPa, and strain rates of 1 × 10^?5 s^?1 to 5 × 10^?4 s^?1. We obtained a power‐law creep for the high temperature and pressure deformation of a ‘dry’ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 10^{3.3 ± 1.0}, n = 3.5 ± 0.4, ΔE =403 ± 30 KJ mol^?1 and ΔV = 27.2 cm³ mol^?1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back‐scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear‐strain conditions, yielding a random LPO similar to that of non‐deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.     

Sio Guyot: A complex volcanic edifice in the western Mid-Pacific Mountains

Sio Guyot, in the westernmost edge of the Mid-Pacific Mountains, is a large, complex volcanic edifice rising to more than 1200 m below sea level. The summit is divided into two flat-topped areas by a WNW-trending sediment-filled trough. Seismic reflection profiles reveal three acoustic units: an upper transparent layer (pelagic cap), a lower opaque layer (reef- and lagoon-derived sediments), and an acoustic (volcanic) basement. Free-air gravity anomalies indicate three eruptive centers or conduits within the main edifice, which apparently was constructed during late Cretaceous time on a broad basement swell or plateau that today is more than 3400 m below sea level (1500 m above regional abyssal depths).     

Extreme value estimates for arbitrary bandwidth Gaussian processes using the analytic envelope

A procedure is presented for the estimation of extreme values of stationary Gaussian random processes with arbitrary bandwidths. This approach is based on the analytic envelope defined by the Hilbert Transform; this envelope is Rayleigh distributed regardless of bandwidth. For experimentally derived data that has been converted into digital form, the Hilbert Transform is approximated using algorithms implemented on a digital computer to produce samples of the envelope's time history. Next, the degree of correlation between these envelope samples is taken into account using a method developed from simulation studies of a series of synthetic Gaussian time histories with varying bandwidths. Once this correlation effect has been estimated, the standard methods of order statistics are applied to these samples using the Rayleigh probability density function. Examples of applying this procedure to experimentally derived data are presented.     

Chemical oceanography, vol. 8: Edited by J.P. Riley and R. Chester. Academic Press, London, 1983, xvi + 398 pp., £45.00/US $75.00 (hardcover)

Representative profiles of inorganic nitrogenous species dissolved in interstitial waters of coral reef sands are presented. Ammonium is the dominant nitrogenous species in these pore waters with concentrations of up to 40 μm. Nitrate is present but in lower concentrations. Nitrite is found only occasionally in trace amounts. Computations of diffusive fluxes and inferences concerning microbial activity are derived from the profile structures. Computed flux rates of nitrogenous species from the sediment to the water column range between 0.75 and 1.37 μM m^?1 h^?1. These inputs may represent a significant source of recycled nitrogen to the primary producers of the coral reef ecosystem.     

Consideration of the oasis analogy for chemosynthetic communities at Gulf of Mexico hydrocarbon vents

The analogy between desert oasis and deep-sea chemosynthetic community arose from the biomass contrast between vents and the relatively depauperate background benthic fauna. Fully developed, the analogy helps pose questions about interactions with the background fauna with respect to resources, colonization, and persistence. The chemosynthetic sites of the Gulf of Mexico provide an opportunity to consider possible interactions between vent and nonvent fauna over a 3000-m depth range. It is postulated that deep chemosynthetic communities require the operation of geochemical transporting and concentrating processes to overcome low levels of in situ methane and sulfide production. Clathrate reservoirs may serve these functions. A few chemosynthetic species at the Gulf of Mexico upper slope sites are related to shallow-water sulfide species, but it can be speculated that the dominant chemosynthetic fauna may have originated in a wide spread deep sulfide biome of the Cretaceous. Generic endemism of consumers is low in Gulf of Mexico sites, suggesting a high level of colonization from the surrounding benthos. Chemosynthetic communities may avoid excessive colonization by predators in spite of the apparent food limitation of the surrounding benthos due to toxicity or an evolutionary mechanism selecting against specialized predators. The abundance of large predators is related to the composition of the surrounding benthos and is high at the Gulf of Mexico upper slope sites. Exclusion of chemosyntheic communities from shallower depths may be due to excessive predation by generalists.