Variability of trace-metal fluxes through the Strait of Gibraltar

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Van Geen, A. and Boyle, A., 190. Variability of trace-metal fluxes through the Strait of Gibraltar. In: L.D. Labeyrie and C. Jeandel (Editors), Geochemical Variability in the Oceans, Ice and Sediments. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 89: 65-79.. Three water masses originating in the Atlantic and entering the Alboran Sea through the Strait of Gibraltar have recently been identified on the basis of salinity and Cu, Ni, Cd and Zn concentrations. The endmembers are (1) Atlantic surface water, (2) North Atlantic Central Water and (3) Spanish shelf water. Spanish shelf water is of particular relevance to the trace-metal composition of the inflow to the Mediterranean Sea because this water mass is highly enriched in Cu, Cd and Zn relative to Atlantic surface water. Here, a conservative mixing model is solved for the above Atlantic endmembers, (with the addition of (4) a Mediterranean deep-water endmember) by weighted least-squares and shown to be consistent with tracer data for 42 surface samples collected in April '86 within the Strait of Gibraltar. Sensitivity of the solution to errors in the data and the assumptions of the model are discussed in detail. Uncertainties in the proportions of metal-enriched Spanish shelf water and NACW are (at most) on the order of 6 and 16%, respectively, and often smaller depending on the composition of a given sample. The inversion shows that Spanish shelf water is present predominantly in the northern half of the Strait and contributes up to 55% to Alboran Sea surface samples. Determining a representative composition of the inflow is complicated, however, by rapid change in the proportion of the three Atlantic endmembers present in the Strait of Gibraltar: entrainment of Spanish shelf water through the Strait roughly doubles between April 11 and 17. We show that the timing of collection of these samples minimizes a potential bias in endmember distributions simply due to variable tidal currents. The increase in entrainment of Spanish shelf water from neap tide to spring tide could, therefore, reflect a significant shift to the north of source waters to the Atlantic inflow over the course of a week. The data show that entrainment of Spanish shelf water is a significant source of the increased Cu, Cd and Zn concentrations observed in the Mediterranean relative to open Atlantic surface water, and this source may even account for the greater part of the Mediterranean enrichments.