Radium as internal tracer of Mediterranean outflow water

Radium-226 and Ra-228 data are presented for the Western Mediterranean Sea and the NE Atlantic. In the NW Mediterranean Sea, the near-shore surface waters of the Ligurian Current contain much higher Ra-228 concentrations than surface waters collected in the open Ligurian seawater. The gradient of Ra-228 along a Villefranche - Calvi transect has been used to calculate values of the horizontal eddy diffusion coefficient perpendicular to the flow direction of the Ligurian Current of about 10(6) cm(2) s(-1). The observed decay of Ra-228, as the Mediterranean Deep Water flows toward the Gibraltar Straits, is in good agreement with the assumption of a rapid turnover of the Mediterranean basin. Concentration profiles of Ra-226 and Ra-228 were measured in the Mediterranean Outflow Water (MOW) off the Gibraltar Straits in the NE Atlantic basin at two stations, 36 degrees 30 degrees'N - 15 degrees 35W and 36 degrees 27'N - 10 degrees 5'W. These sites showed the hydrographic characteristics of Meddies (MOW which propagates as eddies in the NE Atlantic) at intermediate depth. At the center of the Meddles, around 1000 m depth, the activity of the short half-life Ra-228 increases up to 0.9x10(-2) disintegrations per minute per liter, higher than usually found in North Atlantic Deep Water. This ''young'' Ra-228 results from the contact, a few months earlier, of the MOW with the Spanish continental shelf in the vicinity of the Gibraltar Straits. The decay of Ra-228, used as an internal tracer of water mass motion, leads to a transit time of the Meddy Yseult of less than 1 year since its formation.