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

利用宇宙成因和放射性成因稀有气体同位素测定大陆高原区域流体流动和气体通量率直到最近，唯一可用的量化区域尺度地下流体流动的方法是利用宇宙成因氯-36或放射性成因稀有气体分析测定古代地下水的年代。然而，宇宙成因氯-36很难在含盐地下水中量化，而放射性成因惰性气体定年依赖于太多约束不佳的假设。基于氪-81的一种新的宇宙成因同位素方法由于输入速率的良好限制而更加精确，并且由于基于激光的原子陷阱痕量分析的发展而成为可能。该项目将比较南非Witwatersrand盆地深层裂缝水中的氪-81测年数据与放射性稀有气体同位素数据，作为正在进行的地球微生物学研究的一部分，已有相当多的背景数据存在，并在附近的Bushveld火成岩杂岩中。研究人员将使用一种新型脱气系统从南非深部矿井的深层裂缝水中采集气体，同时防止空气污染。然后气体样本将被净化并送到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。由地下水测年得出的地下停留时间将使：1)更好地了解与大气水流通过含盐流体的裂隙岩石相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稀有气体、非生物成因碳氢化合物和氢的通量；3)在古气象水的氧和氢同位素中存储的潜在古气候记录的时间圈定。4)对大陆高架高原的区域尺度流体流动有了更清晰的认识，其隆升速率和冷却速度已在已发表的报告中确定；5)对大陆深层生物圈有机碳和无机碳循环速率有了更好的约束；6)对微生物群落的地下停留时间有了更好的约束，具有生物地理和进化意义。本研究首次将氪-81分析应用于作为深部大陆地壳特征的裂缝性岩石流体系统。放射成因和宇宙成因惰性气体分析的结合将改进我们对这些系统中流体流动速率、气体扩散速率、元素生物地球化学循环速率和微生物演化速率的估计。这项研究还将更好地约束Witwatersrand盆地旧矿山基础设施的水预算和通量，这反过来将为这些深部矿山的水淹建模提供科学依据，这导致了今天酸性矿山条件的发展，并将成为未来南非日益严重的问题。

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.