A 282.9-mcd-thick (253.0 mbsf) sequence of Pliocene (~2.8 Ma) to Holocene pelagic sediments was recovered at Site 1240. Biostratigraphic and magnetostratigraphic datums (see Tables T10, T15) were used to construct an age-depth model for this site (Table T20; Fig. F32). Linear sedimentation rates (LSRs), total MARs, and carbonate MARs were calculated at 0.4-m.y. intervals (see "Age Models and Mass Accumulation Rates" in the "Explanatory Notes" chapter).
The definition of the age model at high-sedimentation-rate Site 1240 relied primarily upon magnetostratigraphic and calcareous nannofossil datums (Fig. F32). Microfossil reworking was evident in the upper Pliocene section (see "Biostratigraphy"), and two magnetic reversal boundaries (~70-100 mcd) were poorly defined (see "Paleomagnetism"). Two inflections in the age-depth profile are reasonably well defined, however, at ~140 and ~200 mcd. These inflections correspond very well to lithologic subunit boundaries (Subunit IB; see "Lithostratigraphy").
LSRs range between ~72 and 160 m/m.y., and total MARs range between 1.6 and 8.5 g/cm2/k.y. The LSRs vary slightly around values of 70-100 m/m.y., except for a peak (160 m/m.y.) between 2.0 and 1.6 Ma. MAR values have a similar pattern but are more differentiated as a result of the dry density variations. The main MAR peaks at 2.0-1.6 Ma are not as pronounced as the LSR peak because this interval (lithologic Subunit IB; 142.2-206.4 mcd) has high diatom and organic carbon concentrations (see "Lithostratigraphy," "Biostratigraphy," and "Geochemistry"), which maintain high porosity and lower the grain density. The youngest interval (0.4-0 Ma) also has LSR values proportionally higher than the low MAR values. Again, the low MAR values in this interval result from lowered dry densities, probably related to both increased biogenic opal contents and reduced overburden toward the top of the sediment sequence.
Total MARs at Site 1240 are controlled by the combined variations in calcareous and siliceous microfossil accumulation. Diatom productivity dominated at 2.4-1.6 Ma and after 0.4 Ma, whereas calcareous productivity dominated at all other times.