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Peatland is an important terrestrial carbon reservoir that contains >25% of soil carbon and accounts for 25%-38% of natural methane emissions. Most of this carbon is contained in postglacial boreal peat. Our understandingof the carbon cycle within this reservoir and its links to the atmosphere is therefore restricted to periods of 1 m.y. Spectral analysis of varying lignite color reveals that 120 m of early Miocene lignite from the Gippsland Basin, Australia, contains a 1.7 m.y. record of orbitally paced climate oscillations dominated by the response to obliquity. Use of the regular orbital signal indicates that the average long-term rate of peatland carbon accumulation recorded in the lignite is 27.5 g.m - 2 .yr - 1 . This rate is constant over periods of >100 k.y. and is independent of shorter-term

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.