Hydrodynamic and sediment transport measurements from instrumentation deployed during a 54-day winter period at two sites on the Louisiana inner shelf are presented. Strong extratropical storms, with wind speeds of 7.8 to 15.1 m s-1, were the dominant forcing mechanism during the study. These typically caused mean oscillatory flows and shear velocities about 33% higher than fair weather (averaging 12.3 and 3.2 cm s-1 at the landward site, and 11.4 and 2.7 cm s-1 at the seaward site, respectively). These responses were coupled with mean near-bottom currents more than twice as strong as during fair weather (10.3 and 7.5 cm s-1 at the landward and seaward sites, respectively). These flowed in approximately the same direction as the veering wind, causing a net offshore transport of fine sand. Weak storms were responsible for little sediment transport whereas during fair weather, onshore sand transport of approximately 25-75% of the storm values appears to have occurred. This contradicts previous predictions of negligible fair-weather sediment movement on this inner shelf. 相似文献
Data are presented indicating the complexity and highly variable response of beaches to cold front passages along the northern Gulf of Mexico, in addition to the impacts of tropical cyclones and winter storms. Within the past decade, an increase in the frequency of tropical storms and hurricanes impacting the northern Gulf has dramatically altered the long-term equilibrium of a large portion of this coast. A time series of net sediment flux for subaerial and nearshore environments has been established for a section of this coast in Florida, and to a lesser extent, Mississippi. The data incorporate the morphological signature of six tropical storms/hurricanes and more than 200 frontal passages.
Data indicate that (1) barrier islands can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong category 4 hurricane near landfall); (2) less severe hurricanes and tropical storms can promote rapid dune aggradation and can contribute sediment to the entire barrier system; (3) cold fronts play a critical role in the poststorm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally generated, high frequency, steep waves; and (4) barrier systems along the northern Gulf do not necessarily enter an immediate poststorm recovery phase, although nested in sediment-rich nearshore environments. While high wave energy conditions associated with cold fronts play an integral role in the evolution and maintenance of barriers along the northern Gulf, these events are more effective in reworking sediment after the occurrence of extreme events such as hurricanes. This relationship is even more apparent during the clustering of tropical cyclones.
It is anticipated that these findings will have important implications for the longer term evolution of barrier systems in midlatitude, microtidal settings where the clustering of storms is apparent, and winter storms are significant in intensity and frequency along the coast. 相似文献
As part of a study of estuarine selenium cycling, we measured the concentration, chemical form (speciation), and distribution of particulate selenium under various river flow conditions in the North San Francisco Bay (from the Golden Gate to the Sacramento and San Joaquin Rivers). We also conducted laboratory studies on the accumulation of selenium by phytoplankton, the critical first step in the transformation of dissolved to particulate selenium. Total particulate selenium concentration in the North SF Bay was relatively constant between high and low flow periods, ranging spatially from 0.05 to 0.35 nmol l−1 and comprising between 5 and 12% of the total water column selenium inventory. Mean concentrations were generally highest in the Carquinez Strait–Suisun Bay region (salinity 0–17) and lowest in Central Bay. However, selenium content of suspended particles varied with river flow, with higher content during low flow (9.76 ± 4.17 nmol g−1; mean ± sd; n = 67) compared to high flow (7.10 ± 4.24 nmol g−1; n = 39). Speciation analyses showed that most particulate selenium is organic selenide (45 ± 27%), with a smaller proportion (typically <30%) of adsorbed selenite + selenate and a varying proportion (35 ± 28%) of elemental selenium. Based on the amount of elemental selenium in the seston (total suspended material), we calculate that resuspension of estuarine sediments could contribute 29–100% of particulate selenium in the water column. While selenium content of SF Bay seston (>0.4 μm) is relatively unenriched compared to phytoplankton (13.6–155 nmol g−1 dry weight) on a mass basis, when normalized to carbon or nitrogen, seston contains a similar selenium concentration to SF Bay sediments or phytoplankton cultures. SF Bay seston is thus comprised of selenium-rich phytoplankton and phyto-detritus, but also inorganic clay mineral particles that effectively “dilute” total particulate selenium. Selenium concentrations in algal cultures (11 species) exposed to 90 nmol l−1 selenite show relatively large differences in selenium accumulation, with the diatoms, chlorophytes and cryptophytes generally having lower selenium cell content (3.8 ± 2.7 × 10−9 nmol selenium cell−1) compared to the dinoflagellates (193 ± 73 × 10−9 nmol selenium cell−1). Because phytoplankton are such a rich (but variable) source of selenium, their dynamics could have a profound effect on the particulate selenium inventory in the North SF Bay. 相似文献
A survey has been made of the quantities of small virgin plastic pellets and granules present in the litter stranding on shores of eastern Canada and Bermuda. Whilst particularly abundant on beaches of Bermuda, they are most uncommon on those of Nova Scotia and Sable Island—this, it is interpreted, reflects broad oceanic circulation patterns, lengthy residence times and distant sources, for there are no significant local ones. The polyethylene character of the granules was confirmed through burning characteristics and infra-red spectrophotometry. Polystyrene spherules were not identified. These plastic granules are commonly associated with pelagic tar balls and many have a tarry coating.The granules support a restricted, encrusting pseudoplanktonic biota, including serpulid worms and coralline algae, similar to that found on floating Sargassum. It is concluded, however, that their environmental consequence is not great.Although plastic granules progressively disappear through oxidative ageing and other degradational processes, they are an unnecessary contaminant of marine waters and evidence of evergrowing oceanic litter. 相似文献
Boron, chloride, sulfate, δD, δ18O, and 3H concentrations in surface water and groundwater samples from the Sulphur Bank Mercury Mine (SBMM), California, USA were
used to examine geochemical processes and provide constraints on evaporation and groundwater flow. SBMM is an abandoned sulfur
and mercury mine with an underlying hydrothermal system, adjacent to Clear Lake, California. Results for non-3H tracers (i.e., boron, chloride, sulfate, δD, and δ18O) identify contributions from six water types at SBMM. Processes including evaporation, mixing, hydrothermal water input
and possible isotopic exchange with hydrothermal gases are also discerned. Tritium data indicate that hydrothermal waters
and other deep groundwaters are likely pre-bomb (before ~1952) in age while most other waters were recharged after ~1990.
A boron-based steady-state reservoir model of the Herman Impoundment pit lake indicates that 71–79% of its input is from meteoric
water with the remainder from hydrothermal contributions. Results for groundwater samples from six shallow wells over a 6–month
period for δD and δ18O suggests that water from Herman Impoundment is diluted another 3% to more than 40% by infiltrating meteoric water, as it
leaves the site. Results for this investigation show that environmental tracers are an effective tool to understand the SBMM
hydrogeologic regime.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Résumé Les concentrations en bore, chlorure, sulfate, δD, δ18O, et 3H d’échantillons d’eaux de surface et souterraine prélevés dans le banc de soufre de la mine de mercure (SBMM en anglais)
en Californie, USA, ont été utilisées pour étudier les processus géochimiques et pour fournir des contraintes à l’évaporation
et à l’écoulement des eaux souterraines. La SBMM est une mine de soufre et de mercure abandonnée, adjacente au lac Clear en
Californie et sous laquelle se trouve un système hydrothermal.Les résultats des traceurs autres que le tritium (bore, chlorure,
sulfate, δD, et δ18O) ont permis d’identifier des contributions de six types d’eaux à SBMM. Des processus tels que l’évaporation, le mélange,
l’entrée d’eau hydrothermale et de possibles échanges isotopiques avec des gaz hydrothermaux ont également été identifiés.
Les données de tritium montrent que les eaux hydrothermales et d’autres eaux profondes sont probablement d’age antérieure
à la bombe (avant ~1952), alors que la plupart des autres eaux sont issues de la recharge après ~1990. Un modèle de réservoir
représentant le lac situé dans la partie Herman Impoundment de l’ancienne mine, en régime permanent et basé sur le bore, montre
que 71–79% de l’eau provient des précipitations, le reste provenant de contributions hydrothermales. Les résultats de δD et
δ18O pour des échantillons d’eau souterraine de six puits peu profonds sur une période de 6 mois suggèrent que l’eau de Hermann
Impoundment est encore diluée entre 3% jusqu’à plus de 40% lorsqu’elle quitte le site du fait de l’infiltration d’eau météorique.
Les résultats de cette étude montrent que les traceurs environnementaux constituent un outil efficace pour comprendre le régime
hydrogéologique de la SBMM.
Resumen Se han utilizado datos de concentraciones de boro, cloruros, sulfatos, δD, δ18O, y 3H en muestras de aguas superficiales y subterráneas procedentes de la Mina de Mercurio Sulphur Bank (SBMM), California, USA,
para estudiar los procesos geoquímicos y caracterizar la evaporación y el flujo del agua subterránea. SBMM es una mina abandonada
de azufre y mercurio con un sistema hidrotermal subyacente, cercano al Lago Clear, California.Los resultados de los trazadores
que no son 3H (por ejemplo, boro, cloruros, sulfatos δD, y δ18O) identifican las contribuciones de seis tipos de agua en la SBMM. Se han identificado diversos procesos, como evaporación,
mezcla, entrada de agua hidrotermal y la posibilidad de intercambios isotópicos con gases hidrotermales. Los datos de tritio
indican que las aguas hidrotermales y otras aguas subterráneas profundas son probablemente pre-bombas (previas a ~1952) en
edad mientras que la mayoría de las otras aguas se han recargado después de ~1990. Un modelo estacionario basado en el boro
de la fosa del lago Herman Impoundment indica que el 71–79% de su entrada procede de agua meteórica con restos de contribución
hidrotermal. Los resultados para las muestras de aguas subterráneas procedentes de seis pozos superficiales en un periodo
de 6 meses para δD y δ18O sugieren que el agua procedente de Herman Impoundment se diluye en otro 3% hasta más del 40% debido a la infiltración de
agua meteórica., que sale del sitio. Los resultados de esta investigación muestran que los trazadores ambientales son una
herramienta efectiva para entender el régimen hidrogeológico de la SBMM.
Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of SrNdPb isotopes(87Sr/86Sr 0.70400.7046, 143 Nd/144 Nd 0.5130 and 206Pb/204Pb 19.3619.51), and mantle-like 18O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (87 Sr/86Sr 0.70480.7077,143 Nd/144 Nd 0.51280.5123 and 206 Pb/204 Pb 19.720.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high 18 O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 14% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher 18 O values of the IDS lavas are interpretedas resulting from 1055% assimilationfractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles*Corresponding author. 相似文献