The chemical characterization of dispersed ash and ash layers at DSDP Site 52, Izu-Bonin

As part of an on-going regional project, the focus of this study is the characterization of compositions and fluxes of dispersed ash and discrete ash layers in the northwest Pacific Ocean in the context of variability in time and space. Deep Sea Drilling Project Site 52 is located eastward of the Izu-Bonin-Marianas subduction zone (27.77N, 147.12E, water depth 5744 m). Site 52 was rotary drilled in 1969 during DSDP Leg 6, and its major sediments were initially described as "clay-rich volcanic ash and brown clay with abundant volcanic glass". We therefore selected this site as potential "ash-rich" end member in our regional assessment. We analyzed 60 bulk sediment and 8 discrete ash layer samples (the latter represents all layers that were recovered) by ICP-ES and ICP-MS, from the upper 60 mbsf. Ash layers are only present in the top 13 mbsf, perhaps due to drilling disturbance at deeper depths. No samples were collected between 60 and 69 mbsf because the sediment there was reported as flow-in. At 69 mbsf lithified ash and chert was encountered so drilling was discontinued. In addition to quantifying the abundance of dispersed ash in the bulk sediment, we compare the composition of the dispersed ash component to that of the discrete ash layers. In order to facilitate comparison between ash layers and the bulk sediment, all major element data are reported on an anhydrous basis. Indeed, the major element totals for the discrete ash population (approx. 92 wt. %) and bulk sediment (approx. 88 wt. %) are consistent with the bulk sediment incorporating more alteration products (i.e., authigenic clay). The discrete ash layers show at least two populations of compositions. "Ash 1" broadly is characterized by lower SiO2 (60-62 wt%) with higher TiO2 (0.8-0.9 wt. %), MgO (2.8-3.0 wt. %), Fe2O3 (7-10 wt. %), Sc (19-30 ppm), and V (125-160 ppm). This ash is generally similar to upper crustal materials such as loess and PAAS, but differs in several key diagnostic compositions. The "Ash 2" group is characterized by high SiO2 (69-75 wt%) with low TiO2 (0.08 - 0.4 wt. %), MgO (0.5-1 wt%), Fe2O3 (1.5-3.4 wt%), Sc (2.3-6.9 ppm), and V (10-42 ppm). The two ash layer populations are also distinguished by other elements. The bulk sediment composition within the uppermost 60 mbsf does not vary significantly with depth and overlaps mostly with the composition of Ash 1. It is characterized by SiO2 from 54 to 63 wt %, TiO2 from 0.5 to 1 wt%, MgO from 2.8 to 5.1 wt%, Fe2O3 from 5 to 10 wt%, Sc from 12 to 31 ppm, and V from 75 to 160 ppm. The bulk sediment composition is consistent with varying mixtures of Ash 1 and an upper crustal source such as loess, as well as small amounts of Ash 2. On going study of the ash shard chemistry (analyzed by microprobe) as well as multivariate statistical analyses of the bulk sedimentary data will quantify the degree of mixing between these three sources (and/or others not yet considered), and quantitatively allow us to resolve the relative contributions of Ash 1 and the upper crustal source.