Calibrating the Geologic Time Scale
How do we keep time through Earth history?
I use U-Pb geochronology to provide absolute age constraints on the bio-, chemo-, and magnetostratigraphy used to calibrate the Cenozoic Geologic Time Scale.
Miocene Magnetostratigraphy
As I produced my new age model for CRBG volcanism, I found that my ages through the basalt stratigraphy yielded ages for magnetic field reversals in the Middle Miocene. The interval from 17-14 Ma had been the poorest-constrained portion of the Neogene Geomagnetic Polarity Time Scale. My reversal ages correspond well with recent astronomically tuned records, and my insights were cited in the 2020 Geologic Time Scale to support a fully astronomically-tuned Neogene timescale. Our new dual-chronometer age model for the CRBG provides new age estimates with uncertainties for these critical reversals.
This white ash between CRBG Wanapum Basalt flows provides a close constraint on the base of chron C5Br; field assistant Joshua Murray for scale
A Burdigalian GSSP?
For my final PhD project, I aimed to provide age constraints for the same magnetic reversals I dated in the CRBG from a section elsewhere in the world. I found a number of promising Italian Early and Middle Miocene carbonate sections with interbedded volcaniclastics , which I sampled in 2018. I obtained zircons and produced a new age model for the Early Miocene carbonates of the Bisciaro Formation in the Contessa Valley. Prior legacy Ar/Ar geochronology, combined with unreliable paleomagnetic results and biostratigraphy subject to reworking, had led to an overestimation of the amount of time captured in this section, showing the importance of obtaining reliable absolute ages. My work also shows that the proposal to locate a Global Stratotype Section and Point (GSSP) in the Bisciaro Formation should be rejected, given the stratigraphic complications and clear presence of hiatuses in this section.
Dating Deep-Sea Sediments
While deep-sea sediments are some of our best archives of Earth’s paleoclimate for the last 100 Ma, these have rarely been the subject of a high-precision geochronology study. As an NSF Earth Sciences Postdoctoral Fellow, I produced a new chronology for Site ODP 1000 on the Nicaragua Rise in the Caribbean Sea by performing high-precision zircon geochronology on volcanic ashes deposited at this site. I integrated my ages with a new bulk δ13C and δ18O record across the Miocene Climate Optimum, and created a Bayesian age model for the core that incorporated uncertainty in sample depths resulting from poor recovery. Our Site 1000 ages show that volcanism of the Columbia River Basalt Group (CRBG) large igneous province was coincident with the interval of greatest sustained MCO warmth at this site. Our age model also yields numerical ages for biostratigraphic data in the core.
In my future work, I aim to continue dating ashes in ODP cores to not only understand the timing and tempo of Cenozoic environmental change, but also to obtain absolute ages on magnetic field reversals and biostratigraphic data. These constraints will be used to calibrate other successions that lack dateable material, improving alignment of Earth history records!
Zircon yield from one of the Miocene ashes I dated in Site ODP 1000
