1

QuarterlyThe IRM

The observation While measuring susceptibility versus temperature

(χ-T) curves upon heating above room temperature (RT), a peak is sometimes observed between ~150 and 350 °C. The shape and intensity of the χ-T peak can vary substan-tially. Most often it takes the form of a gradual increase which then somewhat levels out before dropping at the high temperature side, creating an asymmetric plateau (or pedeplain, to be more exact, Fig.1a, modified after Kontny and Grothaus (2017)). However, the peak may also be more subtle and rounded (Fig.1b), and sometimes have a smaller tip within that, yet maintaining the fundamental characteristic increase at ~150 and decrease at ~350 °C. Notably, the feature only appears, or is most prominent, in weathered samples (e.g. Kontny and Grothaus, 2017).

This behavior is accompanied by other rock-magnetic observations: in saturation remanent magnetization versus temperature (MRS-T) curves decreases in magnetization occur at the corresponding temperatures (Van Velzen and Zijderveld, 1995), whereas hysteresis properties measured at different temperatures reveal a decrease in coercivity (BC, BCR) upon heating (Van Velzen and Dekkers, 1999), which is accompanied by an increase in high-field slope while MS remains unchanged.

The details At first glance, when interpreting the susceptibility

curves without any other knowledge of the sample nor other rock-magnetic measurements, the peak can be eas-ily attributed to production of new magnetic minerals with higher susceptibility (magnetite) associated with alteration of the sample. However, this behavior has been observed in very different rock-types, from igneous to sedimentary and even soils, and in conjunction with other rock-magnetic and microscopic observations.

Production of new magnetic minerals does not necessarily explain the reduction of coercivity. Infact, for specimens that possess a large ~150 °C increase in susceptibility, hysteresis measurements do not show significant change in saturation magnetization over the same temperature range, yet they show a large drop in coercivity and an increase in low-field slope that matches the ac susceptibility pattern (Fig. 2). Significantly, the same phenomenon is sometimes observed in pure (but

Spring 2020, Vol. 30 No.1

Dario Bilardello, IRM

cont’d. on pg. 15...

Practical Magnetism II: Humps and a Bump, the Maghemite Song

ISSN: 2152-1972

Inside...Visiting Fellows Reports 2Current Articles 9Note from the IRM 17

partially oxidized) magnetite samples. These observations reasonably demonstrate that it is

not the generation of new magnetite that is responsible for the increase at 150 °C, as would be logical to infer, for example, in samples containing abundant clay min-erals or organic matter (OM), but instead is related to low-temperature oxidation, with formation of core-shell structures, and their behavior upon heating during the χ-T experiments. I further note that these structures are also abundant in many altered soils and paleosols containing clays and/or OM (more below).

The explanation Low-temperature (LT) oxidation of magnetite (Fe3O4)

occurs during weathering at atmospheric conditions, typically below 50 °C. Upon oxidation the composition gradually becomes that of maghemite (γ-Fe2O3), the fully oxidized endmember phase (e.g. O’Reilly, 1984; White et al., 1994).

The oxidation process starts at the surface of the grain where Fe2+ is oxidized to Fe3+ by either addition of oxygen or loss of Fe2+. Oxidation continues as a solid-state diffusion process that is driven by the oxidation gradient, leading Fe2+ to diffuse from the interior of the grain to the surface and leaving behind a vacancy in its place. The process is temperature dependent and is very slow at room temperature (Askill, 1970). The end result is a strong oxidation gradient close to the grain surface, leading to the formation of an oxidized maghemite shell around an unoxidized magnetite core (O’Reilly, 1984). Further liberation of core Fe2+ ions to the surface or to the

Fig. 1. Increase in susceptibility between ~150 and 350 °C for an Andesite specimen (a) and a Trachydacite (b) modified after Kontny and Grothaus (2017). T

1 and T

2 mark dercreases in

susceptibility corresponding to the inversion of maghemite and magnetite’s Curie temperature, respectively, while T

V is magnetite’s Verwey transition.

Temperature (°C) Temperature (°C)

a) b)

2

A B

Anorthosite

DiabaseDiabase

20 μm

D Anorthosite

px

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an

Fe-Ti oxide

30 μm

E Anorthosite

an

an

mag-ilm

20 μm

F Diabase

px

ilm

mag

C

1 mm

Visiting Fellow’sReports

Pairing paleointensity results with co-ercivity spectra: providing support for selection criteria

Yiming Zhang UC Berkeley yimingzhang@berkeley.edu

A few days into the new year of 2020, I arrived in Twin Cities, Minnesota for the first time, with an exciting rock magnetic project to be conducted at the Institute for Rock Magnetism. My research was focused on developing paleomagnetic data from the 1.1 Ga Beaver River diabase and its anorthosite xenoliths of the Beaver Bay Complex from the North Shore of Lake Superior, Minnesota. Before this trip, I had been developing directional and intensity data at the paleomagnetism lab at UC Berkeley. With the help of rock magnetic experiments at the IRM, I was hoping to decipher the cause of a variety of paleointensity results of my samples. Retrieving paleomagnetic information from Precam-brian rocks can aid in our understanding of the long-term evolution of the geodynamo. A recent paleointensity study

Fig. 1: (A) A typical outcrop of an anorthosite xenolith residing in the Beaver River diabase. (B) A hand sample of anorthosite. The large reflective face at the bottom center is a cleavage plane of a centimeter-size plagioclase crystal. The scale is 9 cm in total. (C) Cross-polarized light image of an anorthosite thin section. The second order birefringence of plagioclase indicates a high Ca content. The closely packed plagioclase crystals show recrystallization texture. (D) High-magnification SEM image of Fe-Ti oxides exsolved from a pyroxene crystal included in a plagioclase crystal. (E) Large, interstitial magnetite-ilmenite intergrowths between two plagioclase crystals. (F) A large Fe-Ti oxide grain with magnetite-ilmenite intergrowths next to a pyroxene in diabase. an: anorthite; px: pyroxene; mag: magnetite; ilm: ilmenite.

by Sprain et al. (2018) reported a ca. 1.1 Ga paleomagnetic field strength similar to today based on the volcanics of the Midcontinent Rift System (MRS). That study suggested that this result is consistent with models wherein this time period of the Proterozoic is characterized by a strong geo-magnetic field. However, to further evaluate the evolution of the geomagnetic field during the Precambrian, more paleointensity data are needed. The hypabyssal intrusions of the ophitic Beaver River diabase (BRD) and the Ca-rich anorthosite xenoliths that it hosts are the targets of this study. The diabase contains abundant Fe-Ti oxides which are dominant carriers of paleomagnetic information. The nearly pure anorthosite xenoliths have potential for paleomagnetic investigation because plagioclase hosts can protect the exsolved mag-netic minerals from post-formation alteration (Tarduno and Cottrell, 2005), and thus may faithfully record paleo-intensity information at the time of formation. A typical outcrop of anorthosite xenolith residing in the diabase and a hand sample of the anorthosite are shown in Fig. 1(A, B). Preliminary petrographic analyses show that the anorthosites are dominantly monomineralic with recrys-tallization textures (Fig. 1C). Crosscutting relationships together with high-precision geochronology dates from Swanson-Hysell et al. (2019) bracket the age of the BRD to be between 1091.61 ± 0.14 Ma (the age of the younger Silver Bay intrusions) and 1093.94 ± 0.28 Ma (the age of the Palisade rhyolite). Most thermally demagnetized anorthosites and alter-nating field (AF) demagnetized diabase specimens have minimal secondary components and their magnetization is interpreted to be dominated by a primary thermal remanent magnetization. Characteristic magnetizations

3

Fig. 2: (A) Site-mean directions of anorthosite and diabase plotted on an equal-area plot. (B) Calculated VGPs from the anorthosite and diabase plotted in context of a previously synthesized ca. 1.1 Ga Laurentia APWP from the Midcontinent Rift volcanics (Swanson-Hysell et al., 2019). VGPs from both lithologies fall close to the 1095 Ma pole position. (C) Example Arai plots for diabase. Most diabase and many anorthosite specimens were rejected by selection criteria due to the zigzagging behavior shown in the diabase example plot. (D) Example Arai plots for anorthosite. A typical anorthosite specimen that passes selection shows a straight Arai plot with a high paleointensity estimate before cooling rate correction. Note both specimens in (C) and (D) show dominantly single component magnetization in the inset orthogonal plots. The estimated field intensity is not cooling rate corrected.

from both lithologies yield indistinguishable site-mean directions and the virtual geomagnetic poles (VGPs) fall close to the 1095 Ma pole from the synthesized APWP of Swanson-Hysell et al. (2019), which is in agreement with the geochronological constraints (Fig. 2). I conducted a comparative study of modified IZZI paleo-intensity experiments on both the diabase and anorthosite, with a group treated with low-field AF demagnetization after each in-field step and another group without. Paleo-intensity experiments for most diabase specimens result in non-ideal Arai plots often with poor pTRM checks, making them difficult to interpret for paleointensity es-timates (Fig. 2). The experiments conducted on anortho-site samples, on the other hand, had a high success rate of > 50%. Moreover, the resulting success rate for the anorthosite group with the AF treatment is even higher. The straighter Arai plots are likely due to the AF steps mitigating the exhibition of pTRM tails often associated with non-ideal behavior of multi-domain grains that are

demagnetized by the pre-treatment step. However, as the nearly pure anorthosite samples did not display dramatic alterations that are easily detectable with common petro-graphic techniques, a question rises as why some of the compositionally similar anorthosites passed the paleoin-tensity selection results and some did not. At the IRM, I was seeking the answer to this ques-tion with the help of the vibrating sample magnetom-eter (VSM) systems, including a newly installed Lake Shore VSM which greatly helped improve measurement resolution on samples with weak magnetizations (Fig. 3). Backfield demagnetization experiments were conducted and used to develop coercivity spectra (Fig. 3). The spec-tra were subsequently modeled to fit for the distributions of different populations of magnetic particles using a similar procedure as Maxbauer et al. (2016). Most spectra can be well approximated with models with one or two components with overlapping coercivity ranges (Fig. 3). The dominance of single-component distributions from

synthesized volcanic APWP

poles from this study

Common Mean Test Angle Critical Angle Result 4.8 < 10.0 Pass

A B

D

530

550

565

585

300

100

FailedZigzagging

No �t

Diabase 100

425

500

530

540

585

550

Anorthosite

Banc = 77.5 μT

Pass

C

4

A

B

accepted anorthosite sites

rejected anorthosite sites

diabase sites

single crystals

C

Princeton VSM

Lake Shore VSM

the unmixed coercivity spectra likely suggests minimal alterations or formation of secondary magnetic mineral within the samples. By compiling the values of median destructive field (MDF) from all measured specimens and categorizing them in terms of their sister specimens’ paleointensity results, I found that the anorthosite samples that pass paleointensity selection criteria have distinctively higher MDF values than those that do not (Fig. 3), and all diabase specimens have low MDF values like those of the non-ideal anorthosites (Fig. 3). The higher MDFs (~60 mT) of the anorthosites are similar to what is typical of stoichiometric, single-domain magnetite grains which are favored for paleointensity experiments. Furthermore, the low MDFs displayed in other specimens could be linked

to a dominant population of large interstitial Fe-Ti oxides that are more likely to form multi-domain grains, which have been observed in both the anorthosite and diabase (Fig. 1). In addition, preliminary tests on single anortho-site crystals might suggest that not all single crystals are necessarily better for paleointensity research when com-paring to bulk samples, given their similar low MDFs to the failed diabase and anorthosite specimens. However, more single crystal grains with paired paleointensity data and rock magnetic data may be needed before a general conclusion can be drawn. Taken together, the rock magnetic experiment results support my paleointensity selection criteria in that specimens that produce straight Arai plots and have MDF values similar to stoichiometric single-domain magnetite are preferentially selected. The preliminary paleointensity results yielded consistent estimates. The cooling rate-corrected site-mean paleointensity estimate is about 40 µT, consistent with results from Sprain et al. (2018) that the Earth’s magnetic field strength at the Earth’s surface in the late Mesoproterozoic was close to that of today.

AcknowledgementsI thank Dario Bilardello, Peat Solheid, Mike Jackson, Josh Feinberg, and Bruce Moskowitz for their tremendous help of instrumental operations, data interpretations, and research guidance. I thank the IRM for the generous U.S. Visiting Student Fellowship. I thank Nicholas Swanson-Hysell at UC Berkeley for advising this research. Margaret Avery provided field assistance.

ReferencesMaxbauer, D. P., Feinberg, J. M., and Fox, D. L., 2016, MAX

UnMix: A web application for unmixing magnetic coerciv-ity distributions: Computers and Geosciences, vol. 95, pp. 140–145, doi:10.1016/j.cageo.2016.07.009. Sprain, C. J., Swanson-Hysell, N. L., Fairchild, L. M., and Gaastra, K., 2018, A field like today?: the strength of the geomagnetic field 1.1 billion years ago: Geophysical Journal International, vol. 213, pp. 1969–1983, doi:10.1093/g ji/ggy074.

Sprain, C. J., Swanson-Hysell, N. L., Fairchild, L., and Gaastra, K., 2018, A field like today’s? The strength of the geomag-netic field 1.1 billion years ago, Geophysical Journal Inter-national 213, 1969-1983, doi: 10.1093/gji/ggy074.

Swanson-Hysell, N. L., Ramezani, J., Fairchild, L. M., and Rose, I. R., 2019, Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis: GSA Bulletin, doi:10.1130/b31944.1.

Tarduno, J. A. and Cottrell, R. D., 2005, Dipole strength and variation of the time-averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals: Journal of Geophysical Research: Solid Earth, vol. 110, doi:10.1029/2005JB003970.

Fig. 3: (A) Backfield measurement data acquired using a Princeton VSM on an anorthosite specimen. The noise level on the coercivity plot is high, and a heavy smoothing is needed to make subsequent coercivity modeling interpretable. (B) Backfield measurement data acquired using a Lake Shore VSM on the same specimen as in (A). The inset coercivity unmixing plot shows that the much smoother initial backfield curve resulted in a tight single-component model. (C) Box plot of distribution of the median destructive field of all measured specimens. The anorthosite specimens that pass selection criteria have distinctly higher median destructive field (MDF) than other groups.

5

Rock magnetic properties of Paleocene-Eocene sediments from the Piceance Creek Basin, western Colorado

Daniel P. MaxbauerDepartment of Geology, Carleton Collegedmaxbauer@carleton.edu

Deposition during the Paleocene and Eocene in the Piceance Creek Basin of western Colorado records dy-namics associated with the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) – a global warming event associated with massive perturbations to the carbon cycle (McInerney and Wing, 2011). Fluvial systems in the Piceance Creek Basin changed during the PETM in ways that suggest discharge in river systems increased due to stronger seasonal rainfall events and/or increased mean annual precipitation (Foreman et al., 2012). Over the past several years, we have been working to develop new stratigraphic sections in the Piceance Creek Basin to develop more robust constraints for paleoclimate change during the PETM. One area of focus is on fine-grained deposition along the western margin of the basin in order

Fig. 1: Paleosols in this study are represented by five end member types that can be represented by fine-grained mudstones with variable pedogenic modi-fications. Red mudstones represent paleosols that developed in well-drained floodplains with oxidative weathering environments in upland topographic positions and/or drier climates. Carbonate nodules observed in certain red mudstones are indicative of semi-arid to arid conditions and represent the driest facies in our sequence. Mottled yellow and grey mudstones form during periods of intermediate drainage that oscillate between oxidizing and reducing conditions. Grey and darker organic rich mudstones for during poorly drained conditions in lowland topographic positions below or at the water table or when the climate is most humid. These deposits are characterized by pedogenic features associated with prolonged saturation. The expected iron oxide mineralogy of each paleosol type is indicated in the horizontal color scale bar. Pedogenic features and general supporting information on pedogenic environments can be found in Beverly et al., (2018).

to constrain paleoenvironmental and paleoclimatic con-ditions from floodplain environments. In this report, my aim is to introduce a simple model for paleosol develop-ment, share some of the magnetic data my students and I collected during two trips to the IRM (one as a visiting fellow in August 2019), and discuss how the rock mag-netic properties help to reinforce our interpretations of pedogenic environments.

Depositional setting and paleosol types Paleocene-Eocene deposits in the Piceance Creek Basin are assigned to the Wasatch Formation, which is subdivided into the Paleocene Atwell Gulch Member and Eocene Molina and Shire Members. The Atwell Gulch consists of lithofacies that are characteristic of fluvial, paludal, and lacustrine systems (Donnell, 1969; Johnson and Flores, 2003). Fine-grained deposits in the Atwell Gulch consist of organic-rich shales and paleosols. The brown-black shales are interpreted to be swamp, paludal, and shallow lake deposits (Donnell, 1969; Johnson and Flores, 2003; Foreman and Rasmussen, 2016). Paleosols are common and often associated with variable amounts of yellow, brown, and red mottling features. In the Molina Member, fluvial sand bodies and crevasse splay deposits are more common and paleosols develop atop overbank

6

deposits that are characteristically red and brown in color. Our study reports from the western margin of the Piceance Basin where sand bodies thin and paleosols become more well developed compared with the basin center. The Shire Member lies above the Molina Member and contains a mixture of sand bodies and well-developed red, pink, and purple paleosols (Donnell, 1969; Johnson and Flores, 2003; Foreman and Rasmussen, 2016). We assigned well-developed paleosols in our study area to one of five representative paleosol types (Figure 1) based on pedogenic features described in the field. The principal variable that controls pedogenic develop-ment in this system is drainage (Kraus and Aslan, 1993). In pedogenic environments that are poorly drained (for example, swamp or paludal environments), prolonged periods of saturation encourage reducing environments that favor iron reduction and gley features to develop in soils (Roberts, 2015; Beverly et al., 2019). Better drained environments become increasingly oxidative as soil pore spaces experience shorter intervals of saturation. Along this drainage gradient, there are characteristic changes to soil color, geochemical conditions, and expected iron oxide mineralogy (see Figure 1). For more information on paleosol features and relationships between paleosols and climate and/or environmental conditions interested readers should refer to a recent review by Beverly et al. (2018). For the purposes of this report, it is most relevant to consider how the depositional sequence of paleosol types are related to iron oxide mineral assemblages. The relationships between soil-formed iron oxide minerals and climate has been a topic of interest within the rock magnetic community for decades. For instance, it is well-known that better drained conditions should be associated

with hematite while soils experiencing intermediate to poor drainage tend to accumulate more goethite (Kampf and Schwertman, 1983; Liu et al., 2013). In addition, many studies have evaluated the controls on magnetic enhancement in soils and paleosol sequences (reviewed in Maxbauer et al., 2016a; Orgeira et al., 2011). Magnetic enhancement is a phenomenon where topsoil magnetic susceptibility is enhanced relative to lower soil horizons due to the pedogenic production of ferrimagnetic mag-netite and/or maghemite. It is expected that magnetic enhancement in most systems will increase with more soil moisture to a certain threshold, where more saturated soils in humid climates (greater than ~1200 mm of annual rainfall) have reduced magnetic enhancement (Maher and Thompson, 1995; Geiss et al., 2008; Balsam et al., 2011). Here, we use rock magnetic properties to constrain iron oxide mineralogy of Wasatch Formation paleosols in an attempt to apply these long-standing relationships to constrain paleoclimate conditions during the Paleocene-Eocene in western Colorado.

Rock magnetic properties of Wasatch Formation sedi-ments For all specimens in this study (n = 301), mass-depen-dent magnetic susceptibility (χ, m3kg-1) was measured in an alternating field of 300 Am-1 at a frequency of 465 Hz. Reported data for χ are the average of four replicate measurements. Anhysteretic and isothermal remanent magnetizations (ARM and IRM respectively) were mea-sured for all specimen. ARM was acquired in a peak alter-nating field of 100 mT with a DC bias of 50 µT. IRM was imparted with three pulses of a 100 mT DC field. Data for all bulk properties are displayed in Figure 2 with respect to stratigraphic height in the Wasatch Formation. There is considerable variability in bulk properties – however most paleosol horizons (shaded grey in Figure 2) have weaker low-field remanence and susceptibility compared to overbank deposits serving as soil parent material (non-shaded intervals in Figure 2). A notable exception to this pattern is elevated remanence in paleosols at the base of the Molina Member (Figure 2) where ARM and IRM are both enhanced relative to their parent material, despite susceptibility remaining low. In order to more effectively interpret bulk magnetic properties, a subset of samples from well-developed pa-leosol horizon samples were subjected to measurements of hysteresis properties and backfield remanence curves in saturating fields of 1 Tesla using the Princeton Measure-ment System Vibrating Sample Magnetometer (VSM). The most notable results from analyzing the hysteresis and backfield data are reported here in Figure 3. There is a considerable increase in coercivity for paleosols at the base of the Molina Member, where coercivity reaches ~400 mT from a background of <100 mT for Atwell Gulch paleosols. The increase in coercivity is associated with an increase in the ratio of remanent to saturation magnetiza-tion (Mr/Ms; Figure 3). Coercivity distributions for repre-sentative samples were derived from backfield remanence curves and decomposed to help identify magnetic mineral components (see Maxbauer et al., 2016b). Representative

Fig. 2: Bulk low-field magnetic properties of Wasatch Formation sediments in the Pice-ance Creek Basin, western CO. A.) Magnetic susceptibility (χ 465 Hz), B.) anhysteretic remanenent magnetization (ARM

100 mT), and C.) isothermal remanent magnetization

(IRM100 mT

) were measured for all study specimens (n = 301). Intervals shaded in grey are paleosols. Stratigraphic members of the Wasatch Formation are indicated along the x-axis with stratigraphic height. The dashed horizontal line indicates the approximate location of the PETM onset determined by field stratigraphic relationships.

7

coercivity spectra are displayed in Figure 3 – highlighting that Atwell Gulch paleosols display a mixed mineral as-semblage while Molina Member paleosols are dominated by a single, high-coercivity component.

Magnetic mineral assemblages in Wasatch Formation paleosol horizons To better constrain interpretations of magnetic mineral-ogy from paleosol layers we monitored room temperature remanence imparted with a 2.5 T field (RT-SIRM) using a Quantum Designs Magnetic Properties Measurement System (MPMS) on cooling and warming from 300 K to 20 K (see Figure 3). None of the paleosol samples measured showed a strong signature for the Verwey Transition in magnetite – confirming that the weak sus-ceptibilities observed in paleosols in this sequence are due to low concentration of ferrimagnetic minerals, perhaps maghemite. Atwell Gulch paleosols appear to be dominated by goethite (see Figure 3), which demonstrates characteristic doubling of remanence on cooling during RT-SIRM experiments (Maher et al., 2004). Although, nanomaghemite often behaves similarly to goethite (Smirnov and Tarduno, 2000; Carter-Stiglitz, 2006) and is likely the low-coercivity component observed in the Atwell Gulch. Further, the only sample to display a slight transition around 225 K is a mottled-grey-mudstone in the Atwell Gulch (paleosol “B” in Figure 3), which likely reflects contributions from nanohematite. Molina Member paleosols generally display slight increases in remanence on cooling and lack appreciable change across the Morin (hematite) or Verwey (magnetite) transition temperatures.

Increase in remanence on cooling and reduced or absent Morin transitions are noted from modern red soils and in Paleocene-Eocene paleosols from the Bighorn Basin (Maher et al., 2004; Maxbauer et al., 2016c). Taking the results of both VSM and MPMS measurements together, we interpret the increased coercivity and Mr/Ms observed in Molina paleosols to indicate a magnetic mineral assem-blage that is increasingly dominated by hematite (top right panel of Figure 3) while magnetic properties in the Atwell Gulch reflect a more mixed combination of iron oxides.

Implications for Paleocene-Eocene paleoclimate The rock magnetic data reported here support inter-pretations of drainage conditions based on field observa-tions of mudstone facies in the Wasatch Formation. The Atwell Gulch is primarily dominated by a mixed-magnetic mineral assemblage dominated by goethite and a low-coercivity component (maghemite) that reflects both the multi-colored pedogenic mottling features observed in the field and poor drainage conditions of swamp and paludal environments. There is a rather abrupt shift to more well-drained paleosols at the base of the Molina Member – which is associated with a shift to red paleosol units that are dominated almost entirely by hematite, with the general absence of any low-coercivity component. The shift to well-drained, hematite dominated paleosols at the base of the Molina is reflective of a shift in early Eocene paleoclimates to more arid conditions. Drying in the early Eocene in the Piceance Basin is consistent with paleoclimate proxy records from the nearby Bighorn Basin (Wing et al., 2005; Kraus et al., 2007). In addition,

Fig. 3: Selected results for hysteresis measurements, unmixing, mudstone facies, and RT-SIRM experiments. The leftmost panel shows the stratigraphic height and members of the Wasatch Formation, along with labeled paleosol horizons A-J. The average coercivity and M

r/M

s ratios for each paleosol horizon are displayed with

grey-filled circles and error bars denoting one standard deviation. Coercivity spectra are displayed with respect to a log field. Color for coercivity spectra match mudstone facies assignments, where ORM = organic rich mudstone, MGM = mottled grey mudstone, MYM = mottled yellow mudstone, RM = red mudstone, and RMC = red mudstone with carbonate as described in Figure 1 and in the text. A biplot of M

r/M

s to coercivity is shown in the upper right-hand panel and shows the increasing

dominance of hematite observed in red mudstones at the base of the Molina Member. RT-SIRM experiments are shown in the bottom right-hand panel normalized to remanence on cooling at 20 K and colored according to mudstone facies. Dashed horizontal lines indicate the Verwey Transition of magnetite (120 K) and the Morin Transition for hematite (260 K).

8

these findings support an overall arid to semi-arid climate punctuated by intense seasonal rainfall as the driving mechanism for fluvial system changes in the basin cen-ter of the Piceance Creek Basin (Foreman et al., 2012), rather than an overall increase in moisture. The lack of any appreciable low-coercivity component in the Molina Member further suggests dry climates where pedogenic production of magnetite/maghemite (and thus, magnetic enhancement) is limited (typical of climates with annual rainfall less than ~500 mm; Maher and Thompson, 1995; Geiss et al., 2008). Ongoing work for this project aims to further constrain paleoclimate change in the earliest Eocene through addi-tional geochemical proxies along with continued sedimen-tological work and the development of additional strati-graphic sections to further evaluate these interpretations.

Acknowledgments This research is a collaborative effort with Brady Foreman (Western Washington) and Andrea Erhardt (University of Kentucky). Data reported here were acquired by Carleton College undergraduates Amelia Papajohn, Oren Lieber-Kotz, and Olivia Laub. Thanks to Mike Jackson, Dario Bilardello, and Peat Solheid for help in the lab and useful discussions that included Josh Feinberg, Bruce Moskow-itz, and Subir Banerjee during my visits to the lab.

References Beverly, E.J., Lukens, W.E., and Stinchcomb, G.E., 2018. Pa-

leopedology as a tool for reconstructing paleoenvironments and paleoecology. In: Croft, D., Su, D., and Simpson, S. (eds) Methods in Paleoecology. Vertebrate Paleobiology and Paleoanthropology. Springer, Cham.

Carter-Stiglitz, B., Moskowitz, B., and Jackson, M., 2006. Unmixing magnetic assemblages and the magnetic behavior of bimodal mixtures. Journal of Geophysical Researh. 106, B11. 26397-26411.

Donnell, J.R., 1969. Paleocene and lower Eocene units in the southern part of the Piceance Creek Basin, Colorado. U.S. Geological Survey Bulletin. 1274, 1-18.

Foreman, B.Z., Heller, P.L., and Clementz, M.T., 2012. Fluvial response to abrupt global warming at the Palaeocene/Eocene boundary. Nature. 491, 92-95.

Foreman, B.Z., and Rasmussen, D.M., 2016. Provenance signals in the Piceance Creek Basin: unroofing of the Sawatch Range and extent of the early Paleogene California River System (Colorado, U.S.A.). Journal of Sedimentary Research. 86, 1345-1358.

Geiss, C.E., Egli, R., Zanner, C.W., 2008. Direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils. Journal of Geophysical Research. 113, B11102.

Johnson, R.C., and Flores, R.M., 2003. Piceance Basin Guide-book. Rocky Mountain Association of Geologists: chapter, “History of the Piceance Basin from latest Cretaceous through early Eocene and the characterization of lower Tertiary sandstone reservoirs. Pp. 21-61.

Kampf, N., and Schwertmann, U., 193. Goethite and hematite in a climosequence in southern Brazil and their application in classification of kaolinitic soils. Geoderma. 29, 27-39.

Kraus, M.J., and Aslan, A., 1993. Eocene hydromorphic pa-leosols: significance for interpreting ancient floodplain processes. Journal of Sedimentary Petrology. 63, 453-463.

Liu, Z., Liu, Q., Torrent, J., Barrón, V., Hu, P., 2013. Testing the

magnetic proxy χfd/HIRM for quantifying paleoprecipitation in modern soil profiles from Shaanxi Province, China.Glob. Planet. Chang. 110, 368–378.

Maher, B.A., and Thompson, R., 1995. Paleorainfall reconstruc-tions from pedogenic magnetic susceptibility variations in the Chinese loess and paleosols. Quaternary Research. 44, 383-391.

Maxbauer, D.P., Feinberg, J.M., and Fox, D.L., 2016a. Magnetic minerals assemblages in soils and paleosols as the basis for paleoprecipitation proxies: A review of magnetic methods and challenges. Earth Science Reviews. 155, 28-48.

Maxbauer, D.P., Feinberg, J.M., and Fox, D.L., 2016b. MAX UnMix: A web application for unmixing magnetic coerciv-ity distributions. Computers and Geoscience. 95, 140-145.

Maxbauer, D.P., Feinberg, J.M., Fox, D.L., and Clyde, W.C., 2016c. Magnetic minerals as recorders of weathering, dia-genesis, and paleoclimate: A core-outcrop comparison of Paleocene-Eocene paleosols in the Bighorn Basin, WY, USA. Earth and Planetary Science Letters. 452, 15-26.

McInerney, F.A., and Wing, S.L., 2011. The Paleocene-Eocene Thermal Maximum: A perturbation of carbon cycle, climate, and biosphere with implications for the future. Annual Re-view of Earth and Planetary Sciences. 39, 489-516.

Orgeira, M.J., Egli, R., Compagnucci, R.H., 2011. A quantitative model of magnetic enhancement in loessic soils. In: Petro-vsky, E., Ivers, D., Harinarayana, T., Herrero-Bervera, E., (eds), The Earth’s Magnetic Interior. Springer Netherlands. Pp. 361-397.

Roberts, A.P., 2015. Magnetic mineral diagenesis. Earth Science Reviews. 151, 1-47.

Smirnov, A.V., and Tarduno, J.A., 2000. Low-termperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and mag-netic mineral reduction. Journal of Geophysical Research. 105, 16457-16471.

“It seemed probable that the forma-tion of the red skin is quite inde-pendent of the depth of the liquid on which it is formed [...] Further, if the wetting and drying are repeated, the layer should be progressively thickened [...] To test these ideas, some beads formed of short lengths of glass tube were threaded on a string to form a kind of necklace (figure 1). This necklace was hung on the pulley of an electric motor [...] The bottom of the necklace dipped in a vessel of chalybeate water, and the free part of the length from the liquid to the pulley [...] was exposed to the radiation from an ordinary domestic heat stove, which dried it off. [...] The beads became yellow in say 15 min., then orange, then red by the thickening of the layer.”

Lord Rayleigh, F.R.S. (1946). Experimental production of red and yellow sandstones from cha-lybeate water, Proc. R. Soc. Lond. Ser. A, 186, 411-422.

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A list of current research articles dealing with various topics in the physics and chemistry of magnetism is a regular feature of the IRM Quarterly. Articles published in familiar geology and geophysics journals are included; special emphasis is given to current articles from physics, chemistry, and materials-science journals. Most are taken from INSPEC (© Institution of Elec-trical Engineers), Geophysical Abstracts in Press (© American Geophysical Union), and The Earth and Planetary Express (© Elsevier Science Publishers, B.V.), after which they are sub-jected to Procrustean culling for this newsletter. An extensive reference list of articles (primarily about rock magnetism, the physics and chemistry of magnetism, and some paleomagnetism) is continually updated at the IRM. This list, with more than 10,000 references, is available free of charge. Your contributions both to the list and to the Current Articles section of the IRM Quarterly are always welcome.

Current Articles

Anisotropy and Magnetic Fabrics Curtin, T. M., M. L. Crocker, and G. Wheatley (2020), Magnetic

fabrics preserved by post-glacial sediment in two New York Finger Lakes (USA) revealed evidence for deformation during coring and an erosional unconformity, Journal of Paleolimnology, 63(4), 305-322, doi:10.1007/s10933-020-00117-1.

Das, A., and J. Mallik Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar-Dhule Deccan dyke swarm, International Journal of Earth Sciences, doi:10.1007/s00531-020-01841-9.

Fodor, L. I., E. Marton, M. Vrabec, B. Koroknai, and M. Tra-janova Relationship between magnetic fabrics and defor-mation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps), International Journal of Earth Sciences, doi:10.1007/s00531-020-01846-4.

Hrouda, F., D. Burianek, and O. Krejci (2020), Effect of post-magmatic processes on magnetic fabric of teschenite as-sociation rocks of the Outer Western Carpathians, Journal of Structural Geology, 133, doi:10.1016/j.jsg.2020.104003.

Huo, F. F., R. Q. Shao, N. Jiang, R. Y. Zhang, X. Cheng, W. J. Zhang, B. T. Wei, Y. N. Zhou, and H. N. Wu (2020), Anisotropy of magnetic susceptibility of Mesozoic and Ce-nozoic sediments in the northern margin of Qaidam Basin and its sedimentary-tectonic significance, Chinese Journal of Geophysics-Chinese Edition, 63(2), 583-596, doi:10.6038/cjg2020N0038.

Njanko, T., M. G. Dedzo, J. Tamen, E. B. B. Nke, O. S. K. Kouemo, E. M. Fozing, G. P. Doumsab, and J. Tchakounte (2020), Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS), Journal of African Earth Sciences, 162, doi:10.1016/j.jafrearsci.2019.103728.

Paris, R., S. Falvard, C. Chague, J. Goff, S. Etienne, and P. Doumalin (2020), Sedimentary fabric characterized by X-ray tomography: A case-study from tsunami deposits on the Marquesas Islands, French Polynesia, Sedimentology, 67(3), 1207-1229, doi:10.1111/sed.12582.

Platzman, E. S., R. S. J. Sparks, and F. J. Cooper (2019), Fab-rics, facies, and flow through a large-volume ignimbrite: Pampa De Oxaya, Chile, Bulletin of Volcanology, 82(1), doi:10.1007/s00445-019-1345-2.

Sbaraini, S., M. I. B. Raposo, M. D. Bitencourt, and C. R. Tome (2020), Magnetic fabrics of the neoproterozoic piquiri syenite massif (Southernmost Brazil): Implications for 3D

geometry and emplacement, Journal of Geodynamics, 134, doi:10.1016/j.jog.2019.101691.

Seifivand, A., and M. Sheibi (2020), Ballooning emplacement and alteration of the Chah-Musa subvolcanic intrusion (NE Iran) inferred from magnetic susceptibility and fab-ric, Geological Magazine, 157(4), 621-639, doi:10.1017/s0016756819001158.

ArcheomagnetismDiarte-Blasco, P., A. M. Casas, A. Pocovi, J. J. Villalain, A. Mu-

noz, V. Beolchini, O. Pueyo-Anchuela, and L. Pena-Chocarro (2020), Interpretation of magnetic anomalies of geological and archaeological origins in a volcanic area (Tusculum site, Lazio, Italy): Methodological proposals, Journal of Applied Geophysics, 173, doi:10.1016/j.jappgeo.2020.103942.

Garcia-Redondo, N., A. Carrancho, A. Goguitchaichvili, J. Morales, M. Calvo-Rathert, and A. Palomino (2020), New constraints on the medieval repopulation process in the northern Iberian plateau from the full vector archaeomagnetic dating of two hearths at La Pudia site (Caleruega, Burgos, Spain), Archaeological and Anthropological Sciences, 12(4), doi:10.1007/s12520-020-01041-1.

Goemaere, E., P. Sosnowska, M. Golitko, T. Goovaerts, and T. Leduc (2019), Archaeometric and Archaeological Character-ization of the Fired Clay Brick Production in the Brussels-Capital Region between the XIV and the Third Quarter of the XVIII Centuries (Belgium), Archeosciences-Revue D Arche-ometrie, 43(1), 107-132, doi:10.4000/archeosciences.6429.

Osete, M. L., et al. (2020), Two archaeomagnetic intensity maxima and rapid directional variation rates during the Early Iron Age observed at Iberian coordinates. Implications on the evolution of the Levantine Iron Age Anomaly, Earth and Plan-etary Science Letters, 533, doi:10.1016/j.epsl.2019.116047.

Peng, F., C. Kasse, M. A. Prins, R. Ellenkamp, M. Y. Kras-noperov, and R. T. van Balen (2020), Paleoflooding recon-struction from Holocene levee deposits in the Lower Meuse valley, the Netherlands, Geomorphology, 352, doi:10.1016/j.geomorph.2019.107002.

Pierce, D. E., P. J. Wright, and R. S. Popelka-Filcoff (2020), Seeing red: an analysis of archeological hematite in east cen-tral Missouri, Archaeological and Anthropological Sciences, 12(1), doi:10.1007/s12520-019-00984-4.

Chronostratigraphy/MagnetostratigraphyAnastasio, D. J., A. L. Teletzke, K. P. Kodama, J. M. Pares,

and K. L. Gunderson (2020), Geologic evolution of the Pena flexure, Southwestern Pyrenees mountain front, Spain, Journal of Structural Geology, 131, doi:10.1016/j.jsg.2019.103969.

Botka, D., et al. (2019), Integrated stratigraphy of the Gus-terita clay pit: a key section for the early Pannonian (late Miocene) of the Transylvanian Basin (Romania), Austrian Journal of Earth Sciences, 112(2), 221-247, doi:10.17738/ajes.2019.0013.

Chu, R. J., H. C. Wu, R. K. Zhu, Q. Fang, S. H. Deng, J. W. Cui, T. S. Yang, H. Y. Li, L. W. Cao, and S. H. Zhang (2020), Orbital forcing of Triassic megamonsoon activity documented in lacustrine sediments from Ordos Basin, China, Palaeogeog-raphy Palaeoclimatology Palaeoecology, 541, doi:10.1016/j.palaeo.2019.109542.

Demory, F., et al. (2020), Chronostratigraphy, depositional patterns and climatic imprints in Lake Acigol (SW Anato-lia) during the Quaternary, Quaternary Geochronology, 56, doi:10.1016/j.quageo.2019.101038.

Han, F., J. P. Sun, H. F. Qin, H. P. Wang, Q. Ji, H. Y. He, C. L. Deng, and Y. X. Pan (2020a), Magnetostratigraphy of the

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Upper Cretaceous and Lower Paleocene terrestrial sequence, Jiaolai Basin, eastern China, Palaeogeography Palaeoclimatol-ogy Palaeoecology, 538, doi:10.1016/j.palaeo.2019.109451.

Liu, W., H. C. Wu, L. A. Hinnov, M. Baddouh, P. J. Wang, Y. F. Gao, S. H. Zhang, T. S. Yang, H. Y. Li, and C. S. Wang (2020c), An 11 million-year-long record of astronomically forced fluvial-alluvial deposition and paleoclimate change in the Early Cretaceous Songliao synrift basin, China, Palaeogeog-raphy Palaeoclimatology Palaeoecology, 541, doi:10.1016/j.palaeo.2019.109555.

Wasiljeff, J., A. Kaakinen, J. M. Salminen, and Z. Q. Zhang (2020), Magnetostratigraphic constraints on the fossiliferous Ulantatal sequence in Inner Mongolia, China: Implications for Asian aridification and faunal turnover before the Eocene-Oligocene boundary, Earth and Planetary Science Letters, 536, doi:10.1016/j.epsl.2020.116125.

Xu, Y. C., R. Jiang, Y. Z. Deng, D. B. Kemp, Z. Y. Yang, C. J. Huang, and Z. M. Zhu (2020b), A robust geochronology of the Yangtze River Delta based on magnetostratigraphy and cyclostratigraphy of sediment core ZKA2, Palaeogeography Palaeoclimatology Palaeoecology, 541, doi:10.1016/j.pal-aeo.2019.109532.

Yang, J. L., M. Y. Liang, T. J. Algeo, Q. M. Xu, Y. Z. Hu, H. F. Yuan, and G. Q. Xiao (2020), Upper Miocene-Quaternary magnetostratigraphy and magnetic susceptibility from the Bohai Bay Basin (eastern China) and implications for regional volcanic and basinal subsidence history, Palaeogeography Palaeoclimatology Palaeoecology, 538, doi:10.1016/j.pal-aeo.2019.109469.

Zhang, T., C. M. Zhang, T. L. Fan, L. Zhang, R. Zhu, J. Y. Tao, and M. S. Li (2020b), Cyclostratigraphy of Lower Triassic terrestrial successions in the Junggar Basin, northwestern China, Palaeogeography Palaeoclimatology Palaeoecology, 539, doi:10.1016/j.palaeo.2019.109493.

Environmental MagnetismAyoubi, S., M. J. Samadi, H. Khademi, M. Shirvani, and Y. Gyasi-

Agyei (2020), Using magnetic susceptibility for predicting hydrocarbon pollution levels in a petroleum refinery compound in Isfahan Province, Iran, Journal of Applied Geophysics, 172, doi:10.1016/j.jappgeo.2019.103906.

Badesab, F., P. Dewangan, V. Gaikwad, J. G. Sebastian, and M. Venkateshwarlu A rock magnetic perspective of gas hydrate occurrences in a high-energy depositional system in the Krishna-Godavari basin, Bay of Bengal, Geo-Marine Letters, doi:10.1007/s00367-020-00646-8.

Berndt, T. A., L. Chang, and Z. W. Pei (2020), Mind the gap: Towards a biogenic magnetite palaeoenvironmental proxy through an extensive finite-element micromagnetic simula-tion, Earth and Planetary Science Letters, 531, doi:10.1016/j.epsl.2019.116010.

Brenko, T., S. B. Sostaric, S. Ruzicic, and T. S. Ivancan (2020), Evidence for the formation of bog iron ore in soils of the Po-dravina region, NE Croatia: Geochemical and mineralogical study, Quaternary International, 536, 13-29, doi:10.1016/j.quaint.2019.11.033.

Cejka, T., D. Nyvlt, K. Kopalova, M. Bulinova, J. Kavan, J. M. Lirio, S. H. Coria, and B. van de Vijver (2020), Timing of the neoglacial onset on the North-Eastern Antarctic Pen-insula based on lacustrine archive from Lake Anonima, Vega Island, Global and Planetary Change, 184, doi:10.1016/j.gloplacha.2019.103050.

Chaparro, M. A. E., M. D. Moralejo, H. N. Bohnel, and S. G. Acebal (2020), Iron oxide mineralogy in Mollisols, Aridisols and Entisols from southwestern Pampean region (Argen-tina) by environmental magnetism approach, Catena, 190,

doi:10.1016/j.catena.2020.104534.Chen, J. N., X. G. Mao, Y. H. Shi, and X. M. Liu (2020), Study

on the Late Cretaceous paleoenvironment documented by red beds in the western Fujian province, Chinese Journal of Geophysics-Chinese Edition, 63(4), 1553-1568, doi:10.6038/cjg2020N0375.

Crouch, E. M., C. L. Shepherd, H. E. G. Morgans, B. D. A. Naafs, E. Dallanave, A. Phillips, C. J. Hollis, and R. D. Pancost (2020), Climatic and environmental changes across the early Eocene climatic optimum at mid-Waipara River, Canterbury Basin, New Zealand, Earth-Science Reviews, 200, doi:10.1016/j.earscirev.2019.102961.

Feinberg, J. M., I. Lascu, E. A. Lima, B. P. Weiss, J. A. Dorale, E. C. Alexander, and R. L. Edwards (2020), Magnetic detec-tion of paleoflood layers in stalagmites and implications for historical land use changes, Earth and Planetary Science Letters, 530, doi:10.1016/j.epsl.2019.115946.

Gai, C. C., Q. S. Liu, A. R. Roberts, Y. M. Chou, X. X. Zhao, Z. X. Jiang, and J. X. Liu (2020), East Asian monsoon evolution since the late Miocene from the South China Sea, Earth and Planetary Science Letters, 530, doi:10.1016/j.epsl.2019.115960.

Gao, L., Q. F. Wang, J. Deng, F. G. Chen, S. H. Zhang, and Z. Y. Yang (2019), Magmatic-Hydrothermal Alteration Mecha-nism for Late Mesozoic Remagnetization in the South China Block, Journal of Geophysical Research-Solid Earth, 124(11), 10704-10720, doi:10.1029/2019jb018018.

Garces, M., M. Lopez-Blanco, L. Valero, E. Beamud, J. A. Mu-noz, B. Oliva-Urcia, A. Vinyoles, P. Arbues, P. Cabello, and L. Cabrera (2020), Paleogeographic and sedimentary evolution of the South Pyrenean foreland basin, Marine and Petroleum Geology, 113, doi:10.1016/j.marpetgeo.2019.104105.

Ghandour, I. M. (2020), Paleoenvironmental changes across the Paleocene-Eocene boundary in West Central Sinai, Egypt: geochemical proxies, Swiss Journal of Geosciences, 113(1), doi:10.1186/s00015-020-00357-3.

Hamzeh, M. A., and S. F. Ghasr-Aboonasr (2020), Palaeoen-vironmental changes in the Khuran Estuary of SE coastal Iran during the last two millennia, based on the analysis of a sediment core, Palaeogeography Palaeoclimatology Palaeo-ecology, 542, doi:10.1016/j.palaeo.2019.109563.

Han, X., E. J. Tomaszewski, J. Sorwat, Y. Pan, A. Kappler, and J. M. Byrne (2020b), Oxidation of green rust by anoxygenic phototrophic Fe(II)-oxidising bacteria, Geochemical Perspec-tives Letters, 12, 52-57, doi:10.7185/geochemlet.2004.

Hargan, K. E., S. A. Finkelstein, K. M. Ruhland, M. S. Pack-alen, A. S. Dalton, A. M. Paterson, W. Keller, and J. P. Smol Post-glacial lake development and paleoclimate in the cen-tral Hudson Bay Lowlands inferred from sediment records, Journal of Paleolimnology, doi:10.1007/s10933-020-00119-z.

Henne, A., D. Craw, E. J. Gagen, and G. Southam (2020), Contribution of bacterially-induced oxidation of Fe-silicates in iron-rich ore to laterite formation, Salobo IOCG mine, Brazil, Chemical Geology, 539, doi:10.1016/j.chem-geo.2020.119499.

Hettiarachchi, E., and G. Rubasinghege (2020), Mechanistic Study on Iron Solubility in Atmospheric Mineral Dust Aerosol: Roles of Titanium, Dissolved Oxygen, and Solar Flux in Solutions Containing Different Acid Anions, Acs Earth and Space Chemistry, 4(1), 101-111, doi:10.1021/acsearthspacechem.9b00280.

Hiatt, E. E., P. K. Pufahl, and L. G. da Silva (2020), Iron and phosphorus biochemical systems and the Cryogenian-Edia-caran transition, Jacadigo basin, Brazil: Implications for the Neoproterozoic oxygenation event, Precambrian Research, 337, doi:10.1016/j.precamres.2019.105533.

Hodelka, B. N., M. M. McGlue, S. Zimmerman, G. Ali, and I.

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Tunno (2020), Paleoproduction and environmental change at Mono Lake (eastern Sierra Nevada) during the Pleistocene-Holocene transition, Palaeogeography Palaeoclimatology Palaeoecology, 543, doi:10.1016/j.palaeo.2019.109565.

Hou, K., H. Qian, Q. Y. Zhang, T. Lin, Y. Chen, Y. T. Zhang, and W. G. Qu (2020a), Influence of Quaternary paleoclimate change on the permeability of the loess-paleosol sequence in the Loess Plateau, northern China, Earth Surface Processes and Landforms, 45(4), 862-876, doi:10.1002/esp.4779.

Hou, K., H. Qian, Y. T. Zhang, and Q. Y. Zhang (2020b), Influ-ence of tectonic uplift of the Qinling Mountains on the pa-leoclimatic environment of surrounding areas: Insights from loess-paleosol sequences, Weihe Basin, central China, Catena, 187, doi:10.1016/j.catena.2019.104336.

Jiang, Y. M., Y. Saito, T. K. O. Ta, Z. H. Wang, M. Gugliotta, and V. L. Nguyen (2020), Spatial and seasonal variability in grain size, magnetic susceptibility, and organic elemental geochem-istry of channel-bed sediments from the Mekong Delta, Viet-nam: Implications for hydro-sedimentary dynamic processes, Marine Geology, 420, doi:10.1016/j.margeo.2019.106089.

Jiang, Z. X., C. S. Jin, Z. B. Wang, Q. S. Liu, S. Z. Li, and Z. Q. Yao (2020), Chronostratigraphic framework of the East China Sea since MIS 6 from geomagnetic paleointensity and envi-ronmental magnetic records, Global and Planetary Change, 185, doi:10.1016/j.gloplacha.2019.103092.

Jordanova, D., and N. Jordanova (2020), Diversity and peculiari-ties of soil formation in eolian landscapes - Insights from the mineral magnetic records, Earth and Planetary Science Letters, 531, doi:10.1016/j.epsl.2019.115956.

Li, X. J., J. B. Zan, R. S. Yang, X. M. Fang, and S. L. Yang (2020a), Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Jung-gar Basin: Implications for the provenance of Chinese eolian deposits, Palaeogeography Palaeoclimatology Palaeoecology, 538, doi:10.1016/j.palaeo.2019.109458.

Li, Y. X., B. Gill, I. P. Montanez, L. F. Ma, M. Leroy, and K. P. Kodama (2020c), Orbitally driven redox fluctuations during Cretaceous Oceanic Anoxic Event 2 (OAE2) revealed by a new magnetic proxy, Palaeogeography Palaeoclimatology Palaeoecology, 538, doi:10.1016/j.palaeo.2019.109465.

Melo, V., J. C. de Oliveira, A. H. Batista, V. F. Cherobim, and N. Favaretto (2020), Goethite and hematite in bichromic soil profiles of southern Brazil: Xanthization or yellowing process, Catena, 188, doi:10.1016/j.catena.2019.104445.

Mendez, J. C., and T. Hiemstra (2020), Surface area of ferrihy-drite consistently related to primary surface charge, ion pair formation, and specific ion adsorption, Chemical Geology, 532, doi:10.1016/j.chemgeo.2019.119304.

Mendez, J. C., and T. Hiemstra (2020), Ternary Complex Forma-tion of Phosphate with Ca and Mg Ions Binding to Ferrihydrite: Experiments and Mechanisms, Acs Earth and Space Chemis-try, 4(4), 545-557, doi:10.1021/acsearthspacechem.9b00320.

Menshov, O., S. Spassov, P. Camps, S. Vyzhva, P. Pereira, T. Pastushenko, and V. Demidov (2020), Soil and dust magnetism in semi-urban area Truskavets, Ukraine, Environmental Earth Sciences, 79(8), doi:10.1007/s12665-020-08924-5.

Ortega-Guerrero, B., D. Avendano, M. Caballero, S. Lozano-Garcia, E. T. Brown, A. Rodriguez, B. Garcia, H. Barceinas, A. M. Soler, and A. Albarran (2020), Climatic control on magnetic mineralogy during the late MIS 6-Early MIS 3 in Lake Chalco, central Mexico, Quaternary Science Reviews, 230, doi:10.1016/j.quascirev.2020.106163.

Panwar, S., S. Yang, P. Srivastava, M. Y. A. Khan, S. J. San-gode, and G. J. Chakrapani (2020), Environmental mag-netic characterization of the Alaknanda and Ramganga river sediments, Ganga basin, India, Catena, 190, doi:10.1016/j.catena.2020.104529.

Peskov, A. Y., V. O. Krutikova, E. N. Zakharchenko, V. V. Chakov, M. A. Klimin, A. S. Karetnikov, and A. N. Didenko (2020), Geochemistry and Magnetism of Peat Deposits in the Khor-Kiya Interfluve, Sikhote-Alin (Preliminary Re-sults), Russian Journal of Pacific Geology, 14(2), 169-179, doi:10.1134/s1819714020020050.

Phartiyal, B., R. Singh, P. Joshi, and D. Nag Late-Holocene climatic record from a glacial lake in Ladakh range, Trans-Himalaya, India, Holocene, doi:10.1177/0959683620908660.

Rushton, J. C., D. Wagner, J. M. Pearce, C. A. Rochelle, and G. Purser (2020), Red-bed bleaching in a co2 storage ana-logue: insights from entrada sandstone fracture-hosted min-eralization, Journal of Sedimentary Research, 90(1), 48-66, doi:10.2110/jsr.2020.4.

Scheidt, S., U. Hambach, Q. Z. Hao, C. Rolf, and V. Wennrich (2020), Environmental signals of Pliocene-Pleistocene cli-matic changes in Central Europe: Insights from the mineral magnetic record of the Heidelberg Basin sedimentary infill (Germany), Global and Planetary Change, 187, doi:10.1016/j.gloplacha.2020.103112.

Shah, R. A., H. Achyuthan, S. J. Sangode, A. M. Lone, and M. Rafiq (2020), Mineral Magnetic and Geochemical Mapping of the Wular Lake Sediments, Kashmir Valley, NW Himalaya, Aquatic Geochemistry, 26(1), 31-52, doi:10.1007/s10498-019-09364-9.

Sheng, A. X., et al. (2020), Labile Fe(III) from sorbed Fe(II) oxidation is the key intermediate in Fe(II)-catalyzed ferri-hydrite transformation, Geochimica Et Cosmochimica Acta, 272, 105-120, doi:10.1016/j.gca.2019.12.028.

Shukla, T., M. Mehta, D. P. Dobhal, A. Bohra, B. Pratap, and A. Kumar Late-Holocene climate response and glacial fluc-tuations revealed by the sediment record of the monsoon-dominated Chorabari Lake, Central Himalaya, Holocene, doi:10.1177/0959683620908654.

Silva, P. F., et al. (2020), Multidisciplinary characterization of Quaternary mass movement deposits in the Portimao Bank (Gulf of Cadiz, SW Iberia), Marine Geology, 420, doi:10.1016/j.margeo.2019.106086.

Singh, J., S. J. Sangode, M. F. Bagwan, D. C. Meshram, and A. Dhobale (2020), Episodic ferricretization of the Deccan Laterites (India): Inferences from ore microscopy, mineral magnetic and XRD spectroscopic studies, Journal of Earth System Science, 129(1), doi:10.1007/s12040-019-1334-z.

Smedile, A., F. Molisso, C. Chague, M. Iorio, P. M. De Martini, S. Pinzi, P. E. F. Collins, L. Sagnotti, and D. Pantosti (2020), New coring study in Augusta Bay expands understanding of offshore tsunami deposits (Eastern Sicily, Italy), Sedimentol-ogy, 67(3), 1553-1576, doi:10.1111/sed.12581.

Sridhar, A., B. Thakur, N. Basavaiah, P. Seth, P. Tiwari, and L. S. Chamyal (2020), Lacustrine record of high magnitude flood events and climate variability during mid to late Holocene in the semiarid alluvial plains, western India, Palaeogeography Palaeoclimatology Palaeoecology, 542, doi:10.1016/j.pal-aeo.2019.109581.

Tang, L., W. J. Zhou, X. S. Wang, F. Xian, Y. J. Du, J. Zhou, G. Q. Chu, G. Q. Zhao, and L. Zhang (2019), Multidecadal- to Centennial-Scale Be-10 Variations in Holocene Sediments of Huguangyan Maar Lake, South China, Geophysical Research Letters, 46(13), 7634-7642, doi:10.1029/2019gl082863.

Tecsa, V., J. A. Mason, W. C. Johnson, X. D. Miao, D. Con-stantin, S. Radu, D. A. Magdas, D. Veres, S. B. Markovic, and A. Timar-Gabor (2020), Latest Pleistocene to Holocene loess in the central Great Plains: Optically stimulated lumi-nescence dating and multi -proxy analysis of the enders loess section (Nebraska, USA), Quaternary Science Reviews, 229, doi:10.1016/j.quascirev.2019.106130.

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Wang, S., J. Liu, J. C. Li, G. Xu, J. D. Qiu, and B. Chen (2020), Environmental magnetic parameter characteristics as indica-tors of heavy metal pollution in the surface sediments off the Zhoushan Islands in the East China Sea, Marine Pollution Bulletin, 150, doi:10.1016/j.marpolbul.2019.110642.

Wei, Z., W. Zhong, J. B. Xue, J. Ouyang, S. T. Shang, S. S. Ye, J. Y. Cao, and M. K. Li (2020), Late Quaternary East Asian Summer Monsoon Variability Deduced From Lacus-trine Mineral Magnetic Records of Dahu Swamp, Southern China, Paleoceanography and Paleoclimatology, 35(2), doi:10.1029/2019pa003796.

Xu, X. W., X. K. Qiang, H. Zhao, and C. F. Fu (2020a), Magnetic mineral dissolution recorded in a lacustrine sequence from the Heqing Basin, SW China, and its relationship with changes in the Indian monsoon, Journal of Asian Earth Sciences, 188, doi:10.1016/j.jseaes.2019.104081.

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Gilder, S. A., and F. Lhuillier (2020), Can Paleomagnetism Distinguish Dynamo Regimes?, 48-53 pp.

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Cai, Y. H., J. L. Pei, S. H. Zhang, Y. B. Tong, Z. Y. Yang, and Y. Zhao (2020), New paleomagnetic results from the ca. 1.68-1.63 Ga mafic dyke swarms in Western Shandong Province, Eastern China: Implications for the reconstruction of the Columbia supercontinent, Precambrian Research, 337, doi:10.1016/j.precamres.2019.105531.

Calvagno, J. M., L. C. Gallo, R. N. Tomezzoli, E. O. Cristal-lini, A. D. Farjat, and R. M. Hernandez (2020), A new constraint on the central Andean rotation pattern from paleomagnetic studies in the southern Subandes of Bolivia, Journal of South American Earth Sciences, 98, doi:10.1016/j.jsames.2019.102470.

D’Agrella, M. S., W. Teixeira, R. I. F. da Trindade, O. A. L. Patroni, and R. F. Prieto (2020), Paleomagnetism of 1.79 Ga Para de Minas mafic dykes: Testing a Sao Francisco/Congo-North China-Rio de la Plata connection in Columbia, Precam-brian Research, 338, doi:10.1016/j.precamres.2019.105584.

Derder, M. E. M., H. Djellit, B. Henry, S. Maouche, M. Amenna, R. Bestandji, H. Ymel, S. Gharbi, A. Abtout, and C. Dorbath (2019), Strong Neotectonic Block-Rotations, Related to the Africa-Eurasia Convergence in Northern Algeria: Paleomag-netic Evidence From the Mitidja Basin, Tectonics, 38(12), 4249-4266, doi:10.1029/2018tc005394.

Ernesto, M., P. N. Demarco, P. Xavier, L. Sanchez, C. Schultz, and G. Pineiro (2020), Age constraints on the Paleozoic Yaguari-Buena Vista succession from Uruguay: paleomag-netic and paleontologic information, Journal of South Ameri-can Earth Sciences, 98, doi:10.1016/j.jsames.2019.102489.

Eyster, A., B. P. Weiss, K. Karlstrom, and F. A. Macdonald (2020), Paleomagnetism of the Chuar Group and evaluation of the late Tonian Laurentian apparent polar wander path with implications for the makeup and breakup of Rodinia, Geological Society of America Bulletin, 132(3-4), 710-738, doi:10.1130/b32012.1.

Golovanova, I. V., K. N. Danukalov, V. N. Puchkov, N. D. Sergeeva, and R. Y. Sal’manova (2019), Paleomagnetism of Ordovician-Silurian Volcanics on the Western Slope of the Southern Urals, Doklady Earth Sciences, 489(2), doi:10.1134/s1028334x1912002x.

Gonzalez, V. R., C. G. Puigdomenech, C. B. Zaffarana, H. Vizan, and R. Somoza (2020), Paleomagnetic evidence of the brittle deformation of the Central Patagonian Batholith at Gastre area (Chubut Province, Argentina), Journal of South Ameri-can Earth Sciences, 98, doi:10.1016/j.jsames.2019.102442.

Gulyuz, E., H. Durak, M. Ozkaptan, and W. Krijgsman (2020), Paleomagnetic constraints on the early Miocene closure of the southern Neo-Tethys (Van region; East Anatolia): Inferences for the timing of Eurasia-Arabia collision, Global and Plan-etary Change, 185, doi:10.1016/j.gloplacha.2019.103089.

Jing, X. Q., Z. Y. Yang, D. A. D. Evans, Y. B. Tong, Y. C. Xu, and H. Wang (2020), A pan-latitudinal Rodinia in the Tonian true polar wander frame, Earth and Planetary Science Letters, 530, doi:10.1016/j.epsl.2019.115880.

Li, Y., W. L. Xu, R. X. Zhu, F. Wang, W. C. Ge, and A. A. Sorokin (2020b), Late Jurassic to early Early Cretaceous tectonic nature on the NE Asian continental margin: Constraints from Mesozoic accretionary complexes, Earth-Science Reviews, 200, doi:10.1016/j.earscirev.2019.103042.

Li, Z. Y., L. Ding, T. Zaw, H. Q. Wang, F. L. Cai, W. Yao, Z. Y.

Xiong, Y. Sein, and Y. H. Yue (2020d), Kinematic evolution of the West Burma block during and after India-Asia collision revealed by paleomagnetism, Journal of Geodynamics, 134, doi:10.1016/j.jog.2019.101690.

Mattei, M., F. Cifelli, H. Alimohammadian, and H. Rashid (2020), The role of active strike-slip faults and opposite vertical axis rotations in accommodating eurasia-arabia short-ening in central iran, Tectonophysics, 774, doi:10.1016/j.tecto.2019.228243.

Montano-Cortes, P. C., R. S. Molina-Garza, and A. Iriondo (2019), Paleomagnetism and isotopes of Hf in Lower Cre-taceous rocks of the Guerrero Terrain, Chamela Bay and Cocinas Island (Jalisco, Mexico): tectonic implications, Revista Mexicana De Ciencias Geologicas, 36(3), 289-307, doi:10.22201/cgeo.20072902e.2019.3.1048.

Patranabis-Deb, S., and S. Saha (2020), Geochronology, paleo-magnetic signature and tectonic models of cratonic basins of India in the backdrop of Supercontinent amalgamation and fragmentation, Episodes, 43(1), 145-163, doi:10.18814/epiiugs/2020/020009.

Peskov, A. Y., A. N. Didenko, A. V. Kudymov, A. S. Karetnikov, and M. V. Arkhipov (2019), Paleomagnetism and Petro-chemistry of Sandstones from the Gorinskaya and Pioner-skaya Formations, Zhuravlevka-Amurian Terrane (Northern Sikhote Alin), Russian Journal of Pacific Geology, 13(6), 556-567, doi:10.1134/s1819714019060071.

Philippon, M., D. J. J. van Hinsbergen, L. M. Boschman, L. A. W. Gossink, J. J. Cornee, M. BouDagher-Fadel, J. L. Leticee, J. F. Lebrun, and P. Munch (2020), Caribbean intra-plate defor-mation: Paleomagnetic evidence from St. Barthelemy Island for post-Oligocene rotation in the Lesser Antilles forearc, Tectonophysics, 777, doi:10.1016/j.tecto.2020.228323.

Roberts, J., G. Heij, and R. D. Elmore (2019), Palaeomagnetic dating of hydrothermal alteration in the Woodford Shale, Oklahoma, USA, Geological Magazine, 156(12), 2043-2052, doi:10.1017/s0016756819000360.

Rowley, D. B. (2019), Reply to Comment on “Comparing Paleomagnetic Study Means with Apparent Wander Paths: A Case Study and Paleomagnetic Test of the Greater India versus Greater Indian Basin Hypotheses”, Tectonics, 38(12), 4521-4524, doi:10.1029/2019tc005740.

Shatsillo, A. V., S. V. Rud’ko, I. V. Latysheva, D. V. Rud’ko, I. V. Fedyukin, and S. V. Malyshev (2019), Paleomagnetic, Sedimentological, and Isotopic Data on Neoproterozoic Peri-glacial Sediments of Siberia: A New Perspective on the Low-Latitude Glaciations Problem, Izvestiya-Physics of the Solid Earth, 55(6), 841-863, doi:10.1134/s1069351319060065.

Shcherbakova, V. V., V. G. Bakhmutov, D. Thallner, V. P. Shcherbakov, G. V. Zhidkov, and A. J. Biggin (2020), Ultra-low palaeointensities from East European Craton, Ukraine support a globally anomalous palaeomagnetic field in the Ediacaran, Geophysical Journal International, 220(3), 1928-1946, doi:10.1093/gji/ggz566.

Usui, Y., M. Saitoh, K. Tani, M. Nishizawa, T. Shibuya, C. Kato, T. Okumura, and T. Kashiwabara (2020), Identification of paleomagnetic remanence carriers in ca. 3.47 Ga dacite from the Duffer Formation, the Pilbara Craton, Physics of the Earth and Planetary Interiors, 299, doi:10.1016/j.pepi.2019.106411.

van Hinsbergen, D. J. J. (2019), Comment on “Comparing Paleo-magnetic Study Means With Apparent Wander Paths: A Case Study and Paleomagnetic Test of the Greater India Versus Greater Indian Basin Hypotheses” by David B. Rowley, Tectonics, 38(12), 4516-4520, doi:10.1029/2019tc005525.

van Hinsbergen, D. J. J., T. H. Torsvik, S. M. Schmid, L. C. Matenco, M. Maffione, R. L. M. Vissers, D. Gurer, and W. Spakman (2020), Orogenic architecture of the Mediterranean

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region and kinematic reconstruction of its tectonic evolu-tion since the Triassic, Gondwana Research, 81, 79-229, doi:10.1016/j.gr.2019.07.009.

Vashakidze, G., A. Goguitchaichvili, N. Garcia-Redondo, M. Calvo-Rathert, A. Carrancho, R. Cejudo, J. Morales, V. A. Lebedev, and K. Gabarashvili (2019), Magnetic dating of the Holocene monogenetic Tkarsheti volcano in the Kazbeki region (Great Caucasus), Earth Planets and Space, 71(1), doi:10.1186/s40623-019-1109-4.

Xian, H. B., S. H. Zhang, H. Y. Li, Q. S. Xiao, L. X. Chang, T. S. Yang, and H. C. Wu (2019), How Did South China Con-nect to and Separate From Gondwana? New Paleomagnetic Constraints From the Middle Devonian Red Beds in South China, Geophysical Research Letters, 46(13), 7371-7378, doi:10.1029/2019gl083123.

Xian, H. B., S. H. Zhang, H. Y. Li, T. S. Yang, and H. C. Wu (2020), Geochronological and palaeomagnetic investigation of the Madiyi Formation, lower Banxi Group, South China: Implications for Rodinia reconstruction, Precambrian Re-search, 336, doi:10.1016/j.precamres.2019.105494.

Yang, T. S., et al. (2019), Precollisional Latitude of the North-ern Tethyan Himalaya From the Paleocene Redbeds and Its Implication for Greater India and the India-Asia collision, Journal of Geophysical Research-Solid Earth, 124(11), 10777-10798, doi:10.1029/2019jb017927.

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Rock Magnetism and direct applicationsAbrajevitch, A. (2020), Diagenetic formation of bedded chert:

Implications from a rock magnetic study of siliceous pre-cursor sediments, Earth and Planetary Science Letters, 533, doi:10.1016/j.epsl.2019.116039.

Alva-Valdivia, L. M., P. Guerrero-Diaz, J. Urrutia-Fucugauchi, A. Agarwal, and C. I. Caballero-Miranda (2020), Review of magmatic iron-ore mineralization in central-western Mexico: Rock-magnetism and magnetic anomaly modelling of Las Truchas, case study, Journal of South American Earth Sci-ences, 97, doi:10.1016/j.jsames.2019.102409.

Choe, H., and J. Dyment (2020), Fading magnetic anomalies, thermal structure and earthquakes in the Japan Trench, Geol-ogy, 48(3), 278-282, doi:10.1130/g46842.1.

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Figure 2. VSM data versus temperature (note both °C and K scales) acquired by Mike Jackson at the IRM on a Bishop Tuff specimen during two progressive (red and green symbols) and one repeat (cyan) heating cycles (blue symbols are the measurements on cooling after the first heating step). A) Mass-normalized low-field slope (χ

0) showing

the characteristic, non-reversible peak; B) coercivity data showing the sudden decrease in BC coincident with the increase in χ

0; C) Saturation magnetization (M

S) remains

~constant over the temperature interval across which both the susceptibility peak and the drop in BC occur, despite a decrease in M

S between the two heating curves to 600 °C.

... cont’d. from pg. 1

magnetite-maghemite interface is expected to be hindered as the oxidized shell becomes thicker with time (Ge et al., 2014).

In soils, the slow rates of diffusion (10−12 – 10−20 cm2/s, Sidhu et al., 1977; Tang et al., 2003) are further limited by the low temperature and circumneutral pH. The core-shell model was recently adopted by Ahmed and Maher (2018) for soils and paleosol sequences. They observed the presence of thin rims (1–4 nm thick) of amorphous Fe-bearing oxide material (interpreted as an oxidized maghemite shell) around the magnetite nanoparticles, and used the model to explain the stability of nanoscale magnetite particles over time periods exceeding 5 My (Fig. 3). Specific to soils, magnetites are typically coated with clay minerals, which form strong barriers to oxygen transport to the particles’ surface, and in time become the rate-limiting factor of the oxidation process.

Maghemite has the same crystal structure as magnetite (inverse cubic spinel) but the unit cell is smaller, so that upon oxidation the shell shrinks and is stretched over the core producing cracking. The resulting stresses act to increase the coercivity of the grain (e.g. Özdemir et al., 1993), making it more stable with respect to thermal acti-vation and/or external magnetic fields, which is reflected in heightened unblocking temperature and suppression of magnetic susceptibility (Van Velzen and Dekkers, 1999).

The surface oxidation model was introduced to explain the behaviour in single domain (SD) grains (Knowles, 1981; Housden and O’Reilly, 1990), but the same surficial cracking also occurs in multi-domain (MD) particles (Appel, 1987). In magnetite the effect is easily produced under normal atmospheric conditions, and un-less the temperature is increased or the conditions become more oxidizing, the core-shell structure may persist over geological time, as modeled by Ahmed and Maher (2018) for nanoparticles. According to Askill (1970) the diffusion constant for Fe2+ in the crystal is 12 orders of magnitude higher at 150°C than at room temperature, although other studies indicate much smaller increases with temperature (Gapeev and Gribov, 1990, see also Fabian and Shcherba-kov, 2020); this leads to a reduction of the large oxidation gradient built up by low-temperature oxidation, and if no

further oxidation or reduction occurs at the grain surface, the heating promotes diffusion of iron and vacancies, leaving a more homogeneously oxidized grain with a low overall oxidation degree, i.e. the average oxidation degree of the thin oxidized shell and the large unoxidized core. In other words, the heating step anneals the stress within the maghemite shell and reduces the coercivity of the grain.

For a more in-depth discussion of the annealing of point defects from the magnetite and titanomagnetite lattice, the reader may refer to studies of the magnetic after-effects (MAEs) (e.g. Castro and Rivas, 1999; Walz et al., 2007, and references therein).

Fig. 3. Shrinking-core model simulation of the soil magnetite oxidation reaction by Ahmed and Maher (2018). (A) Schematic representation of a reacting soil ferrite particle (initial particle radius R0) with unreacted magnetite core (r

c), maghemite oxidation rim and an

associated clay film (at t0, R

0 = r

c). (B) Shrinking-core model simulation using pH = 8.0,

PO2

= 0.001 atm, T = 15 °C, air-filled porosity (θ) = 0.5, and variable R0 between 5 and

20 nm. (C) Shrinking-core model simulation using R0 = 10 nm, T = 15 °C, P

O2 = 0.001

atm, θ = 0.5, and variable pH between 3.0 and 9.0.

Temperature [Kelvin]800700600500400

Temperature [C]500400300200100

Ms[A

m2/k

g]

0.25

0.20

0.15

0.10

0.05

0.00

initial heatingfirst coolingsecond heatingthird heating

GB01-htvsm

Temperature [Kelvin]800700600500400

Temperature [C]500400300200100

χ 0[m

3/k

g]

3.0e-6

2.0e-6

1.0e-6

4.2e-22

initial heatingfirst coolingsecond heatingthird heating

GB01-htvsm

Temperature [Kelvin]800700600500400

Temperature [C]500400300200100

B c[m

T]

40

30

20

10

0

initial heatingfirst coolingsecond heatingthird heating

GB01-htvsm

a) b) c)

16

The practicality It was proposed by Van Velzen and Zijderveld (1995)

and Van Velzen and Dekkers (1999) that the post heating coercivity likely reflects that of the grain before oxidation (the OG) and that mild heating may therefore be used as a tool for determining the effects of low temperature oxidation. Van Velzen and Zijderveld (1995) further ex-cluded that the decrease of the coercivity cannot be due to other factors, for example demagnetization of goethite during the same heating step. Likewise, low-temperature oxidation may result in overlap of the coercivity spectra of oxidized magnetite and those of hematite and goethite, and heating to 150 °C can reduce the overlap aiding in discrimination between these phases. Short of inverting to hematite, pure maghemite of course is fully oxidized and, therefore, cannot increase its coercivity further: heating to 150 °C may thus help to discriminate between maghemite, oxidized magnetite and magnetite (Van Velzen and Dekkers, 1999) and can be very useful in conjunction with the observation of the characteristic “humps” in the RTSIRM cooling and heating curves of maghemitized samples (Özdemir and Dunlop, 2010).

Back to the χ-T> RT curves, segmented incremental heating and cooling runs are particularly informative. Fig. 4 shows such an experiment performed by Kontny and Grothaus (2016) for the same specimen reported in figure 1a. The specimen is a shocked andesite that is visibly fractured (at least microscopically) and contains titanomaghemite. The experiment was performed in ar-gon atmosphere to limit oxidation upon heating. For this specimen, the irreversible increase in susceptibility starts around 125 °C and ends above ~150 °C, where the curves become reversible. Above 350 °C the susceptibility starts to decrease irreversibly, up until 450 °C, and the resulting peak was termed the “maghemite bump” by these authors. These features are common across rock types and sedi-ments, and the increase in susceptibility can be entirely attributed to the annealing of maghemite rim stresses, as described above, whereas the decrease corresponds to

inversion of (ti)maghemite to (ti)hematite (e.g. Stacey and Banerjee, 1974; Dunlop and Özdemir, 1997). Gimme the phase with the humps and a bump.

AcknowledgementI thank Mike for sharing the “maghemite bump” data he

collected (or “beaks” as he refers to them), and provid-ing valuable input and constructive criticism to this short article. I also thank Bruce Moskowitz for his thorough review and thoughtful suggestions.

ReferencesAhmed, I.A.M., Maher, B.A., 2018. Identification and paleocli-

matic significance of magnetite nanoparticles in soils, Proc. Natl. Acad. Sci., 115, 8, 1736-1741.

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Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Funda-mentals and frontiers. Cambridge Studies in Magnetism, Cambridge University Press, Cambridge, 573 pp.

Fabian, K., Shcherbakov, V.P., 2020. The magnetization of the ocean floor: stress and fracturing of titanomagnetite particles by low-temperature oxidation, Geophys. J. Int., 221, 2104–2112.

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Fig. 4. Cycled incremental measurements of temperature dependence of magnetic susceptibility, normalized with respect to the initial value, of the andesite sample from Fig. 1a, measured in argon atmosphere in a) 25 °C steps up to 250 °C, b) in 50 °C steps from 250 to 500 °C (Figure from Kontny and Grothaus (2017)).

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The Institute for Rock Magnetism is dedi-cated to providing state-of-the-art facilities and technical expertise free of charge to any interested researcher who applies and is ac-cepted as a Visiting Fellow. Short proposals are accepted semi-annually in spring and fall for work to be done in a 10-day period during the following half year. Shorter, less formal visits are arranged on an individual basis through the Facilities Manager. The IRM staff consists of Subir Baner-jee, Professor/Founding Director; Bruce Moskowitz, Professor/Director; Joshua Feinberg, Assistant Professor/Associate Director; Mike Jackson, Peat Sølheid and Dario Bilardello, Staff Scientists. Funding for the IRM is provided by the National Science Foundation, the W. M. Keck Foundation, and the University of Minnesota.

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Glasgow, p220.Özdemir, Ö, Dunlop, D.J., 2010. Hallmarks of maghemitization

in low-temperature remanence cycling of partially oxidized magnetite nanoparticles, J. Geophys. Res., 115, K02101, doi:10.1029/2009JB006756.

Özdemir, Ö, Dunlop, D.J., Moskowitz, B.M., 1993. The effect of oxidation on the Verwey transition in magnetite, Geophys. Res. Lett., 20, 16, 1671-1674.

Sidhu P.S., Gilkes R.J., Posner A.M., 1977. Mechanism of low-temperature oxidation of synthetic magnetites. J Inorg Nucl Chem 39:1953–1958.

Stacey F.D., Banerjee S.K., 1974. The Physical Principles of Rock Magnetism. Elsevier, Amsterdam, The Netherlands.

Tang J, Myers M, Bosnick K.A., Brus L.E., 2003. Magnetite Fe3O4 nanocrystals: Spectroscopic observation of aqueous oxidation kinetics. J Phys Chem B 107:7501–7506.

Van Velzen, A., Dekkers, M., 1999. Low-temperature oxidation of magnetite in loess-paleosol sequences: a correction of rock magnetic parameters, Studia geoph. et geod. 43, 357-375.

Van Velzen A.J., Zijderveld J.D.A., 1995. Effects of weathering on single-domain magnetite in Early Pliocene marine marls, Geophys. J. Int., 121, 267-278.

Walz, F., Brabers, V.A.M., Brabers, J.H.V.J., Kronmüller, H., 2007. Vacancy–interstitial annihilation in titanomagnetite by thermal annealing, Phys. Stat. Sol. (a) 204, No. 10, 3514–3525 (2007) / DOI 10.1002/pssa.200723005.

White A.F., Peterson M.L., Hochella M.F., 1994. Electrochem-istry and dissolution kinetics of magnetite and ilmenite. Geochim Cosmochim Acta 58:1859–1875.

From the IRM

Last month Mike Jackson retired from his position as polymath Facilities Manager and Research Professor after 25 unabated years at the IRM. Please join us in congratulating Mike for his outstanding scientific achievements and selfless dedication to the IRM and to the many visitors he has worked with over this past quarter of a century. Mike’s relentless genius has been over-arching and doubtlessy caused a transformative hermeneutic of rock magnetism and its applications. The legacy of Mike’s research speaks for itself and no list would do it any justice: a few highlights include his seminal contributions to magnetic anisotropy, the sagacious inclination shallowing corrections, the dexterous detection of remagnetizations in carbon-ates and redbeds, the erudite investigation of mal-leable Curie temperatures in titanomagnetites, the unparalleled development of an extraordinarily elegant magnetic thermal fluctuation tomography, the facile-processing of complex rock-magnetic data through the monumental IRM database, the marvelous didactic Quarterly articles, the percep-tiveness of listening to baroque music on unfalter-ing loop, and, of course, the brilliant rediscovery of pigs drawn whilst-blindfolded. We wish Mike the very best in his next endeavours and sincerely congratulate him for his well deserved retirement!

In savagely impartial alphabetical order,Bruce, Dario, Josh, Max, Peat, and Subir