Jean-Marie Martin

Elemental mass-balance of material carried by major world rivers

Abstract An estimate of average river particulate matter (RPM) composition was based on analyses of more than 40 elements in the Amazon, Congo, Ganges, Magdalena, Mekong, Parana and Orinoco rivers, to which were added literature data for 13 other major world rivers, covering the whole spectrum of morphoclimatic features. Geographic variations of major elements in RPM are mostly linked to weathering types and to the balance between weathering rate and river transport. As a result of chemical erosion, Al, Fe and Ti are enriched in RPM with respect to the average parent rock, while Na, Ca, Mg and Sr are strongly depleted. These figures are directly related to the relative importance of dissolved and particulate transport in rivers; this has been computed for each of 40 elements. In order to study weathering on a global scale, the total observed elemental fluxes (dissolved + particulate) have been computed and compared to theoretical ones. The latter were derived from the elemental content in the average parent rock and the total quantity of weathered material, computed from the Al ratio in RPM and in parent rock. Observed and theoretical fluxes are balanced for the less mobilized elements (rare earths, Co, Cr, Cs, Fe, Mn, Rb, Si, Th, Ti, U and V) for which no enrichment relative to Al is noted in RPM, and for B, Ba, Ca, K, Mg, Na, Sr which are relatively depleted in RPM due to their high dissolved transport. Additional fluxes have been found for Br, Sb, Pb, Cu, Mo, Zn and are possible also for Ni and P. This is reflected by marked enrichments in RPM relative to Al for the poorly or moderately dissolved transports (Pb, Cu, Zn). Several hypotheses involving either the natural origin (volcanic dust, marine aerosols, geochemical fractionation) or the artificial origin (worldwide pollution) are discussed to explain these discrepancies, assuming river transport and weathering either to be in a steady state on a global scale or not. However, none of them can fully account for these additional fluxes. It is most likely that these excesses have multiple origins, anthropogenic or natural or both. The comparison between RPM and deep-sea clay compositions emphasizes the prime influence of river input on oceanic sedimentation of Si, Al, Fe, Ti, lanthanides, Sc, Rb, V, etc. A few elements such as Zn, Sb, occur in excess in RPM as compared to deep-sea clays; in order to balance this excess, a remobilization of these elements out of the sediment can be considered. Finally, the enrichment of Co, Cu, Mn and Ni in deep-sea clays compared to RPM is discussed and attributed to several sources and processes.

First data on trace metal level and behaviour in two major Arctic river-estuarine systems (Ob and Yenisey) and in the adjacent Kara Sea, Russia

Dissolved and colloidal trace element concentrations (Cd, Cu, Fe, Ni and Pb) were determined in the Ob and Yenisey river-estuary systems and in the adjacent Kara Sea in September 1993. The data show a natural low concentration level of ‘dissolved’ (< 0.4 μm) trace metals (Cd, Cu, Fe, Ni and Pb) in the two rivers and in the Kara Sea as compared to world unpolluted rivers and the central Arctic Ocean, suggesting that the region studied is pristine with respect to trace metals. The pathway of trace metals transported from rivers to the ocean seems to be complicated, and largely influenced by biogeochemical processes taking place in the estuarine mixing zone. Colloidal material (104 Daltons-0.4 μm), in addition to its significant contribution to the so-called ‘dissolved’ fraction, has been shown to play a fundamental role in determining the behaviour of both conservative and non-conservative trace metals during estuarine mixing. Hence, colloids may control to a large extent the fate of ‘dissolved’ trace metals as well as their net input from the rivers to the Kara Sea.

Atmospheric input of dissolved and particulate metals to the northwestern Mediterranean

Concentrations of trace elements (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were determined for aerosols, dry deposition, precipitation and total deposition samples collected from five stations on islands and in the coastal zone of the northwestern Mediterranean. Average concentrations of metals are very homogeneous over the sampled area, in particular at the three coastal sites. Cd and Pb are almost entirely of man-made origin, even in Saharan aerosols. For the other metals, the non-crustal fraction is lower in Saharan aerosols than in European aerosols, but there is an important man-made component in the Saharan aerosol, even for metals such as Fe and Cr. This confirms the results of Chester et al. (1992) who concluded that Mediterranean aerosols have a European background upon which are superimposed Saharan inputs. Dry deposition represents an important fraction of the total deposition. Partitioning of total atmospheric deposition between the dissolved and the particulate phases shows that Al, Fe and Cr originating from the atmosphere are mostly in a particulate form in the surface waters. For the other metals studied, the dissolved fraction represents more than 30% of the total input, and for Cd it is almost 100%. Extrapolation shows that more than 50% of the dissolved metals input to the northwestern Mediterranean originates from the atmosphere. Atmospheric input entirely dominates the total external input of pollution-derived elements, such as Pb and Cd. The dissolved input of atmospheric origin is also very important ( > 80%) for elements of terrigeneous origin such as A1.

River versus atmospheric input of material to the mediterranean sea: an overview☆

Abstract River input has long been considered as the major source of chemical elements to the ocean system. However, in recent years, it has become increasingly apparent that atmospheric transport plays a central role in the supply of material to the ocean. This atmospheric input is especially important in shelf seas and semi-enclosed seas such as the Mediterranean, which is close to potential pollution sources in Northern Europe and subjected to the well-known influx of Sahara red dust. This paper summarizes the work performed by the Land-Sea Interactions Group of CNRS (1984–1987) and the EROS-2000 programme (1987–1988) on a variety of elements and compounds in this area, where a combined study of river and atmospheric fluxes has been carried out. It is shown that the atmospheric input of red dust is of the same order of magnitude as the annual downstream flow of rivers discharging to the Western Mediterranean. As far as trace metals are concerned, the atmospheric flux of Cu, Pb and Cd exceeds river input by one to two orders of magnitude. However, the incoming flux through the straits of Sicily and (mainly) Gibraltar might be as important as the atmospheric flux; the residence times of these three metals are either shorter than the water renewal time (Pb, Cu) or equivalent to it, so that their accumulation in the water column is unlikely. As far as artificial radionuclides are concerned, atmospheric input is predominant for 239 Pu + 240 Pu and 137 Cs , whereas 238 Pu is mainly of riverine origin. The fluxes of nitrogen supplied by rivers and rain are approximately equivalent. The biological significance of this nitrogen input is discussed; it is estimated that the atmospheric nitrogen corresponds to 10% of the average ‘new’ production and may reach values as high as 50% during oligotrophic periods.

The distribution and cycling of mercury species in the western Mediterranean

Total mercury concentrations were determined in water samples from ten vertical profiles in the western Mediterranean. Most Hg concentrations ranged from 0.5 to 4 pM, with a geometric mean of 2.26 pM. Such concentrations are in the same range as those measured in the North Atlantic and equatorial Pacific waters. There is no indication that the presence of geothermal activity or cinnabar deposits around the Mediterranean basin induces higher mercury concentrations in waters. Vertical concentration profiles were characterized by a maximum just below the thermocline, which is mainly developed in regions with relatively high primary production in the overlying waters. These observations support the remobilization model according to which, the mercury, associated with degradable organic matter, is solubilized from the particles accumulated in the thermocline layer. Additional measurements of certain mercury species, including reactive mercury (HgR) and gaseous species [elemental mercury (Hg°) and dimethylmercury (DMHg)], were performed on three profiles in the Alboran Sea and the Strait of Gibraltar. While 50% of the total mercury consisted of unidentified organic association, a maximum of the three determined species were observed below the thermocline: up to 0.71 pM, 0.43 pM and 0.30 pM for HgR, Hg° and DMHg, respectively. DMHg and Hg° appear to be formed in the low oxygen zone. A specific methylation rate of 3 × 10−9 s−1 can be estimated, which is six times higher than the values proposed for the North Atlantic waters (Mason et al., 1995a, Water, Air and Soil Pollution, 80, 665–677). The mass balance budget in the western Mediterranean shows that, while the total mercury exchanges at the straits are not unbalanced, mercury enters the Mediterranean as inorganic mercury and is exported to the Atlantic Ocean partially as methylated species. Riverine input is mainly composed by particulate mercury which is readily accumulated in coastal sediments. The dissolved mercury flux from non-marine sources is largely dominated by atmospheric deposition, which is one order of magnitude higher than riverine input. The mercury accumulation rate in the sediments is quantitatively equivalent to the atmospheric inputs, but mainly constituted by terrestrial material deposited nearshore. On the basis of ☐-core analyses and the global model of Mason et al. (1994a, Geochimica et Cosmochimica Acta, 58, 3191–3198) and present results the pre-industrial Hg riverine, atmospheric and sedimentation fluxes are estimated to have been about one-third the current one.

The significant role of colloids in the transport and transformation of organic carbon and associated trace metals (Cd, Cu and Ni) in the Rhône delta (France)

The distribution of organic C and trace elements (Cd, Cu, and Ni) between truly dissolved and colloidal phases was determined in May 1992, in order to better understand their behavior during estuarine mixing in the Rhone delta (France). Colloidal material (104 Daltons-0.4 μm) was separated with an ultra-clean cross-flow filtration system. A considerable part of the organic C and trace metals studied, used to be considered to occur in the so-called “dissolved” pool, is associated with colloidal material. The significant correlation between colloidal organic C and colloidal Cu suggests an important involvement of colloidal organic material in the transport of this element. The correlation between Ni, Cd and organic matter mainly occurs in the truly dissolved fraction, suggesting that the truly dissolved organic C is important for these two elements. Truly dissolved and colloidal organic C and trace-metal fluxes transported from the River Rhone to the northwestern Mediterranean Sea were estimated.

Sources of Cd, Cu, Ni and Zn in Portuguese Coastal Waters.

Abstract In order to identify the major sources of trace metals (TM) in the Portuguese coastal waters, 58 surface water samples were collected during September 1988. The area sampled extended from the Tagus Estuary (down to a salinity of 25) to cape Ste Marie on the southern coast of Portugal. Dissolved metal concentrations in the fully marine waters ranged from 30 to 250 pM for Cd, 0.7–15 nM for Cu, 0.9–20 nM for Zn and 1.8–4.5 nM for Ni. Within the Tagus Estuary (salinity 25), concentrations increased to 3400 pM for Cd, 26 nM for Cu, 14 nM for Ni and 230 nM for Zn. The large-scale distribution of these metals is dominated by two strong continental sources, both probably linked to the exploitation of pyrite ores. In the Tagus Estuary, TM enrichments can be mostly attributed to a pyrite roasting plant located on the shore in front of Lisbon. Concerning the south Portuguese shelf waters, several hypotheses are proposed to explain their elevated metal concentrations. We particularly discussed the likely influence of the Tinto/Odiel rivers located 100 km eastward, an influence well known in the shelf waters of the Gulf of Cadiz. These rivers are extremely metal-rich because of acid mine tailings originating from their catchment. Between these two regions, upwelling of relatively metal-poor water largely contributes to the dilution of the continental inputs. Indeed, water exchanges on the shelf linked to the upwelling involve water fluxes 500 times higher than the Tagus River flow, and renew the coastal waters that are thus cleaned from terrestrial contamination. Contrary to many other upwelling systems in non-contaminated areas, the Portuguese upwelling does not act as a source of trace-metal enrichment of the continental margin waters.

Outflow of trace metals into the Laptev Sea by the Lena River

Abstract Freshwater concentrations confirm the pristine character of the Lena River environment as already pointed-out in a previous study with a limited set of data (Martin et al., 1993). Total dissolved concentrations of the freshwater are 13.8 ± 1.6 nM, Cu, 4.4 ± 0.1 nM, Ni, 0.054 ± 0.047 nM, Cd, 642 ± 208 nM, Fe, 0.2–0.3 nM Pb and 1.2 ± 1.0 nM, Zn. For Zn and Pb, a simple mixing of the Lena River waters with the Arctic waters is observed. Relationships with salinity suggest that for Cu, Ni and Cd, there is a mobilization of the dissolved fraction from the suspended matter, with an increase of the dissolved concentration of 1.5, 3 and 6 times, respectively. For Fe, the total dissolved concentrations follow an exponential decrease in the mixing zone and 80% of the total “dissolved” Fe is removed from the solution. For Cu, Ni, Cd and Fe, the riverine end-members are 20 nM, 12 nM, 0.3 nM and 47 nM, respectively. When considering the input of total dissolved metals to the Arctic Ocean, the fraction attributed to the freshwaters from the Arctic rivers appears to be small (4% of the input of dissolved metal to the Arctic Ocean for Cd, 27% for Cu, 11 % for Ni and 2% for Zn). Metal concentrations in the Laptev Sea and Arctic Ocean are very similar, indicating a generally homogeneous distribution in the areas sampled.

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

Abstract 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.

Level and fate of trace metals in the lagoon of Venice (Italy)

Abstract This paper presents the first results of dissolved (total concentration and hydrophobic fraction) and particulate trace metals (core sediment and suspended matter) in the lagoon of Venice. Both the particulate and dissolved trace metal concentrations are comparable with those measured in pristine rivers and non-contaminated seawater. In the mixing zone between the Silone channel water and the Adriatic seawater, Pb, Zn, Ni, Fe and Cd behave conservatively, indicating the absence of any significant removal or mobilization processes, Cu is significantly released from the particulate to the dissolved form. The mobilization of Cu is associated with the intense production of macroalgae and phytoplankton in the lagoon. The C18 Sep-Pak column extraction method has been used to isolate the hydrophobic organic fraction of dissolved trace metals. This fraction increases in the following order: Cd, Pb