Next Section | Table of Contents

LEG 202 SITE SUMMARIES (continued)

Site 1238


Site 1238 (proposed Site CAR-2C) is located ~200 km off the coast of Ecuador, on the southern flank of the Carnegie Ridge (Fig. F1). The region is draped with pelagic sediment ~400–500 m thick on a bench that slopes gently to the south (Fig. F51). Sediments consist mainly of diatom nannofossil ooze and nannofossil diatom ooze.

Basement at Site 1238 likely consists of basalt formed at the Galapagos hotspot at ~11–13 Ma. A tectonic backtrack path on the Nazca plate moved Site 1238 about 600 km westward and slightly to the south relative to South America, probably to shallower water depths by 12 Ma (Fig. F52).

Today, Site 1238 is situated under the eastern reaches of the equatorial cool tongue in an open-ocean upwelling system near the equator (Fig. F52). The nutrient-rich Equatorial Undercurrent (EUC) supplies waters that upwell here and along the coasts of Peru and Ecuador, driving high primary productivity. Sediments at the site are likely to record changes in upwelling and biological production, as well as long-term changes in upper ocean temperature and pycnocline depth. The modern water depth is appropriate to monitor the PCW south of Carnegie Ridge, roughly at the sill depth of Panama Basin.

The primary objectives at Site 1238 were to provide a continuous sedimentary sequence of Neogene age to

  1. Assess the history of near-surface water masses, including the eastern reaches of the equatorial cold tongue;
  2. Assess changes in biogeochemistry and biota that are linked to variations in nutrients, productivity, and fluxes of organic matter and biogenic sediments to the seafloor;
  3. Monitor temporal and vertical fluctuations of PCW; and
  4. Monitor changes in the occurrence and frequency of volcanic ashes that might be associated with active tectonic phases of the northern Andes.


After leaving Peruvian waters, the vessel proceeded to a rendezvous point in the Gulf of Guayaquil, Ecuador (south of Salinas; 2.5°S, 81.0°W), to pick up an Ecuadorian observer and an ODP engineer and to drop off a Peruvian observer. The 889-nmi transit required 77.5 hr at an average speed of 11.5 kt. After the helicopter transfer of personnel at 0946 hr, the JOIDES Resolution proceeded to proposed Site CAR-2C. The 114-nmi transit required 9.5 hr at an average speed of 12.0 kt.

Three holes offset from each other by 10 m were completed at Site 1238. Hole 1238A was cored with the APC to 204.5 mbsf (Core 202-1238A-22H), followed by the XCB from 204.5 mbsf to refusal at 430.6 mbsf (Core 202-1238A-46X). Hole 1238B was APC cored to 201.0 mbsf, and Hole 1238C was APC cored to 162.5 mbsf to ensure the recovery of a complete stratigraphic section.

Hole 1238A was flushed and displaced with sepiolite mud and then logged with the bit at 99.4 mbsf, first using the triple combination (triple combo) tool string with the Lamont-Doherty Earth Observatory (LDEO) Multisensor Gamma Ray Tool (MGT) on top and then by the FMS-sonic tool string. One pass with the triple combo was conducted from 429 mbsf (total depth) to the mudline, followed by one full pass from 430 mbsf to the bit and one pass from 167 mbsf to the seafloor with the MGT. The two subsequent passes with the FMS-sonic tool string also reached the bottom of the hole. Hole conditions were unusually smooth, and the hole diameter ranged in size from 11.5 to 14.5 in (29–37 cm). The accelerometer (General Purpose Inclinometer Tool) logs showed increasing deviation vertically in the hole below 170 mbsf, reaching 6.5° at the base.

Cores were oriented starting with the third (Holes 1238A and 1238B) or the fourth (Hole 1238C) core in each hole. The nonmagnetic core barrel was deployed on even-numbered cores to Cores 202-1238A-12H and 202-1238C-14H and on odd-numbered cores to Core 202-1238B-11H. Five downhole temperature measurements were taken with the APCT, yielding a temperature gradient of ~12.5°C/100 m.

Scientific Results

A 424.75-m-thick (467.3 mcd) sediment sequence dating back to the middle Miocene (~11 Ma) was recovered from three holes at Site 1238 (Fig. F53). Correlation of magnetic susceptibility and other core logging data between holes documented complete recovery of the section cored with the APC to a depth of 225.4 mcd. A composite section and a spliced record were constructed over this interval. XCB cores from Hole 1238A were appended onto the mcd scale, using a 10% growth rate of mcd relative to mbsf.

The dominant lithologies at Site 1238, nannofossil diatom ooze and diatom nannofossil ooze, reflect primarily biogenic sedimentation in a moderate- to high-productivity pelagic setting. The sediment sequence is characterized by meter-scale alternations of pale olive nannofossil-rich and dark olive diatom-rich intervals, also reflected by variations in core logging properties such as L*, GRA density, magnetic susceptibility, and NGR. Given average sedimentation rates of ~5 cm/k.y., this lithologic banding falls into the frequency range typical of Earth's orbital cycles (Fig. F54). Although shipboard chronologies are not sufficiently detailed to identify specific cycles unambiguously, these observations indicate the likelihood that the site is sensitive to orbital-scale climate changes that are in turn reflected in the major lithologies.

One lithologic unit, divided into two subunits, was defined at Site 1238. Subunit IA spans the upper 403 mcd of the sequence and contains bioturbated nannofossil ooze, diatom nannofossil ooze, and nannofossil diatom ooze with varying abundance of clay and foraminifers. Variability in magnetic susceptibility decreases gradually from 0 to 100 mcd, perhaps in concert with the known reduction in the amplitude of glacial–interglacial climate oscillations in the early Pleistocene.

The Pliocene to Miocene interval from ~1.8 to 7 Ma is marked by relatively high productivity, as indicated by high concentrations of diatoms, organic carbon, and organic pigments in the sediment, along with high sediment mass accumulation rates. The end of this biogenic bloom (~1.8–2.0 Ma) is associated with an increase in biosiliceous microfossils, organic carbon contents, and minima in bulk density and carbonate concentrations. This change was possibly associated with a change in nutrients or upwelling conditions that favored biosiliceous productivity. Several ash layers are present within the upper ~300 m of Subunit IA, including the regionally correlative ash layer L (~230 ka). Increases in the frequency of ash layers at ~5 Ma and during the past 3 m.y. may represent phases of intensified volcanism in the northern Andes.

Subunit IB occupies the base of the sequence (~400–467 mcd) and is characterized by partially lithified diatom and nannofossil oozes interbedded with chalk and occasional chert horizons. Increasing lithification is documented by increases in bulk density (GRA) and decreases in porosity (moisture and density). The presence of chert and micrite indicates significant opal and carbonate diagenesis, respectively. Magnetic susceptibility increases downhole in Subunit IB toward basaltic basement.

Calcareous nannofossils are generally abundant and planktonic and benthic foraminifers are common to abundant, with good to moderate preservation throughout the section, except in the interval of lower Pleistocene and middle Miocene. Benthic planktonic foraminifer ratios generally increase downhole. Diatoms are common to abundant throughout the Pleistocene, Pliocene, and the upper half of the upper Miocene sequence (to 430 mcd; 9 microfossils, likely due to opal diagenesis resulting in chert formation.

The various microfossil groups provide a well-constrained biostratigraphy. The Pleistocene through Pliocene sequence has a sedimentation rate of ~50 m/m.y. In the lower Pliocene–upper Miocene section, the sedimentation rate is ~60 m/m.y., and this section contains a minor component of reworked older microfossils. A marked change in sedimentation rate occurs at 390–415 mcd (7–8 Ma). Below this level, sediments accumulate at an average rate of ~17 m/m.y. Calcareous nannofossils provide a basal age of 10.4–11.6 Ma. Paleomagnetic measurements were compromised by drill string overprints, as well as diagenesis of magnetic minerals, and thus did not provide useful information for age control.

Moderate amounts of biogenic gas, mostly methane but also traces of ethane, result from the limited effects of methanogenesis at this site. Chemical gradients in the interstitial waters reflect the influence of organic matter oxidation by sulfate reduction, although not to complete sulfate depletion. Sulfate decreases to <10 mM by ~75 mcd, coincident with the top of a relatively organic carbon–rich interval. The sulfate decrease is accompanied by increases in alkalinity (>17 mM), phosphate (>15 mM), and ammonium (3 mM). The shallow part of the calcium profile, with a substantial decrease in calcium by ~55 mcd, is controlled by authigenic calcite precipitation driven by the alkalinity increase. The deeper sequence reflects the diffusive influence of basalt alteration, also affecting magnesium, lithium, and potassium profiles. Silicate increases to >1800 µM by ~350 mcd, indicating temperature variation of the solubility of biogenic opal as a control on interstitial water composition here, with sharp decreases in silicate concentrations and incipient chert formation deeper in the site.

The weight percentage of calcium carbonate (CaCO3) ranges between 17 and 94 wt% and averages 60 wt%. CaCO3 concentrations increase with depth and vary rhythmically with ranges of 10–20 wt% along the profile. TOC contents range between 0.1 and 4.3 wt% and vary inversely with CaCO3. The long-term decrease in carbonate concentration is consistent with the migration of the site toward a more coastal setting with greater production of diatoms and organic matter and delivery of terrigenous material that may both dilute and dissolve the carbonate component.

Downhole logging operations experienced smooth borehole conditions and calm seas (peak heave < 2 m), and as a result the data from the density, porosity, and FMS tools that require good borehole contact were of excellent quality. NGR results were highly reproducible between tools and passes and also closely matched the NGR records developed from shipboard core logging. From the base of the drill pipe (84 mbsf) to 209 mbsf, mean values of velocity, resistivity, and density are relatively low, and porosity is high. At greater depths, velocities and densities increase and porosity decreases associated with sediment lithification.

Similar to the core logging data, the downhole logs document cyclic variations in sediment properties on the scale of meters that reflect alternations between carbonate-rich and opal-rich sediments. For example, NGR logs revealed rhythmic low-amplitude oscillations in thorium and potassium. Higher-amplitude oscillations in uranium appear to be well correlated with shipboard measurements of organic carbon in the cores. This correlation points to changes in organic carbon degradation near the sediment/water interface. Organic matter degradation consumes oxygen and drives suboxic diagenesis that concentrates uranium in organic-rich intervals.

A test of the LDEO high-resolution gamma logger was successful and offers the opportunity for detailed correlation of the cores and logs. Even with preliminary analysis at sea, it appears that stratigraphic gaps between XCB cores can be identified, which suggests that the XCB cores can be placed into the framework of logging depths.

Site 1238 met all of its major objectives. With the combination of a complete composite section and a spliced record through the APC interval, excellent logging data to place XCB cores into a true depth framework, and moderately high sedimentation rates, this site will provide excellent opportunities for high-resolution studies of Neogene climate and biogeochemical change. All major fossil groups are present and reasonably well preserved here, which provides both a well-constrained biostratigraphic age model and great potential for the study of near-surface water masses in the eastern reaches of the equatorial cold tongue, including processes of upwelling and paleoproductivity off Ecuador. A well-preserved benthic fauna will facilitate study of deepwater masses. Volcanic ashes are present, which opens opportunities for tephrochronology and for establishing the history of major volcanic events in the northern Andes.

Next Section | Table of Contents