Fe-rich X-Ray Amorphous Material Records Past Climate and Persistence of Water on Mars

doi:10.21203/rs.3.rs-3200798/v1

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Fe-rich X-Ray Amorphous Material Records Past Climate and Persistence of Water on Mars

https://doi.org/10.21203/rs.3.rs-3200798/v1

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The assemblage of secondary minerals in martian rocks can help constrain the characteristics of past surface and subsurface fluids as well as past climatic conditions. X-ray amorphous material is an important part of martian surface materials, making up 15–73 wt.% of sedimentary rocks and eolian sediments in Gale crater, Mars. This X-ray amorphous material is variably siliceous, Fe-rich, and contains volatiles, and it therefore likely contains incipient weathering products. To better understand the implications of this material for past aqueous and climatic conditions in Gale crater and elsewhere on Mars, we investigated X-ray amorphous material formation and longevity within terrestrial Fe-rich soils of different ages in terrestrial mediterranean, subarctic, and desert climates using bulk and selective dissolution methods, Rietveld refinements of powder XRD patterns, and transmission electron microscopy. Results indicate that in situ aqueous alteration is required to concentrate Fe into the clay-size material and to form abundant Fe-containing X-ray amorphous material. Cooler climates promote the formation and persistence of Fe-rich X-ray amorphous material whereas warmer climates promote the formation of crystalline secondary phases. Fe-rich X-ray amorphous material formation and persistence on Mars are therefore consistent with past cool and relatively wet environments followed by long-term cold and dry conditions.

Earth and environmental sciences/Environmental sciences/Environmental chemistry

Earth and environmental sciences/Planetary science/Geochemistry

X-ray diffraction (XRD) measurements by the Chemistry and Mineralogy (CheMin) instrument on the Curiosity rover at Gale crater, Mars have demonstrated that X-ray amorphous material makes up 15% to 73 wt.% of all rock and eolian sediment samples analyzed to date^1,2. This material has been shown to be variably Fe-rich (3.0–43.1 wt.% FeO_T) and Si-rich (3.6–75.9 wt.% SiO₂) and typically Al-poor (≤ 10.46 wt.% Al₂O₃)^1–3, consistent with principally mafic sources^4,5. This material is hereafter referred to as “amorphous” although it likely encompasses amorphous and short-range order materials and nanocrystallites^6–9. While amorphous material can form from a variety of processes including as glass from melt and through serpentinization¹⁰, the presence of volatiles (e.g., H₂O, SO₂) measured by the Sample Analysis at Mars (SAM) instrument suite suggests at least some of this material likely consists of secondary weathering products^1,3.

Despite the prevalence of Fe-rich amorphous material in Gale crater^1–3 and elsewhere on Mars^11–15, the implications of its presence for past climatic conditions on Mars remain poorly understood. Previous work on terrestrial soils has identified amorphous aluminosilicates as precursors to crystalline clay minerals^16–22 and suggested aluminous amorphous material preferentially forms through aqueous alteration under kinetically limiting conditions^16,23 such as colder temperatures^22–27. Variably Fe-rich (0.95–45.17 wt.% FeO_T) and Si-rich (0.23–93.26 wt.% SiO₂) but Al-containing (0.81–55.79 wt.% Al₂O₃) amorphous material has recently been documented in glacial sediments^8,22 and Hawaiian soils and paleosols at the John Day Fossil Beds⁸. However, little work has yet examined climatic effects on the formation and persistence of Al-poor and Fe-rich amorphous material, and such data are crucial for placing constraints on the formation and persistence of these materials on Mars.

In this study, we use chemical dissolution coupled with powder XRD and transmission electron microscopy (TEM) to investigate the impacts of time and climate on the formation and persistence of Fe-containing but Al-poor amorphous material. We examined ultramafic soils developing in the mediterranean climate Klamath Mountains of northern California (mean annual temperature: ≤12.8℃, mean annual precipitation: ~101–118 cm/year)^28–30, the subarctic climate Tablelands of Newfoundland, Canada (mean annual temperature: ≤3.9℃, mean annual precipitation: ~120 cm/year)³¹, and the desert climate of Pickhandle Gulch, Nevada (mean annual temperature: ~14.4℃, mean annual precipitation: ~14 cm/year)²⁸ over an age range from ~ 12 ka to > 50 ka (Figure S1). Our results show that cool and wet conditions lead to greater in situ formation and persistence of Fe-rich secondary amorphous material, suggesting that the presence of abundant Fe-rich amorphous material at Gale crater is consistent with cool and wet conditions during their formation followed by cold and dry conditions promoting their persistence.

Relevance of the Studied Field Amorphous Material to Amorphous Material at Gale Crater, Mars

For the studied field samples to be relevant to the CheMin measured samples, and therefore martian processes, they must similarly exhibit in situ formation of Fe- and Si-rich and Al-poor X-ray amorphous material. Fe- and Si-rich and Al-poor amorphous material was observed directly via TEM observations of Klamath Mountain and Tablelands soil clay-size fraction material (Fig. 1). However, the evidence for amorphous material in Gale crater comes from broad humps in CheMin XRD patterns^1,3. Amorphous hump positions are dependent on composition^1,32,33, with increasing SiO₂ content correlating with larger d-spacings for amorphous silicates³³. Amorphous humps were present in the backgrounds from Rietveld fits of all clay-size fraction XRD patterns (selected as secondary amorphous material has been observed to concentrate in the clay-size fraction²³) (e.g., Fig. 2A) and were centered between 26 and 30 ⁰2θ (Fig. 2B). While not identifying a specific amorphous material, the terrestrial amorphous humps are centered at similar d-spacings as Gale crater amorphous material^1,2 (Fig. 2B). Relative amorphous material abundances determined from integrating the area underneath the amorphous hump in clay-size fraction XRD patterns indicate greater abundances of amorphous material in the subarctic Tablelands (Fig. 2C), consistent with a climatic impact on amorphous material abundance. Bulk dissolution of the clay-size fraction from the Klamath Mountains and Tablelands sites, although not of purely amorphous material, result in similarly Al-poor (0.27–11.67 wt% Al₂O₃) and Fe- (12.87–23.19 wt% Fe₂O₃T) and Si-rich (13.08–50.20 wt.% SiO₂) chemical compositions as the amorphous materials found in Gale crater, Mars (Fig. 3). Increased Fe-content in clay-size fraction material in the wetter Klamath Mountain and Tablelands soils relative to parent material is consistent with precipitation of Fe-rich secondary phases including amorphous material (Fig. 3). Amorphous material in these terrestrial soils thus appears to possess similar chemical compositions and amorphous hump shape and positions as that found at Gale crater, while exhibiting potential differences in abundance resulting from climatic variation. We therefore examine the mineral inputs to the weathering system generating this amorphous material, the conditions that promote formation and persistence of Fe-containing amorphous material, and the implications of such material for Mars.

Mineral Inputs to the Amorphous Material-Forming Weathering System

Secondary material production is inextricably linked to the parent materials from which they form. Collected rock samples interpreted as parent material at the Klamath Mountain sites contain serpentine minerals, along with variable amounts of amphibole, chlorite, enstatite, olivine, plagioclase, and talc (Table 1, Figure S3). The Tablelands parent material is composed of serpentine minerals, chlorite, magnetite, talc, and olivine (Table 1, Figure S3), and the parent material at Pickhandle Gulch contains serpentine minerals, chlorite, and magnetite (Table 1, Figure S3).

Table 1

– Mineral presence in bulk soil, soil clay-size fraction, and parent material determined by oriented and randomly oriented XRD pattern analysis
Sampling Site	Bulk Soil (< 2mm)^a	Clay Fraction (< 2 µm)^a
Klamath Mountains
Eunice Bluff	A, C, G, O, P, Q, S, Ta	A, C, G, P, Q, S, Ta
Eunice Bluff Parent Material	A, C, En, Mg, O, S
Deadfall Lake	A, C, O, P, S, Ta	A, C, G, P, Q, S, Ta
Deadfall Lake Parent Material	A, B, C, P, S, Ta
Swift Creek Late	A, C, P, Q, S, Ta	A, C, G, P, Q, S, Ta
Swift Creek Late Parent Material	O, S
Swift Creek Middle	A, C, P, Q, S, Ta	A, C, G, Q, S, Ta
Swift Creek Middle Parent Material	O, S
String Bean Creek	Mg, P, Q, S, Sm	G, P, Q, S, Sm
String Bean Creek Parent Material	O, S
Tablelands
Devil’s Punchbowl	C, O, Q, S, Ta	C, G, Q, S
Winterhouse Gulch Canyon	C, O, Q, S, Ta	G, Q, S
Winterhouse Gulch Mouth	C, O, Q, S, Ta	G, Q, S
Trout River Gulch	C, O, Q, S, Ta	G, Q, S
Tablelands Parent Material	C, Mg, O, S, Ta
Pickhandle Gulch
Summit	C, Ca, M, P, Q, S, Sm	C, Ca, M, P, Q, S, Sm, Ta
Footslope 1	C, Ca, M, P, Q, S, Sm	C, Ca, M, P, Q, S, Sm, Ta
Footslope 2	C, Ca, M, P, Q, S, Sm	C, Ca, M, P, Q, S, Sm, Ta
Pickhandle Gulch Parent Material	C, Mg, S
Abbreviations: Amphibole (A), Biotite (B), Calcite (Ca), Chlorite (C), Enstatite (E), Goethite (G), Magnetite (Mg), Muscovite (M), Olivine (O), Plagioclase (P), Quartz (Q), Serpentine Minerals (S)^c, Smectites (Sm)^b, Talc (Ta)
^aBulk soil and oriented mount clay-size analysis utilized a Cu Kα instrument, and the randomly oriented clay size analyses utilized a Co Kα instrument.
^bDifferentiation between different phyllosillicate phases and confirmation of the presence or absence of smectites was confirmed for all soils by oriented mount analysis (Section S4 in supplementary online material).
^cDifferentiation of serpentine polymorphs within XRD patterns is given in supplementary online material sections S4 and S5 and individual serpentine minerals based on that identification were used within Rietveld refinements,
*Labelled XRD patterns used for mineral phase ID can be found in supplementary online material (Section S4).

Dust input also potentially contributes to the composition of the secondary phases. Dust inputs to the examined soils were assessed by comparing the bulk soil and clay-size fraction to the parent material mineralogy and chemistry at each location. Minerals were attributed to dust input if they would not have formed in situ through aqueous alteration at surface conditions and were absent from parent material. Relative peak heights indicate minor concentrations of quartz in the Tablelands soil clay-size fractions and of quartz and plagioclase in the Klamath Mountains soil clay-size fractions (Fig. 2A, S4, S6, S8, S10, S12, S14), consistent with relatively minor dust influx at those sites. In contrast, the clay-size fractions at Pickhandle Gulch incorporate abundant phases attributable to dust deposition, including calcite, muscovite, plagioclase, quartz, and a substantial smectite component (Fig. 2A. S17), all common phases in southwestern Nevada dust³⁴. High Al-content in Pickhandle Gulch clay-size fraction material relative to parent material (Fig. 3) is similarly consistent with a high concentration of exogenous dust-borne phases.

Examination of the persistence and absence of parent material minerals within each soil can also help understand contributions from primary mineral dissolution to secondary material formation. In these soils, parent material minerals generally persist in the soils formed from those parent materials (Table 1), with three notable exceptions. Magnetite is present in some parent materials but not within examined soils (Table 1), consistent with dissolution or presence below XRD detection limits. Similarly, pyroxene is present within Eunice Bluff parent material but absent from the soil at the Eunice Bluff site (Table 1), indicating complete dissolution within < 12.1 ka, corresponding to previous estimates of pyroxene persistence within terrestrial soils³⁵, and releasing elemental chemistry likely including Mg, Si, and Fe for secondary phase formation. Olivine is present in the Klamath Mountains at the 12.1 ka Eunice Bluff and Deadfall Lake soils but absent from older (> 15 ka) sites despite being present in parent rock samples of all Klamath sites except Deadfall Lake (Table 1). Olivine dissolution releases Mg, Fe, and Si that would be available for secondary phase formation, and the ~ 12–15 ka persistence time for olivine is roughly equivalent with previous observations of ~ 10 ka olivine persistence time within terrestrial soils³⁵.

In contrast, although relative XRD peak height suggests that olivine is less abundant in the Tablelands than in Klamath Mountain parent materials (Figure S4), olivine is present in all examined bulk soils from the Tablelands to > 20 ka but absent from the clay-size fraction (Table 1), indicating only partial dissolution and increased olivine persistence (> 20 ka) relative to the warmer Klamath Mountain soils (~ 12–15 ka). Taken together, these trends in the persistence of parent material mineralogy indicate Fe, Mg, and Si availability for formation of secondary materials.

The Effect of Climate and Time on the Formation and Crystallinity of Fe-Containing Amorphous Material

To examine the effect of climate and time on the formation and persistence of Fe-containing amorphous material, the abundance of Fe and Fe-associated Si within amorphous material at each field site was compared using a hydroxylamine hydrochloride dissolution (Fe_H and Si_H), which is a combined reduction and acid protonation technique selective for Fe and Si containing amorphous material^36–40. The amount of Fe within (oxyhydr)oxides was similarly examined using a citrate dithionite dissolution (Fe_D), which acts as a reducing agent for Fe³⁺ in Fe³⁺-containing amorphous and crystalline oxides at near neutral pH conditions^36,41–44. Together these measurements allow both a comparison of the amount of Fe in amorphous material, and the extent of crystallinity of Fe-containing phases under different climate conditions in soils of different ages. In addition, extraction by pyrophosphate confirmed minimal organically complexed Fe at all sites (Figure S48). The hydroxylamine hydrochloride and citrate dithionite dissolutions indicate much greater quantities of amorphous Fe, and slightly greater amorphous Si (Figure S45), in the subarctic Tablelands soils than in the warmer Klamath Mountain soils, and a higher degree of Fe-crystallinity within the warmer Mediterranean climate Klamath Mountain soils. Amorphous Fe in the Tablelands soils is ≥ 1.8x parent material values (Fig. 4), indicating in situ formation of Fe-containing amorphous material. Similarly, the ratio of amorphous Fe to the Fe in oxyhydroxides (both amorphous and crystalline) in the Tablelands is > 0.95 (Fig. 4), suggesting that almost all secondary Fe-containing material in the Tablelands soils is poorly crystalline to amorphous. In contrast, amorphous Fe within the ~ 12–50 ka Klamath Mountains soils overlapping in age with the Tablelands soils is generally equal to or less than that found within parent material, and the ratio of amorphous Fe to the Fe in oxyhydroxides is lower than that ratio in parent material (Fig. 4), indicating little formation of Fe-containing amorphous material. Fe-containing secondary material is therefore almost entirely amorphous in the subarctic Tablelands soils while the warmer conditions of the Klamath Mountains promote crystalline Fe-containing secondary material formation.

In the likely older (undated but unglaciated) String Bean Creek Klamath Mountain soil, higher amorphous Fe ranges (6.02–15.96 mg Fe/g soil) and ratios of amorphous Fe to Fe in oxyhydroxides (Fig. 4) indicate both greater amounts of amorphous Fe-containing material and overall lower Fe-crystallinity compared to the younger Klamath Mountain soils. This increase in amorphous Fe-containing material coupled with abundant smectites detected by XRD not found in the younger soils (Fig. 1) might be consistent with the presence of nanocrystalline smectites or gel-like to poorly crystalline smectite precursor phases⁴⁵. However, even in this Klamath Mountain soil richest in amorphous Fe, mean amorphous Fe (11.34 mg Fe /g soil) is still less than half that in the Tablelands soils (25.77 mg Fe/g soil), despite similar parent material values (Tablelands = 12.11 mg Fe/g soil, String Bean Creek = 8.68 mg Fe/g soil). Substantially more abundant Fe-containing amorphous material and lower Fe-crystallinity in Tablelands compared to Klamath Mountain soils thus remains consistent with the subarctic climate promoting formation and persistence of Fe-containing amorphous material.

Amorphous and oxyhydroxide contained Fe-values from the Pickhandle Gulch soils are roughly equivalent to parent material values and the low ratios of amorphous Fe to the Fe in oxyhydroxides (Fig. 4) together indicate high secondary Fe-crystallinity and minimal to no in situ formation of Fe-containing amorphous material. The dry desert climate at Pickhandle Gulch therefore seems to preclude much in situ formation of Fe-containing amorphous material.

Implications for Past Martian Conditions

The amorphous and crystalline secondary weathering products in these Fe-rich and Al-poor soils developing under a range of climate conditions (mediterranean, subarctic, and desert), and over a range of ages (~ 11 ka to > 50 ka) can help interpret the climatic conditions consistent with the widespread presence of Fe-rich amorphous material at Gale crater and elsewhere on Mars (Fig. 5). The minimal Fe-containing secondary material in soils at the desert climate Pickhandle Gulch site suggest wet conditions are necessary for in situ formation of Fe-containing secondary material. Among the wetter field sites, the cooler conditions at the Tablelands favor the formation and persistence of amorphous material, particularly Fe-containing amorphous material, whereas the warmer conditions in the Klamath Mountains favor development of crystalline phases, Fe-(oxyhydr)oxides in the youngest soils and smectites as soil age increases (Figs. 1 and 4).

While there are a range of possible formation mechanisms for the amorphous material at Gale crater³ and chemical variability potentially suggests multiple source materials⁴ and alteration conditions^3,46, we demonstrate that cooler conditions such as those at the Tablelands preferentially form and preserve Fe-rich amorphous material. The production of abundant Fe-containing amorphous material at Gale crater¹ may therefore also be consistent with wet and near-freezing mean annual conditions. The abundant Fe-rich amorphous material within Gale Crater thus bolsters arguments^22,47–50 for the presence of wet but cold conditions during formation followed by long term cold and dry conditions facilitating preservation. Returned samples from Mars will allow a detailed comparison of martian amorphous material with material such as that examined here to better understand past martian environments.

Acknowledgements

We acknowledge the use of resources from the Nevada Plasma Facility Lab (NPFL) at the University of Nevada, Las Vegas. We also acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160. We acknowledge funding from NASA EPSCoR, the Geological Society of America, and the Clay Minerals Society. We also acknowledge funding from the UNLV Jack and Fay Ross family fellowship, Graduate and Professional Student Association, and TTDGRA programs. We acknowledge the tireless work of Benjamin Azua and Daniel Panduro during field sampling.

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1. Field Sites

1.1 Field Site Selection

Four serpentinite bodies in three different climate zones were chosen for investigation to examine climatic influences on chemical weathering of serpentinite bedrock and serpentine soil. These sites include the Trinity Ultramafic Body and Rattlesnake Creek Terrane within the Klamath Mountains of northern California, the Tablelands of Gros Morne National Park in Newfoundland, Canada, and a serpentinite body near the ghost town of Pickhandle Gulch in western Nevada (Table S1, Figure S1).

The modern climate in the Klamath Mountains is mediterranean, featuring hot and dry summers and cold and wet winters. NOAA 1991-2020 climate normals for the Weaverville, CA weather station (Station ID: USC00049490), near the String Bean Creek sampling site in the Rattlesnake Creek Terrane, record mean annual temperatures of 12.8℃, with a mean of 23.2℃ in July and 3.7℃ in December, and an annual average precipitation of ~100 cm^1,2. NOAA 1981-2010 climate normals for the Sawyers Bar Ranger Station (Station ID: USC00048025), north of Trinity Lake and within the Trinity Alps, show a mean annual temperature of 12.8℃, with a mean of 22.9℃ in July and 4.4℃ in December, and an annual average precipitation of ~118 cm^1,2, 1991-2020 normals were not available for that site. The exact climate at the Eunice Bluff and Deadfall lake sites is likely slightly colder due to the high elevation (~2100 m) compared to ~600-650 m elevation for the monitoring stations, similar high-elevation regions of the Klamath Mountains have mean summertime temperatures (July-October) of ~12.7℃ and mean wintertime (November-May) temperatures hovering around freezing (~0.9℃) though with substantial periods of below-freezing conditions and abundant snowfall³. A dispersed coniferous forest covers much of the Klamath Mountains^2,4 and is present at all sampling locations examined within this study. The modern climate of the Tablelands is subarctic; daily mean temperatures at the nearby Cow Head weather station (Station ID: 8401335) maintained by Environment Canada range from -7.0 ℃ in January to 15.7 ℃ in August with a mean yearly temperature of 3.9 ℃, with a yearly average precipitation of 120.4 cm⁵ from 1980-2010 climate normals. Pickhandle Gulch possesses a desert climate, with the nearest NOAA climate station to Pickhandle Gulch, in Mina, Nevada (Station ID: USC00265168), recording 1991-2020 climate normals of a 14.4 ℃ mean annual temperature, ranging from 2.7 ℃ in December to 27.9 ℃ in July, with an average precipitation of 14.2 cm annually¹.

1.2 Klamath Mountains

The Trinity Ultramafic Body covers ~2100 km² in the eastern Klamath Mountains and is composed of a partially serpentinized mélange of harzburgite and lherzolite (60-70%), dunite (15-20%), and plagioclase lherzolite (10-15%) units cut by clinopyroxene-rich dikes in some areas^6,7. Formation of the peridotite sections of the Trinity Ultramafic Body likely occurred around 472 ± 32 Ma with emplacement of the body occurring during the early Devonian^8,9. The Rattlesnake Creek Terrane lies southwest of the Trinity Ultramafic Body and is composed of a mixed sequence of serpentinite basement unconformably overlain by Upper Triassic and Lower Jurassic volcanic, hemipelagic, and clastic units with intruded gabbroic and quartz diorites throughout that trend into amphibolite facies and locally higher metamorphic grade materials^8,10,11. The serpentinized basement consists of peridotite, greenstone, amphibolite, and pillow basalts that have undergone varying degrees of serpentinization¹⁰. The genesis of the Rattlesnake Creek terrane ophiolite and associated facies and their emplacement timing remain poorly understood⁸.

Samples were collected from five sites within the Klamath Mountains, four sites within the Trinity Ultramafic Body (two sites in high-altitude cirques near Mt. Eddy dubbed Eunice Bluff and Deadfall Lake and two sites within the Swift Creek Valley), and one site within the Rattlesnake Creek Terrane dubbed String Bean Creek (Figure S1). The Trinity Alps, a formerly glaciated region of the Northeast Klamath Mountains where several formerly glaciated valleys feature serpentinite bedrock¹², were most recently extensively glaciated coterminous with the late Wisconsinan glacial period of the Laurentide Ice Sheet¹². Recent work using cosmogenic techniques has confirmed Younger Dryas deglaciation in high-altitude cirques within the Trinity Alps by ~12.1 ka^13,14. The deglaciation date of ~12.1 ka is used as the initiation date for pedogenesis in the high-altitude cirque soils at the Deadfall Lake and Eunice Bluff sites, while within the Swift Creek valley, samples were collected from sites determined by Sharp (1960) to be of Late and Middle Wisconsinan ages. The Late Wisconsinan Cordilleran Ice sheet reached its maximum extent sometime between about 15-23 ka, with initiation of retreat around ~15.6 ka¹⁵, with Middle Wisconsinan dates ranging roughly between 25-50 ka^16,17. As moraines within Swift Creek lack cosmogenic dates, we use these age ranges as approximate estimates for the ages of the Late and Middle Wisconsinan features described by Sharp (1960)¹². In contrast to the Trinity Ultramafic Body, previous glaciation has not been established within the Rattlesnake Creek Terrane, and the soil sampled there is undated but likely substantially older than the soils in the Trinity Ultramafic Body.

1.3 Tablelands

The Gros Morne Tablelands are one of four highland plateaus comprising the Bay of Islands ophiolite, part of the Humber Arm Allochthon located on the western flank of the Island of Newfoundland, Canada^18–20. The Bay of Islands ophiolite was emplaced onto the passive continental margin of eastern North America during the Ordovician age Taconian Orogeny^19,21. The plateau edges are defined by sharp u-shaped formerly glaciated valleys with associated glacial landforms including moraines, rock glacier deposits, pro-talus lobes, and diamict^18,22. Vegetation on the Tablelands is sparse, composed primarily of small ground hugging shrubs with stunted pine trees at lower elevations.

Soil samples were collected from three formerly glaciated valleys on the Tablelands (Figure S1). One sampling site was located just up-slope from and within the protruding lobe of a terminal moraine within the Devil’s Punchbowl cirque. The terminal moraine was dated to 17.6 ± 0.3 ka using chlorine-36 isotope analyses²². One sampling site was located on a terrace overlooking highway 471 in Trout River Gulch. Debris benches in the Trout River Gulch valley were cosmogenically dated using chlorine-36 isotopes to between 15 and 20 ka ²², necessitating the establishment of an ice-free valley prior to that point. Quartzite erratics sourced from over 20 km east of Bonne Bay necessitate westward ice flow at the time of deposition¹⁸. As Late Wisconsinan ice flow in the area was directed northward^18,23 this westward flow through Trout River Gulch ceased prior to the most recent glacial maximum. The combination of ³⁶Cl ages and glacial erratics indicate that Trout River Gulch was likely not glaciated during the Last Glacial Maximum^18,22. Finally, two sampling spots were located on diamict deposits in Winterhouse Gulch, a previously glaciated tributary valley that feeds into Trout River Gulch. One sampling site was located at the mouth of Winterhouse Gulch and one halfway up the valley between the mouth and the cirque. Though cosmogenic ages have not been determined for glacial material in Winterhouse Gulch Canyon, the late Wisconsinan age ranges for other glacial deposits on the Tablelands suggest a similar age range is likely for the presence of a valley glacier in Winterhouse Gulch²², and thus for initiation of soil development following glacial retreat. Reconstructions of Late Wisconsinan ice in Atlantic Canada suggest that the icesheet covering Newfoundland reached a most recent maximum extent around 20 ka^23,24 followed by deglaciation that resulted in glaciers retreating away from the coast by ~12 ka ^23,24. Initiation of soil development in Winterhouse Gulch likely falls within this age range, concurrent with all other cosmogenically dated glacial landforms surrounding the Tablelands²², with the soil at the mouth of Winterhouse Gulch likely older than the soil closer to the cirque from retreat of the glacier that formed the valley.

1.4 Pickhandle Gulch

Pickhandle Gulch is a ghost town located in the now defunct Candelaria mining district roughly 50 km northwest of Tonopah, Nevada. A serpentinite body roughly 6,800 ft by 300 to 1,350 ft is located southeast of the Northern Belle and Mount Diablo open pit mines with smaller serpentinite lenses scattered nearby, with some exposures of serpentinite within the nearby Northern Belle mine workings²⁵. Soil cover overlying the serpentinite bedrock is thin (<10 cm) and is mantled by a desert pavement composed of serpentinite clasts, with a sparse vegetation cover primarily composed of sagebrush. Previous literature examining the serpentinite body at Pickhandle Gulch is extremely limited and to our knowledge only one report from the Nevada Bureau of Mines attests to the serpentinite’s presence²⁵. The serpentinite is bounded by faults, with a larger mass likely to exist unexposed in the subsurface²⁵. The age and provenance of the serpentinite is currently unknown; however, the body contains inclusions of lower Triassic Candelaria shale material²⁵. The exposure age of the serpentinite is unknown. At Pickhandle Gulch the serpentinite body is bounded by sharp contacts with fluvial channels at its base and toeslope locations could not be identified. Soil samples were therefore collected from a ridgeline position and from two footslope locations on the sample hillslope to provide a degree of spatial variation as time differentiation between soils did not exist in a similar manner as in the Klamath Mountains or Tablelands (Figure S1).

2. Sample Collection and Processing

2.1 Sample Collection

Within the Klamath Mountains and Tablelands, soil pits were located on toeslopes at the base of large hillsides as these have previously been shown to accumulate amorphous material and secondary clay minerals²⁶ and soil development along a hillside is typically greatest at footslope/toeslope positions²⁷. In the Klamath Mountains, there is extensive tree cover, and we chose to locate pits not immediately adjacent to trees to limit the need to cut through large root structures and to avoid such overt biological impacts. In each location, soil pits were excavated by hand using picks and shovels to within a C horizon, contact with the bedrock, point of refusal, or contact with standing water. In the Klamath Mountains, the Eunice Bluff and Deadfall Lake soil pits were excavated to the point of refusal, while the Swift Creek Late, Swift Creek Middle, String Bean Creek soils were excavated until contact with a C horizon determined by decreasing soil redness in relation to overlying soil material²⁸. In the Tablelands, soil pits were excavated until infilling water was observed in the Winterhouse Gulch Canyon, Winterhouse Gulch Mouth, and Trout River Gulch sites and to contact with bedrock at the Devil’s Punchbowl site. As a result of the shallow nature of the Pickhandle Gulch soils (<~10 cm), soil pits in that field location were excavated to contact with the bedrock.

We collected at least one soil sample from within each soil horizon (see Appendix C). In the Eunice Bluff, Deadfall Lake, and String Bean Creek soils Munsell Color variation allowed for identification of a clear organically enriched A horizon at the surface of each soil pit. We elected to sample in 10 cm depth intervals below the A horizon at those three sites to increase the density of sampling. At the Swift Creek sites in the Klamath Mountains (Late and Middle) one sample was collected from each soil horizon identified in the field. In the Tablelands soil pits, relatively uniform Munsell soil color indicated that A and B horizons had not formed. Because the Tablelands soil profiles were visually undifferentiated, we gathered soil material from four depth intervals within each Tablelands soil pit to provide a higher density of sample material and evaluate possible variation in soil properties with depth that were not visually observable. At Pickhandle Gulch, the presence of small pore spaces indicated the development of Av vesicular surface horizons. We sampled from the Av horizons and from thin underlying C horizons that lacked a similar development of porosity. When referring to our samples in the methods, main text, and supplementary information, we sometimes refer to sampled depth intervals and not to soil horizons, as we sampled multiple different intervals from within some soil horizons in three of the Klamath Mountain sites and from all four Tablelands soils (see Appendix C).

A minimum of ~4 kg of combined bulk soil and gravel material was collected from within each sampled depth interval at all soil pit sampling sites. Parent material bedrock samples were collected from buried clasts from within each soil pit. The collected bulk soil and gravel samples from the Klamath Mountains and Pickhandle Gulch were stored on dry ice during transport and immediately placed in a -20℃ freezer upon return to UNLV. Bulk soil and gravel samples from the Tablelands were packaged and flown back to UNLV and placed in a -20℃ freezer upon arrival but were not packaged on dry ice during transport.

2.2 Parent Material Preparation

We processed one parent material sample each from the Tablelands and Pickhandle Gulch locations as soil pits at these sites were geographically close and developing out of the same ultramafic body. We processed a parent material sample from each Klamath Mountain sampling site as these sites were geographically disparate and developing out of varied ultramafic material. Each parent material sample was trimmed to remove weathering rinds using a cutting saw, crushed within a Bico Chipmunk Jaw Crusher Model 241-36 WD to produce chunks about 3 mm in diameter, and then powdered in a Fritsch pulverisette model 02-101 agate mortar grinder mill. The saw blade used to trim samples was cleaned between uses using a 100% ACS reagent grade ethanol wash. During crushing in the chipmunk jaw crusher, and then again when powdering in the pulverisette, two sub-samples of material were run to pre-contaminate the equipment and those sub-samples were then discarded. A third crushing or powdering run was then conducted using the pre-contaminated instruments and then collected for X-ray diffraction and bulk and selective chemical dissolution analyses. The jaw crusher was thoroughly cleaned between samples using a compressed air blower and a 100% ACS reagent grade ethanol wash. The pulverisette was cleaned between samples with a 100% ACS reagent grade ethanol wash. Following powdering, parent material samples were stored in a -20℃ freezer.

2.3 Soil Preparation and Clay-Size Fraction Separation

After removal from the freezer, each combined soil and gravel sample was first air dried and the <2 mm bulk soil fraction was sieved from the overall sampled material. Gravel content was estimated by massing the <2 mm and >2 mm fractions following sieving. Sub-samples of bulk soil material were then powdered in a Fritsch pulverisette model 02-101 agate mortar grinder mill prior to XRD analysis and application of selective chemical dissolutions. Two sub-samples of bulk soil material were powdered first to pre-contaminate the pulverisette and discarded afterword, and a third sub-sample then powdered and collected for XRD, bulk chemical dissolution, and selective chemical dissolution analyses. The pulverisette was thoroughly washed between samples using 100% ACS reagent grade ethanol. Both powdered and unpowdered bulk soil samples were stored at -20⁰C following powdering.

The clay-size fraction (<2 mm) was separated from the bulk soil following a modification of the methods in Edwards and Bremner (1967). Briefly, ~10 g of soil was mixed with 25 mL of 18.2 MΩ DI water, sonicated for 30 minutes, vortexed, and allowed to settle for 1.5 hours. The suspended load was then pipetted off, NaCl (JT Baker ACS Reagent Grade) was added to the supernatant to a concentration of 1M to promote flocculation, and the samples were allowed to settle for ~30 minutes. Following this, the samples were centrifuged for 3 minutes at 8,000 rpm (8,228 x g) and the clear supernatant decanted from the clay-size fraction pellet. The clay-size fraction pellets then were washed with 18.2 MΩ DI water and centrifuged 3 more times to remove residual salt, frozen at -20℃ for at least 12 hours, and then freeze-dried (LabConco Freezone 4.5 Freeze Dry System).

3. Mineralogical Analyses and Quantification

XRD analyses of randomly oriented powdered bulk soil samples were performed at the UNLV Geosciences Shockwave laboratory on a Proto-AXD Bragg-Brentano type X-ray diffractometer with a Cu K-alpha tube (1.541 Å wavelength), dectris hybrid pixel array detector, 0.4 mm soller slits, and a Ni filter between 4 and 70 ⁰2θ. A knife-blade attachment was added to limit over-saturation of the detector at low angles. Randomly oriented mounts were prepared by top loading the sample into brass or zero background (MTI corp. ZeroSi24D10C1cavity10D) sample stages. Excess material was gently scraped off the top using a razor blade to limit preferred orientation of phyllosilicates. Mineral identification within randomly oriented bulk soil XRD patterns was determined using the program X’Pert HighScore³⁰ and through comparison with d-spacings of crystal lattice planes reported in the American Mineralogist RRUFF database³¹ and the crystallography open database³².

Randomly oriented clay-size fraction samples extracted as described above were spiked to contain 20 wt.% 0.8 to 1-µm diameter α-Al2O3 (Beantown Chemical 127075-25G) and analyzed from 3 to 80 ⁰2θ at the X-ray Diffraction Laboratory at NASA Johnson Space Center in Houston, Texas on a Malvern PANalytical X’Pert Pro MPD X-ray Diffractometer equipped with a Co K-alpha source (45 kV, 40 mA, ½° anti-scatter slit, ¼° fixed divergence slit, 0.01067° step size, and 0.5 s/step). Spiked samples were prepared by combining pre-weighed α-Al₂O₃ and powdered parent material or clay-size fraction material and mixing the two materials together within an agate mortar and pestle by a combination of pressing material in toward the center and a circular motion. Mineral identification within randomly oriented clay-size fraction XRD patterns was determined by using the programs X’Pert HighScore³⁰ and Profex ³³ and through comparison with d-spacings of crystal lattice planes reported in the American Mineralogist RRUFF database³¹ and the crystallography open database³².

Oriented mount XRD measurements were conducted on a Proto-AXD Bragg-Brentano type X-ray diffractometer in the UNLV Geosciences Shockwave laboratory with a Cu K-alpha tube (1.541 Å wavelength), dectris hybrid pixel array detector, 0.4 mm soller slits, a Ni filter, and a knife-blade attachment between 2 and 14 ⁰2θ. XRD analyses were conducted on oriented clay-size fraction material following cation saturation and various chemical and heat treatments to determine the presence or absence of expandable 2:1 clay minerals in each soil. Phyllosilicate mineralogy determinations from oriented mount analysis as described below assist with determinations of phyllosilicate mineral presence, and particularly the presence of expandable 2:1 clay minerals, within XRD patterns acquired from randomly oriented mounts.

To identify whether expandable 2:1 clay minerals were present in the clay-size fraction, oriented mounts of the clay-size fraction was analyzed by XRD after 3 different treatments: 1) Saturation with Mg²⁺ followed by air-drying, 2) ethylene glycol vapor solvation of the air-dried Mg²⁺ saturated mounts, and 3) K⁺ saturation followed by heating to 550 °C (see Appendix C). Mg²⁺ or K⁺ saturations were prepared^34–38 by suspending 300 mg of clay-size fraction material in solutions of either 1N MgCl₂ (VWR ACS Reagent grade CAS 7791-18-6) or 0.1N KCl (OmniPur EMG Chemical Argentometric grade CAS 7447-40-7) in 18.2 MΩ water, shaking intermittently by hand for ~1 hour followed by centrifugation for 3 minutes at 8,000 rpm (8,228 x g), after which the supernatant was decanted, and the samples then washed with a 50:50 18.2 MΩ DI water-to-ethanol mixture and centrifuged for 3 minutes at 8,000 rpm 3 times to remove excess salts.

Three mL of 70% ethanol 30% water solution (Aldon Corp. EE0069 Denatured Ethyl Alcohol) were then added to each sample, and the container vortexed to suspend the clay-size particles. ~1 mL of the resulting slurry was then pipetted onto fused silica slides (Alfa Aesar Quartz microscope slides, Part #42295) and allowed to air dry in a covered container overnight in a fume hood.

Previous methods have called for 3-5 overnight wash cycles using cation saturated solutions followed by rinsing^35,39, or direct application of cation solution to already oriented clays through a suction device onto ceramic slides followed by a DI-water rinse³⁸. To test whether a 1-hour treatment produced similar saturation to the 3+ day treatment, we compared XRD patterns from a 3-day and 1-hour Mg-saturation procedure. We observed similar peak locations between air dried and glycolated XRD patterns from the two saturation durations indicating that a 1-hour saturation procedure produces cation saturation like a 72-hour saturation procedure (Figure S2).

Mg-saturated mounts were analyzed after air drying and again after solvation with ethylene glycol (VWR ethylene glycol ≥99%, CAS: 107-21-1) vapor in a closed container maintained at 60-70 ℃ for at least 12 hours^35,40. K-saturated oriented mounts were analyzed after heating to 550 ℃ for at least 30 minutes⁴⁰. Table S2 gives reference d-spacings used to identify the presence of various phyllosilicates within the oriented mount XRD patterns. Smectite (001) planes expand upon solvation with ethylene glycol³⁵ with the degree of expansion largely controlled by layer charge^{34,35,37,41,42}, with typically observed d(001) expansion of low-charge smectites following solvation with ethylene glycol to between ~16 to ~18 Å^35,37,43. High charge smectites and vermiculite typically do not exhibit (001) expansion or exhibit extremely limited expansion to between ~14 to ~14.5 Å following ethylene glycol vapor solvation^34,35,37,44. Both high- and low-charge smectite (001) planes collapse to ~10 Å following heating to 550℃^34–36.

Serpentine mineral (lizardite, antigorite, chrysotile) XRD peaks commonly overlap, and distinguishing whether one or multiple serpentine polymorphs is present from powder XRD patterns can be difficult. Lizardite, antigorite, and chrysotile presence in XRD patterns was interpreted primarily through the methodology of Peacock (1987)⁷. Identification of antigorite was based on the lack of a asymmetric (020) peaks at ~4.62 Ǻ to 4.57 Ǻ and the presence of a (16 0 1) peak at ~2.53 Å. Although transmission electron microscopy (TEM) analyses detected chrysotile fibers in the clay-size fraction of four soil samples (Figure S), chrysotile identification is difficult and generally not definitive by XRD in when multiple serpentine polymorphs are present and in various states of disorder or incorporate variable amounts of Mg, Fe, Al, Ni, or other elements. Determination from XRD alone may not be definitive when mixtures of these phases are present within a sample⁷. Additional complications in distinguishing individual serpentine phases arise from the common co-occurrence of the serpentine polymorphs^45–47 and from overlap of chlorite and talc peaks in the range of the lizardite and chrysotile (020) peaks and from olivine, which has a major (131) peak at ~2.51 Ǻ⁷, and from a major goethite peak (111) at ~2.44Ǻ⁴⁸. While TEM imagery suggests widespread chrysotile presence, it’s presence is not definitive based on XRD alone.

Clay-size fraction material from at least one depth interval within A and B (when present) and C soil horizons from all soil pits was analyzed through Rietveld refinement (Appendix C) of randomly oriented XRD patterns^49,50 using Profex 5.1³³, based on the BGMN refinement architecture⁵¹, with structure files from the Crystallography Open Database ³² (see Table S3 for crystal structure files used during refinement). Rietveld refinements were conducted on the clay-size fraction samples spiked with 20 wt.% 0.8 to 1-µm diameter α-Al₂O₃ (Beantown Chemical 127075-25G). The Crystallography Open Database crystallographic information files used for Rietveld refinement, as well as the origin of files used for smectites, are given in Table S3. For the Rietveld refinements, XRD patterns were analyzed between 5 and 80 ⁰2θ for samples that lacked smectite content, or when samples possessed smectites from 10 to 80 ⁰2θ to exclude the highly variable smectite (001) planes as in Smith et al. (2018)⁵². Though chrysotile was present in TEM imagery (Figure S49), it was not included in Rietveld refinement due to unavoidable significant overlap between major peaks between different serpentine polymorphs. During fitting, we refined unit cell parameters, scale factor, crystallite size, microstrain, and preferred orientation. Halite resulting from the Na-saturation utilized during the clay-size fraction extraction procedure was observed in several clay-size samples utilized for Rietveld refinement. XRD patterns that exhibited the presence of peaks attributable to halite were cut between 31.6 to 32.2 ⁰2θ, 36.4 to 37.4 ⁰2θ, 52.6 to 54.0 ⁰2θ, and 66.2 to 67.2 ⁰2θ as these represent peaks attributed to halite from comparison with a halite crystallographic information file from the Crystallography Open Database (COD 9006374)⁵³.

Rietveld refinements were utilized to fit a background to each clay-size fraction XRD pattern for the purpose of determining relative amorphous material contributions to each XRD pattern. Rietveld refinement was not utilized to directly calculate X-ray amorphous material abundances via the internal standard method as we could not accurately estimate the density of the various potential amorphous materials present. The prevalence of phyllosilicate phases with highly overlapping peaks and a high potential degree of disorder and chemical variation in octahedral and tetrahedral sites similarly would induce a high degree of uncertainty in any calculated phase abundances. Instead, relative amorphous contributions were determined by calculating a potential amorphous contribution to the fitted Rietveld background as amorphous material is typically part of the background fit of XRD Rietveld refinements^52,54. Following fitting of a background during Rietveld refinement, a baseline was calculated for each fitted background utilizing a SNIP function with 100 iterations in the Profex program³³. This baseline was then subtracted from the fitted Rietveld background to produce an amorphous hump pattern. The area under the central peak of each amorphous hump was then integrated in OriginPro software. As each XRD samples was analyzed using identical instrument parameters and processing methods, the relative area underneath each amorphous hump contribution to the fitted Rietveld backgrounds provides a relative indication of the abundance of amorphous material within each clay-size fraction.

To ensure the presence of X-ray amorphous material in the samples, we analyzed select samples of clay-size fraction material from the Klamath Mountains and the Tablelands via high-resolution transmission electron microscopy (HRTEM). We analyzed material from the Eunice Bluff soil BC horizon (23-33 cm depth interval) from the Klamath Mountains and the Devil’s Punchbowl soil C horizon (20-35 cm depth interval) from the Tablelands. HRTEM sample preparation was performed as follows. A small amount (~15 mg) of clay-size fraction material was added to ~15 mL of 70% ethanol solution. The solution was then sonicated for 5 minutes to disperse aggregates of clay-size particles. A carbon-coated copper TEM grid was then swirled in the mixture several times and allowed to air dry. Prepared samples were then analyzed using a Titan 300/80 (FEI) aberration-corrected TEM at 300 kV accelerating voltage at Arizona State University. Electron-dispersive spectroscopy results were normalized without Cu or C due to contributions to those elements from the grid material. During our analytical run, the selected area diffraction (SAED) detector exhibited problems and could not be utilized. The presence or absence of amorphous material was determined using fast Fourier transform (FFT) imagery. FFT images produce similar 2D patterns to SAED imagery and can be analyzed in a similar manner. Amorphous material was identified based on the absence of lattice fringes in FFT diffractograms and HRTEM images. Nanocrystalline material, still falling under the X-ray amorphous component of XRD data^55,56, was identified based on the presence of crystalline packets less than 100 nm across in HRTEM images with diffuse spots in FFT diffractograms.

4. Chemical Analyses

Bulk soil pH was measured via a 1:1 mass-to-volume ratio of unpowdered <2 mm bulk soil fraction material to 18.2 MΩ water with a Mettler Toledo InLab Expert Pro pH probe⁵⁷. All samples were dried at 105 ℃ overnight for at least 12 hours to dehydrate the samples using ceramic crucibles that had also previously been dried at 105 ℃ overnight for at least 12 hours. Loss on ignition at 550℃ (LOI₅₅₀), a proxy for organic carbon content, was applied to unpowdered bulk soil material following the method of Heiri et al. (2001)⁵⁸. Estimated error for the method is ±2 wt. %⁵⁸. Briefly, 5 grams of unpowdered air-dried bulk soil material was loaded into oven dried ceramic crucibles, heated to 105℃ overnight for at least 12 hours to dehydrate the samples, cooled and weighed, heated to 550℃ for 4 hours to burn off organic carbon, then cooled and weighed again. Loss on ignition at 950℃ (LOI₉₅₀) as in Jones et al. (2000)⁵⁹ was applied to all clay-size fraction and powdered parent material samples to examine total volatile content. 1 gram of freeze-dried clay-size fraction material or powdered parent material oven dried at 105℃ overnight for at least 12 hours to ensure dehydration was heated to 950 ℃ for 2 hours, allowed to cool overnight, then weighed.

To examine trends in crystallinity of Fe-containing materials within each soil site as well as the bulk chemical composition of the parent bedrock and clay-size fraction material, a suite of total and selective chemical dissolutions was applied to material from all sampled depth intervals in all soil pits (depth intervals and soil horizon designations are given in Appendix C).

ICP-MS analyses for bulk major and trace element chemistry of the total digestions of the powdered parent material and clay-size fraction samples were measured at the Nevada Plasma Facility Lab (NPFL) at UNLV using a ThermoFisher ScientificTM iCAP Qc ICP-MS after the following digestion— 50 mg of sample material was dissolved in acid cleaned Savillex beakers in a 1:1 mixture of sub-boiled double distilled HNO₃ and Optima grade HF. Samples were analyzed concurrently with the USGS standard BHVO-1, AGV-1, and BCR-1, and total procedural blanks. Reproducibility and detailed methods of NPFL data is presented in DeFelice et al. (2019)⁶⁰. Off gassing of SiF₄ during dissolution of silicates in HF precludes accurate direct measurements of SiO₂ in our samples⁶¹. SiO₂ content was instead calculated by subtracting the total major element oxides measured by ICP-MS from 100% as in Udry et al. (2019)⁶² after incorporating the volatile content determined by loss on ignition at 950 ℃ from 100% as in Jones et al. (2000)⁵⁹.

Na₂O was excluded from ICP-MS measurements of the clay-size fraction chemistry as the clay-size fraction extraction procedure involved the addition of NaCl to promote flocculation. Na₂O content within parent material samples was measured (see Appendix B) to estimate whether exclusion of Na₂O would substantially alter calculated SiO₂ values, and minimal Na₂O content is present in the parent material samples for Pickhandle Gulch (0.00 wt.%), the Tablelands (0.02 wt.%), and for the Eunice Bluff (0.06 wt.%), Swift Creek (0.01 wt.%), and String Bean Creek (0.02 wt.%) sampling sites in the Klamath Mountains. Na₂O content was slightly higher within the Deadfall Lake parent material (1.40 wt.%) because of the presence of plagioclase, likely including albite, not observed in the bedrock at other sites and likely not contributing to this extent to the soil formed at Deadfall lake based on the decreased Al present in the clay-size fraction from Deadfall lake relative to the parent material, and the presence of olivine in the clay size fraction despite the absence of olivine in the parent material (Figure S3 and Table 1). As SiO₂ content is calculated based on the measured major element oxide concentrations not including Na₂O it should be read as a potential maximum value and is likely very accurate for the soil pits with minimal parent material Na₂O values, but potentially slightly over-estimates the SiO₂ content for the Deadfall Lake soil. Additionally, if plagioclase found within some soils included a significant albite component this would also lead to overestimates of the SiO₂ content.

Selective chemical dissolution techniques were applied to powdered bulk soil samples, oven dried at 105⁰C for 12 hours to ensure dehydration, from all sampled depth intervals from all soil pits (Appendix C) to examine the distribution of secondary Fe within different soil reservoirs. Hydroxylamine hydrochloride, citrate dithionite, and sodium pyrophosphate extractants were applied to study Fe within amorphous material^63,64, amorphous and crystalline (oxyhydr)oxides^57,64,65, and organically bound^64,66 Fe-reservoirs respectively. The hydroxylamine hydrochloride supernatant was also analyzed for Si content.

A sodium pyrophosphate extraction was applied to analyze organically complexed Fe (Fe_P) following the method delineated within Shang and Zelazny (2008)⁶² a modification of the method used by McKeague (1967)⁶⁴. Whether the pyrophosphate technique results in dissolution of purely organic compounds or whether the supernatant potentially includes some Fe from colloidal Fe-(oxyhydr)oxide particles has been called into question⁶⁵. Fe measured by the pyrophosphate technique thus represents a potential maximum amount of organically complexed Fe. Briefly, 0.1M Na₄P₂O₇∙10H₂O (Alfa Aesar ACS grade #33385-22) was brought to pH 10 with Aristar Plus trace metal grade HCl (VWR chemicals #87003-259). 30 mL of the pyrophosphate extractant was added to 300 mg of 105⁰C oven dried powdered bulk soil material and the mixture was shaken overnight for 16 hours at 180 rpm (Eberbach Mod. 6010 shaker table). The samples were then centrifuged at 15,550 x g (10,980 rpm) for 10 minutes and the supernatant decanted from overtop the soil pellet and filtered through a 0.2 µm filter (VWR #28145-499). Samples were analyzed for Fe within five days of the extraction procedure by atomic absorption spectroscopy on a Thermo iCE 3000 Flame Atomic Absorption Spectrometer. The sodium pyrophosphate results are presented in the supplementary online material (Appendix B).

The hydroxylamine hydrochloride extraction to analyze Fe and Si within amorphous material was applied following the methodology of Shang and Zelazny (2008)⁶⁴, a modification of the methods used by Ross et al. (1985)⁶³ and Chao and Zhou (1983)⁶⁷. The hydroxylamine hydrochloride technique acts as a combined reduction and acid protonation technique and is selective for Fe and Si containing amorphous material^{63,64,67–69}. The hydroxylamine hydrochloride technique was chosen rather than the ammonium oxalate in darkness (AOD) method as it extracts Fe in comparable amounts from amorphous material^63,64, because the hydroxylamine technique does not attack Fe within magnetite or chlorites and exhibits minimal potential for dissolving crystalline Fe-(oxyhydr)oxides such as lepidocrocite^63,64,67, and because the hydroxylamine supernatant produces minimal interference during absorbance measurements compared to the oxalate supernatant⁶⁴. 25 mL of 0.25 M ACS grade NH₂OH∙HCl (Beantown Chemical Catalogue #144935) + 0.25 M Aristar Plus trace metals grade HCl (VWR chemicals #87003-253) solution was added to 100 mg of 105⁰C oven dried powdered bulk soil material from each sampled depth interval (Appendix C) within acid washed 50 mL centrifuge tubes. Samples were shaken for 16 hours at 180 rpm at room temperature (Eberbach Mod. 6010 shaker table) and the supernatant was then filtered through a sterile 0.2 µm filter (VWR #28145-499). The filtered supernatant samples were acidified to 2% v/v with trace metals grade HNO₃ (VWR chemicals Aristar Plus #87003-259). The samples were then diluted 10x using a 2% v/v trace metals grade HNO₃ in 18.2 MΩ DI-water solution and analyzed for Fe and Si content via atomic absorption spectroscopy on a Thermo iCE 3000 Flame Atomic Absorption Spectrometer.

The citrate dithionite extraction was applied to examine Fe within both amorphous and crystalline (oxyhydr)oxides^64,65 based on the procedure of the 2004 Soil Survey Laboratory Manual⁵⁷. The dithionite acts as a reducing agent for Fe³⁺ in Fe³⁺-containing amorphous and crystalline materials at near neutral pH conditions^{64,65,70–72}. Some work has suggested that in addition to reduction of Fe from within oxides and oxyhydroxides that reduction with dithionite extracts minor amounts of Fe³⁺ from vermiculite interlayers^73–75 and limited but measurable quantities of structural Fe from within smectites^64,70–72. Briefly, 25 mL of 0.57 M Na₃C₆H₅0₇·2H₂O, 400 mg of Na₂S₂O₄, and 750 mg of 105⁰C oven-dried powdered bulk soil material from each sampled soil depth interval (Appendix C) were combined within acid-washed 50 mL centrifuge tubes and shaken overnight for 12-16 hours at 180 rpm (Eberbach Mod. 6010 shaker table), then agitated vigorously by hand for approximately 30 seconds to ensure suspension of soil particles in solution and allowed to incubate without shaking overnight at room temperature for 12-16 hours. Samples were then centrifuged at 4,000 rpm (2,057 x g) for 15 minutes and the supernatant was decanted from the soil pellet and filtered through a 0.2 µm filter (VWR #28145-499). Samples were acidified to 2% v/v with trace metal grade Aristar Plus HNO₃ (VWR chemicals #87003-259), diluted 100x using a solution of 2% v/v trace metal grade HNO₃ and 18.2 MΩ DI water, and then analyzed for Fe content via atomic absorption spectroscopy on a Thermo iCE 3000 Flame Atomic Absorption Spectrometer.

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BulkScaleAmorphousMarsAnalogsSupplementaryMaterial.docx
Supplementary Figures and Text
BulkScaleAmorphousMarsAnalogsAppendixAXRDPatterns.xlsx
Supplementary Data Set 1
BulkScaleAmorphousMarsAnalogsAppendixBChemicalDissolutionResults.xlsx
Supplementary Data Set 2
BulkScaleAmorphousMarsAnalogsAppendixCSoilDescriptionsandAnalysisTracker.xlsx
Supplementary Data Set 3

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Fe-rich X-Ray Amorphous Material Records Past Climate and Persistence of Water on Mars

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Introduction

Relevance of the Studied Field Amorphous Material to Amorphous Material at Gale Crater, Mars

Mineral Inputs to the Amorphous Material-Forming Weathering System

The Effect of Climate and Time on the Formation and Crystallinity of Fe-Containing Amorphous Material

Implications for Past Martian Conditions

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