Rock Eval Pyolysis : a tool for quantification of organic fluxes in present lacustrine environnements

The main objective of most organic geochemical studies performed on lake sediments is to identify the sources of sedimentary organic matter (OM). Indeed, molecular and isotopic studies allow to characterise major compositional trends between two organic poles: the one deriving from the lake production (algal and microbial OM), the other one deriving from the watershed production (contemporaneous aerial biomass and reworked OM from soil or geological formations). In order to furnish valid information on the palaeo-environmental variation and on the intensity of responsible sedimentary events (anthropogenic or climatic), it is necessary to quantify the different organic contributions and to calculate the sedimentary organic fluxes through the time

Organic composition and environmental conditions in mangrove sediments : a key for reconstructing the evolution of theFrench Guiana coast.

Le littoral guyanais présente une dynamique sédimentaire remarquable résultant du système dispersif amazonien. Une mangrove, essentiellement composée d'Avicennia germinans, se développe sur les bancs de vase. Ceux-ci migrent rapidement vers le NO et limitent la durée de vie de la mangrove. Les lambeaux de forêt résiduels ont pu être datés par analyse d'images Spot successives (Véga, 2000). Les mesures des paramètres physico-chimiques, les données Rock-Eval ainsi que des observations et comptages pétrographiques ont permis d'identifier deux systèmes de fonctionnement de la mangrove : les forêts jeunes (< 9ans), caractérisées par un export tidal important, et la forêt sénescente où l'exportation tidale est limitée. La composition organique du sédiment des mangroves jeunes provient principalement des mattes microbiennes et en quantité moindre du système racinaire de la mangrove. Les mécanismes de dégradation sont des processus suboxiques résultant de l'activité du système racinaire des Avicennia. La composition organique du sédiment de la forêt sénescente provient principalement de débris de végétaux supérieurs, les processus de dégradations sont de type anaérobie

The composition of sedimentary organic matter in relation to the dynamic features of the mangrove-fringed coast in French Guiana.

The sedimentary organic matter content of a series of 2-m-deep cores was examined in relation to the evolution of mangrove forest, on the basis of geochemical analyses and optical observations. Avicennia-dominated forest deposits, developing along the highly dynamic coastline of French Guiana, were collected in five stations based on stage of forest evolution. The sedimentary organic matter in the upper sediment of the youngest mangrove swamp is mainly derived from algal mats with low carbon:nitrogen ratios (C:N ratio, from 6 to 8) and typical greyish amorphous organic flakes as observed in optical studies. Indeed, rare young Avicennia trees are present, and effectively, geochemical parameters do not give evidence of a litter made up of higher plant debris, these rare debris being probably exported by the tides. A slight increase with depth in the first decimetres of both total organic carbon (TOC) content and C:N ratio results from the development of the radial cable root-system of the pioneer Avicennia germinans. Early diagenetic conditions of this young forest are rather controlled by dominant suboxic processes, as suggested by high Eh values (range, 200–400 mV) and local anoxic processes (occurrence of pyrite) in micro-environments: this is mainly due to the oxygen available by roots and crab bioturbation. The organic content of the senescent mangrove sediment is mainly derived from higher plant debris in the uppermost 30 cm, as indicated by relatively high C:N ratios and the predominance of ligno-cellulosic debris. The strong decrease in hydrogen index values results from the degradation of the higher plant debris, losing hydrogen bounds through decay processes. Moderately acidic pH values, low Ehs and the presence of pyrite framboids point towards the reducing decay processes in surficial layers of the senescent mangrove mediated by sulphate-reducing bacteria. Whatever the stage of evolution of the forest, the geochemical characteristics of the sediment below 30 cm are those of the shoreface one, with opaque refractory debris derived from the Amazon river detritus discharge. The sediment collected from dead mangrove forests, subsequently recolonized by pioneer mangroves, contains organic markers, which predate recolonization, recording previous phases of erosion and accretion

Redox conditions and heavy metals distribution in mangrove forests receiving effluents from shrimp farms (Teremba Bay, New Caledonia)

International audiencePurpose The mangroves of New Caledonia, in the south Pacific, act as a buffer between a lagoon of more than 20,000 km2 and the island, which is characterized by ultramafic rocks and lateritic soils that are exploited for their richness in heavy metals. We will provide a better understanding of the redox conditions, and of heavy metal distributions in mangroves receiving shrimp farm effluents. Materials and methods Samples were collected from four areas defined in terms of vegetation composition: a salt flat, an Avicennia marina forest, in which effluents are released; a Rhizophora stylosa forest, and a dead Rhizophora forest. They were collected during times of maximum effluent release. Some measurements on pore water were also done during a period without effluent. Cores (70 cm deep) were collected at low tide with an Eijkelkamp gouge auger, and pore waters were extracted using soil moisture samplers. Physico-chemical parameters (pH, Eh, salinity) were measured by directly inserting the probes into cores. Total nitrogen and total sulfur were also determined. The sedimentary organic content was studied using a Rock-Eval 6 pyrolysis. Finally, heavy metal concentrations were determined, both in the solid and the dissolved phases, using an HR-ICP-AES. Results and discussion The distribution of heavy metals in the core collected in the salt flat is mainly controlled, on the one hand, by the sedimentary organic content, and on the other hand, by the elevation of the area, which induced dessication. The release of effluent within the Avicennia stand induced anoxic conditions on the whole depth profile, while these conditions are suboxic without effluent release, probably inducing different metal speciation. The Rhizophora forests, located at 100 m from the release point, do not seem to show any impact from the effluent phase, as indicated by the redox profiles which show similar results with and without effluent release. Beneath these two stands, conditions are mainly anoxic and sulfidic, as a result of the decomposition of high organic matter content. Conclusions The release of effluent within the Avicennia stand, by modifying the length of waterlogging, clearly modifies the redox conditions. We suggest that the differences in redox conditions between the two periods modify the carrier phase of heavy metals, being mainly associated with sulfides during effluent release. Metals are thus less mobile, and consequently when mangrove receives effluents, they act as a sink for trace metals

Lacustrine organic fluxes and paleoclimatic variations during the last 15 ka : lac du Bouchet (Massif Central, France)

To assess the influence of climatic changes on organic lacustrine sedimentation, two cores recovered from the centre of the Lac du Bouchet were studied by petrographical (palynofacies) and geochemical methods. Only core LDB H was used for estimation of the organic fluxes. The variation of these fluxes with climo-stratigraphic periods showed : low organic fluxes during the Lateglacial, an increase at the beginning of the Holocene, a minimum at the end of the Atlantic period resulting from the climatic cooling, and a maximum at the end of the Sub-Boreal related to the installation of the present climatic conditions. (Résumé d'auteur

Pore-water chemistry in mangrove sediments: relationship with species composition and developmental stages. (French Guiana).

Spatial and seasonal variability of sedimentary salinity, pH, redox potential and solid phase sulphide concentration were investigated in a range of mangrove communities along the coast of French Guiana. Seasonal depth distributions of these parameters and organic content were compared within Avicennia, Rhizophora and mixed mangrove stands at different stages of plant development. Mangrove communities and variable surface water inputs strongly impact sediment and ground water properties. In the upper sediment, changes in salinity are mainly controlled by seasonal conditions, transpiration and proximity of fresh water influx, whereas we suggest that constant basal salinity results from an accumulation of salt that has migrated as a result of density driven convection processes. There are no clear differences between the depth distributions of salinity obtained beneath Avicennia germinans and Rhizophora mangle stands, implying that plant zonations are not primarily controlled by soil salinity in this environment. Nevertheless, R. mangle grows in places subjected to the greatest variability in freshwater influxes, suggesting that Rhizophora might require or withstand occasional inundation by fresh water. Beneath Rhizophora stands, sediment properties reflect anaerobic and sulphidic conditions close to the sediment surface. In contrast, beneath Avicennia stands, sediment geochemistry mostly depends on the stages in forest development, on contents in sedimentary organic matter and on seasonal changes. In the early stage of Avicennia settlement, the sediment at the level of radial, pneumatophore-bearing cable roots, displays permanent suboxic conditions with Eh values reaching 400 mV. These high Ehs are interpreted as an effect of the oxidation produced by the cable root system. The development of mature Avicennia stands results in accumulation of sedimentary organic matter and promotes low Ehs and the reduction of pore-water sulphate. Near cable root level, the oxidation process observed in pioneer mangroves results in a reoxidation of solid sulphides produced previously. During dry conditions, the desiccation of the upper sediment adds its oxidation effects to those of root activity. As a result, suboxic processes dominate in the upper, 20-cm-thick layer; organic matter decomposition and sulphur oxidation strongly acidify the sediment. Below 20 cm, the sediment is anaerobic and sulphidic. Hence, sulphide concentrations depend on the edaphic conditions controlling decay processes and appear to be a consequence rather than a cause of the observed zonation of vegetal species. The small size of A. germinans propagules might have a significant influence on the extensive development of this plant community along the highly dynamic coastline of the Guianas. This study demonstrates that the different properties of pore-water were intimately linked and that the explanation of the evolution of this forest reflects a combination of multiple parameters. Moreover, it appeared that the organic content played a key role along with the species composition and the seasonal variations (waterlogging, desiccation)

Dynamique de la matière organique lors de l'évolution d'une mangrove à palétuviers gris (A. germinans), Guyane française. Un exemple de processus suboxiques en milieu hydromorphe.

La mangrove est un système ouvert écotone couvrant 22 millions d'hectares, sur près de 75% des littoraux tropicaux et subtropicaux. Il s'agit de forêt de palétuviers caractérisées par une forte productivité, développant des systèmes d'adaptation remarquables leur permettant de pallier aux contraintes d'un milieu très sélectif (sursalure, anoxie, houle...). Pour exemple, les Rhizophoraceae s'ancrent dans le substrat grâce à des racines échasses tandis que les Avicenniaceae utilisent des pneumatophores pour la respiration; leur mode de reproduction de type vivipare leur permet de coloniser des surfaces de vase inondées par les marées. Les mangroves ont un rôle prépondérant dans le cycle des nutrients en milieu côtier, et permettent la stabilité du trait de côte, limitant l'action érosive des vagues. Ces dernières décennies, la surface colonisée par les mangroves a nettement diminué du fait de la pression anthropique : exploitation du bois, riziculture ou bien encore utilisation de la mangrove comme décharge

Petrographic and geochemical study of organic matter in surficial laminated sediments from an upwelling system (Mejillones del Sur Bay, Northern Chile).

A study of recent laminated sediments, accumulated in the oxygen minimum zone of Mejillones Bay (Northern Chile), shows that the organic matter is autochthonous and deposited under varying oceanographic conditions. Sedimentary units dominated by light laminations have low values of total organic carbon, total nitrogen and sulphur, and Chaotoceros content, coupled with high values of yellow amorphous organic matter. These were deposited under conditions of low primary productivity, favoring the recycling of the major part of metabolizable organic matter in the water column, and the accumulation only of non-metabolizable organic matter in the bottom sediments. In contrast, a unit rich in dark laminations with high values of total organic carbon, total nitrogen and sulphur, shows high values of brown and black AOM, and Chaetoceros content. This sedimentary unit reflects increased flocculation of organic particles during a period of high productivity and fast transit of organic material through the column water, thus diminishing the recycling of metabolizable organic matter and its accumulation with non-metabolizable organic matter in the sediments. Finally, a unit characterized by presence of both light and dark laminations was formed by abrupt alternations of the two oceanographic states described previously

The fate of organic matter in mangrove sediments subject to variable environmental conditions (French Guiana).

International audienceDuring the four last years, a National research Programme for Coastal Environment (PNEC) was partly devoted to the littoral of French Guiana. Several field trips allowed to sample twinned sedimentary cores from both the mangroves and the unvegetated sediments, and various vascular plants specific to these swamps. An elemental, pyrolytic and optical study of total sedimentary organic matter (OM), completed with stable isotope and molecular analyses, was carried out on these sets of samples and coupled with a study of pore waters (physico-chemical properties and dissolved organic content) collected in the mean time, during contrasted seasons. The development and age of mangrove marshes and forests was assessed by parallel studies performed by ecologists

Étude haute résolution de la distribution et de la granulométrie des constituants organiques sédimentaires dans le Kimméridgien–Tithonien du Boulonnais (Nord de la France). Application à l'analyse séquentielle <br />–––<br />Sedimentary organic matter, accumulation and grain-size in the Kimmeridgian–Tithonian of the Boulonnais area (Northern France). Application to sequence stratigraphy.

La matière organique sédimentaire des formations argileuses du Kimméridgien–Tithonien du Boulonnais a été étudiée sous un aspect multidisciplinaire : pétrographique, géochimique et granulométrique, ainsi que du point de vue de la stratigraphie séquentielle. Les flocons de matière organique amorphe d'origine marine dominent les palynofaciès et leur distribution se corrèle aux variations du COT, du soufre total et des paramètres granulométriques. La forte productivité des eaux de surface a dû induire une intense sulfato-réduction sous l'interface eau–sédiment, pouvant alors provoquer la sulfuration (vulcanisation) de la matière organique sédimentaire d'origine marine, malgré la présence de fer dans le milieu de dépôt. La granulométrie de l'ensemble des particules se corrèle aux variations de ce type de matière organique et aux périodes de haut niveau marin, ce qui serait une nouvelle approche dans la compréhension du découpage séquentiel. The sedimentary organic matter from the Kimmeridgian–Tithonian marlstones of the Boulonnais area is studied through a petrographic, geochemical and a grain-size distribution approach. The abundance of the orange amorphous organic matter (AOM) correlates well with the geochemical parameters and intense sulfurisation reactions could explain its preservation, with a relative iron shortage. This may imply strong surface-water productivity linked to sea-level highstands and a possible decrease in sedimentation rate. The OM grain-size parameters correlate with sea-level variations and the orange AOM distribution in the Argiles de Châtillon formation. This relation could be helpful for the comprehension of the sequential stratigraphy