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Abstract: 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. [Copyright &y& Elsevier]

In the context of the French research project CHACCRA (Climate and Human-induced Alterations in Carbon Cycling at the River-seA connection), living (rose Bengal-stained) benthic foraminifera were investigated at two stations (24 and 67m depth) in the Rhôme prodelta (NW Mediterranean, Gulf of Lions). The aim of this study was to precise the response of benthic foraminiferal faunas to temporal changes of the Rhôme River inputs (e.g. organic and terrigeneous material). Each site was sampled in April 2007, September 2007, May 2008 and December 2008, permitting to observe foraminiferal faunas of the 63-150 and >150 µm size fractions under a wide range of environmental conditions. Obvious variations in foraminiferal faunal composition were observed during the four investigated periods at the shallowest Station A located in the close vicinity of the Rhôme River mouth. After major Rhôme River flood events, different colonisation stages were observed with foraminiferal faunas responding with an opportunistic strategy few days to weeks after the creation of a peculiar sedimentary environment (Leptohalysis scottii, May 2008) or high organic matter supplies (Ammonia tepida, December 2008). Under more stable conditions, relatively diverse and equilibrated faunas grew in the sediments. Species benefited from noticeable input of riverine phytodetritus to the sediment during spring bloom conditions (April 2007; e.g. Bolivina dilatata, Nonionella stella, Stainforthia fusiformis), or high amounts of still bio-available organic matter under more oligotrophic conditions (September 2007; e.g. Ammonia tepida, Psammosphaera fusca). The reduced influence of the Rhôme River input at the farther Station N led to less contrasted environmental conditions during the four sampling periods, and so to less obvious variations in foraminiferal faunal composition. During reduced riverine influence (i.e. low Rhôme discharge), species able to feed on fresh phytodetritus (e.g. Clavulina cylindrica, Hopkinsina atlantica, Nonionella iridea and Nonionella turgida) benefited from eutrophic conditions of the spring bloom (April 2007, May 2008). Conversely, the occurrence of Nouria polymorphinoides under oligotrophic conditions (September 2007, December 2008) was indicative of a benthic environment potentially disturbed by bottom currents. This study put into evidence the extremely rapid response of benthic foraminiferal faunas to strong variations in environmental conditions mostly induced by the Rhôme dynamics. [ABSTRACT FROM AUTHOR]

In the context of the French research project CHACCRA (Climate and Human-induced Alterations in Carbon Cycling at the River-seA connection), living (rose Bengal-stained) benthic foraminifera were investigated at two stations (24 and 67 m depth) in the Rhôone prodelta (NW Mediterranean, Gulf of Lions). The aim of this study was to precise the response of benthic foraminiferal faunas to temporal changes of the Rhône River inputs (e.g. organic and terrigeneous material). Each site was sampled in April 2007, September 2007, May 2008 and December 2008, permitting to observe foraminiferal faunas of the 63-150 and >150 µm size fractions under a wide range of environmental conditions. Obvious variations in foraminiferal faunal composition were observed during the four investigated periods at the shallowest Station A located in the close vicinity of the Rhône River mouth. Different colonisation stages were observed after major Rhône River flood events, foraminiferal faunas responding with an opportunistic strategy few days to weeks after the creation of a peculiar sedimentary environment (Leptohalysis scottii, May 2008) or high amounts of organic matter supplied by a river flood (Ammonia tepida, December 2008). Under more stable conditions, relatively diverse and equilibrated faunas grew in the sediments. Species benefited from noticeable input of riverine phytodetritus to the sediment during spring bloom conditions (April 2007; e.g. Bolivina dilatata, Nonionella stella, Stainforthia fusiformis), or high amounts of still bio-available organic matter under more oligotrophic conditions (September 2007; e.g. Ammonia tepida, Psammosphaera fusca). The reduced influence of the Rhône River input at the farther Station N led to less contrasted environmental conditions during the four sampling periods, and so to less obvious variations in foraminiferal faunal composition. During reduced riverine influence (i.e. low Rhône discharge), species able to feed on fresh phytodetritus (e.g. Clavulina cylindrica, Hopkinsina atlantica, Nonionella iridea and Nonionella turgida) benefited from eutrophic conditions of the spring bloom (April 2007, May 2008). Conversely, the occurrence of Nouria polymorphinoides under oligotrophic conditions (September 2007, December 2008) was indicative of a benthic environment potentially disturbed by bottom currents. This study put into evidence the extremely rapid response of benthic foraminiferal faunas to strong variations in environmental conditions, especially close to the river mouth (Station A), response noticeably reduced farther from the mouth (Station N) due to a weaker impact of riverine input. [ABSTRACT FROM AUTHOR]

River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O2 m-2 d-1) and decrease offshore to values around 4.5 mmol O2 m-2 d-1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern. [ABSTRACT FROM AUTHOR]

The biogeochemical fate of the particulate organic inputs from the Rhône River was studied on a seasonal basis by measuring sediment oxygen uptake rates in the prodelta, both during normal and flood regimes. On a selected set of 10 stations in the prodelta and nearby continental shelf, in situ and laboratory measurements of sediment oxygen demand were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ sediment Diffusive Oxygen Uptakes (DOU) did not show any significant differences except for shallowest organic rich stations. DOU rates show highest values concentrated close to the river mouth (approx. 20 mmolO2m-2d-1) and decrease offshore to values around 4.5 mmolO2m-2d-1 preferentially in a south west direction, most likely as the result of the preferential transport of the finest riverine material. Total Oxygen Uptake (TOU) obtained from core incubation showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Over different seasons, spring summer and late fall, benthic mineralization rates presented this same stable spatial pattern. A flood of the Rhône River occurred in June 2008 and brought up to 30cm of new soft muddy deposit. Right after this flood, sediment DOU rates close to the river mouth dropped from around 15-20 mmolO2m-2d-1 to values close to 10 mmolO2m-2d-1, in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the rapid response to flood deposits in prodeltaic areas which may act as a suboxic sediment reactor and shorten the relaxation time. [ABSTRACT FROM AUTHOR]