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doi:10.2204/iodp.proc.320321.107.2010 Site U13351Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1335 (5°18.735′N, 126°17.002′W; 4327.5 meters below sea level [mbsl]) (Fig. F1; Table T1) is located in the central area drilled during the Pacific Equatorial Age Transect (PEAT) program (IODP Expedition 320/321). Site U1335 (~26 Ma crust) is situated halfway between IODP Site U1336 ~340 km to the northwest and IODP Site U1337 ~390 km to the southeast, ~250 km south of the Clipperton Fracture Zone. Site U1335 is located on a broad plateau within north-northeast–trending abyssal hill topography. Thick sediment deposits cover the abyssal hills, with a thinning sediment cover on the hills. Site U1335 is draped with ~420 m of sediment cover (Fig. F2), estimated to be ~360 m prior to drilling. Water depth in the vicinity of Site U1335 is between 4300 and 4400 m, apart from the topography around seamounts that are 15–20 km away from the drill site (Fig. F1B). Based on stage-pole reconstructions of Pacific plate motion and observations of basement age from previous drilling sites, along with magnetic anomaly maps (Cande et al., 1989), we estimated prior to drilling that Site U1335 is located on 26 Ma crust. The best control on age is information from Deep Sea Drilling Project (DSDP) Site 79, located ~600 km and 4.5° east and 3° south of Site U1335, apparently on the same fracture zone segment, with the Clipperton Fracture Zone to the north. The base of Site 79 reaches the Miocene/Oligocene boundary, or ~23 Ma on the most recent astronomically calibrated timescales. Site U1335 was proposed for drilling to focus on the paleoceanographic events in the late Oligocene and into the early and middle Miocene, including the climatically significant Oligocene–Miocene transition and recovery from the Mi-1 glaciation event. In conjunction with Sites U1336 and U1337, it was also designed to provide a latitudinal transect for early Miocene age slices. At the end of the Oligocene, a significant multimillion year long rise in the oxygen isotope record (Lear et al., 2004) 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., 2006b). 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 isotopic stratigraphy, biostratigraphies of the planktonic microfossils show very little change at all across this boundary. In fact, it is one of the most difficult epoch boundaries to pick using only microfossil biostratigraphies. At Ocean Drilling Program (ODP) Leg 199 Sites 1218 and 1219 this interval was well recovered; however, carbonate preservation still presented a problem for foraminifer stratigraphy. Both sites were deep and well within the lysocline, making the application of temperature proxies such as Mg/Ca ratios in foraminifer tests more difficult (Lear et al., 2008). At the time Miocene–Oligocene sediments were deposited, Site 1218 already resided on 18 m.y. old crust and was ~4100 m deep. Site 1219 was on ~34 m.y. old crust and was ~4.5 km deep (Lyle, Wilson, Janecek, et al., 2002). There was a relative increase in large diatoms near this boundary in the siliceous coarse fraction, suggesting increased productivity; however, detailed high-resolution flux rates across this interval have yet to be determined. A well-recovered section on the latest Oligocene Equator, near the late Oligocene ridge crest as targeted by Site U1335, should provide both the resolution and the preservation required to better describe the changes in the equatorial ocean taking place at this time. We positioned Site U1335 and the other PEAT sites slightly south of the estimated paleoequatorial position at their target ages in order to maximize the time that drill sites remain 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), 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. 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 Eocene–Oligocene transition (Coxall et al., 2005) and Oligocene–Miocene transition (Shackleton et al., 2000; Pälike et al., 2006b; Zachos et al., 2001b). For this objective, Sites U1335–U1337 will form a combined depth transect for Oligocene–Miocene time. Site U1335 had an estimated crustal paleodepth of ~3.3 km during the Oligocene–Miocene transition. All Expedition 320/321 drill sites 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., 2006a). The seismic reflection data (Fig. F2) (Pälike et al., 2008; Lyle et al., 2006) allowed us to optimize the Site U1335 position on seismic Line PEAT-6C-sl-8, which trends north–south. Site U1335 was moved south of the intersection of this line with the east-west Cross-line PEAT-6C-sl-1 to better image basement and obtain a more expanded lower sediment section. We estimated sediment thickness using interval velocities published for DSDP Site 574 by Mayer et al. (1985), which drilling determined later to underestimate the basal interval velocities and therefore total sediment thickness. The subbottom profiler sections image ~20 m of transparent surface sediment and ~100 m of layered sediments in the upper sediment column (Pälike et al., 2008). Site U1335 is located along the flank of a wide valley that trends north–northeast, on elevated seafloor above a moat around the seamounts (Fig. F1B). Sediment cover is thick on the plateau but is highly variable along the edges. There is variable sedimentation along the eastern edge and the thick sediment cover on the plateau ranging from 300 to 600 ms two-way traveltime (TWT) (Lyle et al., 2006; Pälike et al., 2008). Nevertheless, the seafloor is relatively flat because the sediment has filled in the basement topography at ~200 ms TWT. Based on correlation to the central equatorial Pacific seismic stratigraphy of Mayer et al. (1985), middle Miocene sediment has been exposed. A site survey piston Core RR0603-6JC was taken east of Site U1335 (Fig. F1B). The cores recovered mottled brown to light brown cyclic carbonates and siliceous biogenic sediments with carbonate contents between 20 and 85 wt%. Survey piston core sediments range from foraminifer-nannofossil oozes with radiolarians and diatoms to radiolarian-diatom oozes with foraminifers and nannofossils. The age at the base of this core is ~2.4 Ma based on calcareous nannofossil stratigraphy and a tie from the physical property data to the global isotope stratigraphy. | ||
