Proposed Site OSN-2 in the eastern equatorial Pacific (Fig. F2) is in a particularly interesting location for understanding the interplay between ocean chemistry, productivity, climate, and plate tectonics in a fast-spreading environment. The climatic implications were studied in detail with a series of 11 holes during Leg 138. We will be returning to the immediate vicinity of Site 852 from that leg to develop a legacy hole for purposes of supporting a long-term seafloor observatory.
The age of the lithosphere in this region, based on a full spreading rate of 141 mm/yr and an East Pacific Rise subsidence curve, is in the range of 11-12 Ma (Fig. F2, Fig. F3); the age is also consistent with paleoceanographic results from Leg 138. The water depth is 3860 m. Based on seismic profiles (Fig. F4, Fig. F5, Fig. F6, Fig. F7) and drilling during Leg 138, the sediment at Site 852 is 116 m thick and overlies basement with relief, which is quite smooth, probably with much less than 100-m variability. Whereas sediment thicknesses of as much as 400 m could be used to the south, there is no particular advantage in deploying the borehole seismometer beneath the thicker sediment cover. A thicker sediment column will not attenuate seafloor noise, and an increased sediment thickness will only decrease the frequency of reverberations in the sediment column, which could begin to interfere with seismic observations. For example, the two-way traveltime for compressional waves in 400 m of sediment is ~0.5 s, for a frequency of 2 Hz. This is a particularly interesting band for recording earthquakes at teleseismic distances. On the other hand, the two-way traveltime at Site 852 is only 0.15 s, for a frequency of 6.7 Hz, a frequency above that at which the broadband sensor's self-noise exceeds the seafloor noise level (probably 4-5 Hz). There is every reason to believe that the crustal section at the site will be quite typical of Pacific oceanic crust. Figure F8 illustrates the installation of the seismic component of an observatory at Ocean Drilling Program (ODP) Site OSN-1 south of Oahu; a similar procedure will be followed at proposed Sites H2O-1 and OSN-2.
The equatorial Pacific is a region of considerable interest in paleoceanography given the high productivity of the region and the sensitivity of the rates of sedimentation to both climate change and changes in circulation patterns associated with tectonic changes. The circulation pattern is associated with prevalent surface winds and the change in the sign of Coriolis Force at the equator. The trade winds are driven by the warm waters in the west and the cooler waters in the east-the rising air in the west and sinking air in the east drive the easterly winds associated with the trade winds. The trade winds give rise to northern Ekman transport to the north of the equator and southward Ekman transport to the south; this divergence leads, in turn, to upwelling at the equator. Directly on the equator, the effects of rotation vanish and easterly trade winds push the surface water directly, through friction, to the west. The water transported by the winds piles up in the western Pacific with an offset of ~0.5 m, providing the potential for a semiperiodic El Niño. The resultant Intertropical Convergence Zone (ITCZ) results from these effects (Fig. F9). Today, the ITCZ is always north of the equator in the eastern Pacific. The equatorial current system is dependent upon the seasons with the ITCZ at its most northerly position (~10°) from August to December. Figure F10 illustrates a superimposition of the winds on dynamic sea height from satellite altimetry measurements. Figure F10B shows the residual and demonstrates quite clearly the different current regimes discussed above. Figure F11 is a more complex plot showing the Ekman transport in Figure F11A, the wind driven geostrophic component in Figure F11B, and finally, the combination of the two currents in Figure F11C, superimposed on the temperature anomaly. In this case, the surface current at proposed Site OSN-2 is about a knot and the Equatorial Undercurrent lies well to the south. The actual current regime can be checked in the days before arriving on site, but in any case, should not present a problem for drilling.
The high productivity associated with the circulation system acting in conjunction with a component of the absolute motion of the Pacific plate in a northerly direction has resulted in a bulge in the sedimentation, which is asymmetric to the north (Fig. F12). Beginning with the Swedish Deep Sea Expedition, it has become abundantly clear that the sediments in this area record climatic cycles well into the past. Studies of the early piston cores led to the development of the concept of a lysocline and a calcite compensation depth (e.g., Arrhenius, 1952; Bramlette, 1961; Berger, 1971). The advent of the geomagnetic timescale, coupled with additional coring, increased the resolution of these studies substantially. The sedimentation patterns in the area coupled with plate tectonics led to the concept of "plate stratigraphy" (Berger, 1973; Winterer, 1973; Berger and Winterer, 1974), which explained the general features of Cenozoic sediments in the equatorial Pacific.
The equatorial Pacific is an ideal site for one of the initial deployments of a permanent seafloor observatory. The weather in the region is generally fine, and the limited swell reduces the level of surface-induced noise at the seafloor. One of the north-south arrays in the Toga-Tao experiment, which monitors the development and growth of El Niño, is located at this longitude so that ocean weather conditions are well known. Furthermore, in the ideal world in which the National Oceanic and Atmospheric Administration (NOAA) and the University-National Oceanic Laboratory System (UNOLS) come to share ship time and programs, the maintenance of the surface mooring needed for the station would become a shared responsibility.
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