All archive halves of core sections from Holes 1209A, 1209B, and 1209C that did not show a large degree of drilling-related deformation were measured on the shipboard pass-through magnetometer. In all, 546 core sections were measured from 83 cores recovered in the three holes. All but two are APC cores. As at Site 1208, many of the measured cores from the Oligocene-Paleocene and Cretaceous part of the section are in poor condition due to disturbance related to drilling, core recovery, and core splitting.
The natural remanent magnetization (NRM) of core sections was measured at 5-cm intervals, followed by measurement after two alternating-field (AF) demagnetization steps (10- and 20-mT peak fields). When time was available, additional AF demagnetization steps (usually at peak fields of 15 mT) were measured. NRM intensity values typically ranged over three orders of magnitude, from 10-4 to 10-1 A/m. As for samples from other Leg 198 sites, cores from Site 1209 have acquired a steep downward-directed magnetic overprint that masks the primary magnetization. This magnetic overprint appears to be largely removed by AF demagnetization at peak fields of 20 mT. A plot of magnetization intensity after 20-mT demagnetization shows an initial decline of two orders of magnitude in the upper 30 m of the sedimentary section. This decline is followed at greater depths by a recovery in magnetization intensities to ~10-3 A/m (Fig. F16).
Paleomagnetic data acquired from the shipboard pass-through magnetometer from Site 1209 were uninterpretable for the pre-Pliocene section. In the upper ~60 meters composite depth (mcd), however, it was generally possible to recognize Pliocene-Pleistocene polarity chrons (Fig. F17). The polarity zone correlative to C2An (the Gauss Chron) is recognized from ~38 to ~59 mcd, although the upper boundary is poorly defined, and the two short reversed polarity subzones within it are not recognizable. The Gilbert Chron (C3n) is only partially recognizable. For cores below ~60 mcd, the record cannot be reliably interpreted in terms of polarity chrons. Although some parts of cores yield apparently consistent results, the directions are scattered, and patterns in one hole are not replicated in the others. Weak magnetization intensities are not the cause of the poor-quality data. The cores have magnetization intensities at least an order of magnitude above magnetometer noise level, equivalent to a magnetization intensity of ~3 x 10-5 A/m (Fig. F16). The probable cause of the poor data quality is the effect of drilling disturbance on the poorly consolidated Upper Cretaceous and Paleocene-Oligocene sediments. In many places, it was noted that identifiable bedding features in these watery sediments are highly deformed, pushed downward at the edges of the core, sometimes by tens of centimeters. If these cores are to produce a magnetic stratigraphy, it must come from discrete samples to be analyzed on shore.
Although only a few polarity zone boundaries can be recognized, these are sufficient to construct an age-depth curve for sediments in the upper 60 m of the section. The curve is remarkably linear, implying a nearly constant sedimentation rate of ~16 m/m.y. in Pliocene-Pleistocene time (Fig. F18). This sedimentation rate is in good agreement with that derived from biostratigraphy (~13.4 m/m.y.).