Benthic foraminifera were collected in the Rhône prodelta (Gulf of Lions, Mediterranean Sea), an enriched zone with high organic matter content. In June 2005, sediment cores were sampled at depths ...ranging from 20 to 100 m. Four distinct foraminiferal assemblages were determined in the study area, reflecting the geographical distribution of the impact of river supply. The living foraminiferal faunas present a typical picture, with strongly impoverished faunas composed exclusively of stress-tolerant taxa (
Fursenkoina fusiformis,
Bulimina aculeata,
Leptohyalis scottii, and
Adelosina longirostra) in the immediate vicinity of the river mouth. This assemblage is well adapted to a high input of continental organic matter and a minimum oxygen penetration depth into the sediment. To the southwest, under the main corridor followed by the river plume, high organic input with a dominantly terrestrial signature (more refractory) may be stressful for many taxa which need organic matter of a more labile quality. In this area,
Nonion scaphum,
Nonionella turgida and
Rectuvigerina phlegeri are present in low densities. On the edge of this area, these taxa show much higher densities. A greater proportion of marine organic carbon could explain their increasing abundances in this area. Towards the east and towards the deepest stations, in the outer part of the enriched zone, biodiversity increases. Faunas at these stations have intermediate densities and contain a number of taxa (
Cassidulina carinata, Epistominella vitrea, Valvulineria bradyana, Nonionella iridea/bradyi) at the deepest stations;
Bolivina dilatata/spathulata and
Textularia porrecta at the eastern stations) that seem to benefit from more marine organic matter. The comparison of geochemical measurements and foraminiferal data strongly suggests that the spatial distribution of foraminifera in the Rhône prodelta is mainly governed by the quality and the quantity of organic matter reaching the sediment–water interface. Since bottom waters are well oxygenated (215–260 µmol/L), and oxygen penetration into the sediment is less than 1 cm at all stations, benthic ecosystem oxygenation appears to have only a minor impact on regional differences in faunal distribution.
We examined the occurrence of seasonal hypoxia (O
2<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.
Using data collected in 2009, we evaluated the potential for the southeastern Canada Basin (Arctic Ocean) to act as an atmospheric CO2 sink under the summertime ice‐free conditions expected in the ...near future. Beneath a heavily decayed ice cover, we found surprisingly high pCO2sw (~290–320 µatm), considering that surface water temperatures were low and the influence of ice melt was strong. A simple model simulating melt of the remaining ice and exposure of the surface water for 100 days revealed a weak capacity for atmospheric CO2 uptake (mean flux: −2.4 mmol m−2 d−1), due largely to warming of the shallow mixed layer. Our results confirm a previous finding that the Canada Basin is not a significant sink of atmospheric CO2 under summertime ice‐free conditions and that increased ventilation of the surface mixed layer due to sea ice loss is weakening the sink even further.
Key Points
The southeastern Canada Basin has a weak capacity to absorb atmospheric CO2
A shallow mixed layer that warms rapidly after ice loss inhibits CO2 uptake
Ice‐free summers in the Canada Basin will not create a significant new CO2 sink
The silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep ...Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = −0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. Nevertheless, the δ30Si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30ε, between −0.33‰ and −0.41‰, depending on how the contribution of sea‐ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, the δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125–200 m) composed of modified Pacific water and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300–500 m deep). In the Canada Basin Deep Water (below 2000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.
Key Points
Deep Arctic silicic acid isotopic signals are the heaviest of all ocean basins
Riverine inputs decrease the surface Arctic silicic acid isotopic composition
Silicon isotopic distribution in the Arctic follows that of major water masses
Estuaries and deltas are crucial zones to better understand the interactions between continents and oceans, and to characterize the mineralization and burial of different sources of organic matter ...(OM) and their effect on the carbon cycle. In the present study, we focus on the continental shelf of the northwest Mediterranean Sea near the Rhône river delta. Sediment cores were collected and pore waters were sampled at different depths at one station (Station E) located on this shelf. For each layer, measurements of dissolved inorganic carbon concentration (DIC) and its isotopic composition (δ
13C and Δ14C) were conducted and a mixing model was applied to target the original signature of the mineralized OM. The calculated δ
13C signature of the mineralized organic matter is in accordance with previous results with a δ
13COM of marine origin that is not significantly impacted by the terrestrial particulate inputs from the river. The evolution with depth of Δ14C shows two different trends indicating two different Δ14C signatures for the mineralised OM. In the first 15 cm, the mineralized OM is modern with a Δ14COM = 100 ± 17‰ and corresponds to the OM produced during the nuclear period of the last 50 years. Deeper in the sediment, the result is very different with a depleted value Δ14COM = –172 ± 60‰ which corresponds to the pre-nuclear period. In these two cases, the marine substrate was under the influence of the local marine reservoir effect with more extreme Δ14C results. These differences can be largely explained by the influence of the river plume on the local marine DIC during these two periods.
The Southern Ocean is known to be the largest high-nutrient, low-chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually ...downstream of the Kerguelen Islands, a bloom which is sustained partly by iron released from the sediments deposited onto the shelves. In the framework of the KEOPS-2 project, we used radium isotopes (224Ra, T1/2 = 3.66 d; 223Ra, T1/2 = 11.4 d; 228Ra, T1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sediment-derived inputs (including iron and other micronutrients) towards offshore waters. Significant 224Ra and 223Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224Ra and 223Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the polar front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales on the order of several days up to several weeks) and (ii) that the polar front is not a physical barrier for the chemical elements released from the sediments of the Kerguelen Plateau. The Ra data set suggests that iron and other micronutrients released by the shelves of the Kerguelen Islands may contribute to fueling the phytoplankton bloom downstream of the islands, despite the presence of the polar front. However, the heterogeneous distribution of the 224Ra and 223Ra activities in surface waters suggests that this supply across the front is not a continuous process but rather a process that is highly variable in space and time.
A significant fraction of the global carbon flux to the ocean occurs in River-dominated Ocean Margins (RiOMar) although large uncertainties remain in the cycle of organic matter (OM) in these ...systems. In particular, the OM sources and residence time have not been well clarified. Surface (0–1cm) and sub-surface (3–4cm) sediments and water column particles (bottom and intermediate depth) from the Rhône River delta system were collected in June 2005 and in April 2007 for a multi-proxy study. Lignin phenols, black carbon (BC), proto-kerogen/BC mixture, polycyclic aromatic hydrocarbons (PAHs), carbon stable isotope (δ13COC), and radiocarbon measurements (Δ14COC) were carried out to characterize the source of sedimentary organic material and to address degradation and transport processes. The bulk OM in the prodelta sediment appears to have a predominantly modern terrigenous origin with a significant contribution of modern vascular C3 plant detritus (Δ14COC=27.9‰, δ13COC=−27.4‰). In contrast, the adjacent continental shelf, below the river plume, seems to be dominated by aged OM (Δ14COC=−400‰, δ13COC=−24.2‰), and shows no evidence of dilution and/or replacement by freshly produced marine carbon. Our data suggest an important contribution of black carbon (50% of OC) in the continental shelf sediments. Selective degradation processes occur along the main dispersal sediment system, promoting the loss of a modern terrestrial OM but also proto-kerogen-like OM. In addition, we hypothesize that during the transport across the shelf, a long term resuspension/deposition loop induces efficient long term degradation processes able to rework such refractory-like material until the OC is protected by the mineral matrix of particles.
In this paper, we investigate the ecology of live (rose Bengal stained) benthic foraminifera collected at 20 stations ranging from 15 to 100
m depth in the Rhône prodelta (Gulf of Lions, NW ...Mediterranean). These sites were sampled in September 2006, five months after the Rhône River annual flood. Statistical analyses based on foraminiferal communities (>
150
μm) divide our study area into six main biofacies directly related to environmental conditions. Miliolid species are abundant in the relict prodeltaic lobe which is characterised by sand with low organic matter content. Close to the river mouth, the limited oxygen penetration in the sediment combined with important hydro-sedimentary processes constitute stressful conditions for foraminiferal faunas dominated by opportunistic species (e.g.
Leptohalysis scottii). With increasing distance from the river mouth, foraminiferal faunas (e.g.
Nonionella turgida,
Eggerella scabra) adapted to thrive in sediments enriched in Rhône-derived organic matter under more stable hydro-sedimentary conditions appear. In the distal part of the Rhône River influence, benthic species (e.g.
Valvulineria bradyana,
Textularia agglutinans) living in fine sediment enriched in both continental and marine organic compounds emerge. At the deepest stations located in the south-eastern part of our study area, benthic foraminiferal faunas (e.g.
Bulimina aculeata,
Melonis barleeanus,
Bigenerina nodosaria) are highly diverse, underlining stable environmental conditions characterised by marine-derived organic matter supplies and relatively deep oxygen penetration depth in the sediment. We also compare foraminiferal faunas sampled in September 2006 with communities sampled in June 2005, one month after the Rhône River annual flood (
Mojtahid et al., 2009). This comparison suggests that opportunistic species (e.g.
B. aculeata,
Cassidulina carinata,
V. bradyana) have responded to organic matter inputs related to marine primary production in June 2005.
A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using
in situ measurements of ...sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform:
in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO
3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds.
The
in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5
mmol
m
−2
d
−1 at 1300 and 3100
m depth, respectively, and between 1.9±0.3 and 2.4±0.2
mmol
m
−2
d
−1 at 4000
m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1
mmol
m
−2
d
−1, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps.
Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800
m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19
mm
y
−1) found on this site. The Lobe region could receive as high as 19
mol
C
m
−2
y
−1, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.
We present an ecological study of live (Rose Bengal stained) foraminifera from 6 deep-sea stations sampled on the open slope between the Grand Rhône Canyon and the Petit Rhône Canyon (eastern part of ...the Gulf of Lions, NW Mediterranean). The 6 stations describe a bathymetric transect from ∼350 to ∼2000
m depth. The main objective of our study is to investigate the changes of the foraminiferal density, composition and microhabitat along this transect in response to the physico-chemical conditions at and below the sediment–water interface. All our observations underline the general meso-oligotrophic character of our inter-canyon open-slope setting where low-quality organic matter originating from both marine and continental sources settles. The input of organic matter at the sediment–water interface leads to a classical succession of redox reactions within the sediment. The shallowest station (∼350
m) appears as an active sedimentary environment, where coarse sediments characterized by lower-quality organic matter and biogenic material accumulate. The 550-m-deep station presents bioturbated sediments with the highest concentration of labile organic compounds. The deeper stations, between about 750 and 2000
m deep, show decreasing sedimentation rates with water depth and are characterized by a background of low-quality organic matter. The foraminiferal changes recorded along the bathymetric transect are related to a complex association of physico-chemical parameters. We think that the quality of organic matter in the surficial sediment, as expressed by the lipid concentration, is the major parameter controlling the foraminiferal distribution at our open-slope stations. From the 550- to the 2000-m-deep station, the foraminiferal standing stocks and diversity decrease with depth, as a result of the increasing scarcity of labile organic compounds at the sediment–water interface. Oxygen concentration and penetration depth and the intensity of bioturbation seem to play only a secondary ecological role. Other, putative hydro-sedimentary processes (winnowing by strong bottom currents, sand-bed deposition) appear as additional parameters controlling the foraminiferal community structure. At the 350-m-deep station, the live foraminiferal fauna can be considered as a non-equilibrium assemblage thriving in frequently disturbed and food-impoverished sediments. At the 745- and 980-m-deep stations, the occurrence of suspensivorous epibenthic/epilithic species suggests the presence of strong bottom-water current velocities and the related suspension of organic particles.
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