SITE 1256

The sedimentary overburden at Site 1256, in the Guatemala Basin in the eastern equatorial Pacific (Fig. F1), consists of two end-member lithologies, with terrigenous clay in the upper 40 m and biogenic carbonates deeper in the section. The deeper 200-m biogenic carbonate sequence also contains a 4-m-thick laminated diatom mat at ~111 meters below seafloor (mbsf). The carbonates from here to the bottom of the sequence also contain chert nodules. Initial shipboard geochemical results indicate a decrease with depth in the abundance of terrigenous material (with intervals of volcanic ash) into a lithology dominated by calcareous nannofossil ooze, with a large chemical change at the base of the laminated diatom mat at ~115 mbsf. Increased Fe concentrations, manifested by an elevated Fe/Al ratio, indicate a significant metalliferous component near the basement, although the 1-m-thick red-brown oxide-rich sediment layer directly overlying the basalt was not sampled for this study. With a variety of lithologies and sedimentary components, the sediments at Site 1256 provide a suitable sample suite to test the appropriateness of different sample preparation methods for sediments from several different depositional conditions.