Site 574

Abstract

Total core recovered (m): 574-208.93 574A-180.74 574B-9.45 574C-197.35 Core recovery (%): 574-101 574A-100 574B-50 574C-58 Oldest sediment cored:Depth sub-bottom (m): 517. of flat, acoustically well stratified sediments. The site is just north of the crest of the sediment bulge in the equatorial high-productivity belt, and it is the second of a three-site latitudinal transect along 133°W. The site was drilled to provide detailed documentation of the area's migration across the equator and of Tertiary equatorial Pacific paleoceanography. To this end two holes were drilled with the hydraulic piston corer (HPC) to approximately the same depth (574, 0 to 206.5 m; 574A, 0 to 180.2 m), and two holes were rotary drilled (574B, 185 to 194.5 m; 574C, 194.5 to 525.5 m, i.e., to basement).The oldest sediments in the sequence recovered are uppermost Eocene (520 m sub-bottom). Except for minor hiatuses, the sequence is continuous from this age through the Quaternary. The bottom cores contain about 60 cm of basalt, placing basement at 520 m sub-bottom. The sedimentary sequence is divided into a basal metalliferous calcareous unit (502.5 to 520.0 m), a calcareous ooze chalk unit in the middle (84.1 to 502.5 m), and a cyclic siliceous calcareous ooze unit at the top (0 to 84.1 m).All major planktonic microfossils are represented, although the dissolution of the planktonic foraminifers limits their stratigraphic usefulness. Initial data from the foraminifers, diatoms, calcareous nannofossils, and radiolarians indicate that a remarkably complete uppermost Eocene to lowest Oligocene transition was collected near the bottom of Hole 574C (within the metalliferous calcareous sedimentary unit above basement).The sediment accumulation rate is variable, ranging between 5 and 35 m/m.y.; it is low between 0 and 12 Ma, high between 12 and 23 Ma, and low again between 23 and 34 Ma. The mass accumulation rates are highest (about 3 g/cm 2 per 1000 yr.) in the lower Oligocene and at about 12.5 Ma, with a low of 0.28 g/cm 2 per 1000 yr. occurring between 0 and 5 Ma.The records of calcium carbonate content and the sediment's physical properties show major fluctuations in the last 12 m.y. They are more uniform in the older part of the section. Natural remanent magnetization (NRM) intensity is broadly correlative to lithology; magnetization is strongest in the upper cyclic siliceouscalcareous unit and the basal metalliferous unit and extremely weak throughout most of the middle, calcareous unit. The analysis of inorganic geochemistry suggests the presence of a diffusional control in the section^ upper 150 m and of diagenetic reactions in its lower portion.