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

火山爆发可以将大量的火山碎屑物质(火山灰)分散在广泛的地理范围内。当保存在沉积记录中时，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对比，并与喷发活动的规模、来源和时间相关。对这些矿床的沉积学分析使研究与弧形产生、维持和破坏过程有关的基本地质问题成为可能。其中包括：(1)火山喷发的速度、时间、频率和规模；(2)根据喷发方式、喷发规模和休止期确定火山喷发时间序列的系统模式或变化；(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，以及(4)建造火山复合体期间火山活动的性质(岩浆演化、产出率、喷发方式、喷发喷口和产物的时空分布以及构造过程与破坏过程的重要性)。这一信息对于评估活弧型火山的当今危害是至关重要的。然而，尽管在书面历史记录开始之前或在保存完好的空中火山灰瀑布沉积物(通常只代表几千年)之后限制火山弧的长期喷发历史是重要的，但这本身就很困难。建立准确的地质记录的关键是开发一种方法，能够准确、可复制、定量和常规地识别海洋沉积物序列中的这些Tephra层。传统上，笔迹年代学方法被应用于可见的火山灰岩层，其典型的特征是离散的，通常是深色的沉积层。这限制了对火山年代学的研究，只限于能够产生大量矿床的火山源附近的地区。隐语是肉眼看不到的火山灰细层，可能代表较小的喷发，也可能来自更远的来源。现在，为了完全限制火山中心的喷发历史，对隐翅虫的识别变得至关重要。以前，人们通过各种耗时和破坏性的方法来鉴定隐翅虫。这项建议的目标是记录和描述不同的沉积性质，并利用高分辨率连续扫描技术，以毫米级分辨率从周围的沉积基质半定量地确定沉积物记录中隐语的分布和大小。这些技术包括：(1)磁测量，特别是磁化率；(2)反射光谱；(3)使用iTrax系统的岩心XRF扫描；以及(4)计算机断层扫描(CT)。虽然这四种技术已经证明了它们在鉴定非洲菊属上的潜力，但它们很少一起使用，特别是在记录隐翅虫所需的分辨率上。将这三种方法结合起来，将对沉积物的沉积学、地球化学性质、磁学和光谱性质进行全面的评估和评价，独立地证明，所有这些性质都随着Tephra的存在而有很大的不同，还可以评估这些技术对隐Tephra鉴定的有效性。上述技术将应用于在蒙特塞拉特附近钻探的IODP岩芯，以建立蒙特塞拉特的长期年代学记录，并更好地限制小安的列斯岛弧的演化。这项研究将产生：(1)扩大的火山系统驱动蒙特塞拉特的地质历史和相关的危险评估；(2)更好地了解小安的列斯岛弧的演化；(3)对一套可用于以非破坏性手段快速识别沉积序列中的隐翅层的技术进行评估。

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.