Historical record of heavy metals in a highly contaminated Mediterranean deposit: The Besòs prodelta

The Besòs prodelta, located around a highly populated area, receives the discharge of the Besòs River, one of the most contaminated rivers discharging into the Mediterranean sea. The record of heavy metals accumulated in this prodelta is studied to assess the sedimentary behaviour and the history of the anthropogenic heavy metals in the area.

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dated sediments have allowed the identification of the sedimentation patterns and the elaboration of dated heavy metals and cenospheres profiles, which reflected the evolution of the anthropogenic impact in this marine environment. Sediments accumulated during the 18th and 19th centuries were already affected by a moderate heavy metal contamination. This contamination increased drastically in the sediments accumulated during the last decades. This data is well correlated with the increase in industrialisation, population and energy consumption in the Besòs drainage basin during the last century.     

Short-term and interannual variability of redox-sensitive chemical parameters in hypoxic/anoxic bottom waters of the Chesapeake Bay

A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry.Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.     

The morphodynamic modelling of tidal sand waves on the shoreface

An artificial sand wave on the Dutch shoreface of the North Sea has been studied in conditions with relatively strong tidal currents in the range of 0.5 to 1 m/s and sediments in the medium sand size range of 0.2 to 0.5 mm. The sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km, respectively. The sand wave is dynamically active and shows migration rates of the order of a few metres per year. A numerical morphodynamic model (DELFT3D model) has been used to simulate the morphological behaviour of the sand wave in the North Sea. This model approach is based on the numerical solution of the three-dimensional shallow water equations in combination with a surface wave propagation model (wind waves) and the advection–diffusion equation for the sediment particles with online bed updating after each time step. The model results show that the sand wave grows in the case of dominant bed-load transport (weak tidal currents; relatively coarse sediment; small roughness height; low waves) and that the sand wave decays in the case of dominant suspended transport (strong currents, relatively fine sediment, large roughness height; storm waves).     

Estimation of Rapidly Time-Varying Sparse Channels

The estimation of sparse shallow-water acoustic communication channels and the impact of estimation performance on the equalization of phase coherent communication signals are investigated. Given sufficiently wide transmission bandwidth, the impulse response of the shallow-water acoustic channel is often sparse as the multipath arrivals become resolvable. In the presence of significant surface waves, the multipath arrivals associated with surface scattering fluctuate rapidly over time, in the sense that the complex gain, the arrival time, and the Dopplers of each arrival all change dynamically. A sparse channel estimation technique is developed based on the delay-Doppler-spread function representation of the channel. The delay-Doppler-spread function may be considered as a first-order approximation to the rapidly time-varying channel in which each channel component is associated with Doppler shifts that are assumed constant over an averaging interval. The sparse structure of the delay-Doppler-spread function is then exploited by sequentially choosing the dominant components that minimize a least squares error. The advantage of this approach is that it captures both the channel structure as well as its dynamics without the need of explicit dynamic channel modeling. As the symbols are populated with the sample Dopplers, the increase in complexity depends on the channel Doppler spread and can be significant for a severely Doppler-spread channel. Comparison is made between nonsparse recursive least squares (RLS) channel estimation, sparse channel impulse response estimation, and estimation using the proposed approach. The results are demonstrated using experimental data. In training mode, the proposed approach shows a 3-dB reduction in signal prediction error. In decision-directed mode, it improves significantly the robustness of the performance of the channel-estimate-based equalizer against rapid channel fluctuations.     

Geochronology of the American-Antarctic Ridge

A new map of chrons for the American-Antarctic Ridge area has been compiled. Its analysis and the calculations performed showed that the seafloor spreading with respect to its axis started before 85 My B.P. The spreading directions were 115° (chrons C34-C29), 145° (chrons C29-C21), 110° (chrons C21-C5C), and 85° (chrons C5C-C1). The maximum rates of about 4 cm/year were reached earlier than 52 My B.P.; subsequently, a progressive general decrease in the spreading rate has been observed. According to our forecast, the spreading may cease in the following 3.5 My.     

Atmospheric forcing of the eastern tropical Pacific: A review

The increase in marine, land surface, atmospheric and satellite data during recent decades has led to an improved understanding of the air–sea interaction processes in the eastern tropical Pacific. This is also thanks to extensive diagnoses from conceptual and coupled ocean–atmosphere numerical models. In this paper, mean fields of atmospheric variables, such as incoming solar radiation, sea level pressure, winds, wind stress curl, precipitation, evaporation, and surface energy fluxes, are derived from global atmospheric data sets in order to examine the dominant features of the low level atmospheric circulations of the region. The seasonal march of the atmospheric circulations is presented to depict the role of radiative forcing on atmospheric perturbations, especially those dominating the atmosphere at low levels.In the tropics, the trade winds constitute an important north–south energy and moisture exchange mechanism (as part of the low level branch of the Hadley circulation), that determines to a large extent the precipitation distribution in the region, i.e., that associated with the Inter-Tropical Convergence Zone (ITCZ). Monsoonal circulations also play an important role in determining the warm season precipitation distribution over the eastern tropical Pacific through a large variety of air–sea–land interaction mechanisms. Westward traveling waves, tropical cyclones, low latitude cold air intrusions, and other synoptic and mesoscale perturbations associated with the ITCZ are also important elements that modulate the annual rainfall cycle. The low-level jets of the Gulf of California, the Intra-Americas Sea (Gulf of Mexico and Caribbean Sea) and Chocó, Colombia are prominent features of the eastern tropical Pacific low-level circulations related to sub-regional and regional scale precipitation patterns. Observations show that the Intra-Americas Low-Level Jet intensity varies with El Niño/Southern Oscillation (ENSO) phases, however its origin and role in the westward propagation and development of disturbances that may hit the eastern tropical Pacific, such as easterly waves and tropical cyclones, are still unclear. Changes in the intensity of the trade winds in the Caribbean Sea and the Gulf of Mexico (associated with eastern tropical Pacific wind jets) exert an important control on precipitation by means of wind–topography interactions. Gaps in the mountains of southern Mexico and Central America allow strong wind jets to pass over the continent imprinting a unique signal in sea surface temperatures and ocean dynamics of the eastern tropical Pacific.The warm pools of the Americas constitute an important source of moisture for the North American Monsoon System. The northeastern tropical Pacific is a region of intense cyclogenetic activity, just west of the coast of Mesoamerica. Over the oceanic regions, large-scale properties of key variables such as precipitation, moisture, surface energy fluxes and wind stress curl are still uncertain, which inhibits a more comprehensive view of the region and stresses the importance of regional field experiments. Progress has been substantial in the understanding of the ocean and atmospheric dynamics of the eastern tropical Pacific, however, recent observational evidence such as that of a shallow meridional circulation cell in that region, in contrast to the classic concept of the Hadley-type deep meridional circulation, suggests that more in situ observations to validate theories are still necessary.This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.     

A comparative mineralogical study of gas-related sediments of the Gulf of Cádiz

The Gulf of Cádiz area has been extensively surveyed in recent years and several gas-related fluid escape seafloor structures have been identified. In this study, gravity cores, collected during the ANASTASYA/00 and ANASTASYA/01 cruises, on mud volcanoes, hemipelagic sediments and dredged material from diapiric structures, have been studied. A comparative mineralogical analysis by XRD and SEM of samples from different areas has been performed in order to determine whether there is a characteristic mineralogy related to these fluid escape structures, and also to determine the origin of the mud matrix and constrain the depth of the parent units. The mineralogical analysis reflects the different origins of the different units described in the cores: hemipelagic material of the slope, clays that underlie the mud volcanoes and are discharged at the sea bottom surface, and authigenic and diagenetic minerals possibly involved in the anaerobic oxidation of methane in the mud volcano sediments.     

The contribution of atmospheric acid deposition to ocean acidification in the subtropical North Atlantic Ocean

The absorption of anthropogenic CO₂ and atmospheric deposition of acidity can both contribute to the acidification of the global ocean. Rainfall pH measurements and chemical compositions monitored on the island of Bermuda since 1980, and a long-term seawater CO₂ time-series (1983–2005) in the subtropical North Atlantic Ocean near Bermuda were used to evaluate the influence of acidic deposition on the acidification of oligotrophic waters of the North Atlantic Ocean and coastal waters of the coral reef ecosystem of Bermuda. Since the early 1980's, the average annual wet deposition of acidity at Bermuda was 15 ± 14 mmol m^− 2 year^− 1, while surface seawater pH decreased by 0.0017 ± 0.0001 pH units each year. The gradual acidification of subtropical gyre waters was primarily due to uptake of anthropogenic CO₂. We estimate that direct atmospheric acid deposition contributed 2% to the acidification of surface waters in the subtropical North Atlantic Ocean, although this value likely represents an upper limit. Acidifying deposition had negligible influence on seawater CO₂ chemistry of the Bermuda coral reef, with no evident impact on hard coral calcification.     

A probability distribution for depth-limited extreme wave heights in a sea state

A model for the depth-limited distribution of the highest wave in a sea state is presented. The distribution for the extreme wave height is based on a probability density function (pdf) for depth-limited wave height distribution for individual waves [Méndez, F.J., Losada, I.J., Medina, R. 2004. Transformation model of wave height distribution. Coastal Eng, Vol. 50, 97:115.] and considers the correlation between consecutive waves. The model is validated using field data showing a good representation of the extreme wave heights in the surf zone. Some important statistical wave heights are parameterized obtaining useful expressions that can be used in further calculations.