Martine M. Savard

Significance of Aragonite Cements Around Cretaceous Marine Methane Seeps

ABSTRACT Detailed petrography and geochemistry of carbonate precipitates in Cretaceous cold seep mounds from the Canadian Arctic show spectacular early diagenetic products: some still-preserved splays and isopachous layers of fine, acicular aragonite, and large botryoids and crusts of low-magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite. The latter mineralogy is suggested by clear, flat-terminated cathodoluminescence patterns interpreted as ancient crystal growth steps, and the former by rhombohedral terminations. The early cement phases very likely precipitated in cold Arctic water dominated by bicarbonates derived from bacterially oxidized methane: these cements have d13C values around -44. and d18O values of 1.8 to 0.1 PDB. Coexistence of calcite and aragonite early cements in the Cretaceous seep mounds is unusual, because precipitation occurred in high-latitude, cold-water settings, and during a so-called calcite sea mode. As in modern marine hydrocarbon seeps, the chemistry of the Cretaceous system was apparently controlled by chemosynthetic bacterial activity, resulting in high aHCO3-, that promoted precipitation of carbonates. We suggest that, locally, fluctuations in aHCO3-/aSO42- resulted in oscillating aragonite or calcite supersaturation, and hence, controlled the mineralogy of the early precipitates.

Cathodoluminescene at low Fe and Mn concentrations; a SIMS study of zones in natural calcites

ABSTRACT Point analyses of natural calcite cements by SIMS confirm the earlier suggestions that 20 ppm Mn is required to activate luminescence, whereas Fe concentrations in excess of 1400 ppm quench it partly, independently of the Mn content. Below this Fe threshold, luminescence is bright at Mn concentrations in excess of 225 ppm. In the range 20-225 Mn, all types of luminescence--dull, luminescent, and nonluminescent--can occur. This undifferentiated domain is evident also from a compilation of literature data. The type of luminescence is not a simple function of Fe and Mn concentrations; other parameters influence CL. Sinc most diagenetic calcite cements have Fe and Mn concentrations that place them within the field of mixed cathodoluminescence, these results force reassessment of models that equate the type of luminescence with the redox state of the diagenetic environment.

Century-Long Source Apportionment of PAHs in Athabasca Oil Sands Region Lakes Using Diagnostic Ratios and Compound-Specific Carbon Isotope Signatures

Evaluating the impact that airborne contamination associated with Athabasca oil sands (AOS) mining operations has on the surrounding boreal forest ecosystem requires a rigorous approach to source discrimination. This study presents a century-long historical record of source apportionment of polycyclic aromatic hydrocarbons (PAHs) in dated sediments from two headwater lakes located approximately 40 and 55 km east from the main area of open pit mining activities. Concentrations of the 16 Environmental Protection Agency (EPA) priority PAHs in addition to retene, dibenzothiophene (DBT), and six alkylated groups were measured, and both PAH molecular diagnostic ratios and carbon isotopic signatures (δ(13)C) of individual PAHs were used to differentiate natural from anthropogenic inputs. Although concentrations of PAHs in these lakes were low and below the Canadian Council of Ministers of the Environment (CCME) guidelines, diagnostic ratios pointed to an increasingly larger input of petroleum-derived (i.e., petrogenic) PAHs over the past 30 years concomitant with δ(13)C values progressively shifting to the value of unprocessed AOS bitumen. This petrogenic source is attributed to the deposition of bitumen in dust particles associated with wind erosion from open pit mines.

Hydrothermal origin of Devonian conical mounds (kess-kess) of Hamar Lakhdad Ridge, Anti-Atlas, Morocco

Various interpretations have been proposed for the origin of peculiar cone-shaped, finely crystalline, Devonian carbonate mounds of the Hamar Lakhdad Ridge, Anti-Atlas, Morocco, ranging from shallow-water reefs to deep-water mud mounds, formed by biogenic and/or hydrodynamic processes. This study is the first integrated sedimentological and geochemical analysis of these structures. The mounds are cone shaped, steep sided, circular to subelliptical in plan view, and exhibit internal crude bedding parallel to the mound9s outer surface. They occur in a cluster of 48 mounds on top of a volcanic massif. Stable isotope analyses of first-stage isopachous nonluminescent cement have yielded marine values. In contrast, the finely crystalline carbonate that makes up the bulk of the mound and the internally sedimented mud between crusts of early marine cements have significantly lower δ 18 O values, whereas values of δ 13 C are similar to those of the early marine cement. Strontium isotope ratios also clearly distinguish the nonluminescent early marine cements from the finely crystalline material. The 87 Sr/ 86 Sr values for the former (0.707 934–0.709 392) are in or near the Devonian marine range, whereas the ratios for the latter (0.708 515–0.709 656) indicate a more radiogenic Sr source. On the basis of their lithology, architecture, isotope geochemistry, and clustered occurrence on top of a volcanic pile, we propose that the finely crystalline material that forms the mounds and the intermound beds was precipitated from hydrothermal fluids, and that accretion of the material was driven by hydrothermal venting.

Sources of basinal and Mississippi Valley-type mineralizing brines: mixing of evaporated seawater and halite-dissolution brine

Abstract Origins of basinal brines and Mississippi Valley-type (MVT) mineralizing fluids have been separately attributed to evaporation of seawater or dissolution of halite, although brines originating from the two processes are not mutually exclusive in basins. This study shows that the Na/Br Cl/Br diagram cannot distinguish between evaporated seawater and halite-dissolution fluid. Using the Na deficit Ca excess diagram which was previously proposed to characterize fluid-rock interactions of basinal brines, it is shown that most basinal brines including WT mineralizing fluids of the Viburnum Trend deposits were probably initially a mixture of halite-dissolution fluid and evaporated seawater. Using the same diagram, we suggest that the mineralizing brines of the Gays River MVT deposit were derived from an aquifer of clastics underlying a thick succession of evaporites, where halite-dissolution fluid and evaporated seawater could have mixed.

Tree-ring stable isotopes and historical perspectives on pollution--an overview.

Hydrogen (delta2H), carbon (delta13C), oxygen (delta18O) and nitrogen (delta15N) isotopes of tree rings growing in field conditions can be indicative of past pollution effects. The characteristic delta13C trend is a positive shift generally explained by invoking closure of stomata, but experimental studies suggest that increased rates of carboxylation could also generate such trends. In many cases the delta18O and delta2H values decrease in trees exposed to pollution and exhibit inverse coinciding long-term trends with delta13C values. However, some trees exposed to diffuse pollution and experimental conditions can show an increase or no delta18O change even if delta13C values increase. These diverse responses depend on how stress conditions modify physiological functions such as stomatal conductance, carboxylation, respiration, and perhaps water assimilation by the root system. Recent studies suggest that delta15N changes in trees can be caused by soil acidification and accumulation of anthropogenic emissions with isotopic signals deviating from natural N.

Dendrogeochemical distinction between geogenic and anthropogenic emissions of metals and gases near a copper smelter

For countries where metal resources represent an important economic sector, one of the main challenges of environmental research is to distinguish between natural and anthropogenic accumulations of potentially toxic metals in mining districts. The present work aims to evaluate a new environmental monitoring tool combining dendrochronology with natural (Ca, Ca/Mn, δ13C) and anthropogenic (Cd, Pb, 206Pb/207Pb, 208Pb/206Pb) geochemical tracers in tree rings in such a region. We compare spruce trees sampled at a control site near Hudson Bay, with those sampled near the Horne smelter active since 1928 in Rouyn-Noranda. The first effect of smelter emissions is detected by the tree-ring carbon isotope records. The δ13C values obtained on trees near the smelter show major changes immediately after 1928. This is due to the presence of atmospheric SO2 which generates a rapid response of the foliar system. The Ca/Mn ratios in tree-ring pairs of 1936–1937 and younger suggest a SO2-related soil acidification. The concentrations in Cd and Pb show a major increase starting in 1944 which coincides with a decrease of the 206Pb/207Pb ratios. The smelter activities likely generated this increase and the apparent delay of 14 years may have been generated mainly by the residence time of metals in airborne particulates, the buffering effect of the soils and, to a lesser extent, perhaps by mobility of heavy metals in tree stems. The 206Pb/207Pb and 208Pb/206Pb ratios indicate that the growth rings contain at least three types of Pb: natural, derived from the mineral soil horizons; industrial, from coal burning urban pollution; and mining, typical of the volcanogenic massive sulphides treated at the Horne smelter. This new combination of natural and anthropogenic tracers allows recognition of the succession of atmospheric and pedogeochemical changes related to industrial activities in the Rouyn-Noranda mining area.

Lead concentrations and isotope ratios in the exchangeable fraction: tracing soil contamination near a copper smelter

A total of 75 soil samples were collected from podzolic soils at eight sites in northwestern Quebec in order to compare results in contaminated locations near the Horne Copper smelter in Rouyn-Noranda with those near Hudson Bay, 800 km further north, and a priori devoid of pollution. Lead concentrations and Pb isotopic compositions were determined on soil samples leached by 0.25 M HCl. Lead is enriched in the surface organic horizons at all sites in Rouyn-Noranda. Its concentration decreases abruptly from the surface organic horizons to the underlying mineral horizons. The 206Pb/207Pb ratios are low in surface organic horizons, and they increase sharply in the mineral horizons. Along a NE–SW transect, the highest average Pb concentration (869 ppm) in organic horizons is found at the test site, 9 km from the smelter. The lowest average of 39 ppm Pb in surface organic layers is accompanied by the highest average 206Pb/207Pb ratio (1.15) at site 5, 116 km NE of the Horne smelter. At the test site, the highest Pb concentration of 1414 ppm is also accompanied by the lowest 206Pb/207Pb ratio (0.98), which is close to the isotopic composition of Noranda galena (206Pb/207Pb=0.92). Both Pb concentration and isotopic composition indicate that the Horne smelter is the main source of Pb contamination of soils in the Rouyn-Noranda region. The two soil profiles from near Hudson Bay display much lower Pb concentration and different trends of isotope ratios with depth.

Sedimentologic, Diagenetic and Tectonic Evolution of the Saint-Flavien Gas Reservoir at the Structural Front of the Quebec Appalachians

ABSTRACT The Beekmantown Group (Lower Ordovician) of the Saint-Flavien reservoir has produced 162x106 m3 (5.7 bcf) of natural gas between 1980 and 1994. The conversion of the field into gas storage was initiated in 1992 and the pool became operational in 1998. Integration of structural and sedimentologic features, carbonate and organic matter petrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosity evolution in these reservoir dolostones. The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of 2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequently covered by a shoaling succession of subtidal and intertidal limestones to intertidal dolostones. Early dolomitization has produced intercrystalline porosity and preserved some moldic pores in the intertidal facies. Near surface, post-dolomite karstification has created vugs that were subsequently filled by early marine calcite fibrous cement based on the 18O and 13C ratios of calcite. Early burial elements consist of horizontal stylolites, pyrite and sphalerite. Late migrated bitumen was thermally altered or vaporized as native coke under deep burial conditions exceeding 240°C, partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and subsequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The 18O of the poikilotopic calcite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about 260°C). Horizontal shear zones and marcasite-rich vertical stylolites were produced during folding and thrusting. Dissolution has preferentially affected late fracture-filling calcite and generated most of the actual porosity during or soon after the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the low temperature condition of this dissolution (T <100°C). Porosity in the Saint-Flavien reservoir has been mostly produced by fracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous facies at the top of rhythmic cycles that compose the Beekmantown Group. End_Page 126------------------------

Determination of the origin of groundwater nitrate at an air weapons range using the dual isotope approach.

Nitrate is one of the most common contaminants in shallow groundwater, and many sources may contribute to the nitrate load within an aquifer. Groundwater nitrate plumes have been detected at several ammunition production sites. However, the presence of multiple potential sources and the lack of existing isotopic data concerning explosive degradation-induced nitrate constitute a limitation when it comes to linking both types of contaminants. On military training ranges, high nitrate concentrations in groundwater were reported for the first time as part of the hydrogeological characterization of the Cold Lake Air Weapons Range (CLAWR), Alberta, Canada. Explosives degradation is thought to be the main source of nitrate contamination at CLAWR, as no other major source is present. Isotopic analyses of N and O in nitrate were performed on groundwater samples from the unconfined and confined aquifers; the dual isotopic analysis approach was used in order to increase the chances of identifying the source of nitrate. The isotopic ratios for the groundwater samples with low nitrate concentration suggested a natural origin with a strong contribution of anthropogenic atmospheric NOx. For the samples with nitrate concentration above the expected background level the isotopic ratios did not correspond to any source documented in the literature. Dissolved RDX samples were degraded in the laboratory and results showed that all reproduced degradation processes released nitrate with a strong fractionation. Laboratory isotopic values for RDX-derived NO(3)(-) produced a trend of high delta(18)O-low delta(15)N to low delta(18)O-high delta(15)N, and groundwater samples with nitrate concentrations above the expected background level appeared along this trend. Our results thus point toward a characteristic field of isotopic ratios for nitrate being derived from the degradation of RDX.