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doi:10.2204/iodp.proc.320321.106.2010

Paleomagnetism

We studied the paleomagnetism of sediments from Site U1334 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 66 APC cores recovered from Holes U1334A–U1334C. Measurements were made along each section at 5 cm intervals before and after alternating-field (AF) demagnetization of 20 mT. When time permitted, some sections were measured at 1 or 2.5 cm intervals. We found the higher resolution data to be more useful than measuring the 5, 10, or 15 mT demagnetization steps as had been done at the previous sites. We also did not measure archive-half sections of any XCB cores at this site because the shallow paleomagnetic inclination of the sediments along with the relative azimuthal rotation that occurs between adjacent pieces of XCB core (referred to as "drilling biscuits") results in neither useful intensity nor direction data.

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, T15, T16, and T17 and Figures F16, F17, and F18.

Azimuthal core orientation was determined solely by correlating distinct reversal patterns as recorded by paleomagnetic declinations 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 rather detailed age constraints, which significantly limit the range of possible correlations with the GPTS (see "Biostratigraphy"). Once we had confidently identified a unique, unambiguous reversal pattern, the mean paleomagnetic directions for each core were calculated using Fisher statistics (Table T18). Subsequently, data were reoriented so that normal and reversed polarity magnetozones had declinations of ~0° and ~180°, respectively (see "Paleomagnetism" in the "Site U1331" chapter). Reoriented declinations are provided for Holes U1334A–U1334C in Tables T13, T15, and T17, respectively, for data collected after AF demagnetization at 20 mT.

We measured magnetic properties of 188 discrete paleomagnetic samples, with one sample collected from nearly every section in Hole U1334A. Of these, 87 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 T19 and T20, respectively. Magnetic susceptibilities and masses, along with volumes estimated using MAD data (see "Physical properties"), are given in Table T21. This table also includes magnetic susceptibilities from whole-core data for the intervals corresponding to where the discrete samples were taken, which is useful for checking the scale factor, 0.68 × 10^–5 (see "Paleomagnetism" in the "Methods" chapter), for converting the whole-core raw susceptibility meter measurements into true volume-normalized susceptibility values.

Results

Downhole variations in paleomagnetic data from split-core and discrete samples and susceptibility data from whole-core and discrete samples are shown in Figures F16, F17, and F18. The most prominent features of the records are

1. The clear 180° alternations in declination for the upper ~140–150 m CSF of the section, reflecting the magnetic polarity zones (magnetozones);

2. The remanent magnetic intensity and magnetic susceptibility low that occurs between ~135 and 210 m CSF, referred to as the magnetic-low zone; and

3. The general degradation of the paleomagnetic direction within the magnetic-low zone with further degradation at the depth where coring switched from using nonmagnetic to steel core barrels (compare the results above and below the dashed line in Figs. F16, F17, and F18).

The magnetic-low zone can be attributed to reduction diagenesis, whereby oxygen from fine-grained iron oxides (titanomagnetite and magnetite) is used by bacteria to break down organic matter. This mobilizes iron and converts some of the iron oxides into various iron sulfides, most of which have very low magnetic susceptibilities and retain little or no remanent magnetization.

Unlike Sites U1331 and U1332, where the drilling overprint was present regardless of which type of core barrel was used, the drilling overprint was generally weak for Site U1334 cores when nonmagnetic core barrels were used (Cores 320-U1334A-1H through 16H, 320-U1334B-1H through 15H, and 320-U1334C-1H through 15H). In contrast, those cores collected with steel core barrels are highly overprinted, similar to what was observed at Site U1333 (Fig. F19). At Site U1334, the overprint appears more severe than at any of the other sites, which might be related to mineralogy (replacement of the primary iron oxides with iron sulfides). Whatever the cause, even demagnetization at 20 mT fails to remove the overprint fully. As a result, paleomagnetic declination data have notably higher variability, which makes polarity determination much more difficult in the intervals collected with steel core barrels.

Discrete sample demagnetization data from cores collected above ~130 m CSF (those collected with nonmagnetic core barrels and that are above the magnetic-low zone) indicate that the ChRM of the sediments can be resolved by AF demagnetization above ~10 mT (Fig. F20). We interpret this ChRM to be the primary depositional remanent magnetization. Unlike most of the samples from Sites U1331–U1333, most Site U1334 samples have more poorly resolved ChRM directions (more scattered directions along a linear demagnetization path in the orthogonal demagnetization plot). We attribute this mainly to the weaker magnetization of Site U1334 sediments. For example, within the magnetic-low zone, magnetizations are very close to the noise level of the magnetometer. Even more strongly magnetized intervals are only about an order of magnitude above the noise level. Generally, those samples that are sufficiently strongly magnetized yield linear demagnetization paths that decay toward but do not terminate at the origin of the orthogonal demagnetization diagrams (Fig. F20). This offset from the origin most likely reflects measurement artifacts (a small anhysteretic magnetization imparted to samples as they are demagnetized). Even with this artifact, ChRM directions for the more strongly magnetized samples are well constrained. These ChRMs, estimated from the linear demagnetization paths using PCA, agree well with those of coeval intervals of the archive-half measurements (Fig. F16), indicating that magnetic directions from the split cores after 20 mT demagnetization step provide a reliable indicator of the ChRM of the sediments.

Magnetostratigraphy

Cleaned paleomagnetic data provide a series of distinct ~180° alternations in declination and subtle changes in inclination, which, when combined with biostratigraphic age constraints, allow a continuous magnetostratigraphy to be constructed that correlates well with the GPTS. The magnetostratigraphic record extends from the top of Chron C1n (0 Ma) at the mudline of Hole U1334A (0 m CSF) to the top of Chron C11r (29.957 Ma) at 195.06 m CSF in Hole U1334C (Figs. F19, F21, F22; Table T22). The interpretation is complicated in only two intervals. The first complication occurs for the thin Pliocene and Pleistocene section, which was cored only once (in the upper 5 m CSF of Hole U1334A) (Fig. F21A). Either coring deformation or hiatuses make it difficult to connect what appear to be Chrons C1n through C2n in the upper 2 m of Core 320-U1334A-1H to a clear continuous sequence of magnetozones that correlate well from the base of Chron C3r at 5.3 and 1.8 m CSF in Holes U1334A and U1334C, respectively, to the top of Chron C9n (26.508 Ma) at 139.5, 136.2, and 131.9 m CSF in Holes U1334A–U1334C, respectively. Below this, correlation of the magnetozones to Chrons C9r–C12n is more speculative as a result of the more variable declinations in the magnetic-low zone and the larger drilling overprint, as discussed above. We consider it too speculative to correlate the highly variable declinations from Cores 320-U1334A-22H, 320-U1334B-22H, or 320-U1334C-22H with the GPTS. Below these cores, only XCB cores were collected, from which polarity determination is improbable.

Some highlights of the magnetostratigraphy at Site U1334 include resolving a clear sequence of magnetozones corresponding to Chrons C3r–C9n, which yield a total of 250 dated reversals from the three holes and provide detailed chronostratigraphic and sedimentation rate constraints (see "Stratigraphic correlation and composite section"), the identification of a previously observed cryptochron (C5Dr-1n) in all three holes, and the identification of eight possible excursions, seven of which are recorded in at least two of the holes (Table T22; Figs. F19, F21, F22).

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