Determination of regional fluid flow and gas flux rates for a continental plateau using cosmogenic and radiogenic noble gas isotopes

使用宇宙成因和放射性惰性气体同位素确定大陆高原的区域流体流量和气体通量率

• 批准号: 1141435
• 负责人: Thomas Kieft
• 金额: $ 2.48万
• 依托单位: New Mexico Institute of Mining and Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-03-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1141435&HistoricalAwards=false
• 关键词: Determination; regional; fluid; flow; gas

Determination of regional fluid flow and gas flux rates for a continental plateau using cosmogenic and radiogenic noble gas isotopesThomas Kieft, Tullis OnstottNew Mexico Institute of Mining and Technology, Princeton University Until recently, the only available approach for quantifying regional-scale subsurface fluid flow was by dating ancient groundwater using cosmogenic chlorine-36 or radiogenic noble gas analyses. However, cosmogenic chlorine-36 is difficult to quantify in saline groundwater, and radiogenic noble gas dating relies on too many poorly constrained assumptions. A new cosmogenic isotope method based on krypton-81 is more accurate due to well constrained input rates and is made possible by developments in laser-based atom trap trace analysis. This project will compare cosmogenic krypton-81 dating to radiogenic noble gas isotope data in deep fracture water in the Witwatersrand Basin of South Africa, for which considerable background data already exist as part of ongoing geomicrobiology investigations, and in the nearby Bushveld Igneous Complex. The investigators will collect gases from deep fracture waters sampled in deep South African mines using a novel degassing system, while preventing air contamination. Gas samples will then be purified and sent to Argonne National Laboratories for krypton-81 analysis. Other tracers, including radiogenic noble gases, will be quantified by traditional mass spectrometric methods. The subsurface residence times derived from groundwater dating will enable: 1) a better understanding of the physical and geochemical processes associated with meteoric water flow through fractured rock containing saline fluids, 2) a better calibration of the flux of radiogenic noble gases, abiogenic hydrocarbons and hydrogen from the continental crust, 3) the temporal delineation of a potential paleoclimate record stored in the oxygen and hydrogen isotopes of the paleometeoric water, 4) a clearer understanding of regional scale fluid flow for this elevated continental plateau for which the uplift rates and cooling have been determined in published reports, 5) better constraints on the rates of cycling of organic and inorganic carbon by the deep continental biosphere, and 6) better constraints on the subsurface residence times of microbial communities, with biogeographical and evolutionary implications.This study is the first time that krypton-81 analyses will be applied to fractured rock fluid systems that are a characteristic of deep continental crust. The combination of radiogenic and cosmogenic noble gas analyses will improve our estimates of the rates of fluid flow, gas diffusion, biogeochemical cycling of elements and microbial evolution in these systems. This study will also provide better constraints on the water budgets and fluxes in the old mine infrastructure of the Witwatersrand Basin, which in turn will provide the scientific basis for modeling the flooding of these deep mines, which is leading to the development of acid mine conditions today and which will become an increasing problem in the future for South Africa.

使用宇宙成因和放射成因惰性气体同位素确定大陆高原的区域流体流量和气体流量率Thomas Kieft，Tullis Onstott新墨西哥采矿与技术研究所，普林斯顿大学直到最近，唯一可用的量化区域尺度地下流体流量的方法是使用宇宙成因氯-36或放射成因惰性气体分析确定古代地下水的年代。 然而，宇宙成因的氯-36在盐水中很难量化，放射性惰性气体测年依赖于太多约束不佳的假设。 一种新的宇宙成因同位素方法的基础上氪-81是更准确的，由于良好的约束输入速率，并有可能在激光为基础的原子阱痕量分析的发展。该项目将比较宇宙成因氪-81定年法和南非威特沃特斯兰德盆地深层裂隙水中的放射成因惰性气体同位素数据，作为正在进行的地质微生物学调查的一部分，该盆地已经有相当多的背景数据，此外还有附近的布什维尔德火成岩杂岩。 调查人员将使用一种新型的脱气系统从南非深海矿井中取样的深层断裂沃茨中收集气体，同时防止空气污染。 气体样本将被净化并送往阿贡国家实验室进行氪-81分析。 其他示踪剂，包括放射性惰性气体，将通过传统的质谱方法进行量化。 从地下水测年得出的地下停留时间将使：1）更好地理解与通过含盐流体的断裂岩石的大气水流动有关的物理和地球化学过程，2）更好地校准来自大陆地壳的放射性惰性气体、非生物成因烃和氢的通量，（3）古气候水氧、氢同位素中潜在古气候记录的时间勾画; 4）更清楚地了解这一大陆高原的区域尺度流体流动，其隆升速率和冷却已在已发表的报告中确定，5）更好地限制了深层大陆生物圈的有机和无机碳循环速率，6）更好地限制了微生物群落的地下停留时间，具有生物地理学和进化意义。这项研究是第一次氪-81分析将应用于作为大陆地壳深部特征的断裂岩石流体系统。放射成因和宇宙成因惰性气体分析的结合将改善我们对这些系统中流体流动、气体扩散、元素地球化学循环和微生物演化速率的估计。 这项研究还将提供更好的限制水的预算和流量在旧的矿山基础设施的威特沃特斯兰德盆地，这反过来将提供科学依据，模拟这些深矿井的洪水，这是导致发展的酸性矿山条件今天，这将成为一个日益严重的问题，在未来的南非。