ESTABLISHING A CHRONOLOGIC FRAMEWORK FOR INVESTIGATING LATE QUATERNARY CHANGES IN OCEAN CIRCULATION AND PRODUCTIVITY ON THE SOUTHERN ATLANTIC ARGENTINE MARGIN

The southern Argentine Margin is an important region for global ocean circulation because southern sourced intermediate and deep waters flow along it into the Atlantic Ocean and it is a region of tremendous surface ocean biological productivity. To investigate variations of ocean circulation and productivity during late Quaternary, we collected cores of marine sediments from the margin during Expedition TT1909 and are developing a chronological framework for the deposition of these sediments. The establishment of an accurate timescale is fundamental to this study, but this has proven to be difficult due to several factors: downcore variations of sedimentation rate and type, changes in the abundances of microfossils, and apparent hiatuses in the sediment record. We are therefore employing an integrated approach to develop the timescale that relies upon radiometric dating, together with the correlation of downcore records of the stable isotopic compositions of planktonic and benthic foraminifera, the assemblages of planktonic/benthic foraminifera and radiolaria, and sediment physical/chemical properties (magnetic susceptibility, XRF, and gamma ray density) in our cores to those from other localities. Our preliminary results reveal distinct changes in the types and rates of sediment accumulation occur along the margin. Glacial and interglacial intervals are distinguished as follows: interglacials have lower foraminiferal d18O values as well as the presence of subpolar and transitional planktonic foraminiferal assemblages, and glacial periods are defined by the absence of foraminifera and consist of terrigenous material and variable abundances of radiolaria and diatoms. During the late Quaternary, the lithology and microfossil assemblages switch between calcareous, foraminifera-rich sediments of low magnetic susceptibility during interglacials and diatom/radiolaria rich sediments of high magnetic susceptibility during glacials. This preliminary chronology is being constrained by the FAD and LAD of calcareous nannoplankton and ages obtained by radiometric dating.