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doi:10.2204/iodp.sp.320321.2008

Drilling strategy

The program consists of six primary sites and four possible alternate sites (Fig. F3). The sites have been located to intercept the paleoequatorial position at specific age intervals so that the aggregate records can be formed into a continuous high-resolution profile of the Cenozoic equatorial Pacific. Because critical paleoceanographic information is contained only in the biogenic carbonate fraction and because ocean crust deepens with age until it eventually passes below the CCD, each site has been located on young crust that is only marginally older than the target age interval.

Each site will be cored multiple times until stratigraphic continuity of the targeted age section can be assured; as much of the remaining sediment column as is feasible with the given time constraints will also be sampled. In most cases the uppermost sediments are of secondary importance. Based on prior drilling expeditions (ODP Legs 138 and 199), we plan to core the sedimentary sections using the APC system as much as possible. This will provide the best quality cores, allowing for the determination of paleomagnetic orientations. Additionally, at depths where the APC is no longer deployable, we will use the extended core barrel (XCB) system.

There is a possibility that thin chert layers might occur in the basal sections of the Eocene sites, which could make recovery more challenging (see "Risks and contingency"). Logging will be used for correlation of the sediment column to the seismic reflection profiles (see "Wireline logging").

Proposed drill sites

This section contains the essential information about each of the proposed sites (see also "Site summaries"). A complete set of site survey data can be found in the Site Summary Report sent to the Environmental Protection and Safety Panel in June 2006 (eprints.soton.ac.uk/45921).

Primary Site PEAT-1C (early Eocene Equator)

Proposed Site PEAT-1C (Figs. F11, F12, F13) was chosen for survey because it had the right age crust and because the satellite-derived bathymetry map suggested that the crust was slightly elevated above the expected depth for 53 Ma crust. We found a region of abyssal hills bound by a large volcanic rise to the east and several seamounts to the southwest. The abyssal hills are more widely spaced than at previous sites and trend northwest rather than northeast. We noted a change in trend of abyssal hills between proposed Sites PEAT-2C and PEAT-1C, perhaps associated with the Pacific plate reorganization that occurred at ~50 Ma (Rea and Dixon, 1983).

The PEAT-1C survey area has relatively gentle abyssal hills and more uniform sediment cover than some of the other PEAT sites surveyed. The bathymetry ranges between ~4800 to just below 5100 meters below sea level (mbsl). About 150–200 ms two-way traveltime (TWT) of sediment draped all of the topography, with slightly more in the valleys and in the deeper southwest corner of the site. Site PEAT-1C was chosen on a seismic line going down the western valley, in an area where basal reflections were weaker but better imaged than at other sites.

Primary Site PEAT-2C (early Eocene Equator and climatic optimum)

Proposed Site PEAT-2C (Figs. F14, F15, F16) is located in abyssal hill topography on 49–50 Ma crust north of the Clipperton Fracture Zone and just south of the "Mahi-Mahi Fracture Zone" that was discovered during site survey Cruise EW9709 in preparation for Leg 199. There is a general slope in topography to the north, with the southern side of the site being more elevated. The topography is dominated by north-south–trending small ridges and troughs (~5 km width). Bathymetric relief across the abyssal hills is 50–200 m, bottom depth is between 4850 and 5150 mbsl, and sediment cover is ~200 ms TWT. The closest previous drilling locations north of the Clipperton and south of the Mahi-Mahi Fracture Zones comprise ODP Sites 1220 and 1221, ~100 nmi to the west (with basement ages just before the Paleocene/Eocene boundary at 56.0 Ma). North of the Mahi-Mahi Fracture Zone and to the east of Site PEAT-2C are DSDP Sites 42 (basement age ~45 Ma; J. Backman pers. comm., 2006) and 162 (basement age estimated at ~49 Ma), and to the west lies ODP Site 1222 (on 56 Ma crust).

The PEAT-2 survey area was located to be on 49 to 50 Ma basement on abyssal hill topography. The 48-channel stacked and migrated data reveal regions at the flanks of tilted ridges where older horizons are exposed nearer the surface (images in cross-strike Lines 1 and 6). Coring suggests that the surface sediments were formed at ~20 Ma. The biostratigrapic analysis of surface sediments indicates an age no older than 19–23 Ma, but because of barren and reworked samples in the sediment core, it is not possible to obtain any further information about the possible sedimentation rates in the basal section, other than that the minimum sedimentation rate across the entire section is of the order of 6 m/m.y.

Primary Site PEAT-3C (middle Eocene Equator)

Proposed Site PEAT-3C (Figs. F17, F18, F19) is located in abyssal hill topography north of the Clipperton Fracture Zone on 46 Ma crust. There is a general slope in topography to the north, with the southern side of the site being more elevated. Bathymetric relief across the abyssal hills is 75–150 m, bottom depths range between 4800 and 5100 mbsl, and sediment cover is ~200 ms TWT.

The PEAT-3 survey area was located to be above 46 Ma basement on abyssal hill topography. The 48-channel stacked and migrated data reveal a region where the sediment column that had been deposited is eroding away. Outcropping older horizons are common along Line 1 and at the northern ends of the crosslines. Coring suggests that the surface sediments were formed ~20 Ma and give an average sedimentation rate of 7 m/m.y. for the sediment section.

Primary Site PEAT-4C (late Eocene and Eocene/Oligocene boundary)

Proposed Site PEAT-4C (Figs. F20, F21, F22) is located north of the Clipperton Fracture Zone on abyssal hill topography draped with ~270 m sediment. 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 PEAT-4C ranges between 5 and 5.1 km for the depressions between the abyssal hills. The abyssal hill tops range between 4.7 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 depressions is controlled by the amount of sediment cover. The comparison of sediment thickness and clarity of seismic sections led us to choose a location on the middle elevation of one of the abyssal plateaus.

Based upon correlation to the Neogene central equatorial Pacific seismic stratigraphy of Mayer et al. (1985) and the Paleogene equatorial Pacific stratigraphy of Lyle et al. (2002), there is ~200 m (260 ms TWT) of Eocene/Oligocene sediment, for an average Eocene/Oligocene sedimentation rate of about 15 m/m.y. assuming a crustal age of 38 Ma.

Primary Site PEAT-5C (Oligocene Equator)

Proposed Site PEAT-5C (Figs. F23, F24, F25, F26) is located just north of the Clipperton Fracture Zone on abyssal hill topography draped with thick sediment. The location of Site PEAT-5C is ~17 nmi north of the trace of the fracture zone. In order to maintain a position south of the 28 Ma paleoequator, we were constrained to Site PEAT-5C close to the fracture zone. Nevertheless, the survey has shown that Site PEAT-5C is located on abyssal hill topography, with a fabric of the abyssal hills that is oriented slightly west of due north.

Water depth in the vicinity of Site PEAT-5C is relatively shallow for the age of the crust, ranging between 4200 and 4400 m. Surprisingly, there is little or no depth offset between the crust to the south and to the north of the Clipperton Fracture Zone, despite a depth in the middle of the fracture zone trace near 5 km. In fact, the seafloor south of the Clipperton Fracture Zone is perhaps 100 m deeper than that to the north.

Site PEAT-5C is located on abyssal hill topography just north of the Clipperton Fracture Zone. A few oblique ridges and depressed topography occur in the south, showing some interaction between the fracture zone and the Site PEAT-5C region. The site is thickly covered with sediments (300–400 ms TWT). The site has a very thin layer of recent to middle Miocene sediments and most of the sediments are between 13 and 28 m.y. Based upon correlation to the Neogene central equatorial Pacific seismic stratigraphy of Mayer et al. (1985) and the Paleogene equatorial Pacific stratigraphy of Lyle et al. (2002), there is ~120 m of Oligocene sediment, for an average Oligocene sedimentation rate of 13 m/m.y., assuming a crustal age of 32 Ma.

Primary Site PEAT-6C (Oligocene–Miocene transition)

Proposed Site PEAT-6C (Figs. F27, F28, F29) is situated in the central tropical Pacific on a broad deep within north-northwest–trending abyssal hill topography, ~15 and ~30 km west and ~20 km north of three seamounts. Thick sediment deposits cover the abyssal hills, with a thinning sediment cover on the hills. Based on stage-pole reconstructions of Pacific plate motion and observations of basement age from previous drilling sites, along with magnetic anomaly maps (Cande et al., 1989), we determined that Site PEAT-6C is located on 26–27 Ma crust. The best age control is provided by a new aeromagnetic line south of the proposed site (Horner-Johnson and Gordon, 2003) and from the basal age of Site 79, located 4.5° to the east and 3° to the south of Site PEAT-6C, apparently on the same fracture zone segment. The base of Site 79 reaches the Miocene/Oligocene boundary, or 23 Ma on the most recent astronomically calibrated timescales.

Water depth in the vicinity of Site PEAT-6C is between 4300 and 4400 mbsl, apart from the topography around three seamounts that are 15–30 km away from the proposed drill site. Sediment cover is relatively thick but varies around ridges and deeps.

Site PEAT-6C is located along the flank of a wide north-northwest–trending valley, on high ground above the abyss and moat of the seamounts (Fig. F27). Sediment cover is thick in the entire region, ranging from 300 to 500 ms TWT. The seafloor is relatively flat because the sediment has filled in the basement topography (~200 ms TWT).

Primary Site PEAT-7C (early–middle Miocene Equator)

Proposed Site PEAT-7C (Figs. F30, F31, F32) is situated in the central tropical Pacific on a broad plateau within north-northeast–trending abyssal hill topography. Thick sediment deposits cover the abyssal hills, but these deposits are being eroded at the edge of the plateau. Based on stage-pole reconstructions of Pacific plate motion and observations of basement age from previous drilling sites, an aeromagnetic line south of the proposed site (Horner-Johnson and Gordon, 2003), and magnetic anomaly maps (Cande et al., 1989), we determined that Site PEAT-7C is located on 24 Ma crust. The best control on age is information from Site 79, located 1.5° to the east of Site PEAT-7C. The base of Site 79 reaches the Miocene/Oligocene boundary, or 23 Ma.

Water depth in the vicinity of Site PEAT-7C is below 4500 mbsl, marking regionally deep basement. Sediment cover is relatively thick, but varies radically around ridges and deeps.

Site PEAT-7C is located on a plateau between high topography to the south of the site, a gentle but pockmarked ridge to the north, and a deep to the east (Fig. F30). Sediment cover is thick on the plateau but is highly variable along the edges. Sedimentation was variable sedimentation along the eastern edge, and thick sediment covers the plateau (300–600 ms TWT). Nevertheless, the seafloor is relatively flat because the sediment has filled in the basement topography, of ~200 ms TWT. To the north, along PEAT-7C Line 6, the seafloor is dissected by a series of karstlike holes that cut through the seismic layering (Fig. F30). Based upon correlation to the central equatorial Pacific seismic stratigraphy of Mayer et al. (1985), middle Miocene sediment has been exposed.

Alternate Site PEAT-8C (eastern equatorial Pacific)

Proposed Site PEAT-8C (Figs. F33, F34, F35) is situated ~1° north of the Galapagos Fracture Zone, on abyssal hill topography. Based on stage pole reconstructions of Pacific plate motion and observations of basement age from previous drilling sites, as well as magnetic maps (Cande et al., 1989), we determined Site PEAT-8C to be located on ~18 Ma basement. During the AMAT-03 site survey we collected magnetic anomaly data that can be correlated to additional collated observations (Barckhausen et al., 2005; Engels et al., submitted) and confirm the anomaly location. Water depth in the general vicinity is ~4.2 km. The closest previous drilling location north of the Galapagos Fracture Zone for which basement age estimates are available comprises DSDP Site 79 (basement age approximately coincident with the Oligocene/Miocene boundary at 23.0 Ma; J. Backman, pers. comm., 2006). No age information was obtainable from DSDP Site 571, and DSDP Site 572 is projected south of the fracture zone.

Site PEAT-8C is in a region of abyssal hills, with highs to 3900 m and basins to 4400 m, close to the center along a 350° trending and deepening valley. A seamount (water depth = 3.7 km) with surrounding moat was found ~10 km north-northwest of the site, facing downslope of the valley. The proposed site in ~4301 m water depth is surrounded by a half-circle of ~4100 m deep elevated hills toward the southeast. The fabric of the hills matches the magnetic anomaly fabric, and trends north-northeast–south-southwest. Sediment thickness at the site ranges from ~400 ms TWT at the top of the abyssal hills to a maximum of a little more than 550 ms TWT. At Site PEAT-8C (shotpoint 36015 on seismic Line PEAT-8C Line 1) it is ~557 ms TWT. Surface sediments consist of a carbonate-rich clay, ranging to nannofossil ooze with cyclical (meter scale) carbonate content at depth. Smaller numbers of siliceous microfossils were found in the sediment as well.

Operations plan

Drilling will be accomplished during two expeditions; for logistical reasons, one is directed primarily to sample Paleogene sites (western transect, Expedition 320; Table T1) and the other the Neogene sites (eastern transect, Expedition 321; Table T2).

For Expedition 320, the primary objectives are to core and log proposed Sites PEAT-5C, 4C, 3C, and 1C. If time permits, a check shot survey may be conducted at Site PEAT-4C. For Expedition 321, the primary objectives are to core and log Sites PEAT-2C, 6C, and 7C and to conduct a check shot survey at Site PEAT-6C. In principle, all primary sites on one expedition are considered to be secondary priority objectives on the other expedition.

Because the main objective of both expeditions is to reconstruct a complete sedimentary succession, we plan on drilling several holes per site—tentatively three holes per site or more—until stratigraphic coverage is deemed to be sufficient. Detailed operations and time estimates for PEAT Expeditions 320 and 321 are available in Tables T1 and T2, respectively.

The overall plan for each site follows.

Hole A

Use APC coring until APC refusal and deploy the advanced piston corer temperature (APCT)-3 tool three times during APC coring. Once APC refusal is reached, switch to the XCB system and core until basement is tagged. During XCB drilling we plan on deploying the Davis-Villinger Temperature Probe (DVTP) twice. If hole conditions are adequate, wireline logging operations will take place in Hole A, after which we will pull out of the hole and abandon it. In principle, Hole A will be cored continuously until target depth is reached.

Holes B and C and additional holes

The primary goal of these holes is to fill in sediment sections not recovered in Hole A. Depending on time constraints, operations in these additional holes are envisioned as in Hole A but without downhole temperature measurements. If wireline logging operations were not possible in Hole A, then wireline logging operations will take place in one of the additional holes, depending on operational time constraints (see "Wireline logging" for additional information). If time constraints dictate a shorter plan, these holes might be drilled without coring in their upper part, might be only cored through the targeted age interval, and might not reach basement.

The decision to pursue drilling operations in a hole or at a site will have to be taken at time of operations in consultation, whenever possible, with the other Co-Chief Scientists not present on the ship. It is understood that some problems may require rapid decision making and thus must be solved on the ship. The actual section cored and repeated will depend on the typical operational factors: completeness of the sedimentary splice, priorities, and length of time remaining in the expedition. Critical intervals (e.g., Eocene/Oligocene boundary) may be specifically targeted for additional coring.

We plan to core a complete sediment column at each site even if adverse conditions are encountered (e.g., pervasive diagenesis). These intervals may not be recored in later holes. In the unlikely event of pervasive chert deposits, all reasonable efforts will be made to drill forward through the interval, at least in the first hole. However, the primary drilling objective, to obtain well-preserved carbonate sediments, must be kept in mind for time allocation.

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