Site U1338

Abstract

In principle, the age-transect strategy of this expedition would not be complete without data from the Pliocene and Pleistocene. However, in addition to the logistical reasons of cruise length, near-equatorial records have already been recovered during Ocean Drilling Program (ODP) Legs 138 (Pisias, Mayer, Janecek, et al., 1995) and 202 (Mix, Tiedemann, Blum, et al., 2003). This earlier drilling provides information about the development of Northern Hemisphere glaciation. Our last site focuses instead on the interesting events during and just after a mid-Miocene maximum in sediment deposition (van Andel et al., 1975) and temperature (Zachos et al., 2001).

Integrated Ocean Drilling Program (IODP) Site U1338 (2°30.469?N, 117°58.178?W; 4200 m water depth; PEAT-8 site survey) (Fig. F1; Table T1) was sited to collect a 3–18 Ma segment of the Pacific Equatorial Age Transect (PEAT) equatorial megasplice. Site U1338 is on ~18 Ma crust just north of the Galapagos Fracture Zone, 324 nmi (600 km) southeast of Site U1337 (Fig. F1), in abyssal hill topography that strikes 350° (Fig. F1). The topography slopes down to the north-northwest from a regional high in the south. A seamount (3.7 km water depth) with a surrounding moat is found ~25 km north-northwest of Site U1338, at the downslope end of the survey area. Originally a site (proposed Site PEAT-8C) was chosen ~10 km from the seamount. However, alternate proposed Site PEAT-8D was selected and drilled uphill and further away from the seamount to avoid possible turbidites, as were found near seamounts at Sites U1331 and U1335.

Site U1338 is on a minor ridge along Line 1 of the PEAT-8 survey (Fig. F2) under 4200 m of water. In the survey area, sediment thickness ranges from ~400 ms two-way traveltime (TWT; ~320 m) at the top of the abyssal hills to a maximum of a little more than 550 ms TWT (~450 m) within basins. Estimated sediment thickness from the seismic reflection profile using an ODP Site 849 velocity-depth age model is 402 m at the Site U1338 location. The sediment pattern is typical "pelagic drape" and lies conformably on basement. Ridges at the seafloor reflect basement highs even though the sediment layer is about twice as thick as the original relief. Basement topography has been subdued somewhat by preferential infilling of the abyssal valleys. Pits often occur along the edge of the ridge lines away from the location of Site U1338 (Moore et al., 2007). Good examples of pits lie just west of Line 5 at 2°30?N in Figure F1. Moore et al. (2007) attributed these features to a variety of processes that fracture the sediments and establish conduits for warm fluid flow from the basement, and Moore et al. (2007) hypothesized that sediments are partly dissolved by the fluid flow.

Based on stage-pole reconstructions of Pacific plate motion, observations of basement age from previous drilling, and magnetic maps (Cande et al., 1989) we determined Site U1338 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., 2007) and can confirm the anomaly location.

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