A fully calibrated astronomical time scale for the Cenozoic: Dating, climate forcing, and solar system chaos

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To determine the ages of seafloor sediments and sedimentary rocks geologists need a “calendar” of the past. One such calendar, called the astronomical time scale, makes use of the fact that predictable changes in the orbits of the planets drive cyclic changes in Earth’s climate. In this approach observed cycles in sediment composition are matched to climate cycles. The timing of those cycles is then calculated by computing planetary orbits backward in time. The astronomical time scale is a valuable tool because the orbital calculations provide accurate ages for the geologic record. Recent work by the PI provided new astronomical calculations that stretch back to 58 million years before the present (Myr BP). Those calculations were then compared with deep sea sediment records. The comparisons yielded a new history of climate events during the early Eocene (56 to 53 Myr BP). They also yielded new information about the chaotic motions of the solar system. The proposed study will provide a fully tested astronomical time scale and new astronomical calculations extending back through the Paleocene (66 to 56 Myr BP). The astronomical calculations will provide a time scale for the sediment records. At the same time the cyclic patterns observed in the sediments will constrain calculations of the planets’ orbital variations. That new time scale will pave the way to an accurate time scale extending back through the Cretaceous period (145 to 66 Myr BP). It will also help improve dating methods that use radioactive elements. Finally, it will improve our knowledge of the chaotic evolution of our solar system. The project will also support a graduate student.The development of an astronomical time scale (ATS) based on astronomical solutions has transformed the dating of geologic archives and represents the backbone of marine cyclostratigraphy and astrochronology to reconstruct Earth’s history. While the stable 405-kyr cycle of Earth’s orbital eccentricity has been used to provide floating (relative) chronologies across various time scales, one major challenge remains: Development of an absolute, fully calibrated astronomical time scale beyond ~50 Ma, which has hitherto been hampered by the fact that orbital solutions disagree before that age due to solar system chaos. We have recently made progress in providing a new astronomical solution to ~58 Ma by testing against deep-sea sediment archives (~58-53 Ma), which also yielded critical insight into the chronology of early Eocene hyperthermals and solar system chaos. This project will develop and provide a fully calibrated ATS and a new astronomical solution including the Paleocene (~66-56 Ma), which, combined with our recent work, will then span the entire Cenozoic. The key idea of our study is to use marine geology to constrain astronomy, and, conversely, use astronomy to provide key applications widely used in marine geology. Reaching beyond 58 Ma presents new challenges, which will require analyses of multiple geologic records and significant computational resources. Cyclostratigraphic signals are challenging to interpret in deep-sea records across the interval ~66-63 Ma, for which we will integrate land-based marine sections in our analysis (e.g., Zumaia, Spain). The expression of Earth’s orbital eccentricity in the geologic archives will then guide our search for an astronomical solution, using numerical (ensemble) integrations of the orbital motion of the solar system. Specifically, we will use marine deep-sea records of color reflectance and iron intensities from ODP Sites 1262 and 1209 and cyclostratigraphy from land-based marine sections (Zumaia) to reconstruct Earth’s orbital eccentricity across the critical interval ~66-56 Ma. Based on the eccentricity expression observed in the records, we will search for a new astronomical solution that will allow us to extend the fully calibrated astronomically time scale to ~66 Ma. In summary, our study will (1) provide a fully calibrated ATS across the Cenozoic, (2) help gauge radiometric dating calibrations, (3) test correlations between eccentricity amplitude modulation and Paleocene climate events, (4) model forcing of Paleocene climate-carbon cycle feedbacks, and (5) transform our knowledge of the chaotic evolution of our solar system.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.