The oxygen and carbon isotope distribution in the Mediterranean water masses

The oxygen and carbon stable isotope compositions of the present-day Mediterranean waters have been measured in order to evaluate their variability, which is related to the specific climatic and hydrological conditions within the basin. The experimental equation between the delta(18)O value and the salinity of water, based an 300 measurements on surface, intermediate, and deep waters sampled during the VICOMED 2 and 3 cruises in the western, central and eastern Mediterranean, has a slope of 0.27, a value which is significantly lower than the slope of 0.45, as defined in the northeast Atlantic Ocean. This difference in the delta(18)O-salinity relationship, which occurs immediately in the Alboran basin, is basically a characteristic of the climatic regime of the Mediterranean, i.e., of an excess evaporation over fresh water input. The largest variations of these two parameters, delta(18)O Of water and delta(13)C Of Sigma CO2, are observed in the surface waters, mostly in the western Mediterranean. This evolution mirrors the progressive eastward restriction, which separates the less-evaporated and more-productive western basins from the more-evaporated and less-productive eastern basins. The intermediate waters constitute a homogeneous layer. However, their delta(18)O values decrease eastward by 0.35 parts per thousand at maximum, due to progressive dilution by mixing with overlying and underlying water masses; their delta(13)C values decrease also eastward by 0.35 parts per thousand at maximum, due to an increasing input of nutrients issued from the regeneration of sinking organic particles. The deep waters have similar delta(18)O values but slightly higher delta(13)C values (often by less than 0.1 parts per thousand) than the overlying intermediate waters, indicating generally well ventilated conditions due to active winter convection. (C) 1999 Elsevier Science B.V. All rights reserved.