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doi:10.2204/iodp.proc.320321.105.2010 Site U13331Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1333 (10°30.996′N, 138°25.159′W, 4853 meters below sea level [mbsl]) (Fig. F1; Table T1) is located about halfway between IODP Site U1332 to the northwest and the previously cored Ocean Drilling Program (ODP) Site 1218 to the southeast, both ~350 km away. This site is situated ~440 km north of the Clipperton Fracture Zone and 570 km south of the Clarion Fracture Zone (Fig. F1). Site U1333 is over seafloor basement with a estimated age of 46 Ma. The primary coring objective at Site U1333 was to recover a complete sequence of carbonate sediments spanning the middle Eocene to Oligocene so we can evaluate changes in the temperature and structure of the near-surface ocean, bottom water temperatures, and the evolution of the calcium carbonate compensation depth (CCD). One of the additional objectives of the Pacific Equatorial Age Transect (PEAT) program is to provide a depth transect for several Cenozoic key horizons, such as the Eocene–Oligocene transition (Coxall et al., 2005), which is being targeted at IODP Sites U1331–U1334. Site U1333 forms the third deepest paleodepth constraint, with an estimated crustal paleodepth of <4 km and a paleolatitude ~3° north of the paleoequator during the Eocene–Oligocene transition. Good paleomagnetic stratigraphy at ODP Leg 199 sites allowed a significantly improved calibration of nannofossil and radiolarian biostratigraphic datums (Moore et al., 2004; Raffi et al., 2005; Pälike et al., 2005, 2006; Nigrini et al., 2006). From the combined information, a more detailed picture emerged of temporal variations in sediment accumulation through the middle and upper Eocene of the tropical Pacific. These data showed an increase of up to 2–3 times in accumulation rates of siliceous ooze during the middle Eocene (41–45 Ma). There are also several notable periods of highly fluctuating CCD associated with intervals in which carbonate is preserved as deep as 4000 mbsl, or ~700 m deeper than the average Eocene CCD (Lyle, Wilson, Janecek, et al., 2002; Lyle et al., 2005; Rea and Lyle, 2005; Bohaty et al., 2009). These fluctuations occur immediately prior to the Middle Eocene Climatic Optimum (MECO), which is associated with CCD shoaling (Bohaty and Zachos, 2003; Bohaty et al., 2009). Such fluctuations in the CCD are similar in magnitude to those at the Eocene/Oligocene boundary (Coxall et al., 2005). High siliceous sedimentation rates occur near an apparent short reversal in the middle Eocene cooling interval. It is difficult to interpret the cause of such a substantial change in silica flux during a very warm climatic regime. Site U1333 is located in abyssal hill topography north of the Clipperton Fracture Zone (Fig. F1B), with a general deepening of the seafloor toward the north. Bathymetric relief across the abyssal hills is ~75–150 m, and sediment thickness is ~200 ms two-way traveltime (TWT), which coring determined to correspond to ~180 m of sediment. The 48-channel stacked and migrated seismic reflection data (e.g., seismic Line PEAT-3C-sl-3 in Pälike et al., 2008) (Lyle et al., 2006) reveal a region where the sediment column that had been deposited is eroding away. Outcropping older horizons are common along seismic Line 1 and at the northern ends of the cross-lines. Site survey piston Core RR0603-09JC suggested that the surface sediments are composed of ~4 m of zeolitic clay and then 2 m of radiolarian clay overlying early Miocene carbonates. The carbonate section of the piston core consists of nannofossil ooze and radiolarian nannofossil ooze in meter-scale cycles. The base of the core is ~21 Ma based upon the combined radiolarian and nannofossil stratigraphy, with an average sedimentation rate of 7 m/m.y. for the cored section. We drilled Site U1333 slightly west of the intersection between seismic Lines 3 and 8 to maximize the thickness of the deeper section. The crossing point of the seismic lines was just south of a minor basement hill, and the drill site is a relatively small target, ~720 m across. The low amplitudes of the seismic reflections suggest that the sediment is not lithified, fitting in with the shallow depth to basement. An interpretation of the site survey seismic data (Fig. F2) indicated that Site U1333 might penetrate seismic reflectors P2 and P3 of Lyle et al. (2002), with P4 near the sediment surface. We positioned Site U1333 and the other PEAT sites to the south of the estimated paleoequatorial position at a target age that would maximize the time that drill sites remain within the equatorial zone (i.e., ±2° of the equator). This allows for some southward bias of the equatorial sediment mound relative to the hotspot frame of reference (Knappenberger, 2000) and places the sediment interval of maximum interest above the basal hydrothermally altered sediments. To determine the site location, we used the digital age grid of seafloor ages from Müller et al. (1997), heavily modified and improved with additional magnetic anomaly picks from Petronotis (1991) and Petronotis et al. (1994), as well as Deep Sea Drilling Project (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 all backrotated latitudinal positions. | ||
