The principal biostratigraphies, plus a set of 12 paleomagnetic reversals (Tables T7, T8), are defined primarily in Holes 1221A, 1221B, and 1221C. Paleomagnetic reversals are used to calculate the average linear sedimentation rates (LSRs) for Site 1221 through the upper part of the section; however, the disturbed nature of many of the cores required reliance on all the principal biostratigraphies in order to achieve a detailed picture of the LSR through the section (Fig. F10). To this end, we have calibrated radiolarian events and the magnetic record at Site 1220 (see "Biostratigraphy") and have used these events through the middle and upper Eocene. In the lower Eocene, the nannofossil events and the BEE provide the stratigraphic control. The first reliable paleomagnetic reversals are found in Core 199-1221A-1H, along with the youngest radiolarian and nannofossil events. The paleomagnetic stratigraphy extends through the section recovered by APC coring (0-102.5 mcd). The age of the base of the hole is based on the identification of the BEE in Section 199-1221C-11X-3 (Tables T7, T8) and on four nannofossil events.
Based on a simple linear interpolation through the uppermost stratigraphic events (Tables T7, T8), the calcareous and siliceous ooze of lithologic Units I and II (see "Lithostratigraphy") have an LSR near 3.4 m/m.y. The density records from the lowermost 7 m of piston Core EW9709-14P (Lyle, 2000) taken in the survey area can be correlated to those from Hole 1221A (Fig. F11). This correlation is supported by the ages of the radiolarian assemblages from the bottom of Core EW9709-14P and the uppermost cores of Site 1221. The comparison between these two records indicates that the uppermost 5 m of clays collected in the piston core is missing at the location of Site 1221 (Fig. F11).
Within Core 199-1221A-1H, the radiolarians, nannofossils, and paleomagnetic reversals (see Tables T7, T8) indicate that there is a break in deposition just above the level of the top of Chron C15n (6.67 mcd). This break is near the top of Zone RP19, which spans the E/O boundary. Samples at the base of Core 199-1221A-1H are poorly preserved and contain a mixture of Eocene species. At the top of Core 199-1221A-2H, the radiolarians are from Zone RP18, well within the upper Eocene (see "Biostratigraphy"). Together these data suggest that the lower part of the Oligocene (Chron C13n) and part of the upper Eocene are missing at this site (Fig. F10).
The LSR at Site 1221 gradually increases in the siliceous ooze of lithologic Units II and III, reaching nearly 19 m/m.y. in the middle Eocene part of the section (Fig. F10). This relatively high sedimentation rate exceeds the maximum LSR encountered in the middle Eocene of Site 1220 by about a factor of two; however, the overall mid-Eocene LSR at Site 1221 is only slightly higher at Site 1220 (Fig. F10) (see also Fig. F15 in the "Site 1220" chapter). The LSR at Site 1221 apparently drops to ~6 m/m.y. near the top of the lower Eocene with the appearance of the first significant chert layers (see "Lithostratigraphy"). In the lowermost section of chalk, the LSR appears to decrease to ~1.3 m/m.y. This low rate may indicate the presence of hiatuses in the part of the section just above basement.
LSR values may be combined with the dry bulk density (DBD) data from porosity measurements on individual samples (see "Physical Properties") (Table T13) to determine the bulk mass accumulation rates (MARs) of the sediments (Table T9). Sediment with an LSR of 1.0 cm/k.y. and a DBD of 1.0 g/cm3 will have an MAR value of 1.0 g/cm2/k.y. The observed values are rarely this high, so we report the data in milligrams per square centimeter per thousand years (mg/cm2/k.y.). MAR values are essentially zero above the Oligocene carbonate oozes, reflecting the widespread nondeposition in this part of the Pacific (Fig. F12). Lithologic Unit III, dominated by the radiolarian oozes, accumulated at 200-500 mg/cm2/k.y., with the maximum flux rates exceeding 600 mg/cm2/k.y. Unit IV, the basal chalks that contain the P/E boundary horizon, accumulated at a rate of ~160 mg/cm2/k.y.