Subunit IIA

• Intervals: 329-U1367B-2H-1, 10 cm, to 2H-4, 150 cm; 329-U1367C-1H-4, 110 cm, to 2H-5, 110 cm; 329-U1367D-1H-5, 114 cm, to 2H-5, 20 cm; 329-U1367E-1H-5, 60 cm, to 2H-5, 150 cm

• Depths: Hole U1367B = 5.6–11.5 mbsf, Hole U1367C = 5.6–14.3 mbsf, Hole U1367D = 7.1–13.6 mbsf, Hole U1367E = 6.6–14.0 mbsf

• Lithology: nannofossil ooze

Sediment color in Subunit IIA is variable, but it is generally strong brown to pink (7.5YR 5/4, 5/6, 6/3, 6/4, 6/6, 7/3, 7/4) (Fig. F9C), except for intercalating clay-rich thin layers (brown to dark brown; 7.5YR 3/2, 3/3, 3/4, 4/2, 5/3). The color changes gradually at the base of this subunit to brown (7.5YR 5/3). The transition to Subunit IIB is clearly observed in Holes U1367B and U1367E and is not observed in Holes U1367C and U1367D because of whole-round sampling before observation.

Smear slide analyses indicate that Subunit IIA is predominantly nannofossil ooze (Fig. F11C). Most intervals consist of 80%–95% coccolithophores and coccolithophorid debris (Fig. F7). Foraminifers are also recognized by smear slide analyses, but their content is generally <15% of bulk sediment samples. In two intervals of particularly low clay abundance (intervals 329-U1367B-1H-2, 70–72 cm, and 2H-6, 5–7 cm), XRD analyses identified calcite (a: 4.9887 Å, c: 17.05290 Å) as the form of the carbonate (Fig. F10B). The upper middle part of the subunit includes a thick (20–40 cm) bed of clay containing 10%–40% RSO. The clay corresponds to a distinctive uptick in magnetic susceptibility that occurs in Holes U1367B–U1367E. Thin (5–10 cm) layers with relatively high RSO content are repeatedly intercalated throughout this subunit and exhibit distinct color changes (Fig. F9C).

Subunit IIA is poorly consolidated, except for the transitional part to Subunit IIB where RSO content is relatively high. Moist samples of this unit are sticky.

Mottling occurs throughout this subunit. Thin layers reflecting change in nannofossil content and RSO content are also common. Slight to moderate burrowing is evident in intervals 329-U1367B-2H-2, 50–100 cm; 329-U1367C-1H-4, 110–130 cm; 329-U1367D-2H-2, 110–150 cm; and 329-U1367E-2H-2, 105 cm, to 2H-3, 100 cm. Several narrow (3–4 mm) burrows below the thickly bedded clay in the upper middle part of Subunit IIA are particularly noteworthy because they extend as far as 80 cm into the underlying nannofossil ooze (Fig. F9D).

Subunit IIB

• Intervals: 329-U1367B-2H-5, 0 cm, to 4H-1, 0–32 cm; 329-U1367C-2H-5, 110 cm, to 4H-1, 0–9 cm; 329-U1367D-2H-5, 20 cm, to 3H-CC, 10 cm; 329-U1367E-2H-6, 0 cm, to 3H-CC (no recovery; 24.17 mbsf), Hole U1367F (upper contact not cored; lost core at lower contact)

• Depths: Hole U1367B = 11.5 to 22.0–22.32 mbsf, Hole U1367C = 14.3 to 26.2–26.29 mbsf, Hole U1367D = 13.6–25.2 mbsf, Hole U1367E = 14.0–24.17 m (estimated), Hole U1367F (upper contact not cored; lost core at lower contact)

• Lithology: clay-bearing nannofossil ooze to nannofossil marl

The color of Subunit IIB is variable, reflecting changes in concentrations of nannofossils and RSO (Fig. F7). The overall color of the subunit is dark yellow-brown (10YR 3/4) (Fig. F9E). Generally, the colors become darker in the lower part of the subunit, changing from brown (7.5YR 4/4, 5/3) and dark yellow-brown (10YR 3/4) to very dark brown (7.5YR 2.5/2). The colors of intercalated layers containing relatively abundant nannofossils are dark yellowish brown (10YR 4/4), brown (7.5YR 4/3, 4/4, 10YR 4/3), and dark brown (7.5YR 3/3), probably reflecting differences in nannofossil content. The lower part of this subunit exhibits strong brown (7.5YR 4/6) to reddish yellow (7.5YR 6/6) colors. A well-indurated layer (40–120 cm) in the lower quarter of subunit is dark brown (7.5YR 3/3, 3/4, 10YR 4/3), dark yellowish brown (10YR 3/4), and yellow (10YR 7/6) (Fig. F9F).

Smear slide analyses indicate that Subunit IIB contains abundant nannofossil ooze (Fig. F7). Of the 12 slides prepared from samples below Section 329-U1367B-5H-5 (11.5 mbsf), 10 consist of >70% calcareous nannofossils and nannofossil debris. RSO concentration is higher than in Subunit IIA (Fig. F11D) and increases with depth in Hole U1367E (Fig. F7B). Numerous foraminifers from Sample 329-U1367B-2H-CC contain micrite on the exterior surfaces of their shells. A diffractogram from XRD analyses of bulk sediment (Sample 329-U1367B-3H-4, 50–52 cm) includes numerous peaks corresponding to calcite (Fig. F10C). Several peaks of much lower intensity in the lower and higher ranges of 2θ correspond to clay that is most likely smectite.

The upper half of this subunit is poorly consolidated. The lower half is well consolidated. The chalk layer is well indurated.

Subunit IIB contains numerous thin layers (1–3 cm) and laminations (<1 cm) that correspond to changing nannofossil and RSO abundances. These bedding features are visible as lighter (nannofossil) and darker (RSO) layers in core photographs (e.g., Fig. F9E). All of the observed layers in Subunit IIB are bioturbated. The bioturbation forms ichnofabric Classes 2–4 that moderately disrupt to thoroughly destroy bedding features (Fig. F9F).

Sediment/Basalt contact

Components of the sediment/basalt interface were recovered in Holes U1367B–U1367D. Although the steel cutting shoe on the APC core barrel was damaged significantly by hitting hard rock at 24.17 mbsf in Hole U1367E, no sediment was recovered. Rotary drilling in Hole U1367F produced sediment and basalt; however, several meters of core were lost during drilling and the sediment that was recovered was highly disturbed. Therefore, we do not consider those materials to be representative of the sediment/basalt contact.

The recovered sediment was mixed thoroughly with fragmented basalt and volcanic glass. Continuous sections of interfacial sediment and overlying marl were recovered in Holes U1367B and U1367D. However, the 30–60 cm of sediment overlying the interface was highly disturbed in both holes. Therefore, the nature of the contacts and structures associated with the sediment/basalt contact at Site U1367 are unknown. We believe, however, that the recovered sediment is representative of the overall composition of the interface, and those results are presented below.

The sediment/basalt interface is dark yellow-brown (10YR 3/4) clay with nannofossils and microfossils. The dark color of the clay corresponds to a high (35%–40%) abundance of RSO grains that vary in size from 2 to ~200 µm. Other components of the very fine sediment fraction include calcareous nannofossils and discoasters. Silt and sand grains (>4 and >63 µm, respectively) constitute <10% of the sediment and consist of RSO, microfossils (foraminifers, fish teeth, and sponge spicules), volcanic glass, and quartz (in order of decreasing abundance). Although planktonic and benthic foraminifers exhibit a large range of preservation (see “Paleontology and biostratigraphy”), most exhibit signs of alteration. Volcanic glass exists as large subangular fragments that are highly angular, translucent, amber, and free of surface pits or alterations. The volcanic components are described in “Igneous lithostratigraphy, petrology, alteration, and structural geology.”

Discussion

Sediment composition and texture

Smear slide and XRD analyses identify clay, zeolite, RSO, and calcite as the principal components of the clay, ooze, marl, and chalk at Site U1367. Clay and calcite distributions stand in stark contrast to one another and define the two units at Site U1367: Unit I contains RSO and zeolitic clay and Unit II contains nannofossil ooze, marl, and chalk.

Within the broad units outlined above, zeolite distribution and clay-to-carbonate relationships help provide insight into the depositional history of Site U1367. These features are described below.

Zeolite

Although phillipsite never comprises a majority of the sediment, it exists in equal proportions with clay and RSO throughout the uppermost 4–5 m of Unit I. Its abundance decreases sharply in the lowermost 1.5 m of Unit I, and it becomes completely absent in the lowermost 50 cm of the unit. This distribution could result from differences in sediment composition (Bonatti, 1965; Hay, 1966), sediment accumulation rate (Czyscinski, 1973; Glaccum and Boström, 1976), availability of in situ dissolved silica and potassium (Glaccum and Boström, 1976; Sheppard and Gude, 1970), or the age of the phillipsite crystals (Stonecipher, 1976). Each of these parameters is important for reconstructions of the depositional and diagenetic histories of the sediment. However, additional data (petrologic, geochemical, and chronostratigraphic) are needed before the interstitial water-sediment processes that control phillipsite distribution can be used to more fully define the history of Site U1367.

Carbonate

Several abrupt clay-to-carbonate transitions occur in all of the Site U1367 holes and carry possibly important depositional or oceanographic implications. For example, the Unit I/II boundary consists of a large-scale compositional change from metalliferous clay (60% RSO and 40% clay) to nannofossil ooze (>80% calcite) that spans a distance of only 10–15 cm (see Site U1367 core photographs in “Core descriptions”). Slow sediment accumulation on both sides of the contact is indicated by low but persistent abundance of RSO in both the clay and carbonate units (Heath and Dymond, 1977). The contact is also associated with a distinct community of burrowing organisms that mixed clay and carbonate sediment and fragmented most foraminiferal shells. Micritic overgrowths on the foraminiferal shells imply that postdepositional mineralization occurred in the sediment.

Conditions that might account for the Unit II/I carbonate-to-clay transition include an increase in depth of the carbonate compensation depth (Coxall et al., 2005), variable influx of terrigenous and/or biogenic sediment (Glasby, 1991), or evolution of interstitial water chemistry (e.g., Giambalvo et al., 2002). Each of these phenomena, acting exclusively or in consort, might have produced the transition observed from Unit II to I. However, as was the case with zeolite distribution, more thorough studies of sediment petrology, geochemistry, and chronostratigraphy are needed before the origin and implications of the carbonate to clay-transition at Site U1367 can be defined.

Induration

Considering the overall thinness of the total sedimentary succession (<25 m), the high degree of induration in Subunit IIB is impressive. Steel blades and a mallet were needed to extract shipboard sediment samples. One thick (40–120 cm) interval in the middle of Subunit IIB was sufficiently indurated to earn the distinction of being labeled “chalk” (Figs. F7, F8). The cause of variable induration in the sediment is unknown. However, foraminifers from discrete depth intervals of Subunit IIB show evidence of carbonate diagenesis in the form of recrystallization.

Sedimentary structures

Two observations regarding the bioturbation of sediment at Site U1367 are worthy of note:

• Every meter of core is burrowed or mottled. Although there is no way of demonstrating that burrowers occupied Site U1367 continuously since the onset of sediment accumulation, those organisms persisted with sufficient frequency to disrupt all vestiges of bedding. Most burrows are not adequately isolated or preserved to facilitate identification. However, several burrows possess vertical traces with branches and resemble highly elongated Chondrites, whereas several others are likely Trichichnus ichnofossils (Fig. F9D) (Ekdale and Bromley, 1984). The deepest reaching burrows are 60–80 cm long (see photographs of Core 329-U1367E-2H in “Core descriptions”). Given this depth and the extraordinarily slow sediment accumulation rates (see “Paleontology and biostratigraphy”), burrowers only needed to occupy this site once every 0.5 (Unit II) to 20 (Unit I) m.y. to produce the bioturbated textures observed in Site U1367 cores (see Ekdale et al., 1984).

• Ichnofossil traces imply that interstitial water at Site U1367 was frequently oxygenated. The small, soft-bodied organisms responsible for burrowing sediment in pelagic environments similar to those cored at Site U1367 can subsist on 0.1–1 mg/L dissolved O₂ (Ekdale et al., 1984). However, this range of values should be considered conservative for Site U1367 given that a number of the burrowers penetrated the subseafloor nannofossil ooze as deep as 80 cm. Supply of dissolved oxygen to the lower parts of these feeding/resting tubes calls for the overlying bottom water to contain a sufficiently high concentration of dissolved oxygen for the period of time needed to diffuse the oxygen into the interstitial water of the underlying sediment (Savrda and Bottjer, 1989). Although burrowing cannot be used to definitively state that sediment at Site U1367 was continuously oxygenated throughout its history, its association with distinct horizons at all sediment depths leads to the conclusion that aerobic bottom water conditions played a significant role influencing the composition and structure of sediment at Site U1367.

Interhole correlation

Lithostratigraphic correlation among the five APC holes at Site U1367 shows that differences exist among the units and subunits of Site U1367. Several of the differences are artificially created by coring operations, for example,

• The 20.9–21.9 mbsf interval in Core 329-U1367B-3H and the 22.6–25.1 mbsf interval in Core 329-U1367C-3H are flow-in and not in situ sediment, and

• Whole-round sampling created a large number of gaps in Holes U1367C and U1367D that masked unit boundaries and some beds.

Despite these challenges, a tentative correlation of lithostratigraphic units among the holes of Site U1367 was created (Fig. F9).

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