A 31,000-14C YR VEGETATION HISTORY FROM THE PELONCILLO MOUNTAINS, SOUTHEASTERN ARIZONA

In the American Southwest, there is a conspicuous gap in packrat midden coverage in the semidesert grasslands that separate the northern Chihuahuan and Sonoran Deserts north of the USA/Mexico border. This region is characterized by northwest-southeast trending and tilted fault-block ranges separated by closed topographic basins that now contain ephemeral playas, but held large pluvial lakes during the Pleistocene and smaller, short-lived lakes in the Holocene. Questions remain about the timing and causes of lake level changes relative to other paleoclimatic evidence such as vegetation histories from packrat middens. To bridge the midden gap, we developed a packrat midden series spanning the last 31 14C kyr from the Peloncillo Mountains, at comparable elevations (~1300-1500 m) to nearby pluvial lake highstands. Today, these elevations are dominated by semidesert grassland with a mixture of Chihuahuan and Sonoran Desert shrubs, including a disjunct population of jojoba (Simmondsia chinensis) in the Peloncillos. Between 31-13 14C kyr BP, rocky areas just above the playa lakes supported Pinus edulis, Juniperus osteosperma, Juniperus cf. coahuilensis, Quercus turbinella and a rich understory of summer-flowering C4 annuals and grasses, indicating abundant summer rains and relatively warm summers. After ~13 14C kyr BP, P. edulis declines in abundance and disappears briefly at 12 14C kyr BP, coincident with other records of regional aridity during the Bølling-Allerød. A concurrent expansion is seen in the more xeric species Quercus turbinella, Opuntia, Ephedra trifurca, and Rhus. Abundances of P. edulis rebounded briefly at 10.5 14C kyr BP during the Younger Dryas before disappearing along with other mesic woodland species sometime after 10.3 14C kyr BP. Few middens in our series dated from the middle Holocene (8-4 14C kyr BP), a period when middens are scarce across the Southwest. Wetter-than-present conditions at the beginning of the middle Holocene were followed by a general drying trend. The 35 middens from the late Holocene detail the sequential arrival of increasingly xeric species. The absence of the Sonoran Desert shrub Simmondsia from all middens suggests local populations arose from long distance (~100 km) dispersal from more extensive populations to the north or west, most likely in the last few centuries.