Etude expérimentale de la dégradation des traceurs fluorescents sous l'influence des variations de paramètres environnementaux

National audienceLa réalisation des traçages à l'aide de traceurs fluorescents conduit, dans certains cas, à des bilans de restitution très déficitaires. Compte tenu de l'incidence que ce phénomène peut avoir sur la forme de la distribution des temps de séjour, qui est à la base de toute l'interprétation d'un traçage, il devient nécessaire de s'interroger sur le comportement des traceurs fluorescents une fois injectés dans le milieu naturel. Seule l'influence du facteur pH sur la capacité de fluorescence est bien documentée dans la littérature. La photodégradation des traceurs est également un phénomène connu, mais peu de données sont disponibles sur les taux et les vitesses de dégradation. L'effet d'autres paramètres comme la température reste méconnu, surtout pour les traceurs autres que l'uranine. Cet article présente les premiers résultats d'une étude expérimentale de plusieurs mois portant sur l'impact de facteurs environnementaux tels que la lumière et la température, sur la fluorescence des traceurs les plus couramment utilisés (uranine, éosine). Cette étude essaie de quantifier l'impact de chaque paramètre sur chaque traceur, donnant également une idée de la cinétique de dégradation. Nous avons constaté une décroissance très rapide de la fluorescence de l'uranine à la lumière naturelle: une perte de 23 % au bout de 1 h, et de 99 % au bout de 2 semaines pour des concentrations de 25 µg/L. Nous observons également une nette influence de la lumière sur l'intensité de fluorescence de l'éosine, mais avec une décroissance plus lente que pour l'uranine. De manière inattendue, nous avons constaté une influence non négligeable des fortes températures (40 °C) sur la diminution de la fluorescence de l'uranine, alors que rien de tel n'a été observé pour l'éosine. Ces résultats devraient aider à une meilleure compréhension du comportement de chaque traceur dans des conditions naturelles changeantes, pour une adaptation plus adéquate à des contextes variés, et permettre un usage plus informé de ces traceurs dans le cadre des multitraçages

Biominéralisation vs Organominéralisation: Comparaison des processus de minéralisation dans la matière organique, vivante et "morte"

Les dépôts sédimentaires et les organismes fossilisés offrent de nombreux modèles où des minéralisations – essentiellement carbonatées et phosphatées - se développent au sein de matières organiques (m.o.) mortes ("Organominéralisation"1). L'observation au cryo-MEB (microscope électronique permettant l'observation de m.o. hydratées2) a montré que les minéraux néoformés se forment dans une étroite relation géométrique avec la structure de la m.o. Ceci exclut que la minéralisation soit le fait exclusif de populations bactériennes se développant aux dépens des constituants organiques en voie de décomposition, même si le métabolisme de ces populations a un rôle cardinal dans l'acquisition de la composition ionique des solutions en contact avec les constituants organiques

Reply to “Reply to comments on defining biominerals and organominerals: Direct and indirect indicators of life [Perry et al., Sedimentary Geology, 201, 157–179]” by R.S. Perry and M.A. Sephton: [Sedimentary Geology 213 (2009) 156]

International audienceThis is a reply to R.S. Perry and M.A. Sephton's “Reply to comments on defining biominerals and organominerals: direct and indirect indicators of life [Perry et al., Sedimentary Geology, 201, 157–179]” [Sedimentary Geology 213 (2009) 156]

A Jurassic counterpart for modern kopara of the Pacific atolls: lagoonal, organic matter-rich, laminated carbonate of Orbagnoux (Jura Mountains, France)

Two types of laminated sediments are compared in this paper: the Kimmeridgian bituminous laminites of Orbagnoux (French Southern Jura Mountains) and the present-day ‘kopara' of the French Polynesia atolls or Kiritimati (Christmas) Island from the Pacific Ocean. The kopara is made of laminated sediments, several tens of centimetres thick, that cover the floors of most shallow (<2 m deep) lakes and ponds on the rims of atolls in the Tuamotu Archipelago and Society Islands. The millimetre-scale laminations come from the alternation of organic-rich and CaCO3-rich laminae. The top few centimetres host a succession of bacterial populations, from aerobic to strictly anoxic. Most carbonate grains are precipitated in situ, due to bacterial activity (s.l.). The Kimmeridgian bituminous laminae of Orbagnoux are partly made of flat stromatolites that show many similarities with the kopara. This analogy allows us to refine the interpretation of the Southern Jura platform where laminated, organic matter-rich, carbonates were deposited in many places

Defining organominerals: Comment on ‘Defining biominerals and organominerals: Direct and indirect indicators of life' by Perry et al.

International audienceThe paper by Perry et al. (2007, Defining biominerals and organominerals: Direct and indirect indicators of life, Sedimentary Geology, 201, 157-179) proposes to introduce “the new term ‘organomineral'” to describe mineral products whose formation is induced by by-products of biological activity, dead and decaying organisms, or nonbiological organic compounds, to be distinguished from the biomineral components of living organisms. The substantive ‘organomineral', however, is not new: it was first introduced in 1993, with basically the same definition and distinction from biominerals, at the 7th International Symposium on Biomineralization (Défarge and Trichet, 1995, From biominerals to ‘organominerals': The example of the modern lacustrine calcareous stromatolites from Polynesian atolls, Bulletin de l'Institut Océanographique de Monaco, n° spéc. 14, vol. 2, pp. 265-271). Thereafter, more than twenty-five papers by various authors have been devoted to organominerals and organomineral formation (‘organomineralization') processes. Only two of these papers are cited by Perry et al., and without any reference to the definitions, or even the terms ‘organomineral' or ‘organomineralization', which they included. Moreover, Perry et al. tend to enlarge the original concept of organomineral to encompass all minerals containing organic matter, whether these organic compounds are active or passive in the mineralization, which introduces ambiguities detrimental to a fine understanding of present and past geobiological processes. Finally, Perry et al. propose to consider organominerals as indirect biosignatures that could be used in the search for evidence of life in the geological record and extraterrestrial bodies. This latter proposition also is problematical, in that organominerals may be formed in association with prebiotic or abiotic organic matter

How to assess cutover Peatland regeneration with combined organic matter indicators ?

International audienceWhen restored, cutover peatlands can favour biodiversity and carbon (C) sequestration. Within the EU program RECIPE, we aimed to identify combinations of site physico-chemical conditions, vegetation composition and below-ground microbiological characteristics that are beneficial to the long-term biodiversity and C sink function regeneration. To unreveal these characteristics, we assessed the bioindicator value of peat organic matter (OM) physico-chemistry from cutover peatlands at various stages of regeneration. Although OM continues to reflect disturbances in the catotelm deep peat, we show that along the chronosequence the regenerated peat tends to be biochemically and physically similar to the one from the non exploited area of the same site. The combination of several indicators provides an efficient assessment of ecological conditions and makes valuable for the management of cutover peatlands

Christmas Island lagoonal lakes, models for the deposition of carbonate–evaporite–organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 2°N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km2) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Niño-Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2–5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low δ13C values of sedimentary organic carbon (−14 to −17‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate+chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization)

Dynamic of dissolved organic matter and trace elements in a steady state lake bottom layer : molecular size fractionation.

International audienceDissolved organic matter (DOM) in aquatic systems interacts with most processes including redox reactions, trace elements complexation, sorption and sedimentation. The complex nature of DOM in natural waters suggests that all the fractions constituting it do not contribute in the same way to its reactivity. The total stock of DOM can be split by various means like the difference in size of the molecules or the difference in affinity for adsorbent phases. The deep and stable layers of meromictic lakes are of a particular interest for the study of the mechanisms controlling the behaviour of chemical elements in natural systems. Quasi stationary conditions unroll and stratify the processes through a certain thickness of the water column. In the deep layer of the lake Pavin (Massif-Central, France), solutes dispersing from the water-sediment interface (92 meter depth) cross a succession of levels characterized by different physicochemical and microbiological conditions (redox conditions, interactions with neoformed or settling particles, bacterial metabolic types) before to reach the oxic layer around 60 meter depth (Viollier et al., 1995, 1997, Lehours et al., 2003)

Mg-Calcites and dolomite in the Brejo do Espinho lagoon, Rio de Janeiro.

The aim of this research is to study the carbonate sedimentation in a hypersaline coastal (Brejo do Espinho) where dolomite precipitation can be expected. The study allowed to make a general characterization of the sediment and to identify different types of carbonate minerals by mineralogical studies and cryo-scanning electron-microscopy

Role of a cyanobacterial cover on structural stability of sandy soils in the Sahelian part of western Niger

Microbiotic soil crusts, mostly formed by cyanobacteria, are widespread on the surface of fallow land in western Niger. They lie adjacent to completely bare soils. We have investigated the structural stability of these soils by testing aggregate breakdown under fast-wetting, slow-wetting and mechanical breakdown. The tests were effected on aggregates ranging from 3 to 5 mm in size. These experiments were completed by micromorphological examinations under light and scanning electron microscope. For all tests, the fragment size distribution and the mean weight diameter (MWD) revealed the great stability of aggregates from soils with a cyanobacterial cover (MWD 1.82 to 3.10 mm), compared to those from soils devoid of cyanobacterial cover (MWD 0.25 to 1.26 mm). Fast- and slow-wetting of microbiotic soil crust material induced a lesser disaggregation of aggregates compared to mechanical breakdown. On the contrary, fast-wetting and mechanical breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow-wetting. Microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles on the soil surface. Organo-mineral aggregates ranging from 0.10 to 0.12 mm were observed. Below the superficial crusts, filaments and residual organic matter bind tightly soil particles, thus giving a compact structure. The great stability of aggregates of soil with cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter. This is consistent with the positive correlation between MWD values and organic carbon content. The present results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements