Collaborative Research: Testing Orbital Forcing of Terrestrial Greenhouse Climate? U/Pb Zircon Geochronology from the Eocene Green River Formation of Wyoming

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

The origin of cycles in sedimentary rocks has long been debated. For at least the past few million years a convincing case can be made that the cycles are often controlled by the periodic variations in Earth's orbit and rotation as well as interactions with nearby planets that produce climate variations every 20, 40, 100, and 400 ky. However in more ancient rocks the problem is much more difficult because of a lack of independent age control which leads to a-priori corrections for apparent changes in sediment accumulation rates prior to testing the orbital forcing hypothesis. Recurrent evidence for incompleteness of cyclical records exacerbates the problem, because any strata "missing" within orbital timescales cannot be detected by existing pre-Neogene chronologies. If one could document that these cycles occur in rocks that are 50 million years old, for example, it would allow geoscientists to tell time in rocks for the past 50 million years at unprecedented levels of precision by matching models for Solar System dynamics with cycles in rocks. We propose a program, of high-resolution U-Pb zircon geochronology to resolve the orbital forcing debate for one of the classic examples of cyclic sediments, the Early Eocene Green River Formation. These spectacularly preserved rocks were deposited in a large lake ca. 50 million years ago. The goal of the proposed work is to quantitatively test, at high precision, the hypothesis that the cyclicity in the Green River Fm. is orbitally forced, where the forcing is achieved through changes in Earth's climate that are paced by the periodic changes in the Earth's orbit. We propose to use U-Pb zircon geochronology from abundant interlayered ash beds to provide at least one age per ~100 ky interval with expected precision of less than plus or minus 50 ky. In addition and partially related to the debate about the origin of sediment cyclicity, this research will also test climate's role in the stratigraphic architecture of lacustrine strata. The Early Eocene Climate Optimum is the warmest interval preceding the prolonged cooling that culminated in the current glaciated climate, and thus finding reliable, highly resolved climate records is especially relevant in view of future warmer conditions predicted with anthropogenic increases in pCO2. A highly resolved Early Eocene climate history will have far reaching implications for understanding the effect of climate on mammalian evolution and allow detailed evaluation of the robustness of dynamical models for the Solar System to ca. 50 Ma. The proposed research is an outcome of EARTHTIME (www.earth-time.org), an initiative to bring together a broad cross-section of geochronologists, paleontologists, and stratigraphers with the goal of sequencing Earth history at a much-improved level of resolution. The proposed research will provide a new approach to calibrating the Early Eocene part of the time-scale and will allow for detailed cross-calibration between 40Ar/39Ar and U-Pb chronometers.