Previous   |   Next

doi:10.2204/iodp.proc.320321.107.2010

Paleomagnetism

We studied the paleomagnetism of sediments from Site U1335 with a primary focus on determining a preliminary magnetostratigraphy, which can be used to assist in dating the stratigraphic section. To accomplish this, we measured the natural remanent magnetization (NRM) of archive-half sections from 78 APC cores recovered from Holes U1335A and U1335B. Measurements were made along each section at 5 cm intervals before and after AF demagnetization at 20 mT. Many sections were measured at 2.5 cm intervals following 20 mT AF demagnetization, as we found higher resolution data to be more useful than measuring the 5, 10, or 15 mT demagnetization steps with ordinary resolution (see "Paleomagnetism" in the "Site U1334" chapter). For some of the archive-half sections from deeper intervals in Hole U1335B, we only measured the NRM before demagnetization because of their extremely weak magnetizations. No XCB cores were measured at this site.

We processed the paleomagnetic data by removing measurements made within 5 cm of section ends and data from disturbed intervals (Table T11). Cleaned data are presented in Tables T12, T13, T14, and T15 and in Figures F7 and F15.

Azimuthal core orientation was determined solely by correlating distinct reversals patterns as recorded by the paleomagnetic declination in each hole with the geomagnetic polarity timescale (GPTS) (See "Paleomagnetism" in the "Methods" chapter and "Paleomagnetism" in the "Site U1331" chapter). This process is aided by detailed biostratigraphic age constraints, which significantly limit the range of possible correlations with the GPTS (see "Biostratigraphy"). Once we had confidently identified a unique, unambiguous reversals pattern, the mean paleomagnetic directions for each core were calculated using Bingham statistics (Table T16) with the same procedure described in "Paleomagnetism" in the "Site U1332" chapter. Subsequently the data were reoriented so that normal and reversed polarity intervals had declinations of ~0° and ~180°, respectively (see "Paleomagnetism" in the "Site U1331" chapter). Reoriented declinations are provided for Holes U1335A and U1335B in Tables T13 and T15, respectively, for the data collected after AF demagnetization at 20 mT.

We measured magnetic properties of 257 discrete paleomagnetic samples. Of these, 70 samples were subjected to progressive AF demagnetization up to 60 mT. Remanence measurements and characteristic remanent magnetization (ChRM) directions computed using principal component analysis (PCA) are given in Tables T17 and T18, respectively. Magnetic susceptibilities and masses, along with volumes estimated using MAD data (see "Physical properties"), are given in Table T19. This table also includes magnetic susceptibilities from the whole-core data for the depth intervals corresponding to that of the discrete samples, which is useful for checking the scale factor, 0.68 × 10^–5 SI (see "Paleomagnetism" in the "Methods" chapter), for converting the whole-core raw susceptibility meter measurements into volume-normalized susceptibility values.

Results

Downhole variations in paleomagnetic data from split cores and discrete samples and magnetic susceptibility from whole-core and discrete samples are shown in Figures F7 and F15. The most prominent feature of the records is the magnetic intensity and susceptibility low that occurs between ~70 and 110 m and ~210 and 410 m CSF, referred to as the magnetic-low zone. We could not retrieve any reliable paleomagnetic directions from the magnetic-low zone because the remanent magnetic intensity of the zone after 20 mT AF demagnetization is on the order of 10^–5 A/m, which is comparable to the noise level of the super-conducting rock magnetometer onboard the JOIDES Resolution (see "Paleomagnetism" in the "Methods" chapter). Similar to Site U1334, the magnetic-low zone can be attributed to reduction diagenesis (see "Paleomagnetism" in the "Site U1334" chapter).

The drilling overprint was generally weak for Site U1335 cores when nonmagnetic core barrels were used (Cores 320-U1335A-1H through 16H and 320-U1335B-1H through 19H). This is evidenced by relatively shallow inclinations observed before demagnetization (Figs. F7, F15). In contrast, cores collected with steel core barrels are highly overprinted, as noted by the steep inclinations prior to demagnetization (Figs. F7, F15), which is similar to those observed at Sites U1333 (see Fig. F17 in the "Site U1333" chapter) and U1334 (see Figs. F16, F17, and F18 in the "Site U1334" chapter). Regardless of which type of core barrel was used, shallow inclinations and distinct reversal patterns in the declinations were observed after 20 mT demagnetization, except within the magnetic-low zone. In the magnetic-low zone, the overprint appears more severe, which might be related to mineralogy (replacement of the primary iron oxides with iron sulfides). Similar to Site U1334, inclinations remain steep within the magnetic-low zone even after 20 mT demagnetization, in spite of associated large decrease in the remanent intensity (Figs. F7, F15), indicating that steel core barrels have completely remagnetized these sediments.

Discrete sample demagnetization data indicate that the ChRM of the sediments can be resolved by AF demagnetization above 5–15 mT (Fig. F16) except for samples from the magnetic-low zone. We interpret this ChRM to be the primary depositional remanent magnetization. Similar to the Site U1334 samples, many of the Site U1335 samples have poorly resolved ChRM directions (scattered directions along a linear demagnetization path in the orthogonal demagnetization plot) due to weak magnetization of Site U1335 sediments. Nonetheless the ChRM declinations agree well with those of coeval intervals of the archive-half measurements (Fig. F15), indicating that the declinations from the split cores after 20 mT demagnetization provide a reliable indicator of the ChRM of the sediments.

Magnetostratigraphy

The cleaned paleomagnetic data provide a series of distinct ~180° alternations in declination except for the magnetic-low zone. When combined with biostratigraphic age constraints (see "Biostratigraphy"), the data give a continuous magnetostratigraphy from Chrons C1n (0–0.781 Ma) to C5n.2n (9.987–11.040 Ma) from 0 to 65.95 m CSF in Hole U1335A and from Chrons C1n to C5r.1n (11.118–11.154 Ma) from 0 to 66.225 m CSF in Hole U1335B. Below the bottom of the first magnetic-low zone (~70–110 m CSF), the magnetostratigraphy is again interpretable downhole: Chrons C5Br (15.160–15.974 Ma) to C6n (18.748–19.722 Ma) from 155.35 to 208.40 m CSF in Hole U1335A and from Chron C5AAn (13.015–13.183 Ma) to Chron C5Er (18.524–18.748 Ma) from 107.95 to 202.60 m CSF in Hole U1335B. The magnetostratigraphy of the two holes is presented in Table T20 and Figures F17A, F17B, F18A, and F18B. The magnetostratigraphy could not be determined for the second magnetic-low zone (below ~210 m CSF).

Highlights of the magnetostratigraphy at Site U1335 are the identifications of (1) 123 reversals, (2) a previously observed cryptochron (C5Dr-1n) in both holes, and (3) 40 possible geomagnetic excursions (10 of these are recorded in both holes). The Site U1335 paleomagnetic record has high quality and resolution within the Miocene from 0 to 70 m CSF and 110 to 210 m CSF, whereas it has poor quality in the magnetic-low zones. The resolution is high owing to the high sedimentation rates at Site U1335 throughout the Miocene (see Fig. F14). The high-quality, high-resolution records made it possible to recognize the many potential excursions. Not all of the 40 potential excursions may be real geomagnetic features because a few are possibly associated with turbidites even though we excluded the intervals interpreted as turbidites. Considering the relatively high frequency of turbidites at this site, it is possible that some were not identified. Careful shore-based paleo- and rock magnetic investigations are necessary to confirm the reliability of these records.

Top of page   |   Previous   |   Next