Morphologic and climatic controls on soil evolution in the Bitterroot and Sapphire Mountains, Montana

Abstract

To what extent is chemical weathering governed by a landscape's topography? Two neighboring mountain ranges in the northern Rockies of western Montana, USA, provide an ideal natural laboratory in which to investigate the relationship between soil chemical weathering, persistence of soil cover, and topography. We also examine the connection between the topography and climate history. The mountain ranges we explore are the previously glaciated Bitterroot Mountains, which consist of steep, rock-dominated hillslopes, and the neighboring unglaciated Sapphire Mountains which display convex, soil-mantled hillslopes. Soil thickness measurements, soil and rock geochemistry, and digital terrain analysis reveal that soils in the rock-dominated Bitterroot Mountains are less thoroughly weathered than those in the Sapphire Mountains. These differences are even greater when we adjust weathering for rock fragments and consider surface weathering intensity at a landscape scale using our newly developed metric, the rock-adjusted chemical depletion fraction (RACDF) and rock-adjusted mass transfer coefficient (RA t). The Bitterroots overall are 30% less weathered than the Sapphires despite higher mean annual precipitation in the former, with an average RACDF of 0.38 in the postglacial Bitterroots catchment and 0.61 in the nonglacial Sapphire catchment, suggesting that 38% of rock mass is lost in the conversion to soil in the Bitterroots, whereas 61% of rock mass is lost in the nonglaciated Sapphires. Though we find little evidence for modern climate influence on weathering, data suggest that precipitation may influence slope thresholds for soil cover. Forested soils persist on slopes that are 5° higher in the Bitterroots than the Sapphires (25° and 20° respectively), based on land cover data. Because the previously glaciated Bitterroots are less weathered despite being wetter, we conclude that the glacial history of this landscape exerts more influence on soil chemical weathering than does modern climate. However, while previous studies have correlated weathering intensity with topographic parameters such as slope gradient, we find little topographic indication of specific controls on weathering in these complex systems.