Jean Lebel

Early diagenesis of cadmium and cobalt in sediments of the Laurentian Trough

Abstract The depth distributions of hydroxylamine/acetic acid-extractable Co and Cd were determined in box-cores from a series of stations along the 1200 Km length of the Laurentian Trough. These were compared with the concentrations of total Co and Cd and the distributions of extractable Mn and Fe in the cores, as well as with the total Cd concentrations in sedimenting particles collected with a sediment trap. The results indicate a strongly contrasting behaviour of the two elements during early diagenesis, consistent with their respective chemistries and particle-associations. Cobalt occurs mainly in a non-reactive form and is buried with the accumulating sediments. A mobile component of Co is associated with Mn- and Fehydroxides and, like these compounds, follows a redox-sensitive pattern of dissolution in the reducing zone of the sediment—vertical migration—enrichment by precipitation in the oxidized surface layer—and redistribution along the bottom in paniculate form. Paniculate Cd arrives at the sediment surface bound to biogenic material, is rapidly solubilized during aerobic degradation of the organic matter, and migrates vertically both into the reducing zone of the sediments where it precipitates, and back into the water column where it may be recycled through biological processes. In an estuary, Co will be concentrated in the oxidized layer of the sediments and tend to migrate landward, while Cd will be most abundant in sediments underlying regions where plankton production is high relative to dilution by terrigenous particles.

Capture of arsenic by pyrite in near-shore marine sediments☆

Abstract Evidence of pyrite formation during diagenesis of suboxic sediments in the Laurentian Trough, Canada, is presented. The process is slower than in salt marsh sediments probably due to the low sulfate reduction rate of 2–3 nmol cm −3 day −1 measured in the St. Lawrence sediments. A strong correlation is observed between arsenic and pyrite, suggesting the capture of this element in the lattice sites of pyrite with a Fe/As ratio of 10 3 . It is important to consider this association of As with pyrite in the study of the marine geochemistry of Fe and As.

Observations on the diagenetic behavior of arsenic in a deep coastal sediment

Vertical profiles of total dissolved arsenic, manganese and iron, pH, Eh and rates of sulfate reduction were determined in a freshly-collected box core from a 335m depth station in the Laurentian Trough. The relationships observed between the profiles were further examined in the laboratory by measuring these same parameters with time in surficial sediment slurries as the Eh decreased in response to biological activity or chemical alteration.Both field and laboratory observations have shown that arsenic is released predominantly as As(III) into reducing sediment porewaters. This occurs after the dissolution of manganese oxides and at the same time as the dissolution of iron oxyhydroxides and the onset of sulfate reduction. Laboratory experiments indicated that sulfate reduction and the production of sulfide ions are not solely responsible for the release of arsenic to the porewaters, although this process is necessary to create and maintain a highly reducing environment conducive to rapid iron dissolution.The diagenesis of arsenic in Laurentain Trough sediments involves the simultaneous release of arsenic and iron at a subsurface depth, followed by its removal from porewaters by precipitation and adsorption reactions after migration by diffusion along concentration gradients. A qualitative model is presented to describe the behavior of arsenic in coastal marine sediments.

Gravity core shortening and pore water chemical gradients

Abstract Profiles of pore water alkalinity and dissolved Fe, Mn, and PO 4 −3 were obtained for virtually undisturbed box cores and for gravity cores taken at the same stations. Linear alkalinity gradients in the box cores were preserved in the gravity cores, but the steepened gradients in the gravity cores indicate significant shortening. The ratio of the alkalinity gradients provides a measure of the degree of shortening. The relative shortening could also be clearly seen in the Mn, Fe, and PO 4 −3 profiles. We observed virtually no compaction and a linear shortening. The results agree with the mechanism proposed in early studies on gravity corer behaviour, namely the stretching of the sediment in advance of the corer and the recovery of a thinner section of sediment than the in situ interval. We observed no noticeable differences in pore water chemistry between box and gravity core samples, only the increased gradients due to thinning. The shortening (or thinning) factor at individual stations varied between 1.4 and 3.4, however, and for precise work it is suggested that a concurrent box core or other independent measures of the in situ gradients be obtained to correct for the true shortening.

The densities of western Indian Ocean, Red sea and eastern Mediterranean surface waters

Abstract Direct measurements of density were made on 176 surface samples collected in 1979 from the western Indian Ocean, Red Sea, and eastern Mediterranean Sea. The measurements were compared with the values calculated from the conductance salinity in order to evaluate the influence on the density of the partial molar volumes and the partial molar conductivity of the ions whose ionic ratios differ from their values in Standard Seawater. It is concluded that the differences between the measured and calculated densities are practically constant in vast geographical areas, corresponding to the various water masses of the World Ocean. An exception is the Red Sea, where the differences vary rapidly from the south to the north. These differences are always negative for the surface waters studied and can be correlated linearly to the variation of conductivity salinity. They are only partially explained by the total alkalinity and total carbon dioxide changes in these waters.

Selenium profiles in the sediments of the Laurentian Trough (Northwest North Atlantic)

Abstract The distribution of total selenium in sediments was determined at seven stations along the axis of the Laurentian Trough. The mean content of surface sediment (

On the boron content in the sediments from the St. Lawrence estuary off Rimouski, Quebec, Canada

Abstract Analyses of boron in a box core taken in the St. Lawrence estuary demonstrated the mobility of this element within the sediment. A maximum in dissolved interstitial boron was observed ∼8 cm beneath the sediment surface. This maximum was produced by the partial dissolution of solid-phase boron. Solid-phase boron seemed to be variably associated with iron hydroxides, organic material, and incorporated within crystal lattices of clays. After burial, the fraction of boron associated with Fe-hydroxides (max. 20%) decreased with depth until it became negligible. In contrast, the fraction associated with organic material (max. 50%) showed a slight maximum at intermediate depths as though the organic material were taking the place of the hydroxides. Finally, the fraction incorporated in crystalline structures (max. 60%) increased regularly towards the bottom of the core. This increase corresponded to a rate of incorporation of 0.1 mg kg −1 a −1 .

A simplified automated chelometric method for the determination of sulfate in interstitial water and seawater

Abstract A method is described for the determination of sulfate in interstitial water and seawater. After BaSO 4 precipitation, the Ba 2+ excess is titrated with EDTA using an amalgated silver electrode for end-point detection. An accuracy better than 0.5% is obtained using this method.

Contribution a l'etude chimique du materiel dissous et particulaire de l'estuaire de la riviere rimouski

Abstract Temperature, pH, total alkalinity, dissolved oxygen, silicate, nitrate, phosphate and the Mn, Fe and Al contents of suspended particulate matter (SPM) were measured in the Rimouski River estuary as functions of chlorinity during the period May—September 1980. At any given time, total alkalinity (TA) is conservative within the estuary with respect to chlorinity. However, the total alkalinity (TA) of the freshwater is related to river flow. This is attributed to dilution of the river water with bicarbonate-poor rainwater. Generally, pH follows the same pattern observed for TA in freshwater. Dissolved oxygen is usually more concentrated in freshwater and decreases linearly with increasing chlorinity. Freshwater is always saturated or supersaturated with respect to O 2 . Seasonal fluctuations are attributed to temperature variations. There is no evidence for removal of soluble silicate from the freshwater entering the sea. The concentration of silicate in the freshwater is strongly influenced by rainfall. Soil leaching, conditioned by high rainfall increases the concentration of soluble silicate in freshwater. Nitrate behaves similarly whereas phosphate is complicated by the presence of sewage. Analysis of the Mn, Fe and Al contained in the SPM indicates dilution of river-borne particles rich in Mn by others less rich in this element. A decrease in Mn content with increasing chlorinity and SPM concentration as well as increasing concentrations of SPM with increasing chlorinity indicate that the composition is controlled primarily by physical mixing of material from two sources rather than by chemical processes. Within the analytical precision the Fe/Al ratio does not vary with the chlorinity.

Localisation of the saguenay plume by its alkalinity

Abstract The large difference between the alkalinity of the freshwater of the St. Lawrence River and the Saguenay River was used to locate the region of the St. Lawrence estuary that is under the influence of the Saguenay River. Total alkalinity was measured in the St. Lawrence estuary near the mouth of the Saguenay River using net-works of 33 stations at the slack high tide and 23 stations at the slack low tide. The results show that at low tide Saguenay water from a plume extends more than 10 nautical miles from the mouth of the fiord into the estuary. At high tide the plume is restricted to the surface layer as the Saguenay waters are pushed back into the fiord. The alkalinity method has an advantage over classical measurements such as salinity, temperature and θ t , in that it is independent of the upwelling of deep water in these regions.