Carbon Cycling of Glaciated Landscapes on Modern and Geologic Timescales: Investigation with Strontium and Carbon Isotope Geochemistr

The studies presented in this dissertation combine radiogenic and stable strontium isotope ratios (87Sr/86Sr and delta 88/86Sr, respectively), radiogenic and stable carbon isotope ratios (Delta14C and delta13C, respectively), and bulk ion geochemistry to trace the sources and cycling of C in three glaciated environments (New Zealand, Iceland, and Greenland). A primary focus is the characterization of chemical weathering, and the effect that weathering in these locations has on long-term atmospheric CO2 concentrations and global climate. I also investigate biogeochemical cycling of major cations and C, both for the impact on dissolved chemical weathering products and on short-term C fluxes, the latter being relevant for anthropogenic climate change. One of the main tools used in these studies, namely delta88/86Sr values, is a novel isotopic tracer. I developed a method for the measurement of delta88/86Sr values by thermal ionization mass spectrometry. This method generates data with external error bars 50% less than many other methods, while using an order of magnitude less sample. Cumulatively, these studies provide critical new insight into the controls on delta88/86 Sr values at Earth's surface.;My research in Fiordland, New Zealand quantified the sources and controls on riverine cations using Sr isotopes and bulk ion geochemistry. Rivers have high delta88/86Sr values relative to local bedrock, and may be accounted for by the preferential uptake of light Sr isotopes by plants. This was one of the first studies to demonstrate mass-dependent fractionation of Sr isotopes by plants, and proposes that delta88/86Sr values could be used as a biogeochemical cycling tracer.;Furthermore, Sr isotopes and bulk ion geochemistry in Icelandic rivers indicate that solutes are largely controlled by the weathering of hydrothermal calcite. Glacial rivers have substantially higher quantities of calcite-derived cations, likely due to increased physical erosion. A model of the long-term carbon cycle that incorporates a subsurface silicate weathering flux demonstrates that weathering of hydrothermal calcite will not result in long-term atmospheric CO2 sequestration.;Additionally, I investigated subglacial discharge from the Russell Glacier, Greenland Ice Sheet with Sr isotopes and bulk ion geochemistry. Solute geochemistry is best explained by the preferential weathering of silicate minerals such as K-feldspar. This result is in contrast to the commonly held assumption that ice sheets largely weather carbonate and sulfide minerals. Ice sheets may therefore sequester more atmospheric CO2 on long timescales than previously realized.;Finally, C isotopes and bulk ion geochemistry of subglacial discharge from the Russell Glacier, Greenland Ice Sheet reveal that dissolved C is sourced from supraglacial inputs (~50%), carbonate weathering (~20 -- 40%), and respiration of organic matter by microbes (~10 -- 30%). The amount of microbially-respired C varies across the melt season based on the input of young organic material from the supraglacial environment. The amount of young organic C transferred to the subglacial environment may increase in the future. This could drive an increase in dissolved CO2, which may evade to the atmosphere and exacerbate climate change.