PALEOMAGNETISM

Drilling and Core Orientation

Every other core at Site 1265 was recovered with a nonmagnetic core barrel until the first core barrel had to be drilled over (see Table T1; "Operations"). As at other sites, no obvious difference was noticed in the magnetic data between sediments recovered with the nonmagnetic barrel and those with a standard core barrel. All APC cores in Holes 1265A and 1265B were successfully oriented with the Tensor tool with the exception of Cores 208-1265A-1H and 2H, 208-1265B-1H through 3H, and 22H, 23H, 25H, and 26H (see Table T1; "Operations").

Archive-Half Measurements

The archive halves of 70 cores from Holes 1265A, 1265B, 1265C, and 1265D 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, with the notable exception of Sections 208-1265A-7H-3 through 18H-2, which were demagnetized only at 15 mT to speed core flow through the core laboratory. As at other sites, a strong vertical overprint was largely removed by demagnetization to 10 mT.

The working half of Section 208-1265B-10H-2 was demagnetized to test the effect of core scraping on the magnetization of soft-sediment split cores. The archive halves are routinely scraped by the sedimentologists with a metal spatula over a plastic film. This is done to remove surface roughness created by splitting, which shows up in the color reflectance data as an artificial darkening of color. To check that this process does not adversely affect the magnetization of the softer sediments, the archive halves of Sections 208-1265B-7H-5 and 7H-6 were measured both before and after smoothing. It was discovered that a dramatic change in both inclination and intensity resulted after smoothing (Fig. F26A, F26B). Following this discovery, an additional section was tested more thoroughly. Section 208-1265B-10H-2 was demagnetized to 15 mT and measured as a whole round. The section was then split, redemagnetized to 15 mT, and remeasured. The archive half was then smoothed with a plastic spatula, and the working half was smoothed with a metal spatula. The halves were then remeasured, redemagnetized to 15 mT, and measured again. The results show no significant differences between the nonsmoothed and smoothed halves (Fig. F26C, F26D). The large increase in magnetization following smoothing seen in Sections 208-1265B-7H-5 and 7H-6 was not observed either. The inclination data from the archive half (Fig. F26C) closely parallel those of the whole round. The working-half data, however, do show some shallowing compared to the whole-round data. It is concluded from this admittedly limited experiment that the smoothing has little to no effect on the magnetization of the split cores but that the splitting process can occasionally have some effect. The test results on Sections 208-1265B-7H-5 and 7H-6 are difficult to explain, but it is suspected that the inclination record from these sections was anomalous to begin with.

Remanent Magnetization Intensity

The initial NRM before demagnetization of sediments is on the order of 10–3 to 10–2 A/m and primarily reflects the vertical downward overprint, as at other sites (Fig. F27). The overprint is easily removed after alternating-field (AF) demagnetization to 10 mT. After AF demagnetization at a peak field of 15 mT, intensities are reduced by about an order of magnitude (Fig. F27).

The interval from 0 to 55 mcd (lithostratigraphic Unit I) is characterized by low initial MS values (Fig. F28), a concave trend of depositional remanent magnetization (DRM) (a trough at 20–25 mcd) (Fig. F27), and a convex trend of DRM normalized by initial susceptibility (nDRM) (a peak at 20–25 mcd) (Fig. F29). Assuming no change in geomagnetic field intensity, this indicates that a magnetic property change resulting from diagenesis and/or magnetic grain-size changes might be gradually developed in this unit. In the interval from 55 to 316 mcd, however, it appears that intensity changes are largely controlled by changes in ferromagnetic concentrations. Below 316 mcd, negative MS values (Fig. F28) show that calcite is dominating the MS signal.

Magnetostratigraphy

Portions of the magnetostratigraphy at Site 1265 are relatively good compared to the records from Sites 1263 and 1264. Most of the age-depth tie points (Table T10) are taken from Hole 1265A, which generally provides the best inclination record. The Pliocene–Pleistocene data are not particularly well resolved (Fig. F30A), but initial estimates of major boundaries roughly agree with the biostratigraphy (see "Biostratigraphy") (Table T5). The Brunhes/Matuyama boundary (0.781 Ma) is placed at ~3.8 mcd, and it is bracketed by the top of nannofossil datums Pseudoemiliania lacunosa (0.46 Ma) at ~2.32 mcd and large Gephyrocapsa spp. (1.22 Ma) at ~5.07 mcd. The base of Chron C3n, just above the Miocene/Pliocene boundary, appears to fall in a core gap in Hole 1265A sediments but otherwise is well resolved, and the placement at ~32.9 mcd agrees well with the biostratigraphic placement of the Miocene/Pliocene boundary at ~32.7 mcd (see "Biostratigraphy," "Age Model and Mass Accumulation Rates," and Table T17).

Most of the middle to upper Miocene is condensed or poorly resolved, but the lower Miocene to uppermost Oligocene is characterized by a recognizable polarity sequence that is well resolved for Hole 1265A (Fig. F30B). The placement of the top of Chron C6n (18.748 Ma) at ~90.4 mcd was chosen to approximately agree with the bottom of nannofossil datum S. belemnos (18.92 Ma) at ~93 mcd. The sequence from Chrons C5En to C6Cn is nicely resolved with the exception of Chron C6AAn and Subchron C6AAr.1n, which appear to be missing or fall in a core gap. Alternatively, the normal chron, tentatively identified as Chron C6An may, in part, be one of these two events. All three short normals in Chron C6Cn are well resolved, allowing the placement of the O/M boundary between ~124.6 and 125.0 mcd; this is offset from the biostratigraphic placement of the boundary at ~129 mcd.

In the Oligocene, the sequence becomes less identifiable and only Chrons C8n and C9n have been tentatively identified (but are not included in the age-depth model) (Table T18; Fig. F48). As at other sites, Chron C13n is identifiable in sediments from both holes and is constrained by the tops of E. formosa and D. saipanensis.

The inclination record in the Eocene shows a bias toward negative values as at other sites, and no attempt to interpret the data in terms of polarity intervals has been made. Below the P/E boundary, however, Chron C24r is well resolved, as is the top of Chron C25n.

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