HR: 1330h
AN: PP32B-0299    [PDF]
TI: Late Quaternary Vegetation History and Paleoclimate of the U.S.A.-Mexico Borderlands Region From Two New Packrat Midden Series
AU: * Holmgren, C A
EM: holmgren@geo.arizona.edu
AF: Desert Laboratory, University of Arizona & U.S. Geological Survey, 1675 W. Anklam Rd., Tucson, AZ 85745 United States
AU: Penalba, M
EM: penalba@servidor.unam.mx
AF: Universidad Nacional Aut˘noma de M‚xico, Instituto de EcologĦa, A.P.1354, Hermosillo 83000, Hermosillo, Son 83000 Mexico
AU: Aasen Rylander, K
AF: Desert Laboratory, University of Arizona & U.S. Geological Survey, 1675 W. Anklam Rd., Tucson, AZ 85745 United States
AU: Betancourt, J L
EM: jlbetanc@usgs.gov
AF: Desert Laboratory, University of Arizona & U.S. Geological Survey, 1675 W. Anklam Rd., Tucson, AZ 85745 United States
AB: Two new packrat midden (Neotoma spp.) chronologies reveal glacial to interglacial changes in vegetation and climate in the Playas and San Simon Valleys in the U.S.A.-Mexico Borderlands. The Borderlands, where the states of Arizona and New Mexico intersect with each other and the Mexican states of Chihuahua and Sonora, are characterized by several northwest-southeast trending and tilted fault-block ranges separated by closed topographic basins. These basins now contain ephemeral playas, but held pluvial lakes (Animas, Cloverdale, Cochise, Goodsight, Hachita, Palomas, Playas) during the Pleistocene and lesser lakes sporadically in the Holocene. Plant macrofossil and pollen assemblages from middens indicate vegetation along pluvial lake margins consisted of open pinyon-juniper communities dominated by Pinus edulis, Juniperus scopulorum, Juniperus cf. coahuilensis and a rich understory of C4 annuals and grasses. Although both lake and pinyon-juniper expansion across the lowlands have been attributed to greater winter precipitation, the summer-flowering understory, characteristic of modern desert grassland in the Borderlands, indicates at least moderate summer precipitation during the late glacial. The U.S.A.-Borderlands may have been the only area in the western half of the coterminous United States to "green-up" in July and August, and may have offered seasonal refuge from the dry fire season to the north. Late glacial summer precipitation in the Borderlands may explain the concentration of megafauna and paleoindian sites dating from this period in the area. A transition to a warmer, drier climate is inferred from the extirpation of Pinus edulis from the lowlands of the Playas and San Simon Valleys by 10,300 14C yr B.P. The disappearance of pinyon and change to more xeric oak-juniper communities is contemporaneous with other midden sites in the northern Chihuahuan Desert and may have occurred abruptly during the "Clovis-aged Drought" when the water table at nearby Murray Springs dropped to unusually low levels just before 10,900 14C yr B.P. Few middens in our series dated from the middle Holocene (8000 - 4000 14C yr B.P.), a period during which middens are scarce across the Southwest. The gap was previously attributed to hydrologic drought during the middle Holocene and declines in woody perennials and packrat populations. However, beach ridge and lacustrine deposits from Laguna El Fresnal and Laguna Santa Maria indicate wetter than present conditions in the Borderlands during the middle Holocene. The late Holocene is marked by the arrival of Chihuahuan Desert scrub elements and few departures. Desertscrub elements begin to appear by about 4000 14C yr B.P., marking the transition to present-day vegetation. Larrea tridentata and Fouquieria splendens, two of the dominant desert species present at the sites today; both appear later than in surrounding areas.
DE: 1851 Plant ecology
DE: 3344 Paleoclimatology
DE: 9350 North America
DE: 9604 Cenozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting