Causes and implications of the middle rare earth element depletion in the eolian component of North Pacific sediment

Previous studies clearly demonstrate that the detrital fraction of central North Pacific sediment is derived almost exclusively from wind-born particles from the arid and semi-arid regions in Asia. These conclusions are based, in part, on trajectories observed for westerly wind systems and on a grainsize distribution for central North Pacific sediment which can only be explained by eolian transport. These observations have been verified by comparing the mineralogy, neodymium isotopic composition, and trace element geochemistry of the sediment with its Asian source region. However, recent geochemical investigations consistently highlight compositional differences exemplified by a MREE (middle rare earth element) depletion and a lower Th/Sc ratio in eolian material extracted from North Pacific sediment when compared to bulk China loess, the continental analog of the eolian material. These geochemical differences persist even when the bulk loess is subjected to the same extraction procedure. Here we present experiments demonstrating that these compositional differences are caused by a combination of grainsize 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 extraction procedure commonly used to isolate terrigenous material from marine 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 removed by the extraction procedure correlates well with both the observed MREE depletion and the decrease in Th/Sc and is most pronounced in the finest grained samples. These correlations strongly suggest that the extraction procedure dissolves inorganic phosphate phases which are rich in MREEs and Th. A comparison of the compositions of the 2–3 mm loess size fractions with central North Pacific sediment shows that both of these materials are identical.