Rock-Magnetic Stratigraphy of Leg 108 Sites 658, 659, 661, and 665, Eastern Tropical Atlantic

Abstract

The results of detailed rock-magnetic measurements of selected stratigraphic intervals at Ocean Drilling Program Leg 108 Sites 658, 659, 661, and 665 are as follows:1. At Site 658, it is likely that the entire interval was extensively affected by reductive diagenesis, resulting in the loss of fine-grained magnetite. At Sites 659 and 665, we suspect that a similar diagenetic alteration of fine-grained magnetite occurred, but that it was confined to discrete intervals. At Site 661, the uppermost 60-70 mbsf of the section appears to be relatively unaffected by reductive diagenesis; however, we suspect that the low carbonate content of sediments below were affected by oxidative diagenesis/authigenesis, resulting in the production of ultrafine-grained ferrimagnetic material.2. Preliminary spectral analyses of the uppermost 70 mbsf (i.e., unaltered) rock-magnetic record at Site 661 suggest a decrease in the frequency of magnetite grain-size fluctuations after 2.9 Ma.

Sedimentary Magnetism, Environmental Magnetism, and Magnetostratigraphy

“…The advent of this magnetometer has been a boon for shipboard paleomagnetic study and allows whole cores or split core to be measured rapidly. Scientific results from Leg 104 (Bleil, 1989), Leg 105 (Clement et al , 1989), Leg 108 (Tauxe et al , 1989) and Leg 115 (Schneider and Kent, 1990) indicate that the pass‐through magnetometer is capable of producing very high quality magnetostratigraphic records. However, the whole‐core measurements suffer from physical disturbance of the core, rust contamination and pervasive remagnetization often associated with core barrels.…”

Magnetic Diagenesis, Organic Input, Interstitial Water Chemistry, and Paleomagnetic Record of the Carbonate Sequence on the Ontong Java Plateau

“…A general correlation between the sub-bottom depth at which these diagenetic processes occurred and the local organic productivity was suspected, but low total organic carbon (TOC) contents precluded shipboard evaluation of this relationship. Microbially driven reduction of magnetite has been reported by several authors (e.g., Karlin and Levi, 1983;van Vreumingen, 1984;Bloemendal et al, 1989). Sager (1988) suspected that a metastable magnetic iron sulfide, either greigite or pyrrhotite, had been generated in Leg 101 sediments, and that this phase was the carrier of an overprint.…”

Downhole Magnetic Logging in Sediments during Leg 145: Usefulness and Magnetostratigraphic Interpretation of the Logs at Site 884

“…To avoid this problem, two holes are drilled so that by measuring physical parameters such as magnetic susceptibility in each hole, it is possible to correlate between holes and reconstruct the true thickness of the sedimentary pile. Numerous workers have, since ODP Leg 108 (Bloemendal et al, 1988(Bloemendal et al, , 1989) developed a routine procedure on all ODP legs to obtain these results from whole core magnetic susceptibility (see, for example, Ruddiman, Sarnthein, Baldauf, et al, 1988;Mascle, Moore, et al, 1988;Backman, Duncan, et al, 1988;Cochran, Stow, et al, 1989;Prell, Niitsuma, et al, 1989;Peirce, Weissel et al, 1989;Rangin, Silver, von Breymann, et al, 1990). It is far beyond the scope of this paper to present an extensive review of this technique, which can be found with the appropriate references in Robinson (1993).…”