Sediments from Site 1084 form four lithostratigraphic units defined by the changes in the major lithology between clay and ooze (see Fig. 7, Fig. 8). Unit I is composed of moderately bioturbated, intercalated intervals of olive (5Y 4/3), olive-gray (5Y 4/2), dark olive-gray (5Y 4/1), and black (5Y 2.5/1) clays, which contain varying abundances of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are distinguished based on microfossil type and abundance. Unit II is composed of olive (5Y 4/3) clay-rich nannofossil diatom ooze that grades into olive (5Y 4/3) diatomaceous nannofossil ooze and clay-rich nannofossil ooze. Unit III comprises olive (5Y 4/3 and 5Y 5/3) and olive-gray (5Y 5/2) clay, nannofossil clay, and diatom-bearing nannofossil clay. Unit IV is composed of olive-gray (5Y 4/2) and dark olive-gray (5Y 3/2) clayey nannofossil ooze and diatom-rich clayey nannofossil ooze.
Subunit IA is composed of moderately bioturbated olive-gray (5Y 3/2 to 5Y 4/2), very dark grayish brown (5Y 3/3), and olive (5Y 4/3) foraminifer- and diatom-bearing nannofossil clay, and foraminifer-bearing diatom-rich clayey nannofossil ooze. The abundance of microfossils, particularly nannofossils, increases downcore through this subunit. Cores 175-1084A-7H through 11H contain intervals with nannofossil abundances high enough to define the major lithology as a nannofossil ooze. Sediments in Subunit IA have relatively high carbonate and organic carbon contents, which average 37 and 8.2 wt%, respectively (see "Organic Geochemistry" section, this chapter). The boundary between Subunits IA and IB is relatively sharp and occurs between Cores 175-1084A-12H and 13H, 175-1084B-12H and 13H, and 175-1084C-13H and 14H. The contact is marked by an increase in diatom abundance from diatom-bearing and sporadically diatom-rich to continuously diatomaceous clay and a decrease in nannofossil abundance from ooze to nannofossil-rich. Subunit IA contains several intervals of dark olive-brown (5Y 3/3) and black (5Y 2.5/1 and 2.5Y 2.5/1) sediment. These intervals range in thickness between 30 cm and 9 m and are composed of clay, diatom-rich clay, and nannofossil clay (intervals 175-1084A-3H-6, 0–90 cm, 4H-1, 90–98 cm, 5H-6, 0–70 cm, and 7H-4, 80 cm to 7H-5, 70 cm; 175-1084A-11H-5, 0 cm, to 11H-8, 150 cm; 175-1084A-12H-1, 0 cm, to 12H-3, 150 cm; and 175-1084A-12H-6, 0 cm, to 12H-7, 50 cm; see Table 2). In general, these horizons are characterized by lower carbonate contents, which average just below 10 wt%, and higher organic carbon, which range between 8 and 18 wt% (see "Organic Geochemistry" section, this chapter) when compared to the under- and overlying sediment layers.
The contact between Subunit IB and Subunit IA is defined by the increase in diatom abundance from frequent and common to abundant. Its boundary with Subunit IC is marked by a change from clays that are nannofossil-rich clay to those that are diatomaceous. This transition occurs between Cores 175-1084A-36X and 37X. Subunit IB is characterized by frequent and irregular variations in color and lithology over short depth intervals. A comparable variation in color and lithology is not observed in the other subunits. Lithologies frequently alternate between olive (5Y 4/3) nannofossil-rich diatomaceous clay (e.g., Sections 175-1084A-13H-1 through 13H-4), olive (5Y 4/3) clayey nannofossil ooze, and black (5Y 2.5/1) diatom-bearing clay (e.g., Sections 175-1084A-29X-1 through 29X-2). Intervals of these lithologies range in thickness from 3.5 to 8 m (see visual core descriptions, Section 4, this volume). Concentrations of calcium carbonate and organic carbon average 26 and 7.0 wt%, respectively. Dolomitized clay nodules are present in Core 175-1084A-18X. In Subunit IB, the dark olive-brown (5Y 3/3) and black (5Y 2.5/1 and 2.5Y 2.5/1) layers are composed of nannofossil-bearing diatomaceous clay, foraminifer-bearing diatom-rich nannofossil clay, diatom-rich clay, and nannofossil-bearing diatom-rich clay (intervals 175-1084A-14H-6, 130 cm, to 14H-7, 10 cm; 175-1084A-21X-6, 0–94 cm, 22X-1, 22X-CC, and 24X-4, 0 cm, to 24X-CC, 30 cm; 175-1084A-25X-3, 130 cm, to 25X-4, 70 cm; and 175-1084A-29X-1, 0 cm, to 29X-2, 150 cm). The "dark" horizons in Subunit IB are generally more carbonate rich compared with those found in Subunit IA.
Subunit IC contains moderately bioturbated, dark olive-gray (5Y 3/2) and olive (5Y 4/3) nannofossil-rich diatomaceous clay and nannofossil diatomaceous clay. This subunit is characterized by carbonate and organic carbon contents that average 14 and 4.9 wt%, respectively (see "Organic Geochemistry" section, this chapter). Subunit IC is a transitional unit between Subunit IB and Unit II and is lithologically more homogeneous than Subunit IB. Its upper contact is characterized by a change from diatomaceous clay to diatom ooze. This transition occurs between Cores 175-1084A-36 and 37X and is gradual. The lithologic contact between Subunit IC and Unit II occurs between Cores 175-1084A-43X and 44X and marks a transition from diatomaceous clay to clay-rich diatomaceous nannofossil ooze. The absolute change in diatom abundance across this transition is smaller when compared with the change in diatom abundance observed between black and olive horizons within this subunit. This change is gradual and occurs between Cores 43X and 44X. Subunit IC contains only one interval (175-1084A-43X-4, 100–115 cm) of black (5Y 2.5/1) diatomaceous clay.
Unit II contains olive (5Y 4/3) clay-rich nannofossil diatom ooze that grades into olive (5Y 4/3) diatomaceous nannofossil ooze and clay-rich nannofossil ooze. This subunit is characterized by carbonate and organic carbon contents, which average 28 and 3 wt%, respectively (see "Organic Geochemistry" section, this chapter). The boundary between Units II and III is defined as the change in the major lithology from nannofossil and diatom ooze to nannofossil clay. This gradual transition occurs between Cores 175-1084A-53X and 54X. Unit II contains two intervals of black (5Y 2.5/1) nannofossil-bearing diatom ooze at 175-1084A-43X-3, 0–70 cm, 46X-3, 0–94 cm, and 51X, 85–105 cm.
Unit III contains moderately bioturbated olive (5Y 4/3 and 5Y 5/3), pale olive (5Y 6/3), and olive-gray (5Y 5/2) clay, nannofossil clay, and diatom-bearing nannofossil clay. The unit contains few dark olive-gray (5Y 3/2), 50- to 100-cm-thick clay and nannofossil-rich clay intervals. These intervals occur irregularly throughout the unit. Unit III is characterized by carbonate and organic carbon contents that average 42 and 4 wt%, respectively (see "Organic Geochemistry" section, this chapter). The contact between Units III and IV is defined as the change in the major lithology from clay to nannofossil ooze. This boundary occurs between Cores 175-1084A-59X and 60X.
Unit IV is composed of moderately bioturbated olive-gray (5Y 4/2 and 5Y 5/2), pale olive (5Y 6/3) clayey nannofossil ooze, diatom-bearing clayey nannofossil ooze, and diatom-rich clayey nannofossil ooze. Dark olive-gray (5Y 3/2) intervals, 20 to 50 cm in thickness, of diatom-bearing clay-rich nannofossil ooze and diatom-bearing nannofossil clay are present throughout this unit. Unit IV is characterized by high carbonate and organic carbon contents, which average 46 and 3 wt%, respectively (see "Organic Geochemistry" section, this chapter). The contact between Units III and IV is defined as the change in the major lithology from clay to nannofossil ooze. This transition occurs between Cores 175-1084A-59X and 60X.
Smear-slide analyses indicate that the detrital component of the sediments in the four units consists of clay with rare silt-sized, angular and subangular mono- and polycrystalline quartz grains. Subangular feldspar grains are present in trace amounts. Authigenic minerals include framboidal pyrite and dolomite rhombs in rare or trace abundances. The dark horizons generally contain slightly higher abundances of pyrite. Silt-sized cryptocrystalline carbonate grains are generally rare to frequent in abundance and, at times, common. Subunit IA contains trace and rare amounts of quartz and pyrite throughout. Rare silt-sized, cryptocrystalline carbonate grains are found in Subunit IA in Core 175-1084A-7H. Subunit IB contains pyrite in increased abundance and trace amounts of silt-sized quartz grains, feldspar, and mica. In the lower part of Subunit IB, between Cores 175-1084A-25X and 36X, silt-sized cryptocrystalline carbonate grains are consistently present in rare or few abundance. Subunit IC and Unit II contain trace and rare amounts of silt-sized quartz and pyrite grains throughout. Unit III also contains silt-sized, subhedral pyrite grains. Unit IV contains rare pyrite and trace amounts of both silt-sized quartz and cryptocrystalline carbonate grains. Dark horizons exhibit significant compositional variations in the relative abundances of diatoms, foraminifers, and nannofossils (Table 2). Most of the dark layers in Subunit IA have lower abundances of nannofossils compared with Subunits IB and IC and Unit II. The biogenic component of the dark layers is commonly dominated by diatom resting spores. Resting spores are present only in trace abundances in the sediments under- and overlying the black layers.
Color data were measured for all the cores from Holes 1084A, 1084B, 1084C. At Site 1084, total reflectance values range from 20% to 58% (Fig. 9), which represent the alternating carbonate-rich and organic-rich black-colored sediment layers. The highest values in total reflectance were observed in Subunit IA and Unit II, which correspond to nannofossil ooze intervals (see "Description of the Lithostratigraphic Unit," this section). Low values of total reflectance correspond to the presence of black (5Y 2.5/1 and 2.5Y 2.5/1) clay horizons (Fig. 9; Table 2). The lowest values for total reflectance vary between 20% and 25% and were observed in Subunits IA and IB. These sediments have very low calcium carbonate (0–10 wt%) and high organic carbon concentrations (8–18 wt%; see "Organic Geochemistry" section, this chapter; also see Fig. 10). Numerous gaps in the total reflectance record caused by poor core recovery, especially within Subunit IB (between 150 and 300 mbsf), prevents the determination of the cyclicity of the black layers. The general trends in total reflectance can be correlated to the lithostratigraphic units and subunits at Hole 1082A (Fig. 3, "Site 1082" chapter, this volume) and display well-defined cycles that may be used for high-resolution stratigraphy—except within Subunit IB.