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Identifying cryptotephra units using correlated rapid, nondestructive methods: VSWIR spectroscopy, X-ray fluorescence, and magnetic susceptibility

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Abstract
  • Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast, the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time-consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, nondestructive, and quantitative core scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near-infrared band in tephra. Using predetermined mixtures, BDI1000VIS can accurately identify tephra layers in concentrations >15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, 29 potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near-real-time recognition of tephra units, both visible and cryptotephra.
  • Keywords: spectroscopy, tephrochronology, magnetic susceptibility, identification, cryptotephra
  • Keywords: spectroscopy, tephrochronology, magnetic susceptibility, identification, cryptotephra
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  • McCanta, M. C., Hatfield, R. G., Thomson, B. J., Hook, S. J., & Fisher, E. (2015). Identifying cryptotephra units using correlated rapid, nondestructive methods: VSWIR spectroscopy, X‐ray fluorescence, and magnetic susceptibility. Geochemistry, Geophysics, Geosystems, 16(12), 4029-4056. doi:10.1002/2015GC005913
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  • 16
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  • 12
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  • This paper was supported by NSF grants EAR-1347868 to M.C.M., EAR-1347912 and OCE-1260671 to R.G.H., and EAR-1347868 to B.J.T.
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