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doi:10.2204/iodp.proc.308.102.2006

Paleomagnetism

Magnetic measurements

Paleomagnetic and rock magnetic measurements during Expedition 308 included natural remanent magnetization (NRM) and volume magnetic susceptibility (κ), both performed on continuous archive-half core sections.

NRM was measured on board at 5 cm intervals using an automated pass-through cryogenic direct-current (DC) superconducting quantum interference device (SQUID) rock magnetometer (2G Enterprises model 760-R) with an in-line alternating-field (AF) demagnetizer (model 2G600). The archive-half sections were demagnetized in 0, 10, 20, and, in some cases, 30 mT AF field increments (for details see “Paleomagnetism” in the site chapters). The pass-through cryogenic magnetometer and AF demagnetizer are controlled by 2G LongCore Labview software (version 207.3) (Shipboard Scientific Party, 2004).

NRM intensity, declination, direction, and inclination were measured. For the latter two parameters, a core orientation correction was applied using the digital multishot tool (Tensor, Inc.), which is rigidly mounted onto a nonmagnetic sinker bar during APC coring. The Tensor tool consists of three mutually perpendicular magnetic field sensors and two perpendicular gravity sensors. The information from both sets of sensors allows the azimuth and dip of the hole to be measured as well as the azimuth of the APC core double orientation line. The IODP core orientation scheme designates the positive x-axis direction as the horizontal (in situ) line radiating from the center of the core through the space between the double line scribed lengthwise on the working half of each core liner.

Volume magnetic susceptibility measurements were carried out at 1–6 cm resolution on whole-core sections as part of the MST analyses (see “Physical properties”). On the archive-half sections, the AMST analyzed color reflectance at 10 cm resolution (see “Lithostratigraphy”) and additional point magnetic susceptibility at 5–10 cm resolution using a Bartington MS2F sensor at a sensitivity setting of 0.1 instrument units. The instrument automatically zeroes and records a free-air value for the magnetic susceptibility at the start and end of each section run. Instrument drift during a section run is then corrected by subtraction of a linear interpolation between the first and last free-air measurements (Shipboard Scientific Party, 2003).

Magnetostratigraphy

Classical magnetostratigraphy using field reversal correlations (e.g., Cande and Kent, 1992) could not be used to establish an age model because the Brunhes/​Matuyama Chron boundary was not recovered due to high sedimentation rates.

Instead, we used high-resolution variations of the Earth's magnetic field such as changes in declination and inclination. High-resolution rock magnetic records such as NRM intensity and magnetic susceptibility were potentially helpful tools for multiparameter correlations. Such rock magnetic records are capable of exhibiting Milankovitch cyclicities, sometimes in close agreement with δ¹⁸O profiles. This correlation is generally explained by climatic impact on fluxes of magnetically enriched terrigenous and nonmagnetic biogenic or siliciclastic sedimentary components. Paleoclimatic signatures of magnetic susceptibility and various remanence parameters (such as NRM) are increasingly employed for high-resolution core correlation and have been described and used as stratigraphic correlation tools in paleoceanographic studies from all major oceans (e.g., Radhakrishnamurty et al., 1968; Kent, 1982; Robinson, 1986; Bloemendahl et al., 1988; deMenocal et al., 1991; Bickert et al., 1997; von Dobeneck and Schmieder, 1999). Frederichs et al. (1999) outlined the physical and sedimentological principles of rock magnetic climate proxies.

For marine sediments, κ may vary from an absolute minimum of –15 × 10^–6 SI (diamagnetic minerals such as pure carbonates or silicates) to a maximum of ~10.000 × 10^–6 SI for basaltic debris rich in (titano)magnetite. In most cases, κ is primarily determined by the concentration of ferrimagnetic minerals, whereas paramagnetic components such as clays are of minor importance. Enhanced susceptibilties indicate higher concentrations of lithogenic or authigenic ferrimagnetic components. This relation may serve for correlating sedimentary sequences deposited under similar global or regional conditions.

NRM intensity and κ were further compared with the sediment bulk density to perform multiparameter correlation of the rock magnetic shipboard data. Using these multiple logs is suitable to establish a regional stratigraphic correlation and tentatively derive preliminary age estimates. Correlations are attempted by comparing the individual peaks and troughs of the records and constraining the tie points. These magnetostratigraphic tie points (referred to as “MTP”) were then, if possible, correlated to the δ¹⁸O record published by Bassinot et al. (1994). This provided the basis to establish a rough initial chronology and detect Milankovitch cyclicity.

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