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LEG 202 SITE SUMMARIES (continued)

Site 1239


Site 1239 (proposed Site CAR-1C) is located ~120 km off the coast of Ecuador near the eastern crest of Carnegie Ridge (Fig. F1) in the middle of a bench that slopes gently to the south. Just to the east, Carnegie Ridge slopes downward into the Peru-Chile Trench, where subduction is consuming the volcanic seamounts of the Galapagos hotspot track. Basement at Site 1239 likely consists of basalt formed at the Galapagos hotspot (15–18 Ma). Farther north of Site 1239, a series of partially eroded seamounts rises steeply to water depths of 500–1000 m. These bathymetric highs may have been above sea level early in their history, as part of an archipelago similar to the modern Galapagos Islands. The steep basaltic flanks of Carnegie Ridge are mostly bare rock, so some downslope transport of sediment is possible. The tectonic backtrack path on the Nazca plate moves Site 1239 about 600 km westward and slightly to the south relative to South America (Fig. F52) and probably to shallower depths (and perhaps above sea level, which would account for the possible erosion features on the volcanic highs early in the history of this portion of Carnegie Ridge).

The region of Site 1239 is blanketed by pelagic sediments of variable thickness covering rough basement topography. Seismic profiles reveal stratified reflective sediments that drape the underlying bathymetry and fill basement lows (Fig. F55). The sediments are dominated by nannofossil ooze with varying amounts of foraminifers, diatoms, and clay. Several ash layers are present within the sequence.

Today, Site 1239 is situated under the eastern reaches of the equatorial cold tongue in an open-ocean upwelling system near the equator (Fig. F52). The site is close to the equatorial front that separates cool, relatively high-salinity surface waters south of the equator from the warm, low-salinity waters of the Gulf of Panama. The EUC supplies the nutrient-rich upwelling waters that fuel high surface productivity, although nitrate and phosphate are not fully utilized by the phytoplankton. Here, a limited supply of micronutrients such as iron, for which the EUC is today a major source, may play an important role as a regulator of near-surface productivity.

Site 1239 is likely to record changes in upwelling and biological production, as well as long-term changes in upper ocean temperature and pycnocline depth. The surface-ocean properties of the eastern equatorial Pacific are sensitive to interannual to decadal oscillations, such as those of the well-known ENSO, and to longer-term climate changes, such as those of the Pleistocene ice ages. The Site 1239 tectonic backtrack to the west is almost parallel to the equator, so it is likely that the site resided within the highly productive and climatically sensitive region south of the equator throughout its history.

The modern water depth of Site 1239 is within the range of PCW south of Carnegie Ridge but shallow enough that some mixing with remnants of AAIW can be detected here, especially by its relatively low salinity. NPIW is limited to the Northern Hemisphere and is not detected in the modern water column at Site 1239.

The primary objectives at Site 1239 are to provide a continuous sedimentary sequence of Neogene age (as old as 11–12 Ma) to

  1. Assess the history of near-surface water masses, including the eastern reaches of the equatorial cold tongue, upwelling, and paleoproductivity off Ecuador;
  2. Monitor temporal and vertical fluctuations of intermediate water masses near 1400 m water depth; and
  3. Monitor changes in the occurrence and frequency of volcanic ashes, which might be associated with active tectonic phases of the northern Andes.


Three APC holes were drilled at Site 1239. Hole 1239A was deepened with the XCB and logged. APC cores in Hole 1239A extend to 174.4 mbsf, and XCB cores cover the interval to 515.4 mbsf, where the last core (202-1239A-55X) contacted acoustic basement.

Hole 1239A was flushed and displaced with sepiolite mud, and the bit was placed at 95.5 mbsf in preparation for downhole logging. One pass with the triple combo was conducted from total depth (517 mbsf) to the mudline, followed by one full pass from 517 mbsf to the bit and one repeat pass from 155 mbsf to the bit with the MGT. Two subsequent passes with the FMS-sonic tool string also reached the bottom of the hole. The hole diameter ranged from ~11.5 to 15 in, allowing good pad contact. Hole 1239A was another unusually smooth hole for logging, resulting in excellent quality of logging data. As in Hole 1238A, hole deviation increased with depth, reaching 7.5° at the bottom.

The vessel was offset 20 m west, and Hole 1239B was drilled with the APC to 171.3 mbsf and extended to 398.7 mbsf with the XCB. The vessel was offset again 20 m west to core Hole 1239C with the objective to fill stratigraphic gaps left in the first two holes.

The nonmagnetic core barrel was deployed on either even- or odd-numbered cores in each APC hole; it was not used for cores that had to be drilled over to avoid damage to hardware as a consequence of the drilling-over process. The piston cores were oriented starting with the third or fourth core in each hole. One bottom-water and four downhole temperature measurements taken with the APCT indicated a thermal gradient of ~88°C/km. The total cored interval at Site 1239 was 1026.1 m, with 1042.2 m recovered, representing an average recovery of 101.6%.

Scientific Results

A 512.3-m-thick (557.5 mcd) sediment sequence extending into the middle Miocene (~14.6 Ma) was recovered (F56). The biostratigraphic sequence is similar to that at Site 1238, except that a major hiatus was identified, encompassing the interval from ~7.8 to 13.6 Ma.

A composite section was constructed using magnetic susceptibility and other core logging data from three holes. All APC cores were correlated, and the splice documents complete recovery of the upper 188.9 mcd. Most of the XCB cores from Hole 1239B were not measured during Leg 202 because of expected depletion of the D-tube supply later in the leg, which prevented the extension of the splice. However, these cores will be measured postcruise, and based on the signal that allowed correlation of XCB cores from Hole 1239A to the downhole logs in the same hole, we expect that a complete splice to basement can be constructed later. Density and NGR records from borehole and core logging data (including the XCB-cored interval of Hole 1239A) exhibit strong correlation of meter-scale variability. An eld scale was transferred to the XCB cored intervals based on core-log data correlation. This analysis accounts for core expansion, identifies the extent of gaps between cores, and better approximates the "true" depth of the sediment below the seafloor by accounting for small errors in the mbsf scale.

The lithology, as well as the fossil assemblages and abundances at Site 1239, reflect a moderate- to high-productivity pelagic environment, similar to that at Site 1238. The recovered sediments are dominated by foraminifer and nannofossil ooze with varying amounts of diatoms, clay, and micrite. Siliciclastic components include clay minerals and lesser amounts of feldspars and biotite. Almost the entire sequence is characterized by meter-scale cyclic changes in color reflectance (particularly L*) and bulk density that likely reflect changes in relative proportions of biogenic opal and carbonate. Preliminary evaluation suggests that these features are related to the Earth's orbital cycles on the scale of 104 to 105 yr.

Volcanic ash deposition began in the late Miocene to early Pliocene (~5–4 Ma), increasing in frequency over the last 3 m.y. An ash layer observed at 13.3 mcd can tentatively be correlated to the regional ash layer L (~230 ka), consistent with sedimentation rates of 60 m/m.y. inferred from the shipboard biostratigraphic age model. The magnetic susceptibility signal increases in both mean values and amplitude of high-frequency fluctuations within the interval representing the last ~2.5 m.y. (0–120 mcd). This may reflect the previously documented increase in amplitude of orbital-scale climate variability associated with the Pliocene–Pleistocene intensification of Northern Hemisphere glaciation.

Calcareous nannofossils are abundant and well preserved within the top ~100 mcd, but both abundance and preservation decline slightly below this interval. Planktonic foraminifers are common within the top ~100 mcd but decrease in abundance rapidly at greater depths. Diatoms are also present throughout the sedimentary section, although their abundance is relatively low in the Holocene to middle Pleistocene and lower Pliocene to uppermost Miocene intervals in comparison to Site 1238.

High concentrations of diatoms and nannofossils, increased sedimentation rates (averaging 110 m/m.y.), and enhanced accumulation rates of organic carbon and carbonate characterize the Pliocene interval from ~4.5 to 1.8 Ma, suggesting a period of relatively high productivity. The final phase of this period from ~2 to 1.8 Ma (~85–115 mcd) is associated with a drop in carbonate concentrations, a minimum in L* reflectance, and low bulk density. These significant changes point to an environmental change within the upwelling region that favored biosiliceous productivity, possibly associated with a change in nutrient availability and/or upwelling conditions.

The timing of the onset of this Pliocene productivity maximum remains speculative and relies on interpretations of the biostratigraphic age model between ~4 Ma and the top of a hiatus detected between 519.3 and 527.7 mcd (7.8–13.6 Ma, based on calcareous nannofossils). The current age model suggests sedimentation rates of ~60 m/m.y. for the interval from ~7.8 to 4.5 Ma and higher sedimentation rates of ~110 m/m.y for the interval from ~4.5 to 1.8 Ma. Such a change in sedimentation rates would imply a substantial increase in accumulation rates of organic matter and opal and thus would place the onset of this biogenic bloom at ~4.5 Ma. However, the biostratigraphic framework of diatoms and planktonic foraminifers may indicate that the top of the hiatus occurs later, near 6.5 Ma. This alternative age model would imply high sedimentation rates of ~100 m/m.y. (and attendant high mass accumulation rates of biogenic components) immediately above the hiatus and would place the onset of the biogenic bloom just after or perhaps within the hiatus (i.e., >6.5 Ma). Below the hiatus, sedimentation rates from 13.6 Ma to basal sediments dated at 14.6–14.9 Ma are distinctly lower than those above the hiatus.

Steep positive magnetic inclinations of the sediment at this equatorial location indicate that NRM was affected by a drill string magnetic overprint even after AF demagnetization. Although a detailed magnetic stratigraphy could not be resolved shipboard, declination data suggest that postcruise work may be able to resolve at least a few polarity transitions of the Matuyama Chron.

Chemical gradients within interstitial water reflect the influence of organic matter oxidation, the dissolution of biogenic silica and its reprecipitation in authigenic phases, the effects of authigenic calcite precipitation, and the diffusive influence of basalt alteration processes. Sulfate is completely consumed by ~71 mcd, coincident with an increase in methane and approaching an interval of higher organic carbon contents that marks the final phase of the late Pliocene biogenic bloom. Within this interval, alkalinity reaches maximum values from ~92 to ~123 mcd and then declines downhole. Alkalinity consumption is associated with carbonate mineral precipitation and also with the diffusive influence of basalt alteration reactions. Authigenic mineral precipitation is reflected by increased amounts of micrite within the sediments at depths >100 mcd and a decrease in calcium concentrations to minimum values by ~75 mcd.

As at Site 1238, the increase in dissolved silicate with increasing depth reflects enhanced dissolution of biogenic opal that is controlled by an anomalously high temperature gradient. Peak silicate values are reached at temperatures of ~45°C (~510 mcd at this site). The subsequent decrease in dissolved silicate values at greater depth is consistent with diagenetic reprecipitation of silicate in chert and its precursors.

The weight percentage of calcium carbonate ranges between 16 and 87 wt%. The upper 200 mcd of the sediment sequence is marked by high-amplitude variations in carbonate concentrations around an average value of 52 wt%. At greater depths, CaCO3 concentrations increase to ~80 wt%. These long-term changes are consistent with the migration of the site toward more coastal conditions with greater production of diatoms and organic matter and an enhanced supply of siliciclastics. TOC concentrations average 1.3 wt% in the Pleistocene interval, then increase to 2 wt% in the interval of inferred high productivity from 1.8 to 4.5 Ma, and decrease again to values <1.5 wt% in the sequence older than 4.5 Ma. Enhanced input of terrigenous organic matter may have occurred during the Miocene (>5.5 Ma) as indicated by high organic carbon/nitrogen ratios.

The downhole logging provided excellent data from the NGR, bulk density, porosity, and FMS tools. The most striking feature of the logs is the high-frequency variability in density and resistivity throughout the sequence from 110 to 517 mbsf. High sedimentation rates and meter-scale rhythmic changes in density and resistivity are encountered in the upper Miocene to Pliocene interval that are interpreted to reflect orbital-scale changes in carbonate vs. biogenic opal in the sediment. Color banding on the FMS images is present on the same scale, which clearly documents the potential for developing an orbitally tuned timescale for this site.

Site 1239 met all of its major objectives. The site combines a complete composite section through the APC interval of 189 mcd, double-XCB coring that may document complete recovery to basement, and excellent logging data to place the deeper APC cores and the XCB cores into a true depth framework. Moderately high sedimentation rates of 50–100 m/m.y., combined with observations of rhythmic shifts in sediment composition that probably represent orbital-scale variability, confirm that this site will provide excellent opportunities for high-resolution studies of late Neogene climate change and for the study of biogeochemical cycles in the ocean. The interval from 7.8 (or 6.5) to 13.6 Ma is represented by a hiatus. All major fossil groups are present at Site 1239 and are moderately to well preserved. This rich fossil record will provide a solid base for the study of near-surface water masses in the eastern reaches of the equatorial Pacific cold tongue, including processes of upwelling, nutrient utilization, and paleoproductivity of the coast of Ecuador. A well-preserved benthic fauna will facilitate study of deepwater masses. Volcanic ashes present an opportunity for tephrochronology and for establishing the history of major volcanic events in the northern Andes.

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