New Applications of Atmospheric Noble Gases in Hydrogeology, Atmospheric Sciences, Paleoclimate and Tectonics.

Abstract

Atmospheric noble gases are routinely used as a paleoclimate proxy in sedimentary aquifer systems. In contrast, noble gas studies in fractured media are rare. This dissertation pioneers the use of atmospheric noble gases as natural tracers of groundwater flow in fractured systems, and expands their use as a paleoclimate indicator in sedimentary systems. Specifically, dissolved atmospheric noble gases from basal and perched aquifers in the Galapagos Islands of Santa Cruz and San Cristobal are used to identify location and timing of recharge of these fractured, basaltic hydrologic systems. A unique, previously unknown noble gas pattern of Ne, Kr and Xe depletion together with relative Ar enrichment is also identified in high-altitude springs of the Galapagos Islands. The discovery of this unknown noble gas pattern in the Galapagos Islands lead to the first comprehensive study of noble gases carried out in rainwater using samples collected in southeast Michigan, the results of which are presented in this dissertation. This study identifies distinct rainwater noble gas patterns and investigates the association between noble gas composition in precipitation and weather patterns. This rainwater noble gas study also validates the use of noble gases as paleoclimate indicators in sedimentary systems where infiltration is slower than in fractured systems. Subsequently, this dissertation expands the use of atmospheric noble gases as paleoclimate indicators in sedimentary systems through a noble gas temperature (NGT) study carried out in the shallow Saginaw aquifer in southeast Michigan. In addition to providing a detailed ~13.1kyr paleoclimate record, the onset and termination of the Younger Dryas and Mid-Holocene climatic oscillations are identified for the first time based on NGTs. Finally, it is also shown how atmospheric noble gases can be used to provide insights into the thermal history of stable tectonic regions. Specifically, Saginaw aquifer samples located in the main discharge area of the Michigan Basin display large atmospheric noble gas excesses with respect to modern conditions and are shown to be remnants of a previously identified past thermal event. Overall, this dissertation has important scientific implications for the fields of hydrogeology, atmospheric and climate sciences, paleoclimatology and tectonics.