FIRE UNDER ICE: EVIDENCE FOR EXTENSIVE GEOGRAPHIC AND TEMPORAL RANGE OF WILDFIRE IN THE PLIOCENE HIGH ARCTIC

The Pliocene (ca. 2.6–5.3 Ma) was the last time atmospheric CO₂ was similar to present, with associated feedbacks resulting in amplified warmth in the Arctic. Boreal forest together with limited tundra covered much of the Arctic. Here we present evidence for forest fire in the Pliocene Canadian High Arctic using multiple methods to investigate the palaeofire regime.

Macrocharcoal counts over time were conducted at two sites on Ellesmere Island (~78˚N, 82˚W, 350 m above modern sea level, asl; ca. 3.9 Ma). At Beaver Pond, the increase in charcoal concentration, coeval with a change in the dominant vegetation, is indicative of coupled fire-vegetation interactions. At Fyles Leaf Beds, possible annual periodicity was identified in the rhythmic sediments. If accepted, this site records ~2­ fires/century in a low intensity and severity forest-tundra fire regime. Fire scars from wood from the region support this interpretation of fire on Ellesmere Island.

The Early Pliocene but more southerly (~74˚N, 123˚W; ~30 m asl) Banks Island site likely represents a boreal forest well within tree line. Charcoal was recovered from multiple samples but they do not form a temporal record. Samples with sufficient inertinite for reflectance analysis charred at temperatures of 350-400˚ C and 400-500˚ C. At Meighan Island (~80˚N, 100˚W; 20 m asl; ca. 3.2 Ma or ca. 3.8 Ma), samples taken above the estuarine layer also revealed burning. Those sediments likely record an open forest or forest-tundra at a time when tree line crossed Meighen Island. Here the charcoal also recorded low intensity fire.

Thus, throughout the Pliocene, fire was a ubiquitous ecological driver on the more highly vegetated Arctic landscape. Although the fire regime and vegetation were heterogeneous in time and space, it is posited that the Pliocene Arctic was characterized by low intensity, high-moderate frequency fires similar to Siberia today where a common mean fire return interval is ca. 50 years, ranging from <10 to >400 years depending on ecotone and aspect.