A CARBON ISOTOPE RECORD OF ENVIRONMENTAL CHANGE IN A LATE PALAEOCENE MIRE

Thick coal seams have the potential to provide high-resolution records of the terrestrial climatic response to orbital forcing. High-resolution sequential d13C series from a thick Late Palaeocene coal from the Powder River Basin, Wyoming, USA, displays periodic oscillations with an amplitude of 2-4 ‰ about a mean value of -24.6 ‰. Spectral analysis of the d13C depth profile indicates that oscillations occur with a periodicity of 1.85m. Stratigraphy, rates of accumulation and spectral analysis of the maceral data indicate that this periodicity is probably the 40 ka Milankovitch obliquity cycles. The most striking observations are the magnitude and abruptness of the isotopic shifts. The overall shifts are probably related to the combined effects of changing humidity and temperature, leading to changes in vegetation, and atmospheric CO2. Published pollen data indicate that changes in vegetation alone are unlikely to account for the scale of the isotopic shifts, therefore the changes in d13C are probably dominated by vegetation responses to changes in temperature and CO2. Climate change may also affect the geochemistry of the accumulating peat. Peaks in phosphorus concentration are thought to represent situations when the normally efficient cycling of phosphorus within the mire ceases in response to a change to rheotrophic conditions. In the central part of the seam intervals with high d13C correspond to low phosphorus concentrations and high vitrinite concentrations consistent with cooler, ombrotrophic conditions conducive to better preservation of plant material. In contrast, intervals with low d13C are frequently associated with high phosphorus and more inertinite-rich intervals consistent with warmer, rheotrophic conditions. This work highlights the potential of thick coal seams to provide high-resolution palaeoclimate data. The present results illustrate that the North American continental climate was probably responding to obliquity induced changes in temperature and CO2 during the late Palaeocene. This is of particular interest as this was an abnormally warm period in the Earth's history.