Average LSRs at Site 1218 are based on all the principal biostratigraphies plus a good set of paleomagnetic reversals defined in Holes 1218A, 1218B, and 1218C (Tables T13, T14). Only radiolarians are present in the uppermost part of the section. From the lower middle Miocene through the lower Oligocene, all three fossil groups were useful in establishing age control. In most of the Eocene, both radiolarians and nannofossils were present (Fig. F18).
Lithologic Unit I (see "Lithostratigraphy") contains two well-constrained radiolarian datums in Core 199-1218A-1H. This is in marked contrast to piston core EW9709-7P (~15 m long) taken during the site survey cruise and located only a few miles from Site 1218. Most of the sediment contained in this piston core is a nonfossiliferous pelagic "red" clay, but the bottom 3 m contains a middle Miocene radiolarian fauna, and a few nannofossils are preserved in the lower 1-2 m. However, the EW9709-7P piston-cored sequence can be correlated to the upper 67 m of the Site 1218 composite section using the MST GRA density records (Fig. F19). This comparison shows that core EW9709-7P has a sedimentation rate of ~0.87 m/m.y. through most of its length, a rate that is about four times lower than sediments accumulating in lithologic Unit I at Site 1218.
The paleomagnetic reversal data, based primarily on declination data in oriented APC cores (see "Paleomagnetism") (Fig. F12), show a suite of over 70 reversal boundaries that can be used for detailed sedimentation rate determinations from ~20 to 200 mcd (Table T13). These data seem to indicate an interval of extremely rapid deposition, bounded by short intervals of slow deposition at ~6.6 Ma and 28 mcd (Core 199-1218A-3H). The report on radiolarians (see "Biostratigraphy") notes that Cores 199-1218A-2H and 3H contain poorly preserved specimens representing a great variety of Neogene and Paleogene ages. In combination with the paleomagnetic data, this observation suggests that sediments in Cores 2H and 3H represent a rapid depositional event, perhaps associated with a slump (Fig. F18).
The LSRs noted in Figure F18 are based on radiolarian events down to the top of reliable magnetic data at ~20 mcd. Below that point to ~200 mcd, the sedimentation rates are derived from linear fits to the paleomagnetic data. From the base of the Oligocene near 200 mcd to the bottom of the drilled section, sedimentation rates are based on nannofossil events. Average sedimentation rates are 3-4 m/m.y. through most of the Neogene, with the exception of the slumped interval. Average sedimentation rates reach a maximum of >18 m/m.y. during the middle part of the Oligocene. Rates in the Eocene appear to be more variable, ranging from >4 to ~11 m/m.y. The rates are generally higher in the higher carbonate intervals.
LSR values may be combined with the dry bulk density (DBD) data from porosity measurements on individual samples, averaged over the intervals reported (see "Physical Properties") (Table T19) to determine the bulk mass accumulation rates (MARs) of the sediments (Table T15). 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.).
At Site 1218, which has a more extensive paleomagnetic and biostratigraphic datum record than other Leg 199 sites, we have determined MAR values at each horizon where there is a value for DBD (see "Physical Properties") (Table T19). This permits a quantification of the bulk-sediment MARs for the entire 42-Ma record (Fig. F20). Flux values are low in lithologic Unit I, generally <150 mg/cm2/k.y. Lithologic Unit II, dominated by calcareous material, accumulated at 700-2000 mg/cm2/k.y., with the maximum flux rates in the very light brown to white nannofossil ooze of early Oligocene age. Lithologic Unit III, dominated by radiolarian ooze and radiolarite, has much lower MARs of ~300 mg/cm2/k.y. The basal chalk of Unit IV accumulates at rates similar to the carbonates of Unit II.