Average linear sedimentation rates (LSRs) at Site 1215 are based primarily on datums defined in Hole 1215A (Tables T8, T9). They are calculated using mcd (see "Composite Depths") (Table T5) for each datum as defined in Table T8. LSR values are combined with the dry bulk density (DBD) data averaged over the intervals reported (see "Physical Properties") (Table T14) to determine the mass accumulation rates (MARs) of the sediments (Table T10).
LSR values are plotted in Figure F13, with three categories of control points: paleomagnetic reversal boundaries, calcareous nannofossil bioevents, and the Paleocene-Eocene BEE (see "Benthic Foraminifers" in "Biostratigraphy"). Planktonic foraminifers, strongly affected by calcite dissolution, were not used for age control but clearly show the late Paleocene-early Eocene age progression in the recovered carbonate-bearing sediments (see "Planktonic Foraminifers" in "Biostratigraphy"). Reversal boundaries are generally considered as a reference framework for sedimentation rate plots because all fossil events have been calibrated to a single polarity timescale (Cande and Kent, 1995). Consequently, we use the convention of utilizing reversal boundaries as anchor points for the sedimentation-rate history while realizing that there is both a depth and age uncertainty of these data points. The scatter of biostratigraphic indicators in the paleomagnetic reference framework at least partly reflects how well they have been calibrated to the geomagnetic polarity timescale, but other factors inherent in the geologic record may also play a part. For Site 1215, we have given the BEE a status of an anchor point because it has been assigned a precise age estimate derived from cyclostratigraphy (Norris and Röhl, 1999).
In the upper portion of lithologic Unit I (the lighter brown part of the unit), sedimentation rates are thought to be somewhat higher than in the deeper, dark-brown interval. However, there is no microfossil age control to constrain the magnetic stratigraphy. At the highest rate, these might be slightly >3 m/m.y. Comparison with similar North Pacific "red-clay" sections, which have been dated with 87Sr/86Sr stratigraphy on fish teeth, indicates this is likely an upper estimate of the sedimentation rate. It seems more likely that the overall LSR of Unit I is <1 m/m.y. and probably close to 0.6 m/m.y. (J.D. Gleason et al., unpubl. data; Kyte et al., 1993; Janecek and Rea, 1983).
In lithologic Unit II, the average sedimentation rate is ~8.5 m/m.y. However, using only the paleomagnetic datums (Tables T8, T9; Fig. F13), the section can be divided into two discrete intervals: an upper interval (~30-36 mcd) with a rate of ~4 m/m.y. and a lower interval (36-70 mcd) with at rate of ~12 m/m.y. An exceptionally condensed sequence (1 m/m.y.) or, more likely, a hiatus separates these two intervals between the top of Subchron C24n.1n at 52.347 Ma and the base (first evolutionary appearance) of Sphenolithus radians at 53.1 Ma. This nannofossil event has been calibrated and is found in Subchron C24n.3n at DSDP Sites 528 (Walvis Ridge) and 577 (Shatsky Rise). Considering this synchrony between the two ocean basins (South Atlantic and North Pacific), it becomes difficult to envisage why S. radians should appear ~0.75 m.y. later at North Pacific Site 1215, which would be required in order to invoke continuous sedimentation between the top of Subchron C24n.1n and the base of Subchron C24n.3n here.
The interval below the brief hiatus can be subdivided into three segments using the biostratigraphic datums: an upper segment (36-51 mcd) with a rate of 18 m/m.y., a middle segment (51-58 mcd) with a rate of 7 m/m.y., and a lower segment (53-70 mcd) with a rate of 12 m/m.y. If we rely only on the magnetostratigraphy and the BEE, there is a uniform LSR of 12 m/m.y. throughout this interval.
Lithologic Unit III, the "hydrothermal" unit, has a lower LSR of ~3 m/m.y. This rate is based on the dates of Chron 25n as well as the first occurrence of D. multiradians (Tables T8, T9). However, the position of the base of Chron C25n is near the base of Core 199-1215A-9H and may be an unreliable datum. The primary constraint on this rate is the top of Chron 25n and the D. multiradians datum.
By combining LSR values with DBD data, we determine the MAR of the total sediment (Table T10) and, when appropriate 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 square centimeter per thousand years (mg/cm2/k.y.).
The lighter upper portion of lithologic Unit I has a greater average bulk density than the lower portion and accumulates at ~40 mg/cm2/k.y. vs. 20 mg/cm2/k.y. for the dark-brown pelagic clays. The upper portion of the clayey nannofossil oozes (lithostratigraphic Unit II) has a MAR of ~200 mg/cm2/k.y. above the hiatus. Below the hiatus, lowermost Eocene oozes younger than the BEE accumulate at ~750 mg/cm2/k.y., whereas those below have a much higher DBD value and MARs of 1300 mg/cm2/k.y. The hydrothermal ooze unit accumulates at ~150 mg/cm2/k.y., which is one of the few well-constrained estimates of MARs for this particular facies.