Site 1207 is the only site that has been drilled on the Northern High of Shatsky Rise. The sediment recovered was mostly Neogene in age (0–163.8 mbsf) underlain by Campanian and older oozes and cherts. The sediment consists of nannofossil ooze with diatoms, radiolarians, and clay in varying amounts (Bralower, Premoli Silva, Malone, et al., 2002). The samples taken for paleomagnetic analysis were AF demagnetized in 5-mT steps up to either 50, 60, or 70 mT, depending on the intensity of the natural remanent magnetization (NRM). Less than 10% of the NRM remains after demagnetization at these peak fields, indicating a low-coercivity remanence carrier, most likely magnetite. Orthogonal projections of demagnetization data (Fig. F2A) show well-defined components for most of the Neogene section after removal of the steep drilling-related overprint at peak AF fields of 20 mT. Maximum angular deviation (MAD) values are low for most of the section (<10°), indicating well-defined characteristic magnetization components; however, some intervals, particularly the interval between 50 and 60 mbsf (Fig. F3A), have slightly higher MAD values and less well defined components. The interpretation of the magnetic stratigraphy from shipboard and discrete sample data can be accomplished by polarity zone pattern fit to the GPTS (Cande and Kent, 1992, 1995) (Table T2). This pattern fit is satisfactory to the base of Subchron C5An.1n (Fig. F3A, F3B). Below the polarity zone equivalent to Subchron C5An.1n, recovery was intermittent and biostratigraphy indicates a hiatus with Campanian age sediments below (Bralower, Premoli Silva, Malone, et al., 2002). Sedimentation rates average 1–2 cm/k.y. throughout the section with some slightly higher (3–4 cm/k.y.) rates in the Late Pliocene and late Miocene (Fig. F4A). Component declination has been corrected for each core using Tensor orientation data measured shipboard. The mean inclination in normal polarity zones for the site is 57.8°, close to the expected inclination of 56° for a geocentric axial dipole at this site; however, reversed polarity intervals have a mean inclination of –51.1°, shallower than expected. This can be attributed to shallowing of reversed polarity directions by the steep downward-directed drilling overprint, shown clearly in the orthogonal projections (Fig. F2A).
Sites 1209–1212 are located on the Southern High of Shatsky Rise (Fig. F1). Multiple holes were drilled at each site and composite sections were constructed using shipboard MST data. Discrete sample cubes were only collected from Holes 1209A, 1210A, 1211A, and 1212A. Shipboard data from the pass-through magnetometer are consistent between the different holes at each site and confirms the interpretation of the magnetic stratigraphy (see Shipboard Scientific Party, 2002).
As for Site 1207, orthogonal projections from discrete sample data show two components: a steep downward drilling-related overprint and well-defined characteristic components (Fig. F2B–F2E). In most cases the drilling-related overprint was easily removed in peak AF fields of 10–20 mT. Little of the NRM remained at peak fields of 60 mT. MAD values are generally <5° throughout the sections. The expected inclination for the Southern Rise is 51°; again, all the sites show slightly steeper than expected inclinations in normal polarity zones and shallower than expected inclination in reversed polarity zones. The magnetostratigraphic age models indicate mean sedimentation rates between 1 and 3 cm/k.y. for most of the Neogene (Fig. F4B–F4E).
The polarity interpretation at Sites 1209, 1210, and 1211 is unambiguous back to Subchron C3Bn (Table T2; Figs. F5, F6, F7). Below this level, interpretation becomes difficult due to decreasing sedimentation rates leading to a hiatus recognized at all sites between the upper Miocene and Oligocene and older sequences. At Site 1212, a hiatus accounts for the interval between 4 and 5 Ma (Chron C3), and the polarity interpretation can be accomplished to Subchron C4n.2n (Fig. F8). This interpretation of the sequence of polarity zones is confirmed by shipboard biostratigraphy. The interpretation of the polarity stratigraphy was carried out using data measured shipboard augmented with discrete sample cubes. When the magnetostratigraphic data were placed on the composite depth scale, the reversal boundaries were found to be consistent between holes, indicating that there is very little error in the depths of polarity zone boundaries or in composite depth calculations. The magnetic measurements made shipboard do include a small amount of error due to the response function of the shipboard magnetometer. The response function of the wide-access magnetometer used to measure half-cores is ~10 cm, resulting in a centimeter-scale uncertainty in the placement of the reversal boundaries.
Site 1208 is located on the Central High of Shatsky Rise and also provided an expanded late Miocene–Holocene section. The magnetic stratigraphy from Site 1208 will be presented elsewhere (Evans et al., unpubl. data).