A 424.7-m-thick (467.3 mcd) sediment sequence dating back to the middle Miocene (~11 Ma) was recovered from three holes at Site 1238. One lithologic unit divided into two subunits was defined at Site 1238 (Fig. F14). Subunit IA spans the upper ~400 m of the sequence and primarily contains bioturbated nannofossil ooze, diatom nannofossil ooze, and nannofossil diatom ooze with varying abundance of clay and foraminifers (Fig. F15). Intervals of distinct light and dark banding on a meter scale are present throughout this subunit. Several ash layers are present within the upper ~300 m, including the regionally correlative ash layer L (230 ka) (Bowles et al., 1973; Ninkovich and Shackleton, 1975). Magnetic susceptibility is highly variable from ~0 to 80 mcd (Pleistocene and uppermost Pliocene). A gradual downhole decrease accompanied by lower-amplitude variability at depths from ~80 to 105 mcd and low values from ~105 to 400 mcd may be associated with decreased terrigenous input, increased biogenic input, or diagenetic dissolution of magnetic minerals deposited prior to the large Pleistocene glacial events.
Subunit IB occupies the base of the sequence (~400-467 mcd) and is characterized by increased lithification and diagenesis. This subunit contains partially lithified diatom and nannofossil oozes interbedded with chalk and occasional chert horizons. The presence of chert and micrite indicates significant opal and carbonate diagenesis, respectively. Magnetic susceptibility increases downhole in Subunit IB toward basaltic basement. Increasing lithification is shown by increasing bulk density and decreasing porosity near the top of the subunit.
Overall, the alternating diatom and nannofossil ooze lithologies at this site reflect a moderate- to high-productivity pelagic setting. Evidence from total organic carbon (TOC) measurements, estimates of MAR, and the abundance of organic pigments indicate an interval of relatively high productivity from ~2 to 8 Ma as compared to the Pleistocene interval. Orbital-scale variability in this sequence is evidenced by rhythmic meter-scale light/dark banding in the sediment and preliminary time series analysis of magnetic susceptibility, GRA bulk density, and lightness measurements.
A single lithologic unit, Unit I, is defined and divided into two subunits (Table T7) on the basis of visual core description, smear slide analysis, thin section examination, color reflectance, X-ray diffraction (XRD) analysis, NGR, moisture and density (MAD), and GRA bulk density measurements. Subunit IA contains interbedded nannofossil ooze, diatom nannofossil ooze, and lesser amounts of diatom ooze. Subunit IB is characterized by significant lithification and diagenetic alteration of biogenic oozes that, prior to diagenesis, were probably similar to those that are present in Subunit IA.
Subunit IA contains interbedded nannofossil and diatom oozes. Minor components include foraminifers, radiolarians, silicoflagellates, clay, micrite, and pyrite. The sediment color varies gradationally between olive, olive gray, light olive gray, and light gray. Mottling, color banding, and burrow traces, particularly Zoophycos, are common to pervasive (Fig. F16). 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 in the upper ~300 m of this subunit (Table T8; Fig. F14).
The major lithologies of Subunit IA are nannofossil ooze, diatom nannofossil ooze, and diatom ooze. Transitions between nannofossil- and diatom-rich oozes occur on a meter to decimeter scale. Several intervals of rhythmic meter-scale banding between light nannofossil-rich and dark diatom-rich sediment are present throughout Subunit IA (Fig. F17A). Physical properties measurements within these intervals also show the same meter-scale cyclicity (Fig. F17B).
The biogenic oozes in Subunit IA contain varying proportions of calcareous nannofossils, diatoms, foraminifers, radiolarians, silicoflagellates, micrite, and siliciclastic components. Nannofossil abundance ranges from ~10% to 75% in Subunit IA, with a pronounced minimum at 100 mcd that is accompanied by lower carbonate contents (Fig. F15). A generally increasing trend in nannofossil abundance from ~100 to 400 mcd coincides with a gradual increase in CaCO3. Diatom abundances range from ~15% to 20% in the upper 50 m and then approaches a broad maximum of 30%-50% between 50 and 200 mcd. Below ~200 mcd, diatom abundance becomes more variable, with generally lower values (Fig. F15). Foraminifers are more abundant in the upper ~100 m, approaching maximum values of ~40%. At depths >100 mcd, foraminifer abundance decreases. Radiolarians and silicoflagellates are present in amounts of a few percent throughout most of Subunit IA.
Siliciclastic content is highly variable and primarily fluctuates around ~10% throughout Subunit IA (Fig. F15). Clay minerals constitute ~90%-100% of the siliciclastic fraction. Small amounts of feldspars, amphiboles, mica, and pyroxenes are present throughout the subunit. Authigenic components include micrite and pyrite. Micrite-rich intervals (~95-110 and ~155-190 mcd) are often associated with lower nannofossil abundance (Fig. F15). Pyrite is present in minor abundances (up to 5%) throughout the subunit, typically as infill in diatoms and foraminifer tests.
Seventeen ash layers were found in Subunit IA, ten of which are correlative between holes (Table T8; Fig. F14). The ash layers in this subunit range in thickness from 1 to 20 cm and are typically light to dark gray with sharp basal contacts and diffuse upper contacts (Fig. F18). Ash patches often appear just below the ash layers, suggesting bioturbation, although some reworked features may have resulted from the coring and splitting process. The ash layers are composed mainly of silt- to sand-sized clear volcanic glass shards, including unaltered platy and vesicular glass, and, less commonly, palagonite. The most common associated mineralogical components include feldspars, biotite, hornblende, pyroxenes, and pyrite, indicating an andesitic source. Minor amounts (0%-4%) of volcanic glass are also disseminated throughout the sediment of this subunit.
Magnetic susceptibility is relatively high, with larger-amplitude variations from ~0 to 100 mcd. Lower volumes and lower-amplitude variations mark the interval from ~100 to 400 mcd (Fig. F14). GRA and MAD bulk densities are well correlated to each other (r2 = 0.85) (Fig. F19). An increase in bulk density from ~0 to 30 mcd is accompanied by a decrease in porosity that is likely related to compaction, dehydration, and increasing carbonate contents (Figs. F14, F19).
In the a*-b* color space, all color measurements at Site 1238 plot in the "yellow" (b* > 0) domain (Fig. F20). The relatively carbonate-poor interval from ~80 to 102 mcd, which has low lightness values (Fig. F16), still lies in the "yellow" domain but is shifted in the a*-b* space to values of a* > 0 (Fig. F20). The sediment hue in this subunit is extremely homogeneous (i.e., the ratio of a* to b* is constant). Predictive relationships between color reflectance (a*, b*, and L*) and carbonate and TOC via multiple linear regression are relatively weak (i.e., r2 = ~0.6 for each component), reflecting the complexity of the sediment matrix that contains biogenic silica and other chromophores.
Organic pigment absorption features are detectable at 410, 510, 560, and 650 nm in reflectance spectra for sediment of the lower part of Subunit IA (~100-400 mcd). The strongest absorption feature at 650 nm, which is due to chlorins (i.e., chlorophyll-related pigments), persists throughout the sediment column and is most pronounced between ~75 and 200 mcd (Fig. F21).
Subunit IB contains upper to middle Miocene partially lithified biogenic oozes interbedded with chalk and chert. The sediment color ranges from light gray to white with occasional light-green and purplish gray bands.
The dominant lithologies of Subunit IB are diatom-bearing nannofossil ooze, nannofossil diatom ooze with micrite, and nannofossil ooze (Table T7). Diatoms, radiolarians, and silicoflagellates are relatively abundant within the upper half of Subunit IB and disappear below ~430 mcd. The lower part of the subunit is dominated by nannofossils (up to ~80%). Micrite abundance is generally higher than in the overlying subunit and fluctuates between ~10% and 30% (Fig. F15).
Minor lithologies in Subunit IB include chalk and chert. The first occurrence of chalk (Fig. F22A) is at ~430 mcd, coincident with the disappearance of siliceous microfossils. Thin sections of the chalk show abundant foraminifers, many of which are pyritized. Nannofossils are rare to absent, and radiolarians and diatoms are present. Chalcedony is also observed in the thin sections, often as infill associated with foraminifer tests. XRD analysis of the chalk from Sample 202-1238-42X-1, 23-24 cm, shows a high calcite content (~80%-90%) with smaller trydimite (opal-CT) and quartz peaks (Fig. F22B). The quartz peak may be associated with the chalcedony, and the trydimite peak suggests a modest contribution from biogenic silica. Distinct chert horizons are present at the base of Subunit IB in Section 202-1238-46X-1 (Fig. F23).
From the top of Subunit IB (~400 mcd), magnetic susceptibility increases downcore toward basaltic basement (Fig. F14). A lithification-related decrease in porosity and an associated increase in bulk and grain density begin at ~430 mcd, where chalk first occurs, and persist throughout the remainder of Subunit IB (Fig. F19).
Site 1238 lies within an active upwelling zone and is characterized by rapidly accumulating nannofossil and diatom oozes. Changes in upwelling and the supply of nutrients, as well as carbonate dissolution, may be reflected in the varying predominance of siliceous vs. calcareous primary producers throughout Subunit 1A. In general, silicious microfossils and organic carbon concentrations increase from ~8 to 1 Ma, whereas carbonate concentrations decrease. Mass accumulation rates of biogenic components display a maximum between ~6.5 and 3.5 Ma, suggesting a phase of high productivity from the late Miocene to mid-Pliocene (see "Age Model and Mass Accumulation Rates"). The influence of siliceous sediment deposition increases, especially after 4 Ma, culminating in a maximum between ~2 and 1 Ma, an interval that again is marked by increased mass accumulation rates in biogenic components. A significant change in sediment deposition accompanies the mid-Pleistocene climate deterioration at ~1 Ma. The interval of the last 1 m.y. is marked by low MARs associated with a strong decrease in siliceous microfossil and organic carbon concentrations and an increase in carbonate contents. This inference of high productivity, based on the abundance of siliceous sediments, is consistent with organic carbon driving sulfate reduction, which occurs throughout the upper ~400 m of this sequence, but never to completion (see "Geochemistry"). The sediments offer no evidence for episodes of anoxia, based on the continuous presence of burrows and other bioturbation features. The long-term evolution in biogenic productivity does not reflect the predicted changes in productivity that may result from the eastward paleodrift of Site 1238 toward the coastal upwelling region off Ecuador (see "Introduction"). The paleodrift of Site 1238 toward its modern location would suggest an increase in productivity over the last 10 m.y. Thus, the documented variability in productivity may reflect changes in upwelling and nutrients that are driven by changes in oceanography rather than resulting from plate tectonic movement of the site location.
Preservation of carbonate and siliceous microfossils remains good for the majority of this interval. Persistent mottling, gradational color transitions, abundant burrows, and Zoophycos traces all indicate vigorous bioturbation associated with a robust benthic ecosystem supported by generally high organic carbon flux to the seafloor.
Magnetic susceptibility changes from a baseline with negligible values and little change (0 ± 2 instrument units) in the Pliocene interval to increasingly higher values (~5-10 instrument units) and larger-amplitude variability in the Pleistocene interval (Fig. F14). This shift begins at ~90 mcd and continues through the upper Pleistocene sediment sequence, suggesting a possible link to the intensification of the mid-Pleistocene global glaciation, which is marked by the onset of predominant 100-k.y. climate cycles.
Time series analysis on magnetic susceptibility, GRA bulk density, and lightness measurements for the uppermost portion (0.0-58.2 mcd) of the sequence at Site 1238 (representing ~0-1 Ma) using a preliminary biostratigraphic age model shows significant power in and around orbital frequency bands associated with eccentricity, obliquity, and precession (Fig. F24) and thus may be related to glacial-interglacial changes in the relative supplies of biogenic and terrigenous material. These preliminary results are based on linear sedimentation rates (LSRs) interpolated between limited biostratigraphic datums and warrant further analysis once better age control is established.
Higher susceptibility values in the uppermost portion of this sequence may be explained by increased 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 Subunit IA. Decreased input of biogenic material in this interval would also result in higher magnetic susceptibility. Based on preliminary biostratigraphic estimates across this interval, there is no evidence for a significant change in sedimentation rate that would accompany increased terrigenous or biogenic input. Another possible explanation for this change is related to the abundance and dissolution of magnetic minerals. Diagenetic dissolution of magnetic minerals below this interval, related to a higher concentration of TOC and therefore a more reducing sedimentary environment, would yield lower magnetic susceptibility at depths below ~90-100 mcd.
Although variations in the bulk density are largely controlled by carbonate content at this site, the average bulk density in Subunit IA is lower than at previous sites (e.g., by ~0.2 g/cm3 relative to Site 1236, where carbonate was present in a proportion >95%). Porosity in this subunit is also significantly higher than at previous sites (i.e., by ~10% relative to Site 1236, which contained abundant calcium carbonate). The relatively high proportion of biogenic silica at Site 1238 is likely responsible for both the density and porosity differences. Biogenic silica, which has a grain density of 2.0 g/cm3 (diatoms), has a rigid, open structure that tends to keep pore spaces open (Silva et al., 1976).
Volcanic ash deposition began in the late Miocene at Site 1238 and increased in frequency during the last 2 m.y. The accessory mineral composition of ashes at Site 1238 suggests an andesitic volcanic source, most likely from explosive eruptions in northern South America, Central America, and southern Mexico (Ledbetter, 1985). An ash layer containing 80%-90% clear, platy glass with feldspars, hornblende, biotite, and pyroxene, which we infer to be the ash layer L of Bowles et al. (1973) and Ninkovich and Shackleton (1975), is present at 13.15 mcd in this sequence. Based on its distribution in eastern tropical Pacific marine sediments, Ninkovich and Shackleton (1975) hypothesized a volcanic source in northern South America for this ash layer. This ash is observed in all three holes and is consistently 15-20 cm thick at this site (Fig. F17), which is thicker than any other ash layer L deposits recorded in the eastern tropical Pacific from 10°S to 5°N (Ninkovich and Shackleton, 1975). The age of ash layer L is ~230 ka (Ninkovich and Shackleton, 1975), suggesting a late Pleistocene sedimentation rate of 5-6 cm/k.y., which is consistent with the preliminary age model (see "Age Model and Mass Accumulation Rates") at this site.
The presence of partially to completely lithified biogenic material and the increase in authigenic components such as micrite and chalcedony at the base of the sequence recovered at Site 1238 suggests significant postdepositional diagenetic alteration. Recrystallized calcium carbonate in the form of micrite is present at Site 1238 from ~100 mcd to the base of the sequence. Micrite is typically observed in association with the nannofossils that are the major constituents of the oozes at this site. The abundance of micrite increases during periods of diminished nannofossil and increased siliceous microfossil abundances. An increase in productivity, as suggested by higher TOC and biogenic silica (diatom abundance), likely results in dissolution of biogenic calcite and subsequent authigenic precipitation, which is consistent with pore water alkalinity and calcium profiles (see "Interstitial Water Geochemistry" in "Geochemistry").
Light green and purplish gray banding and mottling in Subunit IB are present from ~430 mcd to the base of the sequence. Pyrite abundance within this interval ranged from ~2% to 4%. Previously described similar color banding in carbonate oozes on the Ontong Java Plateau (ODP Leg 130) was associated with Fe-bearing clays, some of which contained Fe sulfides, such as pyrite, depending on local redox conditions (Lind et al., 1993).
At the base of this sequence (~400-470 mcd), the presence of chert and chalcedony provides significant evidence for opal diagenesis. The dissolution of biogenic silica occurs throughout the sequence, but recrystallization is greater at depth, perhaps as a result of warmer ambient temperatures here (see "Geochemistry"). Below ~430 mcd, the sediment is depleted in biogenic silica but contains chalcedony infill and discrete chert nodules. The chalcedony- and chert-bearing intervals correspond to a large decrease in pore water silicate (see "Geochemistry"), which suggests complete dissolution of siliceous microfossils and subsequent migration of dissolved silica in the pore water to sites of reprecipitation. If the thermal gradient remains constant, this diagenetic boundary will likely remain fixed relative to the seafloor but will migrate upward from basement as more sediment is deposited.