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doi:10.2204/iodp.proc.320321.109.2010 Site U13371Expedition 320/321 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1337 (3°50.009′N, 123°12.352′W; 4463 meters below sea level [mbsl]; PEAT-7C site survey) (Fig. F1; Table T1) was sited in order to collect an early middle Miocene segment of the Pacific Equatorial Age Transect (PEAT) equatorial megasplice and is on ~24 Ma crust between the Galapagos and Clipperton Fracture Zones, ~390 km southeast of Site U1335. Site U1337 is 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. F1). Sediment cover is thick on the plateau (300–600 ms two-way traveltime [TWT]; ~200–500 m) (Fig. F2) but highly variable along the edges. Nevertheless, the seafloor is relatively flat because the sediment has filled in the basement topography, with a relief of ~200 ms TWT. To the north, along seismic Line 6, the seafloor is dissected by a series of karstlike holes that cut through the seismic layering (Fig. F1). Based on correlation to the central equatorial Pacific seismic stratigraphy of Mayer et al. (1985), middle Miocene sediment has been exposed. Site U1337 is now north of the equatorial high-productivity zone but should have been within ±2° of the Equator between 8 and 24 Ma. The north–south grain of abyssal hill topography in the vicinity of Site U1337 is typically covered by 300–500 ms TWT of sediment (~250–400 m) but is often interrupted by areas of significant erosion, as in the east of the PEAT-7 survey area (Fig. F1). 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 estimate that Site U1337 is on 24 Ma crust. The best control on age is information from Deep Sea Drilling Project (DSDP) Site 79, ~180 km (1.6°) east and 1.3° south of Site U1337, apparently on the same ridge segment. The base of Site 79 reaches the Miocene/Oligocene boundary, or 23 Ma on the PEAT timescale. Site U1337 was proposed for drilling to study the paleoceanographic events in the early and middle Miocene. The latest Oligocene through the middle Miocene appears to have been a time of relative warmth comparable to the latest Eocene. However, variability in the isotopic record of the early to middle Miocene is larger than that of the Eocene and may indicate more variability in climate and global ice volume. Nevertheless, the early Miocene calcium carbonate compensation depth (CCD) was not as variable as in the Eocene (Lyle, 2003), nor does there appear to have been as high a concentration of atmospheric pCO2 (Pagani et al., 1999). The climatic "optimum" at ~15 Ma comes just before the major development of ice sheets on Antarctica and a marked increase in ice-rafted debris in circum-Antarctic sediments. The early Miocene also marks a major evolutionary change from the relatively static Oligocene planktonic biota. In the equatorial Pacific, the late Oligocene to early Miocene marks the beginning of abundant diatoms in the stratigraphic record (Barron and Baldauf, 1989) and thus may represent a major change in carbon cycling as well. The only major ocean boundary change proposed for the time near the Oligocene/Miocene boundary is the opening of the Drake Passage to deep flow; however, there is some debate as to the exact timing of this event (Barker, 2001; Pagani et al., 1999; Lawver and Gahagan, 2003; Scher and Martin, 2006; Lyle et al., 2007) and its direct impact on the tropical ocean is uncertain. It may be that, as in the Eocene/Oligocene boundary section, the link between poles and tropics lies in the shallow intermediate waters that provide nutrients to lower latitude upwelling regions. For the equatorial region, an even more pertinent question is what changes were occurring in the Miocene tropical ocean that led to this burst of Miocene evolution. |
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