Site 1218

Abstract

Site 1218 (8°53.378´N, 135°22.00´W; 4828 meters below sea level [mbsl]; Fig. F1) is the sole site to be drilled on the 40-Ma transect during Leg 199 and will be used to investigate paleoceanographic processes in the equatorial Paleogene Pacific Ocean during the inferred transition of Earth's climate from the early Paleogene "greenhouse" into the late Paleogene "icehouse." Site 1218 is situated on a basement swell ~3° north of the Clipperton Fracture Zone in the central tropical Pacific Ocean. The age of basement at Site 1218 was poorly constrained prior to Leg 199 because the crust formed near the Eocene magnetic equator so that little magnetic anomaly data are available between the Clipperton and Clarion Fracture Zones (Cande et al., 1989). Thus, prior to Leg 199, our estimate for basement age at Site 1218 (~40 Ma) was based on previous drilling and assumed spreading rates. Nevertheless, at the outset of Leg 199 drilling, the availability of sediment descriptions from Deep Sea Drilling Project (DSDP) Sites 161, 574, and 575 meant that the Cenozoic history of sedimentation in the region of Site 1218 was better constrained than in the regions of Sites 1215, 1216, and 1217. Based on data from these rotary-cored holes, shallow-penetration piston cores taken near Site 1218 (taken on the site survey EW9709-7PC), and seismic profiling (Fig. F2), we expected the sedimentary sequence at Site 1218 to comprise a relatively thick (25 to 35 m thick) section of clays overlying radiolarian and nannofossil oozes to chalks of early Miocene-late Eocene age with little to no chert (estimated total depth ~250-280 meters below seafloor [mbsf]).

Site 1218 was chosen because it is anticipated to have been located on the equator at 40 Ma (at ~0°N, 107°W) based upon a fixed hotspot model (Gripp and Gordon, 1990, for 0- to 5-Ma Pacific hotspot rotation pole; Engebretson et al., 1985, for older poles) and because our interpretation of seismic data suggested that this site afforded the best possibility of penetrating Eocene/Oligocene (E/O) boundary sediments within the depth range of the Ocean Drilling Program's (ODP's) advanced piston corer (APC) system (~180 mbsf in these sediments).

The paleoceanographic and paleoclimatic objectives of drilling the sedimentary sequence anticipated at Site 1218 are as follows: (1) to help define the shift in the Intertropical Convergence Zone through the Paleogene by following the change in eolian dust composition and flux (red clays) through time; (2) to help constrain changes in the calcite compensation depth (CCD) from the late Eocene to the early Oligocene; (3) to obtain a complete Oligocene section of tropical-assemblage carbonate and siliceous microfossils with good magnetostratigraphic control; and (4) to sample the E/O boundary, late Oligocene, and Oligocene/Miocene (O/M) boundary, three of the most climatologically interesting intervals of Cenozoic time. Recovery of deep-sea sediments from above the CCD across the E/O boundary time interval during Leg 199 is a particularly high priority because complete E/O boundary sections for this time interval are not available from the Pacific Ocean.

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ORCID for P.A. Wilson: orcid.org/0000-0001-6425-8906

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