Gérard Sarazin

Geochemical study of a crater lake: Pavin Lake, France — Identification, location and quantification of the chemical reactions in the lake

Abstract A study of the main geochemical characteristics of the Pavin crater lake was performed in September 1992. Associated with previous results on the hydrochemical budget of the lake, the present study points out that: 1. (1) in the mixolimnion, the major reactions are photosynthesis, removal of silica by diatoms, iron oxidation and alkali and alkaline-earth incorporation in a solid phase; 2. (2) the major reduction reactions occur within the sediment and the products of the reaction are brought to the lake by diffusion; 3. (3) some reactions: silica dissolution, manganese oxide reduction, sulfate-sulfide transformations, iron and phosphate precipitation occur within the monimolimnion. 4. A complete budget of transfers and chemical reactions is presented for some components such as silica.

Sedimentation rate and early diagenesis of particulate organic nitrogen and carbon in Aydat Lake (Puy de Dôme, France)

Abstract A core of low-compacted sediment from a eutrophic lake (Aydat Lake) was extracted in order to study the early diagenesis of particulate organic carbon and nitrogen. We use a simple model where the advection rate was estimated with the classical 210 Pb method. We show that the 210 Pb data are not sufficient by themselves to provide a correct estimation of this parameter, but a model which includes the sediment compaction leads to a better understanding of the sedimentation process. Compaction is also taken into account in the diagenetic model which allows us to make an estimation of the carbon and nitrogen mineralization rates.

Organic matter mineralization in the pore water of a eutrophic lake (Aydat Lake, Puy de Dôme, France)

The chemical composition of the pore water from the sediment of a eutrophic lake is dominated by high concentrations of total dissolved CO2 (up to 12 mM), reduced soluble iron (up to 2 mM) and dissolved silica (up to 1 mM). The pH lies within the range of 6.70 ± 0.02; this reflects that the pore water is efficiently buffered by the CO2 acid/base system. This composition is directly related to the main diagenetic reactions which drive the organic matter mineralization i.e. methanogenesis and ferric oxides reduction. Other geochemical processes are of minor importance. A stoichiometric model based on these main reactions allow us: (i) to define a general formula for the organic matter which is close to Redfields one for the C:N ratio, while the C:P ratio is much higher owing to a probable adsorption of phosphorus onto reactive surfaces of the solid and due to heterotrophic bacterial uptake; (ii) to calculate a global first order kinetic constant which drives the organo-polymers breakdown. Due to the strong influence on the trophic status of the lake caused by an excess of phosphate, special attention is devoted to this species. We show that the sediment-water interface is a source of dissolved phosphate when the hypolimnion is anoxic between May and November. This contribution represents about 17% of the river input and should be taken into account in any attempt toward lake restoration.

Interstitial water and sediment chemistries of Lake Aiguebelette (Savoy, France)☆

Abstract Interstitial waters were collected from Lake Aiguebelette sediment, Savoy, France, using an in situ sampler. Major ions, nutrients and trace metals Fe and Mn were measured. Early diagenesis in these sediments leads to the release of all the investigated elements. ΣPO 4 , NH + 4 and H 4 SiO 4 demonstrate significant concentration gradients reflecting organic matter decomposition. Thermodynamic calculations suggest that calcite, which precipitates from the overlying waters, dissolves in the pore fluids. A simplified model is presented to account for the increase of Ca 2+ , Alk and H + concentrations with depth. Mineralogical investigations using SEM, TEM, microprobe analysis and step dissolution of the sediment complete and corroborate the conclusions based on the study of interstitial waters (i.e. calcium carbonate and silica dissolution).

Annual budget of chemical elements in a eutrophic lake, Aydat lake (Puy-de-Dôme), France

An annual survey of the chemical composition both in dissolved and particulate phases of a eutrophic lake was performed from September 1995 to October 1996. All major elements and many trace elements were analyzed in the tributary and in the water column. Element fluxes were determined via sediment traps. Several sediment cores and interstitial water samples were also analyzed. From these data, a method to calculate element budgets was derived. Chemical elements can be classified into five main groups:(1) Low solubility elements are at or below detection limits in dissolved fractions and exhibit a nearly constant ratio to Al in all solid phases: Al, Ti, REE, Be, Th, Zr.(2) High solubility elements occur essentially in the dissolved phase: alkali and alkaline earth, Cl, B. Within this group, the cations exhibit some reactivity: they are removed from solution in the surface layer of the lake and released at the water–sediment interface or within the sediment.(3) biogenic elements are efficiently trapped within the lake, with about 70–75% of Si and P supplied to the lake in the dissolved form and only 25 to 30% exported as dissolved species.(4) Elements with different oxidation states (Fe, Mn, Co) show an extensive recycling within the lake. Inputs represent only 20–50% of the amount released annually into solution.(5) C and N inputs exceed outputs, suggesting that a significant amount of nitrogen escapes from the lake as N2 and some C may escape as CO2.

The distribution of Ni and Co in a eutrophic lake: an application of a square-wave voltammetry method

Abstract Ni and Co are trace transition elements which exhibit very low concentrations in continental water. Therefore, due to these very low amounts, little is known about their geochemical cycle. A square-wave voltammetry method, initially developed for seawater, was adapted to fresh water in order to get reliable analysis of these two elements. This method uses an intermediate complexation step with dimethylglyoxime in order to increase the sensitivity of the polarographic analysis and leads to detection limits close to 1 · 10 −11 M for Co and 3 · 10 −10 M for Ni. Experiments take place in a clean room and blank tests were carried out to make sure that storage bottles do not alter the sample content. This method was used to determine Ni and Co distributions in the water column during the stratification period and in the sediment pore water of a eutrophic lake. Ni concentration is roughly independent of the water column redox status, and is below detection limit in the sediment pore water. This can be related to the presence of sulfide in these waters. Co concentration strongly increases below the chemocline and its profile is close to that of manganese. Co concentration increases downward in the pore water. Co can be adsorbed onto Fe- and Mn-oxyhydroxides and is released into solution when oxides are dissolved.