All cores from Hole 1262A were recovered using two nonmagnetic core barrels. One of the nonmagnetic core barrels was damaged at the base of Hole 1262A, so Hole 1262B and all subsequent holes during Leg 208 were drilled using the remaining nonmagnetic core barrel every other core. The odd-numbered cores were recovered with the nonmagnetic barrel, and the even-numbered cores were recovered with a regular steel barrel. After the first core barrel had to be drilled over (see Table T1; "Operations"), only regular steel barrels were deployed. No obvious differences were observed in the magnetic data between sediments retrieved with the nonmagnetic core barrel and those retrieved with the regular steel barrel.
All cores were oriented with the Tensor tool with the exception of Cores 208-1262A-4H through 7H and 208-1262B-16H through 21H. In these cases, the Tensor tool did not record the time and Tensor tool data from these cores should be viewed with caution.
The archive halves of 54 cores from Holes 1262A, 1262B, and 1262C were measured in the pass-through magnetometer. Natural remanent magnetization (NRM) was measured on all cores. Most cores were demagnetized at 10 and 15 mT. A few cores were demagnetized only at 15 mT to speed the core flow through the core laboratory. Sections that clearly suffered from severe drilling disturbance were either not measured or were measured only at 15 mT.
As many of the sediments were very soft and easily deformed, a test was made to determine if splitting the cores was contributing to noise in the data. One core section (208-1262A-33H-1) was measured at 0, 10, and 15 mT before splitting. The results were compared with those from the archive half, which was demagnetized at 15 mT. No significant differences in the data were found, and all of the remaining cores were split and measured as usual. Note that many cores were much more soupy than the one tested, and it remains possible that these cores were affected by the splitting.
A strong drilling overprint in the downhole direction was observed in the data from nearly all the cores but was largely removed in most cases by demagnetization to 10 mT (Fig. F24). Test Sections 208-1262A-14H-6 (reversed) and 208-1262B-5H-3 (normal) were demagnetized up to 25 mT. It was concluded that 15 mT is sufficient to remove most of the overprint and to determine the characteristic polarity of the sediment without compromising the magnetization of the cores for future study.
Intensities of initial remanent magnetization (prior to alternating-field [AF] demagnetization) in sediments were mostly on the order of 10–2 to 10–1 A/m (Fig. F25). Intensity values after AF demagnetization to 15 mT (and removal of the overprint) were typically an order of magnitude lower. A considerable increase in the component between 0 and 10 mT was observed in Cores 208-1262A-5H through 9H and 17H. High-intensity spikes at the top of each core are interpreted as representing coring-induced magnetization and/or contaminants from the core barrel.
Depositional remanent magnetization (DRM) following demagnetization to 15 mT (DRM15 mT), DRM normalized by initial susceptibility (nDRM15 mT), and differential DRM between 10 and 15 mT (dDRM10–15 mT) curves for Hole 1262A are plotted in Figure F26. The variations in nDRM15 mT and dDRM10–15 mT normalized by susceptibility (ndDRM10–15 mT) display very similar trends, indicating that the ferromagnetic coercivity fraction of 10–15 mT may play a role as a proxy of relative paleointensity.
Magnetostratigraphy is generally interpretable in the nannofossil ooze units. The record is more difficult to interpret in the clay units, which are characterized by much slower sediment accumulation rates and possibly multiple hiatuses in the Neogene. Although the signal in the clay units appears to be well resolved and some features correlate well between holes, no chron identifications have been made in the clay units.
Preliminary chron identifications for the three holes are shown in Figure F27, and age-depth tie points are given in Table T10. In the Pliocene–Pleistocene section, the Brunhes (C1n), Jaramillo (C1r.1n), and Cobb Mountain events appear to be quite well defined and agree with both the biostratigraphy and cyclostratigraphy. Chron C2n appears to be well resolved in Hole 1262B only, but its location is constrained by the nannofossil datum Subzone CN13a assemblage (1.7–1.8 Ma) from 16.01 to 17.22 mcd (see "Biostratigraphy;" Table T5). Several other short normal events are also evident in the Matuyama reversed polarity period and may correlate with the Reunion and other short events observed elsewhere. Whereas the top of Chron C2An appears quite clearly in sediments from both Holes 1262A and 1262B, the base (3.596 Ma) is less well defined, especially for Hole 1262B. The base of Chron C2An is placed in a core gap in Hole 1262A at ~36 mcd. This depth corresponds with the base of the nannofossil datum Subzone CN12aB assemblage (2.83–3.66 Ma) at 35.7 mcd.
The nannofossil ooze unit corresponding to lithostratigraphic Subunit IIB is characterized by predominantly reversed polarity and appears to correlate to Chron C12r. Chron C13n (33.058–33.545 Ma) is placed at the base of this unit and is nicely bracketed by nannofossil datums with the T of E. formosa (32.9 Ma) above and D. saipanensis (34.0 Ma) below.
The next oldest identifiable chrons are in the Eocene, and although this part of the record is not well resolved for any of the holes, the top of Chron C23n (50.778 Ma) is tentatively placed at ~93 mcd. This reversal appears to be relatively distinct in Hole 1262A sediments (Fig. F27B), and the depth roughly agrees with the B of Discoaster sublodoensis (49.3 Ma) and the T of T. orthostylus (51.0 Ma). The base of Chron C24n (53.347 Ma) appears to be resolved only for Hole 1262C at ~120 mcd. Again, this depth roughly agrees with those of the top of nannofossil datum D. multiradiatus (53.0 Ma), the B of S. radians (53.3 Ma), and the B of T. orthostylus (53.4 Ma) from Hole 1262B. Confirmation of this reversal awaits shore-based analysis of discrete samples.
Chrons C25n through C28n are clearly seen and agree well between all holes (Fig. F27D). The base of Chron C29n (64.745 Ma) above the K/P boundary appears to fall in a Hole 1262C core break and is not well resolved for Hole 1262B. The reversal from Chron C29r to C29n is placed at ~215 mcd, based largely on the eccentricity cycles seen in the MS data for Hole 1262B, which put the reversal at ~300 k.y. above the K/P boundary.
A number of fluctuations in MS are seen in Hole 1262A. These features are strongly correlated with characteristic magnetic remanence (DRM15 mT) (Fig. F28) as well as lithologic variations. Preliminary rock magnetic tests were performed on discrete samples associated with MS peaks in the interval 7.8–10.2 mcd from the working half. In most cases, the characteristic component is readily isolated after removal of a low-coercivity component by 15-mT demagnetization. Based on isothermal remanent magnetization (IRM) acquisition and backfield IRM analysis, magnetite is the most dominant ferromagnetic mineral in the sediments over this short interval. Grain size is identified as single domain or pseudosingle domain based on the median destructive fields (MDF) of anhysteretic remanent magnetization (ARM) and IRM (peaks: MDFARM = 20.06 ± 0.06 mT, MDFIRM = 21.83 ± 0.71 mT; troughs: MDFARM = 21.07 ± 0.12 mT, MDFIRM = 20.07 ± 0.07 mT). The magnetic behavior of samples from both peaks and troughs is very similar, indicating that magnetite concentration is the primary control on DRM and MS fluctuations.