THE GEOCHEMICAL BALANCE OF THE RARE-EARTH ELEMENTS AND NEODYMIUM ISOTOPES IN THE OCEANS

Profiles of REE concentrations and neodymium isotope measurements in t he western Indian Ocean are consistent with published data from the ot her major oceans. REE concentrations show a strong hydrographic contro l. The influence of intermediate water masses is particularly signific ant. Surface water concentrations vary with salinity. Particulate REEs are less than 5% of dissolved concentration, except for Ce, where up to approximately 20% of total REEs are present as >0.4 mum particles. In contrast to dissolved profiles, concentration profiles of particula te REEs, including Ce, all increase with depth. REE concentrations cor relate with silica over all but the deepest waters, although, in detai l, correlations are complex. The neodymium isotopic composition of Ind ian Ocean seawater (avg. epsilon(Nd(0)) = -8.3) is intermediate betwee n that for the Atlantic and Pacific oceans. Comparison of REE data fro m the major oceans shows that the HREEs behave more like Si than do th e LREEs. Sm/Nd ratios are near-constant for all oceans, whereas Er/Nd ratios vary between the Atlantic, Indian, and Pacific oceans. The frac tionation of the REEs in surface seawaters can be explained by fractio nation factors relative to silica from approximately 0.12 for La to 0. 05 for Er. Use of a seven-box model shows that the interoceanic variat ions in neodymium isotopic compositions and Nd concentrations cannot b e reconciled unless particle-water exchange is invoked. The degree of exchange required is consistent with particle residence times of appro ximately 1-10 yr.