Volcanic ash in deep marine sediment; a comparison of dispersed ash and adjacent ash layers

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Author(s): Scudder, R. P.; Murray, R. W.; Kutterolf, S.; Schindlbeck, J. C.
Author Affiliation(s): Primary:
Boston University, Department of Earth and Environment, Boston, MA, United States
GEOMAR, Germany
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: 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 sediments, we are undertaking a systematic study of the western Pacific marine sediments. ODP Site 1149 (Leg 185), located immediately east of the Izu-Bonin Arc, consists of aluminosilicate clay and large amounts of volcanic ash (>75 ash layers described in units I and II). In addition to the ash layers, there is abundant dispersed ash (20-50% of the bulk). Using a multi-elemental geochemical and statistical approach we can characterize and quantify this dispersed ash component, and thus complement the original ash layer record by a novel dataset. At Site 1149, our previous work based on refractory trace element end members of potential sources (from the literature) indicate that Chinese Loess, Ryukyu Dacite (Japan), and an average of Izu-Bonin Front Arc material yield the best mixing to explain the bulk sedimentary composition (Scudder et al., 2009, EPSL, 284, 639-648). Contribution of a significant distal Ryukyu Arc component 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). While Scudder et al. (2009) was based on a small number of samples (∼15 samples for complete major, trace, and REE analysis) and a modest element menu, we here present the results from an expansive suite of analyses (>80 samples) allowing us to test the effect of sample number on the statistical results and achieve additional quantitative resolution of volcanic and upper crustal sources (e.g., loess). This further improves our statistical ability to resolve temporal changes that may be related to variation in arc volcanism through time. Most importantly, we are able to discern an additional source(s) of material beyond our previous work that was based on more limited data. Furthermore, we compare the composition of the dispersed ash component to that of the discrete ash layers. We analyzed 26 ash layers, sampled from the entire span of Units I and II. We are able to identify at least two broad compositional groupings. The first ash population is similar to upper crustal material such as loess, though several key diagnostic compositions are distinctive. The second ash layer population broadly exhibits lower SiO2 with higher TiO2, MgO, Fe2O3, and Sc. These two ash groups are consistent with the two ashes identified by Scudder et al. (2009) as the sources of the dispersed ash in the bulk sediment, with the first ash layer population representing the Ryukyu Dacite component and the second population being similar to the Izu-Bonin Front Arc. Additional studies of the ash shard chemistry as well as multivariate statistical analyses of the enhanced bulk sedimentary data set will allow further resolution of sources, and the development of a quantitative inventory of the sedimentary sequence being subducted into the Izu-Bonin arc system.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Igneous rocks; Leg 185; Lithostratigraphy; Marine sediments; North Pacific; Northwest Pacific; ODP Site 1149; Ocean Drilling Program; Pacific Ocean; Pyroclastics; Sediments; Volcanic ash; Volcanic rocks; West Pacific
Coordinates: N312030 N312030 E1432100 E1432100
Record ID: 2015005573
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