IODP Expeditions 303 and 306 Monitor Miocene-Quaternary Climate in the North Atlantic

Introduction The IODP Expeditions 303 and 306 drilling sites were chosen for two reasons: (1) to capture Miocene-Quaternary millennial-scale climate variability in sensitive regions at the mouth of the Labrador Sea and in the North Atlantic icerafted debris (IRD) belt (Ruddiman et al., 1977), and (2) to provide the sedimentary and paleomagnetic attributes, including adequate sedimentation rates, for constructinghigh-resolution isotopic and magnetic stratigraphies.High accumulation rates, reaching 20 cm ky-1, permit the study of millennial-scale variations in climate and in the Earth's magnetic fi eld over the past several million years, when the amplitude and frequency of climate variability changed substantially. Shipboard logging and scanning data (magnetic susceptibility and remanence, density, natural gamma radiation, digital images and color refl ectance) and post-expedition x-ray fl uorescence (XRF) scanning datahave revealed that the sediment cores recovered on Expeditions 303 and 306 contain detailed histories of millennial-scale climate and geomagnetic fi eld variability throughout the late Miocene to Quaternary epochs. The climate proxies will be integrated with paleomagnetic data to place the records of millennial-scale climate change into a high resolution stratigraphy based on oxygen isotope andrelative paleomagnetic intensity (RPI). The paleomagnetic record of polarity reversals, excursions and RPI in these cores is central to the construction of the stratigraphic template and will provide detailed documentation of geomagnetic fi eld behavior

(Table 1) Magnetic properties of sediments from ODP Hole 166-1007B

A detailed rock magnetic study was conducted on the top six cores (above 57.5 meters below seafloor) at Site 1007. The calcareous nannofossil biostratigraphy assigns the studied intervals to an age ranging from early Pleistocene to Holocene. We found two significant conversion points in the Pleistocene based on isothermal remanent magnetization (IRM) acquisition and on alternating field demagnetization of anhysteretic remanent magnetization and IRM. These conversion points correlate strongly with changes in sedimentation rate and lithology. We conclude that the paleomagnetic signals reflect changes of rate of sediment supply and of paleoenvironment on the western edge of the Great Bahama Bank

Neogene calcareous nannofossils in the Caribbean and the eastern equatorial Pacific

Principal components analysis of nannofossil assemblages in five ODP sites in the Caribbean and the eastern equatorial Pacific was used to monitor Neogene surface water circulation changes. In 15.83-10.71 Ma, floral distributions clearly show the existence of the Circum-tropical Current between the Caribbean and the eastern equatorial Pacific and no surface water communication between the northern and southern Caribbean. In 10.71-9.36 Ma, the Circum-tropical Current weakened and the northward intra-Caribbean current had been initiated. Northern and southern Caribbean surface waters again became separated in 8.35-3.65 Ma and the Circum-tropical Current was regenerated. After 2.76 Ma, the northward intra-Caribbean current has completely been established and the Circum-tropical Current disappeared. This suggests that a barrier to the Circum-tropical Current had formed, indicating the final closure of the Isthmus of Panama