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Site U13381

Expedition 320/321 Scientists2

Background and objectives

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.

The target equatorial interval of Site U1338, the middle and late Miocene, exhibits large changes in global temperature and major changes in equatorial Pacific plankton communities and carbon cycle. Large changes in the glaciation state and frequency have recently been found in the middle Miocene (Holbourn et al., 2005; Abels et al., 2005; Holbourn et al., 2007) in the interval between 14 and 16 Ma. There is a wide latitude range of calcium carbonate (CaCO3) deposition during the earliest Neogene, with a relatively sharp transition to a narrower CaCO3 belt after 20 Ma (Lyle, 2003). CaCO3 mass accumulation rates in the central equatorial Pacific recovered from the 18–19 Ma "famine" and in the period between 14 and 16 Ma reached a second maximum in carbonate deposition, which is also evident in the seismic stratigraphy of the equatorial sediment bulge (Mitchell et al., 2003). The middle/late Miocene boundary interval is also marked by a significant increase in carbonate dissolution in the eastern Pacific.

The early/middle Miocene boundary interval is also the warmest interval in the Neogene, but it does not appear to have highly elevated atmospheric pCO2 levels associated with it. Pagani et al. (1999) estimated by alkenones that atmospheric pCO2 was near modern levels (~250 ppm CO2), whereas more recently Kurschner et al. (2008) estimated by leaf stomata that the atmospheric pCO2 level at 15.5 Ma was as high as 650–700 ppm. New sedimentary records are needed to study temperatures and greenhouse gas levels during this important warm interval, as well as to understand the role of the equatorial Pacific in the global carbon cycle.

Site U1338 should have crossed the equatorial region between 10 and 11 Ma and been within the equatorial zone (±2° of the Equator) between 3 and 18 Ma using the Koppers (2001) fixed hotspot stage-poles for Pacific plate motion. Site U1338 should have been at the Equator during the major late Miocene diatom mat interval at Site U1337 (see "Lithostratigraphy" in the "Site U1337" chapter) (Kemp and Barron, 1993) when Site U1337 was located ~1° north of the Equator. We expected higher deposition of diatoms and perhaps larger diatom mat intervals at Site U1338 than at Site U1337 because it was nearer to the Equator.

Site U1338 is also the easternmost PEAT site and is nearest to the Americas. It has always been within the southeast tradewind belt and should have the highest deposition of windblown dust of all the PEAT sites. Changes in aluminosilicate deposition should reflect changes in wind strength and dust source aridity over the Miocene, Pliocene, and Pleistocene. Finally, we used wireline logging and physical property studies to groundtruth seismic stratigraphy for the equatorial Pacific and to determine how well the seismic stratigraphy at Site U1338 conforms to the central equatorial Pacific seismic stratigraphy of Mayer et al. (1985) (see "Downhole logging").

1Expedition 320/321 Scientists, 2010. Site U1338. 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.320321.110.2010

2Expedition 320/321 Scientists' addresses.

Publication: 30 October 2010
MS 320321-110