A 181.44-m-thick (214.46 mcd) hemipelagic sediment sequence from the late Quaternary was recovered from three holes at Site 1235. A single lithologic unit, Unit I, was defined on the basis of visual core description and smear slide analysis (Table T7). Lithologic Unit I is composed primarily of siliciclastic silty clay, with little lithologic variability. Millimeter-scale silt-rich layers and macrofossils (bivalves and gastropods) are present throughout the cored interval. The silt fraction is dominated by feldspar, with lesser amounts of quartz, pyroxenes, and volcanic glass. Biogenic components, mainly nannofossils and diatoms, are low, ranging from 0% to 18% of the sediment. The authigenic components, pyrite and disseminated authigenic carbonate, are present in variable abundance, and carbonate concretions are intermittently present downhole.
Distinct minima in magnetic susceptibility associated with color reflectance maxima in b* (yellowish) and minima in a* (greenish) may indicate reductive diagenesis. Values of NGR are relatively constant until 180 mcd, where they increase by ~20 cps (Fig. F8). Sediment color varies between dark olive gray, olive gray, and, to a lesser extent, gray.
Overall, the sequence at Site 1235 represents rapidly accumulating hemipelagic sediments dominated by siliciclastics. Fossil biogenic components are typical of an active upwelling zone on a continental margin, and the presence of burrows and benthic faunal components reveals no episodes of bottom water anoxia. Microbial degradation of organic matter resulted in the formation of methane (and perhaps methane hydrate) and authigenic minerals deeper in the section.
The dominant lithology defining Unit I is a massive unit of silty clay with dispersed fragments of bivalves and gastropod fossils. Three minor lithologies with irregular distributions have been identified in Unit I: (1) silt-rich layers and patches, (2) authigenic carbonate-rich horizons (nodules and layers), and (3) an ash layer. Color changes are gradual, from dark olive gray to olive gray and gray, with the exception of a few distinct shifts toward light olive associated with the occurrence of carbonate concretions. Unstable iron monosulfides, which dissipated after 1-1.5 hr of exposure, are abundant at depths above 40 mcd. Bioturbation is pervasive. As a result of degassing after recovery, occasional larger voids and abundant small tissues are present in sediments throughout the sections in all three holes.
Siliciclastic components are dominated by clay minerals. Silt contents range from ~10% to 40% (Fig. F9). The primary minerals found in the silt fraction of the silty clay include feldspar, quartz, and pyroxenes; all increase in abundance within the relatively silt-rich intervals (30%-45% silt) observed at 50, 115, and 140 mcd. Enrichment of volcanic glass occurs mainly between 30 and 60 mcd, with a distinct peak at 40 mcd. Below ~130 mcd, percentages increase slightly (Fig. F9).
Calcareous nannofossils, diatoms, and lesser amounts of sponge spicules dominate the biogenic fraction. Foraminifers, radiolarians, and silicoflagellates are rare. Carbonate components, including nannofossils, foraminifers, and carbonate bioclasts, are grouped here as biogenic carbonate fraction, whereas diatoms, radiolarians, sponge spicules, and silicoflagellates make up the biogenic silica fraction (Fig. F9). Both biogenic fractions are more abundant in the upper part of the hole but decrease downhole from 50 mcd. The biogenic silica fraction again increases below 130 mcd, whereas the biogenic carbonate fraction remains relatively low to ~190 mcd, where it too increases (Fig. F9). Authigenic carbonate is common, and occasional carbonate nodules are present in intervals bearing <5% nannofossils and rare to absent foraminifers (Fig. F9). Opaque minerals, mainly authigenic pyrite, range in abundance from 0.5% to 20%. A notable peak in opaque minerals at 98 mcd is associated with an authigenic carbonate-rich interval (Fig. F9).
The most common minor lithology in Unit I comprises silt-rich layers and patches. With the exception of one layer (>1 cm) at 12.76 mcd, silt-rich layers are mostly millimeter scale and are randomly distributed downhole. Regardless of thickness, the silt layers appear more frequently in the top half of the drilled interval. In general, silt-rich layers have bioturbated contacts. Silty patches (0.5-3.0 cm thick and occasionally as long as 40 cm) are frequent and often related to bioturbation and/or represent the infilling of burrow traces. The silt within the discrete layers and patches has a mineralogic composition similar to the silt dispersed in the major lithology.
Authigenic carbonate, as nodules and partially lithified sediment layers, represents the second minor lithology. One of the nodules (interval 202-1235C-10H-1, 20-21 cm) had carbonate concentrations of ~55%. X-ray diffraction results from another nodule (interval 202-1235B-2H-6, 5-10 cm) demonstrated the dominant composition to be dolomite (Fig. F10). The nodules range in size from 2 to 15 cm. The partially lithified authigenic carbonate-rich sediment is present as 10- to 15-cm-thick layers. This lithology, whether a completely formed nodule or a partially lithified layer, is typically present within light olive-colored sediment enriched in authigenic carbonate (Fig. F10). In addition, these intervals have characteristically low values in both magnetic susceptibility and a* color reflectance (Fig. F11). The carbonate-rich layers and nodules are present within intervals of authigenic carbonate-rich light olive-green sediments, which appear below a thick package (0.8-6 m) of highly bioturbated sediment that includes abundant color mottles, silty patches, and long burrows (up to 40 cm) apparently created by macrobenthic fauna (Fig. F12). Three examples of this facies succession, ranging in thickness from 3 to 16 m, are clearly recognizable and correlative among Holes 1235A, 1235B, and 1235C.
A 5- to 6-cm-thick pale ash layer represents the third minor lithology and is present only in Hole 1235B at 150.5 mcd (Fig. F13). The base of the ash has a sharp, scoured contact. Several ash patches 5-10 cm below the ash layer indicate bioturbation. The ash is composed mainly of volcanic glass shards associated with minor amounts of labradorite plagioclase and trace amounts of quartz, mica, amphiboles, and orthopyroxene.
Magnetic susceptibility at Site 1235 varies between 20 and 780 instrument units over most intervals. Four distinct extended intervals of low magnetic susceptibility are present at ~74-77, ~89-103, ~169-178, and ~188-214 mcd (Figs. F8, F11). Loss of magnetic minerals in these low-susceptibility intervals is presumably the result of strong reductive diagenesis (see "Paleomagnetism").
GRA bulk density (Fig. F8) and moisture and density (MAD) measurements (Fig. F14) track one another with a variable offset of ~0.1 to 0.3 g/cm3 (Fig. F15). GRA data are noisy, presumably because of gas-related voids and fissures. The slight downhole increases in both GRA and MAD bulk densities and the decrease in porosity (Figs. F8, F14) are usually ascribed to compaction and dehydration. Bulk density and porosity may be affected by local diagenesis below ~180 mcd (Fig. F14) through textural modifications and cementation (e.g., authigenic carbonate precipitation) (see above and Fig. F11). The uppermost ~50 m of sediments exhibits low bulk density and high porosity (Fig. F14) that could be related to an increased content of biogenic silica (Fig. F9), which tends to keep pore spaces open with a more rigid structure (e.g., Silva et al., 1976).
In the a*-b* color space, all color measurements at Site 1235 plot in the "yellow" (b* positive) domain (Fig. F16). As at Site 1234, they exhibit a weak bimodal distribution, with one trend characteristic of brownish sediments (a* > 0) and the other representing more greenish sediments (a* < 0). Assuming a two-component chromatic system, preliminary predictive relationships between reflectance, carbonate, and total organic carbon (TOC) via a multiple linear regression are weaker than at Site 1233 for carbonate (r2 = ~0.3) but stronger for TOC (r2 = ~0.8).
The greenest sediments are present in diagenetically modified intervals (i.e., in magnetic susceptibility lows) and coincide with relatively high abundances of pyrite and sulfur (see "Geochemistry"). Therefore, we infer that the green hue is the result of an increased Fe2+/Fe3+ ratio in clay mineral structural iron due to intense sulfate reduction (e.g., Lyle, 1983; Giosan, 2001). These diagenetically modified intervals also show a consistent color pattern (Fig. F11), starting with a brownish zone (a* > 0) that passes into an increasingly green zone (increasing a*, but with a* < 0) farther downhole. The brownish zones also contain hematite and probably goethite (Fig. F17).
The entire cored interval falls within nannofossil Zone NN21 (<0.26 Ma) (see "Biostratigraphy"). The hemipelagic sedimentation is dominated by siliciclastic materials containing abundant feldspar, with minor quartz, pyroxenes, and volcanic glass, indicating a dominant Andean source for siliciclastic material. This composition is typical for surface sediments from the Chile margin between 25° and 43°S (Lamy et al., 1998). The dominance of the siliciclastic component and the extremely high sedimentation rates of >800 m/m.y. suggest a significant fluvial contribution by the nearby rivers (Bío-Bío and Itata). Although turbidites are channeled away from Site 1235 in large canyons, the presence of very thin silt-rich layers might be associated with distal turbidite or dilated overflow deposits.
The one volcanic ash layer, composed primarily of volcanic glass and trace labradorite plagioclase, is intermediate in composition and is likely derived from volcanism originating in the nearby Andes. An interval enriched in volcanic glass is present between 30 and 60 mcd, with a pronounced peak at 40 mcd, indicating increased volcanic activity and delivery of disseminated ash during this interval.
Site 1235 currently underlies a highly productive coastal upwelling zone off Chile. The relatively low percentages of biogenic components together with low TOC values (see "Geochemistry" for TOC concentrations) indicate that the signal of highly productive surface waters is strongly diluted by the terrigenous components. Evidence for high surface water productivity includes the common presence of an active macrofaunal population. The pervasive gas-produced fissures and voids also are consistent with the interpretation that abundant biogenic components, including organic carbon, are reaching the sediment, even if they are not well preserved and/or are diluted.
The decrease in biogenic components at >60 mcd may reflect increased dilution by terrigenous sediment or a decrease in either preservation or deposition of biogenic components. We found no systematic trends in the silt/(silt+clay) values or in the presence of silty layers and patches. However, the increased presence of both opaques (pyrite) and authigenic carbonate in discrete intervals is consistent with diagenesis and decreased biogenic carbonate preservation. These intervals likely represent times of enhanced productivity, with organic carbon and carbonate fluxes that led to intense biogenic carbonate dissolution, sulfate reduction, and precipitation of authigenic carbonate. Sulfate reduction is required to generate both the reduced sulfur and increased alkalinity sufficient to drive the formation of authigenic pyrite and carbonate, respectively. Organic carbon is the primary energy source to drive these reactions within the sediment. Variability in the bottom water oxygen characteristics could also drive the sedimentary system toward increased sulfate reduction, producing these diagenetic processes without an intensification of productivity. At present, Site 1235 lies near the boundary between the low-oxygen GUC and oxygen-rich AAIW. If the position of the GUC changed with time, either as the result of sea level changes or general variability in circulation, it may have modulated the oxidation rate of organic matter near the sediment/water interface.
The facies succession, with strongly bioturbated sediment overlying the enrichments in authigenic carbonate, indicates a repetitive pattern in the sequence of sedimentation. The presence of hematite and goethite demands preservation of former oxic zones that lay above strongly reduced green sediments containing authigenic carbonate and pyrite. The mineralogy and color are consistent with zones of strong redox gradients that are typical for periods of drastic change in sedimentation rates (e.g., Thomson et al., 1996; Giosan, 2001).