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

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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.