Two C3/C4 vegetative boundaries and an expanding/contracting Chihuahuan Desert during the past 4000 yr at the Cañada Alamosa Archaeological Project Area, New Mexico

Carbon isotopic measurement (δ13C) as an indicator of C3 or C4 vegetation preserved in soil organic matter and pedogenic carbonate is an important tool for reconstructing paleoclimate. However, while δ13C values of C4 plants (−14‰) generally indicate warm-season grass, δ13C values of C3 plants can indicate trees (−27‰), cool-season grasses (−27‰), or desert shrubs (−27‰). Additional evidence, therefore, is needed to determine which C3/C4 boundary is indicated: the one between C4 grasslands and forests or cool-season grasslands, or the boundary between C4 grasslands and C3 desert shrublands. Unlike C3 forests and cool-season grasslands, C3 desert shrublands are notorious for having bare ground and high erosion rates. At the Cañada Alamosa Archaeology project area, located near a C4 grassland/C3 shrubland boundary (the Chihuahuan Desert boundary), we tested the hypothesis that δ13C values indicating C3 shrubs accompany periods of erosion-sedimentation, and, in contrast, δ13C values indicating C4 grasses correspond to periods of landscape stability and soil formation. The hypothesis is supported (i.e., failed to be disproven) by δ13C values in both soil organic matter and pedogenic carbonate, which indicate progressively greater amounts of C4 vegetation contributed to the higher δ13C values in the A horizons of buried soils. The hypothesis is also supported by macrobotanical analysis of samples from cultural features that show creosote bush (Larrea) and mesquite (Prosopis) reached their peak concentrations ca. 1200 CE during a period of erosion-sedimentation corresponding to the Medieval Warm Period. Linking the hypothesis to periods of an expanding/contracting Chihuahuan Desert, we make the following inferences: (1) The lowest allostratigraphic unit, termed Alamosa I, was deposited from at least 4000 calibrated (cal.) years before present (yr B.P.) to ca. 3100 cal. yr B.P. and records Chihuahuan Desert expansion during the later stages of (or soon after) the Altithermal. Alamosa I is capped by a well-developed soil with radiocarbon dates indicating landscape stability and desert contraction for at least 1000 yr from ca. 3100 cal. yr B.P to 2100 cal. yr B.P. (2) The overlying and less prominent Alamosa II records a renewed period of alluviation (ca. 2100 cal. yr B.P to 1870 cal. yr B.P.) followed by an estimated 500 yr period of landscape stability associated with desert contraction that enabled the formation of a weakly developed soil. (3) The Alamosa III and Valley Floor alluviation indicates a major desert expansion for ~800 yr (ca. 1370 cal. yr B.P to ca. 570 cal. yr B.P.), including the ~400 yr of the Medieval Warm Period, which was the time when the Pueblo population peaked at the project area with village sites up to 500 rooms. By ca. 600 cal. yr B.P. and through the Little Ice Age, soil formation suggests landscape stability in the project area and a retreat of the Chihuahuan Desert; by this time, Pueblo people had left the project area. The modern period of desertification, given the uncertainties of the radiocarbon ages, began after 1500 CE and before 1800 CE and continues today as the Chihuahuan Desert is expanding yet again.