HR: 14:25h
AN: PP23C-04    [Abstracts]
TI: Cerium Anomalies in Fossil Fish Teeth Reveal Changes in Bottom Water Oxygenation
AU: Chun, C O
EM: cchun@ucsc.edu
AF: University of California Santa Cruz, Ocean Sceinces Department 1156 High St., Santa Cruz, CA 95064, United States
AU: * Scher, H D
EM: howie@ucsc.edu
AF: University of California Santa Cruz, Ocean Sciences Department/Institute of Marine Sciences 1156 High St., Santa Cruz, CA 95064, United States
AU: Delaney, M L
EM: delaney@ucsc.edu
AF: University of California Santa Cruz, Ocean Sciences Department/Institute of Marine Sciences 1156 High St., Santa Cruz, CA 95064, United States
AB: Shale-normalized rare earths and yttrium (REY) concentrations of fossil fish teeth in deep sea sediments display prominent negative cerium (Ce) anomalies and positive yttrium (Y) anomalies. These features are ultimately inherited from seawater and strongly indicate that fossil fish teeth preserve a seawater REY signature. In seawater, Ce⁺³ is oxidized to Ce⁺⁴, and Ce becomes depleted relative to the other REY's as it partitions into other phases (e.g., ferromanganese oxyhydroxides). The magnitude of Ce depletion in a water mass is thus related to its oxygen content. We hypothesize that changes in the oxygenation of bottom waters may be revealed by examining downcore variability in the magnitude of the Ce anomaly of fossil fish teeth. To test this hypothesis, REY concentrations were measured on samples of cleaned fossil fish teeth recovered from the late Paleogene to early Eocene sections of Ocean Drilling Program (ODP) Sites 1262 and 1263 (lower and upper Walvis Ridge, ODP Leg 208, South Atlantic Ocean). These sites are vertically offset (early Eocene paleodepths were 3700 and 1700 meters, respectively) and have been extensively studied to characterize the oceanic response to the Paleocene/Eocene Thermal Maximum (PETM). Manganese enrichment factors (Mn EF) determined from total digestions of samples from these sites reveal abrupt changes in the oxygenation of bottom waters across the PETM interval. Mn EF's decrease to crustal values (~1) during the PETM, which reflects the reduction of Mn oxides as bottom water oxygen levels were depleted. Mn EF's begin to increase to 6-8 during the ‘recovery phase' following the PETM. The Ce anomaly for these samples, Ce/Ce*, was calculated according to the geometric approach reported by Lawrence et al. (2006, Aquatic Geochemistry 12, 39-72), where Ce* represents an interpolation of the expected shale-normalized Ce concentration from near neighbors. In this notation, when Ce/Ce* = 1 no Ce anomaly is present. At upper Walvis Ridge downcore Ce anomalies show a distinct excursion from pre-PETM values between 0.7-0.8 to values ~1.0 during the PETM event. The values return to pre-excursion levels during the recovery phase following the PETM. The results of this study closely follow the evolution of Mn EF's at Walvis Ridge sites. The smallest Ce anomalies (i.e., Ce/Ce* = 1) coincide with Mn EF's of 1, which provide independent evidence for low oxygen levels in the bottom waters bathing these locations. These results confirm the hypothesis that Ce anomalies in fossil fish teeth can be used to assess changes in the oxygenation of bottom waters.
DE: 4851 Oxidation/reduction reactions (0471)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4924 Geochemical tracers
DE: 4948 Paleocene/Eocene thermal maximum
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting