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

天然气水合物是水和气体（通常是甲烷）的固态冰状混合物，在压力足够高、温度足够低的情况下形成于海底沉积物中。在水深超过500米的地方可以发现这种情况，在世界范围内，只要有有机物腐烂产生的甲烷或碳氢化合物系统泄漏的甲烷，就可以在大陆斜坡上发现天然气水合物。大量的碳以这种方式积累，但对总量的估计相差好几个数量级，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。这种知识上的差距是一个问题，因为水合物对人类活动和健康有很大的影响。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。水合物分解导致的沉积物稳定性的丧失会破坏人为的海底基础设施。从海底泄漏的甲烷被一些生物系统用作能量，但如果泄漏太多，它会加剧酸化，影响海洋的健康。该项目将利用一种新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，从而填补海上水合物知识的空白。在新西兰北岛南缘，已经收集了大量的地震数据，并推断水合物在广泛的区域内以低浓度和高浓度存在。该项目支持培养一名早期职业科学家。甲烷水合物的电阻率比海底沉积物高4个数量级，且沉积物电阻率随水合物饱和度呈平滑单调变化。我们已经开发了一种深拖曳控制源电磁（CSEM）系统来成像海底电阻率，并已验证其用于量化加利福尼亚，日本和墨西哥湾的海底水合物浓度。通常，CSEM研究的目标是已知的甲烷渗漏和水合物沉积，但在本项目中，数据将在盆地尺度上收集，以捕获广泛的分布，使用Hikurangi俯冲带作为模型，通过外推到全球俯冲带来了解全球水合物储量。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，将检查浓缩水合物的过程，将评估多少水合物位于稳定场的上边缘（因此容易解离），将研究游离气体和天然气水合物之间的关系（两者都具有电阻性，但在地震数据中很容易区分）。并对Pegasus盆地水合物背景含量和水合物总储量进行估算。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.