Holocene fire-related alluvial-fan deposition and climate in ponderosa pine and mixed-conifer forests,Sacramento Mountains,New Mexico,USA

Abstract

We employed ¹⁴C dating of alluvial-fan deposits in ponderosa pine and mixed-conifer forests of the Sacramento Mountains, New Mexico to document Holocene fires and related geomorphic impacts. Rapid aggradation by charcoal-rich debris flows occurred in the middle Holocene (5800—4200 cal. yr BP), indicating episodic sedimentation following severe fires. Fire-related deposition virtually ceased ~4200 cal. yr BP, with most fan deposits indicating slower aggradation with cumulic soil development until 1800 cal. yr BP. From 1800 to 500 cal. yr BP, fire-related sedimentation increased again, although not to middle Holocene levels. A peak in fire-related sedimentation c . 650 cal. yr BP corresponds to widespread severe drought in the southwestern USA. Limited fire-related sedimentation is evident from 500 to 100 cal. yr BP, consistent with `Little Ice Age' climate and tree-ring records indicating frequent low-severity fires, although at least one severe fire burned in this interval. Increased fire-related sedimentation corresponds to generally warmer conditions. We infer that higher climate variability was also involved, including multidecadal wet periods that limited surface fires and allowed stand densities to increase, promoting severe fires in subsequent severe droughts. Fire has contributed significantly to Holocene valley aggradation. Local fan channel incision followed recent fires, but major nineteenth—twentieth century arroyo cutting appears unprecedented during the Holocene.