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doi:10.2204/iodp.proc.313.102.2010

Chronology

Chronology ages (in Ma) for stratigraphic surfaces identified first on seismic profiles and tied to cores (see "Stratigraphic correlation") were obtained by constructing an age-depth plot for each hole. Plots were constructed through correlation of biostratigraphic zonations to the GPTS of Cande and Kent (1995) as given in the BKSA95 timescale (Fig. F12). Details for the biostratigraphic zonation are provided in "Paleontology." The Martini (1971) calcareous nannofossil zonation was tied to the GPTS by the BKSA95; when available, more recent and additional age estimates were used and tied to the BKSA95 unless otherwise noted (Table T1). Planktonic foraminifer zonations tied to the BKSA95 are defined in the BKSA95 (Neogene) and Berggren and Pearson (2005) (Paleogene). Miocene dinocyst zones are correlated to the BKSA95 and defined in de Verteuil and Norris (1996). The Oligocene dinocyst zonation is that of Van Simaeys et al. (2005), who provided correlations to the GPTS, though correlations to the U.S. Atlantic margin must be considered tentative. Error bars (e.g., Fig. F34 in the "Site M0027" chapter) for the entire zone assigned in each site report are given following the BKSA95. In cases where a portion of the zone is preferred (e.g., upper Zone DN5), a dashed line is used for the less preferred portion of the zone. Methods and errors for strontium (Sr) ages and semiquantitative lithology are discussed below.

A total of 229 samples were obtained from core catchers on the L/B Kayd during drilling and processed onshore at Rutgers University (USA) prior to the OSP for Sr isotope stratigraphy and semiquantitative lithology. Samples were disaggregated in a sodium hexametaphosphate solution (5.5 g/L) and washed through a 63 µm sieve, discarding the mud fraction. Mollusk shells and foraminifer tests were picked from this coarse fraction for Sr isotopic analysis. Many barren intervals, limited availability of shipboard samples in certain zones, and time constraints limited Sr isotopes to 98 analyses (Table T11). For each measurement, ~4–6 mg of carbonate was cleaned in an ultrasonic bath and dipped in dilute HCl; carbonate was then dissolved in 1.5 N HCl. Sr was separated using standard ion exchange techniques (Hart and Brooks, 1974). Samples were analyzed on an Isoprobe T Multicollector thermal ionization mass spectrometer (TIMS) at Rutgers University supervised by M. Feigenson. Internal (instrument) precision on the Isoprobe for the OSP data set averaged 0.000006; external precision (approximate sample reproducibility) on the TIMS is approximately ±0.000008 based on replicate analyses of standards. The mean value of the NBS 987 standard is measured for these analyses at 0.710241 normalized to ⁸⁶Sr/⁸⁷Sr of 0.1194.

We assigned ages using the BKSA95 timescale (Fig. F12); calibrations to the Geological Time Scale 2008 (Ogg et al., 2008) were made using the tsConvert utility previously available at www.chronos.org. We used the Oligocene Sr isotope/age regressions of Reilly et al. (2002) and the Miocene regressions of Oslick et al. (1994). The Oslick et al. (1994) regression is only for sections older than 9.9 Ma (Sr isotopic values = <0.708930), but this includes all Miocene analyses completed for the OSP. We also computed ages for all analyses using the "look-up" tables of McArthur et al. (2001).

For the Pleistocene analysis, we derived a linear regression using the data of Farrell et al. (1995), correcting their data to a mean value of NBS987 of 0.710255 and fitting a linear segment to the data between 0 and 2.5 Ma:

Miller et al. (1991) and Oslick et al. (1994) estimated age errors derived from linear regressions of Sr isotopic records. Age errors for 22.8–15.5 Ma are ±0.61 m.y. and for 15.5–9.7 Ma are ±1.17 m.y. at the 95% confidence interval for a single analysis. Increasing the number of analyses at a given level improves the age estimate (±0.40 and ±0.76 Ma for three analyses each in the two intervals; Oslick et al., 1994). The regression for the late Pliocene–Pleistocene (2.5–0 Ma) has an age error of ±0.35 m.y. for one analysis at the 95% confidence interval or ±0.2 m.y. for three analyses at the 95% confidence interval (K.G. Miller, unpubl. analysis of data; Farrell et al., 1995). The regression for the late Oligocene to earliest Miocene (27.5–22.8 Ma) and latest Eocene to Oligocene (34.4–27.5 Ma) have age errors of ±1 and 1.2 m.y., respectively, for one analysis at the 95% confidence interval (Reilly et al., 2002). These errors are applied to the Sr isotope values for the age-depth plots.

A primary goal of Expedition 313 is to provide the ages of surfaces and hiatuses; hence, an additional aspect of chronologic assessment is to attempt to correlate the age-depth patterns of each hole to regional surfaces revealed by seismic profiles. Seismic sequence boundaries (identified by onlap, downlap, erosional trunction, and toplap), as well as other prominent reflectors (Monteverde, 2008; Monteverde et al., 2008), were initially correlated to each hole using a traveltime-depth relationship developed from seismic stacking velocities (see "Stratigraphic correlation"). When necessary, small adjustments (typically <10 m) were made to match the depths of reflectors predicted by this procedure to surfaces observed in cores and/or to MSCL and downhole measurements (see "Stratigraphic correlation" in each site chapter). In all cases, the original depths predicted by seismic interpretation are preserved in the tables given in each site chapter. An approximate average sedimentation rate (ignoring compaction) for each sequence was obtained by a visual best fit between core thickness and ages obtained by combined biostratigraphic and Sr isotopic analysis. Age-depth plots are preliminary in that micropaleontological and Sr isotopic studies are ongoing. In general, age errors for most surfaces are less than ±0.5 m.y., as shown on figures and tables in the site chapters.

Samples analyzed for Sr isotopes also provided coarse fraction semiquantitative lithology data presented on age-depth plots (e.g., Fig. F34 in the "Site M0027" chapter). The cumulative percent of lithologic components of core sediments was computed from shipboard samples. Each sample was dried and weighed before washing, and the dry weight was used to compute the percentage of sand versus silt and clay. The coarse (sand) fraction was dry sieved through a 250 ?m sieve, and the fractions were weighed to obtain a quantitative measure of the percent of very fine and fine versus medium and coarser sand. Sand fractions were examined using a microscope, and a visual semiquantitative estimate was made of the relative percentages of quartz, glauconite, carbonate (foraminifers and other shells), mica, and other materials contained in the sample.

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