SITE 1096

The paleomagnetic results from Site 1096 provide a magnetostratigraphy that correlates well with the GPTS starting from the onset of Subchron C3n.2n (4.620 Ma) at ~489.68 mcd upward through the Brunhes Chron (Fig. F16, F17). As at Site 1095, exceptions occur in a few intervals where core deformation, dropstones, overprints, or low recovery bias or obscure the paleomagnetic signal. Negligible coring gaps occur above ~140 mcd in the section that was double-APC cored in Holes 1096A and 1096B. Below this, gaps of several meters may occur between cores and with some shorter polarity zones may not have been recovered. Also, as at Site 1095, the interval from below Brunhes/Matuyama reversal to just above Subchron C2An.1n, spanning roughly 0.8 to 2.4 Ma, has the most complex magnetization and is the most difficult to interpret.

In Table T2, we list the depth range within which each reversal occurs along with the best estimate of the depth of the reversal boundary. Agreement between magnetostratigraphic ages and biostratigraphic ages, particularly those from diatom events, is excellent (Fig. F16). One exception is that the top of the Lampromitra coronata radiolarian zone indicates an age for the base of the sedimentary section that is ~0.5 m.y. younger than indicated by either the diatom events or the magnetostratigraphy.

Revisions to the shipboard data are noted in Table T2 and are discussed further below. The most significant changes are the identification of Chron C2n, Subchron C2r.1n (the Reunion Subchron), the onset of Subchron C2An.1n, and Subchron C2An.2n. The relative stratigraphic locations of these newly identified polarity chrons and subchrons are compatible with the broader magnetostratigraphic framework constructed during Leg 178 (table T21 from Shipboard Scientific Party, 1999b). Details of the interpretation are given below.

1. The zones with positive inclinations (reversed polarity) between 20 and 30 mcd are probably related to coring deformation. None of the zones with positive inclinations are replicated where duplicate APC cores from Holes 1096A and 1096B exist (Fig. F17). The two positive inclination zones recorded in Hole 1096A are present in the upper 3.5 m of Cores 178-1096A-3H and 4H. Coring deformation is common in the upper meter of many APC cores but can extend downcore particularly when ship heave is large, which was common during Leg 178. The positive inclination zone in Hole 1096B extends from interval 178-1096B-4H-2, 56 cm, to 4H-5, 82 cm. Core photographs (Barker, Camerlenghi, Acton, et al., 1999) show drilling and or core-splitting disturbance, but the disturbance does not appear to extend over all of the interval and does not appear sufficient to remagnetize or reorient the core material so completely.
2. The Brunhes/Matuyama reversal boundary is a sharp contact between steep negative inclinations of the Brunhes Chron (C1n) and the steep positive inclinations of the upper Matuyama Chron (C1r.1r) (Fig. F17). The fluctuations in directions noted in the transition zone at Site 1095 are recorded neither here nor at Site 1101.
3. Neither the Jaramillo Subchron (Subchron C1r.1n) nor the Cobb Mountain Event (Cryptochron C1r.2r-1n) can be confidently identified within a noisy interval that extends from ~60 to 200 mcd. The Jaramillo Subchron could correspond to one of the short negative inclination zones between ~63 and 110 mcd, though these zones could merely represent noise. Although perhaps coincidental, it is interesting that the interval spanning Chrons C1r through C2r is the most complexly magnetized at both Site 1095 and Site 1096. The complexity is manifested in shallower than expected inclinations and in results that conflict in coeval zones from adjacent holes. Both sites are part of the same sediment drift and likely have similar magnetic mineralogy and depositional histories, even though the sedimentation rates are several times higher at Site 1096. As noted by the Shipboard Scientific Party (1999b; "Paleomagnetism" section), several, but not all, of the negative inclination zones from 60 to 200 mcd occur where the magnetization drops by more than an order of magnitude below the background level of ~0.04 A/m. Similarly, the susceptibility drops by about an order of magnitude where the lows in magnetization intensity occur. Instrument noise, lower concentrations of magnetic minerals, coarser magnetic mineral assemblages, abundant dropstones, and APC core deformation could all be factors. Whatever the cause, many of the narrow zones with negative inclinations from ~60 to 200 mcd, particularly those intervals with intermediate or shallow negative inclinations, do not appear to represent normal polarity zones or subzones.
4. The normal polarity zone located at ~146-147 mcd and sampled only in Hole 1096B is interpreted to represent the upper part of Chron C2n. On its own, this zone is narrower than would be expected for Chron C2n given the Pleistocene sedimentation rates at Site 1096. However, the base of Core 178-1096B-16H has sustained drilling disturbance, as is apparent for Section 16H-6 in the core photographs (Barker, Camerlenghi, Acton, et al., 1999). Thus, the reversal that appears to occur at the top of Section 178-1096B-16H-6 (~147 mcd) may be an artifact of the drilling disturbance. Below Core 178-1096B-16H there is an ~7-m coring gap. We suggest that the onset of Chron C2n occurs somewhere within the coring gap. Below the gap, the next reliable paleomagnetic inclinations come from the top of Core 178-1096B-19H, which clearly have reversed polarity and are interpreted to represent part of Chron C2r.
5. We speculate that the normal polarity subzone between ~173 and 174 mcd is Subchron C2r.1n (the Reunion Subchron). We base this speculation on the position of the normal polarity subzone relative to adjacent polarity zones and on the position of the subzone at the base of lithostratigraphic Unit II. As noted by Kyte (Chap. 9, this volume), the Reunion Subchron was identified at the base of lithostratigraphic Unit II at Site 1101 and Unit II is defined similarly at Sites 1096 and 1101, with its base occurring below the last of one or more foraminifer-bearing interglacial deposits. The inclination within the subzone is, however, shallower than other well-defined normal polarity zones, the magnetization and susceptibility are low within the subzone, and a discrete sample within the subzone indicated reversed rather than normal polarity. The subzone is located within the first two sections of Core 178-1096C-2H, of which the upper 105 cm of Section 2H-1 is affected by drilling disturbance, making the identification of the termination of the subzone uncertain.
6. A geomagnetic excursion or anomalous interval, located at 288.82-291.20 mcd (interval 178-1096C-10X-3, 28 cm, to 10X-4, 98 cm), is present in the normal polarity zone that corresponds to Subchron C2An.1n. Several other narrower anomalous intervals (<1 m thick) occur above and below this interval but are probably artifacts given the level of drilling disturbance common in XCB cores.
7. The reversed polarity zone at the top of Core 178-1096C-14X is interpreted to represent Subchron C2An.1r. The top of the zone is not observed because it occurs somewhere in the coring gap between Cores 178-1096C-13X and 14X. We prefer a location near the top of the coring gap because this minimizes sedimentation rate variations. This reversal boundary also defines the onset of Subchron C2An.1n. Given that the reversed polarity zone represents Subchron C2An.1r, it follows that the normal polarity zone below represents Subchron C2An.2n.
8. A normal polarity zone is present in Core 178-1096C-32X (501.47-506.75 mcd) that is also visible in the GHMT log (Williams et al., Chap. 31, this volume). We interpret it as a geomagnetic excursion or as an anomalous zone. Alternatively, it could be interpreted as Subchron C3n.1n, but this results in more abrupt changes in sedimentation rates than our preferred interpretation. If it is Subchron C3n.1n, the lowest normal polarity zone in the hole would more likely be Subchron C3n.3n than our preferred interpretation of Subchron C3n.2n.
9. The locations of reversal boundaries for the termination of Subchron C2An.3n to the onset of Subchron C3n.2n agree with those of the Shipboard Scientific Party (1999b), although some minor modification has been made to the size of the transition zones (Table T2).

Using the revised magnetostratigraphy, we computed sedimentation rates between the identified reversals (Table T2; Fig. F18). As noted by Barker, Camerlenghi, and Acton, et al. (1999), the sedimentation rates increase downhole with the largest increase at the termination of Subchron C2An.1n (2.581 Ma), which occurs at 220.63 mcd. The average sedimentation rate above this depth is 85.5 m/m.y. and below is 181.0 m/m.y.