The sedimentation rates of Site 1151 were estimated using a combination of biostratigraphy and magnetostratigraphy as at Site 1150 (see "Sedimentation Rates" in the "Site 1150" chapter). This estimation is based primarily on datums of diatom zonal boundaries because diatom valves were well preserved and the datums were continuously recognized. The magnetostratigraphy is poorly constrained and at best provides a few reversal boundaries that can be used to adjust the age estimates predicted between diatom datums. A magnetostratigraphy derived from the diatom datums does, however, accurately predict several of the more prominent observed reversal boundaries, and so we consider the two to be in good agreement above 680 mbsf. Below ~680 mbsf, however, the inclination indicates a long interval with normal polarity, which is interpreted as an overprint (see "Paleomagnetism").
As discussed in "Biostratigraphy", calcareous nannofossil datums between 90 and 940 mbsf indicate younger ages than those from diatom biostratigraphy. The age assignments based on calcareous nannofossils at Site 1151 are uncertain for reasons discussed in the previous chapter.
The control points for age assignment were thus chosen from 16 diatom datums and four geomagnetic chron boundaries above 610 mbsf (Table T11, also available in ASCII format). Ages of diachronous events were averaged, but the age of the FO of N. koizumii (diatom) was defined as its youngest value (3.53 Ma) because the bottom of Gauss (C2An / C2Ar, 3.58 Ma) was recognized immediately below this datum. Instead of the LCO of the diatom D. simonsenii (8.6 Ma), the LO of D. katayamae (8.5 Ma) was used because the presence of the latter species was clearer (see "Biostratigraphy"). Since the FO of the diatom D. praelauta was not found, the lowermost depth (1113.46 mbsf) is slightly younger than 16.3 Ma, which is calculated as 16.18 Ma by using the 43-m/m.y. sedimentation rate of the D. lauta Zone.
Sedimentation rates for the upper 100 m of the site are complicated by offsets between diatom datums. In particular, two diatom datums from Holes 1151C and 1151D (LCO of A. oculatus and LO of N. koizumii) are about 20 m shallower than in Hole 1151A (see "Biostratigraphy"). We have used the datums from Hole 1151A because they are more consistent with the location of the Brunhes/Matuyama boundary, which is located at ~78.2 mbsf in Hole 1151C, 80.1 mbsf in Hole 1151D, and at 82-84 mbsf (between Cores 186-1151A-2R and 3R) in Hole 1151A. In these holes, the reversal is defined by a clear change from steep positive to steep negative inclination, though the reversal sequence lower in the section does not follow a standard magnetostratigraphy. Postcruise study will be needed to resolve the conflicting ages.
The average sedimentation rate is 24 m/m.y over the entire interval, though it is 99 m/m.y. over the past 9.5 m.y. This latter value is slightly lower than the 120 m/m.y. rate found for Site 1150 over the past 9.5 m.y. (Fig. F24).
An age-depth plot for Site 1151 indicates that the upper 200 m has a relatively low rate (18 to 152 m/m.y.). As at Site 1150, the sedimentation rate is especially low between 110 and 86 mbsf, which corresponds to the early to mid-Pleistocene. At a depth of between 200 and 920 mbsf the sedimentation rate increases, with the highest values (~240 m/m.y.) between 450 and 800 mbsf. Below this the rate gradually decreases. In the interval at 1027 mbsf there is a hiatus ranging more than 0.2 m.y., and then the sedimentation rate increases gradually downhole to 43 m/m.y.
Similar to Site 1150, Site 1151 has high rates in the uppermost Miocene and low rates below and above this (Fig. F25). The intervals of low rates correspond to the early late Miocene (prior to 8.5 Ma) and the early to mid-Pleistocene (2.0 to 0.78 Ma), respectively.