Synthesis of Oxygen and Carbon Isotopic Results from Site 704: Implications for Major Climatic-Geochemical Transitions during the Late Neogene

Abstract

Composite oxygen and carbon isotopic records of planktonic and benthic foraminifers and fine-fraction (<63 µm) carbonate were used to identify major paleoceanographic events in subantarctic surface and deep waters during the past 10 Ma. The late Neogene record of the subantarctic Southern Ocean is marked by progressive intensification of glacial-inter glacial cycles, migrations of the Polar Front Zone, and changes in deep-water circulation. The two most important times of rapid paleoceanographic change occurred during the late Miocene (between 6.4 and 4.8 Ma) and late Pliocene (especially at —2.5 Ma). In this paper we summarize the major results of oxygen and carbon isotopic analyses of planktonic and benthic foramini- fers and fine-fraction carbonate from Ocean Drilling Program (ODP) Site 704 during the late Neogene. Our objective is to present all of the isotopic data from individual studies in this volume to provide an overview of the paleoceanographic history of the subantarctic South Atlantic during the past 10 Ma. The composite data from Holes 704A and 704B represent a long, continuously sampled isotopic record with which to evaluate the paleoclimatic evolution of the high-latitude Southern Ocean. The hydrographic setting of Site 704 makes its sediment record extremely sensitive to past surface- and deep-water circulation changes in the Southern Ocean. It is located just north of the present axis of the Polar Front Zone (PFZ), which marks a steep gradient in temperature, nutrient content, and productivity of surface waters. For example, over a latitudinal range of only 10° in the subantarctic, the predicted gradient in the oxygen isotopic composition of calcite in equilibrium with surface water is roughly three times that expected for full glacial-interglacial cycles (Charles and Fairbanks, 1990). Bathymetrically, the site is positioned in upper deep waters (2532 m) that are a mixture of Circumpolar Deep Water (CPDW) and North Atlantic Deep Water (NADW). Benthic foraminiferal oxygen and carbon isotopes at Site 704 should be highly sensitive to the relative mixing ratios of these two water masses in the past. METHODS The methods and tabulation of data can be found in the individual manuscripts in this volume. The late Miocene-early Pliocene foraminiferal isotopic data are from Muller et al. (this volume), the Pliocene-Pleistocene foraminiferal isotopic data are from Hodell and Ciesielski (this volume), and the fine- fraction carbonate isotopic data are from Mead et al. (this volume). All planktonic analyses were made on Neoglobo- quadrina pachyderma (sinistral) and benthic analyses were