A HIGH-RESOLUTION TERRESTRIAL RECORD OF ORBITAL CLIMATE FORCING IN A LATE PALAEOCENE COAL

Thick coal seams in the order of 10-100 m thick were formed from peat deposited over periods of 0.1-1.0 Ma and should show a clear imprint of the terrestrial response to the 5-25% insolation changes that result from the combined effects of orbital cycles (Milankovitch cycles). This can be inferred from studies of Holocene peat deposits that indicate peat composition responds to climatic change driven by minor (<1%) shifts in insolation related to sunspot activity. This study tested this with a high-resolution investigation of changes in the relative proportions of vitrinite and inertinite in the 18.3 m thick Upper Palaeocene Wyodak Seam, Wyoming, USA. Changes in the quotient of vitrinite/inertinite are considered to represent changes in the probability of oxidation in the depositional environment. Spectral analysis of the maceral record reveals a typical amplitude modulated precession signal and indicates that peat accumulation responded to orbital climate forcing at precession (18.8-23 ka), obliquity (40 ka) and eccentricity (95-123 ka) periodicities. Approximately 20 precession cycles occur within the seam and this is consistent with stratigraphic age constraints and estimated rates of peat accumulation that indicate that Wyodak seam probably formed over a period of between 0.1-0.5 Ma. Using the precession cycles we can refine the estimated time for accumulation of the Wyodak seam to between 376 ka and 452 ka with a mean estimate of 414 ka. This provides an internal time scale and may indicate that periods of coal formation have been in part controlled by the longer period 400 ka eccentricity cycles. Using the internal time frame estimated long term rate of carbon sequestration by the precursor peatland is 3.8 mol m-2 yr-1 a similar order of magnitude to present day values. The identification of these periodicities illustrates the potential for coal to extend our high-resolution knowledge of the earth's terrestrial climate and in particular aid cross-correlation between the marine and terrestrial sedimentary record.