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doi:10.2204/iodp.proc.320321.103.2010 Site U13311Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1331 (12°04.088′N, 142°09.708′W; 5116 meters below seafloor [mbsl]) (Fig. F1; Table T1) is the northwesternmost site drilled during the Pacific Equatorial Age Transect (PEAT) program (IODP Expedition 320/321). This site is situated ~6° (~700 km) north of the Clipperton Fracture Zone, ~3° (~350 km) south of the Clarion Fracture Zone, and ~170 km (90 nmi) east of the nearest previously drilled Ocean Drilling Program (ODP) Site 1221. The survey for Site U1331 revealed a region of abyssal hills bound by a large volcanic rise to the east and several seamounts to the southwest. At Site U1331, the abyssal hills are more widely spaced than at the other PEAT sites and trend northwest rather than northeast. During the site survey we noted a change in trend of abyssal hills between Site U1331 and our next site (IODP Site U1332), perhaps associated with the Pacific plate reorganization that occurred at ~50 Ma (Rea and Dixon, 1983). Prior to drilling, an interpretation of site survey seismic data (Fig. F2) indicated that Site U1331 might penetrate seismic reflectors P2 and P3 of Lyle et al. (2002). Sediment thickness, based on interval velocities from nearby ODP Leg 199 sites (Busch et al., 2006), was estimated to be ~187 m of sediment above basaltic basement. One of the objectives of the combined PEAT science program is to obtain paleoceanographic records from the Pacific ocean during the time period around the Early Eocene Climatic Optimum (EECO) (Zachos et al., 2001; Shipboard Scientific Party, 2004), targeted by Site U1331. During Leg 199, a north–south transect was drilled across the equatorial region at ~56 Ma. Sites on this transect had generally drifted below the calcium carbonate compensation depth (CCD) by 52–53 Ma. Thus we presently lack calcareous sediments from the region of the equatorial circulation system during this time of maximum Cenozoic warmth (Zachos et al., 2001), elevated atmospheric pCO2 concentrations (Lowenstein and Demicco, 2007), and a shallow early Eocene CCD estimated between 3200 and 3300 mbsl (Lyle, Wilson, Janecek, et al., 2002; Lyle et al., 2005; Rea and Lyle, 2005). Site U1331 is located on crust with an estimated age of ~53 Ma to primarily intercept the interval between 53 and 50 Ma in basal carbonate sediments, an interval that was poorly sampled on the 56 Ma transect of Leg 199 and that had little carbonate recovery above the basal Paleocene–Eocene section. At Site U1331, the oldest sediments and basaltic seafloor of all Expedition 320/321 sites were sampled. Average (noncarbonate) accumulation rates at this time were estimated to be moderate (Moore et al., 2004), showing only slight increases in some of the more northern sites on the Leg 199 latitudinal transect (ODP Sites 1215 and 1220) (Lyle, Wilson, Janecek, et al., 2002). Leg 199 records suggest that the very shallow CCD of this early Eocene time appears to deepen to the north, perhaps suggesting a northern source for bottom waters. Sites targeting this time interval would ideally give us sediments with sufficient biogenic material to better constrain the isotopic and biotic characteristics of the near-surface equatorial waters. We positioned Site U1331 and the other PEAT sites south of the estimated paleoequatorial position at the target age 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 seafloor digital age grid from Müller et al. (1997), heavily modified and improved with additional magnetic anomaly picks from Petronotis (1991), Petronotis et al. (1994), and Deep Sea Drilling Project (DSDP)/ODP basement ages, as well as the magnetostratigraphic data compiled by Cande et al. (1989) and Cande and Kent (1995). From the digital age grid, each point is 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 the paleolatitude on the original grid. An additional important objective of the PEAT program is to provide a limited depth transect for several Cenozoic key horizons, such as the Eocene–Oligocene transition (Coxall et al., 2005). For this objective, Site U1331 will form the deepest paleodepth constraint. 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., 2006). | ||
