Regional‐scale input of dispersed and discrete volcanic ash to the Izu‐Bonin and Mariana subduction zones

Abstract

AbstractWe have geochemically and statistically characterized bulk marine sediment and ash layers at Ocean Drilling Program Site 1149 (Izu‐Bonin Arc) and Deep Sea Drilling Project Site 52 (Mariana Arc), and have quantified that multiple dispersed ash sources collectively comprise ∼30–35% of the hemipelagic sediment mass entering the Izu‐Bonin‐Mariana subduction system. Multivariate statistical analyses indicate that the bulk sediment at Site 1149 is a mixture of Chinese Loess, a second compositionally distinct eolian source, a dispersed mafic ash, and a dispersed felsic ash. We interpret the source of these ashes as, respectively, being basalt from the Izu‐Bonin Front Arc (IBFA) and rhyolite from the Honshu Arc. Sr‐, Nd‐, and Pb isotopic analyses of the bulk sediment are consistent with the chemical/statistical‐based interpretations. Comparison of the mass accumulation rate of the dispersed ash component to discrete ash layer parameters (thickness, sedimentation rate, and number of layers) suggests that eruption frequency, rather than eruption size, drives the dispersed ash record. At Site 52, the geochemistry and statistical modeling indicates that Chinese Loess, IBFA, dispersed BNN (boninite from Izu‐Bonin), and a dispersed felsic ash of unknown origin are the sources. At Site 1149, the ash layers and the dispersed ash are compositionally coupled, whereas at Site 52 they are decoupled in that there are no boninite layers, yet boninite is dispersed within the sediment. Changes in the volcanic and eolian inputs through time indicate strong arc‐related and climate‐related controls.