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At Sites 1232–1235, located in the Chile Basin and on the Chile margin, late Pleistocene sediments were recovered. Because biostratigraphic resolution is limited in this time frame, estimates of sedimentation rates at these sites rely primarily on a correlation of magnetic susceptibility data to previously dated Holocene sequences (Lamy et al., 2001) and to known magnetic excursions, as well as an analysis of paleomagnetic remanence intensity and secular variations in magnetic directions.

Geomagnetic Field Behavior

Paleomagnetic measurements made during Leg 202 on cores from the Chilean margin Sites 1233, 1234, and 1235 document that these sites may provide the highest-resolution long-term record of paleomagnetic secular variation (PSV) and excursional field behavior ever recovered. They also will provide some of the very first records that document high-resolution paleomagnetic field behavior in the Southern Hemisphere. Shipboard measurements indicate that reproducible records of directional PSV have been recovered from four independent holes at Site 1233 (Fig. F12) over the entire 137 mcd, from the uppermost 60 mcd of three independent holes at Site 1234 (Fig. F13), and from the uppermost 30 mcd of three independent holes at Site 1235. The prospect is good that even better final PSV records from these intervals extending to greater depths at Sites 1234 and 1235 will be developed with shore-based paleomagnetic studies.

Shipboard paleomagnetic measurements also clearly suggest that paleomagnetic field paleointensity variations can be determined from these cores. Initial relative paleointensity estimates determined by normalizing the sediment natural remanent magnetization (NRM) to magnetic susceptibility have been developed for all of Site 1233 and the uppermost 60 mcd of Site 1234. The Site 1233 shipboard relative paleointensity and directional PSV records (Fig. F14) suggest that after shore-based paleomagnetic studies we will be able to develop the longest and highest-resolution "total-vector" PSV record ever observed. A close-up of the total-vector PSV record (Fig. F15) from Site 1233 shows that remarkable cyclicity, on the scale of ~3 m (equivalent to ~2 k.y. based on preliminary shipboard age models), exists in both the directional PSV and paleointensity records for more than 30,000 yr of the late Pleistocene. This total-vector PSV record, when finally developed, should provide important new insights into the working of the Earth's outer-core dynamo, which generates the Earth's magnetic field.

The directional PSV data from Sites 1233 and 1234 also document at least two magnetic field excursions. The younger of these excursions probably occurred around 41,000 yr ago, based on an initial timescale (Figs. F14, F15) determined by correlation of the paleointensity data with other sites around the world, and is almost certainly the Laschamp Excursion (Fig. F16). The PSV record of the Laschamp Excursion at Site 1233 is 2.4 m in width (documented in three separate holes) and probably spans a time interval of <1500 yr. That makes this the highest-resolution paleomagnetic record of an excursion ever recovered. Moreover, this excursion is also recorded at Site 1234, more than 400 km away, and the pattern of more normal directional PSV can be correlated between the two sites (Fig. F16). Further shore-based paleomagnetic studies of these Laschamp Excursion records should provide valuable new insight into the workings of the Earth's magnetic field during times of anomalous behavior that may be related to the geomagnetic field reversal process, as well as magnetostratigraphic information in unprecedented detail that will likely provide reference stratigraphic sections for the region and for the world.

Cenozoic Biostratigraphic and Magnetostratigraphic Reference Sections

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.

Site 1237

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 holes drilled 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 on board the ship provide relatively consistent age assignments (Fig. F17). On a broad scale, sedimentation rate increases from ~10 m/m.y. prior to ~7 Ma to ~20 m/m.y. after ~7 Ma (Fig. F17). This sharp rise in biogenic accumulation rate is of special significance for regional and global paleoceanographic reconstructions.

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, such as "Cobb Mountain" within the Matuyama Chron. The polarity data from the 5- to 7-Ma interval and the lowermost part of the sequence between 13 and 31 Ma appear to be promising, although assignments of chrons and subchrons are not certain yet because of several possible interpretations.

Shore-based biostratigraphic studies will refine biostratigraphic datums and identify additional events and shore-based paleomagnetic studies are expected to provide a detailed magnetostratigraphy for most or all of the sequence. Thus, the long and apparently complete sequence at Site 1237 has excellent potential for developing an orbitally tuned cyclostratigraphy for the last 31 m.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.

Site 1241

This site on the Cocos Ridge was triple-APC cored, and a complete Pleistocene to late Miocene sequence (0–360 mcd) that is unaffected by burial diagenesis was recovered. Calcareous nannofossils are abundant and well preserved throughout the sequence. 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 ~26 m/m.y. In the lower Pliocene–uppermost Miocene section, the sedimentation rate increases to ~65 m/m.y. Below this level, a marked decrease in sedimentation rate occurs at ~6–7 Ma and sediments accumulate at an average rate of ~30 m/m.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 Pleistocene to late Miocene 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 Panama seaway, to decipher the interaction between tropical and high-latitude circulation systems, and to delineate the evolution of various groups of marine biota.

Linear Sedimentation Rates

Except for Site 1232, which is heavily affected by turbidites, the 10 other sites of Leg 202 have yielded excellent biostratigraphies. It is expected that postcruise studies will significantly improve these stratigraphies and establish oxygen isotope stratigraphy and orbitally tuned cyclostratigraphy for most of the sites. Starting from our two stratigraphic reference sites (1237 and 1241), we built a unified stratigraphic framework at all of the Leg 202 sites. Age-depth relationships for Sites 1236–1242 are based principally upon biostratigraphic datums, although paleomagnetic age control points provide greater detail when they are available.

Using our shipboard stratigraphic framework, LSRs for Sites 1236–1241 were estimated using corrected cmcds that account for expansion of cores during recovery. The timing of changes in LSRs is partially controlled by the resolution of the age control points, which will be refined based on postcruise work. To first order, LSR estimates are consistent with regional biogenic fluxes. Sedimentation rates at the Nazca Ridge sites (1236 and 1237) and Cocos Ridge site (1241), all of which lie under low-production oligotrophic regimes, are generally low (<30 m/m.y.), whereas sedimentation rates near the highly productive equator (Sites 1238–1240) and on the Central American margin (Site 1242) are much higher (typically 80–140 m/m.y.). Within this basic theme, variations in sedimentation rates through time at the sites respond to both long-term tectonic drift of the sites relative to the continental margin and to variations in the local environment that drive changes in both the production and preservation of biogenic sediment components.

Nazca Ridge (Sites 1236 and 1237)

Sites 1236 and 1237, currently located near the outer edges of the subtropical gyre and eastern boundary Peru upwelling system, respectively, exhibit relatively low LSRs (<2 cm/k.y.) throughout their records (Figs. F18, F19). Elevated LSRs between 20 and 15 Ma at Site 1236 likely result primarily from an increase in gravity-driven transport to the site rather than from an increase in productivity or preservation. Variations in LSR at these sites are relatively small until an increase at ~7.5 Ma. LSRs decrease at Site 1236 from ~4 Ma to present, whereas sedimentation rates at Site 1237 experience a low only between 2 and 1 Ma. The LSRs from both sites, but especially from Site 1237, are relatively high in the interval from ~7 to ~2 Ma, roughly similar to trends previously observed at Site 848, Leg 138 (Pisias et al., 1995). Tectonic backtrack paths indicate that Sites 1236 and 1237 were located initially within the low-productivity subtropical gyre and have been progressively approaching the eastern boundary Peru Current system where upwelling drives higher production and greater biogenic rain to the seafloor. Although tectonic drift can account for the general increase in LSR at younger ages at these two sites, it cannot account for the peaks and valleys in regional sedimentation, which likely reflect regional or global climate (and biogeochemical) changes.

Carnegie Ridge (Sites 1238 and 1239)

Sites 1238 and 1239 are currently located just south of the equator on the easternmost flanks of Carnegie Ridge. These sites backtrack through time to the west (based on their origin at the Galapagos hotspot), so they have been near the equatorial upwelling throughout their history (Fig. F6).

Although Site 1239 contains an ~8-m.y. hiatus between 14.5 and 6.5 Ma, both sites exhibit similar LSR trends with an initial increase from baseline LSRs at ~6.5 Ma and a further increase ~4 Ma, followed by a decrease after ~2.5 Ma. Site 1239, closer to the equator, exhibits LSR values that are consistently higher than those of Site 1238, with the exception of the hiatus and the youngest part of the record. The increase in sedimentation rates younger than ~6.5 Ma at Sites 1238 and 1239 is roughly similar to changes observed at Site 846, Leg 138 (Pisias et al., 1995), with a broad high in sedimentation rates from ~6.5 to ~2.5 Ma.

Panama Basin and Cocos Rise (Sites 1240, 1241, and 1242)

Site 1240 has relatively high sedimentation rates of ~8–13 cm/k.y., consistent with its position directly under the productive equatorial upwelling system. Although the record is short (~3 Ma basal age), a reduction in the sedimentation rate may have occurred within the past ~2 m.y. Site 1242, on the Costa Rica margin, also displays high sedimentation rates of ~6–14 cm/k.y., but the LSR here appears to increase near ~2 Ma. It remains unclear whether these different sedimentation rate histories at Sites 1240 and 1242 are related and oceanographically meaningful or whether sedimentation rate changes at the two sites simply reflect random variations in tectonics and sedimentation.

Modern and late Pleistocene sedimentation rates at Site 1241, from the Cocos Ridge, are relatively low, similar to those at Sites 1236 and 1237. In the past, however, sedimentation rates increased at Site 1241 to values >5 cm/k.y., which is consistent with the site's tectonic backtrack path toward the equator and the Galapagos hotspot prior to ~6 Ma.

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