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doi:10.2204/iodp.proc.320321.108.2010 Site U13361Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program Site U1336 (site survey PEAT-5C; 7°42.067′N, 128°15.253′W; 4286 meters below sea level [mbsl]) (Fig. F1; Table T1) is in the central area drilled during the Pacific Equatorial Age Transect (PEAT) program (Expedition 320/321). Site U1336 (~32 Ma crust) is between Site U1334 ~410 km to the west and Site U1335 ~330 km to the southeast and ~100 km to the west of Deep Sea Drilling Project (DSDP) Site 78. Site U1336 is also ~30 km north of the center of the Clipperton Fracture Zone. The site survey data (Lyle et al., 2006; Pälike et al., 2008) show that Site U1336 is on abyssal hill topography draped with thick sediment (Fig. F2). The fabric of the abyssal hills is oriented slightly west of due north. Water depth in the vicinity of Site U1336 is relatively shallow for the age of the crust, between 4200 and 4400 m. Surprisingly, the crust south of the Clipperton Fracture Zone is only 100 m deeper than the crust north of the zone, despite a water depth of nearly 5 km in the middle of the fracture zone trace. A few oblique ridges and depressed topography occur in the south of the survey area, showing some interaction between the fracture zone and the Site U1336 region. The site is thickly covered with sediment (300–400 ms two-way traveltime [TWT]) (Fig. F2) and has a very thin layer of recent to middle Miocene sediment, with frequent sediment erosion on flanks of abyssal hills. Based on correlation to the Neogene central equatorial Pacific seismic stratigraphy of Mayer et al. (1985) and the Paleogene equatorial Pacific stratigraphy of Lyle et al. (2002), we estimated ~120 m of Oligocene sediment for an average Oligocene sedimentation rate of 13 m/m.y., assuming a crustal age of 32 Ma. The total sediment thickness, using the velocity-depth conversion for DSDP Site 574 (Mayer et al., 1985) was estimated as 253 m prior to drilling. Based on stage-pole reconstructions of Pacific plate motion, observations of basement age from previous drilling sites, and magnetic anomaly maps (Cande et al., 1989), we estimated that Site U1336 is on 32 Ma crust. The best control on age is information from Site 78, ~100 km east of Site U1336. The base of Site 78 reaches the lower Oligocene. Site U1336 targets the Oligocene and is on lower Oligocene crust. This interval of time is noted by heavy oxygen isotopes and a relatively deep calcium carbonate compensation depth (CCD) (Zachos et al., 2001a, 2001b; Pälike et al., 2006a, 2006b; Lyle, 2003). There was probably ice on Antarctica during this interval, but not the large ice sheets found there in the middle Miocene. There is no compelling evidence for ice sheets in the Northern Hemisphere during the Oligocene and early Miocene. Thus, there was apparently a relatively low global ice volume, relatively cold bottom waters, a relatively cold South Pole, and a relatively warm North Pole. This scenario of a "one cold pole" world has given rise to speculation on the impact of interhemispheric temperature imbalance on pole to Equator temperature gradients and on the symmetry of the global wind systems. The extent to which such an imbalance may have affected trade winds, the position of the intertropical convergence zone, and seasonal shifts in this zone should be seen in the wind-driven currents of the equatorial region. Older low-resolution DSDP data (Hays et al., 1972) indicate relatively high but variable sediment accumulation rates during the Oligocene and better carbonate preservation south of the Equator (van Andel, 1975). In the Ocean Drilling Program (ODP) Leg 199 equatorial transect (Lyle, Wilson, Janecek, et al., 2002), the highest accumulation rates encountered (>15 m/m.y.) occurred in the lower part of the Oligocene, but these were in sites north of the Oligocene Equator or on relatively old (and therefore deep) crust. Thus we expected better fossil preservation and a thicker carbonate section at the Oligocene Equator. Studies of Oligocene sections from Leg 199 (Lyle, Wilson, Janecek, et al., 2002) and from other ODP sites (e.g., Paul et al., 2000; Zachos et al., 2001b) indicate the presence of strong eccentricity and obliquity cycles in carbonate preservation and suggest a strong (southern) high-latitude influence on the carbonate record. These cycles are leading to the development of an orbitally tuned timescale that reaches to the base of the Oligocene (Pälike et al., 2006b). Such a timescale makes it possible to develop a very detailed picture of equatorial geochemical fluxes and of the degree of variability in the equatorial system of the Oligocene. Site U1336 also targets paleoceanographic events in the late Oligocene and into the early Miocene, including the climatically significant Oligocene–Miocene transition and the recovery from the Mi-1 glaciation event. In conjunction with Sites U1335 and U1337, it was also sited to provide a latitudinal transect for early Miocene age slices. At the end of the Oligocene there is a significant multimillion year long rise in the oxygen isotope record (supplementary material in Pälike et al., 2006b), which is closely followed by a relatively short, sharp increase in oxygen isotope values that has been interpreted as a major glacial episode (Mi-1) (Zachos et al., 1997, 2001a, 2001b; Pälike et al., 2006a, 2006b) and correlated to a pronounced drop in sea level (Miller et al., 1991). This event is very close to the Oligocene/Miocene boundary and has now been astronomically age calibrated in several ocean basins (Shackleton et al., 2000; Billups et al., 2004; Pälike et al., 2006a). Although there are clear periodic isotopic signals indicating major changes in ice volume, ocean temperatures, and/or ocean structure, this biostratigraphic boundary has always been somewhat of an enigma. Unlike the major changes in the isotopic stratigraphy, the biostratigraphies of planktonic microfossils show very little change at all. In fact it is one of the most difficult epoch boundaries to pick using only microfossil biostratigraphies. Given the Clipperton Fracture Zone geographical constraint, we positioned Site U1336 as close as possible to the estimated paleoequatorial position at the target age in order to maximize the time that the drill site remains within the equatorial zone (i.e., ±2° of the Equator), to allow for some southward bias of the equatorial sediment mound relative to the hotspot frame of reference (Knappenberger, 2000) and to place the interval of maximum interest above the basal hydrothermal sediments. We located the site using the digital age grid of seafloor age from Müller et al. (1997), heavily modified and improved with additional magnetic anomaly picks from Petronotis (1991) and Petronotis et al. (1994) and DSDP/ODP basement ages. For this grid, each point is then backrotated in time to zero age, using the fixed-hotspot stage-poles from Koppers et al. (2001) and Engebretson et al. (1985) and the paleopole data from Sager and Pringle (1988). From the backtracked latitudes for each grid point we then obtained the paleoequator at the crustal age by contouring of latitudes. One of the common objectives of the PEAT program for all sites is to provide a limited depth transect for several Cenozoic key horizons, such as the Oligocene–Miocene transition (Shackleton et al., 2000; Pälike et al., 2006a; Zachos et al., 2001b). For the Oligocene–Miocene transition objective, Sites U1334–U1337 will form a combined depth transect for Oligocene–Miocene time. Site U1336 has an estimated crustal paleodepth of ~3.7 km during the Oligocene–Miocene transition, ~300 m deeper than Site U1335 and ~500 m shallower than Site U1334. Expedition 320/321 drill sites all have in common the objective to improve and extend the extensive intercalibrated bio-, magneto-, chemo-, and astronomical stratigraphies for the Cenozoic (e.g., Shackleton et al., 2000; Pälike et al., 2006b). Seismic reflection data (Fig. F2) (Pälike et al., 2008; Lyle et al., 2006) allowed us to optimize the Site U1336 position on seismic Line PEAT-5C-sl-6, west of the intersection with the north–south Cross-line PEAT-5C-sl-1, to better image basement and obtain a more expanded lower sediment section. We estimated sediment thickness using interval velocities published for Site 574 by Mayer et al. (1985), which drilling determined later to underestimate the basal interval velocities and therefore total sediment thickness. Based upon correlation to the central equatorial Pacific seismic stratigraphy of Mayer et al. (1985), middle Miocene sediment has been exposed. Site survey piston Core RR0603-7JC was taken east of Site U1336 (Fig. F1B). The cores recovered mottled brown to light brown alternating carbonates and siliceous biogenic sediments. The age at the base of this core is ~13 Ma, based on radiolarian biostratigrapy. |
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