Magnetostratigraphic interpretations for Sites 1095, 1096, and 1101 (Fig. F1) presented here are similar to, though supercede, those in the Leg 178 Initial Reports volume (Barker, Camerlenghi, Acton, et al., 1999). The most significant changes are based on (1) assessment of postcruise paleomagnetic measurements made on U-channel samples; (2) reinterpretation of paleomagnetic data collected during Leg 178 from archive-half core sections and from discrete samples (~7 cm3 of sediment in plastic cubes) taken from working-half core sections (Barker, Camerlenghi, Acton, et al., 1999); (3) additional processing and interpretation of the magnetic logging data (Williams et al., Chap. 31, this volume); (4) use of the meters composite depth (mcd) scale of Barker (Chap. 6, this volume), which has allowed us to place paleomagnetic data from multiple cored intervals into a common depth scale; and (5) incorporation of revised biostratigraphic events as summarized by Iwai et al. (Chap. 36, this volume).
We discuss the changes below and present complete revised magnetostratigraphies for Sites 1095, 1096, and 1101 (Tables T1, T2, T3). The pattern of polarity zones at these sites can be correlated with polarity chrons of the geomagnetic polarity timescale (GPTS) without requiring abrupt large changes in sedimentation rates or hiatuses. Hence, much of the interpretation is accomplished by fairly straightforward matching of the polarity zonation to the GPTS. The pattern matching is further facilitated by using the most complete stratigraphic sections available, which is accomplished through stratigraphic correlation between multiple holes cored at a site. Through such correlation, the data from all holes at a site are placed within a common stratigraphic framework and in a common depth scale, referred to as the meters composite depth (mcd) scale (see Acton et al., Chap. 5, and Barker, Chap. 6, both this volume). Without this, the relationship of reversals recorded in one hole relative those in other holes at the same site is often difficult to establish. Without independent age constraints, such as those provided by the biostratigraphic events that have known correlations to the GPTS, uncertainty in the pattern matching would exist in several intervals. The biostratigraphic data generally support our magnetostratigraphic interpretations built on pattern matching and together provide consistent ages for the Neogene sedimentary sections. Throughout this paper, we use the polarity chron ages and nomenclature of Cande and Kent (1995).
We do not present magnetostratigraphic interpretations for other Leg 178 sites. As discussed in "Paleomagnetism" in the "Site 1097" and "Shelf Transect" chapters of Barker, Camerlenghi, Acton, et al. (1999), only a few polarity zones can be discerned in the paleomagnetic data from the continental shelf (Sites 1097, 1100, 1102, and 1103) and these yield no independent age constraints. In contrast, Palmer Deep Sites 1098 and 1099, located within the inner shelf basin, give a detailed paleosecular variation (PSV) record of the Holocene and latest Pleistocene (Brachfeld et al., 2000), all of which falls within the uppermost portion of the Brunhes polarity chron. Currently, there are no comparable well-dated PSV records from the high latitudes of the Southern Hemisphere to correlate with the detailed directional changes observed in the Palmer Deep records. Instead, the chronology for the Palmer Deep sediments are constrained directly by 14C ages (Domack et al., 2001), making the Palmer Deep PSV records a tool for dating Holocene sedimentary sections that are being and will be cored elsewhere around Antarctica.
Similarly, more detailed chronologies than those provided by magnetic polarity stratigraphy can be obtained for Pleistocene sediments at the continental rise sites through correlation of the Leg 178 relative paleointensity records with global mean paleointensity records, such as Sint-800 (Guyodo and Valet, 1999), or with single high-quality records such as those recovered during Leg 162 in the North Atlantic (Channell et al., 1997; Channell, 1999). Additional independent chronologies can be obtained by using rock magnetic proxies, such as variations in magnetic susceptibility, that appear to correlate with oxygen isotope records. Guyodo et al. (2001) used both susceptibility and relative paleointensity to build a chronology for the interval recording the Jaramillo Subchron (C2r.1n) at ODP Site 1101. The importance of the magnetostratigraphy is evident even in that study, as it is the magnetostratigraphy that provides the key tie points, particularly the reversal boundaries for the Jaramillo Subchron.