Collaborative Research: Expanding the Tephrochronologic Record of the Northern Lesser Antilles Arc: Rapid Identification of Cryptotephra Using Multiple Methods

爆炸性的火山爆发可以将大量的火山碎屑物质（tephra）分散到广泛的地理范围内。当沉积记录保存下来时，这些层通常作为重要的地层标志层，可以在空间和时间上进行关联，并与喷发活动的震级、来源和时间有关。对这些矿床的沉积学分析使研究与电弧的产生、维持和破坏过程有关的基本地质问题成为可能。这些包括:(1)火山喷发的速率、时间、频率和震级；(2)确定火山喷发时间序列在喷发风格、喷发震级和休止期方面的系统模式或变化；(3)评估陆上记录的完整性，特别是如果海洋记录包含未在陆上存档的事件；(4)火山复合体建设期间的火山作用性质（岩浆演化、生产速度、喷发风格、火山活动）；喷发喷口和产物的时空分布，以及构造与破坏过程的重要性)。这一信息对于评估目前弧活火山的危害至关重要。然而，尽管在有文字记载的历史记录开始之前或保存完好的地下火山瀑布沉积物（通常只代表几千年）之前限制火山弧的长期喷发历史很重要，但它本身就很困难。建立准确地质记录的关键是开发一种能够准确、可复制、定量和常规地识别海洋沉积物序列中这些温层的方法。传统上，温度年代学方法已应用于可见的温度层，其典型特征是离散的，通常是深色的沉积层。这就限制了温度年代学研究只能局限于能够产生大量矿床的火山源附近的地区。隐隐层是肉眼看不到的细灰层，可能代表较小的喷发，也可能代表更远的喷发源。为了全面了解一个火山中心的喷发历史，隐隐体的鉴定现在变得至关重要。以前已经通过各种时间密集和破坏性的方法鉴定了隐绦虫。本提案的目标是利用高分辨率连续扫描技术，以毫米尺度分辨率从周围的沉积基质中记录和表征不同的沉积性质，并半定量地确定沉积物记录中隐层的分布和大小。这些技术包括：(1)磁测量，特别是磁化率；(2)反射光谱；(3)核心XRF扫描采用ITRAX系统；(4)计算机断层扫描（CT）。虽然这四种技术已经证明了它们识别麻风的潜力，但它们很少被一起使用，特别是在记录隐麻风所需的分辨率上。将这三种方法结合起来，可以对沉积物的沉积学、地球化学、磁场和光谱特性进行全面的评估和评价，所有这些特性都已被证明随着麻蝇的存在而发生实质性变化，并且还可以评估这些技术对隐麻蝇鉴定的有效性。所描述的技术将应用于在蒙特塞拉特钻取的碘同位素岩心，以建立蒙特塞拉特的长期温度年代学记录，并更好地限制小安的列斯弧的演变。这项研究将产生：(1)扩大推动蒙特塞拉特火山系统的地质历史和相关的危害评估；(2)加深了对小安的列斯岛弧演化的认识；(3)评价了一套可用于通过非破坏性手段快速识别沉积物序列中隐球菌层的技术。

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.