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Ocean Drilling Program Site 704 in the subantarctic South Atlantic was drilled to investigate the response of the Southern Ocean to climatic and Oceanographic developments during the late Neogene. Stable oxygen and carbon isotopes of fine-fraction (<63 μm) carbonate were analyzed to supplement similar analyses of benthic and planktonic foraminifers. The fine fraction is generally composed primarily of coccoliths, and isotopic analyses of the fine fraction were made to complement the foraminiferal analyses. The isotopic curves thus generated suggest paleoceanographic changes not recognizable by the use of benthic and planktonic foraminifers alone. The global Chron 6 carbon isotope shift, found at 253-244 mbsf (6.39-6.0 Ma) at Site 704 in the planktonic and benthic record, is seen in the fine-fraction δC record as a gradual decrease from 255 mbsf (6.44 Ma) to 210 mbsf (4.24 Ma). At 170 mbsf, mean δ 1 8 θ values of Neogloboquadrina pachyderma increase by 0.6°/oo-0.7°/oo (Hodell and Ciesielski, this volume), reflecting decreased temperature and increased continental ice volume. Accumulation rates increase by 3.3 times above this depth (which corresponds to an age of 2.5 Ma), suggesting increased upwelling and biologic productivity. Carbon isotopic values of fine-fraction carbonate decrease by about 1.5°/oo at 2.6 Ma; however, no change is recorded in the δC of N. pachyderma. The fine-fraction δC shift slightly precedes an average l°/oo decrease in δC in benthic foraminifers. The cause of the benthic δC shift (most likely due to a change in deep water circulation; Hodell and Ciesielski, this volume) is probably not directly related to the fine-fraction shift. The fine-fraction shift is most likely caused by (1) a change in the upwelling to productivity ratio at this site, with increased upwelling bringing lighter carbon to surface waters, more productivity, and higher sedimentation rates and (2) a change in the particle composition of the fine fraction. The increased upwelling is probably due to a northward migration of the Antarctic Polar Front to a position nearer Site 704.

Fine-Fraction Carbonate Oxygen and Carbon Isotope Results from Site 704: Implications for Movement of the Polar Front during the Late Pliocene