VALLES CALDERA, NEW MEXICO, USA: DATING AND DEFINING THE RATE OF FORMATION OF SOILS AND WILDFIRE ACTIVITY USING LUMINESCENCE

About 1.25 million years ago a volcanic eruption in New Mexico’s Jemez Mountains created the Valles Caldera, one of seven super volcanoes found in the world. The Valles Caldera Preservation Act of 2000 created the Valles Caldera National Preserve. The Valles Caldera is particularly susceptible to slope erosion induced by wildfires. Gullies that feed the bottom grassland locations are reactivated after a fire season when protective vegetation is burned off and the steep slopes of the caldera are subjected to increased erosion. In July 2011, the Las Conchas Fire, started by an electrical power line on nearby private land, burned 120 km² of the Valles Caldera National Preserve.

As part of broader effort aimed at “the critical zone” work in the Preserve, a recent c.a. 3 meter exposure was found in a gully, after the 2011 fire, which revealed alternating light and dark bands of material (c.a. 15 – 25 cm). The abrupt nature of the contact between different colored material strongly suggests deposition events and not in situ formation of organic soil matter since the mollic soil formation would provide a more gradual decline in dark material. Two competing theories for the formation of the layers are that they reflect the fire history of the caldera or they show soil formation in a local wet spot (or marsh) that is periodically buried by alluvium or debris flows after fires.

We dated fifteen samples for optically stimulated luminescence (OSL) of the dark bands and three OSL samples of the light bands. Particle size analyses reveals that the material is 80% sand, 5% clay and 15% silt, with some variations, but no large swings, which is puzzling if the darker bands are organic soil. Also, elemental concentrations reveal no substantial difference in major or minor elements between light or dark layers. When dated with OSL, material near the bottom is about 7,000 years old and material near the top is about 2,500 years old but there is no apparent pattern to the formation of dark bands. A young mollisol was sampled further upstream, along with a modern marsh deposit. Research into known “paleo” fires from other studies, data from the recently sampled sediment, and other luminescence characteristics will be presented to determine what geomorphic process produced the unusual display of banding seen in the gully walls.