CAUSES AND IMPLICATIONS OF THE MIDDLE RARE-EARTH ELEMENT DEPLETION INTHE EOLIAN COMPONENT OF NORTH PACIFIC SEDIMENT

Previous studies clearly demonstrate that the detrital fraction of cen tral North Pacific sediment is derived almost exclusively from wind-bo rn particles from the arid and semi-arid regions in Asia. These conclu sions are based, in part, on trajectories observed for westerly wind s ystems and on a grainsize distribution for central North Pacific sedim ent which can only be explained by eolian transport. These observation s have been verified by comparing the mineralogy, neodymium isotopic c omposition, and trace element geochemistry of the sediment with its As ian source region. However, recent geochemical investigations consiste ntly highlight compositional differences exemplified by a MREE (middle rare earth element) depletion and a lower Th/Sc ratio in eolian mater ial extracted from North Pacific sediment when compared to bulk China loess, the continental analog of the eolian material. These geochemica l differences persist even when the bulk loess is subjected to the sam e extraction procedure. Here we present experiments demonstrating that these compositional differences are caused by a combination of grains ize fractionation during transport and partial dissolution of REE- and Th-bearing phosphatic phases during the extraction procedure.Two bulk loess samples from China were separated into several different grain size fractions and a split of each fraction was subjected to the extra ction procedure commonly used to isolate terrigenous material from mar ine sediment. All extracted and unextracted sample pairs were analyzed for P, Th, Sc, Fe, La, Ce, Sm, Eu, Yb, and Lu. The amount of P remove d by the extraction procedure correlates well with both the observed M REE depletion and the decrease in Th/Sc and is most pronounced in the finest grained samples. These correlations strongly suggest that the e xtraction procedure dissolves inorganic phosphate phases which are ric h in MREEs and Th. A comparison of the compositions of the 2-3 mm loes s size fractions with central North Pacific sediment shows that both o f these materials are identical. Copyright (C) 1998 Elsevier Science L td.