Natural Remanent Magnetization

The natural remanent magnetization (NRM) of the archive half of each core section was initially measured then remeasured after alternating-field (AF) demagnetization at selected levels. Hole 1237A sections were AF demagnetized at 10, 15, 20, and 25 mT. Sections from Holes 1237B, 1237C, and 1237D were demagnetized at 20 and 25 mT. Sections obviously affected by drilling disturbance were not measured.

The NRM intensity before and after demagnetization shows the same general downhole trend with about 1.5 orders of magnitude less intensity after 25-mT AF demagnetization than before (Fig. F27). NRM intensities after AF demagnetization are in the 10-3 A/m range at the top of each hole. These intensities decrease by an order of magnitude through the upper 60 mcd, below which values range from ~3 x 10-4 to low as 1 x 10-5 A/m. At ~150 mcd, just below the transition from lithologic Subunit IIA to IIB (see "Lithostratigraphy"), intensity increases by an order of magnitude over just a few meters. NRM intensities from ~150 mcd to the base of the cored section are consistently within the middle 10-4 to low 10-3 A/m range (Fig. F27). The overall trend in NRM intensity is similar in pattern to the magnetic susceptibility profile.

Steep positive inclinations observed prior to demagnetization and resulting from the drill string magnetic overprint are mostly removed after 25-mT AF demagnetization. If unremoved, these would hamper polarity interpretations, as they are of the same sign as reversed polarity. The resulting downhole inclination pattern for Holes 1237C, 1237B, and 1237D shows clear alternations of negative and positive inclinations throughout (Fig. F28).

For the upper 100 mcd, the inclination pattern can be correlated between Holes 1237B and 1237C and with the overlapping part of Hole 1237D (Fig. F28). Clear interpretation of polarity chrons and subchrons, based on inclination, can be made to the base of the Gilbert Chron 3n (5.32 Ma). The Matuyama (2r)/Brunhes (1n) boundary, as well as the boundaries of the Jaramillo (1r.1n) Subchron, Olduvai (2n) Chron, and the Reunion (2r.1n) Subchron are all clearly identifiable in at least one and often two holes. The Gauss (2An)/Matuyama (2r) boundary is found in Hole 1237C, and the Kaena (2An.1r) and Mammoth (2An.2r) Subchrons are observed in Hole 1237B. The subchrons of the Gilbert (Cochiti, 3n.1n; Nunivak, 3n.2n; Sidufjall, 3n.3n; and Thvera, 3n.4n) are easily identifiable in Hole 1237B cores and in the overlapping cores of Hole 1237D, which were cored using the nonmagnetic barrel. The normal polarity Cochiti and Thvera Subchrons are not observed in Hole 1237C, where the mcd equivalent depths (Cores 202-1237C-8H and 10H) were cored with the normal steel (magnetic) core barrel. These cores have steeper than expected positive inclinations that suggest these normal polarity subchrons may have been overprinted by the core barrel's magnetization (Fig. F29). The interpretation of the magnetic stratigraphy for the interval 0-100 mcd and the resulting correlation to the geomagnetic polarity timescale (GPTS) (Cande and Kent, 1995) (Fig. F29; Tables T15, T16) produce age estimates that are consistent with the biostratigraphic datums (see "Biostratigraphy") and significantly augment the shipboard age models (see "Age Model and Mass Accumulation Rates").

From 100 to ~160 mcd, the correlation of the inclination pattern between holes, the interpretation of the magnetic stratigraphy from these, and its correlation to the GPTS are less clear (Fig. F30). Normal (negative) polarities are likely present but not clearly recorded. The combined effects of low NRM intensities (Fig. F27), drilling overprints, and intermittent reducing conditions in these sediments are probably at least partially responsible. To reduce the noise in this part of the record, we stacked and smoothed (10-point running mean) the inclination data from Holes 1237B, 1237C, and 1237D based on the mcd scale (Fig. F30). This procedure is similar to that used at Site 1236 (see "Paleomagnetism" in the "Site 1236" chapter; also see Fig. F29).

The stacked record allows an interpretation of the polarity to be made (Fig. F30; Table T16) that, when correlated to the GPTS (Cande and Kent 1995), provides ages consistent with the biostratigraphic datums (see "Biostratigraphy"). Below ~160 mcd, the increase in NRM intensity (Fig. F27) also results in a clearly defined inclination pattern that is correlative from Hole 1237B to 1237C (Fig. F30). The polarity within this interval is easily interpreted and correlated to the GPTS. The predominant normal polarity interval between ~175 and 181 mcd is correlated to Chron 5n. Not only are the polarity chrons identifiable, but a consistent pattern of polarity transitions is observed between Chron 4r (~162 mcd) and Chron 5Ar (~199 mcd). All polarity chrons and subchrons are identifiable within this interval (Table T15), providing exceptional stratigraphic control (Table T16).

Below 200 mcd the inclination data are not obviously correlative between Holes 1237B and 1237C (Fig. F31). Therefore caution must be employed with any polarity interpretation made. An interpretation is made based on the stacked inclination record (Fig. F31) that is generally consistent with the biostratigraphic datums (see "Biostratigraphy") down to a depth of ~250 mcd. However, within this interval, alternative interpretations are possible and the placement of polarity boundaries is equivocal. From ~250 to 280 mcd, the inclination records between Holes 1237B and 1237C are discrepant. The stacked record does not result in a record that resembles the GPTS within the interval as defined by the biostratigraphic datums. It is possible that a reevaluation of the mcd scale may be required. Inclination data for Holes 1237B and 1237D are similar on the mcd scale, whereas data from Hole 1237C are not, further suggesting that the composite depth correlation between Holes 1237B, 1237C, and 1237D may need reevaluation.

Below ~280 mcd, inclination can be correlated between Holes 1237B and 1237C, and the stacked record can be used for polarity interpretation to ~330 mcd, which is suggested to be the base of Chron 9n (27.972 Ma) (Fig. F32). Shore-based work will allow significant refinement of these interpretations. Much of the ambiguity could result from the low level of AF demagnetization used, which has not fully removed the drill string overprint, especially in intervals where the steel core barrel was used.