Marine CSEM study of the southern Hikurangi Margin: A first step towards estimating the global gas hydrate carbon budget

希库朗吉边缘南部的海洋 CSEM 研究：估算全球天然气水合物碳预算的第一步

• 批准号: 1916553
• 负责人: Steven Constable
• 金额: $ 73.49万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-08-01 至 2021-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1916553&HistoricalAwards=false
• 关键词: Marine; CSEM; study; southern; Hikurangi

Gas hydrate is a solid ice-like mixture of water and gas, usually methane, that forms in seafloor sediments when pressure is high enough and temperature is cold enough. Such conditions are found in water depths greater than about 500 m, and gas hydrate is found on continental slopes world-wide wherever there is methane produced from the decay of organic matter or leaking from hydrocarbon systems. Vast amounts of carbon have accumulated this way, but estimates of the total quantity vary by several orders of magnitude, mainly because of limitations in using seismic methods to assess hydrate concentration, especially at lower concentrations. This gap in knowledge is a problem, because hydrate can have a big impact on human activity and well-being. Changes in sea level or temperature can cause hydrate to decompose, possibly contributing to seafloor landslides that can trigger tsunamis. Loss of sediment stability from hydrate decomposition can damage man-made seafloor infrastructure. Methane leaking from the seafloor is used as energy by some biological systems, but if too much escapes it can add to acidification and impact the health of the oceans. This project will begin to plug the gaps in knowledge about offshore hydrate by using a novel non-seismic geophysical method to image hydrate under hundreds of square kilometers of seafloor off the southern margin of New Zealand's North Island, where abundant seismic data have been collected and hydrate is inferred to exist in both low and high concentrations over broad regions. The project supports the training of an early career scientist.The electrical resistivity of methane hydrate is four orders of magnitude greater than seafloor sediment, and sediment resistivity varies smoothly and monotonically with hydrate saturation. We have developed a deep-towed controlled source electromagnetic (CSEM) system to image seafloor resistivity and have validated its use for quantifying seafloor hydrate concentrations off California, Japan, and the Gulf of Mexico. Typically CSEM studies target known methane seeps and hydrate deposits, but in this project data will be collected on a basin-scale to capture the broad distribution, using the Hikurangi subduction zone as a model to develop an understanding of the global hydrate inventory by extrapolation to subduction zones worldwide. By combining existing seismic data with the new CSEM data, through a collaboration with colleagues in New Zealand, the processes that concentrate hydrate will be examined, how much hydrate sits at the upper edge of the stability field (and thus vulnerable to dissociation) will be assessed, the relationships between free gas and gas hydrate (both resistive but easily distinguished in seismic data) will be studied, and the background levels of hydrate and the total hydrate inventory of the Pegasus Basin will be estimated.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

天然气水合物是水和气体（通常是甲烷）的固体冰状混合物，当压力足够高且温度足够冷时，它会在海底沉积物中形成。 这种情况存在于水深超过 500 m 的地方，并且在世界各地的大陆坡上都发现了天然气水合物，只要有有机物腐烂产生的甲烷或碳氢化合物系统泄漏的地方。 大量的碳以这种方式积累起来，但对总量的估计存在几个数量级的差异，这主要是因为使用地震方法评估水合物浓度的局限性，特别是在较低浓度的情况下。 这种知识差距是一个问题，因为水合物会对人类活动和福祉产生重大影响。海平面或温度的变化会导致水合物分解，可能导致海底滑坡，从而引发海啸。水合物分解导致沉积物稳定性丧失可能会损害人造海底基础设施。 从海底泄漏的甲烷被一些生物系统用作能源，但如果泄漏太多，就会加剧酸化并影响海洋的健康。该项目将通过使用一种新颖的非地震地球物理方法对新西兰北岛南缘数百平方公里海底下的水合物进行成像，开始填补有关近海水合物的知识空白，那里已经收集了丰富的地震数据，据推测，水合物在广大地区以低浓度和高浓度存在。该项目支持对早期职业科学家的培训。甲烷水合物的电阻率比海底沉积物大四个数量级，沉积物电阻率随水合物饱和度平滑且单调地变化。 我们开发了深拖可控源电磁 (CSEM) 系统来对海底电阻率进行成像，并验证了其在量化加利福尼亚州、日本和墨西哥湾附近海底水合物浓度方面的用途。 通常，CSEM 研究针对已知的甲烷渗漏和水合物沉积物，但在本项目中，将在盆地规模上收集数据以捕获广泛的分布，使用 Hikurangi 俯冲带作为模型，通过外推来了解全球水合物库存全球俯冲带。 通过与新西兰同事的合作，将现有的地震数据与新的 CSEM 数据相结合，将检查水合物浓缩的过程，有多少水合物位于稳定场的上边缘（因此容易解离）。评估后，将研究游离气体和天然气水合物（两者都是电阻性的，但在地震数据中很容易区分）之间的关系，并估计水合物的背景水平和飞马盆地的水合物总库存。该奖项反映了 NSF 的法定使命通过使用基金会的智力优点和更广泛的影响审查标准进行评估，并被认为值得支持。