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doi:10.2204/iodp.proc.325.102.2011

Paleomagnetism

Magnetic susceptibility

Magnetic susceptibility is an indicator of the strength of the transient magnetism within a sample in the presence of a magnetic field (Nagata, 1961). All mineral grains are “susceptible” to be magnetized when submitted to a magnetic field, and the level of magnetic susceptibility is a function of the concentration and composition of the “magnetizable” materials in a sample. Materials that create an opposite magnetic field (low), which has a negative magnetic susceptibility, are called “diamagnetic,” consisting mainly of organic matter, glass, pure carbonate, and water. Materials that react with (and are slightly attracted to the applied magnetic field are called “paramagnetic.” Examples of these are aluminum, sodium, magnesium, and most of the clay minerals. The important paramagnetic minerals in marine sediments include clay minerals (e.g., illite, chlorite, and smectite), ferromagnesian silicates (e.g., biotite, tourmaline, pyroxene, and amphiboles), iron sulfides (e.g., pyrite and marcasite), iron carbonates (such as siderite and ankerite), and other iron- and magnesium-bearing minerals (Ellwood et al., 1989, 2000, 2007). When a material is able to keep its own magnetization, it is called “ferromagnetic” (e.g., magnetite and hematite).

The study of susceptibility in contrast with rock magnetic analyses provides quantitative and qualitative information about the paramagnetic and ferromagnetic materials in the sediments. Therefore, the presence of cyclic trends in magnetic susceptibility recorded in marine environments can be related to fluxes of detrital sediments resulting from environmental changes attributed ultimately as Milankovitch-driven cyclicity derived from variations in the orbital parameters of the Earth (deMenocal et al., 1991; Weedon et al., 1999; Ellwood et al., 2000, 2007). This can in turn be related to periodical enhancement or reduction of either detrital or eolian origin components, or both, which are transported to the marine environment and distributed by ocean currents (Ellwood et al., 2006). Therefore, this cyclic appearance of magnetic susceptibility due to astronomical causes can be used as a chronological control on geologic timescales (Jovane et al., 2006, 2010). However, the astronomical forcing “footprint” is often superimposed upon other high-frequency local and temporal signals (i.e., tectonic, eustacy, sea level changes, etc.), that also affect the fluxes of magnetic material into the marine environment.

Magnetite and greigite can be produced as intracellular crystals inside of the magnetotactic bacteria. These particular bacteria live in the oxic–anoxic transition zone of waters and sediments (Kopp and Kirschvink, 2008). How this component relates to corals and coral reef systems has never been studied.

Low-field magnetic and mass-specific susceptibility (χ) was measured on minicores, discrete standard IODP paleomagnetic plastic boxes (2 cm × 2 cm × 2 cm), and selected coral samples at the Bremen Core Repository, using a KLY 2 (AGICO) Kappabridge with an operating frequency of 920 Hz and a magnetic induction of 0.4 mT (noise level of 2 × 10^–10 m³/kg). Low-field bulk magnetic susceptibility values obtained have been iteratively checked with MSCL and color reflectance data in order to study their relationship with continuous proxies (e.g., density, volume-specific magnetic susceptibility, and resistivity) and the color of the sediments. Samples were obtained as 2.5 cm (1 inch) minicores drilled perpendicular to the split face of the rock cores. Samples were spaced at irregular intervals of ~1 m in the rubble material sections, with the object of collecting at least one specimen per section depending on sample availability. When material was rubble coral, loose pieces of the corals were collected. For sections that comprised continuous material, discrete minicores were collected with a spacing interval of 10 cm. Low-field magnetic susceptibility data were recorded as corrected mass-specific units (× 10^–8 m³/kg). Additionally, the diamagnetism of sample holders and paleomagnetic boxes (if used) has been removed in relation to the very low susceptibility values of the corals and sediments studied. The accuracy of the Kappabridge was checked using a calibration standard with a bulk susceptibility of 1165 × 10^–6 SI. This calibration piece was run in the Kappabridge every six core sections.

Oriented paleomagnetic samples were recovered, where possible, from all core intervals in which the up–down orientation had been preserved. Unfortunately, some intervals were composed exclusively of coral rubble material, so the azimuthal orientation of each individual core section was often random, and shorter intervals within each core section were obviously rotated relative to each other. As a result, any potential paleomagnetic analysis is limited to measure inclination and relative paleointensity.

Natural remanent magnetization and environmental magnetism

One important aspect of the Expedition 325 program is the recovery of high-quality paleomagnetic data to improve the existing paleointensity stacked curve (Valet et al., 2003, 2005). Results from IODP Expedition 310 demonstrate that these records can be recovered from corals and recent carbonate sediments of coral reefs such as Tahiti (e.g., Lund et al., 2010; Ménabréaz et al., 2010). Therefore, paleomagnetism and magnetostratigraphic studies are important observations needed to fulfill the expedition objectives of obtaining

1. The origin of the magnetic components,

2. A magnetic record (magnetic susceptibility and rock magnetism) useful for correlation between cores,

3. Well-intercalibrated paleointensity records, and

4. Information regarding environmental changes of sediments and corals in relation to sea level changes and climate.

There are some fundamental conjectures in using paleomagnetic directions to resolve the core azimuth of the material recovered. The direction and the inclination of the geomagnetic field in which the characteristic remanent magnetization was acquired must be known and should show a normal inclination. Coral and sediment samples collected during Expedition 325 encompass the time intervals from the present day back to a maximum of >30 calibrated years before present (cal y BP; years before 1950 AD), which is long enough for the Earth’s magnetic field to be modeled satisfactorily by a geomagnetic axial dipole for the timescales of Quaternary to Neogene (McElhinny, 2007). The carbonate sediments collected are recent and unconsolidated; thus, the presence of a potential shallowing of inclination can be excluded. The sample interval resolution is sufficiently long for geomagnetic secular variation to be averaged out, and in principle we know that the bedding is horizontal. Thus, cleaned paleomagnetic data will be characterized by shallow inclinations, consistent with the sites being near the paleoequator (15°–20°S; paleolatitude = ~28° to ~36°), and by normal inclinations.

Magnetic minerals in a carbonate setting are driven by detrital origin relative to environmental changes in the nearby continent (Verosub and Roberts, 1995). However, other contributions must be considered, as magnetic minerals can also be supplied into the system from biogenic or other origins such as (1) magnetotactic bacteria production (Kopp and Kirschvink, 2008), (2) precipitation, (3) dehydration, (4) crystallization as authigenic oxides (e.g., magnetite) and as hydroxides (e.g., goethite and hematite), or (5) chemical or microbial reduction (anoxic basin and fecal pellets) (Jovane et al., 2009).

Paleomagnetic analysis was performed at the University of Bremen. Samples contained in discrete standard IODP paleomagnetic plastic boxes (2 cm × 2 cm × 2 cm) were measured using the pass-through 2G Enterprises automated cryogenic magnetometer (Model 755), with internal diameter of 4.2 cm and equipped with three direct-current superconducting quantum interference device (DC-SQUID) sensors (noise level ≤ 1 × 10^–9 emu). Natural remanent magnetization (NRM) was measured on selected box samples with the highest magnetic susceptibility. Standard stepwise alternating-field (AF) demagnetization was also conducted at steps of 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, and 60 mT to obtain a NRM record of the samples studied. We recognize that only a few samples are characterized by high NRMs, and these appear to be associated with layers of high values of magnetic susceptibility among the different lithological facies. Demagnetization data that were obtained from the AF stepwise method have been examined using orthogonal vector component diagrams (Zijderveld, 1967; Kirschvink, 1980).

The demagnetization method used does not remove the magnetization for the entire core section. Other methods, such as thermal demagnetization experiments, could be used to remove the overprinting that may be related to the presence of high-coercivity magnetic minerals such as hematite and goethite, and so reduce the NRM intensity. However, the overprinting cannot be erased with standard AF demagnetization, and there are still uncertainties regarding how the secondary overprint was acquired and why some samples do not demagnetize at all and others have the potential for demagnetization. The component of any drilling-related overprint that may remain will have a negative effect on both inclination and declination results. However, samples for which data analysis suggests no overprinting, or for which much of the drilling overprint has been removed, will be used to conduct further studies, such as paleointensity experiments.

In order to investigate the environmental magnetic signal carried by the finer magnetic materials in question, we conducted anhysteretic remanent magnetization (ARM). Environmental magnetism provides qualitative and quantitative information of concentration, composition, and grain size of the magnetic particles, which can be directly related to the sedimentary processes or geological systems. The initial approach to study the environmental magnetic signal is to define rock magnetic parameters such as quantity, quality, and dimension of magnetic grains by means of ARM experiments. This is an induced artificial magnetization comprising the demagnetization of 100 mT with a superimposed 50 µT direct-current bias. In relation to the physical properties, ARM experiments magnetically excite only the finer magnetic minerals, and for this reason, when ARM data are normalized for the concentration of the total magnetic minerals, the results are able to provide information of the magnetic grain size.

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