Evidence for intense REE scavenging at cold seeps from the Niger Delta margin

International audienceFor many trace elements, continental margins are the location of intense exchange processes between sediment and seawater, which control their distribution in the water column, but have yet to be fully understood. In this study, we have investigated the impact of fluid seepage at cold seeps on the marine cycle of neodymium. We determined dissolved and total dissolvable (TD) concentrations for REE and well-established tracers of fluid seepage (CH4, TDFe, TDMn), and Nd isotopic compositions in seawater samples collected above cold seeps and a reference site (i.e. away from any fluid venting area) from the Niger Delta margin. We also analyzed cold seep authigenic phases and various core-top sediment fractions (pore water, detrital component, easily leachable phases, uncleaned foraminifera) recovered near the hydrocast stations. Methane, TDFe and TDMn concentrations clearly indicate active fluid venting at the studied seeps, with plumes rising up to about 100 m above the seafloor. Depth profiles show pronounced REE enrichments in the non-filtered samples (TD concentrations) within plumes, whereas filtered samples (dissolved concentrations) exhibit slight REE depletion in plumes relative to the overlying water column and display typical seawater REE patterns. These results suggest that the net flux of REE emitted into seawater at cold seeps is controlled by the presence of particulate phases, most probably Fe-Mn oxyhydroxides associated to resuspended sediments. At the reference site, however, our data reveal significant enrichment for dissolved REE in bottom waters, that clearly relates to diffusive benthic fluxes from surface sediments. Neodymium isotopic ratios measured in the water column range from εNd ~−15.7 to − 10.4. Evidence that the εNd values for Antarctic Intermediate waters (AAIW) differed from those reported for the same water mass at open ocean settings shows that sediment/water interactions take place in the Gulf of Guinea. At each site, however, the bottom water εNd signature generally differs from that for cold seep minerals, easily leachable sediment phases, and detrital fractions from local sediments, ruling out the possibility that seepage of methane-rich fluids and sediment dissolution act as a substantial source of dissolved Nd to seawater in the Gulf of Guinea. Taken together, our data hence suggest that co-precipitation of Fe-Mn oxyhydroxide phases in sub-surface sediments leads to quantitative scavenging of dissolved REE at cold seeps, preventing their emission into bottom waters. Most probably, it is likely that diffusion from suboxic surface sediments dominates the exchange processes affecting the marine Nd cycle at the Niger Delta margin

Environmental regulations and consequences in terms of growth and welfare

Cette thèse en essais propose une analyse des conséquences de différentes règlementations environnementales sur la croissance et le bien-être des agents économiques. Nous considérons trois instruments de régulation, correspondant à nos trois chapitres: les accords volontaires, les taxes pigouviennes et l'utilisation jointe taxe-subvention.Le premier thème de travail s'intéresse à la profitabilité de différents types d'accords volontaires visant à réduire la production d'un bien non-efficient en utilisation d'énergie. Nous définissons un accord volontaire optimal pour la société et analysons les effets de cet accord sur le marché de l'énergie. Notre conclusion est que l'accord entraîne une baisse de la quantité d'énergie d'équilibre et du prix. Cependant, un effet de détournement estimé à 30% se produit.Le deuxième thème de travail valide l'hypothèse de Porter, selon laquelle une règlementation environnementale stricte mais bien pensée peut engendrer des bénéfices sociaux et des bénéfices privés pour les firmes qui y sont soumises. Plus précisément, nous analysons les effets de l'instauration de différentes taxes environnementales dans le contexte particulier d'un lac superficiel. Nous montrons que l'instauration d'une taxe peut amener les agents économiques à converger vers un état stationnaire supérieur au sens de Pareto.Le troisième thème de travail s'attaque au paradoxe vert, qui énonce que les politiques de lutte contre le changement climatique peuvent provoquer, au contraire de leur objectif, une accélération du phénomène de réchauffement. Nous utilisons un modèle de ressources, combinant une ressource exhaustive, dont l'utilisation engendre des émissions polluantes et une ressource renouvelable, non polluante. L'équilibre sur le marché de l'énergie définit un temps d'arrêt endogène de la production de l'énergie sale, si le stock initial de cette énergie est faible. Si ce stock initial est important, l'économie converge vers un état stationnaire où la quantité produite d'énergie polluante est nulle. L'introduction d'une taxe sur l'énergie sale et d'une subvention à l'énergie propre entraîne une prolongation du régime d'utilisation jointe des deux énergies. A tout instant, la quantité d'énergie exhaustive consommée (et donc la pollution) diminue avec la règlementation environnementale. Nous réfutons le paradoxe vert.This essay thesis proposes an analysis of the consequences of different environmental regulations on growth and on the economic agents' welfare. We consider three instruments of regulation, corresponding to our three chapters: voluntary agreements, pigovian taxes, and the joint use of tax and subsidy.The first work theme is devoted to the profitability of different voluntary agreement types, the aim of which is to reduce the production of the good which is not efficient in terms of energy use. We define a voluntary agreement optimal for society and analyse its effects on the energy market. Our conclusion is that the agreement leads to a decrease of the energy quantity and of the price. However an estimated leakage effect of 30% is likely to occur.The second work theme validates the Porter Hypothesis, in which a strict and well-thinking regulation can produce social benefits and private benefits for firms which are subject to the regulation. More precisely, we analyse the effects of the set up of different environmental taxes in the particular context of a shallow lake. We show that a tax set up can make the economic agents converge to a Pareto-superior steady state.The third work theme is concerned with the green paradox, which states that policies against global warming can cause an acceleration of the rise phenomenon, contrary to their objective. We use a resource model combining an exhaustible resource which creates polluting emissions, and a renewable, non-polluting, resource. The energy market equilibrium defines an endogenous stopping time of the dirty energy's production, if the initial stock of this energy is relatively weak. If this initial stock is high, the economy converges to a steady state in which the consumed quantity of polluting energy is null. The introduction of a tax on polluting energy and of a subsidy on clean energy leads to a prolongation of the joint use's system. At any time, the consumed quantity of polluting energy (and so the pollution) decreases with the environmental regulation. We refute the green paradox

Discriminating fluid sources in Miocene cold seep systems using REEs in authigenic carbonates

Authigenic carbonates are a common feature in cold seep environments, where fluids enriched in methane and other hydrocarbon gases escape from the seafloor. Seep-carbonates have been reported worlwide both in modern and ancient sedimentary deposits. In the Northern Apennines (Italy), numerous outcrops of seep-carbonates are particularly well-preserved. Evidences from paleoecological, sedimentological, geochemical and isotopic (O and C stable isotopes) analyses cleary show that they were derived from the microbial oxidation of methane-rich fluids. REE patterns and abundances in fossil seep-carbonates may provide additional informations for better constraining the origin and the composition of the fluids from which they have precipitated.Here, we report REE data for a series of Miocene carbonate samples recovered from various geological settings in the Northern Apennines. Samples were leached with 5% HNO3, prior to analysis by SF-ICPMS using the Tm addition method [1]. Total REE concentrations (ΣREEN) in our studied carbonates are very similar to those reported for modern authigenic carbonates, suggesting negligible post-depositional diagenetic alteration. The shale-normalized REE patterns vary significantly amongst the different authigenic carbonate samples analysed in this study. These data indicate that they were formed from fluids having distinct REE signatures. These results, coupled with other geochemical, petrographic and mineralogical data, allow us to reconstruct the variation of fluid seepage activity in the Northern Appenines during the Miocene

Investigation of miocene cold seep sources using rare earth element in authigenic carbonates

Seep-carbonates are describe

REEs in authigenic carbonates: A new proxy for tracing fluid sources at cold seeps

International audienceNot Availabl

Carbon and oxygen isotope analysis of hydrate-associated Oligocene authigenic carbonates using NanoSIMS and IRMS

Petrography and stable C and O isotope compositions of methane-derived carbonates from the Polish Outer Carpathians were analyzed. Two methods were employed in isotopic determinations. Conventional IRMS measurements on powdered bulk samples were performed with high precision (errors ~0.06‰ for δ13C and ~0.08‰ for δ18O, 1σ), whilst in situ measurements were made using NanoSIMS with a high spatial resolution (5 μm), but with low precision (errors ~3.0‰ for δ13C and ~2.3‰ for δ18O, 2σ). The NanoSIMS study proved very useful in identifying the sources of the fluids precipitating the carbonates. They were formed predominantly by the anaerobic oxidation of biogenic methane (AOM). The very fine spatial resolution of the analyses revealed highly variable δ13C values, even in individual crystals, from extremely negative related to AOM to positive related to methanogenesis (Me). Other methods, including laser ablation and standard SIMS, would fail to detect this variability and would not indicate the true sources of the fluids, because they would show average values. Therefore, any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the δ13C values are extreme, i.e. resulting either from AOM or Me. Unusual clast-like druses from the carbonates were analyzed in detail with NanoSIMS. They are filled with strongly 13C-depleted (down to −54‰) and 18O-enriched (up to 6‰) fringe calcite which precipitated on the walls of the druses from heavy-oxygen-containing water, released from the dissociation of gas hydrates. The rocks can thus be termed clathrites. The bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks. Precipitation of the carbonate cements started when hydrates were still present, so they are a type of melt-and-seal fabric. The recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane-derived carbonates. The isotopic results have demonstrated that NanoSIMS can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements. This provides a basis for the application of NanoSIMS to tracing diagenetic pathways from various, even complexly formed, authigenic precipitates