Previous   |   Next

doi:10.2204/iodp.proc.317.105.2011

Paleomagnetism

Paleomagnetic analyses at Site U1353 included routine measurement and partial demagnetization of natural remanent magnetization (NRM) of archive section halves and some discrete samples from the working halves of cores. Rock magnetic experiments were also performed on these discrete samples. All depths in this section are reported in m CSF-A.

Section-half measurements

NRM was measured on archive section halves from Holes U1353A and U1353B before and after alternating-field (AF) demagnetization at 20 mT peak fields. Hole U1353A was short and entirely overlapped by Hole U1353B. Measurements for Hole U1353A were also affected by common flux jumps on the y-axis; therefore, only results from Hole U1353B are presented in Figure F18.

NRM intensities typically vary between 10^–2 and 10^–3 A/m and generally increase with depth, whereas reliable magnetic susceptibility from Whole-Round Multisensor Logger (WRMSL) measurements (loop sensor) ranges from 0 to ~100 instrument units. Several of the highest susceptibility intervals are associated with drilling disturbance and were not measured on the superconducting rock magnetometer (SRM). Drilling disturbance includes cave-in at the tops of cores (similar to Site U1351) and fluidized sands, which retained no original orientation. Two characteristic and relatively thick sandy layers displaying high susceptibility values occur within the uppermost 40 m of Site U1353 and were not measured on the SRM (Fig. F18). The first 10 cores of Hole U1353B (0–67.6 m) were recovered with nonmagnetic core barrels. Declination varies between cores but is consistent, both before and after demagnetization, within individual cores. NRM inclinations for these cores are steep and positive and are inferred to result from an axial drilling overprint without a significant radial overprint (Fig. F19A). AF demagnetization at 20 mT revealed moderately steep negative inclinations and scattered declinations among cores (Fig. F19A), consistent with normal polarity in the southern hemisphere. Cores from Hole U1353B are not azimuthally oriented (due to harsh coring conditions), so declinations could not be corrected.

From Core 317-U1353B-10H (67.6 m) downward, standard (magnetic) steel core barrels were used because of their greater strength. AF demagnetization reduced the intensity of the samples, but declinations are consistently clustered around north and inclinations are steeply positive both before and after demagnetization (Fig. F19B), which is consistent with a pervasive drilling overprint.

Discrete measurements

NRM demagnetization

Stepwise AF demagnetization (to 80 mT) of discrete samples from Hole U1353B revealed a steep component that was removed at or before the 20 mT demagnetization step (Fig. F20). Some samples then show a characteristic component that demagnetizes to the origin (Fig. F20A), whereas demagnetization at 20 mT peak fields reduced the intensities of other samples to the noise limit of the SRM (Fig. F20B–F20C). One sample (~152 m) with relatively high NRM intensity appears to unblock a characteristic downward component after a short, poorly defined, steep component was removed at low fields (Fig. F20D).

IRM acquisition and demagnetization

In order to investigate the magnetic mineralogy of the sediments, a series of isothermal remanent magnetization (IRM) experiments were conducted on the suite of samples for which NRM had been AF demagnetized. IRM acquisition follows a steep curve, with all samples appearing to saturate between 400 and 600 mT applied fields (Fig. F21). Backfield IRM reveals a variable coercivity of remanence within the sample set. Samples from 46.56 and 152.38 m show higher coercivity (~80 mT), whereas the other three are lower (~40–50 mT).

A 1 T IRM was imparted twice more on this sample set, and the samples were demagnetized with alternating fields (stepwise to 80 mT; Fig. F22A–F22B) and then thermally (stepwise to 500°C; Fig. F22C–F22D). Under AF demagnetization, samples from 46.53 and 152.35 m have a convex-up demagnetization profile and higher median destructive fields (MDFs; 55–60 mT) than the other samples, which have concave-up profiles and MDFs of ~35–40 mT. The remanence of all samples significantly drops from 250° to 390°C. Samples from 46.53 and 152.35 m lose >90% of their remanence by 390°C, whereas the remaining samples lose <80% by this point. Magnetic susceptibility was monitored for all samples after heating to >250°C. An increase in susceptibility was seen at 390°C in all but the sample from 152.35 m, and all samples show a considerable increase in susceptibility at 500°C, after which thermal demagnetization was stopped.

These rock magnetic investigations reveal variations in the magnetic behavior of the discrete samples that reflect variations in magnetic mineralogy and/or grain size. Samples from 46.53 and 152.35 m show high magnetic susceptibility, high coercivity, and correspondingly high MDF and lose a large proportion of their remanence between 250° and 390°C. The remaining samples have lower susceptibility, coercivity, and MDF and lose a smaller percentage (50%–60%) of their remanence over the same interval.

The responses of these two groups of samples reflect lithology (Fig. F22E). Samples at 46.53 and 152.35 m are gray mud, whereas the others are green mud/silt. The rock magnetic parameters of all samples are consistent with the presence of variable concentrations of iron sulfides (particularly the significant drop in remanence at ~300°–340°C). The green mud/silt samples, however, retain a higher percentage of their remanence beyond 360°C and have slightly lower coercivities, suggesting the presence of another magnetic fraction (e.g., magnetite), which could be a product of changing depositional environment or postdepositional processes and would, for example, be consistent with more oxic conditions associated with the green sediments.

Magnetostratigraphy

It was not possible to define a magnetostratigraphy at Site U1353. A pervasive drilling overprint exists in all samples recovered with magnetic core barrels, preventing the identification of a characteristic component. Where nonmagnetic barrels were used with the APC system (to Core 317-U1353B-10H [67.6 m]), the removal of an unstable, steep positive overprint reveals a characteristic negative component interpreted to represent the Brunhes Chron. This is consistent with biostratigraphic results, which predict the Brunhes/Matuyama boundary to lie between Samples 317-U1353B-12H-CC and 14X-CC. No evidence for reversed characteristic magnetizations was identified at Site U1353B.

Top of page   |   Previous   |   Next