Average LSRs at Site 1217 are based primarily on paleomagnetic reversals and nannofossil and radiolarian datums as defined in Holes 1217A through 1217C (Tables T7, T8). A few nannofossil datums are critical to determining the age at the base of the section. Radiolarian datums are not sufficiently well tied to the paleomagnetic timescale to allow their use in this part of the section. LSRs are calculated using the mcd scale (see "Composite Depths") (Table T4) for each datum used in constructing the sedimentation rate plots (Figs. F15, F16). The values of these sedimentation rates are combined with the dry bulk density (DBD) data from porosity measurements on individual samples averaged over the intervals reported (see "Physical Properties") (Table T13) to determine the mass accumulation rates (MARs) of the sediments.
Density measurements on piston core EW9709-4P, taken in the site survey area, show a good similarity with density measurements from the spliced density record measured in cores from Site 1217 (Fig. F16). The EW9709-4P record spans a similar depth range in Site 1217 (down to ~14 m); however, there are no biostratigraphic or paleomagnetic datums that constrain this correlation.
From ~20 to 80 mbsf, the paleomagnetic data (see "Paleomagnetism") (Fig. F11) show several distinct reversals. The uppermost distinct reversal (at 23.27 mbsf) in Hole 1217A is taken to be the base of Chron 12n. If we accept this point as the shallowest age control below the seafloor, we estimate the average LSR of the nonfossiliferous pelagic clays to be ~0.76 m/m.y. (Fig. F15). The interpretation for the base of Chron 12n leads to the identification of the next deeper normal interval in Hole 1217A as Chron13n, which lies at the base of the Oligocene. This identification fits with the underlying spacing and length of several normal and reversed interval and with the thin part of the lithologic Unit I containing a layer of nannofossil-rich carbonate ooze (see "Lithostratigraphy"). This ooze, all from nannofossil Zone NP22, we take to represent the dramatic deepening of the CCD at the Eocene-Oligocene transition (see "Biostratigraphy").
Using identifications of the magnetic reversals, sedimentation rates in the clay underlying the carbonate are ~3.3 m/m.y., increasing to ~4.4 m/m.y. downhole as the more radiolarian-rich sediments of lithologic Unit II are encountered downcore (Fig. F17). In the lower part of the more well-dated section (~60-80 mcd), sedimentation rates appear to increase to 10 m/m.y. (Fig. F17). However, sedimentation rate estimates for this rather short interval are subject to substantial errors arising from even a small error in age or depth estimates. The overall LSR estimate for fossiliferous and cherty lower section of lithologic Units II and III is 4.8 m/m.y. (Fig. F15).
By combining LSR values with DBD, we determine the MAR of the total sediment (Table T9) and, when appropriate geochemical concentration data are available, the MAR of each sedimentary component. Sediment with an LSR of 1.0 cm/k.y. and a DBD of 1.0 g/cm3 will have a MAR value of 1.0 g/cm2/k.y. The observed values are rarely this high, so we report the data in milligrams per centimeter squared per thousand years (mg/cm2/k.y.). The accuracy of this calculation is ±10%-20%.
Lithologic Unit I (pelagic clays) is characterized by LSR and MAR values that increase downhole: LSR from 0.76 m/m.y. in the upper 22.5 m (Fig. F15) to 4.4 m/m.y. in the interval just above 52 mcd (Fig. F17). Because of downhole increases in DBD, the total mass flux increase from 45 mg/cm2/k.y. (in the age range 0-31 Ma) progressively to 150 mg/cm2/k.y. (between 35 and 38 Ma) (Figs. F15, F17). This is the first record clearly showing clay fluxes that increase downhole. Prior records indicate downcore decreases in the MAR of the red clays during middle Cenozoic time (Janecek and Rea, 1983; Kyte et al., 1993; J.D. Gleason et al., unpubl. data). The clayey carbonate ooze that occurs within Unit I has a MAR value of 135 mg/cm2/k.y. (Fig. F17).
The radiolarian oozes of Subunit IIA accumulated relatively slowly (115 mg/cm2/k.y.) between ~37.7 and 40 Ma then more than double (260 mg/cm2/k.y.) for a brief period in the middle Eocene between ~40 and 42 Ma (Fig. F17). Older radiolarian oozes and clayey radiolarian oozes accumulated more slowly, at ~120 mg/cm2/k.y. (Fig. F17). These values represent the first definition of a middle Eocene maximum in the accumulation rate in this facies. Subunit IIB spans ~14 m.y. of time and accumulated at a gross overall rate of ~120 mg/cm2/k.y.
The lower Eocene chalks of Unit III, near the base of the hole, accumulated at ~570 mg/cm2/k.y. (Fig. F17).