Neodymium isotopes in the Mediterranean Sea: Comparison between seawater and sediment signals

Nine depth-profiles of dissolved Nd concentrations and isotopic ratios (epsilon(Nd)) were obtained in the Levantine Basin, the Ionian, the Aegean, the Alboran Seas and the Strait of Gibraltar. Thirteen core-top sediments and Nile River particle samples were also analyzed (leached with 1 N HCl, acetic acid or hydroxylamine hydrochloride). The seawater epsilon(Nd) values become more radiogenic during the eastward circulation in the Mediterranean Sea. The relationship between salinity and the seawater epsilon(Nd) shows that the Nd isotopic signature is more conservative than salinity in the Mediterranean Sea. The water mass with the highest epsilon(Nd) (-4.8) is found at about 200 m in the easternmost Levantine basin. The average epsilon(Nd) value for deep waters is -7.0 in the eastern basin, 2.5 epsilon-units higher than in the western basin. By examining the sensitivity of seawater epsilon(Nd) to Nd inputs from the Nile, we conclude that the most significant radiogenic Nd source is partially dissolved Nile River particles. The Nd flux from the Nile River water has a minor influence on the Mediterranean seawater eNd. Except for the easternmost Levantine Basin, the leachate epsilon(Nd) values are consistent with the seawater values. In the easternmost Levantine Basin, the leachate eNd values obtained with HCl leaching are systematically higher than the seawater values. The relationship between leachate and residual epsilon(Nd) values indicates that the HCl leaching partially dissolves lithogenic Nd, so the dissolution of Nile River particles is the cause of the observed shift. Some eNd values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with HCl leaching. Although the reason for this shift is not clear, Sr-87/Sr-86 successfully detects the presence of a nonmarine component in the leachate. Our results suggest that leaching performance may vary with the mineralogy of marine sediments, at least in the case of the Mediterranean Sea. Copyright (C) 2004 Elsevier Ltd.