A 447.8-mcd thick (394.2 mbsf) upper Miocene (~12 Ma) to Holocene pelagic sediment sequence was recovered at Site 1241. Biostratigraphic datums (see Table T10) were used to construct an age-depth model for this site (Table T16; Fig. F40). Linear sedimentation rates (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).
Relatively good agreement exists among microfossil datums from the top ~100 mcd of the section recovered at Site 1241 (Fig. F40). We relied primarily, although not exclusively, upon available calcareous nannofossil datums to define the age-depth model. The increasing scatter in biostratigraphic data downhole results partly from decreasing calcareous microfossil abundance and from increased carbonate dissolution below 365 mcd (see "Biostratigraphy"). The age model trend below 250 mcd (~7 Ma) is not very reliable. Had we chosen to emphasize the planktonic foraminifer datums, a significantly different model would have resulted in that interval.
LSRs range between 18 and 60 m/m.y, and total MARs range from ~1 to 5.6 g/cm2/ky. All rates have a minor peak at 10-9 Ma and a rather well constrained main peak at 7-6 Ma. After 6 Ma, the LSRs and MARs decline gradually. Diverging trends between LSR and MAR are obvious in the intervals from 11 to 9 Ma (below 340 mcd) and from 7 to 6 Ma (~185-250 mcd) as a result of higher porosity and biogenic opal concentrations. No divergence is seen for the top of the sequence although porosity is characteristically high. This trend can be explained by a relatively high average grain density that results from the increased siliciclastic content in the top 30 mcd of the sediment sequence.
The oldest interval (11-9 Ma) with relatively high concentrations of biogenic opal and organic carbon corresponds to the "Miocene carbonate crash," an interval of poor carbonate preservation recognized in equatorial East Pacific sediments below 3000 m water depth (Lyle et al., 1995). Even though Site 1241 was at a paleowater depth above the lysocline at that time (see "Introduction"), benthic and planktonic foraminiferal tests also appear strongly affected by carbonate dissolution over this interval (see "Biostratigraphy"). The apparent high biogenic opal deposition from 11 to 9 Ma has no equivalent at the other Leg 202 sites but is consistent with a paleolocation near the equator at that time, driven by drift of the Cocos plate (see "Introduction"). Given the high concentrations in diatoms and organic carbon and the suffered carbonate losses, the resulting total biogenic accumulation rate would clearly suggest an interval of enhanced productivity. Carbonate dissolution was likely enhanced by an increase in the rain ratio of organic carbon to carbonate to the seafloor (Archer, 1991).
High carbonate MARs returned by 7 Ma, when carbonate MARs also increased at other sites. However, at this site the carbonate MARs declined after 6 Ma, when the rates still increased at Southern Hemisphere Sites 1236 through 1239, indicating that this site experienced different regional oceanographic conditions during that time. A similar early decrease in LSRs at Sites 852-854 (Shackleton et al., 1995) suggest a significant difference in the timing of peak production in the Northern and Southern Hemispheres.