HR: 0800h
AN: PP41D-0779    [Abstracts]
TI: Reconstruction of the Eocene Arctic Ocean Using Ichthyolith Isotope Analyses
AU: Gleason, J D
EM: jdgleaso@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AU: * Thomas, D J
EM: dthomas@ocean.tamu.edu
AF: Texas A&M University, Department of Oceanography, College Station, TX 77843-3146, United States
AU: Moore, T C
EM: tedmoore@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AU: Waddell, L M
EM: waddelin@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005, United States
AU: Haley, B A
EM: bhaley@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz Institute for Marine Sciences, Kiel, DEU 24148,
AB: Nd, Sr, O and C isotopic compositions of Eocene fish debris (teeth, bones, scales), and their reduced organic coatings, have been used to reconstruct water mass composition, water column structure, surface productivity and salinities of the Arctic Ocean Basin at Lomonosov Ridge between 55 and 44 Ma. Cleaned ichthyolith samples from IODP Expedition 302 (ACEX) record epsilon Nd values that range from -5.7 to -7.8, distinct from modern Arctic Intermediate Water (-10.5) and North Atlantic Deep Water. These Nd values may record some exchange with Pacific/Tethyan water masses, but inputs from local continental sources are more likely. Sr isotopic values are consistent with a brackish-to-fresh water surface layer (⁸⁷Sr/⁸⁶Sr = 0.7079-0.7087) that was poorly mixed with Eocene global seawater (0.7077-0.7078). Leaching experiments show reduced organic coatings to be more radiogenic (>0.7090) than cleaned ichthyolith phosphate. Ichthyolith Sr isotopic variations likely reflect changes in localized river input as a function of shifts in the Arctic hydrologic cycle, and ⁸⁷Sr/⁸⁶Sr values might be used as a proxy for surface water salinity. Model mixing calculations indicate salinities of 5 to 20 per mil, lower than estimates based on O isotopes from fish bone carbonate (16 to 26 per mil). Significant salinity drops (i.e., 55 Ma PETM and 48.5 Ma Azolla event) registered in oxygen isotopes do not show large excursions in the ⁸⁷Sr/⁸⁶Sr data. Carbon isotopes in fish debris record a spike in organic activity at 48.5 Ma (Azolla event), and otherwise high-productivity waters between 55 and 44 Ma. The combined Sr-Nd-O-C isotopic record is consistent with highly restricted basin-wide circulation in the Eocene, indicative of a highly stratified water column with anoxic bottom waters, a "fresh" water upper layer, and enhanced continental runoff during warm intervals until the first appearance of ice rafted debris at 45 Ma.
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4835 Marine inorganic chemistry (1050)
DE: 4948 Paleocene/Eocene thermal maximum
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting