Critical Zone Architecture and the Last Glacial Legacy in Unglaciated North America

Abstract:

As fresh bedrock is exhumed into the Critical Zone and intersects with water and life, rock attributes controlling geochemical reactions, hydrologic routing, accommodation space for roots, surface area, and the mobile fraction of regolith are set not just by present-day processes, but are predicated on the ‘ghosts’ of past processes embedded in the subsurface architecture. Easily observable modern ecosystem processes such as tree throw can erase the past and bias our interpretation of landscape evolution. Abundant paleoenvironmental records demonstrate that unglaciated regions experienced profound climate changes through the late Pleistocene-Holocene transition, but studies quantifying how environmental variables affect erosion and weathering rates in these settings often marginalize or even forego consideration of the role of past climate regimes. Here we combine seven downscaled Last Glacial Maximum (LGM) paleoclimate reconstructions with a state of the art frost cracking model to explore frost weathering potential across the North American continent 21 ka. We analyze existing evidence of LGM periglacial processes and features to better constrain frost weathering model predictions.  All seven models predict frost cracking across a large swath to the west of the Continental Divide, with the southernmost extent at ~ latitude 35° N, and increasing latitude towards the buffering influence of the Pacific Ocean. All models predict significant frost cracking in the unglaciated Rocky Mountains. To the east of the Continental Divide, models results diverge more, but all predict regions with LGM temperatures too cold for significant frost cracking (mean annual temperatures < 15 °C), corroborated by observations of permafrost relics such as ice wedges in some areas. Our results provide a framework for coupling paleoclimate reconstructions with a predictive frost weathering model, and importantly, suggest that modeling modern Critical Zone process evolution may require a consideration of vastly different processes when rock was first exhumed into the Critical Zone reactor.