CARBON 13 ISOTOPIC RECORDS OF FOSSIL PACKRAT PELLETS AND ELEVATIONAL MOVEMENTS OF UTAH AGAVE REVEAL YOUNGER DRYAS COLD PERIOD IN GRAND CANYON, ARIZONA

Carbon isotopic composition of rodent fecal pellets was measured on 92 packrat (Neotoma spp.) middens from the Grand Canyon, Arizona. The pellet samples, ranging from throughout the last 33,000 years, reflect the abundance of cold-intolerant C4 and CAM species relative to the predominant C3 vegetation in the packrat’s diet. The chronology displays a dramatic dip followed by a sharp increase corresponding to the global Bolling/Allerod-Younger Dryas-early Holocene sequence. Much of the variability in δ ¹³C is correlated with the abundance of plant macrofossils of the regionally dominant CAM species, Utah Agave (Agave utahensis), within the middens. Plant macrofossil analysis of the same middens shows that Utah Agave was depressed to extremely low elevations during the Younger Dryas and then quickly rebounded to higher elevations at its termination. The modern geographic distribution of Utah Agave was compiled and compared to seasonal averages of monthly Twentieth Century climate values for North America interpolated to a 1 km grid. Spatial statistical analyses of these datasets compute the climatic limiters of Utah Agave’s modern distribution. The results demonstrate that the upslope distribution of Utah Agave within the Grand Canyon today is controlled by cool season minimum temperatures. These data suggest that cool season minimum temperatures were about 4.5 ^oC below late Twentieth Century values during the late Wisconsinan. Then, starting between 13,856 and 12,740 Cal yr B.P., temperatures dropped to 7.7 ^oC below modern. As the Younger Dryas sharply terminated, temperatures quickly warmed. Midden samples with calendar year means dating to 11,766 and 11,675 yr B.P. display a temperature increase of at least 4.3 ^oC between them. This warming trend continued, reaching values near the late Twentieth Century means by 9970 yr B.P. These data demonstrate the application of high precision AMS radiocarbon dating to a large series of packrat middens, spatial quantification of climatic limiters to a sensitive plant species, as well as the rapid and extreme climate cooling of the Younger Dryas in this southwestern terrestrial record.