The role of fire in dinosaur-dominated ecosystems : examples from the Late Cretaceous of Alberta

Wildfires are a major component of modern ecosystems, however the role of fire in deep time is still being explored. A database of Cretaceous charcoal localities highlights a reduced charcoal record in the Campanian and Maastrichtian compared to that expected given modelled atmospheric oxygen levels. Temporal distribution and relative abundances of charcoal have provided the first detailed information on fire activity spanning c. 11 million years of the Late Cretaceous in Alberta, Canada. A novel approach to charcoal quantification has been developed, allowing relative abundances to be calculated whilst compensating for charcoal fragmentation. The Campanian Oldman and Dinosaur Park Formations were investigated, (117 lithological units spanning 1.8Ma). Charcoal, dominated by gymnosperm wood, was distributed throughout the sedimentary succession in all lithologies, indicating that the Campanian wildfires were not restricted to specific environmental settings. This represents the first documented charcoal occurrence within the Dinosaur Park Formation. The Maastrichtian Horseshoe Canyon, Battle and Scollard Formations were investigated (60 samples spanning 9.5Ma). Greater charcoal abundances occurred in the lower part of the sedimentary succession but climatic variations exerted little or no control over wildfire occurrence. Samples from recent wildfires contained uncharred and partially charred plant debris addition to charcoal, highlighting a probable bias within the late Cretaceous assemblages. Elevated charcoal abundances were recorded in four vertebrate deposits within the Dinosaur Park Formation. Water flows and hyperconcentrated flows, responsible for the formation of these vertebrate deposits, may have been enhanced due to rainfall events influencing the burnt landscape, leading to post-fire destabilisation of slopes. Flooding events following wildfires may be the causal mechanism for the formation of other vertebrate assemblages. Thirteen samples from recent wildfires (Rodeo-Chediski fire -2002, Medano fire -2010 and the Scultz fire -2010) were investigated to further understand the composition of wildfire derived plant debris. Uncharred and partially charred plant debris was recorded in addition to charcoal, highlighting a probable bias within the Late Cretaceous assemblages. Relative abundances of charcoal could not be shown to 5 be influenced by proximity to burned area, vegetation stands, forest types or the size of channels. Further research into potential controls on charcoal distribution needs to be considered by wildfire researchers. This research has highlighted the previously unrecorded extent and importance of wildfire activity through the Late Cretaceous and demonstrated the need for greater investigation of sedimentary successions to prevent underestimation of past wildfire occurrence.