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The paleomagnetic measurement procedure at Site 503 was similar to that described for Site 502 (See preceding chapter). Each core section was measured with the longcore spinner magnetometer at 10-cm intervals. In addition, one or more discrete samples were taken from each core section for measurement of the total magnetic vector and its stability against progressive AF demagnetization. There were noteworthy differences in conditions at Site 503, however, that affected the quality and interpretation of the magnetic data and require comment. The most serious problem we encountered was the presence of rust scale from the drill string. Although the dark flecks typically were concentrated near the top of every recovered sediment core, they also smeared down a meter or more between the core liner and sediment, even when the sediment showed no indication of drilling disturbance. Individual rust scales proved to be highly magnetic—presumably because they incorporate small pieces of unoxidized metal. The anomalously high remanent intensities, several orders of magnitude above the uncontaminated sediment values, and scattered remanent directions observed in long-core magnetic measurements on many cores from Site 503 could be attributed to the presence of rust scale. The presence of rust scale was a serious problem in obtaining meaningful long-core spinner magnetometer measurements on cores from Hole 5O3A and somewhat less so in cores from Hole 503 B. The problem was very minor at Site 502, apparently because almost the same length of drill string had been used on the previous leg so that most of the rust scale had already scraped off during coring. Site 503 was located in deeper water than Site 502, and the additional drill pipe required had not been used for several months and simply rusted longer in the ocean spray. Consequently, long-core magnetic data from at least the topmost 1.5 meters section of most cores at Site 503 was of little value for magnetostratigraphic interpretations. Since the rust contamination did not affect the interior parts of undisturbed sections of the sediment, it was possible in part to complement the reliable longcore magnetic data from lower parts of each core with discrete sample measurements from the upper section of each core. This technique is illustrated in Figure 1.

Magnetostratigraphy of Equatorial Pacific Site 503 Hydraulic Piston Cores