Dispersed ash in deeply buried sediment from the northwest Pacific Ocean: An example from the Izu–Bonin arc (ODP Site 1149)

Abstract

A multi-elemental geochemical and statistical methodology was applied to the sedimentary sequence recovered at Ocean Drilling Program Site 1149 (Leg 185) to characterize and quantify the abundance of dispersed ash–as distinct from discrete ash layers–in sediment from a critical region of the Northwest Pacific. Visual descriptions of sediment from the past 11 Ma describe both clay and volcanic ash (25–35 wt.%) as the dominant components, with many discrete layers observed. Factor analysis of the multi-element dataset indicates that statistical variability is best explained by mixing between three factors. Multiple linear regressions based on refractory trace- and rare earth element end-members of potential sources identified from the literature suggest that wind delivered Chinese loess, Ryukyu Dacite, and an average of Izu–Bonin Front Arc material yield the best statistical mixing that explains the bulk sedimentary composition. All three sources are characterized by gradual increases beginning from ~ 5–6 Ma to 4.5 Ma with occasional spikes in each of the sources. Ryukyu Trough extension has been active since ~ 6 Ma which may help to explain the increase in the Ryukyu Dacite source at Site 1149 as both the abundance and MAR rapidly increase at ~ 3.5 Ma. Comparing the total dispersed ash accumulation rate to the sedimentation rate of the discrete layers documents paired increases beginning at ~ 3.5 Ma, although the dispersed ash increases by a greater relative amount (8–9 fold) than does the sedimentation of discrete layers (~ a factor of four). Collectively, the ability to resolve the dispersed component and study its compositional changes through time, in the context of coupling (or lack thereof) with discrete ash layer records, adds an additional parameter to studies of geochemical budgets (“Subduction Factory”) as well as to the arc- and tectonic evolution of the region.