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doi:10.2204/iodp.proc.303306.108.2006 LithostratigraphySix holes were drilled at Site U1308 (Table T1). Core recovery at Site U1308 was 95.4%, although numerous cores were adversely affected by crushed core liners. Core disturbance caused by elevated ship heave adversely affected core quality in the lower part of Hole 1308A. All cores were recovered using the APC. The sediments at Site U1308 are composed of varying mixtures of terrigenous and biogenic components (primarily nannofossils, clay minerals, and quartz) (see “Site U1308 smear slides” in “Core descriptions;” Fig. F3). The most common lithologies are nannofossil ooze, foraminifer nannofossil ooze, nannofossil ooze with silty clay, silty clay nannofossil ooze, nannofossil silty clay, silty clay with nannofossils, and silty clay. Most contacts between these lithologies are burrowed or gradational. However, thin (≤10 cm) silty clay layers commonly display sharp lower contacts. Abundances of terrigenous components as estimated from smear slides are quartz, 0%–55%; detrital carbonate, 0%–39%; feldspars, 0%–10%; clay minerals 0%–70%; heavy minerals (especially hornblende), 0%–10%; and volcanic glass, 0%–15%. No discrete ash layers were observed. Dropstones are rare throughout the upper 165 m of Site U1308 (~1 per 3 m of core) but are slightly more abundant in the upper 100 m (~1 per 2 m of core) (Fig. F4). The interval below 210 mcd is virtually devoid of dropstones. Dropstones display a wide range of lithologies, including acidic intrusive and metamorphic (granites, gneisses, and granitoids), basic igneous and/or metamorphic (basalts and metabasalts), and sedimentary and metasedimentary (sandstone and limestone/dolostone) rocks. Abundances of biogenic components, as estimated from smear slides, are nannofossils, 0%–95%; foraminifers, 0%–25%; diatoms, 0%–25%; radiolarians, 0%–trace; and sponge spicules, 0%–5%. Total carbonate contents range from 12 to 94 wt% in these cores (see “Geochemistry;” Table T32). Pyrite (usually associated with burrows) and iron oxide coatings on grains are present throughout, constituting the only authigenic sediment components observed. Between 168 and 191 mcd, portions of the sedimentary succession exhibit inclined bedding. This is most apparent in the greenish gray diagenetic horizons that either gently dip (~10°–15°) or are subparallel. However, very sharp inclined lithologic contacts were observed in Holes U1308B, U1308C, and U1308E (Fig. F5). These features were the only observed evidence of sediment disturbance not induced by coring at Site U1308. The sediments at Site U1308 are divided into two units. Unit I is composed of a Holocene–Upper Pliocene sequence of interbedded biogenic and terrigenous sediments with dropstones. Unit II is composed of Upper Pliocene–upper Miocene sediments, rich in nannofossil ooze and devoid of dropstones. Subunit IIA is an Upper Pliocene nannofossil ooze interbedded with terrigenous sediment-rich layers but at a lower frequency than Unit I. Subunit IIB is entirely composed of lowermost Upper Piocene–uppermost Miocene nannofossil ooze that exhibits very little color change. The character of sediment physical properties, including natural gamma radiation (NGR), magnetic susceptibility, and density, exhibits significant changes at each lithologic boundary (see “Physical properties”). Description of unitsUnit I
Primary biogenic lithologies within Unit I are nannofossil ooze, foraminifer nannofossil ooze, and nannofossil ooze with silty clay. Minor biogenic lithologies include nannofossil ooze with diatoms and nannofossil ooze with clay and diatoms. Interbedded with the biogenic lithologies, at decimeter to millimeter scale, are terrigenous lithologies composed of silty clay, silty clay with nannofossils, nannofossil silty clay, and minor nannofossil clay. Sediments from 0 to ~1.75 mcd exhibit a light yellow-brown to olive-brown color and are interpreted from smear slide data as surface-oxidized equivalents of the underlying lithologies. Bioturbation is present throughout Unit I; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Discrete burrows and macroscopic pyritized burrows were also observed. The bioturbation index ranges from rare to abundant, but most lithologies display common bioturbation. Contacts between these lithologies are generally bioturbated to gradational, although centimeter- to decimeter-scale silty clay layers within each hole commonly exhibit bioturbated upper contacts and relative sharp lower contacts. The color of nannofossil-rich lithologies ranges from white (N/8 and 5Y 8/1) to dark gray (5Y 4/1) and occasionally light olive-gray (5Y 7/2) to olive-gray (5Y 5/2) and light greenish gray (5GY 7/1) to greenish gray (5GY 6/1), with darker colors typically indicating increased terrigenous content. Terrigenous lithologies are darker, typically ranging from gray (5Y 5/1) to dark gray (5Y 4/1), olive-gray (5Y 5/2) to dark olive-gray (5Y 4/2), and, rarely, grayish brown (2.5Y 5/2) to dark grayish brown (2.5Y 4/2). Thin black (5Y 2.5/1) silty clay beds are also present. Calcium carbonate content averages 59 wt% and ranges between 12 and 90 wt% in Unit I (see “Geochemistry;” Table T32). Unit IISubunit IIA
Subunit IIA is composed of nannofossil ooze, nannofossil ooze with silty clay, and silty clay nannofossil ooze. The contact between Unit I and Unit II is gradational. Except for one dropstone at 239.7 mcd (215.6 mbsf) in Hole U1308C, Subunit IIA lacks dropstones (Fig. F4). Calcium carbonate content averages 75 wt% and ranges between 68 and 80 wt%, which is a higher average and lower variability than that of Unit I (see “Geochemistry;” Table T32). In Subunit IIA, the sediment lightness (L*) variable and physical properties (magnetic susceptibility, NGR, and density) exhibit reduced amplitude fluctuations relative to those of Unit I (Fig. F6). As determined by visual core description, sediment color ranges from gray, olive-gray, and greenish gray (5Y 5/1, 5Y 6/1, 5Y 5/2, and 5GY 5/1) to light gray, light olive-gray, and light greenish gray (5Y 7/1, 5GY 7/1, and 5Y 7/2) within the upper portion of Subunit IIA, becoming predominantly white (5Y 8/1, N/8) to light gray (5Y 7/1) at the base of Subunit IIA. Greenish gray horizons spaced ~5 to ~10 cm apart are common. Alterations in color occur less frequently than in Unit I, and contacts are typically gradational to bioturbated. Bioturbation is present throughout most of Subunit IIA; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Discrete burrows were also observed. The bioturbation index ranges from rare to abundant. Subunit IIB
Smear slide analysis indicates that Subunit IIB is composed entirely of nannofossil ooze (Fig. F3) dominated by white (N/8) color. The Subunit IIA/IIB boundary is easily recognized by ~10 cm gradational lightening of sediment color from light gray (5Y 7/1) in Subunit IIA to white (N/8) in Subunit IIB, which is apparent in both the L* values and observed Munsell colors. Calcium carbonate content averages 91 wt% and ranges from 85 to 94 wt% in Subunit IIB (see “Geochemistry;” Table T32). Correspondingly, sediment NGR and magnetic susceptibility values decrease whereas density increases at the Subunit IIA/IIB boundary (see “Physical properties”). Greenish gray color bands are present but more diffuse and infrequent than in Subunit IIA. Bioturbation is present throughout most of Subunit IIB; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Discrete burrows were also observed. The bioturbation index ranges from rare to common, but most lithologies display moderate bioturbation. DiscussionThe distribution of abundant lithologies at Site U1308 is presented in Figure F7. Of the 1189 m of sediment recovered at this site, 1044 m (88%) is nannofossil ooze and related derivatives. Silty clay and its variations compose 145 m (12%) of the sediments. The sedimentary succession recovered at Site U1308 was divided into units and subunits based on the relative abundance of terrigenous material within the sediments. Lithological determinations based on smear slide data indicate that terrigenous content of the sediment increases upsection (“Site U1308 smear slides” in “Core Descriptions;” Fig. F3) and is accompanied by lower CaCO3 content (see “Geochemistry;” Table T32). In addition to increasing terrigenous input, the amplitude of lithologic variation increases in the upper part of the succession. This variation is well expressed in the NGR, magnetic susceptibility, and density records (see “Physical properties”). The terrigenous component imparts a distinct color change to the sediments recovered at Site U1308. As such, L* provides an accurate proxy of the terrigenous component and easily displays variations downcore. Two distinct units are identified based on the variability of reflectance. Unit I is characterized by high-frequency and high-amplitude variability in lithology shown by L* values ranging between 25% and 80% (Fig. F6). In contrast, L* variability in Unit II is lower with values between 40% and 70% in the upper part of this unit. Unit II was divided based on the appearance of terrigenous components as indicated by sedimentary color. The L* variable records this a stepwise shift in the mean L* value from ~60% in Subunit IIA to >75% in Subunit IIB. The timing of lithologic unit and subunit boundaries is constrained by shipboard age determinations. The base of Unit I (201 mcd) is close to the Matuyama/Gauss boundary (2.58 Ma; ~193 mcd). The base of the Mammoth Subchron (3.33 Ma) is located at ~264 mcd, close to the base of Subunit IIA (262 mcd) (see “Paleomagnetism”). Nannofossil datum level-derived ages are in agreement with the ages based on magnetostratigraphic reversals (see “Biostratigraphy”). The stepwise increase in terrigenous content in Subunit IIA is consistent with the development of significant ice in the northern hemisphere at ~3.3 Ma (Maslin et al., 1995). At this time, benthic foraminifer δ18O records indicate pronounced deepwater cooling and/or ice sheet growth (Tiedemann et al., 1994). Onset of large-scale glaciation could have provided a possible source for this terrigenous sediment through either ice rafting of fine-grained regolith or increased eolian depositon associated with higher regional/hemispheric wind speeds. The stepwise increase in NGR at this depth indicates increased clay mineral content (see “Physical properties”). Heinrich events H0–H6 and H11 (Termination II) are recognizable in magnetic susceptibility data (see “Physical properties”). H1, H2, H4, and H5 are readily identifiable in split core sections because of their lighter color, typically olive-gray (5Y 5/2 and 5Y 4/2), and sharp basal contacts (Figs. F8, F9, F10, F11). The positions and thicknesses roughly correspond to those identified at Site 609 (Bond et al., 1992). Millennial-scale variability in the magnetic susceptibility record reflects the high-frequency climate recorded in fluctuations documented by previous studies from this region (e.g., Bond et al., 1997, 1999, 2001). Furthermore, the variations continue downcore in the magnetic susceptibility records, indicating that millennial-scale variability may be a persistent feature of the climate system. |
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