Robust modern gradients in sea surface temperature (SST) and biological productivity across the eastern equatorial Pacific (EEP) Ocean between 80° and 140°W derive from vigorous upwelling of cold and nutrient‐rich waters. However, during the mid‐Pliocene warm period (3.0–3.3 Ma), the zonal SST gradient within the EEP was reduced to <0.5°C while the productivity gradient remained as strong if not stronger than modern. We consider the oceanographic mechanisms and external climate forcings responsible for the evolution of the SST‐productivity coupling. We synthesize existing records and present new reconstructions of alkenone biomarker SST and productivity from four EEP sites. The progressive SST gradient strengthening over the Plio‐Pleistocene contrasted with the stronger Late Pliocene surface productivity gradient relative to modern. Dominance of the oceanic tunnel over surface forcings best explains these observations: SST changes reflect extratropical water masses cooling toward the present while productivity proxies suggest these waters became less nutrient‐rich.
The modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80° and 140°W. During the globally warmer late Pliocene (3.0–3.6 Ma), the EEP may have experienced permanent El Niño‐like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east‐west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone‐based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east‐west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital‐scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90° and 120°W was reduced from the modern 1.8°C gradient to 0.9°C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X‐ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Niño SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient‐rich subsurface waters in a warmer climate.
more » « less- PAR ID:
- 10498979
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 39
- Issue:
- 3
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
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