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doi:10.2204/iodp.proc.320321.106.2010 Site U13341Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1334 (7°59.998′N, 131°58.408′W; 4799 meters below sea level [mbsl]) (Fig. F1; Table T1) is located ~380 km southeast of previously drilled Ocean Drilling Program (ODP) Site 1218 (~42 Ma crust) in the central area drilled during the Pacific Equatorial Age Transect (PEAT) program (IODP Expedition 320/321). Site U1334 (~38 Ma crust) is situated ~100 km north of the Clipperton Fracture Zone on abyssal hill topography draped with ~280 m sediment (Fig. F2). The fabric of the abyssal hills within the sites is oriented either due north or slightly east of due north. Water depth in the vicinity of Site U1334 ranges between 5.0 and 5.1 km for the depressions between the abyssal hills. The abyssal hills range between 4.70 and 4.85 km water depth and generally show a thicker and more consistent sediment cover than the basins. In fact, a significant amount of the bathymetric difference between hills and basins is controlled by the amount of sediment cover. The comparison of sediment thickness and clarity of seismic sections led us to select a location on the middle elevation of one of the abyssal plateaus. Site U1334 sediments were estimated to have been deposited on top of late middle Eocene crust with an age of ~38 Ma and target the events bracketing the Eocene–Oligocene transition with the specific aim of recovering carbonate-bearing sediments of latest Eocene age prior to a large deepening of the calcium carbonate compensation depth (CCD) that occurred during this greenhouse to icehouse transition (Kennett and Shackleton, 1976; Miller et al., 1991; Zachos et al., 1996; Coxall et al., 2005). The Eocene–Oligocene transition experienced the most dramatic deepening of the Pacific CCD during the Paleogene (van Andel, 1975), which has now been shown by Coxall et al. (2005) to coincide with a rapid stepwise increase in benthic oxygen stable isotope ratios, interpreted to reflect a combination of growth of the Antarctic ice sheet and decrease in deepwater temperatures (DeConto et al., 2008; Liu et al., 2009). So far the most complete Eocene/Oligocene boundary section recovered from the equatorial Pacific has been at Site 1218 on 42 Ma crust; however, it is far from exceptionally preserved. Carbonate weight percentages drop markedly below the boundary and reach 0 wt% near 34 Ma during a time of apparent global shoaling of the CCD just prior to the Eocene–Oligocene transition and CCD deepening (Bohaty et al., 2008). This prevented the recovery of information about paleoceanographic conditions prior to the Eocene–Oligocene transition and also has implications for the interpretation of paleotemperature proxies such as Mg/Ca ratios in foraminifer shells that were bathed in waters with very low carbonate ion concentrations (Lear et al., 2008). The integrated stratigraphy from Site 1218 has been correlated to the planktonic foraminifer marker extinction of Hantkenina in exceptionally well preserved shallow clay-rich sediments from Tanzania described by Pearson et al. (2008), who demonstrated that the Eocene/Oligocene boundary falls within the middle plateau of the stable isotope double-step described by Coxall et al. (2005), just prior to the base of magnetochron C13n (Fig. F3). Data from Site 1218 allowed the astronomical time calibration of the entire Oligocene (Coxall et al., 2005; Wade and Pälike, 2004; Pälike et al., 2006b), but the lack of carbonate in the uppermost Eocene at this site made the detailed time control now available for the Oligocene much less certain for the late Eocene. Site U1334 is located on crustal basement age of ~38 Ma and crossed the paleoequator shortly thereafter. It was located to provide the missing information about the crucial chain of events prior to and during the Eocene–Oligocene transition. We positioned Site U1334 and the other PEAT sites 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 Site U1334 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. 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. The nearest older drill locations in this plate segment bounded by the Clipperton Fracture Zone nearby to the south and the Clarion Fracture Zone to the north are DSDP Site 160 (with a crustal age of ~35 Ma) to the east and Site 1218 (with a crustal age of 42 Ma) to the west, lending support to our backrotation modeling. 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). For this objective, Site U1334 will form the youngest and shallowest paleodepth constraint, with an estimated crustal paleodepth of ~3.5 km during the Eocene–Oligocene transition. Site U1334 also targets the Oligocene/Miocene boundary and the Mi-1 event (Zachos et al., 2001; Pälike et al., 2006a), again as part of a limited depth transect. Site U1334 is estimated to have been ~4.2 km below sea level at that time (~23 Ma). 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., 2006b). The 48-channel stacked and migrated data (Fig. F2) (Pälike et al., 2008; Lyle et al., 2006) allowed us to optimize the Site U1334 position on the intersection of seismic Lines 1 and 6 from the AMAT-03 survey. Site U1334 was located at the crossing position because the sediment and basement are well imaged. Any additional thickness away from the cross of Lines 1 and 6 was primarily caused by additional Miocene sediment on top and not by the section of primary interest below. We estimated sediment thickness using interval velocities published for ODP Site 574 by Mayer et al. (1985). 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 survey piston Core RR0603-8JC was taken west of Site U1334 (Fig. F1B). The survey piston core lithology consists of a surface layer of siliceous clay over upper Miocene to upper middle Miocene radiolarian oozes, devoid of carbonate. The age at the base of this core is ~12.4 Ma based on radiolarian assemblages. An interpretation of the site survey seismic data (Fig. F2) indicated that Site U1334 might penetrate seismic reflectors Orange (O), P5, and P4 of Lyle et al., (2002). The predicted ages for the latter two can be confirmed by coring (see "Science summary"). |
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