PARTICULATE AND DISSOLVED ND IN THE WESTERN MEDITERRANEAN-SEA - SOURCES, FATE AND BUDGET

The concentration and isotopic composition of Nd in water and particle s collected in the western Mediterranean Sea are studied by two comple mentary approaches. The first examines local vertical profiles and tim e series; the second considers the global Nd budget of the whole weste rn Mediterranean Sea. These two approaches are used to quantify the Nd inputs and the dissolved/particulate exchange processes in the water column. Two profiles of Nd in seawater in the Ligurian Sea taken in Ma y and October 1992 show an average epsilon(Nd)(0) = -9.6 +/- 0.5. Seaw ater from the Strait of Sicily, representative of the eastern waters f lowing into the western basin, is more radiogenic [epsilon(Nd)(0) = -7 .7 +/- 0.6]. Profiles of particulate matter collected in sediment trap s in coastal (Gulf of Lions) and offshore (Ligurian Sea) environments are also shown. Particles are enriched in Nd and are more radiogenic n ear the coast than offshore. Measurements of Nd concentration and epsi lon(Nd)(0) of external sources to the western Mediterranean Sea compar ed with the literature data demonstrate that particulate flux of atmos pheric Saharan origin are more rich ([Nd] = 38 +/- 10 mug/g) and less radiogenic [epsilon(Nd)(0) = -13.0 +/- 1.0] than riverine particulate discharge ([Nd] = 21.5 +/- 4.4 mug/g; epsilon(Nd)(0) = -10.1 +/- 0.5), allowing to trace Nd particulate inputs in the water column. Nd atmos pheric flux appears to be the major source into the whole western basi n, although lateral advection of riverine material is the prevailing p rocess in the coastal environment. Offshore, the vertical propagation of an important Saharan dust event has been recorded for two months in sediment traps at 80, 200 and 1000 m. The evolution of the resulting negative epsilon(Nd)(0) peak along depth and time shows that the parti cles reach 200 m on a time scale of one week. For the first time, the Nd budget in the western Mediterranean basin is constrained by both co ncentrations and isotopic compositions measured in particles and seawa ter. Surface budget requires a remobilization of 30 +/- 20% of particu late Nd input. In deep water, dissolved Nd concentrations are balanced by a scavenging of 10 +/- 20% of the sinking particulate flux. On the other hand, the deep isotopic compositions suggest an exchange betwee n 30 +/- 20% of the sinking particles and the deep waters. The hypothe sis of a non-stationary regime for the surface waters in the Ligurian Sea is also considered.