Changes in Fire Activity and Geomorphic Processes During the Early Neoglacial in Southern New Mexico

The Sacramento Mountains contain numerous small alluvial fans produced by low-order tributaries in ponderosa pine dominated mixed-conifer forests. Arroyos up to 12 m deep in the main valleys expose a 9000 year record of geomorphic change within these fans. Identification and dating of fire-related fan deposits allows us to examine the changing importance of fire-related geomorphic processes through time, and assess the role of climate change in modulating wildfire activity. Data from the middle Rio Peñasco valley show that prior to 4500 cal yr BP fans are dominated by poorly sorted facies containing large clasts and abundant charcoal, typical of fire-related debris-flow deposits. After 4500 BP, charcoal is less common and sediments are finer-grained with features indicative of gradual aggradation and cumulic soil development. Although tree-ring fire-scar studies indicate a low-severity surface- fire regime during the 400 yrs prior to Euro-American settlement, we have identified several fire-related debris- flow deposits dating to the late Holocene that indicate high-severity fires. Fire-related deposits, however, account for a much smaller volume of sediment than prior to 4500 BP. Although the early part of the record is less complete, decreased fire-related sedimentation prior to 6000 BP is also suggested. Increased fire-related sedimentation 6000-4500 BP indicates that fire regimes were significantly different during the middle Holocene, when many paleoclimate records from the region indicate a generally warmer climate. Most recently, deep arroyos, which predate recent severe fires, have formed along the main valley. The lack of comparable paleochannels in the stratigraphic record suggests that these arroyos are the unprecedented result of postsettlement land-use changes. The distinct decrease in fire-related sedimentation after 4500 BP corresponds to a shift towards cooler and wetter conditions at a number of sites in the southwestern USA and northern Mexico. Furthermore, some of the recent episodes of enhanced fire-related sedimentation also correspond to periods of warmer temperatures and increased aridity. This suggests that the impact of fire-related geomorphic processes on this environment is closely linked to climatic conditions and that geomorphic change is likely to be accelerated by warmer and drier conditions.