river chemistry
organic sulfur transformations in the Efri Haukadalsá, Iceland
The upper valley and headwaters of the Efri Haukadalsá catchment in Western Iceland. Our sampling transect ranges from the snowpack visible in the top-right to Haukadalsvatn, a lake at the end of the river.
The nearly-uniform and isotopically-constrained basaltic bedrock of Iceland simplifies the identification of non-conservative sulfur processes, such as microbial sulfate reduction and/or sulfate assimilation, compared to river systems hosting multiple weathering lithologies. In this project, by combining sulfate sulfur isotope measurements with synchrotron spectroscopy, I untangled the relative importance of source isotope variability from biological transformations in a basaltic floodplain. The research has implications for using the dissolved concentration and isotopic composition of sulfate in rivers to quantify the role of sulfide oxidation during chemical weathering globally. A first-author manuscript based on the sulfur side of this research was published in Earth and Planetary Science Letters (pdf), and a second-author manuscript focused on carbon dynamics was published in Geochemistry, Geophysics, Geosystems in 2020 (pdf).
pyrite oxidation in the Narayani River, Nepal
Chemical weathering in the Nepal Himalaya has been studied intensively as a mechanism of Cenozoic cooling. Previously, ~70% of Himalayan river sulfate has been estimated to derive from oxidation of sulfide minerals; if correct, the associated sink of alkalinity may offset the carbon dioxide drawdown from silicate weathering. In this project, I used the dissolved chemistry and sulfate sulfur isotope composition of Himalayan river waters to quantify the fluxes of alkalinity and dissolved inorganic carbon due to inorganic chemical weathering. A first-author manuscript based on this work was published in Geochimica et Cosmochimica Acta (pdf).
Calcium, magnesium, and sodium+potassium ternary diagram for samples from the Langtang-Trisuli-Narayani River system (large points) and samples published previously (smaller gray triangles).
sulfur release during permafrost degradation in the Koyukuk River, Alaska
Elevation map of the Koyukuk River, hosted in discontinuous permafrost near Huslia, AK, with indicated sampling locations.
The degradation of organic-rich permafrost due to anthropogenic climate change can source pre-aged carbon dioxide to the ocean-atmosphere system. However, little is known about whether changes in silicate weathering and sulfide oxidation may accompany permafrost melting and accentuate or offset modern warming trends. In this project, I focused on the Koyukuk River in Alaska, a tributary to the Yukon River underlain by discontinuous permafrost, using sulfate sulfur isotope ratios, radiocarbon measurements, and synchrotron spectroscopy to study sulfide oxidation and organic sulfur cycling during permafrost degradation. A first-author manuscript describing this work was published in Global Biogeochemical Cycles (pdf).