Introduction

Our study focuses on concretions from the Jurassic Navajo Sandstone in Utah, with the primary aim of investigating their potential as recorders of long-term magnetic field variations. Specifically, we address the following research questions:

  1. 1.

    Can these concretions preserve records of magnetic field fluctuations, including possible reversals?

  2. 2.

    What insights can the magnetic properties of these concretions provide about their formation processes and timescales?

  3. 3.

    How do the magnetic signatures of these concretions compare to those of marine ferromanganese nodules, and what implications does this have for understanding terrestrial concretion formation?

  4. 4.

    What are the implications of our findings for interpreting similar formations on Mars?

The presence of magnetic reversals within these concretions would provide, for the first time, a means to evaluate the timescale of iron concretion formation. Our observations suggest that the accumulation of iron in these terrestrial concretions may occur over timescales comparable to those of iron-manganese nodules found on the ocean floor. Marine ferromanganese crusts and nodules (MFC/N) on the deep-sea floor, which form through the migration of manganese and iron from reducing to oxidizing environments, grow slowly at rates usually less than 1 cm/Myr achieving size of several centimeters1. Recent studies have shown that these nodules can record long-term environmental changes, including magnetic reversals2,3.

The formation of terrestrial concretions has been the subject of various hypotheses. Chan et al.4,5 and Beitler et al.6 proposed diagenetic models involving the mobilization of Fe(II) from iron-coated sand grains and subsequent reprecipitation as iron oxides under oxidizing conditions. This process is thought to occur in three steps: (1) initial deposition of thin iron oxide films on quartz grains, (2) iron reduction and bleaching due to introduction of reducing fluids, and (3) reprecipitation of iron oxides as dense cements upon mixing of reduced waters with oxygenated meteoric groundwater. While the potential role of iron-reducing bacteria has been suggested7, definitive evidence of biomediation in concretion formation remains elusive8.

While these models provide a foundation for understanding concretion formation, they do not fully account for the potential of concretions to record long-term magnetic field variations. Our study aims to bridge this gap by examining the magnetic properties of concretions in detail, with the goal of better constraining their formation processes and timescales.

The geological setting of our study area, the Grand Staircase-Escalante National Monument in southeastern Utah, provides an ideal natural laboratory for investigating these hypotheses. The Jurassic Navajo Sandstone exposed in this region is characterized by widespread occurrence of iron oxide concretions varying in size from millimeters to several centimeters in diameter. The formation of these concretions is believed to follow a three-step diagenetic model involving initial iron oxide deposition, subsequent mobilization by reducing fluids, and finally, reprecipitation as dense cements4,5,9.

In this study, we employ a multi-faceted approach to investigate the magnetic properties and formation processes of these concretions. We utilize alternating field demagnetization, physical abrasion techniques, and chemical analysis to probe the internal structure and magnetic signatures of the concretions. By comparing our results with models of concretion formation and magnetic field behavior, we aim to shed light on the timescales and mechanisms involved in concretion development.

This interdisciplinary approach combines elements of rock magnetism, sedimentology, and planetary science to enhance our understanding of these enigmatic geological features and their significance in both terrestrial and extraterrestrial contexts.

Materials and methods

Study area and sample collection

Concretions were sampled from geographically distinct locations in the Grand Staircase-Escalante National Monument, southeastern Utah (Fig. 1). The sampling site in the Jurassic Navajo Sandstone was identified and shown to us by Dr. Marjorie Chan (Chan et al.)5. The Grand Staircase is a natural weathering feature several hundred kilometers long from west to east and nearly 2.8 km high from sea level at the northernmost plateau to southernmost at approximately 1.2 km from sea level10.

Three concretions were selected for detailed analysis: RB01, RB05, and RB10 (Fig. 2).

Fig. 1
figure 1

Location of the study area. The main map shows the southwestern United States, with the sample site indicated by a yellow pin in southern Utah. Major cities and state boundaries are labeled for reference. The inset map (top left) provides a detailed view of the ‘RED BREAKS 02’ sampling location within the Grand Staircase-Escalante National Monument. Geographic coordinates are provided for both maps. Scale bars indicate 500 km for the main map and 500 m for the inset. North arrows are included for orientation. Satellite imagery courtesy of Landsat/Copernicus, with data from SIO, NOAA, U.S. Navy, NGA, GEBCO, LDEO-Columbia, NSF, and NOAA (Google Earth Pro, 7.3.6.9796, https://www.google.com/earth/about/versions/ ).

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Fig. 2
figure 2

Concretions collected from the Red Breaks site, Utah. (A) RB01, diameter 9.5 mm, weight 1.2262 g, showing a rough, reddish-brown surface. (B) RB05, diameter ~ 50 mm, weight ~ 17 g; top image shows the concretion in situ, demonstrating its geological context, bottom image after extraction for laboratory analysis. Red arrows indicate orientation lines. (C–J) RB10, diameter 36 mm, weight 45.5 g, displaying various views and cut sections. (C) Top view where red arrow highlights visible layering, (D) bottom view, (E) edge view, (F–J) cut sections revealing internal structure, including the contrast between the outer rind and inner core (visible in G, I, J). Note the color variations from reddish-brown (RB01, RB05) to greenish grey (RB10), reflecting differences in mineralogy or weathering. Observe the textural differences between the rough surface of RB01 and the smoother surfaces of RB05 and RB10. Scale bars = 1 cm for A, 2 cm for B-J. The visible layering, color variations, and internal structures provide crucial information about the formation processes and potential for preserving magnetic signatures over time. These concretions demonstrate the range of sizes and internal structures found at the study site, offering insights into formation processes and potential magnetic recording capabilities.

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Sample characterization

RB01 measured 9.5 mm in diameter and weighed 1.2 g.

RB05 measured approximately 50 mm in diameter and weighed about 17 g. It was extracted from the original rock with the direction of deposition recorded relative to the surroundings, following the orientation method described by Butler11.

RB10 measured 36 mm in diameter and weighed 45.5 g. This concretion was chosen for its thicker ferrous cover to avoid premature disintegration during abrasion.

Magnetic measurements

All magnetic measurements were performed using a 2G cryogenic rock magnetometer 755 4 K (SRM), which measures magnetic remanence by detecting changes in magnetic flux using superconducting quantum interference devices (SQUIDs). This system provides high-sensitivity measurements of magnetic moment in three orthogonal directions, outlined in Kirschvink et al.12.

Alternating field (AF) demagnetization

This technique progressively removes magnetic remanence by exposing the sample to alternating magnetic fields of decreasing amplitude, revealing the stability and directional components of the natural remanent magnetization (NRM).

  • RB01: AF demagnetization was conducted between 5 mT to 50 mT with 5 mT steps, and from 50 mT to 100 mT with 10 mT steps, following the protocol of Zijderveld (1967).

  • RB05: AF demagnetization was performed between 2 mT to 10 mT with 2 mT steps.

Isothermal remanent magnetization (IRM)

This method characterizes magnetic mineralogy by measuring the sample’s ability to acquire magnetization in a strong field at room temperature.

  • RB01 was magnetized at room temperature in any orientation with a pulsed magnetic field (1.26 T), using ASC Impulse Magnetizers (USA), following the method described by Dunlop (1972).

Physical abrasion

This technique reveals the spatial distribution of magnetic carriers by progressively removing outer layers while maintaining orientation control.

  • RB05 and RB10: The outer surface was abraded using a diamond grinder, removing approximately 1 mm of material in each step, adapting the technique described by van der Voo and Torsvik (2012). The uniformity of material removal was monitored by measuring the concretion diameter after each abrasion step using a sliding caliper in multiple orientations to obtain an average diameter. Orientation control was maintained throughout the process using a white arrow marked on an arbitrarily chosen top surface of each concretion; this arrow was redrawn when it became less visible during the abrasion process. Magnetic remanence was measured after each abrasion step.

Sectional analysis

This approach examines spatial variations in magnetic properties through controlled sample dissection.

  • RB10 was cut into top, middle, and bottom sections. The middle section underwent physical abrasion and subsequent magnetic measurements, following a modified version of the method described by Tikoo et al.13.

Thermal demagnetization of the remanence

  • Thermal demagnetization was performed on a representative sample up to 700 °C to confirm magnetic mineralogy (Supplementary Figure S2). The thermal demagnetization behavior, showing complete demagnetization by 700 °C, supports hematite as the primary magnetic carrier.

Chemical analysis

X-Ray Fluorescence (XRF) spectrometry was performed on each concretion using a VANTA device, following the methodology of Potts and Webb14. For RB5 and RB10, both the sand core and ferrous cover layer were analyzed separately. Three beams (accelerating voltage 0–40 KV, 0–10 KV, 0–50 KV) were used, with a measuring time of 30 s each, to better fluoresce both heavier and lighter elements.

Magnetic modeling

Finite element magnetic modelling (FEMM) was used to model a concretion section cutout from a 1 cm diameter concretion (Fig. 3), following the approach of Baltzis (2008)15. It was conducted specifically to understand how progressive removal of material (either through natural weathering or experimental abrasion) affects the overall magnetic signature of layered concretions. The model incorporated three distinct concentric reversed magnetizations. Magnetic properties of hematite were prescribed, and the program created an automatic mesh of points to solve for magnetic parameters, including magnetic field lines and magnetic intensity.

Fig. 3
figure 3

Finite element modeling magnetics (FEMM) model of the magnetic effect of abrading the outermost part of a concretion section containing three layers of opposing magnetization. (A) Image shows three layers of concretion’s magnetic material, each with alternating upward and downward magnetizations (white arrows). Two internal layers have a thickness of 1 mm and the outermost layer has a thickness of 0.5 mm. (B) The plate shows how the magnetic field lines (represented by contour lines) change when decreasing the thickness of the outmost layer (e.g., by abrasion). The decreasing thickness of the outermost layer is shown under each image. Yellow arrows indicate locations of magnetic poles around the concretion section, representing local concentrations of magnetic field lines. Red arrows show the overall dipole signature, indicating the net magnetic polarity of the entire concretion section. The four images in the lower part show details of the overall magnetic change from downward magnetization to upward magnetization as the outermost layer thickness decreases, demonstrating the sensitivity of the net magnetic signature to small changes in layer thickness. This transition is significant as it illustrates how surface weathering can alter the magnetic properties of concretions, potentially affecting paleomagnetic interpretations. Color gradients represent magnetic field intensity, with the scale shown on the right (units: A/m). The abrasion process modeled here simulates natural weathering of concretions, providing insights into how environmental factors may influence magnetic signatures over time.

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Data analysis

Magnetic data were analyzed using Remasoft software (AGICO), as described in Chadima and Hrouda (2006)16. This software was used to generate stereonet projections, vector component (Zijderveld) plots, and normalized intensity decay curves for each sample.

Results

The magnetic analysis of concretion RB01 (diameter 9.5 mm, weight 1.2262 g) revealed multiple magnetic components, evidenced by the characteristic behavior during AF demagnetization (Fig. 4A). The normalized intensity decay curve shows three distinct magnetic components: a soft component manifesting as a sharp intensity decrease up to 10 mT, an intermediate component showing relative stability between 10 and 40 mT, and a hard component revealed by intensity changes above 45 mT. This natural component structure disappears in the saturated sample (RB01s, Fig. 4B), where the demagnetization curve shows more uniform behavior. These characteristics suggest the presence of multiple magnetic carriers with different coercivities, though specific mineral identification would require additional techniques such as such as thermomagnetic analysis or Mössbauer spectroscopy. Our identification of magnetic mineralogy therefore remains tentative, based on coercivity characteristics and comparisons with previous studies of similar formations17. The magnetic carrier is interpreted to be predominantly hematite based on not only AF demagnetization behavior but also on thermal demagnetization results (Supplementary Figure S2), which show characteristic unblocking temperatures near 675 °C.

During alternating field (AF) demagnetization, the natural remanent magnetization (NRM) exhibited directional changes (Fig. 4A). Upon magnetic saturation, RB01’s magnetization (RB01s label in Fig. 4A denotes sample RB01, and similarly other sample labels, after it was magnetically saturated) increased from 0.33 × 10^-6 A/m to 210.0 × 10^-6 A/m (Fig. 4). The saturated sample demonstrated high stability against AF demagnetization, with magnetization dropping by only 30% when subjected to the maximum field of 100 mT.

Concretion RB05 (diameter ~ 50 mm, weight ~ 17 g) displayed changes in magnetization with each step of physical abrasion of its outer layers (Fig. 5). Initial AF demagnetization up to 10 mT resulted in a slight (~ 5%) increase in magnetic intensity with exceptional directional stability (Fig. 5A). The abrasion process revealed complex magnetic behavior (Fig. 5B). The first 3 mm of abrasion led to a > 300% magnetic intensity increase and a directional migration exceeding 90 degrees. Subsequent abrasion steps showed alternating patterns of intensity increases and decreases, accompanied by significant directional changes. These changes continued until the concretion lost its integrity.

RB10 (diameter 36 mm, weight 45.5 g) was analyzed in sections: whole (RB10), upper (RB10u), middle (RB10m), and lower (RB10l) (Fig. 6). Each section exhibited distinct magnetic directions: RB10u pointed upward, RB10l pointed downward, the whole concretion (RB10) pointed slightly above the horizontal eastward direction, and RB10m pointed slightly below the horizontal in the North-east direction. Physical abrasion of the middle section (RB10m) revealed an increase in magnetization intensity after removing 3 mm of thickness.

Finite element magnetic modelling (FEMM) of a concretion section cutout (Fig. 3) provides crucial context for interpreting both our experimental results and the potential effects of natural weathering on paleomagnetic signals. The model showed that small changes in the thickness of the outermost layer could lead to significant changes in the overall dipole signature, transitioning from downward to upward magnetization.

X-ray fluorescence (XRF) spectrometry results (Fig. 7) showed that the concretions contained major amounts of silicon and iron (See Methods). In the sand core measurements of RB5 and RB10, the iron content significantly decreased compared to their respective crusts. The concentration of iron in the concretions related to their intensity of respective magnetizations. The ratio of titanium to iron was found to be negligible across all samples. Other elements were present in smaller quantities, contributing to the overall composition of the concretions.

Fig. 4
figure 4

Magnetic demagnetization analysis of concretion RB01 using Remasoft software (AGICO). (A) Alternating Field (AF) demagnetization of the Natural Remanent Magnetization (NRM) up to 100 mT. (B) AF demagnetization after imparting a Saturation Isothermal Remanent Magnetization (SIRM) in a 1.3 T external pulse magnetic field. For both A and B, the upper left shows a stereonet projection (equal area) of directional changes during demagnetization, with solid/open circles representing lower/upper hemisphere projections. The upper right displays a vector component diagram (Zijderveld plot) showing horizontal (blue, XY plane) and vertical (green, ZY plane) projections of the magnetization vector. The lower left presents a normalized intensity decay curve, and the lower right shows the demagnetization data table. NRM exhibits scattered directional changes and lower intensity (Mmax = 3.51 A/m) compared to the more stable, single-component SIRM (Mmax = 207 A/m). These results indicate the presence of multiple magnetic components in the natural state and reveal the concretion’s capacity to acquire strong magnetization, providing insights into its magnetic mineralogy, domain state, and potential for paleomagnetic recording.

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Evidence for complex magnetic history

Our evidence for multiple magnetic recording events comes from three independent lines of investigation:

  1. 1.

    Spatial variation: different sections of individual concretions (particularly RB10) show distinct magnetic directions that are unlikely to result from a single recording event.

  2. 2.

    Outside to inside consistency: the systematic changes in magnetic properties when removing the surface layer in our concretion samples suggest a time-sequential recording process rather than random variation.

  3. 3.

    Physical properties: the presence of both soft and hard magnetic remanence components, combined with the layered structure of the concretions, supports multiple recording episodes.

Alternative interpretations of these observations are possible, including self-reversal processes during initial formation. However, the combination of these three lines of evidence most consistently supports a multi-stage formation process.

Fig. 5
figure 5

Vector magnetic analysis of concretion RB05 using Remasoft software (AGICO). (A) Alternating field (AF) demagnetization up to 10 mT. (B) Magnetic behavior during 14 successive abrasions with a diamond wheel, each removing approximately 1 mm of the outer layer. For both A and B: Upper left shows equal-area stereonet projections of magnetic directions (solid/open symbols indicate lower/upper hemisphere); upper right displays vector component (Zijderveld) plots with horizontal (blue) and vertical (green) projections. Lower left in A shows normalized magnetic intensity change vs. AF steps (in mT), while in B it shows changes over 14 abrasion steps (0 being pre-abrasion, followed by 13 abrasion steps). Lower right presents step-wise data. The contrast between A and B demonstrates the minimal impact of AF demagnetization on the concretion’s magnetization compared to the significant changes induced by physical abrasion. This comparison reveals that the concretion’s magnetic carriers are more resistant to alternating magnetic fields than to physical removal, suggesting a stable and complex distribution of magnetic minerals within the concretion layers. These results provide crucial insights into the robustness of the concretion’s magnetization against AF demagnetization and its sensitivity to physical alteration, which has important implications for understanding its potential in preserving palaeomagnetic information.

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Fig. 6
figure 6

Magnetic analysis of concretion RB10 using Remasoft software (AGICO). (A) Natural Remanent Magnetization (NRM) measurements of RB10 in various configurations. The stereonet (upper left) and vector component plot (upper right) show magnetic directions for the whole concretion, middle, upper, and lower parts (refer to Fig. 2 for part locations). The lower left panel displays relative magnetization intensities of each part. Note that the upper section has two measurements: one with and one without sand fill. (B) NRM measurements of the middle section of RB10 (RB10m) after successive abrasions. The stereonet (upper left) and vector component plot (upper right) illustrate changes in magnetic direction during the abrasion process. The lower left panel shows the decay of magnetization intensity with each abrasion step. In both A and B: Stereonets use solid/open symbols for lower/upper hemisphere projections. Vector component plots show horizontal (blue) and vertical (green) projections. This comparison reveals the spatial variability of magnetic properties within the concretion (A) and the effects of physical abrasion on its magnetic signature (B), providing insights into the concretion’s internal magnetic structure and its response to weathering processes.

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Fig. 7
figure 7

Elemental composition analysis of concretions RB1, RB5, and RB10. The bar chart displays element concentrations in parts per million (ppm) for various chemical elements. Each concretion sample is represented by a different color: RB10 (blue), RB10sand (orange), RB5 (gray), RB5sand (yellow), and RB1 (light blue). ‘Sand’ suffix indicates measurements of the sand fill within the concretions. Elements are arranged along the x-axis, with major elements (Mg, Al, Si, P, K, Ca, Ti, Mn, Fe) followed by derived values (LE: Light Elements, likely representing organic matter and water content; Fe/Ti: iron to titanium ratio). The y-axis shows concentration in ppm, with a maximum value of 700,000 ppm. This chart allows for direct comparison of elemental compositions across different concretion samples and their associated sand fills, highlighting variations in mineral content and potential formation processes.

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Discussion

Concretions as recorders of magnetic field fluctuations

The magnetic analyses of concretions RB01, RB05, and RB10 reveal complex magnetic behavior that provides insights into their formation processes and potential for recording paleomagnetic information.

Iron oxide transformation and temporal constraints

Our magnetic analyses is interpreted within the context of iron oxide transformation pathways. The initial precipitation occurs as Fe(OH)3·nH2O, which subsequently transforms to porous cryptocrystalline goethite (See Supplementary Figure S01) with low or non-existing remanent magnetization due to its small size (super-paramagnetism), and eventually converts to hematite through dehydration processes (see thermal magnetic remanence decay curve consistent with hematite in Supplementary Figure S2), picking up the ambient field, and developing the magnetic remanence component. The dehydration timescale (τ) follows the relationship:

\(\tau \,=\,\tau 0{\text{ }}\exp (E/R*(1/T - \,1/T0))\)

where τ0 is the timescale at reference temperature T0, T is the dehydration temperature, E is the activation energy, and R is the gas constant. Using values from Goss (1987): τ0 = 1000 min, E = 250 kJ/mol, T0 = 400 K, we calculate that at ambient temperature (T = 300 K), the dehydration timescale approaches 0.1Gyr. This extended transformation period supports our interpretation of the complex magnetic signatures observed in the concretions.

The presence of both goethite and hematite in our samples, along with their distinct magnetic behaviors, suggests that we are observing different stages of this transformation process within individual concretions. This provides a natural explanation for the multiple magnetic components we observe and supports the possibility of recording magnetic field variations over extended periods.

Formation timescales and magnetic recording

Recent models of concretion formation, particularly those by Yoshida et al. (2018)18, suggest relatively rapid initial formation through CaCO3 precursor dissolution, with timescales of up to 10,000 years. While these models effectively explain the physical formation of concretion structures, our magnetic data suggest a more complex temporal sequence. We propose a two-stage process:

  1. 1.

    Initial formation: rapid precipitation of Fe(OH)3 following CaCO3 dissolution, occurring over thousands of years as modeled by Sirono et al. (2021)

  2. 2.

    Extended transformation: subsequent dehydration and transformation of iron oxides, occurring over much longer periods as demonstrated by our dehydration rate calculations.

This two-stage model reconciles the rapid physical formation with our observed magnetic properties. The multiple magnetic components and contrasting orientations we observe likely reflect this extended transformation period, during which the concretions could record changing magnetic field directions. The presence of both goethite and hematite, with their distinct magnetic behaviors, supports this interpretation.

RB01’s magnetic behavior, characterized by both soft and hard magnetic coercivities, suggests the presence of multiple magnetic carriers or domains within the concretion. The significant increase in magnetization upon saturation indicates that the natural remanent magnetization (NRM) represents only a small fraction of the concretion’s total magnetic capacity. This discrepancy could be attributed to the presence of opposing magnetic components that partially cancel each other out in the natural state, a phenomenon observed in other geological materials19.

The complex magnetic behavior observed in RB05 during physical abrasion provides strong evidence for the presence of multiple magnetic layers within the concretion, each potentially recording different magnetic field orientations.

The sectional analysis of RB10 further supports the hypothesis of layered magnetic structure within the concretions. The contrasting magnetic directions observed in different sections indicate that different parts of the concretion may have formed at different times or under different magnetic field conditions, a phenomenon also observed in some sedimentary rocks (Kodama, 2012)20.

The magnetic carriers in these concretions formed through progressive oxidation and dehydration, as revealed by backscattered electron imaging (Figure S1). Three distinct mineral phases are observed: primary detrital quartz grains (dark grey/black, 100–200 μm), a pervasive goethite cement network (light grey), and discrete hematite crystals (bright white, 10–20 μm) preferentially located at quartz-goethite interfaces.

The goethite cement creates a characteristic porous network structure throughout the concretion, filling spaces between original quartz grains. While volumetrically dominant, this goethite is largely superparamagnetic at room temperature21, preventing it from carrying a stable magnetic remanence. However, its abundance represents a significant reservoir for potential future hematite formation, suggesting these concretions remain “active” recorders.

The spatial distribution of hematite crystals, particularly their occurrence at quartz grain boundaries, provides strong evidence for their formation as a secondary phase through dehydration of the goethite cement. These interfaces served as energetically favorable nucleation sites during transformation. The resulting hematite crystals (10–20 μm) fall within the stable single-domain size range capable of recording and maintaining magnetic remanence22.

The coexistence of abundant goethite cement with scattered hematite crystals displaying multiple magnetic components suggests a remarkably long-lived and gradual recording process. Multiple dehydration events created hematite at different times, with early-formed crystals preserved while leaving sufficient goethite for continued transformation. This process architecture explains both the presence of multiple magnetic components within single concretions and their potential to record magnetic field reversals over extended time periods. The stability of goethite under consistent oxidizing conditions, combined with its gradual transformation to hematite during discrete dehydration events, creates a natural mechanism for preserving a long-term record of magnetic field variations.

These findings collectively suggest that the Utah concretions formed through a complex, multi-stage process that likely occurred over an extended period. The presence of multiple magnetic regions with contrasting orientations indicates that these concretions may serve as valuable records of paleomagnetic field variations, potentially including reversals or excursions (Tarduno et al., 2015)23.

Insights into concretion formation processes and timescales

This complex magnetic signature aligns with the diagenetic model proposed by Chan et al.5 and Beitler et al.6, but our findings suggest a more intricate history than previously recognized. The presence of both soft and hard magnetic coercivities indicates that this process may occur multiple times over an extended period, challenging notions of rapid formation.

The observation of multiple magnetic components, combined with our SEM evidence showing scattered hematite crystals within a goethite matrix, suggests a prolonged formation process. The BSE images reveal discrete hematite crystals (10–20 μm) preferentially formed at quartz-goethite interfaces through localized dehydration events, indicating multiple episodes of magnetic recording rather than a single formation event. This mechanism could span extended time periods, similar to the formation of marine ferromanganese nodules1,2,3,24.

The FEMM modeling results demonstrate the sensitivity of the overall magnetic signature to small changes in the thickness of magnetized layers. This finding has significant implications for interpreting the magnetic signals of weathered concretions, as it suggests that surface erosion could substantially alter the apparent magnetic properties, a concern also raised in studies of magnetic anomalies25,26.

Comparison with marine ferromanganese nodules

While our study focuses on terrestrial concretions, the observed prolonged growth process shows similarities to the formation of marine ferromanganese nodules. Both types of formations appear to record long-term environmental changes, including magnetic field variations.

Marine ferromanganese nodules typically grow at rates of 1–10 mm/Myr24, accumulating iron and manganese oxides from seawater over millions of years. Similarly, our findings suggest that terrestrial iron oxide concretions may form over extended periods, potentially spanning thousands to millions of years. This slow growth rate allows both types of formations to potentially record long-term geomagnetic field variations.

Recent studies by Yi et al.2,3 have demonstrated that marine ferromanganese nodules can preserve paleomagnetic records spanning millions of years. They found that nodules from the eastern Pacific recorded geomagnetic field changes over the past 4.7 million years, including multiple polarity reversals. Our observations of multiple magnetic layers with contrasting orientations in terrestrial concretions suggest a similar potential for recording long-term field variations.

The magnetic properties of marine nodules and terrestrial concretions also show some similarities. Both exhibit complex magnetic behavior, often with multiple magnetic carriers2. However, while marine nodules typically contain a mix of iron oxides and hydroxides, including magnetite and maghemite27, our terrestrial concretions appear to be dominated by hematite. Our concretions show distinctive magnetic properties characteristic of hematite, including high coercivities (> 100 mT). The BSE imaging reveals bright, high-atomic-number crystals (10–20 μm) at quartz-goethite interfaces, consistent with hematite formation through goethite dehydration. The softer magnetic component (coercivities < 10 mT) likely represents magnetite inclusions within the original quartz grains that have acquired viscous remanence, a common feature in sedimentary rocks19. Chan et al.5,28 reported that concretions from the Navajo Sandstone are primarily composed of iron oxides, with hematite being the dominant phase. They also noted the presence of minor amounts of goethite in some samples. This mineralogical composition is consistent with the reddish-brown color observed in our samples RB01 and RB05, which is characteristic of hematite-rich formations.

The formation mechanisms of these two types of geological features differ significantly. Marine nodules form through direct precipitation from seawater, often nucleating around a core object29, while terrestrial concretions form through diagenetic processes in sandstone environments. Despite these differences, both processes appear to result in layered structures capable of recording magnetic information over time.

The comparison between terrestrial concretions and marine nodules opens up intriguing possibilities for paleoenvironmental studies. Just as marine nodules have been used to reconstruct past ocean circulation patterns and redox conditions30, terrestrial concretions might provide insights into past groundwater conditions, fluid flow patterns, and redox changes in continental settings.

Further research comparing the magnetic signatures and formation processes of these terrestrial concretions with marine nodules could provide additional insights into long-term environmental changes in different geological settings. Such comparative studies could help elucidate the factors controlling magnetic signal acquisition and preservation in these slow-growing geological formations, potentially enhancing our ability to extract paleoenvironmental and paleomagnetic information from both terrestrial and marine records.

Implications for Martian “blueberries”

The similarities between these terrestrial concretions and the “blueberries” observed on Mars by the Opportunity rover raise possibilities for Martian paleomagnetic studies (Squyres et al., 2004)31. Our observations of complex magnetic behavior in terrestrial concretions provide a framework for understanding how similar formations might preserve magnetic signatures, though the different environmental conditions on Mars must be considered when making such comparisons32. The complex magnetic behavior observed in our terrestrial samples suggests formation over extended periods, challenging notions of rapid precipitation events on Mars. If Martian blueberries formed over similar timescales, they could offer valuable information about past aqueous environments and their evolution33. Our FEMM modeling demonstrated the sensitivity of magnetic signatures to small changes in layer thickness, implying that weathering of Martian blueberries could significantly alter their apparent magnetic properties - a crucial consideration for future in-situ measurements. Moreover, the potential for long-term, gradual formation suggests these concretions could be promising targets in the search for preserved biomarkers, if microbial life ever existed on Mars34. To test these hypotheses, future Mars missions could prioritize in-situ magnetic measurements of blueberries at different depths, sample return missions targeting these concretions, and development of non-destructive imaging techniques to study their internal structure in situ. Such investigations could significantly enhance our understanding of Mars’ geological and magnetic history, potentially providing unprecedented insights into the planet’s past environments, magnetic field evolution, and the history of water and habitability35.

Interpretation of multiple magnetic components

The observed patterns in our concretions can be interpreted in several ways:

  1. 1.

    Multiple formation events: the distinct magnetic directions in different layers could represent recording during different periods of Earth’s magnetic field.

  2. 2.

    Chemical alteration: post-formation chemical processes might have created layers with different magnetic properties, though this would not easily explain the systematic directional changes observed.

  3. 3.

    Physical weathering: while weathering could affect magnetic properties, the consistent stratigraphic patterns argue against this as the primary cause.

The combination of spatial variation, stratigraphic consistency, and distinct magnetic components most strongly supports a multi-stage formation process, though we acknowledge that additional studies, particularly thermal demagnetization experiments, may help discriminate between these possibilities.

Future research directions

Future research should focus on expanding the sample size, including concretions from diverse geological settings, and incorporating absolute dating techniques to better constrain the timing of concretion formation. While we observe variations in iron content (XRF) between various concretion material regions, the spatial resolution of our current chemical analyses does not allow for detailed layer-by-layer correlation with magnetic directions.

Our findings point to specific questions about the formation sequence of magnetic minerals in these concretions. The presence of both high- and low-coercivity components suggests multiple phases of magnetic mineral formation, but the relative timing remains uncertain. Future work should prioritize establishing this chronology through careful regional sampling and targeted dating of the magnetic phases.

Conclusion

This study provides systematic evidence of complex magnetic behavior within iron oxide concretions from the Jurassic Navajo Sandstone in Utah. Our multi-technique analysis, combining alternating field and thermal demagnetization, physical abrasion, chemical analysis, and magnetic modeling, reveals multiple magnetic components with contrasting directions within individual concretions. The complete thermal demagnetization by 700 °C confirms hematite as the primary magnetic carrier.

The magnetic patterns we observe could result from several processes: sequential recording of ambient field changes during formation, chemical alterations affecting magnetic properties post-formation, physical weathering or diagenetic processes. Our scanning electron microscopy reveals discrete hematite crystals (10–20 μm) within a goethite cement matrix, supporting a two-stage formation model. This model involves rapid initial precipitation of iron hydroxides followed by extended mineral transformation to hematite, potentially explaining both the physical structure and the complex magnetic signatures we observe. The presence of both phases suggests this transformation process may record magnetic field changes over extended periods.

Future studies should focus on comprehensive rock magnetic analyses to fully characterize domain states and magnetic stability. Investigation of larger sample sets from diverse locations within the Navajo Sandstone, combined with absolute dating techniques, would help constrain formation timing and regional variability. Establishing these concretions’ reliability as paleomagnetic recorders will require additional analytical approaches.

Our observations may have implications for understanding similar formations on Mars, particularly the hematite spherules or “blueberries” observed by the Opportunity rover in Meridiani Planum. The identification of both goethite and hematite phases in our samples, coupled with evidence for progressive mineral transformation, suggests similar processes could have operated during Martian concretion formation. However, extrapolation from terrestrial to Martian conditions requires careful consideration of the different environmental and geological contexts, including lower temperatures and atmospheric pressure, different aqueous chemistry and pH conditions, absence of significant magnetic field during potential formation periods, and different weathering conditions. These factors make direct comparisons speculative until further in-situ analyses of Martian concretions becomes available.