Detection of noninteracting single domain particles using first‐order reversal curve diagrams

Abstract

[1] We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the H c axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

Enhanced primary productivity and magnetotactic bacterial production in response to middle Eocene warming in the Neo-Tethys Ocean

“…The coercivity distribution has a broad peak between 3 and 25 mT, with a maximum at 15-20 mT (Fig. 7) that falls within the range expected for magnetite magnetosomes (Egli, 2004;Kopp and Kirschvink, 2008;Egli et al, 2010). Outside the 63.2-65.5 msl interval, the magnetic signal of the samples is much weaker and no meaningful FORC distribution could be obtained from these samples (Fig.…”

“…7) at H b = 0 that indicate negligible magnetic interactions and a dominance of non-interacting SD particles (Roberts et al, 2000) that is characteristic of intact magnetosome chains (e.g. Egli et al, 2010;Roberts et al, 2011;Jovane et al, 2012;Larrasoaña et al, 2012;Roberts et al, 2012;Yamazaki, 2012;Yamazaki and Ikehara, 2012). The coercivity distribution has a broad peak between 3 and 25 mT, with a maximum at 15-20 mT (Fig.…”

“…For samples with natural remanent magnetization above 10 − 6 Am 2 /kg we performed only one FORC run using a vibrating sample magnetometer (Princeton Measurements Corp., PMC) at NOCS, whereas for weak samples we performed multiple runs with a PMC alternating gradient magnetometer to increase the signal-noise ratio of the FORC distribution . FORC diagrams derived from single runs were calculated using the MATLAB routine of Egli et al (2010). FORC diagrams obtained with multiple runs were stacked and calculated using the MATLAB routine of Heslop and Roberts (2012).…”

“…Hysteresis loops and first-order reversal curve (FORC) diagrams were analyzed for eight samples from the MCA section spanning the MECO interval and from intervals immediately before and after the warming event. FORCs were measured with an averaging time of 200 ms, a smoothing factor (SF) of 4, and using the input parameters of Egli et al (2010) (H c1 = 0 mT, H c2 = 110 mT; H u1 = −15 mT, H u2 = +15 mT; δH = 0.63 mT). For samples with natural remanent magnetization above 10 − 6 Am 2 /kg we performed only one FORC run using a vibrating sample magnetometer (Princeton Measurements Corp., PMC) at NOCS, whereas for weak samples we performed multiple runs with a PMC alternating gradient magnetometer to increase the signal-noise ratio of the FORC distribution .…”

“…High-resolution FORC measurements following the specifications of Egli et al (2010) can be used to infer the presence of biogenic magnetite (e.g. Egli et al, 2010;Roberts et al, 2011Roberts et al, , 2012Jovane et al, 2012;Larrasoaña et al, 2012;Yamazaki, 2012;Yamazaki and Ikehara, 2012). FORC distributions are nearly identical in the high-magnetization intervals between 63.2 and 65.5 msl (Figs.…”