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The natural remanent magnetization (NRM) of high sedimentation rate sediments provides significant information about paleomagnetic secular variation of the Earth's magnetic field and can also potentially be used for stratigraphy. However, NRM acquisition depends on conditions inherent to the depositional environment. In addition to recording a precise annual chronology, varved sediments reflect marked annual sedimentary changes. The Earth's magnetic field does not vary significantly over such a short period, so magnetic changes recorded by varves are expected to reflect the influence of depositional parameters on the recording process. We focus here on a sequence of 27 ± 1 varves from the former proglacial Lake Ojibway (∼8.5 ka cal BP) from which individual cm‐thick summer and winter beds were sampled. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. A mean inclination shallowing of 24.3° is observed in winter beds, along with an 11.3° shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20° occur between successive beds. Summer beds are thicker than winter beds and have stronger magnetic susceptibility, higher Ca/Fe ratios and coarser sedimentary and magnetic grains. This grain size pattern reflects the input of coarser detrital particles during summer, while the finer fraction remained in suspension until it was deposited in winter. A combination of differential compaction between the winter and summer beds, seasonally varying physical and magnetic properties of sediments, and delayed NRM acquisition explains the variable and coercivity‐dependent inclination shallowing. Key Points The seasonal variations have an impact on the remanent magnetization in varved sediments

Volcanic rocks, preserving paleorecords of Earth's magnetic field, are essential to constrain the working of the geodynamo, provided their primary signal was not biased. Using a thermomagnetometer, we simulate a situation where a sample's primary record, carried by a thermoremanence (TRM, acquired by cooling in air from 600°C to room temperature), is partly overprinted by a chemical remanence (CRM, acquired by 200 hr of isothermal exposure at 400°C). This situation leads to two directional and intensity components (in the form of linear segments) in the Zijderveld and Arai‐Nagata diagrams. In the case of unstable titanomagnetite grains prior to CRM acquisition, we show that both components can be strongly biased by up to ∼50° for paleodirections and ∼50% for paleointensities. In such a worst‐case scenario, the secondary CRM strongly overprints the primary TRM, rendering the common interpretation of Zijderveld and Arai‐Nagata diagrams in terms of characteristic components invalid. Plain Language Summary Volcanic rocks, the magnetic minerals of which can acquire a thermoremanent magnetization (TRM) from Earth's magnetic field during their initial cooling, are essential to constrain the working of the geodynamo through Earth's history. However, if the rock is subsequently reheated at moderate temperature in another ambient field, the initial record can be partly overprinted by a chemical remanence (CRM). Starting from a TRM applied on materials of various thermostability, we reproduced in laboratory conditions the acquisition of a CRM by annealing the rock in a controlled ambient field for 200 hr at 400°C. Rock‐magnetic and structural analyses at regular intervals, supplemented by continuous measurements of the remanent magnetization, indicate the creation of new phases as a result of oxidation processes. The paleomagnetic analysis of the final products reveals the existence of two distinct components that can be associated with the initial TRM and the secondary CRM. Whereas the paleomagnetic record of the initial TRM is trustworthy for the most stabilized initial products, directional (up to 50°) and intensity (up to 50%) biases are observed for the least stabilized initial products, illustrating a critical example where the interpretation of characteristic components (linear segments in the interpretation diagrams) is invalid. Key Points We overprint a thermoremanent magnetization (TRM) by cooling from 600°C with a chemical remanent magnetization by 200 hr exposure at 400°C Chemical remanent magnetization was produced by the creation of cation‐deficient titanomagnetite phases, then stabilized by oxyexsolution Paleodirections and intensities are strongly biased in the worst scenario, making the interpretation of characteristic components invalid

How and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols. Plain Language Summary Remanent magnetizations preserved in sedimentary rocks serve as a continuous record of Earth’s magnetic field history and play a fundamental role in understanding the Earth system. It is commonly assumed that magnetic minerals align with the magnetic field as a particle settles through the water column, known as a depositional remanent magnetization (DRM). However, diagenesis can lead to chemical growth of magnetic minerals, known as a chemical remanent magnetization (CRM). CRM lacks stratigraphic continuity and can obscure or completely overprint the original magnetization any time after sediment deposition, leading to a magnetic record that is uncorrelated with the age of the rock. Yet, CRMs go largely unrecognized. Theory and experiments in our paper document that CRMs record the magnetic field six times more efficiently than DRMs. Our work provides a way to distinguish the two through routine laboratory protocols. Key Points Recording efficiency of chemical remanent magnetization (CRM) is six times higher than depositional remanent magnetization (DRM) Undetected chemical remanences lead to overestimated relative paleointensity estimates Comparison of natural and laboratory magnetization and demagnetization behavior help identify chemical remanent magnetizations in sediments

The movement history of boulders is crucial for the reconstruction of paleo‐tsunamis. We report findings from viscous remanent magnetization studies of the boulders on Tongatapu Island, aiming to reconstruct their reworkings. Two boulders exhibited viscous remanence, whereas two larger boulders lacked viscous components but exhibited stable remanence. Both the viscous and stable components deviated from the geomagnetic field direction. These observations indicate that: (a) the boulders with a viscous component were reworked before the latest event, which could have reworked all boulders, and (b) the magnitude of the latest event was larger than that of an earlier event. The reworked timing indicated that the event occurred between 3,000 years ago and the fifteenth century. The difference in the wave height required to move boulders on the eastern and western coasts suggests that the source of the earlier tsunami was likely an eruption due to volcanoes along the Tonga Ridge. Plain Language Summary This study explores prehistoric tsunamis on Tongatapu Island by examining large rocks transported during the past tsunamis. Upon analyzing the magnetic records of the rocks, we discovered evidence of multiple rock movements, shedding light on the recurrence and magnitude of past events. Magnetic measurements and previously dated latest movement ages revealed that a tsunami occurred between 3,000 years ago and the fifteenth century. Considering the mobility and immobility of the large rocks and their sizes, the data suggest a smaller paleo‐tsunami magnitude than that of the fifteenth‐century event that transported all the rocks; this expands our understanding of natural hazard history and has implications for assessing tsunami risks in the region. The approach adopted here provides a valuable model for investigating historical natural disasters and their impact on coastal areas, offering insights to enhance global tsunami hazard assessment strategies. Key Points Viscous remanent magnetization of large boulders indicates multiple reworkings, enhancing our understanding of past tsunamis in Tongatapu Combined magnetic records and published radiocarbon ages suggest tsunamis occurred between 3,000 years ago and the fifteenth century Remanence and wave height estimates show that the older tsunami was smaller than the fifteenth‐century event, with a possible volcanic source

Multipolarity remanence in greigite‐bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite‐bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single‐polarity” to “multi‐polarity” samples. The inward alteration of sulfide grains is topotactic and is size‐dependent with higher alteration in smaller grains. We propose a multi‐phase self‐reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self‐reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications. Plain Language Summary Some magnetic minerals in nature can be magnetized opposite to the external geomagnetic and planetary magnetic fields—a peculiar phenomenon called “self‐reversal.” A self‐reversal magnetization process is typically observed to occur in igneous rocks during cooling in an external field. Here, using magnetic and microscopic analyses we demonstrate that sediments containing authigenic ferrimagnetic iron sulfide mineral—greigite—can acquire a self‐reversed magnetization during progressive surface alteration of greigite nanoparticles. Surface alteration produces new “magnetic shells” that are magnetized opposite to the magnetization of the parent greigite core through magnetic interactions due to the close contact between the core and shell. Post‐depositional sedimentary processes, for example, percolation of fluids or oxygenation could trigger surface alteration that leads to “self‐reversal” and complicate the primary magnetization records. This self‐reversal mechanism can explain the commonly reported anomalous magnetization records of authigenic greigite; it is very useful for correct interpretations of tectonic and paleoenvironmental processes, and geological age frames of iron sulfide bearing sediment sequences. Key Points Surface alteration of diagenetic greigite to magnetite and pyrrhotite causes magnetic softening and multipolarity remanence The original microtextures are preserved during iron sulfide alteration and the alteration extent is size‐dependent A new multi‐phase self‐reversal model of diagenetic greigite is proposed that has broad geochronological and geophysical implications

Inclination is the angle of a magnetization vector from horizontal. Clastic sedimentary rocks often experience inclination shallowing whereby syn‐ to post‐depositional processes result in flattened detrital remanent magnetizations relative to local geomagnetic field inclinations. The deviation of recorded inclinations from true values presents challenges for reconstructing paleolatitudes. A widespread approach for estimating flattening factors (f) compares the shape of an assemblage of magnetization vectors to that derived from a paleosecular variation model (the elongation/inclination [E/I] method). Few studies exist that compare the results of this statistical approach with empirically determined flattening factors and none in the Proterozoic Eon. In this study, we evaluate inclination shallowing within 1.1 billion‐year‐old, hematite‐bearing red beds of the Cut Face Creek Sandstone that is bounded by lava flows of known inclination. Taking this inclination from the volcanics as the expected direction, we found that detrital hematite remanence is flattened with f=0.650.560.75$f=0.6{5}_{0.56}^{0.75}$whereas the pigmentary hematite magnetization shares a common mean with the volcanics. Using the pigmentary hematite direction as the expected inclination results in f=0.610.550.67$f=0.6{1}_{0.55}^{0.67}$ . These flattening factors are consistent with those estimated through the E/I method f=0.640.510.85$\\left(f=0.6{4}_{0.51}^{0.85}\\right)$supporting its application in deep time. However, all methods have significant uncertainty associated with determining the flattening factor. This uncertainty can be incorporated into paleomagnetic poles with the resulting ellipse approximated with a Kent distribution. Rather than seeking to find “the flattening factor,” or assuming a single value, the inherent uncertainty in flattening factors should be recognized and incorporated into paleomagnetic syntheses. Plain Language Summary The magnetization of ancient sedimentary rocks provides great insight into Earth's past. Earth scientists use these rocks to understand how Earth's magnetic field has flipped through time and to reconstruct how continents have moved. Hematite is a common mineral which gives many sandstones a red color—leading geologists to refer to them as “red beds.” While hematite is a reliable magnet through time, the magnetic directions recorded by hematite grains can be shallower than the geomagnetic field (i.e., they are flattened). Magnetization steepness is how Earth scientists determine the latitude where rocks were deposited as the magnetic field gets steeper toward the pole. We need ways to correct for magnetization shallowing in sedimentary rocks. In this study, we compared the steepness of magnetic directions held by hematite to that of lava flows that formed in the same time interval. Magnetic directions from lava flows are not flattened so this comparison allows us to determine the shallowing amount. We compare it to a statistical method and see that the results are indistinguishable within the appreciable uncertainty of the methods. Earth scientists should include the uncertainty associated with inclination shallowing when they report ancient pole positions determined from such flattened magnetic directions. Key Points Inclination shallowing is empirically quantified in 1.1 Ga clastic sedimentary rocks bracketed by volcanics Detrital hematite remanence is flattened by a factor of 0.610.550.67$0.6{1}_{0.55}^{0.67}$relative to unflattened pigmentary hematite Flattening factor uncertainty is present in all methods and should be incorporated into the uncertainty of sedimentary paleomagnetic poles

Palaeo-monsoon and palaeoclimate conditions over Southeast Asia are a matter of debate despite notable studies on the continental and oceanic sedimentary record. The present study investigates the environmental magnetic and geochemical records preserved in the deep marine sediments of the northeastern (NE) Arabian Sea to elucidate the erosion history of the western Himalayas and its link with the prevailing hydroclimatic conditions since the late Miocene. For this, the sediment core retrieved during International Ocean Discovery Program (IODP) Expedition 355 at Site U1457 in the NE Arabian Sea has been explored. The results reveal that the hydroclimatic conditions were predominantly arid during the late Miocene, except for humid intervals from 6.1 Ma to 5.6 Ma. Humid climate conditions in the Indus River Basin returned during the mid-Pliocene and continued to the Pleistocene with an intense chemical weathering regime from 1.9 Ma to 1.2 Ma. The dominant sediment source to the NE Arabian Sea at Site U1457 during the late Miocene and the Pliocene was the Indus River, while during the Pleistocene, mixed sediments brought by the Indus River and the Peninsular Indian rivers were observed. The sediment contribution from a chemically less altered mafic source (the Deccan basalts) increased between 1.2 Ma and 0.2 Ma, possibly linked to a weak Indian Summer Monsoon. The summer monsoon wind strength and associated shift in the Inter-Tropical Convergence Zone (ITCZ) influenced the dominant sediment provenance at Site U1457 of the Laxmi Basin.

Greigite (Fe3S4) is a ferrimagnetic iron‐sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo‐ and rock‐magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well‐defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In‐situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid‐state FeS to greigite transformation with an activation energy of 78–90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 and 100 K, respectively. The mixing trend coincides with empirical data reported for natural greigite‐bearing sediments, suggesting a common SP endmember size of 5–10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20–50 nm determined by X‐ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments. Plain Language Summary Sediments provide continuous records of Earth's ancient magnetic field, which lend insights into the workings of the geodynamo and help to establish the geologic time scale through global magnetostratigraphic correlation. Greigite is a magnetic iron sulfide mineral that commonly forms after deposition, thereby remagnetizing the sediment and complicating interpretation of the magnetic record. Understanding greigite formation and detecting its presence is fundamental for obtaining reliable records of the paleomagnetic field, yet knowledge of how greigite grows and how its magnetic properties evolve during growth remains limited. This article outlines a novel approach to form greigite in sediments and to monitor its growth kinetics, grain size and magnetic remanence acquisition. The magnetic properties of the synthetic sediments resemble those of natural greigite‐bearing sediments and match well with theoretical calculations, which can help quantify grain sizes in sedimentary greigite. The reported method and our results contribute to a better understanding of greigite formation and chemical magnetic remanence acquisition in sediments. Key Points We present a new method to grow greigite in aqueous sediments and create a chemical remanent magnetization under controlled conditions Greigite grain sizes of 20–50 nm span the superparamagnetic to single domain threshold, consistent with theoretical predictions Our experimental hysteresis data coincide with calculated mixing curves allowing better quantification of greigite particle sizes in nature

—A new approach to the solution of kinetic equations describing the process of formation of anhysteretic remanent magnetization (ARM) is proposed, which made it possible to accelerate the numerical calculation of the process of formation of ARM by two orders of magnitude for uniaxial oriented non-interacting single-domain particles, while practically not yielding in accuracy to a strict numerical solution. It follows from the calculation results that the susceptibility of ARM is entirely determined by the magnitude of the particle’s coercivity parameter . The data of the previous approximate calculations of ARM value are compared with the exact results presented here and it is shown that the discrepancy between the exact data and the approximate estimates increases with the growth of g , but remains relatively small, within 23%. The proposed algorithm for the rapid calculation of kinetic equations allows us to analyze with physical rigor the method of pseudo-Thellier estimation of paleointensity for an ensemble of single-domain particles, which is supposed to be done in subsequent works.

—The pseudo-Thellier method is numerically simulated based on a rigorous solution of kinetic equations for uniaxial randomly oriented noninteracting single-domain grains. Laboratory experiments have been performed to determine the relative paleointensity B anc with thermoremanent magnetization (TRM) created in samples of igneous rocks in random fields B rand ; the domain structure of grains of these samples varies from single- to multi-domain. Both predicted and measured pseudo-Arai plots can be divided into two quasi linear segments, one in the relatively low-coercivity region B c < 40–50 mT and the other at higher alternating field (AF) amplitudes. The determinations of relative paleointensity B anc on TRM-bearing igneous rocks, estimated from low-coercivity segments of pseudo-Arai plots, give fairly good results with a linear correlation coefficient R = 0.8 between the true field B rand and B anc determined by the pseudo-Thellier method. It is shown that when thermal fluctuations for relatively small and magnetically soft particles (which corresponds to low blocking temperatures) are taken into account, a significant difference is observed between the coercive force B cr of a grain and the actual field of its magnetization (demagnetization). The main conclusion of the study is that applying the pseudo-Thellier method to igneous rocks is promising. Further development of the methodological and practical aspects of this approach could yield interesting results, particularly when analyzing samples that are unstable to changes in magnetic minerals during the classical Thellier procedure.