F. Gadel

Carbon isotope composition and geochemistry of particulate organic matter in the Congo River (Central Africa): Application to the study of Quaternary sediments off the mouth of the river

Organic carbon isotopes and the geochemistry of organic matter have been studied in suspended material from the Congo River and some of its tributaries and in a core located at 2000-m depth, off the mouth of the river. The organic carbon isotopic composition of suspended material from the Congo tributaries identifies the floral cover and the lithologic and pedologic characteristics of their watersheds. Suspended material from the Congo River in the Stanley Pool emphasizes the sole influence of organic matter derived from C3-rich plants. Outside the Congo estuary, differences of isotopic composition between suspended material near the surface and that near the bottom emphasize the respective marine or continental origin of particulate organic carbon. The carbon isotopic composition of the marine deposits records biozonal fluctuations of the recent Quaternary. Shifts in the isotopic composition of organic matter may be explained by an increase of terrigenous organic matter derived from C3 type plants during the wet biozones Z and X whereas during the drier biozone Y the associated organic matter shows a greater influence of the productivity of the oceanic biomass. However, a possible contribution of C4 plant-derived particulate organic matter can be assumed for the very top of the Y biozone, introducing a 13C-enriched signal, indicative of marine planktonic input.

Lignin-derived phenolic compounds in the particulate organic matter of a French Mediterranean river: seasonal and spatial variations

Abstract Samples from the Tech river (French Mediterranean river) were analysed for particulate organic carbon (POC) and lignin phenols, to characterise spatial and seasonal variations. A decreasing gradient of lignin phenols (from ca. 0.8 to 0.5 mg 100 mg OC−1) was observed downstream, together with an increase of p-hydroxyl (H) phenols. Seasonal variations in total and lignin–phenols composition were very noticeable and seemed to be connected directly to rainfall variations. Lignin-phenol concentrations were higher in the wet periods (up to 12.1 μg/l) while the dry periods were characterised by the predominance of H phenols over lignin-phenols (Λ) ( H Λ ratios reaching values of 245 during summer). This predominance of H phenols was also associated with high chlorophyll a concentrations, which may indicate an autochthonous (and fresher) source of these phenols to the water column. Mean vanillic acid/vanillin ratios were higher than those of fresh plant material (0.4 for POM against 0.2 for fresh plant tissues), indicating that the lignin component had already been degraded before entering the river. The syringyl/vanillyl (S/V) and cinnamyl/vanillyl (C/V) ratios indicate a predominance of angiosperm+gymnosperm woody material in the lower sector and the presence of some angiosperm non-woody material in the mountain sector, corresponding to alpine and sub-alpine vegetation.

Determination of marine monosaccharides by high-pH anion-exchange chromatography with pulsed amperometric detection

Abstract In this study, a method was developed for the determination of ten monosaccharides in marine particulate matter utilizing high-pH anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD) Samples were analyzed using a Dionex CarboPac PA1 column with a flow-rate of 1 ml/min and addition of 380 m M NaOH post-column. The effect of NaOH concentration (between 0.5 and 50 m M ) on the monosaccharide separations expressed by the capacity factor ( k ′) was tested. The results showed that one isocratic elution was unfit to discriminate properly arabinose, fructose, fucose, galactose, glucosamine, glucose, mannose, ribose, rhamnose and xylose. Thus, two isocratic elutions, at 2.5 and 15 m M NaOH, were necessary to separate and quantify with significant selectivity (α) and resolution terms ( R s ), respectively, glucose, mannose, xylose and fucose, rhamnose, arabinose, glucosamine, galactose, fructose and ribose. The method is linear for all sugars over the concentration range tested (25–50 ng per injection or 1–10 mg/g) expected in marine concentrated samples, and reproducibility was found to be satisfactory (1.7–4.8%), except for ribose (27%). Monosaccharide determinations from two kinds of marine matrix (hydrolyzates of surface sediment and of suspended particulate matter) are presented.

Dissolved organic matter and UV absorption in a tropical hyperhaline estuary

Several surveys were carried out in a hyperhaline estuary, the Casamance River (Senegal, West Africa). Light absorption (A) by water in the near-UV (250–360 nm) was measured. Absorption spectra (semi-log plot) are described by slope, S∗, and by A254. Using literature data, we show the possibility of calculating dissolved organic carbon concentration [DOC] and mean molecular weight (MW) from A254 and S∗. The “upstream” portion of the estuary is hydraulically isolated. [DOC] and MW are high. Dissolved organic matter (DOM) originates mostly from Phragmites swamps, and is concentrated during the dry season, despite flocculation and photolysis. New, more labile DOM is brought in during the rains. The seaward portion receives high- to medium-MW DOM from mangrove swamps, and loses some of it to the sea by tidal transport. In the median portion, low-MW DOM originates from phytoplankton. Aquatic primary production can partly be explained by DOM distribution.

Tracers and constituents indicating the nature of organic fluxes, their origin and the effect of environmental conditions

Abstract The nature of particulate organic matter was studied in suspended material sampled by bottles, particles collected by sediment traps and deposits from deltaic and open sea ecosystems of the northwestern Mediterranean. Elemental analyses were combined with pyrolysis-gas chromatography-mass spectrometry and with analysis of individual compounds such as phenols separated by high-performance liquid chromatography. In the Rhoˆne delta, a multilayered system was observed. The surficial turbid layer was enriched with fresh material of river origin. Organic matter was more altered and richer in phenols in the bottom nepheloid layer. The river regime determined the nature and quantity of suspended material: when in spate, degraded organic matter previously deposited in the river bed was transported to the sea, thereby inducing an increase of pyrolysis derived aromatic hydrocarbons. On the other hand, phenolic aldehydes increased in the bottom nepheloid layer. When water level was low, organic matter seemed to be of more local origin. The content of phenols and nitrogen-containing compounds increased. The influence of the Rhoˆne River decreased off the mouth, when terrestrial markers were diluted by products deriving from phytoplanktonic activity. Compared with suspended material, trapped organic matter was coarser, more degraded and contained more aromatic hydrocarbons. It was different in nature and size, indicating that it was trapped over a longer period. Deposits contained altered organic matter resulting from degrading processes in the water column. Sediments showed a double trend off the mouth of the river; an increase in nitrogen-containing compounds, indicating a more marine character, and a decrease in phenols and carbohydrates deriving from the terrestrial ligno-cellulosic complex. In the southwestern part of the Gulf of Lions, in the Teˆt prodelta, organic matter from sediment traps was fresher than in the Rhoˆne delta. Phenols and some carbohydrates rapidly decreased from the prodelta due to a lower runoff. During the spate, suspended material was rapidly deposited and sediments were enriched in terrestrial phenols. In the open sea environment, in the Lacaze-Duthiers Canyon at 645 m, in summer, the euphotic zone was rich in amino-sugars and contained a large diversity of phenols deriving from fish fecal pellets. In winter/spring, the development of phytoplanktonic blooms in surficial layers led to high contents of nitrogenous compounds. In deeper layers, the nature of organic matter was different from surficial layers in summer and more homogeneous in winter, although a flux of degraded material rich in pyrolysis-derived aromatic hydrocarbons and poor in nitrogenous compounds progressively sank towards the bottom. The two marine prodeltas were compared: organic matter was more degraded in suspended material and sediments collected in the submarine delta of the Rhoˆne River. Organic material originating from the river was transported further offshore, as indicated by a higher content of aromatic hydrocarbons and phenols. In the southwestern part of the Gulf of Lions, the prodelta reflected the influence of local rivers, with lower discharges.

Evidence for an early land use in the Rhône delta (Mediterranean France) as recorded by late Holocene fluvial paleoenvironments (1640-100 BC)

Abstract The overall objective of this paper is to describe the late Holocene (1640–100 BC) sedimentary and biological evolution of the Rhône–delta–plain, to interpret the sedimentary facies and palynofacies as the result of the effects of fluvial dynamic fluctuations and relative sea level change and to evaluate the paleohydrological constraints in the development of the land use and settlements of the Camargue. Focus is made on the upper part of VIII core drilled on NE of the Vaccares lagoon. By combining sedimentology, palynology, magnetic susceptibility and archeological data, this study allowed to identify the superposition of three types of paleo-environments (marsh, fluvial floodplain, levee/crevasse splay). This sequence indicates a gradual extension of fluvial environments between the end of the second millennium BC and the 1st century BC. The variability of fluvial dynamic is evident during this period with important flood events which contrast with periods of low flow. Pollen record can be a good marker of the fluvial dynamic variability. The expression of the riparian tree pollen grains in the coarser floodplain deposits could correspond to increased fluvial influence and probably to erosion of riverbank during flood events. The local plants are associated to the low energy sedimentary environments. Focuses are made on the relations between the evolution of the environment and land use. The development of the cereal culture in the floodplain of the Rhone delta has been demonstrated between 1640–1410 and 100 BC. The last alluviation of the Rhone perturbs the research of the archaeological sites in the central part of the delta but the existence of the rural villages from the first part of the first millennium BC is highly possible.

Characterisation of riverine particulate organic matter by pyrolysis-gas chromatography-mass spectrometry.

Particulate matter samples were collected in the mountainous section and river mouth at the Tech River basin, south France, during flood (December 1996) and summer (September 1997) periods. Suspended material was analysed by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and 23 identified pyrolysis products were divided into five main fractions: aminosugars, aromatic hydrocarbons, polysaccharides, phenols and nitrogenous compounds. Analysis of relative amounts of these fractions revealed neither significant spatial nor temporal changes in the particulate organic matter composition. Nevertheless, their specific composition showed that during flooding there is a certain homogeneity in the composition of the riverine particulate organic matter, with more degraded material of pedogenic (allochthonous) character, and during the summer the results suggest the presence of two components, allochthonous and autochthonous.

Evidence for an early land use in the Rhône delta (Mediterranean France) as recorded by late Holocene fluvial paleoenvironments (1640–100 BC)Preuves d’une mise en culture précoce de la plaine deltaïque du Rhône (France méditerranéenne) enregistrées par les séquences fluviales de l’Holocène récent (1640–100 av. J.-C.).

Abstract The overall objective of this paper is to describe the late Holocene (1640–100 BC) sedimentary and biological evolution of the Rhone–delta–plain, to interpret the sedimentary facies and palynofacies as the result of the effects of fluvial dynamic fluctuations and relative sea level change and to evaluate the paleohydrological constraints in the development of the land use and settlements of the Camargue. Focus is made on the upper part of VIII core drilled on NE of the Vaccares lagoon. By combining sedimentology, palynology, magnetic susceptibility and archeological data, this study allowed to identify the superposition of three types of paleo-environments (marsh, fluvial floodplain, levee/crevasse splay). This sequence indicates a gradual extension of fluvial environments between the end of the second millennium BC and the 1st century BC. The variability of fluvial dynamic is evident during this period with important flood events which contrast with periods of low flow. Pollen record can be a good marker of the fluvial dynamic variability. The expression of the riparian tree pollen grains in the coarser floodplain deposits could correspond to increased fluvial influence and probably to erosion of riverbank during flood events. The local plants are associated to the low energy sedimentary environments. Focuses are made on the relations between the evolution of the environment and land use. The development of the cereal culture in the floodplain of the Rhone delta has been demonstrated between 1640–1410 and 100 BC. The last alluviation of the Rhone perturbs the research of the archaeological sites in the central part of the delta but the existence of the rural villages from the first part of the first millennium BC is highly possible.

Nature and distribution of phenolic compounds in water and sediments from Mediterranean deltaic and lagunal environments

The nature and distribution of phenolic compounds were studied in suspended particles, sediment trap material and deposits in deltaic, marine and lagunal environments of the northwestern Mediterranean sea (Gulf of Lions).In deltaic environments (Rhône delta), the river flow controlled amounts of terrestrial run-off offshore as characterized by the abundance of phenolic compounds. A clear distinction appeared between surface water layers and deep layers. In the former, organic matter was less degraded and richer in phenolic acids than in the latter, where an abundant lithogenic phase was associated with degraded organic material rich in phenolic aldehydes, partly due to anthropogenic pollution. During the spate period, some compounds, such as phenolic aldehydes, were good indicators of hydrodynamic conditions, as they were carried down from the river bed to the prodelta. There, associated with the organo-mineral complex, they sank down to reach the benthic nepheloïd layer and flowed out offshore.Considering the chemical nature of phenolics, suspended material contained more labile phenolic compounds such as syringyl phenols. In contrast, vanillyl phenols, which are rather refractory components, were more concentrated in sediments.In the Lacaze-Duthiers Canyon (south of the Gulf of Lions), trapped material from the euphotic zone was relatively rich in phenols of marine origin. Similarities in phenolic composition appeared between trapped organic material in bottom layers of the Têt prodelta and of the side of the Canyon, reflecting a transfer of particles by advective currents from the shelf to the deep sea.In deposits containing relatively abundant phenols, humic acids were richer in phenols than fulvic acids.In lagunal environments, phenol contents in suspended material and deposits were relatively high, resulting from the abundant vegetal cover. Highest dissolved phenol contents occurred in water at the end of summer and early autumn on the edges of the Canet lagoon where abundant debris of decaying plants accumulated. Their nature depended essentially on the extent of phanerogam populations (Ruppia maritima, Phragmites australis), which are their major sources. In this area, fine deposits contained humic fractions richer in phenols than coarse sediments were.

PY-GC-MS analysis of organic matter in suspended material and deposits of the submarine delta of the rhone river (France)

Abstract In the Rhone delta the nature of suspended material depends on depth and river regime. Off the river mouth, an increase in nitrogenous compounds and aromatic hydrocarbons is observed in the pyrograms of suspended material and deposits. The decrease of phenolics can be related to diminishing terrestrial run-off. Fulvic acids are enriched in nitrogenous compounds and carbohydrates. Humic acids concentrate phenolics and aromatic hydrocarbons. On the Catalonian coast the higher rate of nitrogenous substances in offshore sediments is explained by outer sampling location. Aromatic hydrocarbons are less abundant indicating fresher organic matter than the more evolved material of the Rhone river.