# Hypotheses for the water and metal fluxes to the rippled Amapari Marker Band, Gale Crater, Mars

Edwin S. Kite, Patrick J. Gasda, Christophe J. Tino, Catherine Weitz, L. Thompson, B.M. Tutolo, C.A. Mondro, W.H. Farrand, S. Gupta, J. Schieber, et al.

► To cite this version:

Edwin S. Kite, Patrick J. Gasda, Christophe J. Tino, Catherine Weitz, L. Thompson, et al. Hypotheses for the water and metal fluxes to the rippled Amapari Marker Band, Gale Crater, Mars. Earth and Planetary Science Letters, 2025, 660, pp.119347. 10.1016/j.epsl.2025.119347. hal-05089127

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# Hypotheses for the water and metal fluxes to the rippled Amapari Marker Band, Gale Crater, Mars

E.S. Kite<sup>a,∗</sup>, P. Gasda<sup>b,†</sup>, C.J. Tino<sup>c,‡</sup>, C. Weitz<sup>d,‡</sup>, L. Thompson<sup>e</sup>, B.M. Tutolo<sup>c,‡</sup>, C.A. Mondro<sup>f,‡</sup>, W.H. Farrand<sup>g</sup>, S. Gupta<sup>h</sup>, J. Schieber<sup>i</sup>, W.E. Dietrich<sup>j</sup>, N. Mangold<sup>k</sup>, K.W. Lewis<sup>l</sup>, R.S. Sletten<sup>m,‡</sup>

## ARTICLE INFO

Keywords:

Mars

Planetary science

## ABSTRACT

Early Mars was habitable, at least intermittently, but major questions remain about how much water flowed and for how long. The paleoclimate evolution of Mars is captured by the stratigraphic record in Gale crater (Milliken et al. 2010). Climbing through mostly aeolian deposits reflecting arid conditions within Gale crater, the Mars Science Laboratory *Curiosity* rover encountered wave-rippled lake sediments of the basin-spanning Amapari Marker Band (AMB) that have very high metal enrichments (Fe, Mn, Zn). What caused the association between relatively wet primary depositional environment, and metal enrichment? Tentative, but reasonable extrapolation of rover metal data across the AMB suggests an excess Fe mass of 0.2 Gt. Transporting this Fe likely required ~10,000 km³ of water flow, much more than the volume of the lake, across >10³ yr. Deposition of the Fe could be due to a redox or pH front within or just beneath the lake. One possible basin-scale synthesis involves a climate excursion consisting of initial cooling then subsequent warming: initial cooling permits wind scour in Gale basin and ice build-up on Gale's rim, while subsequent melting fills the lake and mobilizes Fe. Alternatively, the data can be explained by water-table fluctuations. In either case, the metal enrichment likely contributed to the hardness of these rocks, aiding wave-ripple preservation.

## 1. Introduction

Early Mars was intermittently wet, but interpretations of how much water was available and for how long vary widely. At the low end, it has been argued that mineralogy is consistent with very low water:rock ratio under low-pH conditions and a total time-integrated water column of as little as hundreds of meters (e.g. Berger et al. 2009, Zolotov and Mironenko 2016). At the high end, some authors advocate for at least tens of

Myr of habitable conditions, or even an ocean as late as 3 Ga (Ramirez and Craddock 2018; Schmidt et al., 2022). This uncertainty matters because searching for past life on Mars is a major NASA goal, and the sustained existence of any surface life would have depended on water availability and persistence. Rover data, and ultimately sample return data, are needed to test hypotheses generated from orbiter analyses.

Gale crater was chosen for exploration by the Mars Science Laboratory *Curiosity* rover because it apparently captures the paleoclimate

<footnote><sup>a</sup> University of Chicago, Chicago, IL, USA</footnote>

<footnote><sup>b</sup> Los Alamos National Laboratory, Los Alamos, NM, USA</footnote>

<footnote><sup>c</sup> University of Calgary, Alberta, CA, USA</footnote>

<footnote><sup>d</sup> Planetary Science Institute, Tucson, AZ, USA</footnote>

<footnote><sup>e</sup> University of New Brunswick, Fredericton, NB, Canada</footnote>

<footnote><sup>f</sup> California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote><sup>g</sup> Space Science Institute, Boulder, CO, USA</footnote>

<footnote><sup>h</sup> Imperial College London, London, UK</footnote>

<footnote><sup>i</sup> University of Indiana, Bloomington, IN, USA</footnote>

<footnote><sup>j</sup> University of California, Berkeley, Berkeley, CA, USA</footnote>

<footnote><sup>k</sup> UMR6112, LPG/CNRS, Nantes Université, France</footnote>

<footnote><sup>l</sup> Johns Hopkins University, Baltimore, MD, USA</footnote>

<footnote><sup>m</sup> University of Washington, Seattle, WA, USA</footnote>

<footnote>* Corresponding author.
E-mail address: kite@uchicago.edu (E.S. Kite).</footnote>
evolution of Mars from an intermittently wet planet early in its history,
to the modern environment which is very cold and completely dry
(Milliken et al. 2010). Gale crater contains a 5km-high mountain, Mount
Sharp / Aeolis Mons, composed of near-horizontal sedimentary rock
layers (the Mount Sharp group). Since approximately sol 750 of the
mission, *Curiosity* has been ascending the slopes of Mount Sharp,
traversing a total of >0.7 km of topographic elevation to date. The
sedimentary rocks of the Mount Sharp group in Gale crater investigated
by *Curiosity* since sol 3000 are texturally familiar from Earth analogs
(primarily aeolian) (Gupta et al. 2023), but compositionally are a
mixture that is exotic relative to Earth. Basaltic minerals like pyroxene
are intermixed at sub-centimeter scale with highly soluble salts

including Mg-sulfates, with SO₃ frequently >10 wt %, and the
clay-sulfate transition seen in orbital spectroscopy is essentially iso-
chemical in elemental composition over >200 m of topographic eleva-
tion (Rampe et al. 2020; Berger et al. 2023).

Interstratified within these strata is the Amapari Marker Band (AMB), an erosionally-resistant dark layer that orbital mapping suggests spans Mount Sharp / Aeolis Mons (Fig. 1a) (Weitz et al. 2022). The AMB was found by *Curiosity* to be a marked excursion from the near-uniform composition of the Mount Sharp group explored up to that point: it is a metal-rich deposit (up to 47.5 wt % FeO<sub>T</sub>, 1.5 wt % MnO, 2.2 wt % Zn) (Thompson et al. 2024). Moreover, the bottom of the AMB also marks a step change in elemental composition relative to the rocks below

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Map of Gale crater. Red corresponds to mapped Marker Band outcrop (Weitz et al. 2022). Convex hull of mapped outcrops (inferred, red hatched zone) is 1500 km². Solid gray line corresponds to the edge of the modern (wind-eroded) sedimentary rock mountain, Mount Sharp. The sedimentary basin (approximately corresponding to the area within the gray dashed line) comprises about one-half of the area of the Gale crater topographic catchment. "*" = Location of panels b/c. (b) Oblique view of ripples, average spacing 4.5 cm. Part of PIA25732. NASA/JPL-Caltech/MSSS (see Mondro et al. 2025a for a detailed analysis). (c) Side view of rippled AMB from Mars Hand Lens Imager (MAHLI) 3645MH0008570001301796R00/3645MH0008570001301798R00 (target "Mixiguana," modified after Gupta et al. 2023). (d) Part of the MSL Team stratigraphic column, showing context of Amapari Marker Band (AMB). CA = Canaima drill sample, TC = Tapo Caparo drill sample. Materials underlying AMB (Catrimani member) and overlying AMB (Chenapau member) both show planar bedding, scours, and low-angle cross-bedding, which together with other textures suggest aeolian deposition.</figcaption></figure>
(Thompson et al. 2024) that persists for >150 m above the AMB, and isotopic composition and mineralogy (Burtt et al. 2024; Tutolo et al. 2025) also show a sustained step change above versus below the AMB.

The AMB is divided into distinct parts and the focus of this study is only on the wave-rippled (lower) portion of the AMB. The stratigraphic context of the AMB is given in Fig. 1d. MSL made four overarching discoveries at the AMB: metal enrichment (Thompson et al. 2024), wave ripples (Mondro et al. 2025a), abundant siderite (Tutolo et al. 2025), and highly enriched carbon and oxygen isotopes in carbonate-derived CO₂ (Burtt et al. 2024). The first two discoveries are from the wave-rippled part of the AMB and are the focus of this paper. The siderite and isotopic anomalies correspond to drillsite sampling of post-rippled-AMB materials and are not the focus here.

Relative to the aeolian deposits above and below, the AMB is also sedimentologically remarkable (Fig. 1). The 0.2 m-thick rippled AMB (Fig. 1b, 3 in Fig. 2) corresponds to a shallow (<2 m deep) lake (Mondro et al. 2025a, 2025b). Aeolian deposits are found just below the AMB and resume within a few meters above the AMB (1 and 4 in Fig. 2) (Mondro et al. 2025a). Evidence exists for subaerial scour (deflation) of unknown duration prior to lacustrine deposition, marked by topographic scour and a spectrally and morphologically distinct zone interpreted as an alteration zone (2 in Fig. 2) (Farrand et al. 2023). The shallow-lake episode also corresponds to rapid sediment aggradation, recorded by vertically stacked ripple crests (Mondro et al. 2025a) (Fig. 1b). Immediately overlying the ripple structures are sediments with dark-toned nodules (interpreted to contain 42 wt % FeO, ~0.6 wt % Zn; Thompson et al. 2024) that might have formed beneath a deeper lake (Gasda et al. 2023). However, within a few meters of stratigraphic elevation above the ripple structures, sedimentological evidence for aeolian deposition resumes, indicating that the water table was again mostly below the surface.

These observations raise many questions: what mobilized the iron? Why are these metals mobilized and not others? How much water was involved? Are there Earth analogs? What caused the metals to

precipitate out? What caused the excursion to relatively wet primary depositional environment, and what caused the metal enrichment? Why do they occur within the same stratigraphic layer? What explains the rarity of the marker band - the only strongly metal-enriched laterally extensive layer encountered so far in 700 m of stratigraphy?

Here we focus on the questions of water flux and flow timescale, while considering the other questions. We use topographic-catchment-scale mass balance (Brown 2009; Siebach and Grotzinger 2014) to assess hypotheses for the water and metal fluxes required to explain the rippled AMB. We make the following assumptions. First, we infer that the AMB continues within Mount Sharp (Weitz et al. 2022) (Fig. 1a) and that *Curiosity* data are representative of its enrichment and thickness, if not depositional setting. Second, we assume that the stratiform Fe enrichment corresponds to metal addition (Thompson et al. 2024), and that this addition occurred prior to lithification (on Earth, such enrichment is usually syndepositional to early diagenetic, e.g. Maynard 1983). With these assumptions, the challenge is the first basin-scale Mars metal enrichment problem: what hypotheses can explain the water and metal fluxes to the Amapari Marker Band?

## 2. How much metal?

The strong metal enrichment and $1.5 \times 10^3 \text{ km}^2$ inferred extent of the Marker Band (Weitz et al. 2022) motivate a mass balance calculation. Alpha Particle X-ray Spectrometer (APXS, Gellert et al. 2009) data from ~1 cm depth (drill fines) within the rippled layers show ~40 wt % FeO, ~2 wt % Zn, and ~1 wt % MnO (Thompson et al. 2024). This shows that the metal enrichments are not just a coating but intrinsic to the rock. The metal enrichments were detected over a horizontal span of >130 m. We have no reason to think that this metal content is unrepresentative based on orbital data for Marker Band tone, erosional resistance, or mineralogy (although depositional facies may have varied across the basin), making extrapolation of these data to parts of the Marker Band not visited by the rover reasonable (although tentative).

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Reconstructed history: Amapari Marker Band strata record changing deposition rate and depositional facies. Hatching corresponds to sub-AMB alteration zone. Dotted line corresponds to end of possible deep-lake deposits. 1-4 (discussed in text) correspond to (1) pre-AMB aeolian deposition, (2) pre-AMB scour, (3) rippled AMB deposition, and (4) post-rippled-AMB deposition. See Fig. 4 for a sketch and Fig. S1 for alternative hypothesis.</figcaption></figure>
The ~0.2 m thick rippled AMB (Mondro et al. 2025a), if extrapolated across the mapped-from-orbit spatial extent of outcrops at Mount Sharp, corresponds to 0.3 km³. Assuming 3 g/cc rock density, the mass of metal-enriched rocks is ~1 Gt, a mass of metal-rich rock comparable to Earth's largest Zn/Pb ore deposits (Leach et al. 2010). Relative to surrounding Mount Sharp rocks, the extra FeO is +20 wt %, an excess Fe mass of 0.2 Gt (and 20 Mt for Zn).

From a mass balance perspective, no special source rock is needed to account for the excess metals. For example, the ~0.2 m thick rippled AMB is ~250% enriched in Fe relative to strata underneath the AMB, so leaching of <1 m of strata from the Mount Sharp group could provide the Fe. No compositional evidence for leaching is observed, so in practice the leached interval must have been >10 × thicker in order to dilute the signal below the geologic noise threshold, but this is still a small fraction of the thickness of the subjacent Mount Sharp group. Similar numbers apply to Mn. The sub-AMB rocks are already 10 × to 20 × enriched in Zn compared to Mars basalt (Treiman and Filiberto 2015), so leaching of <10 m would suffice for Zn, although the observation of Zn enrichments elsewhere in the crater (e.g., Lasue et al. 2016) means that leaching must have been patchy. Metal sourcing does not strongly constrain the metal enrichment process.

### 3. How much water to transport the metal?

Metal enrichment involves leaching/dissolution from a source rock, aqueous fluid transport, and deposition. Elevated Fe suggests that relatively reducing, neutral to acidic groundwater transported these metals, because Fe is very insoluble in higher-pH or oxidizing fluids (Kite and Melwani Daswani 2019). To estimate a lower bound on water volume, we need an upper solubility limit. Effective metal solubility depends on factors such as redox, temperature, pH, and kinetics (Yardley 2005;

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Histogram of iron concentration data for nonglacial ferruginous lakes on Earth. Horizontal bars show estimates of Precambrian ocean composition at the time of banded iron formation. The differences between Precambrian estimates correspond to equilibrium with different iron-bearing minerals, or different kinetic parameterizations. Light purple corresponds to non-Fe-rich lakes. See Table S1 for details (Data sources: Canfield, 2005; Eroğlu, 2018; Halevy et al., 2017; Holland, 2007; Jiang and Tosca, 2019; Tosca et al., 2016).</figcaption></figure>

Zhong et al. 2015; Loche et al. 2024).

Fig. 3 and Table S1 compile Fe-rich near-surface fluid data from mostly circumneutral lakes (plus some acid lakes) on Earth (Dickson and Giblin, 2009; Long et al., 1992; Lyons et al., 1990; Swanner et al., 2020), omitting glacially-affected lakes (which can have higher concentrations). The horizontal bars are estimates of Precambrian ocean composition at the time of banded iron formations. These Earth analog waters almost all contain <100 ppmw Fe. This suggests a lake-scale upper limit of ~100 ppmw Fe, which for the AMB requires >1 km water column (>20 % of Mount Sharp's volume) to move the metal. Mount Sharp's porosity is estimated at 18±6% (Johnson et al. 2021), so explaining the enrichment requires flushing an amount of water equivalent to most of the Gale crater sediment column pore space.

One option to explain how Mars waters might have approached this upper solubility limit is acidification from sulfide oxidation, previously proposed by (e.g.) Burns & Fisher (1993). An alternative is the acidity-generating reaction of dissolved Fe-bicarbonate with Ca-sulfates (Peiffer et al. 2009), although the latter process yields calcite which has not been observed at Gale.

Could this water requirement of >1 km column be lowered? Higher temperatures (T = 150 °C) only decrease water demand by a factor of a few based on analogs (Yardley 2005) and calculations (Zhong et al. 2015), with 150 °C already hotter than expected for burial heating of Mount Sharp fluids (Borlina et al. 2015). Another possibility is that the AMB ground-truthed by MSL might be unusually metal-rich. However, the AMB is erosion resistant by definition, so then an explanation for how the rest of the AMB is erosion resistant without metal enhancement would be required. A simpler explanation is that the metal enrichment contributed to the hardness of these rocks, and that this is why a 20 cm-thick layer forms an erosionally resistant bench that is so striking from orbit (Weitz et al. 2022). Another possibility is that Fe was sourced from aeolian dust (Brimhall et al. 1988), or from ultramafic ash, and then leached by very acidic (starting pH < 1) fluids (Loche et al. 2024). However, the immediately overlying rhythmically laminated unit has a mineralogy inconsistent with low pH (Tutolo et al. 2025). Overall these possibilities appear unlikely to reduce the water demand below 1 km column. We conclude >10⁴ km³ (1 km column) of water flow was probably involved in metal transport for the AMB. This is approximately as much as the total pore space in the sediments beneath the AMB at the time the AMB was deposited.

One kilometer column of water flow is a small number for Earth's geologic history, as it corresponds to the average precipitation during 1 Kyr on land on Earth. However, the amount of water needed to explain Mars' geologic record of sedimentation and aqueous alteration is not clear. Earth has a vigorous hydrologic cycle in which, for example, on average an amount of water equal to that of Earth's ocean fell as rain on each Gale-crater-sized area of land during Earth's Cenozoic. Hesperian Mars was much drier than this (and some hypothesize that Hesperian Mars was exceedingly dry; Berger et al. 2009, Zolotov and Mironenko 2016), with estimates ranging over many orders of magnitude (e.g., Kite 2019). As a result, this is a significant geologic constraint on Mars' early climate (Kite 2019).

One kilometer column of water flow is a large quantity of water for an event relatively late in Mars' history of aridification, and is within the range of estimates for the water column needed to form clay weathering profiles from much earlier in Mars history (Loizeau et al. 2018). In stark contrast, low water-to-rock-ratio alteration has formerly been inferred for most of Mars' subsequent history (Ehlmann et al. 2011, but see also Mangold et al. 2019). A plausible way to reconcile these data is intermittency (e.g., Kite et al. 2019).

If metals accumulated during the depositional interval of the rippled marker band, then accumulation timescale can be estimated using the constraint that the lake was shallow enough (<2 m) for wave ripples to leave a trace in the underlying sediment (Mondro et al. 2025a). An approximately 1 km column water is needed to transport Fe, yet 1 km is much too deep for ripples to form. Gale is a topographic basin with no
outlet for water. Therefore, in order to maintain a shallow lake, evap-
oration must have kept pace with water supply (Irwin et al. 2015). For
<1 m/yr evaporation rate, which is reasonable for Early Mars (Irwin
et al. 2015), the lake duration is >10³ years. Whether or not repeated
episodes of fill and evaporation were encompassed within the broader
interval of flow, this scenario concentrates dissolved metals while
removing H₂O.

4. What can explain the rise in water and metal fluxes in this stratigraphic interval?

Global deep crustal groundwater flow models suggest upwelling rates of 0.03–0.1 m/yr at Gale, though permeability is poorly constrained (Horvath and Andrews-Hanna 2021). In these models, the groundwater evaporates on approach to the surface, and dissolved ions form minerals (for example iron oxides). However, these minerals are dispersed over a wide stratigraphic range (diluted in the stratigraphic record) because the surface is aggrading upwards due to sediment deposition.

In this conceptual picture, a depositional hiatus could concentrate
ions at a particular stratigraphic interval. At the rates output by global
deep-crustal groundwater flow models, and adopting the upper limit on
Fe concentration from §3, a depositional hiatus of >(1–3) × 10⁴ yr (with
evaporative concentration of ions at the water table) could provide the
Fe seen at the AMB. An alternative is compaction brine expelled as
previously-deposited sediment compacts during a depositional hiatus
(Kooi and de Vries 1998), but this hypothesis does not explain the lake
event. In either case, metals would precipitate before accumulation of
the overlying rippled sediments, requiring subsequent Fe reworking in
these scenarios.

A second, and perhaps preferred, basin-scale explanation involves
climate cooling followed by climate warming. This temperature swing
would have different effects on the rim versus the basin floor, because
(taking into account the elevation of AMB above the lowest points in
modern Gale) the basin is ~3 km below the rim (Fig. 1a). First, cooling

allows snow/ice accumulation on the rim, consistent with some global
climate models, depending on parameters including obliquity (Steele
et al. 2017). On the basin floor, the hydrologic cycle slows with less
liquid water supply. As a result, sediment cannot be aqueously cemented
because water is not available, but wind scour can continue. This causes
subaerial exposure and forms the scour surface. Next, subsequent
warming melts ice, increasing catchment liquid water supply and sedi-
ment flux, especially during (hypothetical) spillovers between smaller
lakes within sub-basins (Fig. 4). In present-day Alaska, permafrost
melting beneath an O₂-rich atmosphere (Zahnle et al. 2008) releases
large amounts of Fe, Mn and Zn from sulfide-bearing soil, turning rivers
orange (O'Donnell et al. 2024). Fe could move by: 1) flushing
pre-enriched sedimentary-basin brine, 2) transporting Fe/Zn-rich rego-
lith, or 3) shallow groundwater dissolution and transport (analogs here
include saline lakes in W Australia; Benison and Bowen 2015). This
works if Fe ultimately precipitates within a meter of the surface or 10 m
down. Melting may flush out deep basalt groundwaters as well, but these
are less likely to contribute enough Fe for the AMB because
basalt-buffered pH is high and Fe minerals tend to be very insoluble at
high pH (Kite and Melwani Daswani 2019). For this reason, we do not
consider metal input from groundwater derived from beyond Gale's
topographic rim.

These two hypotheses have common elements: for example, both
involve water level changes. As Warren (2016) noted, brine upwelling
through hematite-rich units can scavenge metals, consistent with he-
matite below the AMB (Rampe et al. 2020).

Disfavored explanations (Table 1) include a sediment supply hiatus
(which does not explain the lake), prompt climate effects from a volcanic
eruption (which predicts little warming; Kerber et al. 2015), and the
direct climate effects of an asteroid impact (although indirect effects
remain possible). Fe delivery from hillslopes as solids struggles to
explain why Ti is not also enriched in the rippled AMB: in this scenario,
Fe moves with solids, but Ti also moves with solids so it should be
co-enriched, contrary to observations. Asteroidal Fe would give excess
Ni, which is not observed.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Cooling-then-warming hypothesis. In this hypothesis, cooling allows snow/ice accumulation on the rim. On the basin floor, sediment cannot be aqueously cemented because water is not available, but wind scour can continue. Next, subsequent warming melts ice, increasing catchment liquid water supply and sediment flux, especially during (hypothetical) spillovers between smaller lakes within sub-basins, allowing formation of wave rippled lake deposits. See text for details. See Fig. S1 for alternative hypothesis. Figure credit: Daniel Zhou.</figcaption></figure>
<caption>**Table 1**</caption>

Data-model comparison table. Explanation of symbols: ✓ = this model can explain this observation; ? = special, but plausible circumstances may be required to produce this observation from this model; x = this model does not plausibly explain this observation. Caveat: it is not logically required to explain all observations within a single scenario: more complicated scenarios are possible (e.g., a lake that deposits some of the materials, then a postlithification mineralization event in the deep subsurface) (Data sources: Irwin, 2013; Mondro et al., 2025a; Thompson et al., 2024; Turbet, 2018).

<table><thead><tr><th></th><th>Asteroid impact (direct climate perturbation)</th><th>Fluctuating water table (Fig. S1)</th><th>Change in lithology of silicate clasts transported to AMB site (which did occur based on Cr/Ti, Fe/Mn, etc.; Thompson et al. 2024)</th><th>(Reversed) excursion to lower T (Fig. 4)</th><th>(Reversed) excursion to higher T</th><th>Temporary hiatus in sediment supply</th><th>Hydrologic change due to changing distance to Northern Ocean shoreline</th></tr></thead><tbody><tr><td>Spectroscopically distinct zone just below marker band (Interpretation: alteration front)</td><td>?</td><td>✓</td><td>x</td><td>?</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Basal contact of marker band: interpreted as erosional (Mondro et al. 2025a)</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td></tr><tr><td>Shallow (&lt;2 m) open-water lake, rapid aggradation indicated by preservation of ripple form sets.</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>x</td><td>? / ✓</td></tr><tr><td>Addition of 20 wt% FeO equivalent to rippled AMB</td><td>x (no Ni, no chondritic signature)</td><td>✓</td><td>x (Disfavored by lack of Mg and by Ti dilution signal.)</td><td>✓</td><td>?</td><td>?</td><td>x</td></tr><tr><td>Addition of 2 wt% Zn to rippled AMB</td><td>x (no Ni, no chondritic signature)</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>x</td></tr><tr><td>Highly variable Fe/Mn within rippled marker band (and within marker band as a whole), based on CCAM LIBS data, suggesting redox variability during lake deposition.</td><td>?</td><td>✓</td><td>x</td><td>?</td><td>?</td><td>?</td><td>✓</td></tr><tr><td>Marker band elevation varies by &gt;1 km (orbital mapping)</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td></tr><tr><td>Notes/other issues</td><td>Only ~5 Chicxulub-scale impacts occurred after Gale (Irwin 2013). Direct climate perturbation is modeled to be small (e.g. Turbet 2018).</td><td>Fe delivery from hillslopes as solids (by reworking of regolith) struggles to explain Ti dilution (because Ti is also immobile, at least on Earth).</td><td></td><td></td><td>Does not explain sub- Marker- Band erosion (unless these are fluvial scours).</td><td>Does not explain lake.</td><td>Assumes a stable, long-lived Northern Ocean.</td></tr></tbody></table>

**5. What process precipitated the metals?**

One potential model is precipitation arising from evaporative concentrated brine water or fault-exhaled water crossing a pH or redox front within a lake or shallow sedimentary environment (Gasda et al. 2023). In this scenario, metals transported via reducing, acidic or neutral pH fluids precipitated shortly after introduction to the lake.

Indeed, associations of Fe and Mn (but not Zn) are common in preindustrial terrestrial lake settings (e.g., Marcus et al. 2004), with metals accumulating at or just below the lake floor. Potential analogs include ferruginous lakes and Australian saline lakes of varying, mainly acidic, pH containing sulfates and Fe oxides, some with "ironstone-like" sediments similar to the AMB (e.g. Benison and Bowen 2015, Fang et al. 2024). Based on laboratory data (Benjamin et al. 1996), adsorption of Zn
onto Fe-oxide-coated sands may explain the co-location of Zn and Fe.

Another possible compositional analog is the sediments that are found beneath the Lower Brine of the Atlantis II Deep (Red Sea), which is devoid of reduced sulfur—these sediments are highly enriched in metals (average on a salt-free basis 26 wt % Fe₂O₃, 1.7 wt % Mn, 2.0 wt % Zn; Laurila et al. 2014). Anschutz & Blanc (1995) report 700 kg/m²/kyr mass accumulation rate for Atlantis II Deep metalliferous sediments. At Atlantis II Deep, iron oxides and diagenetic clays are reported to be major metal-hosting minerals (Laurila et al. 2015). The Atlantis II Deep sediments average ~0.5 wt. % Cu, much more than is seen at Gale, but this Martian Cu-poverty can be explained as the result of preferential dissolution of Ol and Px on Mars (McAdam et al. 2008), as Ol and Px host very little Cu relative to Zn and Mn (Yang et al. 2015). Because the cause of the metal enrichment of the Atlantis II deep brine is hydrothermal activity, this analog predicts that more evidence for hydrothermal activity should be present at Gale (Morris et al. 2016; Yen et al. 2021).

Fe can be oxidized by several mechanisms. Surprisingly, photooxidation can keep pace with rapid sedimentation (Tabata et al. 2024), with estimates of 0.3–0.4 mm/yr ferrihydrite accumulation at pH 7 (Heard 2021). Even if oxidation temporarily lagged sedimentation, prelithification enrichment could occur via metal-rich brine plumes moving through sediments, akin to Sedimentary Exhalative ore (SEDEX) formation on Earth. However, Mn deposition might require redox decoupling and evaporative concentration (Heard 2021, but see also Loche et al. 2024).

Our study assumes that pre-lithification processes deposited the metals. Post-lithification enrichment scenarios cannot be ruled out but may be less likely to produce such large Fe enrichments (Bekker et al. 2010).

## 6. Discussion

The scenario outlined above can explain the association between relatively wet primary depositional environment, and metal enrichment, at the AMB. It is compatible with the context of a mostly aeolian stratigraphy elsewhere in the Mount Sharp group.

However, while the hypotheses proposed can explain some observations of the AMB, significant issues remain unresolved. We highlight three here.

(A) Orbital mapping reveals >1 km of elevation variance in the AMB, which may indicate the lake was patchy within the metal-rich outcrop region (Fig. 4). Post-depositional modification processes, such as differential compaction (Gabasova and Kite 2018), can account for part of this elevation variance. In addition, lower-crustal flow might tilt the Marker Band toward the north by hundreds of meters (Nimmo 2005), and loading of the plains north of Gale could flexurally tilt Gale toward the north. As an alternative explanation of the >1 km of elevation variance, it has been suggested that this elevation range could correspond to a transgression (lake deepening from meters to >1 km, Dietrich et al. 2023).

(B) There is no evidence from orbiter data for additional Marker Bands ahead along Curiosity's path. Why are metal-enriched bands rare? We do not know, but one speculative possibility is that the apparent switch from aeolian deposition, to scour, and then back to wet conditions at Gale was caused by changes in Mars' Myr-mean obliquity (for example, a decline in Myr-mean obliquity followed by a rise). In contrast to cyclic changes in Mars' obliquity that occur on 10⁵ yr timescales, changes in Myr-mean obliquity are modeled to be very infrequent (with 250 Myr simulations typically undergoing only a single such transition, https://vo.imcce.fr/insola/earth/onlin e/mars/mars.html). See Fig. 2 in Mansfield et al. (2018) for examples.

(C) Scenarios for preserving soluble Mg-sulfates below the AMB (Chipera et al. 2023) if groundwater fluxes were high include: (1) Mg-sulfate emplacement after Fe enrichment, or alternatively (2) the

groundwater was confined to a thin near-surface layer and did not encounter the deeper Mg-sulfate layers, or (3) the groundwater was routed by fractures/faults that bypassed the Mg-sulfate.

## 7. Conclusions

Mass balance calculations suggest the AMB contains an excess 0.2 Gt of Fe, indicating a flux of at least 10,000 km³ of metal-transporting waters to generate these enrichments. Two plausible hypotheses involve either 1) a temperature swing facilitating first ice accumulation then melting and runoff, or 2) water table fluctuations linked to other environmental changes, perhaps with Fe derived from aeolian dust (Brimhall et al. 1988). In either scenario, the metals were concentrated by aqueous processes into this specific stratigraphic sweet spot. The first scenario can be tested if more Fe-enriched layers are found at Mt Sharp, because it predicts that lake deposits should be co-located. Although AMB metal enrichments currently have no economic value, their existence demonstrates Mars hosted processes capable of generating extensive deposits with Fe and Zn concentrations not much less than those found in ores on Earth. The source rock for the metals remains unknown and could be the strata lower down in the Mount Sharp group, or alternatively rocks and regolith on Gale Crater's rim.

The Amapari Marker Band provides a striking example of metal enrichment on Mars. This distinctive lacustrine deposit stands out for its lateral extent exceeding 1500 km², stratigraphic confinement within a narrow interval, and non-trivial thickness of metal-enriched sediments (Gasda et al. 2023). The high concentrations of Fe, Mn, and Zn discovered by the *Curiosity* rover highlight a process operating on ancient Mars that on Earth can lead to the formation of metal ores (Ciazela et al. 2022). With hindsight, very little of the AMB story could have been understood solely from orbiter data (although orbiter data complements the on-ground observations). Yet the cumulative water flow (km column) implied by this 0.2 m thick layer is as large as that needed for the most spectacular lake overflows on Mars (Stucky de Quay et al. 2020). These constraints are needed to test climate change hypotheses (e.g. Wordsworth et al. 2021, Kite 2019, Kite et al. 2021). For example, high water columns rule out the hypothesis of impact kinetic energy as a source of meltwater (e.g. Toon et al. 2010), because it is energetically insufficient. This points to the need for continued rover (and/or helicopter) exploration to reconstruct and understand the cessation of wet events on Mars, the largest known environmental catastrophe.

### CRediT authorship contribution statement

E.S. Kite: Writing – review & editing, Writing – original draft, Investigation. P. Gasda: Writing – review & editing, Investigation. C.J. Tino: Writing – review & editing, Investigation. C. Weitz: Writing – review & editing, Investigation. L. Thompson: Writing – review & editing, Investigation. B.M. Tutolo: Writing – review & editing, Investigation. C.A. Mondro: Writing – review & editing, Investigation. W.H. Farrand: Writing – review & editing, Investigation. S. Gupta: Writing – review & editing, Investigation. J. Schieber: Writing – review & editing, Investigation. W.E. Dietrich: Writing – review & editing, Investigation. N. Mangold: Writing – review & editing, Investigation. K.W. Lewis: Writing – review & editing, Investigation. R.S. Sletten: Writing – review & editing.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Edwin S. Kite reports financial support was provided by NASA. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
## Acknowledgements

We thank the entire MSL team. Particular thanks to M. Loche, B. Hallet, and A. Vasavada for comments. We acknowledge Daniel Zhou, who made Fig. 3. Funding: National Aeronautics and Space Administration (NASA). Participating Scientist Program grant 80NSSC22K0731 to Kite.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119347.

## Data availability

All data is available via the Planetary Data System (PDS).

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