A 360.6-mcd-thick (317.4 mbsf) Oligocene (~31 Ma) to Holocene pelagic sediment sequence was recovered at Site 1237. Biostratigraphic and magnetostratigraphic datums (see Tables T10, T16) were used to construct of an age-depth model for this site (Table T21; Fig. F38). Linear sedimentation rates (LSRs), total mass accumulation rates (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 magnetic reversal datums are well constrained and in general agreement for the upper 200 mcd (Fig. F38). Here, the age-depth model relies primarily on paleomagnetic data. Below 200 mcd (~12 Ma), the agreement between biostratigraphic datums becomes less certain, presumably because biostratigraphic datums are no longer astronomically tuned, and magnetic data are no longer available. For the interval deeper than 200 mcd, we relied primarily (although not exclusively) upon calcareous nannofossil datums to define the age-depth model. Had we chosen to emphasize the planktonic foraminifer datums, a significantly different age model would have resulted for this interval.
Determining the age-depth trend was particularly difficult from ~250 to 300 mcd, where few nannofossil datums exist. At ~300 mcd, calcareous nannofossil biostratigraphy (see "Biostratigraphy") indicates the potential presence of a 1-m.y. hiatus, an event not noted in the other microfossil groups or in lithologic or physical properties data. A straight line was therefore fit for this interval. The presence of a hiatus at this depth would slightly steepen the curves above and below this interval, and result in slightly higher LSRs and MARs.
LSRs range between ~5 and 23 m/m.y., and total MARs range from ~0.6 to 2.4 g/cm2/k.y. LSRs, total MARs, and carbonate MARs are low before 8 Ma, rise to their peaks at 5-6 Ma, and decline from 5 to 2 Ma. All rates have closely similar trends from 30 to ~15 Ma. Relatively constant LSR and MAR values in this interval result from the smooth age model chosen because the coarse time resolution and large uncertainties of the biostratigraphic datums did not allow a more detailed definition. Significant fluctuations might have occurred in reality.
From 15 Ma to the present, the LSR, total MAR, and carbonate MAR trends diverge. The divergence of carbonate MAR from total MAR reflects continually increasing amounts of noncarbonate components during a period of increasing carbonate MAR (15-5 Ma) as well as the subsequent period of declining carbonate MAR (5-1 Ma). Diatoms are absent below 195 mcd (>12 Ma), and abundance increases from barren to common at ages <9 Ma (<170 mcd) (Fig. F26). Siliciclastic material increases from ~5% to ~80% at ages <12 Ma (<190 mcd), and TOC increases from 0 to ~2% after 3 Ma (<60 mcd) (Fig. F35). The gradually increasing divergence of LSR from MARs at ages <15 Ma can be explained by changing mineral proportions and sediment fabric (more opal and clay and less carbonate), decreasing overburden, and the associated increase in porosity and decrease in dry density. The interval of slightly elevated rates from 13 to 11 Ma might represent effects of the middle Miocene climatic changes, but the present age model is too simplistic to yield detailed interpretations.
Peak carbonate MARs in the interval 5-7 Ma likely represent enhanced productivity of calcareous organisms, because a similar peak in carbonate MAR is present within the records of other Leg 202 sites (see Figs. F20 and F34 in the "Leg 202 Summary" chapter) as well as elsewhere in the equatorial Pacific (Farrell et al., 1995; Pisias et al., 1995), where it is interpreted as a production signal. The substantial reduction in carbonate MAR from 5 Ma to the present is not associated with visual evidence for dissolution of calcareous microfossils (see "Biostratigraphy") and is interpreted as primarily related to a decrease in the productivity of calcareous organisms.
A general increase in the accumulation of both biogenic and terrigenous components toward the younger portion of the record would be expected as a result of the gradual tectonic drift of Site 1237 toward South America. This backtrack path shows that Site 1237 moved out of the oligotrophic subtropical gyre close to the highly productive coastal upwelling regions of Peru (see "Introduction") and into the dust distribution areas of the northern Chile deserts (see "Lithostratigraphy"). The general increases in biogenic opal after 15 Ma, increase in terrigenous input after ~5 Ma, and the abundance of TOC and a further increase of opal after 3 Ma (Fig. F39) are therefore most likely the result of increasing proximity to South America. However, regional oceanographic changes that are expressed as a so-called "opal shift" in the equatorial Pacific (Farrell et al., 1995) may also be expressed at Site 1237.