In this characterization brief, report here comprehensive major and trace element analyses of over 45 sediment samples from Ocean Drilling Program Site 1149, located seaward of the Izu-Bonin trench. The combination of these core analyses with a complete set of geochemical logging data enables us to calculate the bulk composition of the sedimentary column subducting at the Izu trench with high accuracy (uncertainties ≤13%). Izu sediment has lower concentrations than global subducting sediment for most elements, due to ∼50% dilution by biogenic material (∼45% opal and ∼10% carbonate), but is relatively enriched in Ba, Pb, and rare earth elements (REE), except Ce, due to nonterrigenous inputs. Sediments subducting into the Izu and Mariana trenches differ compositionally due to ocean island–sourced volcaniclastics in Marianas sediments, enriching them in Nb, Ta, Ti, and LREE, and continentally derived eolian material in Izu sediments, enriching them in Cs, Rb, Th, and U. These differences predict along-strike variations in sediment input that should be manifested in the composition of volcanic output from the Honshu-Izu-Bonin-Mariana arc systems. Such variations are observed as an increase in Th/La in both sediments and arc volcanics from the Marianas in the south to Honshu in the north.