Site 1236 (proposed Site NAZCA-10A) is located atop Nazca Ridge, a fossil hotspot track with its modern expression at Easter Island (Fig. F1). Based upon a fixed hotspot model and on magnetic anomalies of the surrounding oceanic crust, basement ages are expected to be 30–35 Ma. The ~20-km-wide plateau occupied by Site 1236 is punctuated by small volcanic spires a few hundred meters high that represent eroded remnants of an archipelago of volcanic islands. The plateau and smaller seamounts formed as the site moved eastward over the hotspot. This anomalously shallow section of Nazca Ridge occurs where the Nazca Fracture Zone, a zone of crustal weakness, intersects the hotspot track. About 20 km to the southwest of Site 1236, a flat-topped guyot, which is presumably younger than the plateau surface, rises to within 350 m of present sea level. The existence of this relatively well preserved shallow bathymetric feature adjacent to the deeper, more eroded plateau surface suggests multiple episodes of volcanism in the region.
A tectonic backtrack path on the Nazca plate moved Site 1236 ~20° westward toward the center of the subtropical gyre by 30 Ma (Fig. F45). Thermal subsidence would predict that the site was at shallower water depths during the early Miocene and perhaps above sea level very early in its history.
The total thickness of sediment at Site 1236 is ~200 m. Seismic data reveal a well-stratified pelagic sequence with strong to moderately reflective layers from the sediment surface to ~120 mbsf (Fig. F46). At depths from ~120 to ~200 mbsf, deformed but almost continuous layers of moderate reflection appear to smooth the relief of the acoustic basement.
Today, Site 1236 is situated near the western edge of the northward-flowing Peru-Chile Current in an oligotrophic region of the subtropical gyre and at depths suitable for monitoring intermediate-water chemistry in the open ocean. Given its tectonic backtrack to the west, toward the center of the gyre, we expected relatively low biogenic sedimentation rates (<10 m/m.y.) and minor amounts of terrigenous sediment, except for volcanics and possible shallow-water sediment early in its history.
The primary objectives at Site 1236 are to provide a continuous sequence of Neogene and Quaternary sediments to
Three APC holes were cored at Site 1236, to a maximum depth of 173.1 mbsf (189.7 mcd) in Hole 1236A. Hole 1236A was advanced with the XCB to a total depth of 207.7 mbsf (223.2 mcd), where basement was reached. Hole 1236B was piston cored to 122.8 mbsf, where mechanical problems were encountered and a pipe trip was required to recover a broken float-valve system. In Hole 1236C, 13 more APC cores were recovered (to a refusal depth of 167.3 mbsf) to span coring gaps that remained from the previous two holes. A total of six downhole and one bottom-water temperature measurements indicated a normal thermal gradient for sediment overlying oceanic crust (38°C/km). Cores were oriented starting with either the third or fourth APC core in each hole. The nonmagnetic core barrel assembly was deployed on every other core in each APC-cored interval, in even-numbered Cores 202-1236A, 2H to 20H; odd-numbered Cores 202-1236B-1H to 13H; and even-numbered Cores 202-1236C-2H to 18H.
A 223.2-m-thick sedimentary sequence at Site 1236 spans the entire Neogene and the late Oligocene to ~28 Ma (Fig. F47). A composite section was constructed on the basis of high-resolution core logging data (reflectance and magnetic susceptibility), and complete recovery was documented to ~129.7 m (~17–18 Ma). Color reflectance (a*) was the most useful stratigraphic tool for correlation at this site.
Pelagic sedimentation is prevalent at Site 1236, but the impact of volcanic evolution of Nazca Ridge during the late Oligocene is also clearly represented. An early interval of pelagic sedimentation at relatively shallow water depths (hundreds of meters) probably indicates subsidence of the young volcanic plateau prior to 28 Ma. Glassy volcaniclastic sediment, likely air fall ash, suggests a late stage of volcanic activity near 28 Ma. Nevertheless, we cannot exclude the possibility that the volcanic layers may originate from the erosion of older topographic highs. Well-preserved guyots near Site 1236 rising nearly a kilometer above the main plateau of Nazca Ridge may also represent the remnants of an active volcanic phase that followed primary ridge construction. Shipboard stratigraphy suggests an age of this volcanic interval coeval with the well-known tectonic reorganization that created the Nazca plate ~28 m.y. ago. Volcanic sediments were deposited at the earliest stage, and nonpelagic calcareous (carbonate platform) materials were transported to Site 1236 until the middle Miocene (~17 Ma).
Pelagic sediments at Site 1236 are dominated by calcium carbonate, with an average of 95 wt%. Some isolated minima in weight percent CaCO3 correspond to discrete volcanic ash layers and zones of slightly increased siliciclastic contents. The lowest CaCO3 contents are visible at the base of the record where the sediments contain a significant siliciclastic and volcaniclastic component in addition to authigenic glauconite.
Four major lithologic units are defined at Site 1236. Unit I corresponds to the last 17 m.y. and contains primarily pelagic nannofossil ooze. Two isolated layers at depths of ~65 and ~98 mcd consist dominantly of containing neritic grains and represent a waning supply of gravity flows from a nearby carbonate platform to Site 1236 in the late Miocene. A minor siliciclastic fraction corresponding to increased magnetic susceptibilities in the uppermost ~40 mcd may represent eolian material transported from South America during the Pleistocene and Pliocene. Presence of goethite and hematite, detected via reflectance spectrometry, also suggests that an eolian component, originating probably on the South American continent, reached the site via the southeast trade winds.
Unit II (~17 to ~24 Ma) consists mainly of carbonates characterized by high GRA bulk density and reflectance (L* lightness) as well as low magnetic susceptibility. These sediments are comprised primarily of nannofossil ooze (Subunit IIB) and unlithified calcareous mudstone (Subunit IIB), consistent with the presence of a pelagic sedimentary environment at Site 1236 since the late Oligocene. Frequent intercalation of well-sorted nonpelagic calcareous sediments (neritic grains and micrite), which sometimes fine upward, points to gravity current supply of detritus from nearby carbonate platforms. Micrite, which becomes dominant in Subunit IIB, reflects in situ recrystallization of nannofossils.
Allochthonous coarse-grained calcareous sediments in Unit III, including recrystallized larger benthic foraminifers, bryozoan fragments, and some remains and encrustations of red algae, indicate proximity to a carbonate platform. The presence of glauconite within Unit III sediments is consistent with a shallow-water environment below the storm wave base. Volcaniclastic constituents with relatively high magnetic susceptibility are present through the late Oligocene, suggesting proximity to volcanic islands at this time.
Unit IV is a fine-grained chalk with abundant nannofossils and foraminifers overlying basalt (inferred basement), with no evidence for input of nonpelagic material. We infer, based on the presence of authigenic glauconite, a relatively shallow open-ocean environment, on the order of hundreds of meters below sea level.
Calcareous nannofossil and planktonic foraminifer biostratigraphies indicate that the upper Pleistocene to upper Oligocene succession is essentially complete. Calcareous nannofossils are generally abundant and are moderately to well preserved throughout the interval. Some of the standard nannofossil datums are missing, but alternative events were used to reconstruct the biostratigraphic succession.
Planktonic foraminifers are common to abundant down to ~150 mcd (19–20 Ma). Below this depth, foraminiferal abundance decreases and preservation deteriorates markedly. Benthic foraminiferal assemblages document an oligotrophic, pelagic environment from Pliocene to middle Miocene time. Older intervals of the lower Miocene and upper Oligocene contain higher numbers of buliminids and nodosariids, reflecting shallower water depth and proximity to the oxygen minimum zone. The presence of poorly preserved larger benthic foraminifers, which have modern equivalents that host algal endosymbionts, indicates redeposition from a nearby carbonate platform during the Oligocene and early Miocene.
Diatoms are only found close to the two ash layers and indicate either increased diatom production near the time of ash deposition or anomalous preservation as a result of the presence of ash. A typical low-latitude diatom assemblage with an age of between 13.5 and 12.5 Ma is present at 82.3 mcd. The presence of benthic and neritic diatoms at 82.3 and 206.7 mcd indicates redeposition from shallow-water environments.
Interstitial water chemistry at Site 1236 is consistent with low input of organic matter in an oligotrophic setting. Diagenesis of opal and calcite is minimal, as reflected in silicate, calcium, and magnesium measurements of interstitial waters. Sulfate concentrations are high, >23 mM. Alkalinity, phosphate, and ammonium concentrations are all low. Methane is <2 ppmv, near the detection limit, and organic carbon content is <0.2 wt%. Interstitial water geochemistry varies little with lithologic unit except in the deepest sedimentary unit overlying presumed basement, where alkalinity, silicate, and boron decrease and manganese increases.
The NRM intensities are similar to the magnetic susceptibility profiles and reflect the major lithologic changes at Site 1236. Prior to demagnetization, inclinations are steeply positive, characteristic of a drill string–induced magnetic overprint. Much of the overprint is removed after demagnetization at either 20 or 25 mT, with a clearer polarity record in cores taken with the nonmagnetic core barrel. A preliminary magnetostratigraphy was established for the last 13 m.y. that can be correlated to the geomagnetic polarity timescale, significantly augmenting the biostratigraphy and providing a well-constrained chronology.
A long history of oligotrophic conditions with little input of nonbiogenic sediments at Site 1236 has resulted in low sedimentation rates averaging <10 m/m.y. Winnowing processes may have contributed to the overall low sedimentation rates on this bathymetric rise. Sedimentation rates of ~5 m/m.y. are typical for the middle and late Miocene (~15 to ~6.5 Ma) and for the last ~4 m.y. Higher rates of 10–15 m/m.y. occur from ~6.5 to ~4 Ma and from ~25 to 17 Ma. The late Oligocene and early Miocene maxima in sedimentation rates reflect the influence of nearby carbonate platforms and/or islands, which supplied significant amounts of siliciclastic, volcanic, and nonpelagic calcareous material until ~17 Ma. The late Miocene to early Pliocene peak in sedimentation rates from ~6.5 to ~4 Ma, however, is controlled by changes in pelagic sedimentation. Given no detectable changes in fossil preservation in this interval, high sedimentation rates likely reflect an interval of increased productivity. A similar interval of rapid accumulation was found at Sites 846–850 in the equatorial Pacific (3°S–1°N, 90°–115°W) and is often referred to as the late Miocene to early Pliocene biogenic bloom. All of the sites that contain this bloom event are influenced by either the Peru-Chile Current or its westward extension into the South Equatorial Current, suggesting that the source of the bloom is in the Southern Hemisphere. The ultimate cause of this extraordinary event, which has been hypothesized to relate to the closure history of the Isthmus of Panama and/or changes in deep-sea circulation, remains a question for postcruise research.
Site 1236 has clearly met our shipboard objectives of providing an essentially complete Neogene to Quaternary sequence relatively unmodified by burial diagenesis and recovered with the APC. The tight framework of biostratigraphic and magnetostratigraphic age controls together with a good preservation of calcareous microfossils extending to the late Oligocene will provide a solid base for detailed studies that aim to reconstruct the long-term history of sea-surface and intermediate-water characteristics in the southeast Pacific.