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doi:10.2204/iodp.proc.303306.102.2006 BiostratigraphyPreliminary ages were assigned based primarily on core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. Ages for calcareous nannofossil, foraminiferal, diatom, radiolarian, and palynological events in the late Miocene to Quaternary were estimated by correlation to the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995). Biostratigraphic events, zones, and subzones for nannofossils, planktonic foraminifers, and diatoms are summarized in Figure F7. The Pliocene/Pleistocene boundary has been formally located just above the top of Olduvai (C2n) magnetic polarity subchron (Aguirre and Pasini, 1985) and just below the first occurrence (FO) of Gephyrocapsa caribbeanica (Takayama and Sato, 1993–1995). We used the FO of G. caribbeanica to mark the Pliocene/Pleistocene boundary. The Miocene/Pliocene boundary has not yet been formally defined. We tentatively used the last occurrence (LO) horizon of Discoaster quinqueramus for the boundary. The FOs of Globorotalia margaritae (planktonic foraminifer) and Thalassiosira convexa (diatom) also mark the approximate position of the stage boundary. Details of the shipboard methods are described below for each microfossil group. Calcareous nannofossilsThe zonal scheme established by Martini (1971) was used for upper Miocene to Quaternary sequences during Expedition 303. In addition, the Upper Pliocene–Quaternary biohorizons defined by Sato et al. (1999) were used for more detailed correlations. Age estimates for all upper Miocene–Quaternary datums were based on and correlated to the GPTS of Cande and Kent (1995). These zonal scheme and nannofossil biohorizons are shown in Figure F7. MethodsStandard smear slide methods were used for all samples, using Norland optical adhesive as a mounting medium. Calcareous nannofossils were examined under a light-polarizing microscope at 1250× magnification. We followed the taxonomic concepts summarized in Takayama and Sato (1987; Deep Sea Drilling Project Leg 94). Calcareous nannofossil preservation was assessed as follows:
The total abundance of calcareous nannofossils for each sample was estimated as follows:
Nannofossil abundances of individual species were recorded as follows:
ForaminifersPreliminary ages were assigned based on the presence of planktonic foraminifers in core catcher samples. The zonal schemes of Weaver and Clement (1987) were applied to the late Neogene and Quaternary samples of Expedition 303 (Fig. F7). Because FOs and LOs of planktonic foraminifers are diachronous across the temperate to subpolar North Atlantic (Weaver and Clement, 1987; Spencer-Cervato and Thierstein, 1997), we did not assign absolute ages to these occurrences. Taxonomic concepts for Neogene taxa were adopted from Kennett and Srinivasan (1983). Benthic foraminifers provide limited biostratigraphic age control as currently applied to Expedition 303 samples. The LO of the benthic foraminifer Stilostomella was noted whenever possible. Most stilostomella species disappeared from the global ocean at different latitudes during the interval of 1.0–0.6 Ma (Hayward, 2001). Taxonomic assignments on the generic level follow Loeblich and Tappan (1988). Benthic foraminifers were identified mainly to determine past changes in hydrography and surface water productivity. Oxygenation and carbon flux are the main factors controlling abundance and species composition in deep-sea assemblages (Jorissen et al., 1995; Altenbach et al., 2003). MethodsFrom each core catcher, 10 cm3 of sediment was analyzed for planktonic and benthic foraminifers. Unlithified sediment samples were soaked in tap water and then washed over a 63 µm sieve. Semilithified material was soaked in a 3% H2O2 solution before washing. Washed samples were dried at 60°C and analyzed under a Zeiss Stemi SV 11 binocular microscope. Planktonic and benthic foraminifers were analyzed from the same residues. Cleaned sieves were put into a sonicator for several minutes to avoid contamination between successive samples. Planktonic foraminiferal abundance in relation to total residue was categorized as follows:
Preservation was categorized as follows:
Planktonic foraminiferal species abundance in a random sample of 100–300 specimens from the >63 µm size fraction was defined as follows:
DiatomsThe Neogene and Quaternary diatom zonation used for the high-latitude sites of Expedition 303 was that earlier proposed by Baldauf (1985) and Koç et al. (1999) (Fig. F7). The late Miocene–Holocene part of these zonations is based on the presence of a warm-temperature diatom assemblage in the North Atlantic similar to that recorded from the eastern equatorial Pacific (Burckle, 1972, 1977; Baldauf, 1985; Barron, 1985). Typical diatom assemblages preserved in the sedimentary record can be used as tracers of the corresponding hydrographic conditions of the surface waters. Abundant diatoms characterize fertile surface waters of high latitudes and coastal upwelling areas. In the North Atlantic north of ~50°N, diatom assemblages are species rich and are the main contributors to the biogenic silica preserved in the sediment (Baldauf, 1985; Koç et al., 1999). MethodsTwo types of slides were prepared for diatom analysis, depending on overall abundance. For areas of high abundance, smear slides were prepared from a small amount of raw material in a core catcher. At a low concentration of diatom valves and/or abundant clay, selected core catcher samples were boiled in a solution of H2O2 and sodium pyrophosphate to remove organic matter and to disperse the clay-sized material, followed by treatment with hydrochloric acid to remove CaCO3. The treated samples were then washed several times with distilled water. In each case, aliquots of raw and cleaned samples were mounted on microslides using Norland optical adhesive. All slides were examined with phase-contrast illumination at a magnification of 1000× for stratigraphic markers and paleoenvironmentally sensitive taxa. The counting convention of Schrader and Gersonde (1978) was adopted. Overall diatom abundance and species-relative abundances were determined based on smear slide evaluation, using the following convention:
Preservation of diatoms was determined qualitatively as follows:
RadiolariansRadiolarian biostratigraphy during Expedition 303 was based on the radiolarian zonation documented by Goll and Bjørklund (1989). Primary datums from the late Miocene–Pleistocene are listed in Table T1. MethodsCore catcher samples were treated with a 5%–8% solution of hydrochloric acid to dissolve all calcareous components. The solution was sieved through a 63 µm mesh screen. The residue was disaggregated by gentle boiling in a 5%–8% hydrogen peroxide solution and a few grams of Calgon and sieved again. The dried residue was divided equally by a simple splitter into smaller aliquots to obtain an appropriate number of specimens for a slide. One aliquot of residue was randomly scattered on a slide on which thin gum tragacanth was spread. A few drops of Norland optical adhesive were added to the strewn-slide and a 22 mm × 40 mm glass coverslip applied. Overall radiolarian abundances were determined based on strewn-slide evaluation at 20× objective lens, using the following convention:
The abundance of individual species was recorded relative to the fraction of the total assemblages as follows:
Preservation was recorded as follows:
Palynology: dinoflagellate cysts, pollen, and other palynomorphsPalynological sample preparations yielded several types of organic-walled microfossils (i.e., palynomorphs). They included mainly dinoflagellate cysts (dinocysts), pollen, and spores from terrestrial plants and organic linings of benthic foraminifers. In some marine environments, phycoma of prasinophyte algae and acritarchs are also recovered. During this expedition, we were looking at all palynomorphs, but more attention was paid to dinocysts as paleoecological and biostratigraphic indicators. No calibrated zonal schemes have been established based on dinocysts for the Neogene interval. Nevertheless, many studies during the last decades have documented the late Cenozoic stratigraphic distribution of organic-walled dinocysts at several locations at middle to high latitudes in the North Atlantic Ocean. Regional schemes are available for Baffin Bay (Head et al., 1989a; de Vernal and Mudie, 1989a; Piasecki, 2003), the Labrador Sea (Head et al., 1989b; de Vernal and Mudie, 1989b, 1992), the Greenland Sea (Matthiessen and Brenner, 1996; Poulsen et al., 1996), the Norwegian Sea (Mudie, 1989; Manum et al., 1989), and the North Sea area (Head, 1998a, 1998b; Head et al., 2004; Louwye et al., 2004), in addition to a few other locations in the North Atlantic (e.g., Harland, 1979; Mudie, 1987; Powell, 1988; de Vernal et al., 1992; Head and Norris, 2003; Head and Westphal, 1999). Encountered dinocyst events were compared with these previous investigations and with the broad-scale succession of events and zones established by Powell (1992) and Williams et al. (1998). MethodsApproximately 5 cm3 of sample was processed using a simplified palynological treatment, avoiding HF treatments. The procedures included ultrasonic treatment, sieving on a 106 µm mesh sieve to discard coarse material, and sieving at 10 µm to eliminate clay and fine silt particles. The fraction between 15 and 106 µm was treated with HCl (10%) to remove carbonates. The residue was then submitted to differential settling in water. In some cases, heavy liquid separation with polytungstate solution was needed. In many cases, ultrasonic treatments were needed to deflocculate clays. Residues were sieved again over a 10 µm mesh sieve and mounted between slides and cover-slides with glycerine jelly. Tablets of Lycopodium spores were included in the sample at the beginning of the preparation to allow a semiquantitative concentration calculation. The slides were examined on a Zeiss Axioplan microscope equipped with differential interference contrast with magnification ranging from 250× to 630×. Taxonomic identifications were verified at 1000× magnification. Abundance of each palynomorph group (i.e., dinocysts, pollen, spores, etc.) was calculated from relative proportions of the Lycopodium spores after scanning the slides. Abundances are reported as follows:
Special attention was given to dinocysts, which were counted and identified to the species level when possible. The relative abundance of taxa was defined as follows:
When counts were too low (<50 specimens) for percentage calculation, only the presence of specimens (P) and the observation of single specimen (o) are reported. Preservation of dinocysts, and palynomorphs in general, was determined qualitatively. It is reported as follows:
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