Charles Gobeil

A SEDIMENT AND ORGANIC CARBON BUDGET FOR THE CANADIAN BEAUFORT SHELF

Abstract The Arctic Ocean accounts for 20% of the worlds continental shelves. Because the Arctic is sensitive to global change, budgets of organic carbon for its shelves are of immediate interest. The Mackenzie Shelf of the Canadian Beaufort Sea is the best North American proxy for the enormous Eurasian Shelves (large area, large river input), and the only site for which a complete organic carbon budget can be attempted, due to an extensive data base. A mass balance for the Mackenzie Shelf has been constructed for sediments, terrestrial organic carbon, and primary produced carbon. We have considered allochthonous inputs from the Mackenzie River, from coastal erosion, from smaller rivers, from groundwater, from the atmosphere and import by ice. The Mackenzie River dominates the supply to the Beaufort shelf of inorganic sediment (127 Mt a −1 ) and paniculate and dissolved terrestrial carbon (2.1 Mt a −1 POC, 1.3 Mt a −1 DOC). The combined input from all other sources contributes only about 5% of the Mackenzie load. Using sediment accumulation data we estimate that about half of the sediment supply is trapped in the delta, about 40% on the shelf and the remainder escapes the shelf edge by various processes. Autochthonous primary production in the delta and on the shelf adds a further 3.3 Mt a −1 of particulate organic carbon. A box model has been constructed to account for sediment, terrestrial organic carbon and primary produced carbon. Whereas about 60% of the terrestrial POC is preserved in delta and shelf sediments, it appears that most (97%) of the primary produced carbon is recycled and not preserved in sediments. Confidence in the budget should be improved by focusing future research on the determination of modern sedimentation rates on the delta and shelf, measurement of organic carbon content of deltaic sediments, determination of primary production on the shelf, and determination of the relative proportions of terrestrial and marine organic carbon preserved in sediments.

Cadmium diagenesis in Laurentian Trough sediments

The depth distributions of Cd, Mn and Fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the Laurentian Trough. The results reveal a surface layer in which the content of solid-phase Cd first decreases but then increases sharply with depth. In contrast, Mn decreases regularly over the same depth interval. In the oxygenated zone of this layer, Cd, probably associated with organic matter, is released to the pore water, resulting in concentrations that are more than an order of magnitude higher than in the overlying water column. Some of the dissolved Cd is returned to the water column and some migrates downward and is precipitated at depth. This part of the Cd cycle is virtually complete within the surface layer, the base of which corresponds to the base of the zone of Mn enrichment. In the subsurface layer, solid-phase Cd and Mn show little or no concentration change. However, dissolved Cd, which reaches often non-detectable concentrations (<0.05 nM) in this layer, increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer. The presence of dissolved Cd complexes (revealed by 1.5 to 8 fold increases in electrochemically active Cd in pore water following UV-treatment) as well as inconsistent distributions of dissolved and solid-phase Cd, indicate complexities in diagenesis that merit further investigation.

Diagenetic separation of cadmium and manganese in suboxic continental margin sediments

Abstract The vertical distribution of Cd differs from that of Mn in sediment cores taken from the continental margins of the Arctic and Atlantic oceans: the oxic surface sediment is depleted in Cd and enriched in Mn, whereas the subsurface sediment is enriched in Cd and depleted in Mn. In some cores the distributions of solid-phase and dissolved Mn are offset: the porewater profiles indicate that Mn is being precipitated relatively deep in the sediment, whereas the solid-phase distributions indicate that Mn has been precipitated very close to the sediment surface in the recent past. At one deep site, located near the base of the continental slope (3000 m), the offset may be caused by a relatively recent decrease in ocean productivity, a corresponding decrease in the organic carbon flux to the sediment, and a subsequent migration of the redox boundary deeper into the sediment. However, at a site where the upper sediment layers have accumulated during the last few decades, this explanation cannot account for the offset distributions. We propose that a fluctuating input of organic matter to the sediment on seasonal or decadal timescales causes the position of the redox boundary to fluctuate. The fluctuating redox boundary pumps dissolved Mn upward and concentrates it as Mn oxide in a distinct layer at the upper limit of the redox excursion; at the same time, it pumps Cd downward and concentrates it in a distinct layer at the lower limit of the redox excursion. The net result of a fluctuating redox boundary is to increase the spatial separation of Cd and Mn during the early diagenesis of suboxic sediments.

Mercury Fluxes at the Ocean Margins

A mass balance budget for mercury at Ocean margins is proposed on the basis of recent data on exchange fluxes with terrestrial, atmospheric and marine reservoirs. The largest single mercury flux consists of particulate species from rivers (~5 Mmol a-1), which is, primarily, unreactive and quickly buried in nearshore sediments. The most important source of mobile mercury for ocean margins is the atmosphere directly via deposition and indirectly via upwellings (total ≈ 5 Mmol a-1). The direct atmospheric input to coastal zones rivers (~2 Mmol a-1), exceeds considerably the dissolved inputs from rivers (~0.13 Mmol a-1). The evasion of elemental mercury from coastal surface waters is balanced by the mercury deposition from the atmosphere, but geographical differences exist suggesting a net mercury transfert to the higher latitudes. The overall residence time for mercury on the continental shelves is about 4 months. This is less than the mean residence time of water on the shelves (~1.3 year) and confirms the known reactivity of mercury in aquatic environments. However, two fractions of mercury are actively recycled (through the atmosphere and through the organic carbon recycling). The highly productive zones associated with frontal stuctures near the shelf edges appear to be very active in redistributing mercury species between ocean and coastal areas. One consequence is that the main source of methylmercury for coastal waters is upwelled oceanic waters.

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.

Sources and burden of lead in st. Lawrence estuary sediments: isotopic evidence.

The depth distribution of the concentration of lead has been determined in sediments from seven stations located along the longitudinal axis of the Laurentian Trough in the St. Lawrence Estuary. In the sediments collected at one of the sites, we also measured the stable isotope composition of the total Pb as well as that of the Pb found in sedimentary fractions operationally defined by a sequential extraction procedure. The results reveal three isotopicallydistinct sources of lead to these sediments: two natural and one from recent industrial pollution. While the two types of natural lead most likely derive from crystalline rocks of the Canadian Shield, the industrial lead has an isotopic composition typical of that measured in the atmosphere of urban regions in Canada

Recent change in organic carbon flux to Arctic Ocean deep basins: Evidence from acid volatile sulfide, manganese and rhenium discord in sediments

Evidence of recent large-scale change in redox conditions in Arctic Ocean basin sediments is found in profiles of solid phase acid volatile sulfide (AVS), Mn, and Re. AVS occurs at 2.5-5.5 cm in sediment cores collected from all major Arctic basins implying that presently there is a sufficient supply of organic carbon (OC) to deplete O 2 and form sulfide. Re, which is mobile under oxic conditions but precipitates under suboxic or anoxic conditions, is not found enriched in the sediments suggesting that AVS is a recent product of enhanced OC fluxes. Estimated diffusion rates suggest that AVS production is recent and that a change from oxic to anoxic diagenesis has taken place within the past 50 y. The most likely origin for such widespread change is the ice climate, and we infer that reduction in ice cover over the past several decades has led to enhanced OC fluxes to the sea floor.

Accumulation of heavy metals (Pb, Zn, Cu, Cd), carbon and nitrogen in sediments from Strait of Georgia, B.C., Canada

Abstract Cores collected from three sites in Ballenas Basin in the Strait of Georgia and at a remote site in Jervis Inlet were measured for 210 Pb, metals, C, N, δ 13 C, stable Pb isotopes and 137 Cs to establish modern records of basin contamination. At one site, in Ballenas Basin, duplicate cores were analyzed in detailed section. Sedimentation rates ranged from 0.17 to 0.9 g cm −2 year −1 (0.5-1.8 cm year −1 ). The stable Pb isotope data indicate that the increased Pb burden in the top of the cores derives mostly from Canadian leaded gasoline. The chronology of Pb recorded in the sediments closely matches the history of Pb emissions from gasoline consumption around the Strait of Georgia Basin. Cu and Zn show recent (post-1920) increases in sediment burden; Cd does not. From C/N ratio and δ 13 C records, we infer an increased burden in terrestrial carbon toward the sediment surface. We believe the additional carbon to come from pulp-mill effluent discharged into Strait of Georgia waters.

Dissolved mercury behaviour in the Saint Lawrence estuary

Abstract Dissolved mercury concentrations have been measured in the waters of the St Lawrence estuary. The typical concentration of the riverine end-member is 12·0±3·0 p m ; the oceanic end-member samples exhibit a mean mercury concentration of 2·4 p m . The graphical pattern of the relationship between mercury concentration and salinity shows a departure from a dilution line. We suggest that a removal of mercury from the dissolved phase during the estuarine mixing is responsible for this observation. Based on the results, the actual input of dissolved mercury from the St Lawrence River to the Gulf is evaluated to be approximately 0·52 T a −1 .

Early diagenesis of lead in Laurentian Trough sediments

Abstract The depth distributions of various solid-phase and porewater fractions of Pb, Fe and Cd in boxcores from three stations in the Laurentian Trough demonstrate that Pb is subject to early diagenetic change. High total-Pb concentrations, compared to background levels, indicate that much of the Pb in these cores is of anthropogenic origin. Patterns of mobilization and fixation, and of changes in the chemical reactivity towards extradants, are consistent from core to core. Pb is mobilized both near the sediment surface and at intermediate depth. In these zones dissolved Pb concentrations attain levels of 8–13.5 nM, which are two orders of magnitude greater than in the overlying bottom waters of the Laurentian Trough (0.1 nM). A fraction of Pb, soluble in a pH 5 sodium acetate-acetic acid solution, is produced diagenetically within the sediment. It is most abundant at intermediate depth, where it accounts for as much as 31% of the total Pb. Another fraction, soluble in hydroxylamine hydrochloride-acetic acid solution, decreases in abundance with depth. Flux calculations indicate that significant amounts of Pb are subject to postdepositional transformation. Parallels in the vertical distribution of the various fractions suggest that some Pb, like Cd, is mobilized near the sediment surface during the aerobic degradation of organic matter and that other forms are implicated in the redox-controlled cycles of Fe diagenesis.