High-resolution terrestrial record of orbital climate forcing in coal

Pre-Quaternary terrestrial climate records in which time has been calibrated using astronomical cycles are, with the exception of lacustrine proxies, poorly represented in the geological record. This omission is a significant gap in our knowledge of ancient climate systems. Here we present new evidence of orbital periodicities in an 18.3-m-thick late Paleocene coal and conclude that coal has the potential to provide continuous time-calibrated terrestrial climate data. Spectral analysis of changes in the relative proportions of vitrinite to inertinite, two environmentally sensitive coal macerals, reveals several characteristic frequencies, some of which display evidence of amplitude modulation every five to six cycles. Combining this observation with depositional time limitations derived from present-day rates of carbon accumulation in mires, we interpret the characteristic frequencies as resulting from precession and obliquity and their influence on oxidation and decay. Using the inferred precession component of the data to derive an internal time scale, we estimate that the Wyodak coal was deposited over a period of ∼414 k.y. with a long-term carbon sequestration rate of 29 g m-2 yr-1. The identification of an internal astronomical time scale in coal is an important step toward realizing the potential for coal to extend our high-resolution knowledge of Earth's terrestrial climate back to the formation of the first peat deposits at 360 Ma.