Reconstructing the thermocline in the south Atlantic subtropical Gyre over the last 4 Ma
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. The Pliocene presents a paradox because despite atmospheric pCO2 and other boundary conditions being similar to today average global temperatures were 3-4°C warmer, sea level was 35 meters higher, and ice sheets were reduced compared to today. Sea surface temperature (SST) in eastern equatorial and coastal upwelling regions were 2-9°C warmer than today, while western equatorial warm pool SSTs were similar to today in both the Pacific and Atlantic Oceans. Consequently, zonal and meridional SST gradients were reduced in the early Pliocene. A current hypothesis is that a deeper global thermocline during the early Pliocene led to warmer SST in upwelling regions, which caused climate-warming feedbacks such as increased atmospheric water vapor and a reduction of highly reflective stratus clouds that contributed to early Pliocene warmth. Existing SST and subsurface temperature records are from the tropical oceans and coastal upwelling sites, which could have been affected by tectonic events (closing of the central American seaway and restriction of the Indonesian throughflow), and local wind changes, respectively. Evidence of a shoaling thermocline during the Pliocene through the Pleistocene at a non-upwelling subtropical site would indicate that the thermocline has shoaled globally. This study will provide a 4 million year record of the depth of the subtropical Atlantic thermocline. We will present δ18O records for Globorotalia crassiformis (a subsurface species) and G. sacculifer (a mixed layer species) at ODP site 1264 (28.53°S; 2.85°E, 2505 m water depth) located on the Walvis Ridge in the south Atlantic subtropical gyre. When the thermocline at this site is deep, the δ18O gradient between surface and subsurface species is small because there is no strong temperature gradient between the depth habitat of G. crassiformis and G. sacculifer. When the thermocline shoals, the temperature gradient between the depth habitat of G. crassiformis and G. sacculifer increases, leading to an increased δ18O gradient. In this way, changes in the δ18O gradient between G. crassiformis and G. sacculifer at ODP site 1264 will record relative changes in the global thermocline over the past 4 Ma.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP13A1817K
- Keywords:
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- 4922 PALEOCEANOGRAPHY / El Nino;
- 4954 PALEOCEANOGRAPHY / Sea surface temperature;
- 9325 GEOGRAPHIC LOCATION / Atlantic Ocean