Fire history of the unique high-elevation Snowmastodon (Ziegler Reservoir) site during MIS 6–4, with comparisons of TII to TI in the southern Colorado Rockies

Paleoecological records detailing fire and vegetation histories during previous interglacials are extremely rare. We present a unique, high-resolution, 10-m long record of fire from a high elevation conifer-dominated site – the Snowmastodon (Ziegler Reservoir) site – in the southern Rocky Mountains, USA, for the period spanning MIS 6 to MIS 4. The charcoal particle influx (CHAR) over the entire Snowmastodon record documents that fire was a significant process on this high elevation landscape during MIS 5e-MIS 5a, just as it is today, but during a period without human influence. Both vegetation and precession cycles influence fire occurrence, with warm stages (MIS 5e, 5c, 5a) exhibiting higher CHAR than cool stages (MIS 6, 5d, 5b, 4). CHAR abruptly increases at the MIS 6-MIS 5e boundary, contemporaneously with a rapid increase in arboreal pollen. A strong positive correlation during MIS 5e between CHAR and Pinus pollen strongly suggests control of charcoal production and deposition through burning of Pinus forests within the region. Subsequently, elevated CHAR is correlated to abundant Abies concolor in MIS 5c, and Picea in MIS 5a, while minimal CHAR occurs during MIS 5b when local vegetation was sagebrush steppe. Comparison of the Termination II (TII) Snowmastodon record with Termination I (TI) from several nearby published sites shows mostly similar patterns of developing forest preceding increased burning, with a lag of several hundred years. This may signify the development of a new fire regime tied specifically to the new vegetation type, as climate warmed and fine fuels accumulated in a forested community.