Preliminary age models were created using shipboard biostratigraphic and paleomagnetic data (Fig. F17) to aid initial interpretation of the recovered sediment sequences and sampling of the cores. It is expected that postcruise studies will significantly improve the models by establishing oxygen isotope stratigraphy and orbitally tuned cyclostratigraphy, as planktonic and benthic foraminifers are abundant enough throughout the Holocene to Oligocene sequences and most sites are marked by cyclic changes in lithology.
At Chile margin Sites 1232-1235, where we recovered high-resolution Pleistocene records <0.26 Ma, the common stratigraphic approach provided no additional age control points to refine the age models. Sites 1233-1235, however, revealed unprecedented high-resolution records of pronounced centennial- to millennial-scale variability in paleomagnetic intensities. Here, future studies are expected to strike a new path in establishing a high-resolution late Pleistocene stratigraphy, as centennial- to millennial-scale fluctuations in paleomagnetic secular variation and intensity are global in extent and on such scales may provide a more powerful tool than stable isotope stratigraphy. Following this approach resulted in an improved preliminary shipboard age model for the upper Pleistocene sequence at Site 1233.
Using our shipboard stratigraphic framework (Fig. F17), LSRs and MARs were computed as described in "Age Models and Mass Accumulation Rates" in the "Explanatory Notes" chapter and in the "Age Model and Mass Accumulation Rates" sections of the site chapters and as summarized in Figures F18 and F19. The late Cenozoic variability in LSRs and MARs at Leg 202 sites responds to both long-term tectonic drift of the sites relative to the continental margin and to variations in local environments that drive changes in production and preservation of biogenic sediment components and the supply of terrigenous sediments.
At Sites 1232-1235, located in the Chile Basin and on the Chile margin, upper Pleistocene sediments that accumulated rapidly and at extremely high rates were recovered. At Chile Basin Site 1232, paleomagnetic and biostratigraphic age control suggests a basal age of >0.46 and <0.78 Ma for the turbidite-dominated sediment sequence. A conservative estimate of the LSR is, therefore, ~475 m/m.y. As a first approximation, the average recurrence time of turbidites would be on the order of hundreds of years, possibly triggered by large earthquakes and instabilities at the continental slope associated with climate-induced century- to millenial-scale fluctuations in sediment supply from the continent.
The almost turbidite-free hemipelagic sediment sequences recovered from the Chile margin Sites 1233-1235 are all younger than 0.26 Ma, and the average LSR is estimated to be 82 cm/k.y. at Site 1233, ~79 cm/k.y. at Site 1234, and ~70 cm/k.y. at Site 1235. A more detailed age model was developed at Site 1233. Here, good correlations to a nearby 14C-dated Holocene sequence (Lamy et al., 2001) and to a previously dated magnetic paleointensity record (Stoner et al., 2002) as well as the presence of the well-dated Laschamp Excursion justified the transfer of additional age control points (for details, see "Age Model and Mass Accumulation Rates" in the "Site 1233" chapter). This preliminary shipboard model suggests a basal age of ~0.17 Ma for the sediment sequence at site 1233. The late Pleistocene to Holocene LSRs and MARs range from 40 to 160 cm/k.y. and from 40 to 150 g/cm2/k.y., respectively (Fig. 18C).
The input of siliciclastic material accounts in general for >90% of the total sediment deposition at Site 1233 and largely reflects the predominantly fluvial supply of terrigenous material from the Andes and the Coastal Range province (see "Lithostratigraphy" in the "Site 1233" chapter). A pronounced maximum in MARs between 50 and 10 ka (up to ~150 g/cm2/k.y.) (Fig. F18C) suggests enhanced terrigenous sediment discharge during marine isotope Stages 3 and 2, possibly associated with temporal changes in continental rainfall patterns and/or refocusing of fluvial sediment discharge during the glacial sea level lowstand. Distinctly lower rates mark the late Pleistocene interval from 50 to 170 ka (40 g/cm2/k.y.) and the Holocene (~60 g/cm2/k.y.). We cannot exclude the possibility that the tripling of noncarbonate MARs during MISs 2 and 3 is a random event, perhaps driven by a newly formed channel that favored a higher sediment flow to Site 1233.
The MARs of carbonate and organic carbon range from 1 to 9 g/cm2/k.y. and from 0.3 to 1.2 g/cm2/k.y., respectively (see "Age Model and Mass Accumulation Rates" in the "Site 1233" chapter). Such high rates clearly reflect the influence of the high-productivity region off southern Chile. Both carbonate and organic MARs suggest a maximum in biogenic productivity for the LGM and, although slightly reduced, a high level for the Holocene.
The sediment sequences recovered at Nazca Ridge span the entire Neogene and the Oligocene to ~28 Ma at Site 1236 and to ~31.5 Ma at Site 1237 without any detectable stratigraphic breaks (Fig. F17). The age models are based on a framework of paleomagnetic datums and biostratigraphy (for details, see "Age Model and Mass Accumulation Rates" in the "Site 1236" chapter and "Age Model and Mass Accumulation Rates" in the "Site 1237" chapter). Sites 1236 and 1237, currently located near the outer edges of the oligotrophic subtropical gyre and the nutrient-rich eastern boundary Peru upwelling system, respectively, exhibit relatively low LSRs (0.4-2.4 cm/k.y.) and MARs (<2.5 g/cm2/k.y.) throughout their late Cenozoic history (Fig. F18A).
Carbonate MARs and total MARs are nearly identical at Site 1236, as carbonate concentrations are >90%. Slightly elevated carbonate MARs between 25 and 12 Ma (Fig. F20A) resulted from gravity-driven transport of predominantly calcareous neritic material to the site rather than from an increase in productivity or preservation, as Site 1236 was located in the center of the oligotrophic subtropical gyre well above the lysocline during its early history.
A major characteristic of both sites is the simultaneous increase in carbonate MAR at ~9 Ma that culminated in a brief maximum at ~7 to 5 Ma followed by a decrease into the Pleistocene (Fig. F20A). Given no detectable changes in the preservation of calcareous nannofossils, the late Miocene to early Pliocene maximum suggests an interval of enhanced surface productivity. A similar increase and maximum in carbonate MAR is also recognized at equatorial East Pacific sites from Leg 138 (Fig. F20B) and is often referred to as the Miocene to early Pliocene biogenic bloom (Farrell et al., 1995b). Sites 1236 and 1237 trace this event for the first time into the southern East Pacific. At equatorial Sites 849-851 (Leg 138), this event was also accompanied by a pronounced maximum in biogenic opal MAR (Fig. F21), which is not observed at Nazca Ridge. Instead, biogenic opal MARs at Site 1237 dramatically increased over the last ~6 m.y. from 0.02 to 0.8 g/cm2/k.y. (Fig. F21B). Organic carbon MARs at Site 1237 strongly increased over the last 4 m.y., when organic carbon contents increased from the detection limit of 0.1 wt% (using shipboard techniques) to 1.9 wt%.
The long-term Pliocene to Holocene increase in both organic carbon and biogenic opal MAR is considered to be associated with the progressive paleodrift of the site toward the eastern boundary upwelling system (Fig. F6), as farther westward at Site 1236 diatoms are almost absent and organic carbon contents are <0.1 wt%. The paleodrift of both sites and the modern regional changes in surface nutrients and productivity along their track would predict a slight and continuous increase in productivity over the last ~30 Ma at Site 1236 and a fourfold increase at Site 1237 (Figs. F22, F23). Although this cannot explain the late Miocene to early Pliocene maximum in productivity, the predicted increase in nutrients, especially in silicate, at ages <7 Ma may serve as an explanation for the observed increase in biogenic opal MARs at Site 1237 (Fig. F21B).
The nonbiogenic MAR at Site 1236 is the lowest of the Leg 202 sites (<0.05 g/cm2/k.y.) (Fig. F20A) and reflects a minimal amount of eolian siliciclastics (biogenic opal and organic carbon contents are negligible). This is consistent with the site's position far away from the Atacama Desert, the major dust source for this area. At Site 1237, the Oligocene and early Miocene siliciclastic MARs are as low as at Site 1236 but slightly increase at ~13 Ma to a higher level of ~0.1 g/cm2/k.y. and then increase more rapidly from ~9 Ma to a late Pleistocene/Holocene maximum of 1 g/cm2/k.y. The enhanced supply of eolian siliciclastics since ~13 Ma may partly result from the eastward migration of Nazca Ridge, which moved Site 1237 closer to the dust source. Although tectonic drift can account for the general increase, it cannot account for short-term changes in dust flux, which likely reflect regional changes in aridification, wind strength, and wind direction.
The sediment sequences recovered from Carnegie Ridge range in age from the Holocene to ~11 Ma (Site 1238) and to ~15 Ma (Site 1239). The construction of the age models relies primarily on calcareous microfossil datums and suggests an upper Miocene hiatus at Carnegie Ridge (Fig. F17) encompassing the interval from ~13.5-8 Ma at Site 1239 and extending from the base of Site 1238 to ~8 Ma (for details, see "Age Model and Mass Accumulation Rates" in the "Site 1238" chapter and "Age Model and Mass Accumulation Rates" in the "Site 1239" chapter)
Sites 1238 and 1239 are currently located just south of the equator on the easternmost flanks of Carnegie Ridge. The tectonic backtrack path moves the sites to the west almost parallel to the equator (Fig. F6), and thus both sites remained within the highly productive equatorial upwelling zone throughout their history. Accordingly, Sites 1238 and 1239 are marked by relatively high Neogene LSRs (up to 10 cm/k.y.) (Fig. F18A). Site 1239, closer to the equator and to the continent and at shallower depths, exhibits MARs (2-8 g/cm2/k.y.) that are higher than those at Site 1238 (1-5 g/cm2/k.y.) except in the interval 0-1 Ma.
Both sites are characterized by similar trends in carbonate and noncarbonate MARs with distinctly higher values at Site 1239 (Fig. F20A). Rates of carbonate deposition gradually increased from the end of the hiatus (~8 Ma), peaked at 6-3 Ma, and then progressively declined over the last 3 m.y. The maximum in carbonate MARs occurred ~1 m.y. later and persisted ~2 m.y. longer than that observed between 7 and 5 Ma at Sites 1236, 1237, and 1241 and at most of the Leg 138 sites (Fig. F20). This temporal difference may partly result from the paleodrift of Sites 1238 and 1239. The modern pattern of local changes in sea-surface characteristics along the track of Site 1239 would predict a maximum in biogenic productivity from 6 to 3 Ma terminated by the eastward drift of the site out of the highly productive equatorial cold tongue (Fig. F24). The drift-related trends for Site 1238 would predict a generally higher productivity than at Site 1239, thus higher carbonate MARs, and a mid-Pleistocene productivity maximum (Fig. F25), which is in conflict with both absolute values and major trends in carbonate MARs. Hence, regional and temporal oceanographic changes and nutrient budgets may have contributed to the observed variations in carbonate MARs at Carnegie Ridge but cannot explain the full pattern. An additional explanation for the relatively young peaks in carbonate MARs at Sites 1238 and 1239 is their relatively shallow water depth, compared to the Leg 138 sites, which suggests progressive development of corrosive bottom waters through Neogene time.
The noncarbonate MARs at Carnegie Ridge exhibit a broad maximum between 6 and 1 Ma, with highest values between 4 and 3 Ma (Fig. F20A). For this interval, the results from Leg 138 (Fig. F20B) suggest an interesting eastward shift (~4.4 Ma) in the locus of maximum noncarbonate and biogenic opal MARs along the equatorial high-productivity belt from 110°W (Sites 849 and 850) into the Galapagos region (Site 846). At Sites 849 and 850, maxima in both noncarbonate and biogenic opal MARs correspond to the interval of the biogenic bloom between ~7 and 5 Ma. Further eastward and closer to Sites 1238 and 1239, maxima in noncarbonate and biogenic opal MARs occurred between 3 and 1.5 Ma, with a distinct peak at ~1.9 Ma (Farrell et al., 1995b). The maximum in noncarbonate MAR at the easternmost Sites 1238 and 1239, however, occurred in the early Pliocene interval just between the maxima observed at the Leg 138 sites. Whether the trends in noncarbonate MARs at Carnegie Ridge can be ascribed to biogenic opal or terrigenous siliciclastic MARs is difficult to assess without any precise biogenic opal contents. Careful inspection of smear slide data suggests a strong variability in biogenic opal contents on shorter timescales. A distinct trend is only recognized at Site 1238, suggesting a phase of enhanced biogenic opal deposition between 1 and 3.5 Ma, which would closely match the maximum in biogenic opal MARs observed in the Galapagos region (Site 846). An exceptionally strong minimum in grain density data at both sites consistently suggests a short-term interval of enhanced biogenic opal deposition at ~1.9 Ma. Although the documentation of this event will require finer-resolution age control, such a maximum was also recognized in the Galapagos region (Site 846) and the Panama Basin (Site 1240).
At present, long-term variations in siliciclastic MARs at Carnegie Ridge cannot be assessed, except for the younger part of the record. The significant late Pliocene to Holocene increase in magnetic susceptibility may indicate that the supply of siliciclastics dominated the noncarbonate MARs at Carnegie Ridge during that interval.
At Panama Basin Site 1240, an extended sequence of upper Pliocene (~2.8 Ma) to Holocene sediments was recovered. Age control was excellent and relied primarily upon magnetostratigraphy and calcareous nannofossil datums (Fig. F17B). The site remained below the highly productive equatorial upwelling zone during its entire history. Accordingly, the sediment deposition is largely controlled by variations in biogenic opal and carbonate accumulation and reflects high LSRs and MARs that range between 7 and 16 cm/k.y. and between 4 and 8 g/m2/k.y. (Fig. F18B). Most remarkable is a pronounced maximum in carbonate and noncarbonate MARs at the Pliocene/Pleistocene boundary between 2 and 1.6 Ma (Fig. F20B). The maximum in noncarbonate MARs largely results from high contents in organic carbon (as much as 3 wt% and biogenic opal as indicated by smear slide data, high porosities, and low grain densities (see "Lithostratigraphy" in the "Site 1240" chapter). This interval of enhanced biogenic productivity was also recognized at Carnegie Ridge (Sites 1238 and 1239) and in the Galapagos region (Site 846) although less pronounced and dominated by biogenic opal deposition. This event seems to be restricted to the easternmost equatorial Pacific, as it is not documented farther westward at sites from Leg 138.
Sites 1241 and 1242 at Cocos Ridge recovered continuous sediment sequences that reach back into the middle Miocene (11-12 Ma) and the Pliocene (~2.5 Ma), respectively. The construction of the age model was primarily based on calcareous nannofossil datums (Fig. F17). Magnetic data were useful back to ~1 Ma at Site 1241.
At Site 1241, the Miocene to Pliocene LSRs are relatively high (3-6 cm/k.y.) and decrease in the Pliocene interval to <3 cm/k.y. (Fig. F18A). This general trend is consistent with the northeastward paleodrift of the site away from the highly productive equatorial upwelling zone (Fig. F6). The modern pattern of local changes in productivity along the track of Site 1241 would predict high rates in biogenic productivity from ~11-10 Ma that rapidly drop between 10 and 7 Ma to a relatively low level in productivity at ages <7 Ma (Fig. F26). This trend is mainly reflected by the noncarbonate MAR but not by the carbonate MAR (Fig. F20A). The oldest interval from 11 to 9 Ma is marked by the relatively high concentrations of organic carbon and biogenic opal (see "Biostratigraphy" and "Geochemistry" both in the "Site 1241" chapter) suggesting that the maximum in noncarbonate MARs during this period reflects high biogenic productivity when the site was near the equator. Benthic and planktonic foraminiferal tests appear strongly affected by carbonate dissolution over this sequence, which corresponds to a well-known interval of poor carbonate preservation in the equatorial east Pacific and in the Caribbean, the "Miocene carbonate crash" (Lyle et al., 1995).
A major characteristic of the carbonate MAR is the pronounced maximum at 7-5 Ma (Fig. F20A) that is mirrored by the noncarbonate MAR and accompanied by increased deposition of biogenic opal from 6 to 7 Ma (see "Lithostratigraphy" in the "Site 1241" chapter). This maximum in productivity is not predicted by the tectonic backtrack but is a widespread feature that has been documented in sediment records from the eastern equatorial Pacific (Leg 138; Farrell et al., 1995b) (Fig. F20B) and in the eastern South Pacific at Sites 1236 and 1237 (Fig. F20A).
The Pliocene and Pleistocene interval is marked by a continuous decrease in total and carbonate MARs, with the lowest rates in the upper Pleistocene to Holocene interval. This trend is not shown at Site 1242, where all rates exhibit a pronounced and broad maximum between 2.5 and 0.5 Ma, with peak values centered at 1.6-1.2 Ma. Noncarbonate MARs are higher than carbonate MARs by a factor of three. Distinct minima in grain density values and in siliciclastic contents at ~1.4-1.6 Ma (see "Lithostratigraphy" in the "Site 1242" chapter) suggest that the maximum in noncarbonate MAR may result primarily from increased biogenic opal contents rather than from a maximum in terrigenous sediment supply. The inferred maximum in biogenic MAR may provide evidence for variations in upwelling and biological production on the Costa Rica margin.