Short-Term Benthic Recolonization after Dredging in the Harbour of Ceuta, North Africa

Abstract. The benthic recovery after dredging (area: 2625 m²) was studied in a polluted and enclosed area of the harbour of Ceuta, in which the recolonization through the water column (larvae and adult bedload transport) could be limited by the lack of renewal. The benthos was sampled at two sites (control and dredged) using a van Veen grab and adopting a BACI (Before, After, Control, Impacted) approach. Five samplings were conducted after dredging (3, 15, 30, 90, 180 days). The proportion of gravel in the sediment of the dredged site increased after dredging, while the organic matter decreased. The impact on the community was estimated at species level, using both univariate and multivariate analyses. The maximum negative effect on benthic macrofauna was a reduction by 65% for species richness (15 days after dredging) and by 75% for abundance (3 days after dredging). Between 15 and 30 days after dredging, the abundance of some species such as the molluscs Parvicardium exiguum and Retusa obtusa and the polychaete Pseudomalacoceros tridentata increased considerably in the dredged site, while typical ‘opportunistic’ species such as Capitella capitata were disfavoured by the disturbance. For this small‐scale dredging, about 6 months are required for the disturbed area to re‐establish a sediment structure and a macrobenthic community similar to the undisturbed area. Small‐patch dredging operations are proposed in harbour management whenever possible, since they allow a quick re‐adjustment of the initial sediment structure and benthic communities.     

Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties

Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.     

The Spatial Distribution of Caprellidea (Crustacea: Amphipoda): A Stress Bioindicator in Ceuta (North Africa, Gibraltar Area)

Abstract. The caprellid (Crustacea: Amphipoda) community associated with the alga Cystoseira usneoides (L.) Roberts 1967 was studied on a spatial scale in relation to the influence of environmental factors on the coast of Ceuta (North Africa) using multivariate analyses. Twenty-two stations were sampled and five environmental factors were tested (hydrodynamics, silting, suspended organic matter, organic matter in silt and suspended solids). The spatial distribution of the caprellid community reflected the physico-chemical conditions of the coast. Phtisica marina Slabber 1769 was the only species present at the stations of the harbour of Ceuta, characterised by the lowest values of hydrodynamics and the highest values of silting, suspended organic matter and suspended solids. The most exposed stations (high hydrodynamics and low values of silting, dissolved organic matter and suspension solids) were mainly dominated by Caprella danilevskii Czerniavskii 1868 and C. penantis Leach 1814. These two species have developed a "parallel" posture that enables them to attach strongly to the substratum and thus inhabit areas subjected to heavy exposure or strong currents. Caprella acanthifera Leach 1814 and Pseudoprotella phasma (Montagu 1804) preferred stations characterised by moderate values of silting and suspended solids, such as those located in the coastal areas in front of the city of Ceuta, but these species were not found at the most stressed harbour stations.     

Miocene extensional detachments in the outcropping basement of the northern Alboran Basin (Betics) and their tectonic implications

Whether or not there are extensional detachment faults in the Alboran basement can be tested directly because a part of the Alboran Basin is now emerged. These detachments, related to crustal thinning beneath the Alboran Basin, occurred from the Aquitanian to Tortonian. The resulting extensional geometries can be described in general terms. During the Serravalian a considerable southwest extension of the basin took place, accompanied by south-southeast extension in the northern Gibraltar Arc. Other detachments affected by Serravalian extension can be found. The spreading of the Alboran was nearly coeval with roughly westward migration of the Gibraltar mountain front.     

Stratigraphic framework of gas accumulations in the Ría de Pontevedra (NW Spain)

Knowledge of marine geological environments in which shallow gas is accumulating is becoming increasingly important in global studies of climate change because a measurable proportion of the total methane source comes from continental margins. Previous studies have revealed that coastal environments represent important geological environments where microbial methane is being generated, is accumulating, and is being released. In the Ría de Pontevedra, at least 4.5 km² of seafloor in the innermost part of the ría is underlain by sediments containing natural gas. Seismic interpretation contributes new findings for the definition of periods and geological environments in which the gas could have been generated, and is accumulating and released in the Ría de Pontevedra. Groundtruthing the seismic data (facies, environments) makes it possible to identify favourable geological environments for gas generation in the sedimentary infill of the Ría de Pontevedra. Sequence stratigraphy based on high-resolution seismic profiles and post-Last Glacial Maximum sea-level records makes it possible to establish the stratigraphic architecture of the ría and to define the periods in which gas could have been generated. The results of this study show that the sedimentary infill is composed of a fifth-order sequence developed since the Last Glacial Maximum. Within this sequence, gas appears to have accumulated in the Holocene deposits associated with the latest transgressive and highstand system tracts. Seismic analysis shows that gas could have been generated in different geological environments in the Ría de Pontevedra. If coastal environments at times of lower sea level were similar to those of the present, organic-rich mud deposits (deposited mainly in lakes, estuaries and floodplains) could have survived transgression and remained buried as potential gas sources in the inner part of the ría.     

Spin rate estimation of sounding rockets using GPS wind-up

Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna. In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas. Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR.     

Simulations of 3D DC borehole resistivity measurements with a goal-oriented <Emphasis Type="Italic">hp</Emphasis> finite-element method. Part II: through-casing resistivity instruments

We simulate direct current (DC) borehole resistivity measurements acquired in steel-cased deviated wells for the assessment of rock formation properties. The assumed data acquisition configuration considers one current (emitter) and three voltage (collector) electrodes that are utilized to measure the second difference of the electric potential along the well trajectory. We assume a homogeneous, 1.27-cm-thick steel casing with resistivity equal to 10^− 5 Ω· m. Simulations are performed with two different numerical methodologies. The first one is based on transferring two-dimensional (2D) axisymmetric optimal grids to a three-dimensional (3D) simulation software. The second one automatically produces optimal 3D grids yielded by a 3D self-adaptive goal-oriented algorithm. Both methodologies utilize high-order finite elements (FE) that are specially well-suited for problems with high-contrast coefficients and rapid spatial variations of the electric field, as it occurs in simulations that involve steel-cased wells. The method based on transferring 2D-optimal grids is efficient in terms of CPU time (few seconds per logging position). Unfortunately, it may produce inaccurate 3D simulations in deviated wells, even though the error remains below 1% for the axisymmetric (vertical) well. The method based on optimal 3D grids, although less efficient in terms of CPU time (few hours per logging position), produces more accurate results that are validated by a built-in a posteriori error estimator. This paper provides the first existing simulations of through-casing resistivity measurements in deviated wells. Simulated resistivity measurements indicate that, for a 30° deviated well, measurements in conductive layers 0.01 Ω· m) are similar to those obtained in vertical wells. However, in resistive layers (10,000 Ω· m), we observe 100% larger readings in the 30° deviated well. This difference becomes 3,000% for the case of a 60° deviated well. For this highly-deviated well, readings corresponding to the conductive formation layer are about 30% smaller in magnitude than those in a vertical well. Shoulder effects significantly vary in deviated wells.     

Water balance in the playa-lakes of an arid environment,Monegros, NE Spain

The playa-lakes of the Monegros desert in north-east Spain are saline wetlands in an arid environment, a rare phenomenon in Europe. These extremely valuable habitats are threatened by changes associated with agricultural expansion and incorporation of new irrigated areas. An understanding of the present hydrologic regime will enable changes to be identified, particularly those brought about by flooding and pollution caused by irrigation surplus. This study sets out to show the results of applying a daily water balance in three selected playa-lakes. The balance was in two parts and consisted of: (1) the average balance for all the endorheic basin using the BALAN_11 program, and (2) the water balance in some playa-lakes, applying discharge flows obtained from the previous balance. The resulting volumes of water were converted to water depths and contrasted with reference volumes taken from field and Landsat images. The model was calibrated by applying various hypotheses of function which enabled the results to be adjusted. The proposed balance is an acceptable reproduction of field water measurements during this period, and underlines the consistency of the conceptual model. The methodology used is appropriate for understanding the playa-lakes function and for monitoring them for conservation purposes.