ABSTRACTEstuaries and deltas are crucial zones to better understand the interactions between cont... more ABSTRACTEstuaries 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 (δ13C and Δ14C) were conducted and a mixing model was applied to target the original signature of the mineralized OM. The calculated δ13C signature of the mineralized organic matter is in accordance with previous results with a δ13COM 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...
Deep Sea Research Part I: Oceanographic Research Papers, 1998
Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main are... more Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main area of organic matter deposition in the Cape Blanc upwelling region. The faunal analyses suggest that at the shallowest station (1200 m) the benthic ecosystem is permanently influenced by the upwelling, whereas at the deepest stations (3010 and 2530 m depth) the ocean bottom is subject to significant organic influxes only in summer. The vertical zonation of foraminiferal species in the sediment shows a close correspondence with the depth distribution of oxic respiration, nitrate and sulphate reduction. It is suggested that this linkage is caused by the presence of various stocks of anaerobic and sulphate-and nitrate-reducing bacteria. Deep infaunal foraminiferal species are thought to feed selectively, either on the bacterial stocks or on nutritious particles produced by bacterial degradation of more refractory organic matter. As such, foramininiferal microhabitats are only indirectly controlled by pore water oxygen concentrations.
Journal of Experimental Marine Biology and Ecology, 2003
Sediment and water can potentially be altered, chemically, physically and biologically as they ar... more Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists.
Interactions of C, N, P and S Biogeochemical Cycles and Global Change, 1993
Sediments are major reservoirs of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S). In fac... more Sediments are major reservoirs of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S). In fact, with the exception of N, marine sediments form the largest reservoirs of these elements at the Earth’s surface (Mackenzie et al., 1992). On geological time scales, sedimentary burial in the oceans is one of the critical sink mechanisms controlling the global distribution and cycling of C, N, P and S (e.g., Holland, 1984; Berner, 1989; Berner et al., 1992; Kump, 1992). Thus, there is a need to understand and quantify the processes that affect burial of these elements in sediments.
Periphytic biomass has an important influence on the water quality of many shallow streams. The p... more Periphytic biomass has an important influence on the water quality of many shallow streams. The purpose of this paper is to synthesize the knowledge obtained on periphyton during the PIREN Seine research program. Periphyton was sampled using chl a measurements by acetone extraction and oxygen measurements with microelectrodes. The experiments reveal the presence of an important fixed biomass ranging between 123 and 850 mgchl a m(-2) and the mean gross production (photosynthesis) is shown to range between 180 and 315 mgC m(-2) h(-1). An independent approach was performed using the ProSe model, which simulates transport and biogeochemical processes in 22 km of the Grand Morin stream. A strong agreement between in situ measurements and the model results was obtained. The gross production obtained using ProSe is 220 mgC m(-2) h(-1) for the periphyton, which matches the experimental data. Although the net photosynthetic activity of the phytoplankton (0.84 gC gC(-1) d(-1)) is higher than the periphytic one (0.33 gC gC(-1) d(-1)), the absolute periphytic activity is greater since the mean biomass (3.4 gC m(-)(2)) is 10 times higher than the phytoplanktonic one (0.3 gC m(-2)), due to the short residence time of the water body (1.5d).
A model based on transport reaction equations has been developed, introducing a new bacterial kin... more A model based on transport reaction equations has been developed, introducing a new bacterial kinetic term for the oxidation of organic matter by oxygen. This formulation is known as the Monod rate law. As it depends on both the oxygen and the organic carbon concentrations, it allows in a single model the representation of sediments undergoing totally oxic or oxic plus suboxic diagenesis. It results, however, in a system of coupled nonlinear second-order ordinary differential equations. As the common numerical solutions to such systems are not straightforward, it has infrequently been used in diagenetic modeling. By applying a numerical iterative method which utilizes the previous computation step to solve nonlinearity, the numerical instabilities of the usual methods were avoided. In order to illustrate the wide application field of this model to various diagenetic situations, verifications and sensitivity studies were performed. Rather large variations in bioturbation coefficients and kinetic constants led to small changes in the distribution of oxygen and organic carbon. On the other hand, small variations in the organic carbon rain rate or bottom water oxygen concentrations resulted in large changes in the calculated depth of the oxic-suboxic boundary. In contrast, the effect of varying sedimentation rate was negligible. Organic carbon preservation was very sensitive to sedimentation rate, the flux of organic carbon, and the bottom water concentration of oxygen. This emphasizes the importance of these three parameters and the key role of their precise determination for the quantification of the organic carbon recycling in the sediments. The effect of near-surface compaction was also estimated in order to assess the influence of physical processes on the early diagenesis of organic carbon and oxygen. The inclusion of this compaction produced a maximum departure of 30% in the oxygen penetration depth and may multiply the carbon preservation by almost a factor of 2 in certain cases. Finally, the effect of varying the kinetics as a function of burial was predicted. It showed that organic carbon might be preserved under oxic diagenetic conditions when an appropriate rate of organic carbon is reached. This addresses the question of the formation and the preservation of refractory carbon in sediments, but more experimental work needs to be done in order to confirm or disprove this hypothesis. 1.
Deep Sea Research Part II: Topical Studies in Oceanography, 2001
A coupled one-dimensional biogeochemical/physical model is developed to follow the organic matter... more A coupled one-dimensional biogeochemical/physical model is developed to follow the organic matter #uxes from the upper ocean to the sea #oor. The biogeochemical model is a nitrogen-based sevencompartment model including nutrients, phytoplankton, zooplankton, two pools of dissolved organic matter, and two size classes of detrital material. Particle dynamics are considered through the water column as well as organic matter deposition and mineralization in the super"cial sediments. The model is applied at the EUMELI oligotrophic site (213N, 313W) where di!erent seasons were sampled in 1991}1992 and sediment trap data collected continuously over the same period. The model, forced with the reanalyzed ECMWF #uxes for these years, reproduces satisfactorily the weak seasonal variability of phytoplankton concentration as well as the exported nitrogen #uxes. Annual primary production (65 g C/m/yr) is sustained mainly by remineralization of DON and zooplankton excretion. Export production at 150 m is ensured by large particles, the DON export contributing only 31% of the total export. The POC export represents 1.3% of the primary production. Including nutrient horizontal advection in the model to mimic any lateral Ekman transfer from the enriched neighboring subtropical gyre (5.5 mmol N/m/yr over the "rst 150 m estimated from optimization) induces an annual primary production of 73 g C/m/yr, closer to Morel et al.'s (Deep-Sea Res. I 43(8) (1996) 1273}1304) estimate (110 g C/m/yr). Estimated mean carbon #uxes at 1000 and 4400 m depth compare well with sediment trap data, 2 mg C/m/d and 1 mg C/m/d, respectively. Remineralization and disaggregation are the dominant processes below 150 m, aggregation playing a minor role. Observed
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Estuaries are important nutrient filters between rivers and coastal zones. However, the quantific... more Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 µM) to low values downstream (less than 10 µM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NO x-/O 2 between downstream and upstream (0.03-1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al. 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15-35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5-10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37-78% of total nitrate reduction in the upstream part of the estuaries.
Contaminated systems in which uranium (U) concentrations slightly exceed the geochemical backgrou... more Contaminated systems in which uranium (U) concentrations slightly exceed the geochemical background are of particular interest to identify natural processes governing U trapping and accumulation in Earth's surface environments. For this purpose, we examined the role of early diagenesis on the evolution of U speciation and mobility in sediments from an artificial lake located downstream from a former mining site. Sediment and pore water chemistry together with U and Fe solid state speciation were analyzed in sediment cores sampled down to 50 cm depth at four locations in the lake. These organic-rich sediments (~12 % organic C) exhibited U concentrations in the 40-80 mg.kg-1 range. The sediment columns were anoxic 2-3 mm below the sediment-water interface and pore waters pH was circumneutral. Pore water chemistry profiles showed that organic carbon mineralization was associated with Fe and Mn reduction and was correlated with a decrease in dissolved U concentration with depth.
ABSTRACTA better understanding of the dynamics of different particulate organic matter (OM) pools... more ABSTRACTA better understanding of the dynamics of different particulate organic matter (OM) pools in the coastal carbon budget is a key issue for quantifying the role of the coastal ocean in the global carbon cycle. To elucidate the benthic component of this carbon cycle at the land-sea interface, we investigated the carbon isotope signatures (δ13C and ∆14C) in the sediment pore waters dissolved inorganic carbon (DIC) in addition to the sediment OM to constrain the origin of the OM mineralized in sediments. The study site is located at the outlet of the Rhône River (Mediterranean Sea), which was chosen because this river is one of the most nuclearized rivers in Europe and nuclear 14C can serve as a tracer to follow the fate of the OM discharged by the river to the coastal sea. The ∆14C results found in the pore waters DIC show a general offset between buried and mineralized OM following a preferential mineralization model of young and fresh particles. For example, we found that the ...
ABSTRACTEstuaries and deltas are crucial zones to better understand the interactions between cont... more ABSTRACTEstuaries 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 (δ13C and Δ14C) were conducted and a mixing model was applied to target the original signature of the mineralized OM. The calculated δ13C signature of the mineralized organic matter is in accordance with previous results with a δ13COM 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...
Deep Sea Research Part I: Oceanographic Research Papers, 1998
Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main are... more Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main area of organic matter deposition in the Cape Blanc upwelling region. The faunal analyses suggest that at the shallowest station (1200 m) the benthic ecosystem is permanently influenced by the upwelling, whereas at the deepest stations (3010 and 2530 m depth) the ocean bottom is subject to significant organic influxes only in summer. The vertical zonation of foraminiferal species in the sediment shows a close correspondence with the depth distribution of oxic respiration, nitrate and sulphate reduction. It is suggested that this linkage is caused by the presence of various stocks of anaerobic and sulphate-and nitrate-reducing bacteria. Deep infaunal foraminiferal species are thought to feed selectively, either on the bacterial stocks or on nutritious particles produced by bacterial degradation of more refractory organic matter. As such, foramininiferal microhabitats are only indirectly controlled by pore water oxygen concentrations.
Journal of Experimental Marine Biology and Ecology, 2003
Sediment and water can potentially be altered, chemically, physically and biologically as they ar... more Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists.
Interactions of C, N, P and S Biogeochemical Cycles and Global Change, 1993
Sediments are major reservoirs of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S). In fac... more Sediments are major reservoirs of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S). In fact, with the exception of N, marine sediments form the largest reservoirs of these elements at the Earth’s surface (Mackenzie et al., 1992). On geological time scales, sedimentary burial in the oceans is one of the critical sink mechanisms controlling the global distribution and cycling of C, N, P and S (e.g., Holland, 1984; Berner, 1989; Berner et al., 1992; Kump, 1992). Thus, there is a need to understand and quantify the processes that affect burial of these elements in sediments.
Periphytic biomass has an important influence on the water quality of many shallow streams. The p... more Periphytic biomass has an important influence on the water quality of many shallow streams. The purpose of this paper is to synthesize the knowledge obtained on periphyton during the PIREN Seine research program. Periphyton was sampled using chl a measurements by acetone extraction and oxygen measurements with microelectrodes. The experiments reveal the presence of an important fixed biomass ranging between 123 and 850 mgchl a m(-2) and the mean gross production (photosynthesis) is shown to range between 180 and 315 mgC m(-2) h(-1). An independent approach was performed using the ProSe model, which simulates transport and biogeochemical processes in 22 km of the Grand Morin stream. A strong agreement between in situ measurements and the model results was obtained. The gross production obtained using ProSe is 220 mgC m(-2) h(-1) for the periphyton, which matches the experimental data. Although the net photosynthetic activity of the phytoplankton (0.84 gC gC(-1) d(-1)) is higher than the periphytic one (0.33 gC gC(-1) d(-1)), the absolute periphytic activity is greater since the mean biomass (3.4 gC m(-)(2)) is 10 times higher than the phytoplanktonic one (0.3 gC m(-2)), due to the short residence time of the water body (1.5d).
A model based on transport reaction equations has been developed, introducing a new bacterial kin... more A model based on transport reaction equations has been developed, introducing a new bacterial kinetic term for the oxidation of organic matter by oxygen. This formulation is known as the Monod rate law. As it depends on both the oxygen and the organic carbon concentrations, it allows in a single model the representation of sediments undergoing totally oxic or oxic plus suboxic diagenesis. It results, however, in a system of coupled nonlinear second-order ordinary differential equations. As the common numerical solutions to such systems are not straightforward, it has infrequently been used in diagenetic modeling. By applying a numerical iterative method which utilizes the previous computation step to solve nonlinearity, the numerical instabilities of the usual methods were avoided. In order to illustrate the wide application field of this model to various diagenetic situations, verifications and sensitivity studies were performed. Rather large variations in bioturbation coefficients and kinetic constants led to small changes in the distribution of oxygen and organic carbon. On the other hand, small variations in the organic carbon rain rate or bottom water oxygen concentrations resulted in large changes in the calculated depth of the oxic-suboxic boundary. In contrast, the effect of varying sedimentation rate was negligible. Organic carbon preservation was very sensitive to sedimentation rate, the flux of organic carbon, and the bottom water concentration of oxygen. This emphasizes the importance of these three parameters and the key role of their precise determination for the quantification of the organic carbon recycling in the sediments. The effect of near-surface compaction was also estimated in order to assess the influence of physical processes on the early diagenesis of organic carbon and oxygen. The inclusion of this compaction produced a maximum departure of 30% in the oxygen penetration depth and may multiply the carbon preservation by almost a factor of 2 in certain cases. Finally, the effect of varying the kinetics as a function of burial was predicted. It showed that organic carbon might be preserved under oxic diagenetic conditions when an appropriate rate of organic carbon is reached. This addresses the question of the formation and the preservation of refractory carbon in sediments, but more experimental work needs to be done in order to confirm or disprove this hypothesis. 1.
Deep Sea Research Part II: Topical Studies in Oceanography, 2001
A coupled one-dimensional biogeochemical/physical model is developed to follow the organic matter... more A coupled one-dimensional biogeochemical/physical model is developed to follow the organic matter #uxes from the upper ocean to the sea #oor. The biogeochemical model is a nitrogen-based sevencompartment model including nutrients, phytoplankton, zooplankton, two pools of dissolved organic matter, and two size classes of detrital material. Particle dynamics are considered through the water column as well as organic matter deposition and mineralization in the super"cial sediments. The model is applied at the EUMELI oligotrophic site (213N, 313W) where di!erent seasons were sampled in 1991}1992 and sediment trap data collected continuously over the same period. The model, forced with the reanalyzed ECMWF #uxes for these years, reproduces satisfactorily the weak seasonal variability of phytoplankton concentration as well as the exported nitrogen #uxes. Annual primary production (65 g C/m/yr) is sustained mainly by remineralization of DON and zooplankton excretion. Export production at 150 m is ensured by large particles, the DON export contributing only 31% of the total export. The POC export represents 1.3% of the primary production. Including nutrient horizontal advection in the model to mimic any lateral Ekman transfer from the enriched neighboring subtropical gyre (5.5 mmol N/m/yr over the "rst 150 m estimated from optimization) induces an annual primary production of 73 g C/m/yr, closer to Morel et al.'s (Deep-Sea Res. I 43(8) (1996) 1273}1304) estimate (110 g C/m/yr). Estimated mean carbon #uxes at 1000 and 4400 m depth compare well with sediment trap data, 2 mg C/m/d and 1 mg C/m/d, respectively. Remineralization and disaggregation are the dominant processes below 150 m, aggregation playing a minor role. Observed
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Estuaries are important nutrient filters between rivers and coastal zones. However, the quantific... more Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 µM) to low values downstream (less than 10 µM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NO x-/O 2 between downstream and upstream (0.03-1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al. 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15-35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5-10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37-78% of total nitrate reduction in the upstream part of the estuaries.
Contaminated systems in which uranium (U) concentrations slightly exceed the geochemical backgrou... more Contaminated systems in which uranium (U) concentrations slightly exceed the geochemical background are of particular interest to identify natural processes governing U trapping and accumulation in Earth's surface environments. For this purpose, we examined the role of early diagenesis on the evolution of U speciation and mobility in sediments from an artificial lake located downstream from a former mining site. Sediment and pore water chemistry together with U and Fe solid state speciation were analyzed in sediment cores sampled down to 50 cm depth at four locations in the lake. These organic-rich sediments (~12 % organic C) exhibited U concentrations in the 40-80 mg.kg-1 range. The sediment columns were anoxic 2-3 mm below the sediment-water interface and pore waters pH was circumneutral. Pore water chemistry profiles showed that organic carbon mineralization was associated with Fe and Mn reduction and was correlated with a decrease in dissolved U concentration with depth.
ABSTRACTA better understanding of the dynamics of different particulate organic matter (OM) pools... more ABSTRACTA better understanding of the dynamics of different particulate organic matter (OM) pools in the coastal carbon budget is a key issue for quantifying the role of the coastal ocean in the global carbon cycle. To elucidate the benthic component of this carbon cycle at the land-sea interface, we investigated the carbon isotope signatures (δ13C and ∆14C) in the sediment pore waters dissolved inorganic carbon (DIC) in addition to the sediment OM to constrain the origin of the OM mineralized in sediments. The study site is located at the outlet of the Rhône River (Mediterranean Sea), which was chosen because this river is one of the most nuclearized rivers in Europe and nuclear 14C can serve as a tracer to follow the fate of the OM discharged by the river to the coastal sea. The ∆14C results found in the pore waters DIC show a general offset between buried and mineralized OM following a preferential mineralization model of young and fresh particles. For example, we found that the ...
Uploads
Papers by C. Rabouille