| 英文摘要: | Explosive volcanic eruptions can disperse large amounts of pyroclastic material (tephra) over a wide geographic range. When preserved in the sedimentary record, these layers often serve as important stratigraphic marker beds that can be correlated spatially and temporally and related to the magnitude, source, and timing of eruptive activity. Sedimentological analysis of these deposits enable investigation of fundamental geologic questions related to arc generation, maintenance, and destruction processes. These include: (1) the rates, timing, frequency and magnitude of volcanic eruptions, 2) identification of systematic patterns or variance in the time series of volcanic eruptions in terms of eruptive style, eruption magnitude, and repose periods, (3) assessing the completeness of the onshore record, especially if the marine record contains events not archived onshore, and (4) the nature of volcanism during the construction of a volcanic complex (magma evolution, production rate, eruptive styles, spatio-temporal distribution of eruptive vents and products, and importance of constructional vs. destructional processes). This information is essential to inform assessments of present-day hazards from active arc volcanoes. However, while constraining the long term eruptive history of a volcanic arc before the start of written historical records or beyond well-preserved subaerial tephra fall deposits (often representing only a few thousand years) is important, it is inherently difficult.
Key to establishing an accurate geologic record is the development of a methodology capable of accurate, replicable, quantitative, and routine identification of these tephra layers in marine sediment sequences. Traditionally, tephrochronologic methods have been applied to visible tephra beds that are typically characterized by discrete, often dark-colored depositional layers. This limits tephrochronology studies to areas proximal to volcanic sources capable of producing voluminous deposits. Cryptotephras, fine layers of ash that may not be visible to the naked eye, may either represent smaller eruptions or eruptions from more distal sources. Identification of cryptotephra has now become essential in order to fully constrain the eruptive history of a volcanic center. Previously cryptotephra have been identified by a variety of time-intensive and destructive methods. The goal of this proposal is to document and characterize different sedimentary properties and semi-quantitatively identify the distribution and magnitude of cryptotephras in the sediment record at millimeter scale resolution from the surrounding sedimentary matrix using high resolution continuous scanning techniques. These techniques include: (1) magnetic measurements, particularly magnetic susceptibility; (2) reflectance spectroscopy; (3) core XRF scanning using the ITRAX system; and (4) Computer Tomography (CT) scans. While these four techniques have demonstrated their potential for the identification of tephra, they have rarely been used together, especially at the resolution required to document cryptotephra. Combining these three methodologies will provide a comprehensive assessment and evaluation of the sedimentological, geochemical, magnetic, and spectral properties of sediment, all of which have independently been shown to vary substantially with the presence of tephra and also allow evaluation of the efficacy of these techniques for cryptotephra identification. The described techniques will be applied to IODP cores drilled off Montserrat to establish a long term tephrochronologic record of Montserrat and to better constrain the evolution of the Lesser Antilles arc. This research will generate: (1) an expanded geologic history of the volcanic system driving Montserrat and an associated hazard assessment; (2) a better understanding of the evolution of the Lesser Antilles arc; and (3) an evaluation of a set of techniques that can be used to rapidly identify cryptotephra layers in sediment sequences by non-destructive means. |