HR: 1340h
AN: PP43A-0626    [Abstracts]
TI: Extraordinary Climatic Greenhouse Conditions in Middle Miocene: Evidence From Xenoliths in Basalts
AU: * Zou, H
EM: hzou@ess.ucla.edu
AF: University of California at Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA 90095 United States
AU: McKeegan, K D
EM: mckeegan@ess.ucla.edu
AF: University of California at Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA 90095 United States
AU: Xu, X
EM: xsxu@nju.edu.cn
AF: Nanjing University, Department of Earth Sciences, Nanjing, 210093 China
AU: Zindler, A
EM: zindler@magnet.fsu.edu
AF: Florida State University, Department of Geological Sciences, Tallahassee, FL 32306 United States
AB: Although Paleoclimate records have been obtained mostly from the study of sedimentary rocks, some high-temperature rocks may contain useful climatic information. Here we show that unusual tridymite-hercynite xenoliths in Niutoushan basalts from the southeast coastal area of China preserve evidence of extraordinary climatic greenhouse conditions in middle Miocene. These xenoliths are composed of hercynite (FeAl$_{2}$O$_{4}$), tridymite (SiO$_{2}$) and glasses, and are characterized by positive cerium anomalies and extremely high Al$_{2}$O$_{3}$ (32-34 wt.%) and total iron oxide (20-22%). Their chemical and Nd, Sr, Pb and O isotopic compositions suggest that these xenoliths represent preserved aluminous lateritic paleosols that are not genetically related to host basalts. These lateritic paleosols with strongly desilicated minerals were formed by intense chemical weathering under warm and humid tropical conditions (with annual average temperature of $>$$19\deg$C and the annual rainfall of $>$165 cm) in SE China during the interval from 17 to 15 million years (Ma). The formation age of the paleosols corresponds to a period characterized by slow uplift of the Himalayan-Tibetan Plateau region (and thus less consumption of CO$_{2}$) after 17 Ma, and eruptions of 17-15 Ma Columbia River flood basalts, the Vogelsberg basalts, and eastern China basalts (and thus more input of CO$_{2}$ into the atmosphere). The contributions of Niutoushan and nearby basalts to our understanding of middle Miocene climate are (1) to preserve the geochemical record of lateritic paleosols captured and transformed in the basalts; and (2) to provide robust constraints on the ages of the lateritic paleosols.
DE: 3344 Paleoclimatology
DE: 1040 Isotopic composition/chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting