HR: 14:55h
AN: PP43C-06    [Abstracts]
TI: Permanent El Nino-Like Climate State During the Warm Pliocene - Evidence From Changes in East Pacific Equatorial Upwelling
AU: * Tiedemann, R
EM: rtiedemann@ifm-geomar.de
AF: IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Steph, S
EM: ssteph@ifm-geomar.de
AF: IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Sturm, A
EM: asturm@ifm-geomar.de
AF: IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Groeneveld, J
EM: jgroeneveld@ifm-geomar.de
AF: IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AB: Here, we present Pliocene biogenic opal, planktonic foraminiferal stable isotope and Mg/Ca records from tropical east Pacific ODP-Sites 1239 and 1241. Site 1241 is positioned below the North Equatorial Countercurrent northeast of Galapagos at 6°N. Site 1239 is situated under the eastern reaches of the equatorial cold tongue, in an open ocean upwelling system near the equator, where the largest ENSO signal occurs. Accordingly, the SST gradient between both sites is weak during modern El Nino events and upwelling-induced biogenic productivity is strongly reduced due to weak southeast trades. Our records cover the time interval from 5.0-2.5 Ma. This interval was probably characterized by a permanent El Nino-like climate state as suggested by modeling studies and recent paleoceanographic evidence. δ¹⁸O and Mg/Ca records of G. tumida indicate a simultaneous shoaling of the thermocline from 4.6 to 4.0 Ma at Sites 1241 and 1239. The latitudinal difference in mixed layer δ¹⁸O ( G. sacculifer) was weak during the early Pliocene (El Nino-like conditions) and increased after 3.8 Ma. Biogenic opal accumulation rates were relatively low between 5.0 and 3.5 Ma, but increased significantly after 3.5 Ma. Maxima in biogenic opal accumulation rates correspond to minima in planktonic δ¹³C. This points to enhanced upwelling of nutrient-enriched subsurface water, favouring biogenic productivity after 3.5 Ma. We interpret the delay between the thermocline shoaling and increased upwelling as a function of trade wind intensity. The change at 3.5 Ma may indicate a shift from El Nino-like conditions towards more modern-like La Nina conditions.
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4870 Stable isotopes (0454, 1041)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4922 El Nino (4522)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005