Mathematics – Logic
Scientific paper
Aug 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2173v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 8, pp.2173-2191
Mathematics
Logic
3
Scientific paper
Dissolved Cu, Cd, and Zn concentrations in continental shelf waters of the Gulf of Cadiz are much higher than reported for other coastal areas. Direct observations of the metal enrichments in this region were made in March-April 1986 and October 1986; indirect observations (in the Alboran Sea) suggest similar enrichments occurred in June 1982 and September 1980, and it is probable that these enrichments are a persistent feature of the region. Zinc concentrations varied by more than two orders of magnitude between Atlantic water entering the Strait of Gibraltar (<1 nmol/kg) and Spanish shelf water (up to 160 nmol/kg). Copper and cadmium were also highly enriched in Spanish shelf water (up to 47 and 1.3 nmol/kg, respectively), and concentrations of these metals varied co-linearly with Zn. Chemical profiles show that metal enrichments over the Spanish shelf extended from the surface down to about a 45 m depth. In contrast, Ni, phosphate, silicate, and 228 Ra/ 226 Ra all occurred at levels typical of other shelf waters. Copper, cadmium, and zinc concentrations in nearby Moroccan shelf water were only a factor of 2 to 3 higher than off-shore surface Atlantic water. A steep surface Cu, Cd, and Zn concentration gradient was observed in the axis of the Strait of Gibraltar denoting the boundary between off-shore Atlantic and Spanish shelf water. Metal:metal and metal:salinity relationships immediately west of the Strait of Gibraltar show that variations in the composition of surface water can be described by conservative mixing of three sources: (1) metal-depleted surface Atlantic water, (2) Spanish shelf water, highly enriched in Cu, Cd, and Zn, and (3) subsurface North Atlantic Central Water, enriched in Cd and slightly enriched in Ni. Because the Atlantic inflow through the Strait is the main water source for the Mediterranean Sea, enrichments in the Gulf of Cadiz influence metal concentrations of the whole basin. Some temporal metal variation is observed in Spanish shelf water: Zn concentrations decreased by a factor of two between April and October 1986. Although metal concentrations increase systematically with decreasing salinity over the Spanish shelf, Iberian rivers cannot account for the Cu, Cd, and Zn enrichments. An anthropogenic source of any type seems unlikely because the loss of Cu, Cd, and Zn by entrainment of shelf water through the Strait of Gibraltar is equivalent to a significant fraction of total metal consumption on the Iberian peninsula. A diagenetic shelf sediment source may be more plausible, but it would have to be of much greater magnitude than on other shelves. One mechanism that may enhance metal fluxes from shelf sediments is a coastal "metal trap" driven by an estuarine-type circulation pattern and biological production along the Iberian coast. This process is illustrated here by a simple box model; if this process occurs along the whole Iberian peninsula, Cd enrichments (but perhaps only part of the observed Cu or Zn enrichments) could be sustained against the considerable advective metal sink through the Strait of Gibraltar.
Boyle Edward A.
Moore Willard S.
van Geen Alexander
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