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Site U13321

Expedition 320/321 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1332 (11°54.722′N, 141°02.743′W; 4924 meters below sea level [mbsl]) (Fig. F1; Table T1) is located ~120 km east and slightly south of IODP Site U1331, near the northwesternmost area drilled during the Pacific Equatorial Age Transect (PEAT) program (IODP Expedition 320/321). This site is situated on 50 Ma crust ~750 km north of the Clipperton Fracture Zone, ~380 km south of the Clarion Fracture Zone, and ~270 km northeast of the nearest previously drilled Ocean Drilling Program (ODP) Site 1220 (56 Ma crust).

The Eocene was a time of extremely warm climates that reached a global temperature maximum (the Early Eocene Climatic Optimum [EECO]) near 52 Ma (Zachos et al., 2001; Shipboard Scientific Party, 2004). During that time, atmospheric pCO2 concentrations were elevated (Lowenstein and Demicco, 2007) and the early Eocene calcium carbonate compensation depth (CCD) was very shallow, estimated between 3200 and 3300 mbsl (Lyle, Wilson, Janecek, et al., 2002; Lyle et al., 2005; Rea and Lyle, 2005). From this temperature maximum there was a gradual climatic cooling through the Eocene to the Eocene/Oligocene boundary. Throughout the Eocene, the CCD lay near 3.2–3.3 km depth, albeit with potentially significant short-term fluctuations (Lyle et al., 2005). Thus, although recovering carbonate sediments from the equatorial region is a substantial challenge, it is not impossible if the depth of the East Pacific Rise lay near the global average of 2.7 km.

During the early Eocene, a very shallow CCD and typical rapid tectonic plate subsidence of young crust near the shallow ridge crest conspired to make the time window during which carbonate is preserved very short (~2 Ma) before each site sinks below the CCD (Rea and Lyle, 2005). Thus, although good records of pelagic carbonates during and just after the Paleocene/Eocene Thermal Maximum (PETM) were recovered at ODP Leg 199 sites (Lyle, Wilson, Janecek, et al., 2002; Raffi et al., 2005; Nuñes and Norris, 2006), the time period of the EECO (Zachos et al., 2001) and the shallowest CCD is not well sampled. In combination with Site U1331, which is located on crust with an age of 53 Ma, Site U1332 is located on crust with an expected age of ~50 Ma to intercept the interval between 50 and 48 Ma in biogenic sediments above the CCD. Thus, Site U1332 forms the second oldest time slice component of Expedition 320/321.

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 U1332 will form the second deepest paleodepth constraint, with an estimated crustal paleodepth of ~4 km during the Eocene–Oligocene 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., 2006).

Site U1332 is located in abyssal hill topography with a general slope in topography to the north. The topography is dominated by small ridges that trend north–south and troughs of ~5 km width (Fig. F1B). Bathymetric relief across the abyssal hills is 50–200 m, and sediment cover is around 200 ms two-way traveltime (TWT), or ~155 m using the velocity model developed by Busch et al. (2006).

The 48-channel stacked and migrated data (e.g., seismic Lines PEAT-2C-sl-1 and PEAT-2C-sl-6 in Pälike et al., 2008) (Lyle et al., 2006) reveal a region at the flanks of tilted ridges where older horizons are exposed nearer the surface. The site survey piston coring suggested that the surface sediments were formed at ~20 Ma. Site survey seismic Line 6 (Fig. F2), on which Site U1332 is located, suggests ~160 m of sediment above basement. An interpretation of the site survey seismic data (Fig. F2) indicated that Site U1332 would penetrate seismic Reflectors P2 and P3 of Lyle, Wilson, Janecek, et al. (2002).

We positioned Site U1332 and the other PEAT sites to the 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 digital 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 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.

1Expedition 320/321 Scientists, 2010. Site U1332. In Pälike, H., Lyle, M., Nishi, H., Raffi, I., Gamage, K., Klaus, A., and the Expedition 320/321 Scientists, Proc. IODP, 320/321: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.320321.104.2010

2Expedition 320/321 Scientists' addresses.

Publication: 30 October 2010
MS 320321-104