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

合作研究：扩大北小安的列斯群岛弧的地热年代记录：使用多种方法快速鉴定隐壳虫

• 批准号: 1709889
• 负责人: Molly McCanta
• 金额: $ 5.67万
• 依托单位: University of Tennessee Knoxville
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2019-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1709889&HistoricalAwards=false
• 关键词: Collaborative; Research; Expanding; Tephrochronologic; Record

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.

火山喷发可将大量火山碎屑物质（火山灰）散布到广阔的地理范围内。当保存在沉积记录中时，这些层通常充当重要的地层标记床，可以在空间和时间上进行关联，并与喷发活动的强度、来源和时间相关。对这些矿床的沉积学分析可以研究与电弧产生、维护和破坏过程相关的基本地质问题。其中包括：(1) 火山喷发的速率、时间、频率和强度，2) 识别火山喷发时间序列中喷发类型、喷发强度和休止期的系统模式或方差，(3) 评估陆上记录的完整性，特别是当海洋记录包含未在陆上存档的事件时，以及 (4) 火山复合体建造过程中火山活动的性质（岩浆演化、生产率、喷发）类型、喷发口和产物的时空分布以及构造与破坏过程的重要性）。这些信息对于评估当今活弧火山的危害至关重要。然而，虽然在有书面历史记录之前或在保存完好的地下火山灰沉积物（通常仅代表几千年）之外限制火山弧的长期喷发历史很重要，但它本质上是困难的。建立准确地质记录的关键是开发一种能够准确、可复制、定量和常规识别海洋沉积物序列中这些火山灰层的方法。传统上，火山灰年代学方法已应用于可见的火山灰层，其典型特征是离散的、通常是深色的沉积层。这将岩石年代学研究限制在靠近能够产生大量沉积物的火山源的区域。隐火山灰是肉眼看不见的细小火山灰层，可能代表较小的喷发或来自更远源的喷发。为了充分限制火山中心的喷发历史，对隐毛虫的识别现在变得至关重要。此前，人们通过各种耗时且破坏性的方法来鉴定隐齿类。该提案的目标是记录和表征不同的沉积特性，并使用高分辨率连续扫描技术从周围的沉积基质中以毫米级分辨率半定量地识别沉积物记录中隐石的分布和大小。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系统进行岩心XRF扫描； (4) 计算机断层扫描 (CT) 扫描。虽然这四种技术已经证明了它们在识别火山灰方面的潜力，但它们很少一起使用，特别是在记录加密火山灰所需的分辨率时。结合这三种方法将对沉积物的沉积学、地球化学、磁性和光谱特性进行全面评估和评估，所有这些特性均已独立证明会因火山灰的存在而发生很大变化，并且还可以评估这些技术的功效用于隐花鉴定。所描述的技术将应用于在蒙特塞拉特岛附近钻探的 IODP 岩心，以建立蒙特塞拉特岛的长期年代学记录，并更好地限制小安的列斯群岛弧的演化。这项研究将产生：(1) 驱动蒙特塞拉特岛的火山系统的扩展地质历史和相关的灾害评估； (2)更好地了解小安的列斯群岛弧的演化； (3) 对一组可用于通过非破坏性手段快速识别沉积物序列中的隐石层的技术进行评估。