Lithologically influenced geomorphic responses to Holocene climatic changes in the Southern Colorado Plateau, Arizona: A soil-geomorphic and ecologic perspective

Abstract

The semiarid landscape occupied by the Navajo and Hopi peoples on the Colorado Plateau in northeastern Arizona in the southwestern United States is characterized by an extensive network of deeply incised arroyos. Since the early 20th century, many researchers have proposed that the recent formation of arroyos in this region and also many attributes of modern vegetation communities were caused directly by overgrazing of vegetation by domestic livestock of the Navajo. Other researchers, however, have proposed other causes for such features, such as climatic change. We believe that the landforms, soils and vegetation of a small area located on Antelope Mesa in this region, underlain by the highly erodible materials of the Miocene Bidahochi Formation, may have been more sensitive to minor climatic changes of the Holocene than landscapes of massive Mesozoic sandstones that dominate the Colorado Plateau. In the Antelope Mesa area, the presence of actively filling channels rather than arroyos in the upper parts of many drainage basins and associated soils and ecologic patterns indicate that the aggradation (1) was initiated in downstream reaches and within the past two centuries, (2) may be linked to recently accelerated slope erosion, and (3) is unrelated to past or ongoing grazing. This suggests the ongoing aggradation may be related to recent minor climatic changes. Geochronologic and soil-geomorphic evidence indicate that the most recent cycle of arroyo incision and filling may be a small-scale analogue for larger-magnitude, older cycles that produced regionally recognizable, paired terraces that are attributable to previous Holocene climate changes. We propose that climatic change, and more specifically, increases in precipitation, caused an acceleration in the erosion of the steep, typically minimally vegetated slopes of the Bidahochi Formation. The beginning of the ‘Neoglacial Period’ (ca. 2–3 ka), effects of which are documented by other proxy records in this region, may be the climatic change that triggered widespread deposition of a large, mid-Holocene alluvial unit in this region. Alternating episodes of eolian activity and soil formation in the study area and in this general region provide independent evidence of Holocene climatic changes. Our interpretations of the origin of the cut-and-fill cycles that emphasize the role of drainage basin lithology and differ significantly from previous interpretations emphasize the linkage of climatic changes, groundwater levels and arroyo incision. This study reveals the importance of integrating soil and ecologic studies with geomorphologic research; such an approach may be critical in helping understand how anthropogenically induced climatic changes of the next century could impact geomorphic processes and the ecology of arid and semiarid regions.