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doi:10.2204/iodp.proc.303306.112.2006 LithostratigraphyAt Site U1313, a 308 m sequence of pelagic sediment was recovered in four holes. The major sediment is composed of nannofossil ooze with varying amounts of foraminifers and clay- to silt-sized terrigenous material. Major lithologies range from silty clay nannofossil ooze to nannofossil ooze, with minor foraminifer nannofossil ooze, silty clay nannofossil ooze with diatoms, and nannofossil silty clay. Smear slides indicate that noncalcareous sediment contains biogenic silica, quartz, and clay and generally makes up 5%–40% of the sedimentary sequence (Fig. F6). Sediments recovered are generally undisturbed. Lithologic units for the Site U1313 sedimentary sequence are defined on the basis of data obtained from smear slide examination, visual observation of sediment color, percent carbonate (see Fig. F27 in “Geochemistry”), lightness parameters of spectral reflectance measurements (L*), and physical property data (see “Physical properties”). Two major lithologic units were identified. Unit I consists of Holocene–upper Pliocene alternating nannofossil ooze, silty clay nannofossil ooze, and nannofossil ooze with clay. Unit II extends to the total depth of each hole. It consists of upper Pliocene–uppermost Miocene nannofossil ooze. Unit I can be further divided into subunits on the basis of changes in the percentage of and variability in clay and nannofossil content. Figure F7 shows a summary of the lithologic units at Site U1313. Description of unitsUnit I
This unit represents deposition during the Holocene–late Pliocene. Dominant lithologies are silty clay nannofossil ooze, nannofossil ooze with clay, and nannofossil ooze, which are gray (5Y 6/1), light gray (5Y 7/1), and white (5Y 8/1 and N9). The unit is defined by centimeter- to decimeter-scale variability in detrital clay and biogenic carbonate with decreased variability downcore. Lithologic variability occurs visually in the form of distinct cyclic color changes at centimeter to decimeter scale. This is reflected by large-amplitude shifts in carbonate content (Fig. F27), as well as large variability in gamma ray attenuation (GRA) density and magnetic susceptibility and intensity measurements (Figs. F18, F32, F35). Occasional millimeter- to centimeter-scale diffuse bands of green and gray (5G 7/2 and N4–N5) are distributed throughout the unit. Boundaries between intervals are often gradational and bioturbated. In general, bioturbation/mottling is rare to moderate, and burrows up to 2–3 cm in size are mainly located across contacts between contrasting colors. Dropstones frequently occur throughout Unit I (see below). Subunit IA
This subunit is characterized by the largest-scale amplitude fluctuations in detrital clay and biogenic carbonate in Unit I. This is reflected by distinct color changes and shifts in both L* (Fig. F7), carbonate (Fig. F27), GRA density (Fig. F35), and magnetic susceptibility (Fig. F32). The uppermost ~65 cm of the succession is yellowish brown (10Y 5/4) and consists of nannofossil silty clay. This interval represents the Holocene, and the color is related to sediment oxidation due to circulation of oxygen-rich seawater. The remainder of the subunit consists of nannofossil silty clay, silty clay nannofossil ooze, nannofossil ooze with clay, nannofossil ooze, and subordinate intervals of silty clay nannofossil ooze with diatoms and nannofossil ooze with diatoms and clay. Color varies from white (N9 and 5Y 8/1) to light gray (5Y 7/1) and gray (5Y 6/1) with subordinate light olive-gray (5Y 7/2) and olive-gray (5Y 6/2). Color changes are usually gradational and bioturbated and reflect variation in carbonate versus clay content. In general, bioturbation is rare to moderate. Typical abundance of biogenic and terrigenous components, as estimated from smear slides, are nannofossils (30%–90%), clay (0%–35%), detrital calcite (0%–30%), detrital quartz (0%–20%), diatoms (1%–10%), foraminifers (0%–10%), sponge spicules (trace to 10%), and radiolarians (trace to rare). Percent carbonate also displays the strong amplitude variability demonstrated by nannofossil ooze and detrital clay content (30 wt%) (Fig. F27). Figure F8 shows an X-ray diffractogram with peaks identified for minerals present in the carbonate-free fraction of Sample 306-U1313A-4H-2, 147–148 cm (27.17 mbsf). Relative intensity of peak heights from baseline indicates that the sample is dominated by detrital quartz. Other significant peaks are attributed to feldspar and clay minerals. Although no oriented glass slides were prepared from the clay size (<4 µm) fraction, the presence of clay minerals can be discerned as significant from a number of prominent peaks between 0° and 40°2θ. A broad peak from 4° to 8°2θ and a sharp peak at 20°2θ are characteristic of biotite mica and vermiculite and muscovite mica, respectively. A relatively strong peak at 12°2θ corresponds to the 001 h-k-l plane for kaolinite. The presence of biogenic silica is recognized from an amorphous hump between 20° and 30°2θ. Subunit IB
This subunit consists of nannofossil ooze, nannofossil ooze with clay, and silty clay nannofossil ooze. Overall, percent biogenic carbonate is higher in Subunit IB, whereas detrital clay content is lower. Both are defined by decreased variability. This is reflected in reduced amplitude fluctuations of L* (Fig. F7), percent carbonate (Fig. F27), magnetic susceptibility (Fig. F32), magnetic intensity (Fig. F18), and GRA density (Fig. F35). Colors vary from white (N9 and 5Y 8/1) to light gray (5Y 7/1) and gray (5Y 6/1), reflecting decimeter-scale variation in clay and biogenic carbonate content. Typical abundance of biogenic and terrigenous components, as estimated from smear slides, are nannofossils (60%–98%), clay (0%–20%), foraminifers (2%–10%), detrital calcite (1%–5%), detrital quartz (1%–5%), diatoms (trace), and radiolarians (trace). Unit II
Unit II consists of nannofossil ooze of late Pliocene–latest Miocene age. The change from Unit I to Unit II is gradational over ~40 m. It is defined by a change toward higher values and a reduction in variability of L* (Fig. F7) and is coincident with a downcore reduction of percentage detrital clay and an increase in percentage biogenic carbonate, as well as decreased variability in both (see “Site U1313 smear slides” in “Core descriptions”). The transition is coincident with a consistent change in central tendency in percent carbonate, magnetic intensity, and resistivity. Decimeter-scale color banding is no longer apparent, and consistently high carbonate content is reflected in coloration, which varies only in shades of white (5Y 8/1 and N9). Occasional millimeter- to centimeter-scale diffuse bands of green and gray (5G 7/2 and N4–N5) are distributed throughout the sequence. Smear slides suggest that as much as 95% of the lithology is composed of nannofossil ooze with clay contents <5%. Carbonate content averages 94 wt%, with a range of 89–96 wt%. In Unit II, bioturbation is rare to moderate when observed. Lack of color contrast in this uniform lithology makes it difficult to estimate the degree of bioturbation, which may be totally bioturbated or homogenized. However, micronodules of pyrite are often present and likely located near burrows, and they may produce black streaks on the cut surface of the cores as a result of the core splitting process. Pyrite is also sporadically present in the form of burrow casts and nodules. Pyrite origin is probably related to local reducing conditions associated with organic matter complexes. Foraminifer “event” bedsA number of millimeter- to centimeter-scale foraminifer nannofossil ooze beds occur in both units. Figure F9 demonstrates that the majority of these beds can be correlated with variable thickness across all four holes. These beds typically have a sharp, occasionally erosive, pyrite-rich base and, most commonly, gradational upper contacts. A number show normal grading. The most noteworthy of such layers is a decimeter-scale thick bed at 100.62–100.91 mbsf in Hole U1313A, 101.49–101.73 mbsf in Hole U1313C, and 103–103.29 mbsf in Hole U1313D (Fig. F10; Table T2). An equivalent layer was not observed in Hole U1313B and is shown to have been lost in a core break. The layer displays normal grading with a scour at the base and has been interpreted as a turbidite. It is not certain if all of these beds are the result of deposition by turbidity currents, although they probably represent reworked material. Ash bedsA prominent ash layer was identified at 32.97–33.02 mbsf in Hole U1313A, 32.62–32.34 mbsf in Hole U1313B, 32.68 mbsf in Hole U1313C, and 32.85–32.86 mbsf in Hole U1313D (see Figs. F9, F11). It is associated with a large positive magnetic susceptibility anomaly (see “Physical properties”). Smear slides show that these layers are composed almost exclusively of volcanic glass, opaque minerals, and minor quartz grains. In Hole U1313A, the ash layer displays a sharp erosive base and gradational upper contact. These factors, together with its variable thickness between the four holes, suggest that this ash layer does not represent deposition from settling through the water column but more likely reworking of older ash deposits, either locally or by turbidity currents. DropstonesFigure F9 and Table T3 summarize the occurrence of gravel-sized grains at Site U1313. The gravel-sized grains are interpreted as ice-rafted debris (IRD). Correlations can be made between regular intervals of IRD in meters below seafloor across all four holes in Unit I. Preliminary analysis of these dropstones shows that they typically range in size from 2 to 10 mm but can be as large as 30 mm, especially in sediments of Subunit IA. Only two gravel-sized grains were found in Unit II. A millimeter-scale carbonate clast at 157.27 mbsf in Hole U1313A (Section 306-U1313A-18H-1, 7 cm) has been attributed to core flow-in and is not in situ. A 30 mm clast of basic igneous origin occurs at 176.32 mbsf in Hole U1313A (Section 306-U1313A-20H-1, 12 cm). Given its location at the top of Core 306-U1313A-20H, it also may not be in situ. Two of the most prominent events of increased IRD input are shown in Figures F12 and F13. Subangular to subrounded carbonate gravel-sized grains occur in a decimeter-scale interval of silty clay nannofossil ooze at 57.60–58 mbsf in Hole U1313A (interval 306-U1313A-7H-4, 20–60 cm), 57.50–57.90 mbsf in Hole U1313B (interval 306-U1313B-7H-3, 110–150 cm), 58.65–59.15 mbsf in Hole U1313C (interval 306-U1313C-7H-6, 95–145 cm) (Fig. F12), and 57.54–57.90 mbsf in Hole U1313D (interval 306-U1313D-7H-1, 54–90 cm). In Figure F13, a mixture of carbonate and mafic igneous gravel-sized grains occur in a matrix of decimeter-scale silty clay nannofossil ooze at 88.23–88.48 mbsf in Hole U1313A (interval 306-U1313A-10H-5, 103–128 cm) and 87.60–87.80 mbsf (interval 306-U1313D-10H-4, 120–140 cm) and 88.83–89.08 mbsf (10H-3, 33–58 cm) in Hole U1313D. DiscussionThe preliminary age model for Site U1313 indicates that lithologic changes described above may be related to northern hemisphere climate instability, with the main lithologic division between Unit I and II recording the onset of major northern hemisphere glaciation during the late Pliocene. Unit II is characterized by uniformity in composition and color with high biogenic carbonate content. It is interpreted to record stable open-water pelagic sedimentation during the latest Miocene and late Pliocene. According to biostratigraphic and paleomagnetic results, the base of Unit I is placed at 2.6–2.7 Ma. Unit I, which is characterized by an increased but variable detrital clay content as reflected in decimeter-scale color banding, therefore, may record the evolution of northern hemisphere glaciation from this time (Raymo, 1992). An increase in the percentage and variability of the sediment detrital component may be explained by an expansion of northern hemisphere continental ice sheets and subsequent increased ice-rafting and eolian deposition connected to enhanced regional/hemispheric wind speeds. The onset of full glacial–interglacial conditions at this time is also reflected in accelerated North Atlantic bottom water cooling at 2.8 Ma as determined from Mg/Ca ratios in fossil ostracodes from Site 607 (Dwyer et al., 1995). A distinctive increase in the occurrence of dropstones found in this unit may be important in identifying IRD events, further highlighting that northern hemisphere ice sheet instability forms an important part of the sediment's paleoclimate record during the Holocene–late Pliocene. Further evidence for glacial–interglacial climate controlled changes in detrital input comes from a strong correlation of the lightness record with the global benthic oxygen isotope stack for the last 3.4 m.y. (Lisiecki and Raymo, 2005) (Fig. F24). This preliminary correlation makes it possible to place the timing of the transition from Subunit IB to IA at 0.9 Ma within the “Mid-Pleistocene Revolution,” which marks the transition from the 41 to 100 k.y. world (Imbrie et al., 1993). This also is reflected in more distinct maxima and minima in biogenic carbonate and detrital clay components and is mirrored by a distinct change in percent carbonate variance. |
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