Radium as internal tracer of Mediterranean Outflow Water

Abstract

Radium‐226 and 228Ra 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 228Ra concentrations than surface waters collected in the open Ligurian seawater. The gradient of 228Ra 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 106 cm2 s−1. The observed decay of 228Ra, 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 226Ra and 228Ra were measured in the Mediterranean Outflow Water (MOW) off the Gibraltar Straits in the NE Atlantic basin at two stations, 36°30′N ‐ 15°35′W and 36°27′N ‐ 10°35′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 Meddies, around 1000 m depth, the activity of the short half‐life 228Ra increases up to 0.9 × 10−2 disintegrations per minute per liter, higher than usually found in North Atlantic Deep Water. This “young” 228Ra 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 228Ra, 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.