Sea surface temperature and salinity in the south Atlantic subtropical gyre over the last 4 Ma
Abstract
The early Pliocene (3-5 Ma) is the most recent time in the Earth's history when climate was significantly warmer than today. Even though atmospheric pCO2 (360-410 ppm) was similar to today, the average global temperature was 3-4°C warmer, ice sheets were smaller, and sea level was approximately 35 m higher than today. Recent studies show that reduced zonal and meridional sea surface temperature (SST) gradients played a role in sustaining Pliocene warmth and increasing SST gradients coincides with Pliocene-Pleistocene climate cooling. Ocean circulation is another important factor in climate transitions. Changes in shallow overturning circulation (ventilated thermocline) and ocean heat transport are hypothesized to be responsible for increasing Earth's sensitivity to orbital forcing and the onset of Pleistocene glaciations. However, most early Pliocene SST records are focused in the northern hemisphere and tropical upwelling regions, while data from the subtropical southern hemisphere is scarce. We present G.sacculifer Mg/Ca and δ18O records from ODP site 1264 (28.53°S; 2.85°E, 2505 m water depth), located on the Walvis Ridge in the south Atlantic subtropical gyre and within the influence of the Agulhas leakage. The Mg/Ca SST record displays no long-term trend over the past 4 Ma. The δ18O record indicates that sea surface salinity (SSS) gradually decreased from the early Pliocene to today. In the modern ocean the Agulhas Current is a western boundary current in the Indian Ocean that sheds rings of salty Indian Ocean waters into the SE Atlantic. The current is driven by a high air pressure system (the downward portion of the Hadley cell) over the south Indian Ocean and therefore responds to climatic shifts. Previous work indicates that weaker Hadley circulation in the early Pliocene lead to expanded subtropical gyres. This would have enhanced inter-ocean flow from the Indian to the Atlantic Ocean, resulting in higher SSS. Contraction of the south Indian Ocean subtropical gyre due to strengthening Hadley circulation from the early Pliocene to today would have resulted in a decreasing influence of the Agulhas Current, and therefore reduced SSS in the SE Atlantic. Increasing foraminifera abundance and decreasing nannofossil content from 4 Ma to today at ODP site 1264 implies a change from turbulent conditions in the Pliocene to calm gyre-like conditions in the modern ocean, consistent with a reduced Agulhas current influence. We conclude that the SST and SST records at ODP site 1264 reflect a decreasing influence of the Agulhas Current on the south-eastern Atlantic over the last 4 Ma.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP13A1816W
- Keywords:
-
- 4900 PALEOCEANOGRAPHY;
- 4954 PALEOCEANOGRAPHY / Sea surface temperature