BACKGROUND AND OBJECTIVES

Site 1213 is located on the southern flank of the Southern High of Shatsky Rise at 3883 m water depth. Basement under the site formed in the latest Jurassic within Magnetochron M20 (~145 Ma) and is close to the oldest part of Shatsky Rise (Nakanishi et al., 1989). Site 1213 is the southernmost and deepest site on the Shatsky Rise transect.

The site is located on seismic line TN037-17B (see Fig. F35 in the "Leg 198 Summary" chapter). Correlation of this profile with the summit region of the Southern High and the seismic units of Sliter and Brown (1993) is somewhat uncertain. Precoring interpretation suggested a relatively thin seismic Unit 1 (Neogene) with prominent reflectors unconformably overlying a thin Unit 3 (Upper Cretaceous) with dipping and somewhat discontinuous reflectors and a relatively thick Unit 4 (mid-Cretaceous) and Unit 5 (Lower Cretaceous). The paleodepth of Site 1213 was ~2.8 km in the mid-Cretaceous, assuming normal subsidence rates for oceanic crust (e.g., Thierstein, 1979).

The major goals of Site 1213 drilling were to core a relatively deepwater Upper Cretaceous and Lower Cretaceous sequence as well as to obtain fresh volcanic rocks from basement underlying the sedimentary sequence for geochemical and radiometric analyses. Because this section exists at shallow burial depths, it can be reached without coring through an extensive ooze-chert sequence. Moreover, from the seismic profile, basement appears to be shallow at this site, between 400 and 600 mbsf, depending on velocity estimates for the deep sedimentary section. The site will be contained in broad leg-based objectives that include

1. Reconstructing changes in the properties of surface and deep waters including the character and stability of intermediate and deepwater circulation and vertical thermal gradients during Cretaceous intervals of extreme warmth.
2. Understanding water column stratification during mid-Cretaceous Oceanic Anoxic Events (OAEs) as well as obtaining complete records of organic-rich sediments suitable for detailed paleontological and geochemical investigations. These data will allow us to determine more fully the response of marine biotas to abrupt environmental changes and to constrain changes in carbon and nutrient cycling during the OAEs. The Shatsky Rise transect hopefully will allow us to determine whether the organic-rich sediment distribution conforms to an oxygen minimum zone model or to a deep basinal model.
3. Investigating the significance of unconformities in the stratigraphic section. Are they related to local changes in currents or to regional/basinal fluctuations in the CCD?
4. Determining fluctuations in the CCD through the Cretaceous, comparing them to other records from the North Pacific as well as from other ocean basins, and interpreting them in a paleoceanographic framework.
5. Recovering volcanic rocks from Shatsky Rise, thus providing samples to determine the age and geochemistry of basement. Compositional data from Shatsky Rise basement and other large igneous provinces will also be compared in light of the different models for their formation.