Site 1237 (proposed Site NAZCA-17A) is located on a relatively flat bench on the easternmost flank of Nazca Ridge, ~140 km off the coast of Peru (Fig. F1). Nazca Ridge, a fossil hotspot track with its modern expression at Easter Island, terminates just outboard of the Peru-Chile Trench, where it is deformed and subducted beneath Peru. Eastern Nazca Ridge is covered by a thick drape of pelagic sediment to its shallowest reaches. The seismic record reveals well-stratified reflective layers, which clearly drape the underlying bathymetry from the sediment surface to acoustic basement (Fig. F48). Prior to drilling, basement ages were expected to be between 40 and 45 Ma, based upon a hotspot model and magnetic anomalies of the surrounding oceanic crust. The oldest material recovered at the site was ~31.5 Ma.
The tectonic backtrack path on the Nazca plate has moved Site 1237 about 20° westward and 7° southward relative to South America over the past 32 m.y. (Fig. F45). Considering the thermal subsidence over this time span, the site may have been within a few hundred meters of sea level in its early history.
Today, Site 1237 is situated near the eastern edge of the northward-flowing Peru-Chile Current, a major conduit of cool-water transport from high to low latitudes. The site's position near the productive upwelling system of Peru suggests that it may record changes in upwelling and biological production during the past few million years. The site underlies the path of eolian dust that originates in the Atacama Desert of Chile, as indicated by the spatial quartz distribution in the southeast Pacific that reflects the pattern of the southeast trade winds. Given the tectonic backtrack to the west toward the center of the oligotrophic gyre and assuming that there were no compensating changes in the environment, we expect that accumulation rates of both biogenic and eolian sediments at Site 1237 were smaller in the past and increased with time.
The modern water depth of Site 1237 reflects the transition zone between the relatively oxygen-rich (nutrient poor) remnants of CPDW, which enter the Peru Basin as bottom water through the Peru-Chile Trench, and the relatively oxygen-poor (nutrient rich) PCW. During its early history, the site may have occupied depths of the modern PCW or its paleoequivalent.
The primary objective at Site 1237 was to provide a continuous and complete sedimentary sequence of the late Cenozoic to
The 432-nmi voyage from Site 1236 to Site 1237 was accomplished in 39.0 hr at an average speed of 11.1 kt. At 0340 hr on 26 April, the vessel left Chilean waters and entered the territorial waters of Peru.
Four APC holes were drilled. While drilling, permission was granted from ODP/TAMU headquarters to deepen the hole by APC coring as much as 30 m beyond the approved maximum depth of 300 m. Hole 1237A recovered a full core barrel, but the mudline was not recovered, and so the hole was terminated. Hole 1237B recovered the mudline, and piston coring advanced until the wireline parted at the rope socket while attempting to recover Core 202-1237B-13H (110.0–119.5 mbsf). Two extra wireline trips were required to recover the Tensor tool, sinker bars, and barrel containing Core 202-1237B-13H, respectively. APC coring resumed and deepened the hole to refusal at 317.4 mbsf (Core 202-1237B-34H). This represented the deepest penetration with the APC since Leg 162. To our knowledge, only four other piston-cored holes in ODP have exceeded a penetration of 300 m.
The vessel moved 10 m west before Hole 1237C was initiated. The hole was deepened to 315.3 mbsf, where Core 202-1237C-33H failed to achieve a full stroke. After the vessel moved 10 m west, Hole 1237D was initiated with the objective to spot core four intervals to cover coring gaps left in the previous holes.
Cores were oriented starting with the third core in each hole. A total of 15 core barrels in Hole 1237B and 16 core barrels in Hole 1237C were drilled over when a force of 60 kilopounds could not retrieve the cores. The nonmagnetic core barrel was deployed on even-numbered cores up to and including 202-1237B-20H, on odd-numbered cores up to and including 202-1237C-17H, and on Cores 202-1237D-2H, 4H, 6H, and 8H. The nonmagnetic core barrel was not used below these intervals to prevent possible damage to hardware as a consequence of the drilling-over process. One bottom-water and nine sediment temperature measurements were taken with the APCT in Holes 1237B and 1237C, which indicated a thermal gradient of ~33°C/km.
The 360.65-mcd-thick pelagic sequence recovered at this site spans the interval from the early Oligocene to the Holocene (0 to ~31.5 Ma) without any detectable stratigraphic breaks (Fig. F49). The composite depth section documents complete recovery for the entire sequence. The hole-to-hole correlation was based on high-resolution core logging data, including magnetic susceptibility, reflectance, natural gamma radiation, and GRA bulk density.
The upper 100 mcd of the sequence reflects pelagic sedimentation with a minor terrigenous (probably eolian) component that decreases downhole. Such a gradual trend, likely a consequence of tectonic drift of the oceanic plate toward the continent, is recorded by a gradual transition from nannofossil-bearing (silty) clay to nannofossil ooze. At greater depths, the sedimentary sequence is dominated by nannofossil ooze. In the interval >150 mcd, calcium carbonate contents are high (>90 wt%), organic carbon values are low (< 0.3 wt%), and sedimentation rates are ~10 m/m.y., typical for oligotrophic settings far from the continents. Micrite increases at depths >148 mcd.
A marked color change from more greenish younger sediment to reddish older sediment containing goethite and hematite is present at ~164 mcd (~9 Ma) (Fig. F50). This oxide component is carried in eolian dust transported to the site via the southeast trade winds from arid regions of South America. The abrupt green to red color change at ~9 Ma appears to result from diagenetic reduction processes rather than a change in the dust source area because the reddish color signal persists into the Pleistocene section at Site 1236, located ~500 km southwest of Site 1237. At ages of <8 Ma, both siliciclastic and biogenic accumulation rates increase at Site 1237, suggesting that eastward drift continues to enhance the eolian dust component, even though the reddish color is gone.
At approximately the same time as the color change, discrete ash layers and dispersed ash begin to appear in the sediment. Fifty-five ash layers were deposited during the last 9 m.y. This transition near 9 Ma may record the onset of intense volcanism and accompanying tectonic uplift of the Andes. If so, the topographic barrier of an early mountain range would have enhanced the steering of westerly and trade winds along the coast, resulting in stronger meridional flow, enhanced eolian transport, coastal upwelling, and biogenic productivity within the eastern boundary current.
The enhanced rain of organic carbon that follows from high productivity would also support the hypothesis of a diagenetic boundary that reduced the reddish oxides. High production continued and expanded into a Miocene to early Pliocene biogenic bloom (~8 to ~4 Ma). A similar but shorter interval of rapid accumulation was found at Site 1236, as well as in the equatorial Pacific (6.7–4.5 Ma). A variety of previous hypotheses have linked the rise and fall of this event to changes in global nutrients associated with erosion and chemical weathering of the rising Tibetan Plateau, reorganizations in deep ocean circulation, and closure of the Isthmus of Panama. Although we are not yet able to identify a unique cause of the biogenic bloom, our shipboard results help to pose new hypotheses by tracing the event into the eastern boundary current of the southeast Pacific and by providing detailed chronological controls based on biostratigraphy and magnetostratigraphy that reveal structure in the bloom event and suggest an early onset in the South Pacific.
Carbonate concentrations decrease from ~90 to ~10 wt% in the interval <4.5 Ma, and Neogene carbonate accumulation rates at Site 1237 reach their lowest level during the last 2 m.y. (0–41 mcd). The Pliocene–Pleistocene interval contains significant amounts of both terrigenous material and siliceous microfossils with various contributions from calcareous microfossils. Siliciclastic accumulation rates increased again at ~3 Ma and may reflect an enhanced eolian supply to Site 1237. This increase was apparently paralleled by the intensification of the Northern Hemisphere glaciation, which was associated with enhanced subtropical aridification and intensified trade wind circulation, partly due to a steeper pole-to-equator temperature gradient.
Calcareous nannofossil and planktonic foraminifer biostratigraphies indicate that a complete upper Pleistocene to upper Oligocene succession was recovered. Most of the standard nannofossil and planktonic foraminiferal zonal markers as well as some nonstandard nannofossil markers can be used to reconstruct the biostratigraphic succession. Calcareous nannofossils at the base of the hole suggest an age younger than 31.5 Ma. Diatoms provide additional biostratigraphic control down to ~136.7 mcd.
Calcareous nannofossils and foraminifers are generally abundant or common and well to moderately well preserved throughout. 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.
Marked changes in the relative proportions of benthic foraminiferal species within the Pleistocene–upper Pliocene assemblage indicate variations in carbon fluxes at the seafloor that are probably related to temporal and spatial fluctuations of the coastal upwelling system and/or shifts in subsurface water masses. In contrast, the lower Pliocene to upper Oligocene benthic foraminiferal assemblage characterizes an oligotrophic, pelagic environment. The diatom assemblage down to 59 mcd is typical of a coastal upwelling zone on a continental margin. Below this depth, typical upwelling forms are present only occasionally with some oceanic forms.
The paleomagnetic stratigraphy at Site 1237 is excellent, with clear definitions of all chrons and subchrons over the last 5 m.y. and from 7 to 13 Ma. Even fine-scale features and short polarity subchrons are hinted at, like the "Cobb Mountain" within the Matuyama Chron. The polarity assignments for the interval from 5 to 7 Ma and for the lowermost part of the sequence between 13 and 31 Ma are still preliminary because the polarity sequence allows several possible interpretations. The age interpretations are consistent with the biostratigraphy where clear geomagnetic correlation to the geomagnetic polarity timescale is observed. This site offers an excellent potential to derive an orbitally tuned timescale that can be tied to the magnetostratigraphy and biostratigraphy to provide a reference section for the South Pacific.
Chemical gradients in the uppermost sediments reflect a minor influence of organic matter diagenesis, a limited degree of biogenic opal dissolution, and a minor signature of biogenic calcite diagenesis. Organic matter diagenesis, driven by microbially mediated oxidation reactions, has a relatively minor influence on interstitial water composition. Sulfate undergoes a limited degree of reduction, primarily in the uppermost sediments, by no more than 4 mM from typical seawater values of ~29 mM. Alkalinity has a shallow maximum of >5 mM and declines with depth to ~2.2 mM. Phosphate concentrations are ~9 µM in the shallowest sediments and decline to <2 µM. Ammonium concentrations have peak values of >250 µM. The shallow and relatively minor maxima in alkalinity, phosphate, and ammonium, along with minor sulfate reduction, result from degradation of organic matter in the upper ~70 mcd. Dissolved silicate concentrations have highest values in the depth range where diatoms are reported to have the best preservation and abundance, averaging ~800 µM in the upper ~70 mcd and declining to an average of 325 µM in the deeper interval barren of diatoms. Authigenic mineral precipitation reactions have a limited influence on interstitial water geochemistry at this site, with no indications of calcite precipitation or dolomite formation in the calcium and magnesium profiles. Strontium concentrations increase to a maximum of >200 µM, with the profile similar in character, although with a smaller maximum value, to those in calcium carbonate rich equatorial sites driven by biogenic calcite recrystallization.
Site 1237 clearly surpassed our shipboard objectives by providing a complete upper Paleogene to Neogene sequence relatively unmodified by burial diagenesis and fully recovered with the APC. Good preservation of calcareous microfossils extending to the base of the Oligocene and the tight framework of biostratigraphic and magnetostratigraphic age controls will provide an excellent base for a broad spectrum of detailed studies that aim to reconstruct the long-term history of Andean uplift and continental climate as well as the evolution of upwelling, biota, biogeochemistry, and sea-surface and intermediate-water characteristics in the southeast Pacific.