Plio-Pleistocene SST reconstruction: essay for the Southwest South Atlantic

GARCÍA CHAPORI, NATALIA; LAPRIDA, CECILIA

Buenos Aires

Congreso; Fourth Congress R.C.A.N.S.; 2008

In the oceans sea surface temperature (SST) and salinity variations are ultimately driven by climate so that paleoceanographic reconstructions allow to recognize climate variations. The reconstruction of ocean history has employed a large variety of methods. Planktic foraminifera have been extensible used because of their widespread geographical and geological occurrence (Hale and Plaufmann, 1999). Their spatial distribution is controlled by physiological requirements related to salinity, productivity, feeding preferences and specially temperature, the dominant factor (Bé and Hamilton, 1967; Bé and Tolderlund, 1971). Planktic foraminifera are excellent proxies of Quaternary climate because Recent and Quaternary faunas are practically the same and have similar ecologic requirements, so one is able to draw conclusions with respect to Quaternary environments by comparing Quaternary faunas with Recent ones. This is the base of the Modern Analog Techniques. The SP1251 core, 877 cm long, is located at 38°29.7’S-53°40.7’W at 3400m depth in the southwestern South Atlantic, in the Convergence Zone. This is an excellent place to perform paleoceanographic reconstructions because this zone is sensitive to climatic change. Nineteen samples were recovered and their foraminifers analyzed. The age estimation of the core is based on the presence of G. tosaensis at 60 cm which, even in low proportion, indicates Upper Pliocene-Lower Pleistocene (Boltovskoy, 1973; Watanabe et al., 2005). Assemblages were compared with a data set of modern samples (SPECMAP archive #1) to determine the paleotemperatures based on the most similar modern samples (MAT) using PaleoAnalogs 3.0 software (Theron, 2004). Southern Ocean cores are characterized by alternations of cold-water faunas composed almost entirely of left-coiling G. pachyderma, with warmer water faunas which include several other species (Kennett, 1970), and this core is not an exception. The core reflects a prominent cold period with summer SSTs between 5-11°C and winter SSTs between 2.6-6.4°C. Nowadays, the summer SST in the area is 20-22°C (Martinez Avellaneda, 2005; Pathfinder, NOAA) and the winter SST is 8-12°C (Pathfinder, NOAA). A moment of warmer waters was observed at 60-74 cm depth, with summer SSTs of 12-14°C and winter SSTs of 7-8°C. This reconstruction fits well with the Plio-Pleistocene boundary climatostratigraphy event proposed by Shackleton (1997) (around 2.5 Ma), and ratified by Mauz (1998) as a level that marks the onset of the prominent cold period around the marine isotope stages 100, 98 and 96.