Current Projects

Precambrian seawater oxygen isotope composition
and continental emergence:
When did the continents emerge? The presence of continents - above sea level - is one of the most fundamental and important features of the Earth. We don't know when they became emergent, however. In this project, we are using hydrothermally altered oceanic crust, from modern to the Archean, as a repository of ocean oxygen isotopes, which in turn reflect the presence of continental crust, weathering, and the water cycle. We reproduce this record using analyses of oxygen isotopes from several field sites, using an inverse model to estimate water composition, and integrating these over Earth history using a model of seawater oxygen isotope evolution and exchange.

Paleoaltimetry and paleohydrology using calderas:
The same approach used to estimate seawater oxygen isotope composition can also be used in continental settings. Here, the oxygen isotope value records precipitation, which relates to the elevation and atmospheric circulation influencing ancient hydrothermal systems. We are currently working in the San Juan Mountains, Colorado, to constrain their recent uplift history. We hope to expand this work to a number of mountain ranges around the world, including the Andes and Cascades!

Glacial Periods through time:
We are interested in how the biosphere and climate react and respond to glacial periods throughout Earth history. There are two current sub-projects: 

1. Neoproterozoic nitrogen cycle: We are comparing Marinoan units from Namibia and Utah, and so far have found support for an active nitrogen cycle and aerobic ocean during the glaciation. This is difficult to reconcile with a so-called "hard Snowball'', or totally  ice-covered ocean. 
2. Characterizing Archean glaciations: Recently, we have been studying a diamictite near the Stillwater Complex in Montana, which may represent the oldest glacial rocks on Earth. These rocks were interpreted as glacial in the 1970s, with alternate ideas posed in the 1980s, but never conclusively characterized. Recent field work supports a glacial origin, and preliminary zircon dating suggests deposition at about 2.9 Ga, similar to glacial deposits in South Africa. The hunt for Archean diamictites continues! 

​Geologic Nitrogen Cycle: 
Nitrogen is historically considered to be a noble element residing primarily in the atmosphere. Over the last 10-20 years a new, more dynamic view of the nitrogen cycle has emerged, highlighting links between the atmosphere, biosphere, and geosphere. We study this fascinating cycle in two main ways: ​

1.  Earth system box modeling: incorporating both geologic and biologic cycling of N. Important fluxes include N fixing and denitrification in the ocean, incorporation into crust during hydrothermal alteration, and volatilization during subduction. New work on this model includes adding atmospheric processing, isotopic fractionation, and mantle processes.
2. Nitrogen contents of geologic materials: Using a series of glacial tills spanning the last 3 Ga of Earth history as a proxy for the N evolution of the continental crust. Our results, both isotopic and concentration, show that the N content has increased over time, hinting at a sequestration of atmospheric N into the geosphere. This work is ongoing, and future samples to be analyzed include ancient subduction zones and paleo-seafloor.

Past Projects. 

• ​Measurement of geologic NH4+: We are adapting a fluorometric technique, common in aquatic sciences, for use in quantifying geologic NH4+ concentrations. The goal of this research is to establish a relatively quick and easy way to analyze this difficult to measure species in rocks and minerals. PDF Link​
• The Nitrogen Budget of Earth: This project compiled and comprehensively synthesized the published record of N analyses in geologic materials. Based on these analyses, in conjunction with N-Ar systematics in basalts and xenoliths, we show that the majority of N in the Earth is contained in the mantle. This work also calculates a N budget based on comparison to chondrites, and estimates both a core N content and discusses using the moon as an analog for the early Earth mantle. This work has is published in Earth Science Reviews, and is posted to arXiv. 
• Development of Oxygen Isotope analysis in sulfate: 
• Thermal and isotopic history of a small granodiorite pluton: 
• Oxygen and carbon isotopic investigation of a dinosaur fossil site: