A 223.2-mcd-thick (207.7 mbsf) sequence of Neogene and upper Oligocene (~28 Ma) mostly calcareous sediments was recovered at Site 1236. Biostratigraphic and magnetostratigraphic age datums (see Tables T7, T11) were used to construct an age-depth model for this site (Table T14; Fig. F29). 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).
Both biostratigraphic and magnetostratigraphic age datums are well constrained and in general agreement for the upper 90 mcd (Fig. F29). Almost no reworked specimens were observed (see "Biostratigraphy"). Large error bars on the magnetostratigraphic datums result from the method used for identifying polarity chrons (see "Paleomagnetism"). Below 90 mcd, age datums become less frequent, preservation becomes worse in some intervals (especially at depth >150 mcd), some standard middle and early Miocene biostratigraphic zonal markers are absent, and the agreement between magnetostratigraphic and biostratigraphic datums becomes less certain. The downhole transition to more scattered datums occurs at ~11-12 Ma, approximately when biostratigraphic datums are no longer astronomically tuned. The age-depth relationship relies primarily upon available magnetostratigraphic data, which is not available below ~90 mcd. In the deepest section (90-222 mcd), a nearly straight line was used to approximate the age-depth relationship because of a paucity of reliable biostratigraphic datums.
Three specific calcareous nannofossil outliers were not included in the generation of this age-depth model (see "Biostratigraphy"). These included the last occurrence of T. rugosus (~49.4 mcd; 5.35 Ma), which was considered only because the LO of D. quinqueramus (5.56 Ma) was not observed near the Miocene/Pliocene boundary. Another outlier resulted from the LO of C. floridanus at 71.8 mcd (~13 Ma), whose age may be more than 1 m.y. younger at Site 1236 than has been observed elsewhere. The two calcareous nannofossil outliers at ~170 mcd (LOs of R. bisecta and Z. bijugatus) may represent anomalous appearances or reworking and are not supported by other microfossil datums.
Site 1236 LSRs and both total and carbonate MARs follow the same general trend for the last 25 m.y. (Fig. F29). LSRs range between ~3 and 11 m/m.y., and MARs range between 0.2 and 1.6 g/cm2/k.y. Values are high at the base of the record but decrease gradually until ~11 Ma. A peak in LSRs (~9 m/m.y.) and MARs (~0.9 g/cm2/ky) occurs between 6-5 Ma. The rates then decrease gradually toward minimum values at the top of the record.
Some variation in dry density occurs, represented by the changing relationship between LSRs and total MARs. The accuracy of dry density measurements between ~24 and 17.5 Ma (119.5-206.5 mcd) may have been compromised in intervals of unconsolidated grainstones because the moisture contents may not have represented saturated in situ conditions. Carbonate MARs closely mimic total MARs for essentially the entire record, indicating that only minimal input of noncarbonate components has occurred throughout the site's history.
The relatively high accumulation rate of lithologic Unit II is related to downslope gravity flows from nearby carbonate platforms (see "Lithostratigraphy"). Gravity flows are apparent at least through ~17.5 Ma (the top of Unit II). Two isolated intervals of unlithified wackestone also are present as shallow as 65 and 99 mcd, indicating that at least some influx of neritic grains through gravity flows occurred as recently as ~11 Ma. Therefore, at ages >17.5 Ma, and possibly >11 Ma, inferences cannot be made about surface water productivity from MARs and LSRs, as the production signal cannot be distinguished from gravity-driven accumulation.
The interval of low carbonate MAR (~0.4 g/cm2/k.y.) from 8 to 12 Ma coincides with the so-called "carbonate crash," which is an interval of poor preservation recognized in equatorial east Pacific sediments below 3000 m water depth (Lyle et al., 1995). Given the shallow paleowater depth of Site 1236 at <1300 m well above the lysocline and no evidence for a change in carbonate preservation (see "Biostratigraphy"), the low carbonate MARs may reflect the oligotrophic environment at the eastern boundary of the subtropical gyre rather than a link to the "carbonate crash." Carbonate MARs rise from 9 to 6 Ma and peak between 6 and 5 Ma (0.9 g/cm2/k.y). This peak in MAR matches a similar event, referred to as the "biogenic bloom," in the equatorial Pacific that is inferred to reflect anomalously high productivity (Farrell et al., 1995; Pisias et al., 1995). A long-term trend of increasing productivity from ~9 to ~5 Ma could result from gradual eastward tectonic drift of the site toward the South American margin, but drift cannot explain the discrete peak in MAR. Similar trends occur in LSRs and MARs at other Leg 202 sites (see "Age Model and Mass Accumulation Rates" in the "Site 1237" chapter) and in the LSRs of some Leg 138 sites (especially Sites 848 and 846; see fig. 18, in the Leg 138 summary chapter (Shipboard Scientific Party, 1992). This MAR peak may indicate widespread trends in biogenic sedimentation. Site 1236 MARs and LSRs decrease from 5 to 0 Ma, indicating a gradual decrease in productivity at this site from a late Miocene and Pliocene maximum.