Publication

Subduction of oceanic sediments is known to have a large impact on the chemical evolution of the continental crust and mantle. Therefore, bulk chemical composition of subducting sediments has long been studied to elucidate geochemical cycles through subduction zones. Recent researches on oceanic sediments have discovered thin layers (<1 m in thickness) with extraordinarily high concentrations of rare-earth elements and yttrium (REY) in pelagic clay, demonstrating that the bulk chemical compositions of the pelagic clay are more heterogeneous than previously thought. However, in previous studies on subduction recycling, sampling densities (>5 m interval) were not high enough to detect such compositional peaks and thus, the heterogeneities in pelagic clay were not fully considered. Here we present new, high-resolution (0.25?1.5?m interval) bulk chemical compositions of unlithified sediments obtained from Ocean Drilling Program Site 1149. We identified distinct pelagic clay layers highly enriched in several characteristic elements, including a layer reaching a total REY concentration of 7500?ppm. Then, the average composition of the sediment column at Site 1149 was evaluated. Compared to the previous estimation, our high-resolution data significantly revise the average composition of the pelagic clay unit (e.g. +27% for Co, +32% for P2O5, +20?48% for REY). This in turn affects the average composition of whole sediment column (e.g. +19% for Co, +9% for P2O5, +11?15% for REY). Our results indicate that the high-resolution analyses of pelagic clay are very important for understanding global geochemical cycles.