HR: 17:00h
AN: U12B-04    [PDF]
TI: A Tropical Climate Puzzle: Plio-Pleistocene Obliquity-Dominated Temperature and Productivity Records From the Eastern Equatorial Pacific
AU: * Lawrence, K T
EM: Kira_Lawrence@Brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AU: Liu, Z
EM: Zhonghui_Liu@Brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AU: Herbert, T D
EM: Timothy_Herbert@Brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AB: A recent hypothesis suggests that the cooling of Earth's climate during the Cenozoic lead to a change in the ocean's thermal structure and a gradual shoaling of the thermocline. At about the same time as continental ice sheets began forming in the Northern Hemisphere ($\sim$3 Ma) these oceanographic changes shifted the ocean's heat budget out of equilibrium with the local insolation field changing both regional and global climate. In this state of local disequilibrium, low- and high- latitude climate are linked because heat gained in low-latitude upwelling zones must be balanced by heat lost at high-latitudes. Thus, variations in the distribution of insolation at high latitudes associated with changes in obliquity, are felt in the tropics as well as at high latitudes. To test this theory, we characterize changes in sea surface conditions in one of the Earth's major upwelling zones from 4 Ma to the present. We document sea surface temperature (SST) and coccolithophorid productivity variations at Ocean Drilling Program Site 846 ($3\deg$ S, $91\deg$ W) in the Eastern Equatorial Pacific (EEP) using the alkenone organic proxy. We find that SSTs in the EEP ranged from $\sim$20 to $27\deg$C and that productivity varied significantly, with greater values occurring during glacial periods. Our results indicate that a long-term cooling trend of $\sim$$1\deg$C/Myr occurred from the early Pliocene to the present. Our temperature record shows the characteristic late Pleistocene high-amplitude 100-kyr dominated "sawtooth" pattern observed in most climatic records. However, the typically lower amplitude interval of the Pliocene and early Pleistocene is interrupted by high-amplitude excursions (2-$4\deg$C coolings) occurring with $\sim$400 kyr periodicity. These excursions are accompanied by large increases in productivity and in some cases correspond to orbital eccentricity minima. Spectral analysis indicates the persistent presence of strong spectral power in the obliquity (41-kyr) band and negligible spectra power in the precessional (23-kyr) band. We find a late-Pleistocene transfer of power from 120 and 80 -kyr frequency bands (sub-harmonics of the 41-kyr signal) to the 100-kyr band. The continuous strength of obliquity, typically a 'polar' signal, and the absence of precession, typically a 'tropical' signal, in climate records from the EEP supports the implication of a mechanism linking tropical and high latitude climates during the Plio-Pleistocene. The presence of obliquity variations in these climate records prior to 3 Ma implies that this phenomenon may have preceded the onset of glaciation in the Northern Hemisphere. Finally, the response of tropical SST and productivity to eccentricity forcing suggests the presence of significant non-linear feedbacks in the Plio-Pleistocene climate system.
DE: 4267 Paleoceanography
SC: U
MN: 2003 Fall Meeting