Site 1242 (proposed Site COC-4A) is located at 1364 m water depth in a shallow graben within the structurally complex intersection between Cocos Ridge and the Mesoamerican Trench. The crust underlying the site was probably formed at the Galapagos hotspot ~15–16 m.y. ago. A tectonic backtrack path on the Cocos plate moves Site 1242 southward and to the west, placing it close to the equator and near the ancestral Galapagos hotspot early in its history (Fig. F6).
Sediments at Site 1242 are predominantly hemipelagic clay that fill a subsiding basin, ~460 m thick. The basin geometry is complex, however, and is clearly affected by faulting (Fig. F62). Occasional ash layers are present and may record the history of volcanism in Central America.
At its present location, Site 1242 is under the warm, relatively low salinity waters of the intertropical convergence in the Panama Basin. Nutrients at the sea surface are low, and biological productivity is relatively low for a continental margin setting. Site 1242 is likely to record changes in the pool of warm and relatively low salinity surface waters north of the Panama Basin. In the Pleistocene, it will also provide evidence for variations in upwelling and biological production on the Costa Rica margin. Older sediments here may provide a record of the tectonic and oceanographic responses to Isthmus of Panama closure during the late Miocene and Pliocene, along with a record of volcanism in Central America based on the accumulation rates of ash.
The bottom-water mass at the seafloor is associated with the lower reaches of an anomalously thick oxygen minimum zone between North Pacific Deep Water and remnants of NPIW. These waters of North Pacific origin are relatively depleted in oxygen and enriched in nutrients. The region has one of the deepest and strongest oxygen minimum zones in the world and is thus an important site for denitrification.
Given the relatively shallow depth of Site 1242 above the regional lysocline, variations in carbonate dissolution here should be controlled by the relative rates of carbonate rain from surface waters and organic carbon degradation within the sediments.
The primary objective at Site 1242 is to provide a continuous sedimentary sequence of Pliocene to modern sediment in order to assess variability of upper ocean processes at high resolution, including variations in the Atlantic to Pacific salinity contrast and dynamics of the intertropical convergence associated with late Neogene climate changes. The site is well located to study long-term changes associated with denitrification in low-oxygen subsurface water masses.
We drilled four APC holes at Site 1142, offset by 20 m, with penetration depths of 176.0, 169.9, 166.5, and 91.5 mbsf. Several short intervals were drilled without coring to optimize overlap of cores in adjacent holes. Holes 1242A and 1242B were advanced with the XCB to 250.8 and 256.0 mbsf, respectively. APC cores were oriented, starting with the third or fourth core. The nonmagnetic core barrel was deployed on even- or odd-numbered APC cores in each hole. A total of seven downhole and one bottom-water temperature measurements indicated a high thermal gradient of ~20°C/100 m.
A composite sediment sequence of 287.74 mcd was recovered at Site 1242, spanning the interval from the middle Miocene to Holocene (Fig. F63). A splice documents complete recovery for the upper 196.37 mcd. All APC cores and one XCB core can be correlated to the splice. Below the splice, two cores from the two XCB intervals are correlated to each other, and a floating splice is defined to the bottom of the drilled interval.
The upper 280.6 mcd at Site 1242 consists of fine-grained, homogeneous, hemipelagic sediments that represent the past ~2.5 m.y. Based on shipboard stratigraphy, sedimentation rates range from ~60 to 200 m/m.y. Relatively high CaCO3 and low siliciclastic content characterize the upper Pliocene interval. Carbonate content decreases gradually, and siliciclastic content increases in the Pleistocene and Holocene intervals. This sedimentary record may be influenced by the tectonic movements of Site 1242 toward the continental margin, the uplift of the Central American cordillera following closure of the Isthmus of Panama, the focusing of terrigenous material, and the long-term environmental changes affecting both continental rainfall and marine productivity. The upper Pleistocene interval is characterized by relatively low sedimentation rates of ~60 m/m.y and low carbonate content. The early Pleistocene to Pliocene increase in mean sedimentation rate is paralleled by a gradual increase in carbonate concentrations. Magnetic susceptibility varies significantly on decimeter to meter scales throughout this sequence, suggesting changes in the relative supplies of terrigenous and biogenic material to Site 1242. These changes may in part reflect millennial- to orbital-scale changes in productivity and/or climate.
Increased ash layer frequency between ~30 and 120 mcd and ~160 and 210 mcd indicates increased volcanic activity from ~0.4 to 1.1 Ma and ~1.3 to 1.6 Ma, respectively. Clear platy and vesicular glass and intermediate accessory mineral compositions suggest an andesitic volcanic source, most likely in Central America.
Calcareous nannofossils and foraminifers document a relatively continuous sequence that accumulated rapidly (~110 m/m.y.) over the past ~2.5 m.y. A hiatus spanning the time interval of ~2.5–13 Ma is detected. This interval coincides with the closure of the Isthmus of Panama. Middle Miocene sediments are found below the hiatus, from 280.6 mcd to the base of the sequence at 287.7 mcd, representing pelagic conditions with a relatively high abundance of siliceous microfossils. We infer higher Miocene productivity that may be related to the southward tectonic backtrack of Site 1242 on the Cocos plate toward the equatorial upwelling zone.
Calcareous nannofossils are relatively rare at Site 1242, largely due to dilution by clay and some silt-sized detrital grains. Preservation is moderate. Planktonic foraminifers are abundant to common in the upper ~70 m, but abundance decreases markedly downhole. Planktonic foraminifers are rare from 269 to 277 mcd, just below the major unconformity. Preservation is moderate to good in the upper 99 mcd but deteriorates markedly below this depth. The percentage of benthic foraminifers relative to total foraminifers is initially relatively low (<10%) in the upper 109 mcd, but increases markedly downhole, reaching ~99% between 234 and 257 mcd. Benthic foraminifers are better preserved than planktonic foraminifers, but in both groups preservation deteriorates downhole, particularly from 215 to 269 mcd. Diatoms are rare at Site 1242, with the exception of abundant Ethmodiscus fragments from ~208 to 215 mcd, representing thin diatom ooze layers. A thin layer at 281 mcd contains a middle–late Miocene diatom assemblage dominated by Thalassiothrix.
Paleomagnetic measurements can not yet constrain age models at Site 1241 but offer the potential for a good record of magnetic paleointensities through time.
Chemical gradients in the interstitial waters at Site 1242 reflect the influence of organic matter oxidation by sulfate reduction, authigenic mineralization, the dissolution of biogenic silica, and the diffusive influence of basalt alteration reactions at greater depth. Sulfate decreases to nondetectable levels by 34.4 mcd, coincident with the rise in methane in the gas samples. The disappearance of sulfate coincides with an increase in barium, apparently driven by dissolution of barite. Organic matter oxidation by sulfate reduction and methanogenesis results in significant increases in alkalinity, phosphate, and ammonium. The alkalinity increase drives authigenic mineral precipitation, resulting in minimum calcium concentrations of 2.5 mM at 34.4 mcd. The deeper part of the calcium profile is dominated by a large increase with depth, linearly correlated to a decrease in magnesium. Dissolved silicate increases with increasing depth to >1400 mM from the dissolution of biogenic opal. Calcium carbonate contents range between 5.7 and 51.2 wt%, averaging 24.4 wt%. The calcium carbonate profile shows a long-term increase downhole. TOC contents range between 0.9 and 3.5 wt%, averaging 1.6 wt%, with a large decrease in the uppermost ~10 mcd. TOC/total nitrogen ratios average 7.2, indicating that the organic matter is dominated by material of marine origin.
Site 1242 has clearly met the shipboard objectives of providing a continuous sedimentary sequence of Pliocene to modern sediments to assess the variability of upper ocean and subsurface water mass processes at high resolution. The relatively high sedimentation rates of 60–200 m/m.y. and the abundant fossil record suggest that this site will provide an excellent history of climate and geochemical changes near the Intertropical Convergence Zone.