IGPP Seminar Series

Deep time calibration of the astronomical timescale

by Prof. Bruce Runnegar


Earth's orbital parameters (eccentricity, axial obliquity, axial precession) follow cycles that range in duration from tens of thousands to millions of years. Whereas the history of most of these cycles cannot be retrieved by long numerical simulations that extend into deep time, the 400 ka eccentricity cycle is more nearly metronomic and may, in principal, be used to parse the Phanerozoic into 1,331 subequal fractions of geological time. If each of these successive intervals could be identified by means of paleoclimate signals in one or more sedimentary basins, then contextual evidence (fossils, volcanic ash beds, magnetic properties, etc.) could be used to trace and subdivide the interval globally. This concept raises interesting questions. How accurate are the astronomical calculations in deep time? How are trivial changes in orbital eccentricity transmitted to the sedimentary rock record via the climate system on million year timescales? Is there any value in having such an accurate timescale for the past half billion years of Earth history? And how could the development of such a timescale be approached? The 400 ka eccentricity cycle already forms the backbone of an astronomically-tuned geological timescale that extends from the present-day surface of the Mediterranean depositional basin into the medial Tertiary. Recently, our long integration (Va03.R7) was used to "tune" the K-T boundary section at Zumaia, Spain. Using this procedure alone, the estimated age of the mass extinction event is 65.78 million years. The EARTHTIME consortium now plans to use this interval for a "proof-of- concept" study that will involve using observed magnetic reversals to correlate the Zumaia section with the central Denver Basin where collaborative efforts among many argon-argon and uranium-lead labs are expected to provide a succession of high-resolution radiometric ages. With such a temporal framework firmly established, it will then be possible to explore and perhaps understand the biosphere's response and recovery following the K-T catastrophe.
Tuesday, 07 November 2006
3845 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM