PALEOSOLS AS A TOOL FOR UNDERSTANDING THE EXPANSION AND CONTRACTION OF THE CHIHUAHUAN DESERT

Tracking the expansion and contraction the Chihuahuan Desert—North America’s Largest Desert—provides information about natural cycles of climate change and environmental conditions in which prehistoric people lived. Paleosols have been one of the major tools for understanding the movement of the Chihuahuan Desert at the Cañada Alamosa archaeology project in southern New Mexico which is located on the oscillating Chihuahuan Desert/grassland boundary. Warmer temperatures and lower precipitation cause the desert to expand and engulf the Cañada Alamosa site, during which times erosion/sedimentation increases and isotopically distinct C3 shrubs replace C4 grasses. In contrast, cooler-wetter climates are periods of soil formation. (1) The greatest expansion was in the late Middle Holocene based on alluvial deposits with charcoal dating back to 3695 cal BP and colluvial deposits with maize dating back to 3952 cal BP (some of the oldest corn found in the Southwest). (2) Following this greatest period of expansion was the greatest period of contraction when a 800 to 1000-year span of landscape stability allowed the formation of a well-developed soil on “Alamosa I” sediments. (3) The Alamosa I paleosol was abruptly buried during a renewed period of desert advancement beginning ca. 2500 cal BP after which a 400-year contraction enabled the formation of the “Alamosa II” paleosol. (4) During that 400-year hiatus, one of the longest and most severe megadroughts in the last 2000 which set up the landscape for a period of erosion-sedimentation that came with an ensuing pluvial. (5) The last major desert expansion based on charcoal in “Alamosa III” sediments was underway by 1358 cal BP when sedimentation continued for some 800 years throughout the Medieval Warm Period. (6) During the Medieval Warm Period, the Pueblo population peaked and structures with up to 500-rooms were built, some of which contained Larrea charcoal, seeds, and flowers providing direct evidence for the desert’s presence at the site. (7) By 500 cal BP (CE 1450), now within the Little Ice Age period, a last retreat of the Chihuahuan Desert occurred when Pueblo people abandoned the site. (8) By the 1770s, before the introduction of domestic livestock, arroyo cutting and fan building were underway, but were soon exacerbated by overgrazing.