Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay

Spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter, hydrographic and nephelometric observations, gathered during the ECOFER experiment (1989–1991) in the Cap-Ferret Canyon on the Aquitanian margin of the Bay of Biscay. While characteristics of the deep water masses were stable from one year to another, large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope. The slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean. A predominantly northward along-slope current with occasional reversal characterizes this circulation. The nephelometric structures also showed seasonal changes in the overall suspended particulate matter content, but recurrent features, such as the presence of intermediate nepheloid layers at the shelf-break depth and various depths along the slope (∼500, 1000 and 2000 m), were observed. These nepheloid layers extended off the slope to about 10–30 km, but especially laterally along the slope. Their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope. The intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the M₂ internal tide.     

Quaternary sedimentary regime on the Berthois Spur (Bay of Biscay)

A study of cores from the Celtic Margin shows that high terrigenous accumulation rates occurred at the end of isotopic stage 3 and during isotopic stage 2. These high rates prevailed during rapid changes from relatively warm conditions (Älesund interstadial) to maximum glacial condition. The input of terrigenous material may have culminated with the blockage of the North Sea by an ice sheet and the diversion of NW European flumes toward the Celtic Margin. The uppermost sediments are in a state of overconsolidation as the consequence of erosional processes following the episod of high sediment supply.     

Fine-grained sediment budget on the continental margin of the Bay of Biscay

Under present-day conditions, rivers are the main source of fine sediments dispersed to the Bay of Biscay. They deliver about 2.5×10⁶ t yr⁻¹ of continental fine sediments, 60% of which is derived from the Gironde estuary. Of this flux, 65% is believed stored on the shelf. Two kinds of mud fields can be found in the Bay of Biscay: coastal mud and shelf mud belts. The total mass of fine sediments stored during the past 2000 years is 3.2×10⁹ t. Consequently, about 0.9×10⁶ t yr⁻¹ could reach the shelf edge and eventually the open sea. From this amount of displaced material and the deposition surface areas, an evaluation of sediment fluxes across the margin during the late Holocene period is discussed. This evaluation is compared with results obtained from ECOsystéme du canyon du cap-FERret (ECOFER) data from sediment traps and surficial box cores.     

The impact of the last European deglaciation on the deep-sea turbidite systems of the Celtic-Armorican margin (Bay of Biscay)

The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the ‘Fleuve Manche’ palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.     

Benthic response of Munnopsurus atlanticus (Crustacea Isopoda) to the carbon content of the near-bottom sedimentary environment on the southern margin of the Cap-Ferret Canyon (Bay of Biscay,northeastern Atlantic Ocean)

The response of benthic organisms to organic carbon fluxes in a continental margin region was studied by investigating the diet of the suprabenthic isopod Munnopsurus atlanticus, which is well represented on the southern margin of the Cap-Ferret Canyon (Bay of Biscay). The grain-size distribution, foraminiferal assemblages, particulate organic carbon and pigments found in the sediment and in the gut of the isopods were analyzed. These results suggest that M. atlanticus feeds on benthic agglutinated foraminifers which are in a high “nourishment state” and represent a link between primary and secondary producers.     

Past and present sedimentary activity in the Capbreton Canyon, southern Bay of Biscay

Located in the south-eastern part of the Bay of Biscay, the Capbreton Canyon incises the continental shelf up to the 30 m isobath contour, and acts as a natural conduit for continental and shelf-derived sediments. EM1000 multibeam bathymetry shows two main features characterising the canyon — a deeply entrenched meandering channel, bordered by fluvial-like terraces constituting large sediment traps. A dataset of cores and seismic profiles together with a multibeam bathymetry map has enabled the characterisation of recent sedimentary activity in the axial channel and on the terraces. Data analysis evidenced the major role of the canyon head in recent sediment dynamics. This part of the canyon is a temporary reservoir for sediments, accumulated by coastal hydrodynamic processes. Exceptional climatic, tectonic or hydrodynamic events can mobilise the sediments and generate gravity-driven flows. Under the present-day sea-level highstand conditions, these flows are not powerful enough to bring their bedload to the deep sea, and are confined mainly to the upper part of the canyon. Turbidity currents model the axial channel pathway and are at the origin of terrace formation. Terraces in the Capbreton Canyon are not typical but rather are reduced to confined levees. Three factors control the vertical growth of a terrace: (1) the amount of overspilled sediments brought by turbidity currents, (2) hemipelagic sedimentation and (3) terrace height. The amount of sediment spilling over a terrace decreases with increased terrace elevation. Concurrently, the proportion of hemipelagic fallout depositing on a terrace increases. Terraces are considered to be fossil when the height of the terrace prevents further deposition by overspilling. The terraces studied in this paper are interpreted as having formed during the Holocene, implying that the sediment dynamics of the Capbreton Canyon is continuous through time. Highstand periods differ from lowstand periods because they show a decrease in the energy of erosive processes. Temporal variations in erosive and depositional processes in the canyon are controlled by the Adour River, which delivers large amounts of sediment to the system.     

Oxygen and organic carbon fluxes in sediments of the Bay of Biscay

The relationship between particulate organic carbon (POC) concentrations measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC content as a proxy of paleo-environmental conditions. The objective of our study was to compare POC concentrations, POC mineralization rates calculated from O₂ consumption and POC burial rates. Benthic O₂ distributions were determined in 58 fine-grained sediment cores collected at different periods at 14 stations in the southeastern part of the Bay of Biscay with depths ranging from 140 to 2800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM), assuming that O₂ consumption solely was related to heterotrophic activity at the sediment–water interface. Heterogeneity of benthic O₂ fluxes denoted changes in time and space of fresh organic material sedimentation. The most labile fraction of exported POC engendered a steep decrease in concentration in the upper 5 mm of vertical O₂ profiles. The rupture in the gradient of O₂ microprofile may be related to the bioturbation-induced mixing depth of fast-decaying carbon. Average diffusive O₂ fluxes showed that this fast-decaying OM flux was much higher than buried POC, although diffusive O₂ fluxes underestimated the total sediment oxygen demand, and thus the fast-decaying OM flux to the sediment surface. Sedimentary POC burial was calculated from sediment mass accumulation rate and the organic carbon content measured at the top of the sediment. The proportion of buried POC relative to total exported POC ranged at the most between 50% and 10%, depending on station location. Therefore, for a narrow geographic area like the Bay of Biscay, burial efficiency of POC was variable. A fraction of buried POC consisted of slow-decaying OM that was mineralized within the upper decimetres of sediment through oxic and anoxic processes. This fraction was deduced from the decrease with depth in POC concentration. At sites located below 500 m water depth, where the fast-decaying carbon did not reach the anoxic sediment, the slow-decaying pool may control the O₂ penetration depth. Only refractory organic material was fossilized in sedimentary records at locations where labile OM did not reach the anoxic portion of the sediment.     

近200年桑沟湾养殖海域柱状沉积剖面的污染有机质记录与评价

通过对取自桑沟湾南部和北部养殖海域2个站位的柱状沉积物样品进行TOC和TN测定,分析了桑沟湾沉积物中TOC和TN的分布情况,结合沉积物年代序列的测定,估算了工业革命之前、之中及规模化养殖之后桑沟湾海域沉积物中水生有机碳和总氮的含量,并对近200年来桑沟湾养殖海域有机质沉积情况进行了评价。结果显示:19世纪70年代以前,TOC含量较稳定,TN含量随深度增加而缓慢降低;19世纪70年代至20世纪70年代,TOC,TN含量波动均较大;20世纪70年代以后,TOC含量逐渐减小并趋于稳定,TN含量随养殖规模的扩大显著增加,且扇贝养殖区表层沉积物中TOC,TN的含量均低于海带养殖区。近200年桑沟湾沉积物中有机质来源变化趋势为混源-陆源-混源,近40年来的规模化养殖明显增加了水生有机碳及水生总氮在TOC和TN中所占比例。对桑沟湾沉积物中有机质污染评价结果显示,大规模的养殖活动增加了底质中沉积物TOC和TN的含量,海带筏式养殖对沉积物中的TN造成了一定的累积性自身污染。     

Effects of commercial dragging on sedimentary organic matter

The effect of commercial dragging on sedimentary organic matter was examined in two field experiments using different types of gear. A heavy scallop dredge caused two types of organic matter translocation—some of the surficial organic matter was exported from the drag site and the remaining material was mixed into subsurface sediments. Phospholipid analysis indicated decreases in various classes of microbial, with relative increases in the contribution of anaerobic bacteria to the microbial community. An other trawl that largely remained above the sediment-water interface caused little change in organic matter profiles, although ⁷Be profiles suggest an export of the surficial horizon. Sediment mixing by some types of gear will likely result in burial of labile organic matter and hence may shift sediment metabolism toward microbial and anaerobic food chains.     

Amino acid biogeochemistry and bacterial contribution to sediment organic matter along the western margin of the Bay of Bengal

Six sediment cores collected from various water depths and sampling locations along the western margin of the Bay of Bengal (BOB) were investigated for the total hydrolysable amino acids (THAA) and d-amino acids (d-AA) to understand their distribution, digenetic alteration and bacterial contribution to organic matter (OM). Irrespective of their location, THAA concentrations and yields generally decreased and mol% glycine increased with increasing water depth indicating that OM was degraded during its transit through the water column. Amino acid based degradation index (DI) indicated that OM of the surface sediments of shallow stations, BOB-1 to BOB-3 was relatively fresher than that of deeper stations, BOB-4, BOB-5 and BOB-6. The concentrations and mol% of the d-AA varied from 0.04 to 0.76 µmol gdw⁻¹ and 0.3 to 8.5 mol%, respectively. Contribution of bacterial peptidoglycan amino acids to THAA (% THAA_pep/THAA) ranged between 4.0% and 55.0%. Both % THAA_pep/THAA and mol% d-AAs were significantly (p<0.01) higher in the surface sediments and decreased with sediment core depth. Based on the d-AA yields, bacterial OM accounted for 1.5–15.6% of TOC, and 3.7–50.0% of TN of the sediments of BOB.     

Nutrient fluxes to the Bay of Biscay from Cantabrian rivers (Spain)

Nutrient fluxes to the Bay of Biscay from the Cantabrian basin have been quantified for the first time. Data between 1981 and 1995 of the main 16 Cantabrian rivers from the COCA monitoring programme have been used. Values of water flow and dissolved inorganic nitrogen (DIN), phosphate and silicate concentrations have been taken. Equations are proposed to quantify the fluvial nutrient contributions to the Cantabrian Sea. The annual average of continental outputs to the Bay of Biscay from the Cantabrian basin is 16.1 × 10⁹ m³ of freshwater, 1.0 × 10⁹ mol of N in DIN, 0.062 × 10⁹ mol of phosphate and 1.2 × 10⁹ mol of silicate. In comparison with the French rivers, those of the Cantabrian have small fluxes and their outflow is very disperse, not forming large coastal plumes. From April to September, when the primary production is relatively important, the DIN contribution to the Cantabrian coastal reservoir is 10%. Coastal fertilisation due to continental waters could be considered as negligible and only influences areas very close to river mouths, except for the Nalón River. Its flux represents 33 % of nitrate, 39 % of phosphate and 15 % of silicate of the total continental inputs of nutrients to the Cantabrian Sea.     

Origin of sedimentary organic matter in the north-western Adriatic Sea

In order to evaluate the origin and the transformation of organic matter on the shallow shelf of the NW Adriatic Sea, organic carbon, total nitrogen and stable isotope ratios of organic carbon were analysed in riverine suspended matter and sediments as well as in marine suspended and sedimentary organic matter, in marine phytoplankton and zooplankton.The deposition of organic matter is influenced by fine sediment concentration. Surface sediments were characterised by highly variable biogeochemical conditions on the sea floor, whereas sub-surface sediments showed a more homogeneous hypoxic/anoxic environment.Low C_org/N ratio and high organic carbon and nitrogen concentrations in riverine suspended organic matter indicate an important contribution of freshwater phytoplankton within rivers, particularly during low flow regimes, which adds to the marine phyto- and zooplankton at shelf locations.In order to evaluate the importance of terrestrial, riverine and marine sources of OM in shelf sediments, a three end-member mixing model was applied to shelf surface sediments using ¹³C/¹²C values for organic matter and N/C ratios. The model showed an elevated contribution of terrestrial organic substances at intermediate depths (10–15 m), mostly corresponding to an area of coarser grain-size, whereas the riverine and marine organic fractions were mainly accumulating near the coast and offshore, respectively.     

Sedimentology of box cores from the Cap-Ferret Canyon area (Bay of Biscay)

Sedimentological and geochemical investigations of 45 box cores collected in various morphological settings of the Cap-Ferret Canyon (Bay of Biscay) are presented to document accurately present-day sedimentary processes on the eastern Atlantic continental slope. The magnitude and variations through time and space of the canyon's channelling or sinking effect on fine-grained particles behaviour in comparison with sediment flux across the continental margin was particularly considered and discussed:1. All the parameters (grain-size, carbonate and water content, major and trace elements), measured both in surface sediment and downcore, demonstrate that the characteristics at the sediment interface vary with water depth and with the morphological setting.2. Surface sediment is generally coarser-grained, more terrigenous and deposited at higher rate in the canyon than outside. The terrigenous particle supply must be preferentially directed and trapped within the canyon's depression due to present-day dynamic conditions.3. The downcore gradients reflected in grain-size variations yield information on settling processes. The coarse-grained population has the characteristics of a winnowed sediment similar to those on the outer shelf, while the fine-grained population has grain-size spectra very similar to the present-day fine-grained suspensions.4. The carbonate particles are partly derived from direct pelagic production (distinct grain-size distribution) and, like terrigenous grains, are partly reworked (similar downslope decrease in the coarse grained fraction). The relatively low CaCO₃ content observed in the canyon, and its downward increase up to values observed at shallower depths, may result from a channelling of terrigenous suspensions within the canyon.5. At the present high sea-level stand, the canyon should become a trap for sediments without much gravity remobilisation, as indicated by a lack of sedimentary structures in box cores. However, a simple increase in sediment trapping can hardly account for the downcore gradients observed in the box cores. These trends, which are observed on other continental margins (Monaco et al., 1993, Journées spécialisées de la Soc. Géol. France: Géosciences Marines, 16–17 December 1994, Abstract p. 83.), indicate a probable increase in terrigenous supplies and/or in settling energy.     

Biological and granulometric controls on sedimentary organic matter of an intertidal mudflat

A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.     

The partitioning of organic carbon fluxes and sedimentary organic matter decomposition rates in the ocean

The partitioning of annual organic carbon fluxes from five stations located in the vicinity of the Pacific-Antarctic Ridge and the Peru continental margin suggests that 35–85% of the total near-bottom organic carbon flux is utilized at or near the sediment-water interface. These estimates have large uncertainties, but illustrate that assessments of organic carbon utilization can be made by several stepwise approaches which are generally applicable to a wide spectrum of marine environments.In one approach, the mineralization of organic carbon from the sediments was predicted from both sedimentary organic carbon and pore water nutrient profiles with comparable results. Neglecting sediment mixing, the rate constants of the anoxic sediments off Peru range from 0.1 × 10^?3 to 4 × 10^?3 y^?1, and rate constants derived for oxic SW Pacific sediments range from 3 × 10^?4 to 7 × 10^?4 y^?1. As with other values reported for sulfate reducing sediments by Toth and Lerman (1977) and for oxic central Pacific sediments by Müller and Mangini (1980), log-log plots of rate constants vs. sedimentation rate define two parallel linear relationships for oxic and anoxic sediments, respectively. The apparently enhanced rates for oxic environments may result from large benthic organisms which redistribute a portion of the available detritus and in doing so convert it into more easily accessible and metabolizable organic matter. In low-oxygen environments, bottom feeders and infauna are less abundant and more likely to irrigate rapidly accumulating sediments.     

Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Chromophoric dissolved organic matter (CDOM) is the light absorbing fraction of dissolved organic carbon (DOC). The optical properties of CDOM potentially permit remote sensing of DOC and CDOM, and correction for CDOM absorption is essential for remote sensing of chlorophyll a (chl a) in coastal and estuarine waters. To provide data for this purpose, we report the distributions of CDOM, DOC, and chl a from seven cruises in Chesapeake Bay in 1994–1997. We observed non-conservative distributions of chl a and DOC in half of the cruises, indicating net accumulations within the estuary; however, there were no net accumulations or losses of CDOM, measured as absorption at 355 nm or as fluorescence. Freshwater end member CDOM absorption varied from 2.2 to 4.1 m⁻¹. Coastal end member CDOM absorption was considerably lower, ranging over 0.4–1.1 m⁻¹. The fluorescence/absorption ratio was similar to those reported elsewhere for estuarine and coastal waters; however, in the lower salinity/high CDOM region of the Bay, the relationship was not constant, suggestive of the mixing of two or more CDOM sources. Chl a was not correlated with the absorption for most of the cruises nor for the data set as a whole; however, CDOM and DOC were significantly correlated, with two groups evident in the data. The first group had high CDOM concentrations per unit DOC and corresponded to the conservative DOC values observed in the transects. The second group had lower CDOM concentrations per unit DOC and corresponded to the non-conservative DOC values associated with net DOC accumulation near the chl a maximum on the salinity gradient. This indicates the production of non-chromophoric DOC in the region of the chl a maximum of Chesapeake Bay. In terms of remote sensing, these data show that (1) the retrieval of the absorption coefficient of CDOM from fluorescence measurements in the Bay must consider the variability of the fluorescence/absorption relationship, and (2) estimates of DOC acquired from CDOM absorption will underestimate DOC in regions with recent, net accumulations of DOC.     

Comparison of polycyclic aromatic hydrocarbon distributions and sedimentary organic matter characteristics in contaminated, coastal sediments from Pensacola Bay, Florida

In this study, we examined the distribution of polycyclic aromatic hydrocarbons (PAHs) in a contaminated coastal area and the characteristics of the natural organic matter in tandem. We present a detailed study of PAH concentration, distribution, and organic matter characteristics of three core samples from Pensacola Bay, Florida. Solid-state 13C Nuclear Magnetic Resonance (NMR), pyrolysis gas chromatography coupled with mass spectrometry (GC-MS), and tetramethyl ammonium hydroxide (TMAH) thermochemolysis GC-MS were applied to obtain structural details about the sedimentary organic matter. Elemental compositions (carbon and nitrogen) and estimates of black carbon contents are also reported. These coastal sediments were found to contain more PAHs in the upper 15 cm layers than in the bottom 15-25 cm samples. The samples that contained the most PAHs also contained the least amount of aromatic carbon and contained a significant amount of paraffinic carbon. Lignin-derived pyrolysis and TMAH thermochemolysis products were abundant and generally higher in all of the samples in comparison to those reported for modern coastal sediments, indicating a large flux of terrestrial carbon. The black carbon contents were found to range from 4.3% to 6.8%, which are significantly lower than other reports of black carbon in sediments, which represent as much as 65% of the total organic carbon content. The low black carbon content suggests that this type of refractory carbon may not be as responsible for regulating PAH distribution as indicated by other researchers.