Dispersed volcanic ash in marine sediment: Revisiting an old issue with a new geochemical and statistical approach
Abstract
The presence of dispersed volcanic ash in pelagic marine sediment (as differentiated from ash found in discrete layers) has been known since the 1970’s. Most previous studies have assessed the dispersed component through sedimentological and petrographic methods. As part of an effort to quantitatively determine the amount, and chemical composition, of dispersed ash in pelagic sediment, we have begun a systematic study of the tectonically active western Pacific. ODP Site 1149 (Leg 185), located immediately east of the Izu-Bonin Arc, consists of aluminosilicate clay and large amounts of volcanic ash (25-35 wt%), most obviously found in ash layers). In addition to the ash layers, however, there is abundant (~20% - ~50%) mixed and dispersed ash. Through the use of a multi-elemental geochemical and statistical methodology we are able to characterize and quantify this dispersed ash component, and thus generate a novel ash record that complements the record provided by the ash layers. At Site 1149, factor analysis indicates that mixing between three factors best explains the statistical variability. This is consistent with sedimentological and other studies that indicate loess and two other ash components. Multiple linear regression treatments based on refractory trace element end members of potential sources (identified from the literature) more specifically suggest that Chinese Loess, Ryukyu Dacite, and an average of Izu-Bonin Front Arc material yield the best mixing that explains the bulk sedimentary composition. That the distal Ryukyu Arc contributes substantial material to the sediment eastward of Izu-Bonin (i.e., Site 1149) is surprising, yet is required by our chemical results, and is consistent with the previous work of Egeberg et al. (1992). Comparing the records of discrete ash layer sedimentation rate (cm/m.y.), as well as thickest ash layer and average ash thickness in 0.5 m.y. time slices, to the record of dispersed ash accumulation rate (g/m2/m.y.), provides interesting comparative information to the studies of discrete layer and dispersed ash coupling. At Site 1149 we find that dispersed ash accumulation and discrete layer sedimentation rates show a paired increase from ~3 Ma. However, the dispersed ash increases by a significantly greater amount (8-9 fold) than that of the discrete layers (~4 fold). This suggests that the dispersed record may be responding to different volcanic forcings than that of the discrete layers. Also, temporal trends in the thickest ash layer and average ash thickness do not match the dispersed ash record. Changing plate kinematics led to the increases in volcanism seen in the dispersed ash component. Current Ryukyu Trough extension has been active since ~6 Ma and the area was first subjected to collision at ~3 Ma. Overall, the regional quantification of dispersed ash provides additional insight into the nature of sediments headed to their fate in the western Pacific “Subduction Factory”, as well as of the tectonic evolution of the region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T23A1873S
- Keywords:
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- 1051 GEOCHEMISTRY / Sedimentary geochemistry;
- 3060 MARINE GEOLOGY AND GEOPHYSICS / Subduction zone processes;
- 8413 VOLCANOLOGY / Subduction zone processes;
- 8455 VOLCANOLOGY / Tephrochronology