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doi:10.2204/iodp.proc.319.102.2010 BiostratigraphyCalcareous nannofossilsDuring Expedition 319, only calcareous nannofossils were used to date core catcher and cuttings samples. When necessary, we took and examined additional samples from the cores to further refine ages. We report the depth of each cuttings sample referring to its bottom depth (i.e., the bottom of the 5 m sampling interval). Zonation and biohorizonsFor Expedition 319, we applied a biostratigraphic zonation of calcareous nannofossils based on the zonal schemes of Martini (1971) and Okada and Bukry (1980) that were modified by Young (1998). Our application of zonal markers and additional datums follows Expeditions 315 and 316 for biostratigraphic consistency and subsequent correlation (see "Biostratigraphy" in Expedition 315 Scientists, 2009, and Expedition 316 Scientists, 2009). Each nannofossil datum was assigned an astronomically calibrated age based on Raffi et al. (2006). The astrochronological frame for the Neogene and Quaternary follows the International Commission on Stratigraphy (ICS) 2004 timescale (Lourens et al., 2004). The timescale and biostratigraphic zones of calcareous nannofossils are summarized in Figure F12 and Table T10. Downhole contamination common in cuttings often poses problems in recognition of a zonal boundary defined by a first occurrence (FO) datum because such a boundary may stratigraphically appear significantly lower than its in situ location. To circumvent this problem, a last occurrence (LO) datum stratigraphically close to the FO, if available, was selected to approximate the zonal boundary; otherwise, the biozone was combined with adjacent zones. In addition, though some FO datums can be considered in situ if observed in normal biostratigraphic order as constrained by LO datums, many FO datums listed in Figure F12 may not be reliable in cuttings. The unreliability of FO datums in cuttings reduces the resolution of biostratigraphy. We applied two additional criteria to resolve the reworking of zonal markers, which tends to make the assemblage appear older. First, we defined a datum based on the continuous occurrence of a taxon, whereas its sparse occurrence is considered reworked. Second, we used changes in assemblage composition and specimen size to evaluate the occurrence of zonal markers. Taxonomic remarksTaxonomy followed the compilation of Perch-Nielsen (1985) and Young (1998). Previous work (Raffi et al., 1993, 2006; Raffi, 2002) suggested grouping species in the genus Gephyrocapsa by size. This is because Gephyrocapsa species show a great variability in size and other morphological features (e.g., relative size of the central opening and orientation of the bar), creating discrepancy in identification among different authors (Su, 1996). Accordingly, Gephyrocapsa is divided into four major groups by maximum coccolith length: small Gephyrocapsa (<3.5 µm), medium Gephyrocapsa I (3.5–4 µm), medium Gephyrocapsa II (4–5.5 µm), and large Gephyrocapsa (≥5.5 µm). Some important morphologic features (e.g., bar orientation) were also considered during the analysis. In addition, Reticulofenestra pseudoumbilicus should have a coccolith length >7 µm. Sample preparation and analysisWe prepared smear slides from cuttings sampled at 5–30 m spacing and core catcher samples within the cored interval, following the standard method with photocuring adhesive as a mounting medium. A simple concentration technique was adopted from Expedition 316 (see "Biostratigraphy" in Expedition 316 Scientists, 2009) for samples that contained coarse materials or few to rare nannofossils. This technique involves suspending and settling sediments in distilled water to remove sands and silts before making smear slides. In addition, we used a "mixing" technique for cuttings samples in order for the subsamples to better represent the 5 m sampling interval. We soaked >20 g of drilling mud-sediment mixture (when no hard rock pieces were present) or clean chips with ridges in distilled water, stirred to disaggregate rock chips (ground with a mortar and pestle when needed), and suspended the sample materials. The suspension was allowed to settle for 60 s, and then we drew ~3 mL of the suspension using a transfer pipet for making standard smear slides. Slides were examined using a Zeiss Axio Imager.A1m microscope under cross-polarized light and transmitted light at 1250× magnification. A Joel JCM-5700 Carry Scope scanning electron microscope was employed to confidently identify Emiliania huxleyi. We estimated relative abundances of individual species/genus and assemblages based on observations from a traverse at 1250x magnification that generally parallels the long axis of the slide. Usually more than three traverses were browsed for zonal markers and rare species, and >600 fields of view (FOVs) were examined per sample. A letter code was given to each abundance category and defined as follows:
The average preservation state of the nannofossil assemblage in each sample was qualitatively categorized and defined as follows:
We recorded abundance, preservation, and bioevents for each sample in the J-CORES database. Some additional samples from core sections were scanned only for biostratigraphic zonal markers without recording the abundances of the entire assemblage. These samples were not included in the range chart but may be included in the nannofossil event list. |
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