A 560.7-mcd-thick (515.4 mbsf) sequence of Miocene (~15 Ma) to Pleistocene pelagic sediments was recovered at Site 1239. Biostratigraphic datums (Table T9) were used to construct an age-depth model for this site (Table T16; Fig. F37). 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).
The generation of the age-depth model for Site 1239 (Fig. F37) relied upon all available microfossil datums, which generally agree well with few outliers. A hiatus is present at ~530 mcd, spanning the interval 14.6 to ~8 Ma. Frequent and persistent reworking was noted in the upper Miocene-lower Pliocene interval (see "Biostratigraphy")
A decision was made to use the majority of the planktonic foraminiferal and diatom datums in the interval 460-520 mcd, rather than a single calcareous nannofossil datum (the top of the R. pseudoumbilicus [>7 µm] absence interval) because low abundance may make this datum unbelievable. This decision resulted in relatively high LSRs (~55 to ~75 m/m.y.) and MARs (~4.5 to ~6 g/cm2/k.y.) during the interval 7 to 5 Ma.
Given the low-precision age control points, this age-depth model is not fully constrained in all the 1-m.y. intervals used for the calculation of LSRs and MARs, and this dictates caution when interpreting variations.
LSRs range between ~10 and 104 m/m.y, and total MARs range from ~1 to 8 g/cm2/k.y. All rates display a broad peak between 6 and 2 Ma that is driven by both carbonate and noncarbonate MARs. Maximum rates are indicated at 4-3 Ma (194-306 mcd), but the details of the broad peak are somewhat dependent on choices made in the definition of the age model and the 1-m.y. sampling of the model. If the model is sampled at 2 m.y., the maximum carbonate MAR occurs earlier, between 6 and 4 Ma. This pattern is generally consistent with trends predicted by eastward tectonic drift, which suggests relatively high production when the site was centered under the productive south equatorial current from 4 to 6 Ma (see "Introduction"). However, regional oceanographic changes and nutrient budgets may also contribute to these variations in MARs, consistent with similar peaks in carbonate MAR at other sites, although elsewhere peak MAR is older, typically occurring at 5-7 Ma (Farrell et al., 1995; Pisias et al., 1995). The relative divergence of the LSRs from the total MAR is most pronounced for that last 1 m.y. as a result of decreasing overburden and associated high porosity near the top of the record, and where all rates are at a maximum (4-3 Ma), which has no obvious lithologic explanation. The relatively smooth trend of LSRs and MARs at Site 1239 reflects the relatively homogeneous lithology of the entire sediment sequence recovered here.