PALEOMAGNETISM

Paleomagnetic studies conducted aboard the JOIDES Resolution during Leg 210 consisted of routine measurements of natural remanent magnetization (NRM) and magnetic susceptibility of sediments and rocks. NRM was measured on all archive halves of recovered sediments and rocks and on discrete samples taken from the working halves. Stepwise alternating-field (AF) demagnetization was conducted on all archive halves and on some discrete samples in an attempt to isolate stable components of remanence. A few discrete samples were thermally demagnetized in an effort to obtain their primary remanent magnetization and to identify magnetic carriers. Magnetic susceptibility was measured on archive halves and, in a few cases, on discrete samples.

Laboratory Instruments

The remanence of archive halves and oriented discrete samples from working halves was measured using a 2G Enterprises pass-through cryogenic direct-current superconducting quantum-interference-device rock magnetometer (model 760R). This magnetometer is equipped with an in-line AF demagnetizer (2G model 2G600) that allows for demagnetization of samples up to peak fields of 80 mT with a 200 Hz frequency. The practical limit on the resolution of natural remanence of core samples is imposed by the magnetization of the core liner itself (~0.01 mA/m). The magnetometer and AF demagnetizer are interfaced to a computer and are controlled by the 2G Long Core software by National Instruments. A Molspin spinner magnetometer was also available on the ship for measuring the remanence of discrete samples. For stepwise demagnetization of discrete samples, the laboratory contains an AF demagnetizer (model D-2000 by DTech Inc.) and a thermal demagnetizer (model TSD-1 by Schonstedt Instrument Co.) capable of demagnetizing specimens to 200 mT and 700°C, respectively. An Analytical Services Company model IM-10 impulse magnetizer (capable of pulsed fields from 0.02 to 1.35 T) and a PARM-2 system by DTech Inc. were available for anhysteretic and isothermal remanent magnetization (IRM) acquisition studies of discrete samples. A Geofyzika Brno Kappabridge KLY-2 magnetic susceptibility meter was available for magnetic susceptibility measurements of discrete samples.

Core Orientation and Sampling Method

The standard ODP core orientation convention was applied for paleomagnetic work during Leg 210. This convention is described as follows. The positive z-axis direction is downhole, parallel to the core. The x-axis forms a line perpendicular to the split face of the core and is directed into the working half (Fig. F12). The positive x-axis direction is used as the reference "geomagnetic north" (also "north" in the structural geologists' reference frame) for the definition of magnetic declination values. The positive y-axis direction is left along the split surface of the archive half when looking upcore (see Fig. F12).

Discrete samples of the softer sediments were taken using oriented standard plastic boxes (7 cm3), with an arrow pointing in the uphole direction (Fig. F12). In order to reduce the deformation of the sediment, the core was cut using a thin stainless steel spatula before pressing the plastic boxes into the sediment. Minicores (10 cm3) were drilled from lithified sedimentary and crystalline rocks using a water-cooled nonmagnetic drill bit attached to a standard drill press. Minicores were oriented as for the samples just described, with an arrow pointing in the uphole direction. Discrete samples used in pilot demagnetization studies were typically taken in increments of one or two per section. However, discrete samples were taken at smaller increments (as little as every 10 cm) to examine geomagnetic reversals, key geologic boundaries, or other intervals of interest.

Remanent Magnetization Measurements

Tests conducted during previous legs (e.g., Leg 131) indicate that in cases where the core is not uniformly magnetized, either because of natural processes or artifacts (voids in the core or differential rotation of segments within the core liner [biscuiting]), the values of declination, inclination, and intensity need careful evaluation. For this reason, at least one discrete shipboard paleomagnetic sample was taken from each section and from each representative lithology for progressive AF and/or thermal demagnetization to characterize the magnetic behavior.

The NRM and remanence after progressive AF demagnetization were routinely measured on all archive halves at 2-cm intervals. For shipboard analyses, measurements within 10 cm of the ends of each section were disregarded because of end effects. Progressive AF demagnetization steps, incremented by 10 mT, were typically 10, 20, 30, 40, 50, and 60 mT, as required, to remove drilling overprints and to resolve the characteristic remanent magnetization. To isolate stable remanence, stepwise thermal demagnetization up to 580°C was also applied to some discrete samples using the Schonstedt thermal specimen demagnetizer. The magnetic susceptibility of the samples was monitored between each temperature step to assess any irreversible mineralogical changes associated with heating.

Perturbations by Drilling

Mechanical disturbance of unconsolidated sediments can occur during rotary coring, which complicates the interpretation of measurements made by the pass-through cryogenic magnetometer. Another persistent problem is the pervasive remagnetization associated with drilling. As noted during previous legs, discrete samples taken from the center of the working half of the core tend to show shallower inclinations. This indicates that the intensity of the drilling-induced overprint increases radially from the center to the edge of the core, and it suggests that the core barrel is the most significant source of the overprint. Isolation of the primary component of magnetization in the discrete samples generally required AF demagnetization of 20 to 30 mT. Thus, the drilling-induced remagnetization can be considered as a viscous remanence, or it may be comparable to a strong-field (>20 mT) IRM.

Starting with Core 210-1276A-59R, a nonmagnetic RCB was used for all odd-numbered cores, whereas even-numbered cores were recovered with a regular RCB. Sedimentary rocks from Cores 210-1276A-58R through 68R in Unit 5 (see "Lithostratigraphy" in the "Site 1276" chapter) have uniform lithology and composition, and these sediments all have a single normal polarity. Thus, we could test the effects of the regular vs. nonmagnetic RCBs on remanence intensity of these cores. The results clearly indicate that cores obtained with a regular core barrel are more strongly magnetized than those obtained with the nonmagnetic core barrel.

Magnetostratigraphy

Where magnetic cleaning successfully isolated the primary component of remanence, paleomagnetic inclinations were used to assign a magnetic polarity to the stratigraphic column. The revised polarity timescale of Cande and Kent (1995), as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons. The timescale of Gradstein et al. (1995) was used for Mesozoic rocks and sediments (Table T6).

Magnetic Susceptibility Measurements and the Königsberger Ratio

Magnetic susceptibility was measured for each whole-core section as part of the MST analysis (see "Multisensor Track Measurements"). Susceptibility was measured on the MST using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 88 cm operating at 0.565 kHz. The sensor was set on SI units, and the data were stored in the Janus database in raw meter units. The sensor coil is sensitive over an interval of ~4 cm (half-power width of the response curve), and the width of the sensing region corresponds to a volume of 166 cm3 of cored material. To convert to true SI volume susceptibilities, these values should be multiplied by 10–5 and then multiplied by a correction factor to account for the actual volume of material that passed through the susceptibility coils. Except for measurements near the ends of each section, the correction factor for a standard full ODP core is ~0.66 (= 1/1.5). The end effect of each core section is not adequately corrected using this procedure.

The magnetic susceptibility of archive halves was also routinely measured using the AMST at 2-cm intervals. The measurements were automatically recorded by the AMST, which permits measurements only at evenly spaced intervals along each section of core. For the two types of susceptibility measurements (MST and AMST), the same type of magnetic susceptibility meter (Bartington Instruments model MS2) was used but with a different sensor.

MST susceptibility measurements made on the whole core are directly comparable with the AMST measurements of archive halves, correcting for the final curated positions and placement of spacers. The susceptibility response is a function of the mineralogy as well as the shape and volume of the magnetic particles within the rocks. Susceptibility provides an indication of the amount of magnetic material in the core and was used to consider locations for taking discrete samples for further paleomagnetic measurements. The volume susceptibility was used in conjunction with the NRM intensity to calculate the Königsberger ratio (Q, the ratio of remanent to induced magnetization) of the samples. The International Geomagnetic Reference Field value at the Leg 210 sites (49,412 nT = 39.59 A/m) was used for calculating Q.

Q = NRM [A/m] /(k [SI] x H [A/m]),

where H is the local geomagnetic field and k is the susceptibility. In general, the Königsberger ratio is used as a measure of stability to indicate a rock's capability of maintaining a stable remanence. At Leg 210 sites, the Königsberger ratio is a useful parameter to evaluate how much the effective magnetization that results from the susceptibility of the rocks contributes to the regional magnetic anomalies; the Königsberger ratio is a linear function of NRM/induced magnetization.

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