Methane Oxidation Rates by AMS: Use of low-level 14C-CH4 and Accelerator Mass Spectrometry to Determine Rates of Water Column Aerobic and Anaerobic Methane Oxidation

我们对海洋中甲烷地球化学的了解在很大程度上依赖于对含氧和缺氧水域和沉积物中甲烷氧化速率的测量。相对较少的氧化速率测量适用于甲烷浓度较低的海水(~10 NM至10微米)。由于14C-CH4的比活度，涉及添加该示踪剂的速率测量需要在高于环境浓度的浓度下添加，并将在几个浓度下测量的速率外推回原位浓度。加州大学欧文分校的一个研究小组正着手克服这些分析限制。在这笔赠款的支持下，他们将利用加速器质谱仪(AMS)的高灵敏度来测量甲烷氧化速率，而甲烷池的大小只有很小的变化。这一方法还将示踪剂14C-CH4的活度水平降低到接近豁免放射性同位素运输和处理条例的水平。他们期望在使用14CCH4测定甲烷氧化率方面取得两项进展：1)对加勒比盆地的甲烷氧化率进行平行测量，涉及低水平的14C-CH4和已建立的~3H-CH4示踪技术；以及2)开发方法，使14C-CH4的活度水平低于规定的危险废物水平。在今后的测量中使用这些低活度示踪剂将简化运输，放宽在装有同位素货车的大型船舶上进行放射性同位素示踪剂实验的要求，并应允许在偏远地点和时间序列测量中更广泛地应用这些测量方法。这个项目将产生一些更广泛的影响。这项研究将涉及一名研究生和一名本科生。这位资深首席调查员是太平洋海洋保护研究所长滩水族馆的董事会成员。MCRI致力于在种族多元化的长滩社区开展外展和教育，这应该会促进与社区团体和学生的联系。这位初级首席研究员和她的其他同事开发了一个短期课程，向年轻科学家介绍放射性碳在碳循环科学中的使用。该课程的目标是(1)在全球碳(C)循环科学方面教育一批新的年轻科学家，(2)向他们介绍AMS放射性碳测量技术和放射性碳在碳循环研究中的应用。

ABSTRACTOCE-0622759Our understanding of methane geochemistry in the ocean depends to a large extent on measurements of methane oxidation rates in oxic and anoxic waters and sediments. Relatively few oxidation rate measurements are available for seawater, which has low methane concentrations (~10 nM to 10 uM). Because of the specific activity of 14C-CH4, rate measurements involving addition of this tracer require additions at concentrations higher than ambient and extrapolation of rates measured at several concentrations back to in situ concentrations. A research team at the University of California at Irvine is setting out to overcome these analytical limitations. With support from this grant, they will exploit the high sensitivity of Accelerator Mass Spectrometry (AMS) to measure methane oxidation rates with only small changes in the CH4 pool size. This approach also lowers the activity level of tracer 14C-CH4 concentrations to levels that are close to being exempt from regulations covering transportation and handling of radioisotopes. They expect to make two advances in the use of 14CCH4 in determination of methane oxidation rates: 1) conducting parallel measurements of methane oxidation rate in the Cariaco Basin involving low-level 14C-CH4 and established 3H-CH4 tracer techniques, and 2) developing methods that will allow the use of 14C-CH4 at activity levels that are below those regulated as hazardous waste. The use of these low tracer activities in future measurements will simplify shipping, relax the requirement that radioisotope tracer experiments be performed on large ships with isotope vans, and should permit wider application of these measurements in remote locations and time series measurements. This project will have a number of broader impacts. The research will involve one graduate student one undergraduate student. The senior principal investigator is active as a board member of the Long Beach Aquarium of the Pacific's Marine Conservation Research Institute. MCRI's commitment to outreach and education in the ethnically-diverse Long Beach community should facilitate contact with community groups and students. The junior principal investigator and her other colleagues have developed a short course to introduce young scientists to the use of radiocarbon in carbon cycle science. The goals of the course are (1) to educate a new group of young scientists in the science of the global carbon (C) cycle and (2) to introduce them to the technology of AMS radiocarbon measurements and the applications of radiocarbon to C cycle studies.