Carbon, Oxygen and Uranium Isotopes in Pedothem Carbonates Reveal Anomalous North American Atmospheric Circulation 70,000 to 55,000 Years Ago

Friday, 18 December 2015
Poster Hall (Moscone South)
Erik Oerter1, Warren D. Sharp2, Jessica Leigh Oster3, Angela Ebeling4, John W Valley5, Reinhard Kozdon5,6, Ian J Orland5,7, John Charles Hellstrom8, Jon D Woodhead8, Janet Hergt8, Oliver Chadwick9 and Ronald Amundson10, (1)University of California Berkeley, Berkeley, CA, United States, (2)Berkeley Geochronology Center, Berkeley, CA, United States, (3)Vanderbilt University, Earth and Environmental Sciences, Nashville, TN, United States, (4)Wisconsin Lutheran College, Milwaukee, WI, United States, (5)University of Wisconsin Madison, WiscSIMS, Dept. of Geoscience, Madison, WI, United States, (6)Rutgers University New Brunswick, New Brunswick, NJ, United States, (7)University of Minnesota Twin Cities, Dept. of Earth Sciences, Minneapolis, MN, United States, (8)University of Melbourne, School of Earth Sciences, Parkville, VIC 3010, Australia, (9)University of California Santa Barbara, Santa Barbara, CA, United States, (10)University of California Berkeley, Environmental Science, Policy and Management, Berkeley, CA, United States
Climate conditions in North America during the past two glacial cycles remain uncertain in part because long, well-dated, continuous paleoclimate records are limited in number and sparsely distributed. Here we present the first continuous, millennial resolution paleoclimate proxy record derived from C, O and U isotopes in mm-thick pedogenic carbonate clast-coatings (pedothems), which are widely distributed in semi-arid to arid regions worldwide. Carbonate 230Th/U ages were obtained from laser ablation ICP-MS spots of 93um diameter size, which also yielded 234U/238Ui proxy values for soil water infiltration and paleoprecipitation. The U-series transects were coupled to δ13C and δ18O values obtained by ion probe on ~10um diameter spots. Modern soil carbonate in the region has δ18O and δ13C values that are similar to the youngest (early Holocene) samples acquired in the laminations, strengthening the interpretation that the carbonate reflects soil isotopic conditions at the time of formation. Our new record from the Wind River Basin in Wyoming spans the last 120,000 years and confirms a previously hypothesized period of increased transport of Gulf of Mexico moisture northward into the continental interior from 70,000 to 55,000 years ago. Such pronounced meridional moisture transport, which contrasts with the dominant zonal transport of Pacific moisture into the North American interior by westerly winds before and after 70,000 to 55,000 years ago, may have resulted from a persistent anticyclone developed above the North American ice sheet during Marine Isotope Stage 4. Subsequently, the carbonate record suggests that climate in mid-latitude central North America became progressively more arid during the onset of the last glaciation (both δ13C and 234U/238Ui values become more positive), culminating in peak aridity during the LGM. We conclude that pedothems, when analyzed using micro-analytical techniques, can provide unique insights into past terrestrial climates.