Plio-Pleistocene records from the South East Atlantic reveal changes in the Agulhas leakage
Abstract
The early Pliocene (3.5 to 5 Ma) is the most recent time in Earth's history when global climate was significantly warmer than today. Although atmospheric pCO2 (360-410 ppm) was similar to today, average global temperature was 3-4°C warmer, ice sheets were smaller, and sea level was ~35 m higher than today. Zonal and meridional sea surface temperature (SST) gradients were reduced in the early Pliocene, Walker circulation was weaker, and Hadley circulation was weaker and expanded poleward. Changes in shallow overturning circulation (the ventilated thermocline) and ocean heat transport may have played a role in the Plio-Pleistocene climate transition. However, most early Pliocene SST records are in the northern hemisphere, the tropics, and coastal upwelling regions, while data from the subtropical southern hemisphere is scarce. We present Mg/Ca and δ 18 O records from 4 species of planktonic foraminifera (G. ruber , G. sacculifer , G. tumida , and G. crassiformis ) from ODP site 1264 (28.53°S; 2.85°E, 2505 m water depth) located on the Walvis Ridge in the southeastern Atlantic. Combined Mg/Ca and δ 18 O measurements on species with different depth habitats allow us to reconstruct changes in temperature and salinity in the upper water column. The Mg/Ca record of G. sacculifer , a surface dwelling species, shows no long-term trend in SST over the past 4 Ma while the δ 18 O record indicates sea surface salinity gradually decreased from the early Pliocene to today. Mg/Ca and δ 18 O of subsurface species show a similar pattern: no long-term trend in temperature, but a marked freshening from the early Pliocene to today Increased salinity in both the surface and subsurface could be due to increased Agulhas leakage (AL), which delivers relatively salty water to the region. The strength of AL is controlled by the position of the sub-tropical front; a southward shift of the front widens the Agulhas corridor and allows increased AL. A sub-Antarctic SST record displays ~4°C Pliocene-Pleistocene cooling, indicating the equatorward expansion of the Southern Ocean into its modern position. The Plio-Pleistocene salinity decrease corresponds to decreased nannofossil and increased foraminifera content in the core, indicating a transition from well-stratified, chlorophyll-rich AL waters in the early Pliocene to lower productivity water as AL decreased from 4 Ma to the present. We therefore conclude that the Mg/Ca and δ 18 O records and the sediment composition at ODP site 1264 reflect an increase in AL during the early Pliocene warm period and a decreasing influence of the AL as climate cooled over the past 4 Ma.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP23A2040D
- Keywords:
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- 4900 PALEOCEANOGRAPHY;
- 4954 PALEOCEANOGRAPHY / Sea surface temperature