Because the diatom and calcareous nannofossil datum levels used in this study have been tied to the geomagnetic polarity time scale (see "Biostratigraphy" in the "Explanatory Notes" chapter), Site 1150 sedimentation rates can be estimated by combination of biostratigraphy and magnetostratigraphy. 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 ~850 mbsf. Below ~850 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 indicate younger ages than those from diatom biostratigraphy between 150 and 900 mbsf. The age assignments based on calcareous nannofossils at Site 1150 are uncertain because there are few nannofossils and these occur discontinuously, the nannofossils have poor preservation, and the datum levels are possibly diachronous.
The control points for age assignment were thus chosen from eleven diatom datums, five geomagnetic chron boundaries above 800 mbsf, and two nannofossil datums above 80 mbsf (Table T13). 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 the Gauss Chron (C2An/C2Ar, 3.58 Ma) was recognized immediately below this datum. Since the FO of diatom D. dimorpha was not found, the lowermost depth (1180.23 mbsf) is slightly younger than 9.9 Ma. The average sedimentation rate is >119 m/m.y.
In general, the sedimentation rate of Site 1150 is high relative to nearby DSDP Sites 438 and 584 (Fig. F48), suggesting a much greater amount of biogenic and/or detrital input to the basin around this site. An age-depth plot for Holes 1150A and 1150B indicates that the upper 70 m has a relatively high sedimentation rate (108 to 205 m/m.y.), which may be related to lower compaction of the uppermost part (see "Physical Properties"). There is an interval of significantly low sedimentation rates between 70 and 122 mbsf (24 to 42 m/m.y.). It is possible that there is a very short hiatus (<0.42 m.y.) between 120 and 122 mbsf because the FO of the Pleistocene diatom P. curvirostris is recognized in this interval, together with the LO of the late Pliocene fossil N. koizumii (see "Biostratigraphy"). More detailed postcruise study will be necessary to confirm this. In the interval between 122 and 195 mbsf, the sedimentation rate increases to 114 m/m.y. Below this depth the rate decreases to 76 m/m.y., and then increases gradually until 530 mbsf. High sedimentation rates (>200 m/m.y.) occur between 530 and 971 mbsf, with the highest value of 248 m/m.y. in the interval of 529-629 mbsf. Below this the rate suddenly decreases, varying from 43 to 100 m/m.y.
Though sedimentation rates at Site 1150 are high and hiatuses are absent or span a very short interval of time, there is a common trend of the sedimentation rate between Site 1150 and Sites 438 and 584. In particular, all three sites had high rates in the latest Miocene and early Pliocene and low rates before and after this (Fig. F49). On the other hand, two intervals of low rates correspond to the early late Miocene (prior to 7.35 Ma) and the early to mid-Pleistocene (2.0-0.41 Ma), respectively. In general, the intervals of high sedimentation rate during the latest Miocene to the early Pliocene (~7.5-3.0 Ma) correspond to higher content of opal-A (see "Lithostratigraphy"). In addition, a higher concentration of terrigenous minerals occurs in the upper Miocene (~7-6 Ma) and the upper Pleistocene, which also correspond to the higher sedimentation rates. Therefore, increasing siliceous biogenic and terrigenous siliciclastic inputs may both be related to the change of the sedimentation rate.