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: 1347912
• 负责人: Robert Hatfield
• 金额: $ 9.83万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1347912&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.

火山爆发会将大量的火山碎屑物质（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，来源和时间有关。这些矿床的沉积学分析使调查有关弧的产生，维护和破坏过程的基本地质问题。其中包括：（1）火山喷发的速率、时间、频率和规模，（2）根据喷发类型、喷发规模和休眠期确定火山喷发时间序列中的系统模式或变化，（3）评估陆上记录的完整性，特别是如果海洋记录包含陆上未存档的事件，（4）火山杂岩形成过程中火山活动的性质（岩浆演化、产出率、喷发类型、喷发口和产物的时空分布、构造过程与破坏过程的重要性）。这一信息对于评估目前的活弧火山危害至关重要。然而，虽然在有文字记载的历史记录开始之前或在保存完好的陆上火山灰瀑布沉积物（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，定量和常规识别海洋沉积物序列中的火山灰层。传统上，火山灰年代学方法已被应用于可见的火山灰床，其典型特征是离散的，往往是深色的沉积层。这就限制了火山灰年代学的研究，使其局限于能够产生大量沉积物的火山源附近的地区。隐火山灰是一种肉眼看不到的细小火山灰层，可能代表较小的喷发，也可能是来自更远来源的喷发。为了充分限制火山中心的喷发历史，隐火山岩的鉴定现在变得至关重要。以前，隐石层是通过各种时间密集和破坏性的方法鉴定的。这项建议的目的是记录和描述不同的沉积特性，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中半定量地确定沉积物记录中隐晶岩的分布和大小。这些技术包括：（1）磁性测量，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）扫描。虽然这四种技术已经证明了它们在鉴别火山灰的潜力，但它们很少一起使用，特别是在记录隐火山灰所需的分辨率下。结合这三种方法将提供一个全面的评估和评价的沉积物，地球化学，磁性和光谱特性的沉积物，所有这些都已独立地被证明有很大的变化与火山灰的存在，也允许这些技术的有效性评价cryptotephra识别。所描述的技术将被应用到IODP核心钻蒙特塞拉特建立蒙特塞拉特的长期火山灰年代学记录，更好地限制小安的列斯群岛弧的演变。这项研究将产生：（1）扩大蒙特塞拉特火山系统的地质历史和相关的危险评估;（2）更好地了解小安的列斯群岛弧的演变;（3）评价一套可用于以非破坏性手段迅速查明沉积序列中隐石层的技术。