AN ORGANIC MOLECULAR RECORD OF CLIMATE AND FIRE DURING THE LATE PALEOZOIC ICE AGE

The late Paleozoic Ice Age (LPIA) represents the longest “icehouse” state of the Phanerozoic and occurs during a time period associated with long-term changes in atmospheric pO2 and global vegetation. Paleoclimate models and geologic evidence indicate repeat cycles of glacial advance in the late Carboniferous punctuated by intermittent, warm, interglacials, that cycle in lock step with atmospheric pCO2. Paleobotanical remains record widespread shifts in vegetation during these glacial to interglacial cycles as well as a long-term restructuring of global ecosystems through the Late Carboniferous. At present, there is considerable uncertainty over how changes in atmospheric pCO2 and pO2 are related to changes in ecosystem dynamics, global vegetation feedbacks to the earth system, and fire. We present an organic molecular record of paleoclimate and fire through the LPIA from the paleotropical Illinois Basin of North America. Specifically, we analyze the 13C and D of n-alkanes and the distribution of polycyclic aromatic hydrocarbons preserved in well-dated cyclothems. Hydrogen and carbon isotopes of n-alkanes preserved in a series of cyclothems show systematic variability that is similar in magnitude to records of late Quaternary topical climate and hydrologic cycling during recent glacial and interglacials. Second, we observe a decrease in the overall abundance of high molecular weight PAHs (HMW) from ~312 to 304 Myr, and short-timescale cycling in association with changes in atmospheric pCO2. The observed long-term decrease in PAH abundance is coincident with a proposed long-term change in pO2 and may reflect competing effects of atmospheric oxygen, vegetation and hydrology in regulating fire occurrence. In total, isotopic records of hydroclimate and organic molecular indicators of paleo-fire point to longstanding establishment of biologic feedbacks to atmospheric pCO2, climate, ecosystem dynamics, and fire.