Three lithologic units were identified at Site 1146 (Fig. F5). Unit I is composed of greenish gray nannofossil clay that extends from 0 to 242.68 mcd. This unit is relatively enriched in quartz, plagioclase, and chlorite and depleted in calcite compared with the lower units. Unit II consists of light brownish gray clayey foraminifer and nannofossil ooze and foraminifer clay nannofossil mixed sediment that continuously grades down into the green nannofossil clay of Unit III at 553.02 mcd. Unit II is divided into two subunits based on shifts in mineralogy determined by X-ray diffraction (XRD) and carbonate coulometry of sediments from Hole 1146A (Fig. F6). The XRD analyses indicate an increase in illite, kaolinite, and quartz concentrations across the Subunit IIA/IIB boundary. A decrease in the calcite concentration and the disappearance of dolomite also occur across this boundary. Subunit IIA includes sediments from 242.68 mcd through 434.22 mcd and contains slightly more carbonate-rich sediment than Subunit IIB, which continues down to 553.02 mcd. The green nannofossil clay of Unit III dominates below this level to the bottom of Holes 1146A and 1146C (642.31 mcd). This unit is defined by changes in color and "iron sulfide" concentration; there is little mineral change across the Unit II/III boundary.
Unit I mainly consists of clay, nannofossil clay, and clayey nannofossil ooze. The greenish gray nannofossil clay contains abundant foraminifers. Pteropods, including individuals >1 cm across (e.g., Section 184-1146A-2H-2, 102 cm; 7.37 mcd), are common in the upper part of Unit I. Nannofossils are common throughout the unit. Sedimentary structures are rarely noted in the core since the clay is completely homogenized by bioturbation. However, shifts in color toward lighter tones correspond to higher nannofossil content (clayey nannofossil ooze), for example, at interval 184-1146A-7H-4, 0-70 cm (58.4-59.1 mcd), and at interval 184-1146A-9H-5, 10 cm, to 9H-6, 100 cm (80.1-82.5 mcd) (Table T5). Foraminifer turbidites are absent in Unit I but are observed in Units II and III. An ~10-cm-thick red-brown oxidized sediment layer is observed at the top of Cores 184-1146A-1H and 184-1146B-1H and is presumed to be the modern seafloor. This layer was not recovered in Hole 1146C.
The bulk mineralogy as determined by X-ray diffraction (Fig. F6) indicates that the sediments in Unit I are relatively enriched in quartz, plagioclase, and chlorite relative to Units II and III, and dolomite is usually a trace component of the mineral assemblage.
Slightly lighter intervals, with higher carbonate content, are observed throughout Unit I in all three holes (Table T5; Fig. F5). Internal sedimentary features of the light layers include abundant visible foraminifers, common green clay layers (typically 1-3 cm thick), well-defined bioturbation, and slightly yellowish gray patches or mottles, which probably represent traces of bioturbation. Smear-slide estimates indicate calcareous nannoplankton contents of >30%, compared to ~10% in the background clay. The light layers are characterized by a stiff clay, which often shows a rough surface caused by the cutting wire.
Green layers and less distinct green mottles, which resemble the green clay layers observed at Sites 1143, 1144, and 1145 (see "Lithostratigraphy" in the "Site 1143" chapter; "Lithostratigraphy" in the "Site 1144" chapter; and "Lithostratigraphy" in the "Site 1145" chapter), occur frequently in the upper part of Unit I, principally above ~185 mcd (Fig. F7). They are characterized by a stiffer clay than the dominant clay lithology of the section and have a lower water content and higher P-wave velocity.
Bioturbation is intense throughout Unit I. The sediment is generally completely homogenized, and individual burrows are rarely observed. Pyrite-filled burrows are the exception to this pattern. They occur at a frequency of about one to two burrows per section throughout the lower part of the unit. Large pyrite-filled burrows reach several centimeters in length and up to 2 cm in diameter. Pyrite-filled burrows first appear at ~39 mcd, although small, disseminated pyrite crystals are present at shallower depths (Fig. F7). They probably form by the diagenetic replacement of organic matter-rich sediment fills.
Macrofossils are a volumetrically small but regular component of the sediments at Site 1146. Large (>1 cm in length) pteropods of the genus Cavolinia are observed in Cores 184-1146A-2H and 3H (4.85 mcd and 25.5 mcd). Additionally, large benthic and planktonic foraminifers were observed. Diatoms, silicoflagellates, radiolarians, and sponge spicules are commonly observed in smear slides. Several large pyrite nodules preserve organic structures that may be interpreted as xenophyophorians (large agglutinating multicellular organisms, which show some similarities to agglutinated foraminifers). Xenophyophorians are often observed on the sediment surface in the deeper parts of the South China Sea (Hess, 1998).
The stratigraphic distribution of ash layers in the upper part of Unit I exhibits a pattern similar to previous Leg 184 sites: thin, light gray ash layers, often dispersed by bioturbation, as well as isolated pumice clasts (Cores 184-1146A-1H through 8H). In contrast, the lower part of Unit I is characterized by a small number of dark gray or black ash layers with larger pumice clasts (Table T6). Finely dispersed volcanic glass is observed in smear slides from Section 184-1146B-2H-2, 27 cm (10.32 mcd), and this level coincides with a significant peak in magnetic susceptibility (see "Physical Properties").
Black ashes are present in the lower part of Unit I and include thin (<1-2 cm) layers at intervals 184-1146A-9H-7, 28-29 cm (83.28-83.29 mcd), and 184-1146B-12H-5, 57-60 cm (115.87-115.90 mcd). The latter contains large volcanic glass shards as long as 1 cm. A prominent 4-cm-thick black ash interval with a sharp base and with pumice debris in the lower part of the layer is observed at intervals 184-1146A-19H-5, 10-14 cm (181.65-181.69 mcd), 184-1146B-19H-2, 58-62 cm (181.63-181.67 mcd), and 184-1146C-19X-1, 0-4 cm (182.40-182.44 mcd). This ash is easily correlated between the three holes at this site.
Lithologic Unit II differs from Unit I mainly in its significantly higher carbonate content (50-60 wt% instead of 10-30 wt%) and the resulting lighter colors. In addition, the mineralogy shifts from a plagioclase and quartz-rich assemblage to a finer grained and biogenic calcite-rich assemblage (inferred from smear slides). Dolomite occurs in Subunit IIA but not in Subunit IIB. The unit consists mainly of homogeneous to rarely mottled, light brownish gray foraminifer and nannofossil clay mixed sediment. A few intercalations of light bluish green nannofossil clay occur in the lower part of the unit. This unit is subdivided into two subunits based on the mineralogy (Fig. F6).
Subunit IIA is dominated by biogenic calcite, which increases sharply across the Unit I/Unit II boundary and represents an increase in the flux and/or preservation of nannofossil ooze. Calcite remains relatively high until the Subunit IIB boundary at 434.22 mcd, where it decreases; dolomite is not detected below this level. Kaolinite becomes a significant contributor to the mineral composition within Subunit IIB, and quartz also increases at the Subunit IIA/Subunit IIB boundary. These mineralogy changes mark the onset of the gradational change in the lithology from a carbonate-rich Unit II to the green clays of Unit III; they represent either a change in source region or a change in weathering regime at the source area.
The most noteworthy features within lithologic Unit II are several intervals of characteristically bluish green nannofossil clay, which contain large amounts of pyrite as nodular irregular layers or as finely disseminated specular particles within the sediment. Contacts with the overlying and underlying gray or light brownish gray background sediment are generally gradational. In some cases (e.g., interval 1146A-42X-3, 80 cm, through 42X-4, 20 cm; 414.06-414.96 mcd) a broad transition zone is observed, characterized by intense visible bioturbation. Two bluish green intervals contain foraminifer turbidites within the middle of the layer (Fig. F8). One of these turbidites (324.6-324.8 mcd) is directly overlain by a fine-grained green ash layer. A typical pyrite layer with calcite is observed in interval 184-1146A-44X-2, 27-36 cm (431.38-431.47 mcd) (Fig. F9). These units are evident in the reflectance red parameter (a*) as a drastic decrease to more green values (Fig. F5).
Bioturbation is common to abundant throughout the cored interval of Unit II, although individual burrows are difficult to identify because of the homogeneous sediment color. In most cases, faint black "iron sulfide" staining and some mottled intervals are the only visible traces of burrowing. In the lower part of Subunit IIB, a large number of individual Zoophycos burrows are observed.
Volcanic ash is a volumetrically small and rare component of the sediments in Subunit IIB. A stiff, dark gray, altered volcanic ash, with a sharp base and normal grading, is observed at interval 184-1146A-26X-CC, 30-33 cm (258.78-258.81 mcd). This layer is composed of quartz, feldspar, and volcanic glass altered to clay and zeolites, as shown by smear-slide analysis.
The transition between Unit II and Unit III corresponds to an important progressive change in the sediment color. The sediment at the base of Unit II is brownish gray but shows an increasing number of green intervals downsection, becoming a distinct greenish gray below Section 184-1146A-55X-CC, 22 cm (553.02 mcd). We define this color change as the boundary between Unit II and Unit III (Fig. F5, a* values curve). The sediment is clearly poorer in foraminifers in the upper part of the unit compared to Unit II, whereas the proportion of nannofossils seems to increase slightly. The abundance of foraminifers increases again toward the bottom of Unit III. The carbonate content of Unit III is in the range of 30-50 wt% and is clearly lower than in Unit II (50-60 wt%). Unit III is characterized by relatively high concentrations of smectite and illite (Fig. F6).
Bioturbation is intense and most of the sediment is homogenized, although a number of well-defined traces can still be detected. Only a few burrows are replaced by pyrite. The most common trace fossils are Zoophycos and composite burrows with Chondrites, a classic bathyal assemblage. The density of Zoophycos burrows increases toward the bottom of the hole. They are generally filled with brownish sediment and clearly visible (Fig. F10). A well-preserved escape trace is observed in interval 184-1146A-64X-4, 144-150 cm (638.45-638.51 mcd).
Three main lithologic units have been recognized at Site 1146. This division is also shown in the mineralogy and color data. Unit I is characterized by an alternation of darker and lighter intervals, usually with gradational boundaries. According to smear-slide observation and carbonate analysis, the lighter intervals correspond to significant increases in the carbonate content. The uppermost light intervals are clearly correlative between the three holes. Their formation may be caused by changes in marine productivity, the relative amount of terrigenous deposition to the site, or differential dissolution caused by changing bottom-water chemistry and/or glacial-interglacial variation. We propose an interpretation in which the light intervals correspond to the interglacial isotopic stages (Table T5) (e.g., Shackleton and Opdyke, 1976), based on the similarity of the L* value curve and the deep-sea oxygen isotope curve. Smear-slide analyses indicate no noticeable change in the character of the terrigenous material between dark and light layers. In the lower part of Unit I, the correlation of the L* value curve with global isotopic stages becomes more difficult. This could be because of the overall increase of carbonate that results in a lower contrast between light and dark intervals. Unit II is characterized by high intensity in lightness (reflectance L* parameter) as a result of its high carbonate content. Only a few darker intervals (e.g., interval 184-1146C-30X-3, 10-40 cm [299.25-299.55 mcd]) indicate an increase of clay content because of lower primary productivity, higher continental detrital sediment supply, or decreased preservation of carbonate. Unit III does not show a change in lightness, but there is a color shift toward green values at the Unit II/III boundary (Fig. F5).
Despite the variation in carbonate content, no evidence suggests a significant change in the water depth at Site 1146 during the sedimentation of the recovered section. The sediment character is one typical of deposition at bathyal depths on a continental slope, with oxygenated bottom water implying water depths exceeding the oxygen minimum zone (~600 m). However, the bulk mineralogy suggests that either a change in the source of the terrigenous material or a change in the weathering regime of the source region took place over time. The relatively high concentration of quartz and plagioclase, and to a lesser extent chlorite, in Unit I suggests that the terrigenous component in this unit is coarser grained and characteristic of source-area environments where physical weathering was more important than chemical weathering compared with the older sediments.
The relative increase in kaolinite in at the Subunit IIA/IIB boundary suggests a change to a more chemical weathering-dominated source-area environment during the middle to late Miocene. The complete lack of dolomite below the Subunit IIA/IIB boundary suggests either a change in terrigenous source area, or a change in the diagenetic regime in the sediment column.
Overall, the terrigenous component of the sediment suggests aridification of the source regions from the late Miocene through the Pleistocene.