Monday, March 06, 2017 at 2:32PM
Jill A. Marshall
Earth and Planetary Science, UC Berkeley & INSTAAR
Climate regulation of erosion in unglaciated landscapes remains difficult to decipher. Easily observable modern ecosystem processes such as tree throw and precipitation-driven erosion can erase the past and bias our interpretation of landscape evolution. Surprisingly, there is little consideration that temperature, rather than precipitation, may dictate the frequency, magnitude, or style of erosion in unglaciated landscapes during cold intervals. As fresh bedrock is exhumed into the Critical Zone, rock attributes controlling geochemical reactions, hydrologic routing, and the mobile fraction of weathered rock may be predicated on the ‘ghosts’ of past processes embedded in the subsurface architecture. Here, I couple a 50-ky paleo-environmental record with 24 10Be-derived paleo-erosion rates from a new 63 m paleo-lake core from the never glaciated Oregon Coast Range (OCR). The results span the warm forested Marine Isotope Stage (MIS) 3, the cold subalpine MIS 2, and the modern, warm, forested MIS 1. From 46 ka through 28.5 ka, erosion rates increased ~ 4x, from 0.06 mm yr-1 to 0.23 mm yr-1, coincident with declining temperatures. Mean MIS 2 erosion rates remained at 0.21 mm yr-1 and declined with increasing MIS 1 temperatures to the modern mean rate of 0.08 mm yr-1. Paleoclimate reconstructions and a frost-weathering model suggest periglacial processes were vigorous between 35 and 17 ka in the OCR. Motivated by these results, I combine 11 downscaled Last Glacial Maximum paleoclimate reconstructions with a state-of-the-art frost cracking model to explore frost weathering potential across the North American continent 21 ka. All models predict frost weathering across a large swath of unglaciated North America. The results provide a framework for coupling paleoclimate reconstructions with a predictive frost weathering model, and importantly, invite reconsideration of the extent to which past climate regimes manifest in modern landscapes.
INSTAAR's Monday noon seminars are held in conjunction with CWEST and the Hydrology and Water Resources seminar series for the 2016-17 academic year.
Free and open to the public.