Shipboard data suggest that two of the sites with long stratigraphic records, Sites 1237 and 1241, have great potential to provide not only a well-constrained chronological framework for studying the long-term tectonic, climatic, and biogeochemical history of the region, but also excellent stratigraphic reference sections in the Pacific Ocean.
The 360.65-mcd-thick pelagic sequence recovered at Site 1237 spans the last ~31 m.y. (the Holocene through the early Oligocene) without any detectable stratigraphic breaks. The composite depth section based on the four APC-drilled holes at the site documents complete recovery for the entire sequence, which is dominated by biogenic components with a minor terrigenous (probably eolian) component that decreases downhole.
Calcareous nannofossils and foraminifers are generally abundant or common and well to moderately well preserved throughout the sequence. Diatoms are abundant and well preserved down to ~60 mcd, but abundance decreases and preservation deteriorates below ~69 mcd, and diatoms are absent below ~174 mcd. Most of the standard nannofossil and planktonic foraminiferal zonal markers, as well as some nonstandard nannofossil markers, can be used to establish a relatively detailed biostratigraphy. Diatoms provide additional biostratigraphic control down to ~137 mcd. All fossil groups examined during the cruise provide relatively consistent age assignments (Fig. F17). Shore-based biostratigraphic studies will refine biostratigraphic datums and identify additional events.
The paleomagnetic stratigraphy at Site 1237 includes the clear definition of all chrons and subchrons for the 0- to 5- and 7- to 13-Ma intervals. Some fine-scale features and short polarity subchrons are also apparent within the Matuyama Chron. Although the assignments of chrons and subchrons in the intervals from 5 to 7 Ma and from 13 to 31 Ma are not certain yet, shore-based paleomagnetic studies are expected to provide a detailed magnetostratigraphy for most or all of the sequence.
Based on the preliminary biostratigraphic and magnetostratigraphic age model, the sedimentation rate generally increases from ~1 cm/k.y. prior to ~7 Ma to ~2 cm/k.y. after ~7 Ma (Fig. F18A). Despite the relatively low sedimentation rates, the opportunities for an astronomical calibration of the Neogene timescale are good, as core logging data are characterized by high-frequency fluctuations in lithology. First frequency spectra of GRA density fluctuations clearly identified pronounced cycles at orbital periods of precession and obliquity for the time interval from 3 to 4 Ma (Fig. F19), although bioturbation may have smoothed the primary signal at low sedimentation rates ranging from 1.5 to 2 cm/k.y.
Site 1237 should provide not only a well-constrained chronological framework for studying the long-term history of Andean uplift and continental climate, as well as the evolution of upwelling, sea-surface, and intermediate-water properties in the southeast Pacific and the sequence of biotic events, but will also provide an outstanding stratigraphic reference section for improving integrated biostratigraphic, magnetostratigraphic, and cyclostratigraphic timescales.
This site on Cocos Ridge was triple APC cored, and a complete Pleistocene to upper Miocene sequence (0-360 mcd) that is unaffected by burial diagenesis was recovered. Calcareous nannofossils are abundant and well preserved within the sequence of the last 9 m.y. Planktonic foraminifers are abundant to common and reasonably well preserved. Although rare above ~184 mcd, diatoms are consistently present and show better preservation and higher abundance below this depth. Standard nannofossil, planktonic, foraminiferal, and diatom index markers provide tight biostratigraphic age control from the Pleistocene to latest Miocene (~8 Ma) (Fig. F17). The Pleistocene-lower Pliocene sequence has a sedimentation rate of ~2 cm/k.y. From 3 to 6 Ma, the sedimentation rate increases to ~6 cm/k.y. Below this level (7-9 Ma), the sedimentation rate decreases again to an average rate of ~3 cm/k.y.
Site 1241 offers an excellent potential to derive an orbitally tuned timescale that can be tied to the biostratigraphy in order to establish a late Neogene reference section for the equatorial East Pacific. The new timescale will provide integration and intercalibration of datums from both calcareous and siliceous microfossil groups within a single regional chronological framework. It will also provide a useful link between tropical and subtropical biostratigraphic schemes for the Pacific and between timescales from different oceans (such as the Leg 154 orbitally tuned timescale). The timescale will also serve as the basis for high-resolution studies that aim to reconstruct sea-surface and intermediate-water characteristics in the eastern Pacific, to retrace the history of the closure of the Panamanian seaway, to decipher the interaction between tropical and high-latitude circulation systems, and to delineate the evolution of various groups of marine biota.