François Fernex

Wastewater discharge, seagrass decline and algal proliferation on the côte d'Azur

Abstract The green alga, Caulerpa taxifolia , has recently proliferated in areas of the N-W Mediterranean that were previously dominated by the seagrass Posidonia oceanica . Sediments in areas of proliferation had high exoenzyme activities, NH + 4 concentrations and production rates, but very low capacities to transform organic-N into NH + 4 relative to the quantity of organic material available. All sediments supporting C. taxifolia contained large quantities of precipitated phosphorus and exhibited extremely low capacities to transform NH + 4 into NO − 2 and NO − 3 . Low transformative capacities and high phosphate concentrations are characteristic of sediments that have been polluted by urban wastewater. Where C. taxifolia did not exist, or where growth had stabilized, sediments had exoenzyme activities, NH + 4 concentrations and NH + 4 production rates that were proportional to organic-N transformative potential. All sediments contained significant concentrations of non-crystalline sulphide. Caulerpa taxifolia proliferation may be linked to enrichment of substrata by urban wastewater and dead or dying seagrass vegetation.

High metal inputs to closed seas: the New Caledonian lagoon

Abstract The islands of New Caledonia are largely composed of ultrabasic rocks (peridotites). severily weathered, rich in Fe, Mn and Co, and where several ore deposits of Ni and Cr are extensively mined. Sediment cores from the bay of Dumbea, in the south of the main island, and from the northern part of the lagoon (Belep Islands), less affected by the mining activities, were analyzed for their mineral composition and metal concentrations. In the surficial sediments, oxides and silicates, including Fe serpentine and smectites, undergo rapid transformation or neoformation in a short time, in particular in the confined bay of Dumbea. Fe is largely present as goethite, and in deeper layers (60–100 cm) as hematite and magnetite. Chromite can be identified at each horizon. The metal concentrations decrease from the near shore areas, in particular the vicinity of the Dumbea river mouth to the open part of the lagoon. This trend is more important for Ni than for Fe or Cr. Fe ranges from 3.5 to 9% (dry weight), Ni from 200 to 2000 μg/g, Cr from 700 to 2000 μg/g, Co from 20 to 150 μg/g and Mn from 130 to 900 μg/g; yet the concentrations are lower than concentrations found in the ultrabasic rocks or laterites of the watershed. To try to understand the behavior of metals during the sedimentation-diagenesis events, we evaluated the sediment accumulation rate, and used different sequential leaching procedures. Fe, Mn, Ni, Cr and Co are mainly present as, or bound to, oxides or oxyhydroxides, even in the deeper layers ( > 100 cm) where the organic content is relatively high (about 6% of organic C). Metals are mainly transported from the land to the lagoon as oxides and dispersed in the lagoon sediments, where they are diluted with a large amount of carbonaceous sediment. During diagenesis, a significant part of Mn, Co and Ni are dissolved; but, unlike Mn and Co, which seem to coprecipitate with carbonate, most of the Ni is released into the waters of the lagoon. Apparently no horizon of the sediment has undergone significant in-situ metal enrichment.

Sedimentation rates in Lake Chapala (western Mexico): possible active tectonic control

Abstract Lake Chapala, a major natural freshwater reservoir, belongs to the watershed of the Rio Lerma, which originates from the Mexico City area, 450 km eastwards. The east–west-trending Lake Chapala basin is located on the active tectonic feature Citala rift. Sediments of cores collected in the lake were studied for their 210Pb, 226Ra, 137Cs and 239–240Pu activities in order to evaluate sedimentation rates. Profiles of magnetic susceptibility allowed stratigraphic correlations to be made since some fine levels display more or less high susceptibility values. This allows an average sediment accumulation rate to be calculated. The sediment accumulation rate was calculated assuming that, the activity at the interface of a defined site is constant in the course of time, since any global decrease or increase in the rates could not be observed. The direct flux of atmospheric 210Pb was evaluated from the inventory in a horizontal soil: 0.25–0.3 dpm g−1. Except from two cores at the west, the fluxes to sediment were higher than the direct atmospheric flux. The sedimentation rates temporal variations along the cores seem to be mainly related to annual precipitation changes, with rates lower during the dryness epochs (1945–1957 and 1979–1991). The rates measured in the western part of the lake range approximately from 1 to 2 mm year−1. The lowest rates in the central and eastern areas are higher than 2.4–2.5 mm year−1 and exceed 3.5 mm year−1 (0.066 g cm−2 year−1) near the Rio Lerma prodelta. As the lake is not deeper eastward than westward, it can be suggested that relative subsidence, related to the active tectonic framework, compensates the higher sedimentation rate.

Copper, lead and zinc trapping in Mediterranean deep-sea sediments: probable coprecipitation with Mn and Fe

Abstract In the deepest area of the Western Mediterranean Sea (between Sardinia and Minorca), sediments are relatively rich in Zn and Fe. On the periphery of this area, the sediments are rather rich in Cu, Pb and Mn, and to a lesser extent in Zn. In this last case, the vertical profile of Mn in the sediment suggests that the Mn enrichment in the top level (with an Mn concentration of ∼ 1500 μ g g −1 ) is due to an ascending Mn flux from deeper levels, reducing for Mn-oxyhydroxides, to the surficial, 0–4-cm-thick, oxidizing sediment. The concentrations of Cu, Pb and Zn dissolved in the surficial sediment pore waters are too low to allow direct precipitation of mere oxides or hydroxides of these metals; but enrichment in the Mn-rich deep-sea zone can be explained by a coprecipitation process during the precipitation of Mn-oxides or -hydroxides, since Mn dissolved in surficial sediment pore water is present in an apparent slight oversaturation with respect to the solubility product. In the deepest areas, where Zn seems to precipitate with Fe-hydroxides, the surficial sediments appear to be too reducing to allow Mn-oxides to precipitate. Cu and Pb are trapped not so well there. Calculations of chemical equilibria of the species concerned are consistent with these considerations. Since Mn seems to be released from sediments in the deepest zone (with total Mn concentrations μ g g −1 ), it can be assumed that there the surficial sediments correspond nearly to a reducing medium for the Mn-oxides; therefore, the bottom seawaters of these deepest area of the Western Mediterranean Sea seem to have rather low O 2 concentrations, and to be only rarely renewed.

Ammonification rates and 210Pb in sediments from a lagoon under a wet tropical climate : Marica, Rio de Janeiro state, Brazil

AbstractThe ammonification rates in surficial sediments of the Marica lagoon (Rio de Janeiro, Brazil) were estimated using two methods:(1)A direct evaluation of the ammonia productivity in sediments by means of an incubation procedure: the ammonia production rate is deduced from the difference in the ammonia concentrations in interstitial waters which were extracted directly after sampling and after incubation from two samples collected at the same point.(2)Since ammonia results essentially from the destruction of organic nitrogen, the ammonia productivity will be obtained by evaluating the nitrogen destruction rate: the amount of organic N mineralized is assumed to correspond to NHinf4sup+produced. The organic N destruction rate can be calculated by means of the matter balance equation provided the organic nitrogen profile and the sediment accumulation rate are known. The sedimentation rate, evaluated by means of the 210Pb method, differed in various areas of the lagoon. It was 0.35 cm y−1 in the open area of the lagoon (at 1.6 m in water depth), near the mouth of the Mombuca river (site M2); and 0.27 to 0.28 cm y−1 at 1.6 to 1.7 m at a greater distance of the river mouth (site M4). In a confined part of the lagoon (Z1), surrounded by reeds (Typha dominguensis), the rate is lower: 1 to 1.4 mm y−1 in the upper level (from 0 to 4 cm) where the organic matter amount reaches 60 to 80% (dry w.), and 1.1 to 1.2 mm y−1 deeper (below 5 cm) where organic matter corresponds to less than 40% of the sediment.The organic nitrogen destruction rate was calculated to be 0.8 · 10−6 µmol cm−3 s−1 (= 0.07 µmol cm−3 d−1) for the upper sediment level (0–2 cm) of site M4. This computed rate is slightly less than the value 1.1 · 10−6 µmol NHinf4sup+cm−3 s−1 obtained by means of the incubation experiment. For the upper level of the site close to the reeds (Z1), the calculated nitrogen destruction rate was 1.7 · 10−6 µmol cm−3 s−1 (0.15 µmol cm−3 d−1), the ammonification rate obtained by the incubation method 1.4 · 10−6 µmol cm−3 s−1. It appears that the two methods provided similar results.

Geology, Sediments and Soils

Lake Chapala, the major Mexican fresh water reservoir, is located in western Mexico, is a good example of a neotectonic lake with a history linked eastwards to the development of the active Transmexican Volcanic Belt geological province. Lake Chapala crosses the E-W trending Citala rift, which forms with the Tepic-Zacoalco and Colima rifts the so-called Jalisco triple junction. The Lake Chapala basin is tectonically controlled and belongs to a regional intra-arc system of half-grabens. It is assumed that the current Lake Chapala basin has a depth of about 600 m. Shallow (1.40 m) sediments in Lake Chapala are dark and silty with granulometric median ranging from 10 to 50 µm. Sediment samples were studied by isotopic, geophysical, geochemical and micro-paleontological methods. Major oxides in sediments are SiO2 (54.05%), Al2O3 (18.94%), Fe2O3 (6.75) and CaO (2.57%). The major mineralogical composition of sediments is as follows: plagioclases (25.86%), quartz (20.92%), clays (38.5%), carbonates (2.99%) and TiO2 (1.05%). Sedimentation rates (SR) were evaluated from 210Pb profiles assuming constant initial concentrations; and we can conclude that SR is at least two or three times higher on the East side of the lake than in the western part. As water depths on both sides of the lake are similar, we assume there must be a tectonic compensation on the eastern part. This explains why Lake Chapala has a uniform depth in spite of the differences detected in SR. We have estimated an approximate 1 mm yr−1 subsidence at the eastern part of the lake that compensates the higher SR. This tectonic compensation also explains the fact that the lake’s fluvial input and output are so abnormally close to each other. Scarcity of diatoms in shallow sediments of Lake Chapala is due to pH variations in both sediments and water, hence inhibiting their preservation. The major soil types around Lake Chapala are: Vertisol (50%), Pheozem (30%), Lithosol (15%) and Luvisol (5%).

Metal origin and Pb isotopes in water of the mine-draining Mejerda river system, north Tunisia

ABSTRACT Water samples (< 0.45 μm) from the Mejerda hydrographic system were analysed for trace metals and Pb isotopes, together with Pb isotope measurements of galena from the mines of the system. Water was collected (i) within the mines, (ii) in tributaries lying as close as possible to the mines, (iii) along the tributaries, (iv) along the main Mejerda River, and (v) in dammed lakes. All metal concentrations measured in the waters are below the limits given for health purposes of surface water, though some of them are close. Metals such as Al, V, Ni, Cu, Pb, and U are slightly enriched in the uppermost part of the system but they do not exceed 20 ppb (μg l−1), with the exception of Al reaching 120 ppb. In the lower Mejerda sector, a few high Zn concentrations of up to 550 ppb are observed, and up to 80 ppb for Ba. Lead isotopic compositions of 41 water samples show large variations, yielding ratios at 17.736–18.884 for 206Pb/204Pb, 15.504–15.977 for 207Pb/204Pb, and 37.135–39.992 for 208Pb/204Pb. In contrast, Pb isotopic compositions of 11 galena samples from different mines define a surprisingly narrow field, being systematically more radiogenic in 206Pb than water. Compared to water and galena, Pb in Tunisian gasoline has a very different and much less radiogenic Pb isotopic composition at 16.351, 15.493, and 36.135, respectively. No significant contribution of such gasoline Pb can be detected either in the main Mejerda River or the numerous tributaries. The same observation is made for galena Pb, with the particularity that water lying closest to the isotopic compositions of galena was sampled in the lower part of the system where mines are absent. For the upper part of the system, minor contributions from the mines cannot be ruled out. The full set of Pb isotopic data substantiates the ultimate sources of Pb to have evolved with large variations in time-integrated U/Th/Pb ratios, including differences in mantle extraction ages. Simple mixing between two end-members can be ruled out for most of the water Pb data. It is therefore suggested that Pb and other metals of Mejerda water originate from local soils, from where they can be leached from alteration phases. In contrast to water showing a wide variation of ultimate source lithologies, the particularly narrow field of galena Pb isotopic compositions requires Pb extraction from a surprisingly homogenous and strongly U- and Th-depleted reservoir. This reservoir is most likely the Triassic evaporites, essentially gypsum, forming in a well mixed water volume devoid of significant detrital input.