High Latitude and Tropical Climates Linked Prior to the Onset of Northern Hemisphere Glaciation: Evidence From the Eastern Tropical Pacific

HR: 1340h
AN: PP23A-1397 [Abstracts]
TI: High Latitude and Tropical Climates Linked Prior to the Onset of Northern Hemisphere Glaciation: Evidence From the Eastern Tropical 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: The last major climatic transition in Earth's history occurred between the Pliocene and Pleistocene epochs. This transition witnessed a substantial increase in the variance of Earth's climate in response to changes in orbital obliquity at $\sim$3 Ma, approximately coincident with the onset of Northern Hemisphere glaciation. A recent theory suggests that the start of this transition also marks a fundamental shift in the ocean's heat budget from a state of local equilibrium prior to $\sim$3 Ma, to a state in which heat absorbed in low latitude upwelling zones is balanced by heat removed at high latitudes after $\sim$3 Ma. In this remotely balanced state, changes in the distribution of high latitude insolation due to changes in orbital obliquity can be communicated from the high to low latitudes via the thermocline. To evaluate this theory, we use the alkenone organic proxy on sediments from Ocean Drilling Program (ODP) Site 846 ($3\deg$S, $91\deg$劬) in the Eastern Equatorial Pacific (EEP), documenting orbital scale variations in sea surface temperature (SST) and productivity in the largest upwelling zone on Earth over the past 5 Myrs. Our U$^{k'}$$_{37}$ temperature record shows that SST decreased by $\sim$$1\deg$C/Myrs from a high of $28\deg$C in the early Pliocene to a low of $20\deg$C in the late Pleistocene. A dramatic increase in paleoproductivity occurred $\sim$3 Ma followed by a sharp decline $\sim$1.5 Ma. Evolutive spectral analysis of sea surface indices and benthic oxygen isotope data from ODP Site 846 show that before 1 Ma all three proxies were dominated by obliquity variations and that these variations were modulated by the 1.2 Myr obliquity envelope. Evolutionary cross-spectra further indicate that in the obliquity band, the phase relationships between these three climatic indices remain essential constant throughout the entire 5 Myr interval, with EEP SST and paleoproductivity essentially in phase and both of these proxies leading benthic $\delta$$^{18}$O by $\sim$4 kyrs. The constancy of phase between SST, paleoproductivity, and ice volume implies that the physical mechanisms governing the climate of the EEP have persisted for at least the past 5 Myrs and that the high latitudes and the tropics were linked at least 2 Myrs before the onset of Northern Hemisphere glaciation. These results do not rule out a major reorganization of the sources and sinks of the Earth's heat budget, but suggest that if it occurred, it took place prior to 5 Ma. However, the EEP was not entirely unaffected by the onset of Northern Hemisphere glaciation. The major increase in productivity and cooling of SST at $\sim$3 Ma suggest a dramatic strengthening of the EEP upwelling system at this time.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting