Zinc budget in the Mediterranean Sea: a hypothesis for non-steady-state behavior

Abstract

The concentrations of Zn were determined in surface and deep-water samples at 11 sites in the Mediterranean Sea during the PHYCEMED 2 (October 1983) cruise. The Zn vertical profile is marked by a surface concentration (typically 3.0 nmol kg −1 in the Western Basin) higher than in the open ocean where the surface layer is Zn depleted. The subsurface layer (between 30 and 80 m) shows Zn and salinity minumum values along the path of the Atlantic inflow throughout the Western Basin. A weak intermediate maximum is present between 200- and 400-m depth, whereas Zn values are homogeneous in the deep waters from ∼300-m depth, as are T and S values. In deep water, the Zn/P ratios are similar to those of plankton; this is a good argument for a biological Zn cycle, which may explain the intermediate maximum caused by remineralization of organic matter settling from the euphotic layer. The deep layer is expected to be homogeneous as a result of the basin dynamics, whereas the biological cycle will induce a Zn depletion in the surface layer. The relatively high values observed in the surface waters of the Western and Eastern basins are explained by the Zn cycle's non steady state, linked to recent increases in Zn supplied to the sea, which can be estimated as ∼4 × 10 8 mol year −1 in 1983, i.e. about twice the Mediterranean deep Zn outflow over the Gibraltar Sill. Owing to the Zn and salinity minimum values in the subsurface Atlantic inflow, we can conclude that most of the Zn originates from atmospheric and terrestrial inputs around the Mediterranean Sea.