A 512.3-m-thick (557.5 mcd) sediment sequence dating back to the middle Miocene (~14.6 Ma) was recovered from three holes at Site 1239. One lithologic unit (Unit I) was defined at Site 1239 (Fig. F14). The recovered sediments are dominated by foraminifer (0 to ~25 mcd) and nannofossil oozes with varying amounts of clay, diatoms, and micrite. Biogenic constituents vary in abundance and consist primarily of nannofossils with less abundant diatoms, foraminifers, radiolarians, silicoflagellates, and spicules (Fig. F15). Siliciclastic components include clay minerals and lesser amounts of feldspars and biotite. The sediments contain varying amounts of authigenic pyrite and micrite throughout. Interbedded minor lithologies include volcanic ash layers and mafic sand layers.
The lithologies at this site reflect a moderate- to high-productivity pelagic setting. Evidence from total organic carbon (TOC) (in weight percent) measurements, mass accumulation estimates, and the increased occurrence of organic pigments indicates an interval of relatively high productivity from ~1.8 to 3 Ma (90-200 mcd) as compared to the Pleistocene interval. Magnetic susceptibility remains relatively constant and low below ~150 mcd, indicating minimal terrigenous input. A gradual increase accompanied by higher-amplitude variability in the uppermost ~150 mcd may be linked to the intensification of Pliocene-Pleistocene glaciations. Rhythmic meter-scale light/dark color changes are present throughout and are interpreted to reflect changes in the relative proportions of biogenic components (CaCO3 vs. biogenic opal) possibly as a response to orbital climate forcing. Twenty-four ash layers are present within the sequence.
We distinguished one lithologic unit at Site 1239 (Unit I) (Table T7; Fig. F14) on the basis of visual core description, smear slide analysis, thin section examination, color reflectance, and NGR, moisture and density (MAD), and GRA bulk density measurements. The dominant lithology defining Unit I is nannofossil ooze with varying diatom and clay abundance (i.e., clay-bearing nannofossil ooze and clay diatom-bearing nannofossil ooze). Transitions between nannofossil ooze and diatom-bearing nannofossil ooze are present on a meter to decimeter scale. Foraminifer ooze is present in the upper 24.6 mcd. Minor components include radiolarians, silicoflagellates, micrite, and pyrite. Sediment colors include light olive gray, olive gray, dark olive gray, olive, and olive brown in the upper 230 mcd and pale olive, light gray, and pale yellow below that interval. High-frequency (meter scale) dark and light color variability is present throughout the sedimentary sequence, reflecting the transitions between light nannofossil-rich and dark diatom-rich sediment. These meter-scale color changes are all subtle and gradational. Similarly, physical properties measurements within these intervals show parallel meter-scale changes. Bioturbation is common to pervasive throughout the sediments recovered at Site 1239 (Fig. F16). Mottles, Zoophycos traces, and horizontal and vertical burrows are commonly surrounded by diagenetic halos. Burrow fill is often coarser or finer than the surrounding material, suggesting some centimeter-scale sediment redistribution. Hydrogen sulfide gas was released when the cores were split. Small horizontal fissures caused by degassing are present throughout the three holes. Several ash layers are present throughout the unit (Table T8; Fig. F14).
Nannofossil abundance increases from the top (~20%) to ~50%-70% near 100 mcd. Farther downhole, nannofossil contents fluctuate, with average values gradually increasing downhole to values of ~80% at ~500 mcd. Below that level, nannofossils decrease again toward the base of the sequence (Fig. F15). Average calcium carbonate concentrations gradually increase below 100 mcd, reflecting the observed trend in nannofossils (Fig. F15). Foraminifers are most abundant in the upper ~85 mcd, approaching maximum values of >60%. Below ~85 mcd, foraminifer abundance remains between 0% and 10% down to 500 mcd. Diatom abundances are variable and range from ~0% to 30% throughout. Diatoms are absent or rare below 530 mcd. The foraminifer and nannofossil oozes in Unit I contain variable minor amounts of radiolarians, silicoflagellates, spicules, micrite, and siliciclastic components. The combined abundance of radiolarians, spicules, and silicoflagellates are quite variable throughout the sequence with pronounced minima at ~85 mcd and from 250 to 290 mcd (Fig. F15).
Clay minerals constitute ~90%-100% of the siliciclastic component with minor contributions from feldspars, amphiboles, micas, and pyroxenes. Siliciclastic content is highly variable with low mean values (~10%) throughout (Fig. F15). In the top ~10 mcd and bottom ~50 mcd of the sequence, siliciclastics are more abundant.
Unit I also contains the authigenic components micrite and pyrite. Micrite is absent to rare in the upper 40 mcd, increases to ~25% at 180 mcd, and then fluctuates between 0% and 15% downhole. Micrite content is highly variable between 40 and 400 mcd. Micrite-rich intervals are often associated with lower nannofossil abundance (Fig. F15). From 400 to 500 mcd, the micrite content decreases gradually, and below 510 mcd it increases toward the base of the sequence. Pyrite is finely dispersed (0%-2%) throughout and concentrated as framboids in biogenic material such as diatoms and foraminifers. Black sand-sized mafic grains are sparse in the sediments below ~400 mcd, increasing in size and abundance downhole. Below 519 mcd (Cores 202-1239A-52X through 55X), several black mafic sand layers and lithified black sand patches are present.
Twenty-four ash layers are present in Unit I, thirteen of which correlate among Holes 1239A, 1239B, and 1239C (Table T8). The ash layers range in thickness from 1 to 18 cm and are typically light to dark gray with sharp or bioturbated basal contacts and gradational, often bioturbated, upper contacts (Fig. F17). The ash is composed mainly of silt- to sand-sized clear volcanic glass shards, including unaltered platey and vesicular glass, and, less commonly, palagonite. The most common associated mineralogical components include feldspars, biotite, hornblende, pyroxenes, and pyrite.
All physical properties at Site 1239 show depth variability patterns that are generally similar to the ones observed at Site 1238 (Fig. F14) (see "Lithostratigraphy" in the "Site 1238" chapter). Magnetic susceptibility is moderately high with superimposed high-amplitude variability in the top 100 mcd and decreases to low values (generally <15 instrument units) between 100 and 150 mcd. From 150 to 300 mcd, the values are generally low, with pronounced spikes interspersed throughout. Below 300 mcd, a further decrease to <10 instrument units occurs. From 520 to 558 mcd, magnetic susceptibility increases again to moderately high values (10-80 instrument units).
GRA and MAD bulk density correlate well (r2 = 0.8) (Fig. F18). Bulk density increases in the upper ~25 m of the sequence related to compaction and dewatering (Fig. F18). Although bulk density seems to vary with depth similar to carbonate content, the correlation between the two is poor (r2 = 0.3) (Fig. F18).
In the a*-b* color space, all color measurements at Site 1239 plot in the "yellow" domain (Fig. F19). Although L* seems to vary with depth similar to the carbonate content, the correlation between the two is weak (r2 < 0.5). Predictive relationships between reflectance and carbonate and TOC via a multiple linear regression are also weak (i.e., r2 = ~0.6 for both components), reflecting the complexity of the sediment matrix that includes opaline silica and other chromophores.
Organic pigment absorption features are detectable at 410, 510, 560, and 650 nm in reflectance spectra measured in sediment at Site 1239 (Fig. F20). The strongest absorption feature at 650 nm that persists throughout the sediment column is due to chlorins (i.e., chlorophyll-related pigments) and is most pronounced between ~90 and 230 mcd.
Site 1239 is primarily characterized by rapidly (~5-10 cm/k.y.) accumulating pelagic foraminifer and nannofossil oozes. Sites 1238 and 1239 are both located in a similar setting in which changes in the supply of nutrients in the active equatorial upwelling zone are reflected in the varying amounts of siliceous or calcareous primary producers throughout their sequences. The largest shifts in microfossils at Site 1239 roughly coincides with the Pliocene/Pleistocene boundary (~100 mcd), where foraminifers become more abundant and nannofossil abundance decreases. Diatoms also generally becomes less abundant above this horizon. These changes take place within the context of a setting that remained relatively productive. The increase in TOC from 550 to 90 mcd (upper Miocene-lower Pleistocene) parallels a similar increase in U (see "Downhole Measurements"), both of which can be indicators of either a gradual increase in productivity with time or a gradual increase in degradation of organic matter (see "Geochemistry"). Nevertheless, the increased presence of chlorins, higher MARs (see "Age Model and Mass Accumulation Rates"), and a maximum in TOC from ~90 to 200 mcd suggest high productivity within this interval (~1.8-3 Ma) compared to the Pleistocene.
The cyclic pattern of light/dark color changes observed throughout Unit I was also observed at Site 1238. Preliminary spectral analyses of the magnetic susceptibility, GRA density, and lightness measurements between 0.0 and 58.2 mcd of the sequence at Site 1238 have significant concentration of power within orbital frequency bands associated with eccentricity, obliquity, and precession.
The observed susceptibility values in the upper 150 mcd of Unit I are most easily explained by generally higher, more variable input of fine-grained magnetic minerals associated with a terrigenous component because small changes in terrigenous content would be proportionally significant in the biogenically dominated sediment of Unit I. The increase in magnetic susceptibility is paralleled by a decrease in sedimentation rate (see "Biostratigraphy"), possibly associated with a decrease in biogenic components. Hence, less dilution may result in higher siliciclastic concentrations and thus in higher magnetic susceptibility values. On the other hand, diagenetic dissolution of magnetic minerals might have contributed to the low magnetic susceptibility values farther downhole (see discussion in "Lithostratigraphy" in the "Site 1238" chapter). Variability in NGR is probably controlled by U concentration in sediments (see "Downhole Measurements"); the average counts are higher than those at Site 1238. Grain densities are very similar at Sites 1238 and 1239, but average bulk density is higher at Site 1239, probably as a result of lower concentrations of biogenic silica. However, owing to their increased biogenic silica content (e.g., Silva et al., 1976), both Sites 1238 and 1239 have significantly higher porosities than the more southern Sites 1236 and 1237. Lithification-related general decreases in porosity and associated increases in bulk density are apparent in the lower half of the sequence (Fig. F18).
Volcanic ash deposition at Site 1239 began in the late Miocene to early Pliocene, increasing in frequency between the middle-late Pliocene and early Pleistocene. The accessory mineral composition of ashes at Site 1239 suggests an andesitic volcanic source, most likely from explosive eruptions in northern South America, Central America, and southern Mexico (Ledbetter, 1985). Based on sedimentation rate estimates of ~5 to 6 cm/k.y. (late Pleistocene) for Site 1239 (see "Age Model and Mass Accumulation Rates"), ash layer L (230 ka) (Bowles et al., 1973; Ninkovich and Shackleton, 1975) is present at 13.31 mcd in this sequence (Table T8). Based on its distribution in eastern tropical Pacific marine sediments, Ninkovich and Shackleton (1975) hypothesized the existence of a volcanic source in northern South America for this ash layer. Ash layer L is observed in sediment from Holes 1239B and 1239C (the corresponding interval in Hole 1239A spans a core gap). Unlike at Site 1238, where ash layer L was consistently 15-20 cm thick, ash layer L at Site 1239 is between 5 and 8 cm thick.
The shallowest occurrence of mafic sand-sized grains at Site 1239 is at ~400 mcd (~5 Ma). The downhole increase in abundance of these grains below 400 mcd and the presence of lithified sand layers toward the base of the sequence are consistent with a paleoposition of Site 1239 that is closer to the Galapagos hotspot than the modern position of Site 1239.
A 6.8-m.y. hiatus is identified between Sections 202-1239A-51X-CC (7.8 Ma) and 52X-CC (14.6 Ma) based on biostratigraphic datums (see "Biostratigraphy"). Although less well defined, a similar hiatus was found at Site 1238, spanning >12 to ~8 Ma. However, a major difference between the two sites concerns the formation of chalk and chert. Despite penetrating 512 m at Site 1239, no chalk or chert layers were observed, even though they were recovered at a shallower depth at Site 1238 (430 mcd). Pore water geochemical evidence suggests that the lack of chert formation at Site 1239 is most likely related to its lower thermal gradient relative to that at Site 1238 (see "Geochemistry").