Influence of sampling on magnetic susceptibility anisotropy of soft sediments: comparison between gravity and piston cores
© Shimono et al.; licensee Springer. 2014
Received: 20 September 2013
Accepted: 27 December 2013
Published: 27 May 2014
Anisotropy of magnetic susceptibility (AMS) has been used extensively for determining mineral orientatrections. In terms of the sample coordinate, Kmax declinations in the three gravity cores are oriented along the core-splitting surface, whereas Kmax declinations in the three piston cores are perpendicular to the splitting surface. We attribute the artificial AMS to the stress created by the deformation of core liners when being split. When interpreting AMS data from sediment cores, it is necessary to investigate the influence of sampling using the sample coordinates. In this paper, we also report over-sampling and under-sampling of piston cores from a comparison of down-core magnetic susceptibility variations between piston and gravity cores. It is noteworthy that under-sampling as well as over-sampling can occur in the uppermost few meters of piston cores.
Anisotropy of magnetic susceptibility (AMS) has been used extensively for determining mineral orientation fabrics (e.g., Hrouda 1982; Tarling and Hrouda 1993). AMS in marine sediments has been applied to a wide variety of studies including paleocurrents in depositional processes (Liu et al. 2001; Abdeldayem et al. 2004; Parés et al. 2007) and paleostresses in accretional prisms (Housen et al. 1996; Ujiie et al. 2003; Kanamatsu et al. 2012). One of the advantages of the AMS method for studying sediment and rock fabrics is that it is sensitive to very weak deformation and that it can be measured rapidly (Tarling and Hrouda 1993).
Because of its high sensitivity, AMS of marine sediments may easily be influenced by artificial deformation during coring and sampling. Copons et al. (1997) reported laboratory-induced magnetic fabric in lake sediment cores taken with a piston corer. They found that the maximum axis of susceptibility (Kmax) is horizontal and parallel to the core-splitting surface and the minimum axis of susceptibility (Kmin) is perpendicular to bedding planes. They suggested that the observed magnetic fabric was created by the compression applied on the core surface when cores were half-split and subsampled using plastic boxes. On the other hand, Gravenor et al. (1984) proposed that when pushing plastic boxes into soft sediments, Kmax directions could be altered to be perpendicular to the core-splitting surface, which is the pushing direction. Aubourg and Oufi (1999) reported coring-induced AMS in sediment cores taken with an Ocean Drilling Program (ODP) Advanced Piston Corer. They found that Kmax declinations were distributed horizontal and parallel to the core-splitting surface and that Kmin declinations were concentrated almost perpendicular to bedding planes but showed a small tilt. They suggested that the piston coring resulted in a weak alignment of magnetic minerals in a conical fabric with the apex parallel to the vertical caused by friction with a core liner. These studies indicate that to improve the reliability of AMS for geological applications, it is necessary to better understand artificial influences at coring and sampling.
In this study, we compare magnetic susceptibility and its anisotropy between gravity and piston cores taken at the same sites. We report for the first time that artificial AMS caused by deformation of sediments is dependent on sampling methods. We also present thickening/thinning of piston cores compared with gravity cores. Previously, comparison of different sampling methods at the same site was rarely conducted partly because of limited ship time (Yamazaki and Kanamatsu 2007).
Piston and gravity coring
When sea-bottom sampling is conducted, an appropriate corer is selected considering the purpose of the sampling. A piston corer can recover a relatively longer sequence of sediments than that collected by a gravity corer. This leads to a negative pressure produced by the piston, which makes it easier for sediments to enter tubes. However, upward acceleration of the piston disturbs the sediment texture and structure. When the motion of a piston is slower than the penetration of the corer at the beginning of the penetration, the uppermost part of the sediment column is missed and/or shorter core recovery than penetration occurs. A gravity corer can recover the uppermost sediments in principle, although it cannot take long cores because of friction.
The depth scale of marine sediments collected by coring is of key importance for the precise calculation of sedimentation rates and fluxes. However, gravity coring may result in sediment under-sampling, while imperfect piston coring (upward acceleration of a piston) may result in over-sampling. Under-sampling leads to sediment thinning and over-sampling leads to sediment thickening compared with the actual length. See Skinner and McCave (2003) and Széréméta et al. (2004) for details. It should be noted that sediment thinning and thickening are not elastic deformations (stretching and squeezing) because the magnetic susceptibility and density of sediments do not change by either over-sampling or under-sampling (Skinner and McCave 2003; Széréméta et al. 2004).
Materials and sampling
Positions, water depths, core lengths, and coring systems of studied sites
Core length (m)
51° 16.4′ N
149° 12.5′ E
51° 16.6′ N
149° 12.6′ E
53° 17.0′ E
150° 04.7′ E
53° 16.9′ E
150° 04.7′ E
54° 19.0′ E
149° 16.1′ E
54° 19.0′ E
149° 16.1′ E
The core liners for the piston cores consist of polycarbonate tubes with a 7.4-cm inner diameter and were split lengthwise into two halves after core recovery. Gravity cores that do not have a piston do not require airtightness, so acrylic-resin semi-cylindrical trays with an inner width of 11 cm attached together with plastic tapes were used for core liners; they are easier to split than whole-round tubes. The core sediments of each 1-m section were split into halves from the bottom to top using a fishing line. All samples for AMS measurements were taken sequentially from the center of the half-split core surface by inserting cubic plastic boxes with a volume of 7 cm3 by hand. The plastic boxes had sharp inward edges to reduce friction at insertion. The cut surfaces of the cores were lightly scraped away to allow visual core description before sampling. Some samples were taken onboard, and others were taken at sampling parties about 1 month after the cruises.
where η1 = ln(Kmax), η2 = ln(Kint), η3 = ln(Kmin), and η m = (η1 + η2 + η3)/3. During deposition and sedimentary compaction, magnetite grains with a shape anisotropy tend to preferentially fall with their Kmin axes perpendicular to the depositional plane (e.g., Tauxe 1998). Consequently, the magnetic fabric expresses an oblate shape (T > 0) when sedimentary fabric is preserved.
To orient the samples to geographic coordinates, we used paleomagnetic directions. Natural remanent magnetization (NRM) measurements with stepwise alternating field (AF) demagnetization were performed using a cryogenic magnetometer system with an in-line static AF demagnetizer (2G Enterprises model 760, Applied Physics Systems, Inc., Mountain View, CA, USA) in a magnetically shielded room of GSJ, AIST. Generally, the maximum angular deviation (Kirschvink 1980) values were less than 5°, which indicates that stable primary magnetization was preserved (Inoue and Yamazaki 2010; Yamazaki et al. 2013). The mean paleomagnetic inclinations of the cores are close to those expected from the geocentric axial dipole model for the respective site latitudes.
In this study, we refer to two coordinate systems: the sample coordinates that are not oriented with respect to the true north and the geographical coordinates oriented using paleomagnetic directions. For the sample coordinates, we use the right-handed sample orientation convention: +X points vertical upward against the split face of the working halves of the cores, +Y points to the right within the core surface, and + Z points down-core.
With regard to geographical coordinates, Kmax declinations were not consistent between the pairs of gravity and piston cores although they were taken at the same sites (Figure 3). For example, Kmax of the gravity core YK0712-GC9 was dominantly oriented to northeast-southwest directions, whereas Kmax of the piston core MR0604-PC5 dominantly aligned north-south. In the sample coordinates, Kmin axes were generally vertical, and Kmax and Kint axes were mostly horizontal. An interesting observation was that Kmax axes of the gravity corers (YK0712-GC1, GC8, and GC9) were dominantly distributed in the Y-axis direction (90° to 270°), whereas Kmax axes of the piston corers (MR0604-PC7, PC6, and PC5) dominantly aligned the X-axis (0° to 180°; Figure 3). These results indicate that the AMS of these cores represent artificial deformation dependent according to the sampling methods.
An alternative possibility for the cause of the artificial AMS is that stress due to half-splitting and scraping of the cut surface of the cores may have caused the alignment of Kmax declination into the Y-axis direction (Copons et al. 1997). For the gravity cores, the influence of the core liner deformation may have been smaller at the center of the cores than that of scraping because of the larger core diameter. For the piston cores, on the other hand, the influence of the core liner deformation may be stronger than that of splitting or scraping because of the smaller core diameter, which may have resulted in the alignment of Kmax declination to the X-axis direction. This explains the observation that the degree of anisotropy (P′) of piston cores is not larger than that of gravity cores.
Parés et al. (1999) classified three stages of fabric development in mud rocks using AMS: earliest deformation, pencil structure, and weak cleavage. The earliest deformation stage is characterized by the alignment of Kmax declination to a particular direction while the sedimentary fabric is preserved. The magnitude of the extensional or compressional stresses associated with the deformation of core liners in this study may be roughly equivalent to that of stresses in the earliest deformation stage of Parés et al. (1999).
The paleomagnetic directions of our cores were not influenced by artificial AMS; the inclinations of the cores were close to those expected from the geocentric axial dipole model (Inoue and Yamazaki 2010). Rosenbaum et al. (2000) reported that artificial AMS is accompanied by distorted paleomagnetic directions, but a considerably large degree of anisotropy (P′) of their sediments indicated that the deformation was much more severe.
In conclusion, when interpreting AMS data from sediment cores, it is necessary to examine the influence of sampling using sample coordinates. AMS of sediment cores may not necessarily reflect sedimentary environments such as paleocurrent directions or tectonic stress directions. The composition and performance of core liners should be carefully examined to avoid deformation of sediments. Our comparison of down-core magnetic susceptibility variations between the piston and gravity cores revealed over-sampling and under-sampling of the piston cores. It is noteworthy that under-sampling can occur in the uppermost few meters of piston cores; Skinner and McCave (2003) mentioned a possibility of only over-sampling for a piston core. We believe that under-sampling of our piston cores was due to delayed working of the piston at the sediment/water interface.
The cores were obtained through the cooperation of all onboard scientists, marine technicians, officers, and crew of the MR06-04 cruise of R/V Mirai and YK07-12 cruise of R/V Yokosuka. We thank Naomi Harada and Tatsuhiko Sakamoto, the chief scientists of the cruises, for arranging our participation in the project. We thank Yujiro Ogawa and Akira Takada for their encouragement. Constructive comments from two anonymous reviewers greatly helped improve the manuscript.
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