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doi:10.2204/iodp.proc.347.102.2015

Paleomagnetism

The main objectives of the OSP paleomagnetic work were to (1) establish a magnetic susceptibility profile at each site based on discrete samples of known volume and mass; (2) establish, if possible, site-specific paleomagnetic secular variation (PSV) profiles that could be compared for relative dating purposes to regional Holocene master curves based on multiple lake varve chronologies (Snowball et al., 2007) and a late glacial master curve (Lougheed, 2013; Lougheed, et al., in press); and (3) detect geomagnetic field excursions that took place during the latest glacial cycle (reviewed by Laj and Channel, 2007; Singer, 2014). Some background information about the parameters measured and the reason for establishing them is provided below.

Fundamental magnetic properties, magnetic susceptibility, and natural remanent magnetization

Depending on their composition, materials can display one or more of three fundamental responses to the application of an external magnetic field. The weakest response, which is inherent to all materials, is actually negative (i.e., the field induced in the sample has a polarity that opposes that of the applied field). This property is known as “diamagnetism” and is the only response exhibited by, for example, water, organic compounds, most plastics, and some minerals, such as pure quartz and calcite. Collectively, these are called “diamagnets.” A stronger, positive linear response (“paramagnetism”) is exhibited by minerals that are “paramagnetic,” and mineral examples include many members of the iron oxide phase system (e.g., lepidocrocite), the iron sulfide phase system (e.g., pyrite and marcasite), ferromagnesian minerals (e.g., biotite and pyroxene), and iron carbonates (e.g., siderite). Another example of a paramagnet is the hydrated iron phosphate mineral vivianite, which is commonly found as an authigenic phase in organic-rich, reduced freshwater lake sediments.

Pure iron is “ferromagnetic,” and ferromagnetism is the strongest type of positive response to an applied magnetic field. Similar ionic ordering gives rise to “ferrimagnetism” and “antiferromagnetism,” which are important to paleomagnetism because of the capability of “ferromagnets” (e.g., pure iron) and “ferrimagnetic” and “canted anti-ferromagnets” to retain a memory of a magnetic field that they were once exposed to. This memory is called “magnetic remanence.” Natural minerals that behave as “ferrimagnets” at temperatures and pressures normally experienced close to the Earth’s surface include magnetite, titanomagnetite, maghemite, monoclinic pyrrhotite, and greigite.

Magnetic susceptibility is a parameter that defines how easily a material can be magnetized. Geologists routinely measure this parameter at room temperature and pressure, although temperature-dependent measurements may detect changes in crystallographic structure, which causes magnetic transitions and can be diagnostic of specific minerals (e.g., the Verwey transition in magnetite). Magnetic susceptibility is a dimensionless ratio between the intensity of magnetization (M) induced in a sample by an externally applied field (H) of known intensity (i.e., M/H); this ratio can span many orders of magnitude, both positive and negative. If the magnetic susceptibility is expressed on a volumetric basis the volume magnetic susceptibility (κ) is obtained. Magnetic susceptibility may also be expressed per unit mass, as χ, and the SI units are cubic meters per kilogram (m³/kg).

Mineral ferrimagnets have susceptibilities that are many orders of magnitude higher than paramagnets and diamagnets. Thus, even if naturally occurring ferrimagnetic minerals account for only a few parts per thousand of a sample, they can dominate the magnetic susceptibility and determine the ability of it to acquire and carry a natural remanent magnetization (NRM).

In natural sediments, NRM is commonly acquired in two fundamental ways. One way is through the acquisition of a (post-)depositional remanent magnetization (pDRM). pDRMs require that mineral grains fall out of a calm fluid suspension and that the Earth’s ambient magnetic field exerts a torque on them and they align along the direction of the field. They are subsequently buried by nonmagnetic grains and are locked into position close to or just below the sediment/water interface and at a depth that can be dependent on the degree of consolidation and bioturbation. pDRMs are frequently carried by primary ferrimagnetic and antiferromagnetic iron oxides that originate from continental erosion. A second way is through the precipitation of secondary authigenic and diagenetic minerals that are capable of acquiring magnetic remanence when the crystals grow and pass through the superparamagnetic to single-domain grain size threshold. This threshold is mineral specific but is often exceeded in the submicrometer grain size window.

A large range of lithologies was encountered during the offshore phase of the expedition. Processes that lead to the production and transport of clastic magnetic minerals to sedimentary basins in the Baltic Sea include the glacial erosion of the Fennoscandian Shield, which is a varied provenance consisting of predominantly igneous and metamorphic rocks, postglacial weathering, pedogenesis, and isostatic land uplift (the present rate at Sites M0061 and M0062 is ~1 cm/y). Primary productivity and the subsequent degradation of organic matter can cause the reductive dissolution of clastic iron oxides and the diagenesis/authigenesis of iron sulfides, and these processes can decrease the concentration of magnetic minerals or enhance them, respectively. For example, Sohlenius (1996) shows that authigenic greigite formed through the downward diffusion of sulfide from relatively organic rich sediments deposited during the Littorina Sea stage of the Baltic Sea into underlying clays that characterize the Anclyus Lake (Sohlenius, 1996). Reinholdsson et al. (2013) subsequently discovered that laminated sapropels, which formed during the Littorina Sea stage, were magnetically enhanced because of the presence of magnetosomal greigite, which are single-domain grains produced by magnetotactic bacteria (MTB) in the water column, just under the anoxic–oxic transition zone (AOTZ).

Paleomagnetic sampling and measurements

OSP samples for magnetic susceptibility and measurement of the direction and strength of the NRM were obtained using standard plastic IODP paleomagnetic cubic boxes (external dimensions of 2 cm × 2 cm × 2 cm and an internal volume of 7.6 cm³). At Site M0066, additional minicubes were also utilized, measuring 1 cm × 1 cm × 1 cm. The sampling interval (resolution) was restricted by the OSP duration and other sampling programs. With the exception of Sites M0059, M0060, and M0063, the samples were taken from the sections that contributed to the composite splices for each site and at intervals of ~0.5 m, with small adjustments made for voids and sediment disturbances. At Site M0059 samples were taken from all core sections that included a sampling point. Samples within each core section were oriented with respect to each other. At Site M0060, there was only one paleoenvironmental hole drilled and so samples were taken from these cores. At Site M0063, the formation of a composite splice was not possible because of the expanding nature of the gaseous sediments and lack of significant MSCL tie points (see “Stratigraphic correlation”). Therefore, the OSP sampling scheme was targeted toward the holes where the majority of shipboard and postcruise samples were taken to better aid later research. Additional higher resolution sampling, up to every 5 cm, was performed in areas of specific interest.

Magnetic susceptibility measurements and wet density

Magnetic susceptibility was measured using a KLY 2 (AGICO) Kappabridge that operates at a frequency of 920 Hz and a magnetic induction of 0.4 mT (equivalent to a field intensity of 300 A/m), with a noise level of 2 × 10^–10 m³/kg. The Kappabridge was calibrated using a standard with a bulk susceptibility of 1165 × 10^–6 SI, and this procedure was repeated every morning before measurements began and after approximately every 50 samples.

The wet weight of the samples was established with a DISPE TP-200 electronic balance with a precision of 1 × 10^–5 kg. The weight of the paleomagnetic sample cubes and their diamagnetic contribution to magnetic susceptibility was established by measuring five empty cubes. The sample measurements were then corrected according to the respective averages. The volumetric samples and masses were used to make an additional estimate of bulk density that could be cross-checked with the continuous MSCL data.

NRM measurements and alternating field demagnetization

The NRM direction and intensity of discrete samples were measured using the 2G-Enterprises horizontal pass-through super-conducting rock magnetometer (SRM 755–4000) that was made available for the OSP by the University of Bremen. The standard IODP sample cubes used during the OSP were measured in batches of eight or less using the pass-through conveyor.

A series of at least 16 pilot samples was selected from each site to cover the full range of magnetic susceptibility. After measurement of NRM, the samples were sequentially demagnetized by alternating fields (AF) at the following steps: 5, 10, 15, 20, 30, 40, 50, 60, and 80 mT. The magnetic remanence measured between each step. Orthogonal plots and visualization of the demagnetization spectra were produced in PuffinPlot (Lurcock and Wilson, 2012).

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