Researchers have long pondered the question of how climate and climate change influence rates and processes of erosion: this question has become more relevant in the face of ongoing and future anthropogenic warming. We examine evidence of hillslope erosion and sedimentation on alluvial fans over Holocene and Quaternary timescales throughout a range of ecosystems in Idaho, USA. Records of erosion include fire-related and non-fire related deposition on alluvial fans, and landscape-scale analysis of hillslope slope angles. Over Holocene timescales, five independent records of forest-fires and fire-related erosion from sagebrush steppe, pinion-juniper, ponderosa pine, lodgepole pine and mixed conifer ecosystems indicate that sedimentation rates and processes on alluvial fans vary temporally with Holocene climate and spatially with vegetation type. Despite variations in ecosystem type and associated fire regimes, all sites show similar broad-scale temporal patterns. The mid-Holocene (~4-8 ka) is characterized by few fire-related deposits and many non-fire related sheetflooding events (vs. debris flows); this relatively fire-free interval is punctuated by fire peaks and associated sheetflooding ~7-6 ka. As regional paleoclimatic reconstructions generally indicate this time was generally warm and dry the lack of fire is somewhat counterintuitive; however, decreased fuel loads, combined with perhaps a more stable climate may reduce fire and storm intensity and frequency. Late Holocene (last ~3 ka) cooler, wetter and more variable climates (as compared to the mid-Holocene) are characterized by increased fire activity at all sites, and more large debris flows. Medieval droughts correspond with major fire and debris flow peaks ~1000-800 cal yr BP; decadal to annual droughts during the generally cooler and wetter LIA also promote fire peaks ~500-300 cal yr BP. Modern observations of hillslope erosion indicate north-facing or moister slopes are characterized by dense vegetation, steeper slope angles, thicker soils, and episodic but large debris flows, while south-facing or drier slopes have sparse vegetation, thin soils, lower slope angles, and frequent small sheetfloods. Space for time substitutions indicate similar shifts over Holocene timescales. We hypothesize that increased climate variability, where wet intervals of vegetation (fuel) production are followed by severe drought and fire, promote large debris flows. Dry (but stable?) climates are characterized few fires and large debris flows, and more frequent sheetflooding. When Holocene records of erosion are compared with alluvial fan records of sedimentation over the past 100 ka, the Holocene is characterized by very limited deposition. Most fan deposition corresponds with LGM and OIS stage 3 cool/wet intervals. Deposition on fans occurs regardless of whether the contributing basin was glaciated, indicating cool and/or wet climates are responsible for increased erosion, not the direct effect of glaciers. The question of whether cool/wet intervals or warm/dry intervals drive hillslope response is, therefore, very dependent on the timescale of observation. It is likely, however, that increased climate variability may drive erosional response over multiple timescales.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- 1616 GLOBAL CHANGE / Climate variability;
- 1625 GLOBAL CHANGE / Geomorphology and weathering;
- 1815 HYDROLOGY / Erosion;
- 1826 HYDROLOGY / Geomorphology: hillslope