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LEG 202 SITE SUMMARIES (continued)

Site 1240


Site 1240 (proposed Site PAN-2A) is located midway between mainland Ecuador and the Galapagos Islands, north of Carnegie Ridge at 2921 m water depth in the Panama Basin (Fig. F1). The site is in a small east-west–trending trough of basaltic crust formed at the Cocos-Nazca spreading center about 3 m.y. ago (Fig. F57). Tectonic movements over the past 3 m.y. are small; the site backtracks from its current position (~2 km north of the equator) ~40 km to the south and 200 km to the west over the past 3 m.y.

The pelagic sediment cover at Site 1240 drapes the abyssal hills, with a slight thickening of the section within the valleys. Total sediment thickness was estimated at 250–280 m based on seismic survey data (Fig. F58). Dominant sediments are (diatom) nannofossil ooze, with some intervals of nannofossil diatom ooze and occasional ash layers.

Equatorial upwelling driven by southeasterly trade winds is strong over Site 1240, especially in the Southern Hemisphere winter. Nutrient-rich waters of the EUC serve as primary source waters for the modern upwelling. The surface-ocean properties of the eastern equatorial Pacific are sensitive to interannual to decadal variability, such as those of the well-known ENSO events, as well as to longer-term changes associated with the Pleistocene ice ages. Site 1240 is likely to record changes in upwelling and biological production, long-term changes in upper ocean temperature and pycnocline depth, and carbonate dissolution within the Panama Basin.

The primary objectives at Site 1240 are to provide a continuous sedimentary sequence of late Neogene sediment to assess variability of upper ocean processes, including equatorial upwelling, at millennial to orbital timescales.


At Site 1240, four APC holes, offset by ~10 m, were cored to 253.0, 248.2, 80.2, and 31.7 mbsf, with the objective to recover a complete stratigraphic sequence. Some intervals were drilled without coring to achieve the appropriate stratigraphic overlap. The nonmagnetic core barrel was deployed to ~150–200 mbsf in each hole, either on even-numbered or odd-numbered cores. Below this depth interval, the core barrels had to be drilled over in order to retrieve them, and the risk to the nonmagnetic core barrel was considered too great. The piston cores were oriented starting with the third or fourth core in each hole. Fourteen downhole and one bottom-water temperature measurements were collected with the APCT, yielding a low average thermal gradient of 2.56°C/100 m.

Scientific Results

Site 1240 cored a 282.9-mcd-thick sedimentary sequence that provides a continuous record dating back to the late Pliocene (Fig. F59). Sediments consist mostly of nannofossil ooze with varying amounts of diatoms. Siliciclastic components, primarily clay, are rare. A composite depth scale extends to the bottom of the sequence, and a splice documents complete recovery for the upper 278.28 mcd.

Sedimentation rates are ~80 m/m.y. in the uppermost ~140 mcd of the section (0–1.7 Ma) and 120–160 m/m.y. at greater depths, to a basal age of ~2.6 Ma (based on nannofossil datum) or 2.9 Ma (extrapolated from planktonic foraminifer datums).

A single lithostratigraphic unit was defined and subdivided into three subunits. Subunit IA (0–142.2 mcd; 0–1.7 Ma) is composed of extensively bioturbated nannofossil ooze. Subunit IB (142.2–206.4 mcd; ~1.7–2.2 Ma) is enriched in diatoms, siliciclastic components, TOC, and chlorophyll-related pigments relative to the other two subunits. Color banding is frequently present throughout Subunit IB. Darker colors are associated with the presence of pennate diatoms. Subunit IB is also characterized by relatively low values in grain density associated with higher biogenic silica content and high values of TOC (1.1–3.1 wt%), chlorins, and NGR. Physical properties in Subunit IB are distinct and separated by sharp transitions from Subunits IA and IC. Relatively high porosity associated with high abundance of diatoms results in low bulk density and higher P-wave velocity. The color is characterized by high reddish hue (a* > 0) values. Magnetic susceptibility and lightness (L*) are low. Sediments and physical properties in Subunit IC (206.4–282.9 mcd, ~2.2–2.6 Ma) are very similar to those in Subunit IA.

Characteristics of Subunit IB may indicate increased primary productivity between ~1.7 and 2.2 Ma, perhaps associated with enhanced equatorial upwelling and intensified atmospheric and oceanic circulation. Increased siliciclastic content in this interval also suggests effective eolian transport toward Site 1240, supporting a potential intensification of atmospheric circulation.

Eight ash layers were recorded at Site 1240. Grain compositions of most of the ash layers can be associated with volcanism from northern South America. However, some ash layers containing brown volcanic glass indicate that Central America is a possible source region. In addition, the occurrence of an ash layer fully composed of brown glass (~268 mcd) may indicate a hotspot source such as Galapagos.

Calcareous nannofossils are generally abundant and well preserved at Site 1240, but preservation declines between 20 and 46 mcd and below 120 mcd. Foraminifers are common, with moderate to good preservation, except between 163 and 194 mcd, where the percentage of benthic foraminifers relative to total foraminifers also increases. Diatoms are common throughout the section. They are very abundant but severely fragmented in the upper portions of Subunit IB. Persistent reworking of late Miocene microfossils is noted throughout the sequence; however, the three microfossil groups provide a well-constrained biostratigraphic model.

NRM intensities are consistent throughout the sediment column, suggesting that reduction diagenesis plays a minor role in the magnetic properties of this record. Though still apparently affected by a drill string magnetic overprint after AF demagnetization, declinations corrected by the Tensor tool provide polarity information through the Gauss/Matuyama boundary. The magnetic record from Site 1240 provides an interpretable magnetic stratigraphy that is consistent with the biostratigraphic age model.

Methane was first detected in a headspace gas sample at 19.5 mcd. Below this depth, methane increased to a maximum of 57.4 ppmv at 146.5 mcd, followed by a downhole decrease to concentrations <10 ppmv in basal sediments. Very small amounts of ethane were also detected. These gases originate from in situ fermentation of organic matter.

Pore water chemical gradients reflect the influence of organic matter oxidation in sulfate reduction to values about one-half that of seawater. Fluid flows of relatively unaltered seawater in the underlying basement affect pore waters in the deeper section (e.g., with a return to relatively high concentrations of sulfate and calcium). Fluid flow also results in a thermal gradient of ~2.2°C/100 m, about half the normal gradient for sediments above oceanic crust. Pore water silicate increases to generally high values of >1000 mM, but never reaches the values observed at Sites 1238 and 1239, where downhole temperatures were significantly higher.

Site 1240 met its primary goal of providing a continuous sedimentary sequence of upper Neogene sediment to assess variability of upper ocean processes, including equatorial upwelling, at millennial to orbital timescales. Sedimentation rates at this site are clearly high enough to record millennial-scale variability, and the record is continuous as far back as 2.6 Ma, encompassing the Pliocene–Pleistocene interval associated with Northern Hemisphere glaciation. The major changes in lithology at this setting of equatorial divergence are between sediments rich in calcareous nannofossils and those rich in diatoms and organic carbon. Although the influence of preservation may not be entirely ruled out, the evidence is consistent with control by variations in upwelling and production.

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