Greater Water Depths Research Articles

Beyond the Fe-P-redox connection: preferential regeneration of phosphorus from organic matter as a key control on Baltic Sea nutrient cycles

Abstract. Patterns of regeneration and burial of phosphorus (P) in the Baltic Sea are strongly dependent on redox conditions. Redox varies spatially along water depth gradients and temporally in response to the seasonal cycle and multidecadal hydrographic variability. Alongside the well-documented link between iron oxyhydroxide dissolution and release of P from Baltic Sea sediments, we show that preferential remineralization of P with respect to carbon (C) and nitrogen (N) during degradation of organic matter plays a key role in determining the surplus of bioavailable P in the water column. Preferential remineralization of P takes place both in the water column and upper sediments and its rate is shown to be redox-dependent, increasing as reducing conditions become more severe at greater water-depth in the deep basins. Existing Redfield-based biogeochemical models of the Baltic may therefore underestimate the imbalance between N and P availability for primary production, and hence the vulnerability of the Baltic to sustained eutrophication via the fixation of atmospheric N. However, burial of organic P is also shown to increase during multidecadal intervals of expanded hypoxia, due to higher net burial rates of organic matter around the margins of the deep basins. Such intervals may be characterized by basin-scale acceleration of all fluxes within the P cycle, including productivity, regeneration and burial, sustained by the relative accessibility of the water column P pool beneath a shallow halocline.

High matrix interference affecting detection of PAH metabolites in bile of Atlantic hagfish ( Myxine glutinosa) used for biomonitoring of deep-water oil production

The characteristic biology and wide distribution of hagfish species makes them relevant for use in pollution biomonitoring at great water depths, particularly in regions where deep-water oil production may take place. The exposure of fish to petrogenic contaminants can normally be detected from the level of polycyclic aromatic hydrocarbon (PAH) metabolites in bile fluid. Some of these metabolites are strong fluorophores, allowing analytical detection by means of simple fluorometric techniques such as fixed wavelength fluorescence (FF) and synchronous fluorescence scanning (SFS). In the present study bile from Atlantic hagfish ( Myxine glutinosa) collected in pristine areas (Barents Sea and southwestern Norway) displayed strong bile fluorescence levels, suggesting the presence of PAH contaminants. However, gas-chromatography–mass spectrometry (GC–MS) analyses ruled out PAHs as the origin for this fluorescence signal. Rather, the bile of Myxine contains components resulting in unusually strong background fluorescence interfering at the wavelength pairs used for detection of PAH metabolites. Possible background for the observed matrix interference and implications for detection of PAH metabolites in hagfish is discussed.

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Peten Itza, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatan Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Peten Itza, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g−1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 µS cm−1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g−1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 µS cm−1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g−1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.

Micro-tidal coastal reed beds: Hydro-morphological insights and observations on wave transformation from the southern Baltic Sea

This paper investigates the hydro-morphological controls on incident wind-generated waves at, and the transformation of such waves within, two Phragmites australis reed beds in the southern Baltic Sea. Meteorological conditions in combination with geomorphological controls result, over short (<2 km) distances, in significant differences in water level and wave climate to which fringing reed beds are exposed. Significant wave height attenuation reached a maximum of 2.6% m −1 and 11.8% m −1 at the transition from open water into the reed vegetation at the sheltered and exposed sites respectively. Wave attenuation through the emergent reed vegetation was significantly lower in greater water depths, suggesting (1) a reduced influence of bed friction by small shoots/roots and/or (2) drag reduction due to flexing of plants when the wave motion is impacting stems at a greater height above the bed. For a given water depth, wave dissipation increased with increasing incident wave height, however, suggesting that, despite their ability to flex, reed stems may be rigid enough to cause increased drag under greater wave forcing. The higher frequency part of the wave spectrum (>0.5 Hz) was preferentially reduced at the reed margin, confirming the theoretical wave frequency dependence of bottom friction. The possibility of physiological adaptation (differences in reed stem diameter) to water depth and wave exposure differences is discussed. The results have implications for the possible impact of environmental changes, both acute (e.g. storm surges) or chronic (e.g. sea level rise) in character, and for the appropriate management of reed bed sites and delivery of ecological goods and services.

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well-studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time-parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re-deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near-sea floor pore system is a first-order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.

Supporting the Winds of Change

The growing demand for energy from renewable sources is expected to lead to greater construction of offshore wind farms near major coastal cities in the United States, particularly in the northeast. However, the larger turbine systems and greater water depths associated with such installations will place significant demands on wind turbine support structures and foundations. Engineers working on such installations will be challenged to develop innovative, cost-effective designs.

Effects of discount rate and various costs on optimal design of caisson breakwater

In this study, the method developed by Goda and Takagi in 2000 for optimal design of a vertical breakwater caisson is extended to take into account the effects of discount rate and economic damage costs due to long-term harbor shutdown and temporal stoppage of harbor operation. The effect of discount rate is important only at smaller return periods where the damage to the caisson frequently occurs. Among the various costs, the initial construction cost and the economic damage cost due to long-term harbor shutdown caused by extraordinary sliding of caissons are found to be equally important in finding the minimum expected total lifetime cost. On the other hand, the rehabilitation cost and the economic damage cost due to temporal stoppage of harbor operation caused by excessive wave overtopping are not so important in the optimal design of the breakwater. In general, in smaller water depths the optimal return period and the corresponding optimal cross-section of the caisson are determined as those yielding the minimum expected total lifetime cost, while they are determined by the allowable expected sliding distance in greater water depths.

New exposures of Baltic Ice Lake drainage sediments, Götene, Sweden

New exposures created during the construction of highway E20 near Götene, Sweden, reveal poorly sorted gravelly sand overlain and underlain by varved clay. The stratigraphy at Pellagården consists of, from the bottom up, striated gneiss, till, varved marine clay, the gravelly sand unit, and varved marine clay. The varves represent deglacial marine sediment deposited in 40–50 m deep water. The gravelly sand unit contains graded bedding, indistinct horizontal bedding, mud clasts and interstitial mud. It is poorly sorted and poorly organised. The unit has a pebble fabric indicating flow to the northwest. These characteristics and the great water depth suggest that the gravelly sand was deposited from a hyperconcentrated traction current or from concentrated to hyperconcentrated density flows. We interpret the gravelly sand bed to be sediment deposited during the Baltic Ice Lake drainage at around 10,000 14C years BP. The unit likely represents rapidly deposited sediment at the very start of the drainage and does not indicate the duration of the drainage event. The bed was deposited during a single drainage event rather than as a series of events over a few weeks or months. Based on the number of varves and regional ice retreat rates, the ice-margin was 0.2 to 5 km north of Götene at the time of the drainage. These sites represent the first reported occurrences of the drainage sediment in a stratigraphic sequence since the work of Simon Johansson (1926, 1937, and 1941).

Molecular evidence for anaerobic ammonium‐oxidizing (anammox) bacteria in continental shelf and slope sediments off northwest Africa

The presence and abundance of anaerobic ammonium‐oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300‐3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q‐PCR) analysis of anammox‐specific 16S rRNA genes and anammox‐specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g−1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]‐ ladderane monoether‐PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox‐specific 16S rRNA gene copy numbers and the concentration of the C20 [3]‐ladderane monoether‐PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 × 108 copies g−1 sediment and 30 pg g−1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification.

Spatial distribution of Late Pliensbachian (Early Jurassic) calcareous nannofossils within the Lusitanian basin (Portugal)

Quantitative analyses of Pliensbachian calcareous nannofossils have been carried out on a proximal-distal transect in the Lusitanian Basin (Portugal). The studied sections Vale Venteiro near Tomar and Peniche represent proximal and distal environments with respect to the emerged land of the Iberian Meseta (to the East). The upper portion of the Vale das Fontes Fm (Davoei and Margaritatus ammonite Zones) is studied in both sections and correlated by means of ammonite and nannofossil integrated biostratigraphy. A careful analysis of the preservation state of nannofossils is performed. Preservation state is moderate to good in the two settings; changes in nannofossil assemblages are therefore considered as primary. Samples were analysed for nannofossil absolute and relative abundances, species diversity and wt%CaCO 3. The analysis of calcareous nannofossils (absolute abundance, percentage, average abundance) shows changes in the community structure in space (i.e., from proximal to distal), and vertically within the water column. This pattern suggests a partitioning of habitats within the photic zone, and with respect to emerged lands. Proximal environments within the Lusitanian Basin were probably more eutrophic, because of the proximity of emerged lands to the East (Iberian Meseta). This was the source area for nutrients delivered into the basin by river transport. Distal environments were likely characterized by a greater water depth and by a deeper light penetration leading to a relatively more expanded photic zone. The assemblages recorded in the proximal setting exhibit high mean relative abundance of placolith-bearing coccolithophorids ( Lotharingius spp., Biscutum spp. and Similiscutum spp.) and of Schizosphaerella spp., while distal assemblages are dominated by Crepidolithus crassus and Schizospharella spp. Crepidolithus crassus is interpreted as a deep-dweller coccolithophorid, inhabiting preferentially distal and deeper settings in the Lusitanian Basin. Placolith-bearing coccolithophorids were more abundant in proximal settings with respect to emerged lands, under relatively elevated trophic conditions. The probable calcareous dinocyst Schizospharella spp. proliferated in surface waters of both proximal and distal environments.

Rain Loads and Flow Attenuation on Roofs

Basic relationships are derived for calculating the flow attenuation and depth of storm water on roofs. Hyetographs and corresponding hydrographs are specialized to the case of roof drainage and incorporated in methods which are presented for analysis and design of roof drains. Roof storage and outflow are characterized for various roof geometries and outflow devices. The methods are applied to examples found in the literature, including those in standards of Factory Mutual Insurance Company, ASCE, and others. Assumptions implicit in those standards are tested and their limitations demonstrated. A critical duration at which the greatest water depth and outflow occur must be found on a case-by-case basis. The critical durations can differ significantly from the fixed durations specified in standards. The corresponding maximum water depths and peak outflows can also differ accordingly. For determination of roof loads, these results will have their greatest importance in warmer climates in which the rain loads govern over snow loads. An example of design specifically for flow attenuation is also presented.

Comment on “On the cause of the 1908 Messina tsunami, southern Italy” by Andrea Billi et al.

[1] The December 28th 1908 Messina Earthquake has been ranked as one of the most destructive events of the last centuries [Davison, 1936] The damages produced by ground shaking were aggravated by the effects of a remarkable tsunami, with up to 11 m of run-up height, that followed the earthquake [Omori, 1909; Baratta, 1910; Tinti et al., 2004]. The location of the causative fault is still a matter of debate [e.g., Argnani et al., 2009], the modelling of the associated tsunami [Tinti and Armigliato, 2003] allowing uncertainty. [2] Billi et al. [2008] have recently proposed that the tsunami that stroke the coast of the Strait in December 1908 originated from a large submarine landslide (20 km) located offshoreGiardini-Naxos. The hypothesis is based on the study of tsunami arrival times [Platania, 1909; Baratta, 1910] and is supported, according to Billi et al. [2008], by inspection of: a multibeam morphobathymetry [Marani et al., 2004] and of a crustal-scale seismic profile [Scrocca et al., 2004]. [3] The arguments of Billi et al. [2008], however, have weak points that cast doubt on their interpretation. But most importantly, data collected in the last few years by the authors of this comment (A. Argnani and F. Chiocci), and work on tsunami modelling (S. Tinti and his group) cast further doubt on the proposed hypothesis that a large submarine landslide that was caused by the 1908 earthquake is located offshore Giardini-Naxos. These issues will be discussed in the following.

Accumulation of prokaryotic remains during organic matter diagenesis in surface sediments off Peru

Bacterial biomarkers (D‐amino acids and muramic acid) were investigated in surface sediments (0–1 cm) at 22 stations in the Peru margin. Concentrations were used to quantitatively estimate the relative importance of peptidoglycan in the preservation of prokaryotic remains during diagenesis. The bacterial imprint in organic matter was also evident from low molar ratios (average of 1.6) of glucosamine and galactosamine. Estimates of the fraction of biomarkers associated with intact peptidoglycan showed that peptidoglycan became a progressively less important component of prokaryotic remains with ongoing diagenesis, whereas other prokaryotic biomolecules increased in importance. The exact nature of these biomolecules remains unknown, but constant ratios between individual D‐amino acids and amino sugars observed in this study provide novel information on their molecular composition. Examination of total hydrolyzable amino acids revealed systematic compositional changes with increasing water depth and age of the sediment, reflecting increased diagenetic alteration of the organic matter. The time scale integrated in the upper 1 cm of the sediment was ca. 2 yr at the shallow sites and up to 20 yr at the deeper stations. The alteration stages of organic matter ranged from coastal and ocean margin sediments at shallow water depth to organic matter as degraded as hemipelagic surface sediments at greater water depths.

Determination of ultra‐low oxygen concentrations in oxygen minimum zones by the STOX sensor

The methods used until now have not been able to reliably resolve O2 concentrations in oceanic oxygen minimum zones below 1–2 µmol L−1. We present a new amperometric sensor for the determination of ultra‐low O2 concentrations under in situ conditions. The electrochemical STOX O2 sensor contains a primary sensing cathode and a secondary cathode that, when polarized, prevents entry of O2 into the sensor. This arrangement enables frequent in situ zero calibration and confers the sensor with a detection limit of 1‐10 nmol L−1 O2, even during application on a Conductivity‐Temperature‐Depth (CTD) profiler at great water depths. The sensor was used during the Galathea 3 Expedition to demonstrate that the core of the oxygen minimum zone (OMZ) off Peru contained &lt; 2 nM O2. Application in a reactor on board demonstrated that changes in O2 concentrations in OMZ water containing &lt; 200 nmol L−1 O2 could be monitored over periods of hours to days. The linear decrease in O2 concentration in the reactor indicated very low (&lt; 20 nmol L−1) half saturation constants for the O2 respiring microbial community.

Deepwater Completions Offer Great Potential

Guest editorial Deepwater E&amp;P is a growing part of the world's energy supply. Estimates are that deepwater production will grow 78% by 2011, making it one of the leading growth sectors in our industry. The explanation is simple: many of the world's largest untapped reservoirs are in deep water. The "golden triangle" of the Gulf of Mexico, offshore Brazil, and west Africa are joined by newer areas of opportunity in India, Malaysia, Australia, the eastern Mediterranean, offshore Norway, and eastern Canada. There are even more promising possibilities in the salt and subsalt reservoirs located in the basins of the golden triangle. The industry's definition of deepwater exploration and its technical challenges are expanding along with knowledge of the reserves. Ten years ago, deep water was anything more than 1,500 ft water depth. Today, the deepwater frontier is more than 5,000 ft. The frontier of a decade ago is now practically routine. And along with these greater water depths, operators are facing increasing total well depths and higher temperatures and pressures. The deepwater opportunity comes at a price—the high rig rates and capital intensity make it a high-risk and high-reward proposition. And the enormous technical challenges make deepwater wells a focal point for developing and deploying advanced technology in every aspect of well construction and production. Completions technology brings operators more production, lower costs and lower risks, and holds the promise of enabling the future production the world demands. What follows is a snapshot of the deepwater completions picture, with emphasis on those areas that will contribute most to operators’ success. All these elements are interrelated and complementary. Developments in one area contribute to the effectiveness of the others. Ultimately, it is the integrated development of all aspects of deepwater completions that promises to make possible the production the world is counting on over the next several years. High-Rate Fracturing Systems Stimulating production through hydraulic fracturing is a proven way of making deepwater wells economical. Frac packs of long, thick unconsolidated producing intervals in turn depend on the ability to pump frac fluid at a high rate and place proppant in large quantities. Tools and techniques that make access to these zones profitable include using hydrostatic properties of frac fluids to increase bottomhole fracture pressure without unsafe surface treating pressures. This helps achieve the high pressure needed for the fracture without exceeding the pressure rating of surface equipment. Also, tool systems using carbide-protected crossover tool designs are now able to withstand the large volumes of proppant at high rates needed to capitalize on the high-rate fluid pumping. These technologies are valuable in their own right, and they are crucial to the development of single-trip multizone completions.

Deep cyclogenesis by synoptic eddies interacting with a seamount

Strong deep eddies with cyclonic vorticity greater than 0.2f0 were detected using an array of bottom current and pressure measurements in the Kuroshio Extension System Study (KESS) in 2004–2006. Daily maps showed these deep eddies developed locally. As in the Gulf Stream, meandering of the upper baroclinic jet generates deep cyclones and anticyclones by stretching and squashing the lower water column. However, unlike the Gulf Stream, the smaller vertical stretching and greater water depth in the Kuroshio Extension limits the relative vorticity generated by this vertical coupling process to about 0.1f0. In the deep Kuroshio Extension the strong cases of vorticity generation and cyclone development are related to stretching driven when water columns are advected off isolated seamounts in the region. The large observed values of relative vorticity are consistent with a straightforward calculation of deep layer potential vorticity conservation. A barotropic model is used to illustrate the topographic generation of cyclones by ambient currents in synoptic eddies. Positive potential vorticity filaments also develop during the cyclogenetic process with width LR = O(20 km), where LR is the topographic Rhines scale, and travel anticyclonically around the seamount. Observational evidence lends support to the existence of submesoscale filaments, insomuch as current meter records near the flanks of seamounts exhibited bursts of eddy kinetic energy when bandpass-filtered between the inertial period and eight days.

The influence of terrestrial run off on marine biotic communities: An example from a thrust-top carbonate ramp (Upper Pennsylvanian foreland basin, Picos de Europa, NW Spain)

High-diversity marine biotic communities characterize the lower Kasimovian (Upper Pennsylvanian) carbonates of the Las Llacerias Formation in the Ándara Massif (Picos de Europa Province, Cantabrian Mountains, Spain). The carbonates accumulated on a small thrust-top ramp system. Within the shallow, low-energy and euphotic realms, biotic communities were characterized by abundant photozoans (calcareous algae and cyanobacteria) and heterozoans including filter and suspension feeders (calcareous and siliceous sponges, bryozoans, brachiopods, crinoids), diverse other metazoans (rugose solitary and colonial corals, tabulate corals, bivalves, gastropods and echinoids), benthic foraminifers including very abundant small incrusting “tubiforms” and conspicuous swarms of cement-filled spheres (calcispheres or Penella). Small-sized buildups of chaetetid sponges, syringoporids, branching rugose corals, Anthracoporella and phylloid algae commonly developed in these shallow-water environments. Meanwhile, mud mounds formed at greater water depths, mostly below the lower limit of the phothic zone. The occurrence of such diverse biotic communities and the grain composition of the shallow-water deposits, not recorded in the platform-top successions of the antecedent Bashkirian–Moscovian carbonate shelf (Valdeteja and Picos de Europa formations) are interpreted as linked to increased nutrient levels (mesotrophic to mildly eutrophic conditions) in the actual sea-surface water. The nutrients are inferred to have resulted from land-derived fluxes from the rivers draining the adjacent growing Variscan Orogenic chain. Nutrients were subsequently transported into the narrowed marine shallowing foreland basin located in the eastern side of Pangea, within the palaeoequatorial humid belt. Comparison to the Pennsylvanian Auernig Group in the Carnic Alps allows the discrimination between the influence of land-derived versus upwelling-derived nutrients, providing a useful tool in the interpretation of biotic assemblages in the fossil rock record.

Abyssal ostracods from the South and Equatorial Atlantic Ocean: Biological and paleoceanographic implications

We report the distribution of ostracods from ∼5000 m depth from the Southeast and Equatorial Atlantic Ocean recovered from the uppermost 10 cm of minimally disturbed sediments taken by multiple-corer during the R/V Meteor DIVA2 expedition M63.2. Five cores yielded the following major deep-sea genera: Krithe, Henryhowella, Poseidonamicus, Legitimocythere, Pseudobosquetina, and Pennyella. All genera are widely distributed in abyssal depths in the world's oceans and common in Cenozoic deep-sea sediments. The total number of ostracod specimens is higher and ostracod shell preservation is better near the sediment–water interface, especially at the 0–1 cm core depths. Core slices from ∼5 to 10 cm were barren or yielded a few poorly preserved specimens. The DIVA2 cores show that deep-sea ostracod species inhabit corrosive bottom water near the carbonate compensation depth (CCD) even though their calcareous valves are rarely preserved as fossils in sediment cores due to postmortem dissolution. Their occurrence at great water depths may partially explain the well-known global distributions of major deep-sea taxa in the world's oceans, although further expeditions using minimal-disturbance sampling devices are needed to fill geographic gaps.

Stratigraphic signatures due to flood deposition near the Rhône River: Gulf of Lions, northwest Mediterranean Sea

Episodic flood delivery provides the bulk of the solid discharge for many small to moderate river systems, including the Rhône River in the northwestern Mediterranean. Several recent studies have demonstrated that the fate of this sediment depends on the coherence between river discharge and energetic ocean conditions. The deposition of flood sediment in the ocean can be confirmed by common signatures of episodic discharge events: presence of 7Be, physical stratification, and elevated clay content associated with low 210Pb activities. Previous research has indicated that the Rhône River discharge is episodic and generally independent of oceanic conditions. Sometimes the floods coincide with energetic storms and winds from the southeast, which facilitate the movement of sediment towards the southwestern Gulf of Lions. High-resolution coring near the mouth of the Rhône River provides a detailed record of sedimentation associated with past flood events. Cores were collected on two cruises, October 2004 and April 2005, in a study area seaward of the Rhône subaerial delta. Episodic sediment discharge from the Rhône River routinely deposits on the seabed in water depths shallower than 40 m. This is documented by the presence of 7Be in the surficial sediments of physically stratified cores. Through identification of a dilution signature in 210Pb profiles (i.e., increased clay content, decreased 210Pb activity), past flood events are recognized. Greater water depths and distances from the river mouth allow bioturbation to erase these signatures, except in the most extreme events where physical stratification is preserved. Excess 210Pb and 137Cs were ubiquitous in cores from this study, indicating apparent accumulation rates in the range of 2.5 to >10 cm/yr. This study confirms that although flood-event signatures provide a basis with which to examine recent flood deposits of all scales, only the thickest deposits are likely to be preserved over the long term (>100 yr).

Bioassessment of wet-weather pollution impacts on fine sediments in urban waters by benthic indices and the sediment quality triad

Benthic invertebrate assessments can be used to gauge the impact of urban wet-weather flows in receiving waters. Experiences from Cemagref in France have shown that standardized benthic indices (e.g. Oligochaete Index of Sediment Bioindication - IOBS) can be used to reliably determine the ecological status of urban streams and can be incorporated into the new European Water Framework Directive. The Canadian studies on streams and stormwater ponds using chemical analyses, benthic toxicity testing and benthic invertebrate community structure (i.e. the sediment quality triad) comparisons have shown that toxicity was more likely to occur in ponds at sites with higher concentrations of heavy metals and heavier polycyclic aromatic hydrocarbons, and at greater water depths, where fine sediments from urban runoff accumulated. A more comprehensive evaluation of wet-weather flow impacts could be obtained by combining approaches from both countries.