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doi:10.2204/iodp.proc.320321.108.2010

Science summary

Two holes were cored at Site U1336 (proposed Site PEAT-5C; 7°42.067′N, 128°15.253′W; 4286 mbsl) targeting paleoceanographic events in the late Oligocene and into the Miocene, including a focus on the Oligocene–Miocene transition and the recovery of the Mi-1 glaciation event (Zachos et al., 2001b; Pälike et al., 2006b). In conjunction with Sites U1335 and U1337, Site U1336 was also designed to provide a latitudinal transect for early Miocene age slices. Site U1336 provides data toward a depth transect across the late Oligocene and Miocene that allow us to verify and apply a previous astronomical age calibration from Site 1218 (Pälike et al., 2006a).

At Site U1336, advanced piston corer (APC) cores were taken from the seafloor to 184.8 m (Cores 320-U1336A-1H through 21H) and 173.6 m (Cores 320-U1336B-1H through 20H). Nonmagnetic core barrels were used for Cores 320-U1336A-1H through 16H and Cores 320-U1336B-1H through 16H and steel barrels were used for all other cores. Two hard layers, one at ~121 m core depth below seafloor (CSF) (Cores 320-U1336A-14H and 320-U1336B-14H) and one at ~135 m CSF (Core 320-U1336B-16H) caused core loss and prevented the development of a continuous sediment section. Extended core barrel (XCB) cores (320-U1336A-22X through 35X) were taken from 184.8 to 302.9 m CSF in Hole U1336A. We stopped coring before reaching the basement objective because of decreasing rates of penetration, relatively low recovery, and the possibility of obtaining a stratigraphically complete Miocene section by allocating the remaining operational time during Expedition 320 to Hole U1336B.

At Site U1336, ~300 m of pelagic sediments are divided into three major lithologic units (Fig. F3). The sediments are composed mainly of nannofossil oozes, nannofossil chalks, and chert. The lower to middle Miocene sedimentary sequence of Unit I (0–74.54 m CSF) contains more radiolarians, clay, foraminifers, and diatoms relative to the lower Miocene to lower Oligocene sediments below ~70 m CSF. Subtle changes in the relative proportions of these minor components produce meter-scale dark–light color cycles and two diatom-rich layers. Numerous rounded fragments of pumice occur throughout this unit.

Unit II (74.50–189.50 m CSF) is dominated by nannofossil ooze. Sediment color changes downhole from pale yellow to light greenish gray at 92 m CSF. Below this boundary, the color of Unit II alternates between light greenish gray and white to 184.80 m CSF. Oxidation-reduction reactions are responsible for the observed vivid colors and pore water chemistry changes, likely fueled by varied availability of organic carbon. Occasional thin chert layers were encountered below 120 m CSF in Unit II. Mainly broken chert fragments were recovered, except for a small in situ chert fragment at 159.6 m CSF in Section 320-U1336B-18H-4, 106 cm. More abundant chert layers are common in the lower third of the recovered sequence.

Unit III (189.5–299.6 m CSF) was only recovered in Hole U1336A. The dominant lithologies of this unit are light greenish gray and white nannofossil chalk with light greenish gray millimeter-scale color banding and chert layers. The chert shows many different colors including black, dark greenish gray, very dark greenish gray, dark gray, olive-yellow, dark brown, and pink. The Unit II–III transition is identified by the uppermost common occurrence of chert. Below 289 m CSF, nannofossil chalk contains increasing amounts of micrite and the cherts vary in color. The lowermost cherts are olive-yellow, then pink, and, finally, dark brown at the base. The chalk changes color to white below 298.54 m CSF. CaCO₃ contents remain >88 wt% in the chalk layers. Igneous basement was not recovered at Site U1336.

All major microfossil groups were found in sediments from Site U1336, representing a complete biostratigraphic succession at the shipboard sample resolution level of middle Miocene to lower Oligocene sediments. They provide a coherent, high-resolution biochronology through a complete sequence (Fig. F3). Calcareous nannofossils are moderately to poorly preserved throughout the succession. There appears to be a complete sequence of nannofossil zones from Zone NN6 (middle Miocene) through NP22 (lower Oligocene), except for Zone NN3, which could not be resolved. Planktonic foraminifers are present throughout the succession ranging from Zones N12 through O1. They are moderately well preserved in the Miocene and less well preserved in the Oligocene.

Benthic foraminifers are present throughout the section, although abundances are overall quite low. The preservation of tests is moderate in the upper part of Site U1336 (Samples 320-U1336A-1H-CC through 19H-CC, 8.22–170.63 m CSF, and 320-U1336B-1H-CC through 20H-CC, 1.68–174.01 m CSF) but deteriorates below this level. The Oligocene to middle Miocene benthic foraminifer assemblage is relatively diverse and indicates oligotrophic lower bathyal to abyssal paleodepths.

The Oligocene/Miocene boundary is placed between the first occurrence of Paragloborotalia kugleri (23.0 Ma) and the extinction of Sphenolithus delphix (23.1 Ma). The former occurs between Samples 320-U1336A-16H-CC and 17H-2, 38–40 cm (142.96 m CSF) and Samples 320-U1336B-16H-1, 52–54 cm, and 17H-3, 80–82 cm (137.72 m CSF). The top of S. delphix is recognized between Samples 320-U1336A-17X-2, 90 cm, and 17X-4, 90 cm (145.9 m CSF), and between Samples 320-U1336B-16H-CC and 17H-1, 150 cm (137.56 m CSF).

The radiolarian stratigraphy at Site U1336 spans the interval from just above the Zone RN6/RN5 boundary (middle Miocene) to the upper part of Zone RP22 (upper Oligocene) at ~170 m CSF. Below this level the sediments are barren of radiolarians. Above this level the assemblages tend to have good to moderate preservation with intermittent intervals of good preservation in Zones RN3 and RN4 (lower to middle Miocene). The downsection decrease in preservation and ultimate disappearance of the radiolarians below Core 320-U1336A-19H appears to be associated with dissolution and reprecipitation of the biogenic silica as intergranular cement and as chert.

Diatom stratigraphy in Hole U1336B spans the interval from just above the Cestodiscus peplum zone (middle Miocene) in Core 320-U1336B-1H to the lowermost part of the Crucidenticula nicobarica zone (upper lower Miocene) in Core 320-U1336B-7H. Below Sample 320-U1336B-7H-CC, the sediments are barren of diatoms. Above this level the valves tend to be mostly poorly preserved. Sample 320-U1336B-1H-CC contains the highest diversity with Cestodiscus pulchellus as dominant component, accompanied by Cavitatus jouseana and Thalassiosira yabei. Fragments of the large centric diatom Ethmodiscus are present in the upper part of Hole U1336B.

Paleomagnetic measurements were conducted on archive-half sections of 21 APC cores from Hole U1336A and 20 APC cores from Hole U1336B. Measurements of natural remanent magnetization (NRM) above ~80 m CSF in Holes U1336A and U1336B indicate moderate magnetization intensities (~1 × 10^–3 A/m) with a patchy but generally weak viscous remanent magnetization (VRM) or isothermal remanent magnetization (IRM) drilling overprint. Polarity reversal sequences are clearly recognized (Fig. F3). Demagnetization data from discrete samples above ~80 m CSF indicate that the characteristic remanent magnetization of the sediments is identified at the 10–20 mT demagnetization steps. The reversals pattern can be correlated with the geomagnetic polarity timescale (GPTS) from the base of Chrons C5r to C6n (~12 to 19 Ma).

Below ~80 m CSF, a zone of diagenetic alteration involving dissolution of remanence carriers reduces remanence intensities after alternating-field (AF) demagnetization of 20 mT to values close to magnetometer noise level in the shipboard environment (~1 × 10^–5 A/m). In this zone, sediment magnetizations have been partly or entirely overprinted during the coring process and remanence inclinations are sometimes steep after AF demagnetization at peak fields of 20 mT. At ~130–140 m CSF (Cores 320-U1336A-15H through 16H and 320-U1336B-15H) and below ~160 m CSF (Cores 320-U1336A-19H through 21H and 320-U1336B-18H through 20H), polarity reversals are apparently present but the inclinations are steep (as much as 80°), indicating that the drilling overprint has not been effectively removed during shipboard demagnetization.

A complete physical property program was conducted on whole cores, split cores, and discrete samples. Whole-Round Multisensor Logger (WRMSL) (gamma ray attenuation [GRA] bulk density, magnetic susceptibility, P-wave velocity, and electrical noncontact resistivity), thermal conductivity, and natural gamma radiation (NGR) measurements comprised the whole-core measurements. Compressional wave velocity measurements on split cores and moisture and density (MAD) analyses on discrete core samples were made at a frequency of 1 per undisturbed section in Cores 320-U1336A-1H through 35X. Compressional wave velocities were measured toward the bottom of sections. MAD analyses were located 10 cm downsection from carbonate analyses (see "Geochemistry"). Lastly, the Section Half Multisensor Logger (SHMSL) was used to measure spectral reflectance on archive-half sections. Physical properties measurements on whole-round sections and samples from split cores reflect the differences among lithologies drilled at Site U1336 (Fig. F3). Nannofossil ooze with varying amounts of clay, radiolarians, and diatoms makes up lithologic Unit I and is characterized by high-amplitude and high-frequency variations in bulk density, magnetic susceptibility, NGR, and color reflectance. Magnetic susceptibility is highest in Unit I, with values ranging from 5 × 10^–5 to 30 × 10^–5 SI. NGR is also high in this unit, with values to 56 cps near the seafloor. Wet bulk densities are lowest in Unit I, with values ranging from 1.4 to 1.7 g/cm³. Porosity is highest in this interval, ranging from 65% to 80%. The grain density of most of the sediments of Unit I, as well as Units II and III, ranges from 2.6 to 2.9 g/cm³, reflecting the dominance of carbonate constituents at Site U1336. The sediment velocity in Unit I is low, averaging 1500 m/s. The color reflectance of Unit I is marked by luminance (L*) values that are slightly lower and more variable than values determined for sediments in Units II and III.

Below Unit I, a more uniform increase in wet bulk density and decrease in porosity in Units II and III reflects the increasing compaction of the sediments. A slight step increase in wet bulk density marks the transition between Units II and III. In Unit III wet bulk density and porosity average 1.9 g/cm³ and 51%, respectively. Magnetic susceptibility and NGR are low and nearly uniform in Units II and III. Magnetic susceptibility is typically below 5 × 10^–5 SI, and NGR is ~2 cps. Lower clay abundance in Unit II is marked by an increase in L* at the boundary between Units I and II. At 92 m CSF, within Unit II, sharp decreases in the a* and b* reflectance parameters mark the change in sediment color from pale yellow to greenish gray. One of the most pronounced changes in physical properties at Site U1336 is the sharp increase in velocity that accompanies the change from nannofossil ooze to nannofossil chalk at the boundary between Units II and III. The velocity at the base of Unit II is ~1700 m/s. Below 190 m CSF, in Unit III, the rate at which velocity increases with depth increases, ultimately reaching ~2200 m/s at 290 m CSF, near the base of Hole U1336A.

Special Task Multisensor Logger (STMSL) data were collected at 5 cm intervals from Hole U1336B and compared to the WRMSL data obtained at 2.5 cm resolution from Hole U1336A during Expedition 320. Features in the magnetic susceptibility and gamma ray attenuation density are well aligned between Holes U1336A and U1336B to a depth of ~94 m core composite depth below seafloor (CCSF-A; see "Core composite depth scale" in the "Methods" chapter). Below 94 m CCSF-A, the magnetic susceptibility signal drops to very low values but the density data are good enough to sustain a correlation to interval 320-U1334B-14H-4, 122 cm. At this point (138.50 m CCSF-A) sediments recovered in both holes are disturbed.

Paleomagnetic reversals were used to calculate the average linear sedimentation rate (LSR) for the upper 74 m of the section at Site U1336 on the corrected core composite depth below seafloor (CCSF-B; see "Corrected core composite depth scale" in the "Methods" chapter) depth scale. Below 74 m CSF only biostratigraphic datums were used to calculate the average LSR. The LSR at Site U1336 decreases from 15 m/m.y. in the upper Oligocene to 12 m/m.y. in the lower Miocene and stays relatively constant at 9 m/m.y. in the remainder of the section.

Standard geochemical analyses of pore water and organic and inorganic sediment properties were undertaken on Site U1336 samples. Alkalinity is relatively constant at values >2.5 mM in the upper 110 m CSF, with a pronounced decline to 1 mM by 170 m CSF. Sulfate concentrations decrease with depth to values as low as 22 mM. Dissolved manganese has a broad peak in the depth range from ~25 to 120 m CSF, and dissolved iron appears then peaks below 100 m CSF. The increase of dissolved iron occurs where Mn decreases downhole. Concentrations of dissolved silicate increase with depth from <400 to 800 µM.

Highlights

Miocene sedimentary section and cyclic sedimentation

One of the highlights from Site U1336 is the recovery of a thick Miocene carbonate section from the central equatorial Pacific, one of the high-priority objectives of the PEAT program. We recovered the complete early Miocene sequence (7.1 m.y. duration) in a ~110 m thick section, with a sedimentation rate of 12 m/m.y. and the middle Miocene sequence (4.4 m.y. duration) in a ~45 m thick interval with a sedimentation rate of ~21 m/m.y. These high sedimentation rates will facilitate the study of paleoceanographic processes at unprecedented resolution for the equatorial Pacific.

The obvious variations of both color and biogenic composition within nannofossil oozes represent cyclic fluctuations of CCD and upwelling intensity during the middle Miocene through early Miocene. The variable lithology also results in the variations of many petrophysical signals of physical properties including L*, b*, magnetic susceptibility, NGR, and GRA bulk density.

Oligocene–Miocene transition and depth transects

Site U1336 was planned as part of a latitudinal transect for early Miocene age slices and the PEAT Oligocene–Miocene depth transect compound in conjunction with Sites U1335 and U1337. The Miocene sequence at these sites includes the critical intervals of the Mi-1 glaciation and middle Miocene ice sheet expansion (Holbourn et al., 2005; Zachos et al., 2001b; Pälike et al., 2006b). The dominant lithologies of nannofossil ooze and chalk at Sites U1336 and U1335, with good preservation of calcareous microfossils, will allow us to achieve the prime objective for this site.

The Oligocene–Miocene transition in Hole U1336A occurs in homogeneous nannofossil ooze within the alternations of white and light greenish gray ooze. The same alternating sequence is observed above the Oligocene–Miocene transition at Site U1334. Biostratigraphy reveals that the Oligocene/Miocene boundary exists between 142.96 and 145.9 m CSF at Site U1336; this will allow the high-resolution study of this critical interval.

Geochemical front

Site U1336 recovered an interval of greenish gray carbonates that exhibit a distinct peak in dissolved Fe concentrations in pore water with similar characteristics as geochemical alteration fronts at Sites U1334 and U1335. At Site U1336, this zone is ~200 m thick. The remanent magnetization intensity is very weak in most parts of this section (80–180 m CSF). High dissolved Fe and Mn concentrations in pore water are caused by changes in the oxidation state of the sediments. The oxidation-reduction reactions are likely fueled by variable availability of organic carbon in the sediments. This site may provide the opportunity to study organic matter degradation.

Site U1336 migrated from south to north through the equatorial belt of high productivity. Based on paleolatitude reconstructions these geochemical alteration fronts can be mapped to similar equatorial positions between Sites U1334 and U1335, roughly between the Equator and ~4°N.

Chert formation in the early Oligocene

The sequence at Site U1336 includes barren intervals of radiolarian fossils and many thin intercalated chert layers and fragments. The radiolarians decrease in preservation downsection and disappear below Core 320-U1336A-19H. Instead, the sediments contain several chert fragments. Some inferred chert layers occur at ~120–140 m CSF and blocked APC penetration. Below ~190 m CSF, various colored chert layers and fragments occurred within the cores. The chert frequently contains foraminifer tests, reflecting diagenetic process of dissolution and reprecipitation of the biogenic silica.

The dissolution of biogenic silica is the source of porcellanite and chert and, on crust younger than 65 Ma, almost all cherts in the Pacific Ocean lie <150 m above basement. Although we did not recover basement rocks at this site, the sediments became hard, lithified limestones and the drilled section is probably close to basement. The dissolution of silica in the basal sedimentary section is likely associated with the circulation of warm hydrothermal waters in the upper oceanic crust that extend into the lower sediments where they are cut by fractures and faults (Moore, 2008a, 2008b). This site will provide information on chert formation in the equatorial Pacific regions.

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