A 203.7-m-thick sequence (238.9 mcd) of hemipelagic sediments from the late Quaternary was recovered at Site 1234. A single lithostratigraphic unit is primarily defined based on visual core description and smear slide examination (Table T7; Fig. F8). The major lithology of this unit consists of homogeneous dark olive-gray to dark gray diatom nannofossil silty clay and clay, with subtle and gradational color changes. Siliciclastic components are dominated by clay minerals and feldspar. Minor components primarily include quartz, amphibole, pyroxene, mica, and opaque minerals. Authigenic calcite and pyrite are present throughout the sequence. The biogenic fraction is dominated by calcareous nannofossils and diatoms. The ratio of siliciclastic vs. biogenic components generally increases downhole.
Thin silt-rich layers and volcanic ash layers are present as minor lithologies. Magnetic susceptibility is highly variable and reaches high values of up to 600 instrument units. Bulk density shows little downhole trend at Site 1234. As at Site 1233, sediments at Site 1234 are consistent with a hemipelagic upper continental slope setting characterized by high fluvial sediment input from the Chilean Coastal Range and the Andes. Thin silt layers may represent distal turbidites. Some intervals appear to be significantly affected by diagenesis, particularly in the lower half of the recovered sequence.
The major lithologies of Unit I are diatom nannofossil and diatom- or nannofossil-bearing silty clay and clay with little visual variability and few sedimentary structures. The sediment color within Unit I typically ranges from dark olive gray to dark gray. Most color changes are subtle and gradational, indicating small compositional variations within the dominant lithology. Black spots and dispersed mottling originating from disseminated monosulfides are common on the surface of newly split cores. These features fade within 1 to 2 hr after splitting and may introduce noise to the color reflectance data, depending on the time elapsed between splitting and color measurement. Bioturbation is rarely visible within the major lithology, probably as a result of the homogeneous lithology. Macroscopic shell fragments are common, primarily randomly oriented, and partly concentrated in patches.
Clay and feldspar (primarily plagioclase) are the predominant siliciclastic components in the sediments (Fig. F9). Quartz is generally present in minor amounts except for two intervals at ~40 and ~80 mcd. Above these maxima, the quartz/feldspar ratio gradually increases and drops abruptly below. Further, primarily silt-sized siliciclastic components within the major lithology include amphiboles, pyroxenes, glauconite, opaque minerals, and volcanic glass, which are present in variable amounts throughout the sequence.
The ratio of siliciclastic vs. biogenic components generally increases downhole (Fig. F9). Biogenic components primarily consist of calcareous nannofossils and diatoms. Nannofossils are the dominant biogenic component and reach up to >30% the upper ~40 mcd, generally decrease downhole until ~160 mcd, and are absent below that depth (Fig. F9). Foraminifers are present in very minor amounts (mostly 
1%) throughout the sequence. Diatoms are more abundant and present in variable amounts of generally a few percent except for the interval from ~50 to 90 mcd, where up to ~10% is reached. Other siliceous microfossils are present in minor amounts and include ubiquitous sponge spicules and, in some intervals, radiolarians and silicoflagellates.
Authigenic components include relatively abundant pyrite reaching up to ~10% and monosulfidic minerals. Inorganic calcite is present in significant amounts of more than ~5% in the lower half of the sequence and mirrors the decrease in calcareous microfossils. Intervals of increased inorganic calcite contents between ~130 and 140 mcd, 150 and 180 mcd, and 210 and 220 mcd are characterized by higher bulk density and reduced porosity and water content.
Notable minor lithologies within Unit I include silt-rich layers, dark clay layers, and volcanic ash layers. The silt-rich layers (<1 cm thick) are primarily clayey silt composed of clay minerals, feldspar, and opaque minerals. Many of the layers are discontinuous, soupy, or disturbed (e.g., Core 202-1234A-10H). Thin (~1-5 cm) layers of very dark olive-gray clay are also occasionally present (Fig. F10). These layers contain significant amounts of glauconite and opaque minerals and little or no biogenic material. They are often discontinuous, likely as a result of bioturbation. Similar sediment is sometimes present within mottled intervals or as burrow fill.
Eight layers of volcanic ash were recovered in Unit I (Table T8). These ash layers dominantly consist of clear volcanic glass with rare accessory minerals, including andesine or labradorite plagioclases (Table T8). Three ash layers found in Hole 1234A are brown and have diffuse boundaries (e.g., interval 202-1234A-3H-5, 115-122 cm). One ash layer in Hole 1234A (Fig. F11) and two in Holes 1234B and 1234C occur as white patches (e.g., interval 202-1234C-6H-3, 47-53 cm). The others found in Holes 1234B and 1234C are brown and have sharp lower boundaries and diffuse upper boundaries.
The sediments at Site 1234 were slightly to moderately disturbed by the coring process, primarily as a result of gas expansion (methane and hydrogen sulfide) (see "Geochemistry"). Coring disturbance features are expressed as porous, soupy sediment, occasional larger gas voids, and numerous small fissures. In some cases (e.g., Sections 202-1234A-17X-4 and 17X-5) sediments were ejected from the liner and had to be reinserted.
Magnetic susceptibility is highly variable, especially in intervals with higher than average values. Intervals of low magnetic susceptibility exhibit less variability (~120-180 mcd and ~210-220 mcd) (Fig. F8). Lack of magnetic minerals in these intervals is presumably due to reductive diagenesis (see "Paleomagnetism").
GRA bulk density and moisture and density bulk density measurements parallel one another with a variable offset of ~0.1 to 0.3 g/cm3, but they are poorly correlated (Fig. F12). Although GRA bulk density may be strongly affected by coring disturbance, neither type of density measurement shows discernable downhole trends toward higher values that are usually ascribed to compaction. Instead, bulk densities seem highly variable, presumably due to time-dependent diagenesis that can change the physical properties of the sediment locally through textural changes and cementation. Porosity and water content are also variable (Fig. F13), most likely because of localized incipient lithification (i.e., cementation and dehydration).
In the a*-b* color space, all color measurements at Site 1234 plot in the "yellow" domain (Fig. F14). However, sediment colors are more chromatic than at previous sites, and they exhibit a weak bimodal distribution with one trend characteristic to brownish sediments (a* > 0) and the other typical to more greenish sediments (a* < 0). The lightness (L*) (Fig. F8) appears to be low where high total organic carbon (TOC) and high NGR sediments are encountered, but this correlation is not confirmed by a simple regression. Assuming a two-component chromatic system, preliminary predictive relationships between reflectance, carbonate, and TOC via a multiple linear regression are weaker than at Site 1233 (i.e., r2 = ~0.5 for carbonate, and r2 = ~0.7 for TOC).
Sediments at Site 1234 are primarily composed of fine-grained siliciclastics consistent with undisturbed hemipelagic sedimentation, except for occasional thin silt-rich layers that may represent distal turbidites. Pathways of major turbidity currents in the region are known to be confined to large submarine canyons (Thornburg and Kulm, 1987). The calcareous biogenic components diminish and their preservation decreases (see "Biostratigraphy") in the lower half of the recovered sequence, suggesting increasing diagenesis downhole.
Nannofossil biostratigraphy indicates that the cored sequence is younger than 260 ka, suggesting that sedimentation rates are very high at Site 1234. As at Site 1233, these above-average sedimentation rates are most likely the result of high rainfall in the Chilean hinterland. Onshore from Site 1234, the largest Chilean river (Bío-Bío) primarily drains easily erodible source rocks from the Andes. Smaller rivers supply additional material from the Coastal Range. Changes in the ratio of quartz vs. feldspar in the younger part of the sequence (Fig. F9) may indicate changes in the relative contribution of lithologically different source rocks from these source regions.
Based on preliminary shipboard paleomagnetic evidence (see "Paleomagnetism"), the Laschamp Excursion (~41 ka) is located at a depth of ~23 mcd at Site 1234 compared to ~70 mcd at Site 1233 (see "Age Model and Mass Accumulation Rates" in the "Site 1233" chapter), suggesting comparatively lower sedimentation rates at Site 1234 during marine isotope Stages 1 to early 3, which may be consistent with northward decreasing rainfall and correspondingly less fluvial sediment supply in central Chile. The basal age of <260 ka suggests higher sedimentation rates at Site 1234 prior to the Laschamp Excursion, whereas sedimentation rates were significantly lower at Site 1233 (see "Age Model and Mass Accumulation Rates" in the "Site 1233" chapter). This may indicate that changes in transport pathways of terrigenous material and syndepositional sediment focusing might also have influenced siliciclastic sedimentation at both sites.
Site 1234 is located within the highly productive upwelling area off Concepción. For such a setting, contents of biogenic components appear comparatively low as a result of the overwhelming dilution by siliciclastics. Biogenic components are primarily dominated by calcareous nannofossils, except for the interval from ~40 to 90 mcd, where diatoms reach equal amounts. This shift may be related to a change in the extension of the Concepción upwelling cell.
The presence of authigenic monosulfides, pyrite, and inorganic calcite in the sediments at Site 1234 reflects diagenetic processes associated with degradation of organic matter. The increased abundance of authigenic calcite and the disappearance of calcareous nannofossils downhole (Fig. F9) imply an increase of diagenesis in the lower part of the sections, resulting from the dissolution and recrystallization of primary biogenic calcite (see "Geochemistry").
Ash layers observed at Site 1234 are mainly composed of clear volcanic glass shards with minor amounts of andesine or labradorite plagioclases and trace amounts of quartz, mica, amphiboles, and pyroxenes, suggesting a nearby source of intermediate-composition volcanism, such as the Andes. Bioturbation may have resulted in mixed and/or homogenized sediments of volcanic ash and the major lithology suggested by the ubiquitous presence of volcanic glass outside the distinct ash layers. Two ash layers recognized from approximately equivalent depth intervals in Holes 1234B and 1234C (Table T8) provide a useful interhole correlation. These two ash layers were not found in Hole 1234A because of coring gaps. Lack of interhole correlation between the other ash layers found in Hole 1234A may be caused by dispersion of the layers by bioturbation.
