Rare earth element distributions in anoxic waters of the Cariaco Trench

Abstract

The concentrations of dissolved and suspended particulate rare earth elements in the Cariaco Trench are reported. In solution all REE, notably Ce, show a sharp increase just at or below the oxic anoxic interface at 300 meters depth. Particulate concentrations show a complementary decrease at the same depth. The overlying oxic waters exhibit a negative Ce anomaly; the anoxic waters carry a slightly positive Ce anomaly. The particulate Ce anomaly reaches a maximum just above the interface, coinciding with maxima for particulate Mn and Fe and minima for the dissolved Ce anomaly and dissolved Ce, Mn and Fe. Thermodynamic calculations predict that the solubility of Ce(IV)O 2 is greatly enhanced by the steep pϵ-drop at the O 2 H 2S interface whereas oxygenated seawater is grossly oversaturated with dissolved Ce(III). However, the dramatic shifts in pe and absolute total Ce concentration do not affect the relative speciation of Ce in solution, which varies only with pH and not with pϵ because dissolved Ce(IV)-species are negligible. The cycling of Ce across the oxic anoxic boundary is driven largely by its own redox chemistry as with Fe and Mn. The remaining REE, being strictly trivalent, are recycled in association with the dissolution-precipitation of (ferro)manganese oxyhydroxides. The relative turnover rates at the interface are ranked as Mn > Ce = Ce-anomaly > Nd, Sm, Eu, La, Dy, Er . The observed absence of Eu anomalies would render in situ reduction of Eu(III) to Eu(II) unlikely; thermodynamic considerations also rule out the existence of Eu(II) species in low temperature reducing environments.