The trace metal content of recent organic carbon-rich sediments; implications for Cretaceous black shale formation

Aquatic surface chemistry: Edited by Werner Stumm. Wiley, New York. 1987. $69.95 (ISBN 0471822951)

Chemical composition of suspended sediments in World Rivers: New insights from a new database

Trace metal distribution in sediments of the Pearl River Estuary and the surrounding coastal area, South China.

Marine ecosystems' responses to climatic and anthropogenic forcings in the Mediterranean

Concentrations of a number of trace metals have been measured in samples obtained on a series of cruises in the western basin of the Mediterranean, carried out as part of the EROS 2000 project. The Mediterranean distributions of Cd, Cu and Ni, metals which show varying degrees of nutrient-like behaviour in the world ocean, are dominated by lateral transport and vertical mixing processes, rather than biogeochemical cycling. In particular, Cu shows a clear distinction between depleted upper waters with a high proportion of Atlantic Surface Water, and waters of similar salinity but subject to terrestrial influences. The behaviour of Cd is more complex, as a result of both non-conservative estuarine mixing and inputs of Cd-enriched North Atlantic Central Water entrained with the surface inflow through the Gibraltar Straits. This entrainment of relatively small volumes of high-Cd water has a potentially important effect on Mediterranean budgets of this metal. Dissolved Mn, Co and Pb distributions are closer to those found elsewhere in the world ocean. Atmospheric inputs are clearly distinguishable for Mn and Ph, although riverine inputs are dominant in the coastal zone. All the metals discussed show some degree of enrichment around the Straits of Gibraltar and Sicily, apparently a result of a combination of localized influences from highly mineralized adjacent regions.

Distribution and behaviour of some dissolved trace metals in the western Mediterranean Sea

Trace metal concentrations were measured in the Danube River estuary and in the shelf area of the north-western Black Sea. Total dissolved concentrations (<0·45 μM) of the freshwater in the river end-member were found to be: Cd=117 pM, Pb=81 pM, Co=266 pM, Zn=6 nM, Mn=19 nM, Ni=15 nM, Cu=36 nM, Al=384 nM and Fe=20 nM. These concentrations are surprisingly low for an area where serious contamination has been suspected. This is particularly true of Cd, Pb and Zn, which are generally anthropogenic in polluted regions. The observed low concentrations, in particular for Fe, Mn, Co and Al can be attributable to the precipitation of hydrated oxides as a consequence of high pH of the Danube River. Concentrations of metals in the particulate fraction are similar to those in other major rivers of the world.

The evolution of the total dissolved trace metal concentrations in the surface waters within the salinity gradient suggests: (1) no noticeable exchange between the particulate and the dissolved fraction for Cu, Ni and Zn; and (2) evidence for a low solid to liquid exchange for the other metals (Cd, Fe, Mn, Co and Pb) which brings about a two-fold increase in their concentrations from lower to higher salinity. Another source of dissolved material is suspected at around salinity 15. This source may be from localized-patches in the sediment where specific redox conditions may induce a vertical flux of dissolved metals: the bottom waters above these patches are characterized by a diminution of the dissolved O2and high concentrations of metals.

The colloidal fraction (between 10 kD and 0·45 μm) was established on a limited number of samples for Cd, Cu, Mn and Fe and was found to be significant: 40% of Cu, 40% of Cd, ∼50% of Mn and 60% of Fe of the so-called dissolved phase are actually associated with colloidal material. It represents at least 40% of the total dissolved concentration in the fresh water and in the mixing zone. This proportion decreases linearly with salinity for Cd and Cu. Fe is mostly in colloidal form with a maximum (∼80%) between salinity 5 and 10.

The surface distribution of the total dissolved metals throughout the study area show a limited influence of the Danube on the concentrations observed for the ‘ open sea ’ (salinity=18). Most of the riverine flux seems to follow the general circulation to the south-west area. These ‘ open Black Sea ’ concentrations are the same order of magnitude as average surface concentrations in the Mediterranean Sea.

On trace metal geochemistry in the Danube River and western Black Sea

Measurements of dissolved and leachable particulate trace metals (Mn, Fe, Co, Pb, Cd, Zn, Cu and Ni) and total particulate Mn and Fe were made on seawater samples collected from the northwestern Black Sea during the EROS 2000 expedition conducted in July–August 1995. The investigation concentrated on waters of the shelf and shelf edge, but included one deeper water (1440 m) station. In the oxic layer of the deep station, the suspended particulate fractions of Mn and Fe were a major part of the total metal mass, consistent with the presence of the “Fine Particle Layer” which forms on the shelf and spreads all over the Black Sea with intensities decreasing from the coast. Dissolved and total particulate concentrations were, respectively, Mn, 0.69–9.6, 1.2–29; Fe, 0.79–3.03, 2.3–7.4 nM. Dissolved Cu and Ni concentrations were relatively high (1–8 and 8–12 nM, respectively), and did not show any depletion in surface oxic waters, possibly as a result of strong organic complexation. Dissolved Pb concentrations (100–200 pM) were higher than were generally found on the shelf. This was attributed to atmospheric inputs combined with less efficient scavenging of metals in these low SPM waters. The distribution of dissolved Co closely resembled that of dissolved Mn reflecting coupling through oxidation of Mn. Concentrations of dissolved Cd and Zn were low in surface water (0.07–0.09 and 0.9–2.0 nM, respectively), and increases in concentrations with depth were sharply reversed around the top of the redoxcline. For most metals (Mn, Fe, Co, Pb, Cu, Cd, Zn) dissolved concentrations were low in the anoxic layers as a result of solubility by formation of, or association with, solid sulphide phases. Dissolved Ni was not affected by sulphide precipitation. At most of the shelf stations there were clear enhancements of dissolved Mn and Fe in the deepest waters, consistent with other evidence that significant benthic fluxes of these metals arise through the redox conditions in the region of the sediment-water interface. In the shelf water column, dissolved Mn and Fe concentrations ranged between 1.2 and 1350 and between 0.4 and 181 nM, respectively; the highest concentrations were found near the bottom. Particulate concentrations of Mn and Fe were high, implying high oxidation rates of Mn(II) and Fe(II) and/or high supply rates from rivers. Total particulate concentrations of Mn and Fe were 0.7–1050 and 2.3–2650 nM, respectively; the highest concentrations were found in surface and bottom waters. The distributions of particulate Mn and Fe were consistent with the isopycnal transport of Mn and Fe oxyhydroxides from the shelf by the coastal circulation. Distributions of other trace metals (Co, Pb, Cu, Ni, Cd, Zn) were considerably influenced by riverine inputs. Relatively high dissolved and available particulate metal concentrations were generally found in surface waters at stations directly influenced by the Danube River. Some trace metals (Co, Ni, Cd and Zn) were influenced by Mn and Fe cycling and increases in their dissolved concentrations occurred at a number of stations near the sediment–water interface. Dissolved and available particulate metal concentrations (nM) at stations on the shelf were, respectively: 0.171–1.80, 0.003–0.437 (Co); 0.014–0.614, 0.010–1.48 (Pb); 7.6–28.8, 0.048–3.75 (Cu); 11.0–17.5, 0.018–2.10 (Ni); 0.033–0.161, 0.003–0.063 (Cd); 1.01–8.33, 0.135–7.58 (Zn).

Trace metal distributions in shelf waters of the northwestern Black Sea

Mass balance of trace metals in the Adriatic Sea

Measurements of dissolved Mn and Fe and total particulate Mn, Fe and Al were made on seawater samples collected in the Northern Adriatic Sea in different seasons (May 1994, June–July 1994, September 1994 and February 1995), and in the Central Adriatic Sea in June–July 1994. Porewaters were also analysed for dissolved Mn and Fe in February 1994, June 1994, September 1994 and February 1995. Seasonal observations of dissolved Mn and Fe and particulate excess Mn and Fe in the Northern Adriatic Sea have demonstrated pronounced temporal and spatial variability of their distribution. This variability was mainly due to: (a) storm events which broke down stratification of the water column and enhanced resuspension and advective transport of particulate forms, and (b) fluctuations of oxygen concentrations in the water column and sediment–water interface, related to the biodegradation of the organic matter produced during bloom periods. Large inputs of dissolved Mn and Fe were attributed to reductive solubilization and diffusion from the sediment–water interface. Particulate excess Mn and Fe concentrations were also increased by oxidation in the presence of oxygen of dissolved Mn and Fe, which had diffused from the sediment–water interface.

S.P.C. Tankéré

Papers

Distribution and behaviour of some dissolved trace metals in the western Mediterranean Sea

On trace metal geochemistry in the Danube River and western Black Sea

Trace metal distributions in shelf waters of the northwestern Black Sea

Mass balance of trace metals in the Adriatic Sea

Biogeochemical cycling of Mn and Fe in an area affected by eutrophication: the Adriatic Sea