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Major element oxides and Cl of dispersed (invisible to the naked eye) volcanic glass shards were measured in clastic sediments of the central Japan Trench recovered at Site M0090 during International Ocean Discovery Program Expedition 386, Japan Trench Paleoseismology. The glass shards were extracted from two giant piston cores, Core 386-M0090B-1H (Sections 1H-15, 80 cm, to 1H-14, 50 cm) and Core 386-M0090D-1H (Sections 1H-14, 0 cm, to 1H-13, 50 cm), which together represent 180 cm of the sediment. High-resolution sampling with 1 and 5 cm spacing (65 samples) aimed to better define the stratigraphic position of the To-Cu marker ash (~6000 y before present), which had previously been identified within this interval. Electron microprobe analysis reveals low-K volcanic glasses (<1.5 wt% K2O; ~67% of the glass data) and medium- and high-K glasses (>1.5 to 5 wt% K2O; ~33% of the glass data) in all 65 samples. The low-K volcanic glasses display the typical compositional characteristics of volcanic glass from the Towada volcano in Northern Honshu and may be mostly of To-Cu origin. The medium- and high-K glasses are likely an assortment of volcanic glasses produced during various Holocene and Pleistocene explosive eruptions of the Honshu arc volcanoes. Variation in grain size fraction, magnetic susceptibility, bulk density, and natural gamma radiation of the sediment sequence sampled suggests that all glass shards were emplaced in turbidite flow either syn- or posteruptively with a major explosive eruption of the Towada volcano.
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Phases in fine volcanic ash
Abstract Volcanic ash emissions impact atmospheric processes, depositional ecosystems, human health, and global climate. These effects are sensitive to the size and composition of the ash; however, datasets describing the constituent phases over size ranges relevant for atmospheric transport and widely distributed impacts are practically nonexistent. Here, we present results of X-ray diffraction measurements on size-separated fractions of 40 ash samples from VEI 2–6 eruptions. We characterize changes in phase fractions with grainsize, tectonic setting, and whole-rock SiO2. For grainsizes < 45 μm, average fractions of crystalline silica and surface salts increased while glass and iron oxides decreased with respect to the bulk sample. Samples from arc and intraplate settings are distinguished by feldspar and clinopyroxene fractions (determined by different crystallization sequences) which, together with glass, comprise 80–100% of most samples. We provide a dataset to approximate glass-free proportions of major crystalline phases; however, glass fractions are highly variable. To tackle this, we describe regressions between glass and major crystal phase fractions that help constrain the major phase proportions in volcanic ash with limited a priori information. Using our dataset, we find that pore-free ash density is well-estimated as a function of the clinopyroxene + Fe-oxide fraction, with median values of 2.67 ± 0.01 and 2.85 ± 0.03 g/cm3for intraplate and arc samples, respectively. Finally, we discuss effects including atmospheric transport and alteration on modal composition and contextualize our proximal airfall ash samples with volcanic ash cloud properties. Our study helps constrain the atmospheric and environmental budget of the phases in fine volcanic ash and their effect on ash density, integral to refine our understanding of the impact of explosive volcanism on the Earth system from single eruptions to global modeling.
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- Award ID(s):
- 1719875
- PAR ID:
- 10464230
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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