HR: 11:35h
AN: PP52B-06    [Abstracts]
TI: Penetration of Atlantic Walker Circulation Into East Africa During Early to mid-Holocene: Hydrogen Isotope Evidence From Sacred Lake, Mt. Kenya
AU: * Hou, J
EM: juzhi_hou@brown.edu
AF: Brown University, 324 Brook Street, Providence, RI 02912, United States
AU: Russell, J M
EM: James_Russell@Brown.edu
AF: Brown University, 324 Brook Street, Providence, RI 02912, United States
AU: Huang, Y
EM: yongsong_huang@brown.edu
AF: Brown University, 324 Brook Street, Providence, RI 02912, United States
AB: The tropics play a very important role in global climate variability, yet the mechanisms behind the tropical climate variation remain poorly understood. Here, we present a high-resolution, well-dated record from Sacred Lake, Kenya, East Africa. We measured D/H ratios of botryococcenes, a class of highly specific biomarkers produced by freshwater algae ( Botrycoccus braunii) in a sediment core obtained from this open lake. Our main goal is to examine changes in East African rainfall amount and moisture source during the past 18kyr BP. During the late Pleistocene and late Holocene, the hydrogen isotope records track local hydrological variations inferred from numerous lake level and pollen records from the region. However, during the early to mid-Holocene (10-5ka cal yr BP), the D/H values from Sacred Lake were as much as 90 per mil heavier than during the late Pleistocene and late Holocene. If the "amount effect" is the main control on the isotopic compositions of rainfall during the early to mid Holocene, Our data would suggest drier conditions, which is inconsistent with the "African Humid Period" inferred by numerous records of the mid-Holocene. We propose that the high isotopic ratios in precipitation in East Africa during the early to mid-Holocene is due to an eastward shift in the large-scale atmospheric circulation of the tropics. In East Africa, this shift involves a major increase in moisture source from the Atlantic Ocean relative to Indian Ocean. Heavier isotope ratios of precipitation originated from Atlantic Ocean result from the intensive convection and recycling of water vapor over the Congo Basin, as opposed to Indian moisture that traverses dry land masses and losses moisture rapidly. In comparison to the late Holocene, the early to mid-Holocene is characterized by relatively northerly positioning of the ITCZ and intense monsoon systems as well as weak ENSO. These factors combine to shift the walker circulation eastward, allowing the Atlantic moisture to penetrate farther into eastern Africa. Our theory is supported by climate model results, paleoclimate records from the Kilimanjaro ice core and Cariaco Basin, and other records from the Indian and Pacific Oceans. For example, the ice core in Kilimanjaro, East Africa suggest that the oxygen isotope ratios of precipitation were ~ 8 per mil higher during early to mid-Holocene than the late Holocene, which is consistent with our results.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4914 Continental climate records
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting