Previous   |   Next

doi:10.2204/iodp.proc.320321.108.2010

Paleomagnetism

We conducted a paleomagnetic study of archive-half sections of 21 APC cores from Hole U1336A and 20 APC cores from Hole U1336B with the primary objective of determining the magnetostratigraphy of the site and providing chronostratigraphic constraints. To accomplish this, we measured the NRM of each section at either 5 or 2.5 cm intervals (depending on time availability) before and after demagnetization at a peak AF of 20 mT. AF demagnetization at a peak field of 10 mT was also carried out for most Hole U1336B core sections. We processed the paleomagnetic data by removing measurements made within 5 cm (or 7.5 cm when measured at 2.5 cm intervals) of section ends and data from disturbed intervals (Table T13).

The FlexIt core orientation tool was not deployed at the site because the conclusion, derived from Sites U1331 and U1332, that the output of the tool was erratic and inconsistent with expected remanence directions for sediments of this age from this location (we expected Neogene declinations to lie close to north for the normal polarity and close to south for the reversed polarity). The remanence inclination is close to zero, as expected for a site located close to the paleoequator, making inclination not diagnostic of polarity. The lack of core orientation gave us an unwelcome degree of freedom in the assignment of polarity, as each core can be rotated in azimuth through 180° to change the sign of any polarity zone. Azimuthal core orientation had to be determined solely by correlating distinct reversals patterns as recorded by the paleomagnetic declination with the GPTS (see "Paleomagnetism" in the "Methods" chapter and "Paleomagnetism" in the "Site U1331" chapter). Although this process is aided by biostratigraphic age constraints, which limit the range of possible correlations with the GPTS (see "Biostratigraphy"), it is the polarity zone pattern and its fit to the GPTS that provided the basis for the match. Once we had identified a possible reversal pattern, data were oriented so that normal and reversed polarity zones had declinations of ~0° and ~180°, respectively.

We measured magnetic susceptibilities and masses of 138 discrete samples. Volumes of samples were recalculated using sediment MAD data (see "Physical properties") and the magnetic susceptibilities were subsequently normalized using recalculated volumes and masses. Of these samples, 20 were stepwise AF demagnetized and measured at 5 mT steps to a peak field of 40 mT and 10 mT steps to 60 mT.

Results

Downhole paleomagnetic data for Holes U1336A and U1336B are presented in Figures F12, F13, F14, F15, and F16. Measurements of NRM above ~80 m CSF in Holes U1336A and U1336B indicate moderate magnetization intensities (~1 × 10^–3 A/m) with a patchy but generally weak VRM or isothermal remanent magnetization IRM drilling overprint (see "Paleomagnetism" in the "Site U1331" chapter). Polarity reversal sequences are clearly recognized in general. Demagnetization data from discrete samples above ~80 m CSF (Fig. F17; Table T14) indicate that the characteristic remanent magnetization (ChRM) of the sediments is identified at the 10–20 mT demagnetization steps. Principal component analysis directions of the ChRM components above ~80 m CSF agree with measurements of coeval intervals from the archive halves (Fig. F12), indicating that the magnetic directions after 20 mT demagnetization provide a reliable indicator of the ChRM of the sediments.

Below ~80 m CSF, a zone of diagenetic alteration involving dissolution of remanence carriers (see "Paleomagnetism" in the "Site U1334" chapter) reduces remanent intensities after 20 mT AF demagnetization to values close to magnetometer noise level in the shipboard environment (~2 × 10^–5 A/m). In this zone, sediment magnetizations have been partly or entirely overprinted during the coring process and remanent inclinations are sometimes steeply negative after AF demagnetization at a peak field of 20 mT. At ~130–140 m CSF (Cores 320-U1336A-15H through 16H and 320-U1336B-15H) and below ~160 m CSF (Cores 320-U1336A-19H through 21H and 320-U1336B-18H through 20H), polarity reversals are apparently present but the inclinations are steep (as much as 80°), indicating that the drilling overprint has not been effectively removed during shipboard demagnetization.

Magnetostratigraphy

In the upper ~80 m CSF, correlation of polarity zones with the GPTS is guided by the biostratigraphic framework of key nannofossil and foraminifer datums from core catcher and additional samples (see "Biostratigraphy") but is based largely on the polarity zone pattern fit to the GPTS. Reversal depths are provided in Table T15 and polarity interpretation is shown in Figure F16. At the top of Holes U1336A and U1336B, polarity assignments are constrained by the tops of nannofossils C. nitescens and C. premacintyrei, which have ages of 12.12 Ma and 12.45 Ma, respectively. We correlate the uppermost reversed polarity zone to the base of Chron C5r of the GPTS (Fig. F16). Cores 320-U1336A-3H and 320-U1336B-4H contain a ~10 m thick normal polarity zone, which we tentatively correlate to normal polarity Chrons C5ACn and C5ADn; Chron C5ACr was not identified. The base of Core 320-U1336A-4H has an age of 15.7 Ma (see "Biostratigraphy"); therefore, we correlate the reversal sequence within Cores 320-U1336A-4H, 320-U1336B-4H, and 5H with Chrons C5ADr and C5Bn (Fig. F16). The top of the polarity zone corresponding to Chron C5Br occurs at the base of Core 320-U1336-4H and top of Core 320-U1336B-5H, with the base of the polarity zone at the base of Core 320-U1336A-5H and at the top of Core 320-U1336B-6H. Below the polarity zone corresponding to Chron C5Br, our correlation with the GPTS is relatively unambiguous through Core 320-U1336A-9H, which contains the upper portions of Chron C6n. Below ~90 m CCSF-A the magnetization intensity of the sediments decreases below analytical noise level; however, below ~160 m CSF a reversal pattern is discernible (Figs. F13, F15) but difficult to correlate with GPTS in the absence of core orientation data.

Top of page   |   Previous   |   Next