Collaborative Research: Improving the Late Cretaceous-Eocene geomagnetic polarity time scale by integrating the global magnetic anomaly record and astrochronology

Every few tens of thousands to millions of years Earth's magnetic field reverses, with magnetic north and south swapping places. The geomagnetic polarity time scale (GPTS) dates these reversals. Starting in the 1960s, successive versions of the GPTS were based on the global pattern of oceanic magnetic anomalies, which preserve an archive of past field reversals recorded by magnetized volcanic rocks that formed at the spreading axes of mid-ocean ridges. More recently, astrochronology has provided an independent source of information on the GPTS. Astrochronology uses sedimentary cycles driven by astronomical cycles in Earth's orbit and spin axis orientation, and the timing of magnetic field reversals can be estimated in sequences of sediments that recorded the field orientation when they were deposited at the bottom of the sea. The objective of this project is to build a GPTS that combines information from all sources: magnetic anomalies, astrochronology, and radioisotopic dating. The improved accuracy of the resulting GPTS will be crucial to determine rates of plate motion and the timing of past climatic changes recorded in sediment sequences. To broaden its impact, the project will support a scholar in a two-year non-degree program where college graduates from underrepresented groups participate in an active research project to support their applications to graduate school. The science of time scale construction will also be presented to K-12 educators and in public lectures, and the software routines developed in the project will be made available in Astrochron, an open source package that is widely used by students and researchers.

The overarching project goal is to transform GPTS construction from a 'winner-take-all' strategy, where the time scale is based on a single 'best' source of information, to a rigorous integration of different sources of information that exploits the independence of the associated errors. A Bayesian formulation and Monte Carlo sampling methods will be applied to combine all the data, weighing each piece of information on the basis of its uncertainty and propagating uncertainties to the final GPTS. The project will focus on the Late Cretaceous-Eocene (~84-33 million years ago, or Ma), a time interval where the GPTS is still in flux. This period contains a time of extreme warm climate around 50 Ma, which was followed by a cooling trend that resulted in the formation of the Antarctic ice sheet. Moreover, several global tectonic events took place around 50-45 Ma, when seafloor spreading rates decreased in the Indian Ocean as India collided with Eurasia; simultaneously, spreading became faster in the southern Atlantic and northern Pacific, coinciding with a bend in the Hawaii-Emperor seamount chain. An improved time scale is critical to advance our understanding of Cenozoic climatic changes and to explore their possible correlation to global tectonics.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.