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doi:10.2204/iodp.proc.318.102.2011 BiostratigraphyDiatoms, organic-walled dinoflagellate cysts, calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, spores, pollen, and other palynomorphs provided preliminary shipboard biostratigraphic and paleoenvironmental information. Ebridians, silicoflagellates, and chrysophyte cysts provided auxiliary biostratigraphic and/or paleoenvironmental information. All microfossil groups were used to characterize water mass changes. In addition, sporomorphs were used to detect influence from land, and benthic foraminifers were used to constrain paleobathymetry. Shipboard biostratigraphic age assignments were principally based on analysis of core catcher samples. Where necessary, additional toothpick (and/or plug) samples from core material were analyzed for biostratigraphic refinement. Diatoms provided age control for the recent to early Neogene (0–18 Ma) using the constrained optimization (CONOP) average range model by Cody et al. (2008). Cody et al. (2008) use the Gradstein et al. (2004) timescale, which is also adopted for Expedition 318 with the updates outlined by Ogg et al. (2008). This means that all pre-existing biostratigraphic datums tied to older geomagnetic polarity timescales (GPTS) (e.g., Berggren et al., 1995) were recalibrated to the Gradstein et al. (2004) GPTS for Expedition 318 by tying datums to geomagnetic chron boundaries or to a point within chrons (e.g., mid-chron or one-third or two-thirds of a chron, as necessary). We acknowledge the June 2009 ratification of the redefinition of the base of the Pleistocene (see Ogg et al., 2008, and recent revisions), which pushes the Pliostocene/Pleistocene boundary back from 1.806 to 2.588 Ma. Because this change postdates statements in Ogg et al. (2008), we indicate both boundary ages in this report. Diatoms and radiolarians provided good age control for recent through early Oligocene (<34 Ma), whereas palynomorphs (notably dinoflagellate cysts) were the primary source of age control for the Eocene (>34 Ma). Calcareous nannofossils provided age control for the Oligocene and Oligocene/Miocene boundary interval, whereas planktonic foraminifers provided secondary age control throughout. Abundance, preservation, and age assignments were entered into LIMS for all identified microfossil taxa. Smear slides made for diatom analysis were also used to document the presence and abundance of other microfossil groups in order to guide additional shipboard (e.g., foraminifers, radiolarians, and calcareous nannofossils) and postcruise (e.g., calcareous nannofossils) sampling efforts. Siliceous microfossilsMethodsDiatoms, silicoflagellates, ebridians, chrysophyte cysts, and sponge spicules were analyzed from smear slides mounted with Norland optical adhesive 61 (refractive index = 1.56). For radiolarian methods, see the separate section below. Samples with rare to common overall abundance of siliceous microfossils were either disaggregated in distilled water or processed with hydrogen peroxide (H2O2) and/or 10% hydrochloric acid (HCl). Strewn slides were prepared from these samples, and, when necessary, the cleaned material was also sieved at >15 µm to improve viewing. Species identification was carried out with Zeiss Axioplan microscopes using bright field illumination at 400×, 630× (oil), and 1000× (oil) magnification. The counting method of Schrader and Gersonde (1978) was utilized for all diatom specimens. At least two traverses of a 22 mm coverslip were examined (one traverse = 55 fields of view at 630× magnification). An additional scan at 400× was also made to look for age-diagnostic taxa not identified in the first two traverses. Where necessary, taxonomic identification was aided by strewn slide examination at 1000× magnification. Abundance and preservationAbundance of individual taxa was quantified by a count tallied over two 22 mm traverses of a smear slide, using 630× magnification. Qualitative siliceous microfossil group abundances were also determined from smear slides, using 630× magnification. Abundance of groups and individual taxa are categorized as follows:
Preservation of individual siliceous microfossil groups was determined qualitatively and reported in terms of both dissolution/recrystalization and fragmentation as follows: Dissolution:
Fragmentation:
Pyritization was also noted and marked as “X” in a separate column in the relevant site chapter tables. Age assignment and taxonomyInitial shipboard age assignment of individual samples for 0–18 Ma is mostly based on diatoms using the high-resolution quantitative diatom biochronology by Cody et al. (2008). Species ranges and age assignments are given in Figure F6 and Table T2. We primarily employed the CONOP average range model, drawing on information from the CONOP total range model, where appropriate. We followed the taxonomic concepts of marker species specified comprehensively in appendixes 1a and 1b of Cody et al. (2008). For strata older than 18 Ma, particularly for the Oligocene and older, age assignment from siliceous microfossils was less well constrained. However, the nearshore biostratigraphic zonation by Olney et al. (2007) was used for nearshore sites for ~18–35 Ma (Fig. F7). For offshore sites older than 18 Ma, the magnetostratigraphically calibrated scheme of Harwood and Maruyama (1992) (to ~35 Ma) was utilized (we updated published ages from the Berggren et al., 1995, timescale to the Gradstein et al., 2004, timescale). Unpublished information by C.E. Stickley for ODP Leg 189 Hole 1172A provided tentative magnetostratigraphically calibrated age information from diatoms, silicoflagellates, and ebridians to ~45 Ma using the unpublished reinterpreted magnetostratigraphy of Hole 1172A by P.K. Bijl (pers. comm., 2010). Silicoflagellates and ebridians provided additional chronostratigraphic information based on various Southern Ocean sites, including the McMurdo Sound Erratics (Bohaty and Harwood, 2000). RadiolariansMethodsSamples of sediment, 20 cm3 in size, were disaggregated by boiling in 35% H2O2 with a 1% solution of borax, followed by the addition of 10% HCl to remove carbonate (CaCO3), where present. The samples were washed through a 63 µm mesh sieve and the dried residue was scattered on a glass slide (76 mm × 26 mm), on which gum tragacanth was spread thinly over a small area corresponding to that of a cover glass (40 mm × 24 mm). After moistening to fix the radiolarians on the slide, the free specimens were removed by turning the slide over and patting it gently. Canada balsam (refractive index = 1.55) was used to mount the specimens. Abundance and preservationThe abundance of radiolarian specimens in one sample is given as
The preservation of the radiolarian assemblages is indicated as
Counts were made to a total of 500 individuals or more per sample. Individual species of radiolarians expressed as a percentage of total radiolarians per sample is given as
Zonation schemeFor sediments younger than 23 Ma, the radiolarian zonation scheme and zonal boundary datums follow that by Abelmann (1992) and Lazarus (1992) (Fig. F8). For sediments older than 23 Ma, the lower resolution zonation scheme of Takemura (1992) was used. These zones are (upper to lower) Lychnocanoma conica, Axoprunum (?) irregularis, and Eucyrtidium spinosum. Calcareous nannofossilsCalcareous nannofossils were encountered during routine smear slide analysis (for diatoms) within Holes U1356A, U1359A–U1359D (mainly U1359D) and U1361A. A representative selection of light microscope images under cross-polarized light was sent off-ship during the expedition to a calcareous nannofossil specialist for preliminary age-assessment. For additional biostratigraphic information, standard smear slides were processed immediately postexpedition, using Canada balsam mounting medium, and observed in a petrographic polarizing microscope at 1000× magnification. The abundance of calcareous nannofossils per smear slide is expressed as
The abundance of calcareous nannofossils and/or groups of taxa relative to the total assemblage is categorized as
Three preservation categories are defined:
Zonation schemeAs a general biostratigraphic framework, the calcareous nannofossil standard zonation by Martini (1971) and Okada and Bukry (1980) was used, with additional information from Perch-Nielsen (1985) and Bown (1998). Age assignments were taken from Berggren et al. (1995) and adjusted to the Gradstein et al. (2004) GPTS. PalynologyMethodsFive to ten grams of sediment was processed per sample, following standard palynologic laboratory protocols as outlined by Brinkhuis et al. (2003a). Briefly, this includes digestion with 30% HCl and 42% hydrofluoric acid (HF) followed by HCl leaching, with centrifuging after each step. Residues were sieved using a nylon 15 µm sieve and strew-mounted on slides using glycerin jelly. For each sample, at least two slides were prepared. Abundance, preservation, and taxonomyThe following broad palynofacies groups were categorized:
For qualitative estimates of the abundances of each of these palynofacies groups, the following arbitrary scale was used:
When possible, counts of ~150 dinocysts were carried out per sample. Dinocyst taxonomy follows that cited by Fensome and Williams (2004), Fensome et al. (2007, 2008), Clowes and Wilson (2006), Sluijs et al. (2009), and Pross et al. (2010). Terrestrial palynomorphs identified during these counts were also quantitatively registered, attributing them to five broad categories:
Preservation was qualitatively categorized as one of the following levels:
Age assignmentsOver the past ~20 y, numerous marine drill cores and continental sections have been studied for dinocysts throughout the Southern Ocean. The dinocysts provided a suite of regional dinocyst biostratigraphies, such as those from New Zealand (Wilson, 1988; Crouch and Brinkhuis, 2005), Australia (Truswell, 1997), and the Southern Atlantic Ocean (Goodman and Ford, 1983; Wrenn and Hart, 1988). Many of the resulting biozonation schemes are, however, not calibrated to the international timescale (e.g., Gradstein et al., 2004). Moreover, and despite these efforts, the dinocyst biostratigraphy of the Neogene in the Southern Ocean has remained relatively poorly known; in contrast, there are significantly more data available for Paleogene dinocyst events for that region. A first synthesis of magnetostratigraphically calibrated Late Cretaceous–Neogene organic-walled dinocyst events, including those from the Southern Ocean, was provided by Williams et al. (2004). In this synthesis, the southern high-latitude dinocyst bioevents were calibrated to magnetostratigraphically dated successions drilled during Leg 189 using the Gradstein and Ogg (1996) timescale. The most recent update of the integrated magnetostratigraphic dinocyst stratigraphy is presented by Brinkhuis et al. (2009) (Tables T3, T4). This work, which also includes an account of dinocyst events in the high southern latitudes, gives first and last occurrence data for dinocyst taxa calibrated to the Gradstein et al. (2004) timescale. With updates from most recent publications, the data set presented in Tables T3 and T4 currently provides the best reference point available for dinocyst biostratigraphy of the Antarctic margin. Planktonic and benthic foraminifersMethodsCore catcher samples of up to ~20 cm3 were soaked in a sodium hexametaphosphate solution, disaggregated, wet sieved over a 63 µm sieve, and dried in an oven at 60°C. For more consolidated sediments, samples were treated with potassium hydroxide (KOH) and heated on a hot plate to help disaggregation. Foraminifers were separated and identified under a stereo-binocular microscope. The abundance of planktonic foraminifers relative to the total sieved residue is categorized as
Benthic foraminifer species abundances are recorded as
Foraminifer preservation is categorized as
Zonation schemePlanktonic foraminifer zonation and classification follows Berggren (1992a, 1992b) and Berggren et al. (1995), with supplemental ages based on Li et al. (2003). Absolute ages were recalibrated to the Gradstein et al. (2004) GPTS. Taxonomic assignments for planktonic foraminifers follow Kennett and Srinivasan (1983), Leckie et al. (1993), and Pearson et al. (2006). Whereas benthic foraminifers provide limited biostratigraphic age control, they are most useful for paleoenvironmental reconstruction for Expedition 318. Taxonomic assignments for benthic foraminifers follow Rögl, (1976), Kurihara and Kennett (1986), Leckie and Webb (1986), van Morkhoven et al. (1986), Miller and Katz (1987), Webb (1989), Mackensen et al. (1990), Thomas (1990), Schröder-Adams (1991), Mackensen (1992), Mackensen and Berggren (1992), and Webb and Strong (2006). Paleodepth estimates are based on van Morkhoven et al. (1986) using the following categories:
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