BACKGROUND AND OBJECTIVES

Site 1207 is located in lower bathyal (3103 m) water depth close to the most elevated, central part of the Northern High of Shatsky Rise. According to the reconstruction of Nakanishi et al. (1989), basement underlying the site was formed in Magnetochron CM14 in the early Valanginian (~135 Ma). The site is located on seismic line TN037-5A (see Fig. F1).

Site 1207 is the northernmost in the Shatsky transect, some 5° north of the Southern High. Plate reconstructions suggest that this site exited the equatorial high-productivity zone ~5 m.y. before the Southern High (R. Larson, pers. comm., 2001), and thus the uppermost chert horizon should lie in an older part of the stratigraphic section. The paleodepth of this site is ~1200 m in the early Maastrichtian based on the estimate of Barrera et al. (1997) for Site 305. Application of subsidence curves for normal oceanic crust (e.g., Sclater et al., 1971) yields a depth of ~500 m in the mid-Cretaceous and close to sea level at the time that the Northern High was formed. However, the site probably subsided rapidly initially, then followed a less steep curve than typical crust (e.g., McNutt et al., 1990).

The Northern High has not been drilled before; thus, the stratigraphy was unknown prior to our drilling of Site 1207. The ages of reflectors and the major seismic units cannot be correlated with the Southern High with any degree of certainty. Site 1207 drilling was designed to provide knowledge of the stratigraphy of the Northern High as well as correlation of units and reflectors with the Central and Southern Highs. The sedimentary section at Site 1207 is moderately thick (Fig. F1), ~1200 m, assuming velocity estimates from Site 305 and a tentative correlation of units. Tentative predrilling correlation with the Southern High seismic units of Sliter and Brown (1993) suggested a thick seismic Unit 5 (pre-Aptian) that is characterized by discontinuous reflectors, relatively thick Units 4 (Aptian-Turonian) and 3 (Turonian-Maastrichtian), with intervals marked by prominent reflectors separated from intervals with generally continuous, but weak reflectors. Seismic Units 2 and 1 (Neogene) at Site 1207 are relatively thin and show diffuse reflectors. These seismic data suggest that the Cretaceous section at Site 1207 is relatively expanded.

Site 1207 represents the northernmost site of the Leg 198 latitudinal transect and lies in the middle of the depth transect. Combined with the other sites in the transect, Site 1207 sediments will be used to:

1. Reconstruct changes in the properties of surface and deep waters through the Cretaceous and Paleogene. This will help to constrain the character and stability of intermediate- and deep-water circulation, vertical and latitudinal/thermal gradients, and basin fractionation during ancient intervals of extreme warmth.
2. Understand the significance of unconformities in the Shatsky Rise section and determine whether they are related to local or regional changes in ocean circulation (currents) or changes in carbonate production (i.e., CCD variations).
3. Determine long-term changes in biotic communities and how they relate to oceanographic and climatic evolution. In particular, we will monitor changes in the relative abundances of truly tropical and high-latitude planktonic foraminifers, nannoplankton, diatoms, and radiolarian species through time as well as planktonic and benthic (benthic foraminifers and ostracods) indicators for productivity variations.
4. Shed light on the origin of transient climatic events such as the mid-Cretaceous Oceanic Anoxic Events (OAEs) and determine the nature of chemical (i.e. CCD, nutrients, and oxygenation) and physical oceanographic changes (temperature gradients) during these events.
5. Understand water column stratification during mid-Cretaceous OAEs as well as obtain complete records of organic-rich sediments suitable for detailed paleontological and geochemical investigations. These data will allow us to more fully determine the response of marine biotas to abrupt environmental changes and to constrain changes in carbon and nutrient cycling during the OAEs.
6. Determine the timing of the equatorial crossing of Shatsky Rise as well as variations in the intensity of upwelling in this region through time by using the age of chert in the sedimentary section.
7. Determine the paleoceanographic factors controlling the origin and nature of orbital cycles in the Site 1207 sedimentary section.
8. Determine changes in the intensity of atmospheric circulation through time and how they relate to climate deterioration by using the Neogene section at Site 1207, which should contain an excellent clay mineral record.