Seasonal variability of water mass distribution in the southeastern Beaufort Sea determined by total alkalinity and delta O-18

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Macdonald, Robie W. Gratton, Yves Macdonald, Robie/A-7896-2012 Macdonald, Robie/0000-0002-1141-8520 CCGS Amundsen; CASES (Canadian Arctic Shelf Exchange Study) NSERC network (Natural Sciences and Engineering Research Council of Canada); Canadian Fund for Innovation; Canadian Coast Guard; Department of Fisheries and Oceans Canada; NSERC Discovery We thank the officers and crew of the CCGS Amundsen for their support and dedication to the CASES expedition. We are indebted to Constance Guignard, Nes Sutherland, Pascale Collin, Simon Belanger, Jens Ehn, Mike Arychuk and Owen Owens for their care and perseverance in collecting and analyzing the TA, TIC and pH samples at sea. Thanks must go to the CTD data acquisition group for these basic but critical measurements and the calibration of the various probes. Most of the plots and maps in this study were created with the ODV Software [Schlitzer, 2009]. We also thank A. Proshutinsky and two anonymous reviewers who provided constructive comments that helped to improve our manuscript. This study was funded through the CASES (Canadian Arctic Shelf Exchange Study) NSERC network (Natural Sciences and Engineering Research Council of Canada) and a Canadian Fund for Innovation grant to support the upgrade and operation of the CCGS Amundsen. Additional financial contributions were provided by the Canadian Coast Guard, the Strategic Science Fund of the Department of Fisheries and Oceans Canada, and NSERC Discovery grants to A. Mucci and Y. Gratton. 10 AMER GEOPHYSICAL UNION WASHINGTON J GEOPHYS RES-OCEANSWe examined the seasonal variability of water mass distributions in the southeastern Beaufort Sea from data collected between September 2003 and August 2004. Salinity, total alkalinity (TA) and isotopic composition (delta O-18) of seawater were used together as tracers of freshwater input, i.e., meteoric water and sea ice meltwater. We used an optimum multiparameter analysis to identify the different water masses, including the Mackenzie River, sea ice melt (SIM), winter polar mixed layer (PML), upper halocline water (UHW) with core salinity of 33.1 psu (Pacific origin) and Atlantic Water. Computed values of CO2 fugacity in seawater (fCO(2)-sw) show that the surface mixed layer (SML) remains mostly undersaturated (328 +/- 55 mu atm, n = 552) with respect to the average atmospheric CO2 concentration (380 +/- 5 mu atm) over the study period. The influence of the Mackenzie River (fCO(2-SW) > 500 mu atm) was relatively small in the southeastern Beaufort Sea, and significant fractions were only observed on the inner Mackenzie Shelf. The contribution of sea ice melt (fCO(2-SW) 600 mu atm) was usually located between 120 and 180 m depth, but could contribute to the SML during wind-driven upwelling events, in summer and autumn, and during brine-driven eddies, in winter

Study Of The Phytoplankton Plume Dynamics Off The Crozet Islands (Southern Ocean): A Geochemical-Physical Coupled Approach

2169-9291The Crozet Archipelago, in the Indian sector of the Southern Ocean, constitutes one of the few physical barriers to the Antarctic Circumpolar Current. Interaction of the currents with the sediments deposited on the margins of these islands contributes to the supply of chemical elements--including iron and other micro-nutrients--to offshore high-nutrient, low-chlorophyll (HNLC) waters. This natural fertilization sustains a phytoplankton bloom that was studied in the framework of the KEOPS-2 project. In this work, we investigated the time scales of the surface water transport between the Crozet Island shelves and the offshore waters, a transport that contributes iron to the phytoplankton bloom. We report shelf-water contact ages determined using geochemical tracers (radium isotopes) and physical data based on in situ drifter data and outputs of a model based on altimetric Lagrangian surface currents. The apparent ages of surface waters determined using the three independent methods are in relatively good agreement with each other. Our results provide constraints on the time scales of the transport between the shelf and offshore waters near the Crozet Islands and highlight the key role played by horizontal transport in natural iron fertilization and in defining the extension of the chlorophyll plume in this HNLC region of the Southern Ocean

Spatial distribution of benthic foraminifera in the Rhône prodelta: faunal response to organic matter focussing

On many continental shelf areas, the combination of high surface water productivity coupled with limited water depth leads to important organic matter deposits on the sea floor. In the Gulf of Lion, the Rhone River is a major source of nutrients and organic matter. This important supply may create important eutrophication and hypoxia on the benthic environment. In our study, three faunal assemblages occur in relation to the organic enrichment gradient and to the oxygen penetration in the sediment. The first assemblage is situated in the immediate vicinity of the river mouth (1.3 % < Corg < 1.9 %; 1 mm < O2 penetration< 2 mm); the faunas are characterized by a low density and biodiversity; they are dominated by Fursenkoina fusiformis, Bulimina aculeata, Reophax scotti, and A. longirostra. A second assemblage is situated in the intermediate part of the organic enrichment zone (1.0 % < Corg < 1.3 %; 2 mm < O2 penetration< 4 mm) and is characterized by maximum densities and intermediate biodiversity; faunas are dominated by the species Nonionella turgida, Hopkinsina pacifica and Nonion scaphum accompanied in lower proportions by the species Rectuvigerina phlegeri. A third assemblage is situated in the outer part of the organic-rich sediments (0.7 % < Corg < 1.0 %; 4 < O2 penetration< 7 mm). The faunas are characterized by high densities and a high biodiversity; they are dominated by Cassidulina carinata accompanied in lower proportions by the species Epistominella vitrea, Valvulineria bradyana and Textularia porrecta

Benthic foraminifera as tracers of brine production in the Storfjorden "sea ice factory"

The rapid response of benthic foraminifera to environmental factors (e.g. organic matter quality and quantity, salinity, pH) and their high fossilisation potential make them promising bio-indicators for the intensity and recurrence of brine formation in Arctic seas. Such an approach, however, requires a thorough knowledge of their modern ecology in such extreme settings. To this aim, seven stations along a north-south transect across the Storfjorden (Svalbard archipelago) have been sampled using an interface multicorer. This fjord is an area of intense sea ice formation characterised by the production of brine-enriched shelf waters (BSW) as a result of a recurrent latent-heat polynya. Living (rose bengal-stained) foraminiferal assemblages were analysed together with geochemical and sedimentological parameters in the top 5 cm of the sediment. Three major biozones were distinguished. (i) The inner fjord zone, dominated by typical glacier proximal calcareous species, which opportunistically respond to fresh organic matter inputs. (ii) The deep basins and sill zone, characterised by glacier distal agglutinated fauna; these are either dominant because of the mostly refractory nature of organic matter and/or the brine persistence that hampers the growth of calcareous species and/or causes their dissolution. (iii) The outer fjord zone, characterised by typical North Atlantic species due to the intrusion of the North Atlantic water in the Storfjordrenna. The stressful conditions present in the deep basins and sill (i.e. acidic waters and low food quality) result in a high agglutinated = calcareous ratio (A=C). This supports the potential use of the A=C ratio as a proxy for brine persistence and overflow in Storfjorden

Patterns of suspended particulate matter across the continental margin in the Canadian Beaufort Sea during summer

The particulate beam attenuation coefficient at 660&thinsp;nm, cp(660), was measured in conjunction with properties of suspended particle assemblages in August 2009 within the Canadian Beaufort Sea continental margin, a region heavily influenced by freshwater and sediment discharge from the Mackenzie River, but also by sea ice melt. The mass concentration of suspended particulate matter (SPM) ranged from 0.04 to 140&thinsp;g&thinsp;m−3, its composition varied from mineral to organic dominated, and the median particle diameter determined over the range 0.7–120&thinsp;µm varied from 0.78 to 9.45&thinsp;µm, with the fraction of particles &lt;1&thinsp;µm in surface waters reflecting the degree influenced by river water. Despite this range in particle characteristics, a strong relationship between SPM and cp(660) was found and used to determine SPM distributions across the shelf based on measurements of cp(660) taken during summer seasons of 2004, 2008, and 2009. SPM spatial patterns on the stratified shelf reflected the vertically sheared two-layer estuarine circulation and SPM sources (i.e., fluvial inputs, bottom resuspension, and biological productivity). Along-shelf winds generated lateral Ekman flows, isopycnal movements, and upwelling or downwelling at the shelf break. Cross-shelf transects measured during three summers illustrate how sea ice meltwater affects river plume extent, while the presence of meltwater on the shelf was associated with enhanced near-bottom SPM during return flow of upwelled Pacific-origin water. SPM decreased sharply past the shelf break with further transport of particulate matter occurring near the bottom and in interleaving nepheloid layers. These findings expand our knowledge of particle distributions in the Beaufort Sea controlled by river discharge, sea ice, and wind, each of which is sensitive to weather and climate variations.</p

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers

We examined the occurrence of seasonal hypoxia (O2&lt;2 mg l-1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1-10 g C m-2 d-1), and lower off the Rhône River (<1 g C m-2 d-1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia

Oxygen dynamics in shelf seas sediments incorporating seasonal variability

Shelf sediments play a vital role in global biogeochemical cycling and are particularly important areas of oxygen consumption and carbon mineralisation. Total benthic oxygen uptake, the sum of diffusive and faunal mediated uptake, is a robust proxy to quantify carbon mineralisation. However, oxygen uptake rates are dynamic, due to the diagenetic processes within the sediment, and can be spatially and temporally variable. Four benthic sites in the Celtic Sea, encompassing gradients of cohesive to permeable sediments, were sampled over four cruises to capture seasonal and spatial changes in oxygen dynamics. Total oxygen uptake (TOU) rates were measured through a suite of incubation experiments and oxygen microelectrode profiles were taken across all four benthic sites to provide the oxygen penetration depth and diffusive oxygen uptake (DOU) rates. The difference between TOU and DOU allowed for quantification of the fauna mediated oxygen uptake and diffusive uptake. High resolution measurements showed clear seasonal and spatial trends, with higher oxygen uptake rates measured in cohesive sediments compared to the permeable sediment. The significant differences in oxygen dynamics between the sediment types were consistent between seasons, with increasing oxygen consumption during and after the phytoplankton bloom. Carbon mineralisation in shelf sediments is strongly influenced by sediment type and seasonality

Benthic remineralization at the land-ocean interface : a case study of the Rhone River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhone River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (delta C-13(OC)) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhone delta system. Large spatial variations were observed in both the total O-2 uptake (5.2 to 29.3 mmol m(-2) d(-1)) and NH4+ release (-0.1 to -3.5 mmol m(-2) d(-1)) rates at the sediment-water interface. The highest fluxes were measured near the Rhone River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of delta C-13(OC) ranged from -26.83 parts per thousand to -23.88 parts per thousand with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhone mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhone POC input is either buried (similar to 52%) or remineralized (similar to 8%), mostly on the prodelta area. The remaining fraction (similar to 40%) is either mineralized in the water or exported outside the Rhone delta system in dissolved or particulate forms

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea).

International audienc

In situ oxygen uptake rates by coastal sediments under the influence of the Rhone River (NW Mediterranean Sea)

The influence of riverine inputs on biogeochemical cycling and organic matter recycling in sediments on the continental shelf off the Rhone River mouth (NW Mediterranean Sea) was investigated by measuring sediment oxygen uptake rates using a combination of in situ and laboratory techniques. Four stations were investigated during two cruises in June 2001 and June 2002, with depths ranging from 9 to 192 m and over a distance to the Rhone River mouth ranging from 4 to 36 km. Diffusive oxygen uptake (DOU) rates were determined using an in situ sediment microprofiler and total oxygen uptake (TOU) rates were measured using sediment core incubations. There was good agreement between these two techniques which indicates that the non-diffusive fraction of the oxygen flux was minimal at the investigated stations. DOU rates ranged from 3.7 +/- 0.4 mmol O-2 m(-2) d(-1) at the continental shelf break to 19.3 +/- 0.5 mmol O-2 m(-2) d(-1) in front of the Rhone River mouth. Sediment oxygen uptake rates mostly decreased with increasing depth and with distance from the Rhone mouth. The highest oxygen uptake rate was observed at 63 m on the Rhone prodelta, corresponding to intense remineralization of organic matter. This oxygen uptake rate was much larger than expected for the increasing bathymetry, which indicates that biogeochemical cycles and benthic deposition are largely influenced by the Rhone River inputs. This functioning was also supported by the detailed spatial distribution of total organic carbon (TOC), total nitrogen (TN) and C/N atomic ratio in surficial sediments. Sediments of the Rhone prodelta are enriched in organic carbon (2-2.2%) relative to the continental shelf sediments (< 1%) and showed C/N ratios exceeding Redfield stoichiometry for fresh marine organic matter. A positive exponential correlation was found between DOU and TOC contents (r(2) = 0.98, n = 4). South-westward of the Rhone River mouth, sediments contained highly degraded organic matter of both terrestrial and marine origin, due to direct inputs from the Rhone River, sedimentation of marine organic matter and organic material redeposition after resuspension events