A 468.2-mcd-thick (430.6 mbsf) sequence of middle Miocene (~12 Ma) to Holocene pelagic sediment was recovered at Site 1238. Biostratigraphic datums (see Table T9) were used to construct an age-depth model for this site (Table T16; Fig. F43). LSRs, total MARs, and carbonate MARs were calculated at 1-m.y. intervals (see "Age Models and Mass Accumulation Rates" in the "Explanatory Notes" chapter).
The generation of an age-depth model at Site 1238 (Fig. F43) relied primarily upon calcareous nannofossil datums. Outliers were present within all three microfossil groups, although they were most often planktonic foraminifer datums. Had we chosen to emphasize the planktonic foraminifer datums, a significantly different age model would result along with higher LSRs and MARs in the interval 4-5 Ma, and lower LSRs and MARs in the interval 1-3 Ma. Reworking was noted for all fossil groups, especially in the upper Miocene-lower Pliocene interval (see "Biostratigraphy"), and this limits the precision of the age model.
A decision was made to use the LO of planktonic foraminifer Globorotalia fohsi s.l. at 429.8 ± 7.16 mcd (~11.7 Ma) rather than the LO of calcareous nannofossil Coccolithus miopelagicus (456.5 ± 0.75 mcd; 10.4 Ma) based on the relative reliability of the datums (see "Biostratigraphy") and a nearby lithologic change (see "Lithostratigraphy"). Our age-depth model thus predicts a hiatus at ~430 mcd, spanning >12 Ma to ~8 Ma and ending at a similar time as the more clearly defined hiatus at Site 1239. The presence of a hiatus at 430 mcd means that the deepest calcareous nannofossil datum (the LO of Coccolithus miopelagicus) may not be a true last occurrence. Alternatively, if this deepest calcareous nannofossil datum is used to define the age-depth model, the basal age would be <11 Ma and the LSR could be as high as ~20 m/m.y. for the lowermost 40 mcd.
LSRs range between ~10 and 72 m/m.y, and total MARs range between ~1 and 5 g/cm2/k.y. The rates gradually increase from the hiatus ending ~8 Ma and peak at 5-3 Ma. A general increase in LSRs and MARs at younger ages would be predicted by the eastward tectonic drift toward the continental margin (see "Introduction"). At ages <3 Ma (<150 mcd), however, the rates drop rather abruptly and remain lower from 3 to 1 Ma, and this decrease is not predicted by tectonics. During the last 1 m.y. (<56 mcd), the MAR decreased further, whereas the LSR slightly increased. The divergence of LSR and MAR since 1 Ma can be explained by increased siliciclastic (clay) input during that time, decreasing overburden in the top section, and the associated increase in porosity (see Figs. F15, F19).
The broad peak in carbonate MARs from 7 to 3 Ma likely represent changes in the productivity of calcareous organisms because a similar peak at ~7 to 5 Ma is present within the records of other Leg 202 sites (see Figs. F20 and F34 in the "Leg Summary" chapter) and is known from other equatorial Pacific sites where it is interpreted as a production signal (Farrell et al., 1995; Pisias et al., 1995).