Stephen E. Cox

Note that all published papers and most abstracts are available on the CV page. If something is missing, please email and I will happily send a reprint.

I am part of the Turkana Miocene Project, a large, multi-disciplinary project funded through the NSF FRES program to investigate the Miocene in the Turkana Basin, northwestern Kenya. My interest in Turkana started with a 2018 field trip I took with a team through a Columbia Global Centers President's Global Innovation Fund project. Since then, I have dated dozens of basalt samples as part of TMP and other related projects with collaborators at LDEO, Stony Brook, Rutgers, and elsewhere.

Major earthquakes can be difficult to understand and predict because they are relatively rare in the historical record. Working with collaborators at UC-Santa Cruz and GNS New Zealand on a project funded by NSF Geophysics and the New Zealand Catalyst Fund, I am studying ways to date fault motion using the K–Ar system in conjunction with organic biomarker proxies for fault heating. An early look at this work has been published in Geology, and I am using noble gas diffusion experiments to better understand the conditions under which we can provide earthquake ages or minimum ages depending on lithology and earthquake magnitude.

As an undergraduate, I participated in the broad Antarctic sediment provenance research project that is ongoing at Lamont-Doherty Earth Observatory. Now that I am back at Lamont, I have resumed work on on ⁴⁰Ar/³⁹Ar thermochronology and (U-Th)/He thermochronology and fission track dating in collaboration with Pete Reiners and Stuart Thomson at the University of Arizona, Kathy Licht at IUPUI, and Christine Siddoway at Colorado College. I have studied questions such as the history of the Gamburtsev Subglacial Mountains (Cox et al., 2010), the stability of the ice sheet in the Transantarctic Mountains, and the thermal history of East Antarctica.

I am interested in new methods for and applications of cosmogenic nuclide dating, especially in using rare cosmogenic noble gas species (such as helium-3 and neon-21) to address problems that are challenging for other systems because of the cost of AMS analyses or the limitations of dating with radioactive nuclides. I am also interested in improving the relationships of different cosmogenic chronometers for the purposes of burial dating and creating chronologies that span many timescales.

The (U-Th)/He chronometer has become well known in the past twenty years, but it is not the only chronometer based on production of alpha particles by uranium and thorium. While the vast majority of the energetic alpha particles emitted during uranium and thorium decay come to rest as stable helium-4, a small minority react with other isotopes to form new nucleogenic daughter products. For example, alpha particles may react with oxygen-18 to produce neon-21, which is a rare stable isotope of neon. This is the basis for the (U-Th)/Ne chronometer, and we are working to better quantify the production rate of nucleogenic neon and the diffusivity of neon in various minerals to enable this chronometer to be applied to more problems in the geological sciences.

I have worked for years on a research project to improve the chronology of the Wilson Creek Formation at Mono Lake, CA. This formation consists of lake sediments from the last glacial period that are punctuated frequently by rhyolitic ashes, so we dated the constituents of the ashes to better constrain the records preserved in the lake sediments. I applied the (U-Th)/He technique to allanite found in some of these ashes, which had previously only been directly dated by Ar/Ar (Cox et al., 2012). This work continues with both novel methods and improvements in Ar/Ar techniques for dating young ashes; most recently, I am pursuing Ar/Ar dating on the youngest of the Wilson Creek ashes, which are younger than the Last Glacial Maximum, in collaboration with Guleed Ali at the Earth Observatory of Singapore (Ali et al., 2021).

I am interested in the application of the heavier noble gases (krypton and xenon) to geochronology. These gases present analytical challenges due to their high mass and low abundance in most environments, but their many stable and long-lived isotopes and different geochemical properties also present opportunities for expanding the capabilites of noble gas dating.

During my last four years at Caltech, I worked with the JPL Planetary Surface Instruments group on a quadrupole ion trap mass spectrometer. This instrument has been developed for years at JPL by Stojan Madzunkov, Murray Darrach, and others. My contribution was to add the capability of doing static vacuum measurements in a UHV chamber, and to develop the instrument for doing high resolution measurements of small quantities of noble gases. The QIT has very low background and high mass resolution in a small, efficient package that makes it ideal for spaceflight, portable terrestrial applications, and labs with limited budgets.