Winter Wonderland Cave, Utah, USA: A Natural Laboratory for the Study of Cryogenic Cave Carbonate and Thawing Permafrost

Winter Wonderland Cave contains perennial ice associated with two types of cryogenic cave carbonate (CCC) formed during the freezing of water. CCC ne is characterized by relatively enriched δ 13 C values, whereas CCC coarse exhibits notably depleted δ 18 O values indicating precipitation under (semi)closed-system conditions in a pool of residual water beneath an ice lid. Previous work has concluded that CCC coarse forms during permafrost thaw, making the presence of this precipitate a valuable indicator of past cryospheric change. Available geochronologic evidence indicates that CCC formation in this cave is a Late Holocene or contemporary process, and eld observations suggest that the cave thermal regime recently changed in a manner that permits the ingress of liquid water. This is the rst documented occurence of CCC coarse in the Western Hemisphere and one of only a few locations where these minerals have been found in association with ice. Winter Wonderland Cave is a natural laboratory for studying CCC genesis.


Introduction
The cryosphere is responding rapidly to climate warming, and paleoclimate records are critical for understanding the novelty of these responses 1,2 . A persistent challenge is that many paleoclimate archives are biased toward extremes, for instance the most extensive glacial advances 3 , most widespread periglacial conditions 4 , or sea level high-and low-stands 5 . Yet records of past climatic transition are crucial for placing contemporary global change into a longer-term context 6 . This is especially true in Arctic and high mountain environments where temperatures are warming rapidly 7,8 , leading to dramatic diminishment of ice extent 9,10 and degradation of permafrost 11,12 . Thus there is an urgent need for better records of how the cryosphere changed in the past, to clarify when and how rapidly changes occurred, and in what pattern these changes unfolded across the landscape.
A unique proxy demonstrated to provide information about past episodes of permafrost thaw, a transition that is often particularly disruptive to landscapes, ecosystems and infrastructure, is cryogenic cave carbonate (CCC). These minerals form when liquid water enters a cave containing subzero temperature conditions 13 . As this water freezes, solutes are concentrated in the remaining liquid until saturation is reached and precipitation of CCC is induced 14,15 . Although seasonal subzero conditions can be created inside a cave entrance by winter cold, more signi cant permanently subzero conditions are maintained in so-called "ice caves" by ventilation regimes that preferentially allow the ingress of winter air while excluding summer warmth 16,17 . For instance, caves with a single downward sloping entrance can trap cold air through density settling in winter; air that is not replaced by warmer, less dense air in the summer.
Alternatively, caves with multiple entrances at different elevations are susceptible to chimney effects that support freezing conditions where large amounts of cold air are pulled into the cave in winter. Either way, CCC is precipitated when water containing su cient dissolved solutes encounters the subzero conditions. Two types of CCC have been identi ed. One group, referred to as CCC ne , forms when a lm of water freezes on the surface of pre-existing ice 18 . The large surface area of this thin layer facilitates degassing of CO 2 , preferentially removing 12 C, and leaving the remaining water enriched in 13 C through kinetic effects 19,20 . Simultaneously, the preferential incorporation of 18 24 . CCC coarse , therefore, is a critical proxy because it records past episodes of permafrost thaw.
Here we report the rst discovery of CCC coarse in North America from an unusual setting where apparently young CCC is present in association with modern perennial ice. Our work provides an important point of comparison for CCC from well-studied caves in Europe, and documents a setting in which theories for CCC genesis could be evaluated.

Location
The CCC studied in this project was collected from Winter Wonderland Cave (WWC) in the Uinta Mountains of northeastern Utah, USA (Fig. 1). This solution cave has developed in the Carboniferous-age Madison Limestone, a regionally extensive rock unit that in this area consists of ne to coarse-grained dolomite and limestone, with locally abundant nodules of chert 25  Inside WWC temperatures were consistently subzero from 2016-2018 because of a ventilation regime that brings cold winter air in through the main entrance, and inhibits the entry of warm air in the summer. As a result, about half of the cave, which has a mapped length of 245 m, is oored by perennial ice up to 3 m thick. CCC is present as a lag on the ice surface, drapes rocks emerging from the sublimating ice, and occurs as discrete layers within the ice body (Fig. 2a). Additional details about the cave and its climatology are presented in Munroe (2020).
Representative CCC samples, numbered YS-1 through YS-6 (Table S1) were collected from the ice surface along with three samples that were submerged in small (~10-30 cm deep) pools ( Fig. 2b) of water lling depressions in the ice (CP, YP, and TF). Additional samples of Madison Limestone bedrock and water from pools were also collected for analysis.

Results And Interpretation
CCC Morphology.
An array of morphologies is present in the CCC samples from WWC. The smallest size fractions are spherulitic, with large aggregates reaching diameters of 50 µm, and individual spherules from 10 to 20 µm in diameter (Fig. 2c). This observation is corroborated by results from laser scattering, which reveal mean grain sizes from 24 to 42 µm in the <75-µm fraction. Rounded grains are typically clumped in botryoidal aggregates with smooth surfaces, and dumbbell structures are common. Occasionally more sharp-edged morphologies are present, with needle-like points that rarely exceed 5 μm in length. Larger size fractions (250-75 μm) contain rafts of aggregated grains in excess of 200 μm long. The TF samples, which were collected from the water surface, are characterized by arrow-shaped blades 10 to 20 µm long, grouped into thin rafts (Fig. 2d). All of these observations match CCC morphologies reported in the literature. For instance, previous work has established that CCC often occurs as raft-like aggregates of crystals, and spherical forms 21,22,26 . Rafts are generally at and composed of interlocking crystals in a manner similar to oating carbonate minerals reported from non-cryogenic cave environments 27 . Crystal splitting has been invoked as a mechanism for the growth of spherical forms 28 , which are sometimes superimposed on a branching, sheaf-life skeleton 29 . Finally, spherules are reported to display smooth surfaces, and often connect during growth to form dumbbell shapes 21 or chains 22 .
On the other hand, the CCC in WWC is considerably ner than precipitates with similarly depleted δ 18 (Fig. 3a).
Samples YS-3 and YS-4 have δ 13 C averaging 6‰ and an average δ 18 O of -7.3‰ (Table S2), suggesting that both are CCC ne . In contrast, samples YS-1, YS-2, YS-5, YS-6, CP, and YP have lower values of δ 13 C, averaging 3.5‰, and notably more depleted δ 18 O, averaging -16‰. These values identify these samples as CCC coarse (Table S2). Sample TF also plots in the CCC coarse eld (Fig. 3a), although it was collected from a pool that had just begun to freeze, which may explain why its δ 18 (Fig. 3b). In YS-1 and YS-2, CCC coarse is concentrated in the <75-µm fraction. In contrast, YS-4 is consistently CCC ne in all size fractions.
The results of additional analyses permit further interpretation of these samples. XRD reveals that YS-3 and YS-4 (CCC ne ) are a mixture of calcite and quartz (Fig. S1), and rounded quartz grains were visible in the <250-µm size fraction. The presence of quartz is con rmed in the XRF results, where sample YS-4 contains 52% SiO 2 (Table S3) . YS-3 and YS-4 also have much higher chondrite-normalized REE values (Table S4, Fig. S2). These observations, along with the isotope data for size fractions presented above, are evidence for a detrital component in these CCC ne samples, which were collected from bedrock shelves above the current ice surface (Fig. 1). In contrast, XRD analysis indicates that the CCC coarse samples are composed solely of calcite, as are the bedrock samples (Fig. S1). Major element analysis (  (Fig. S4). Second, samples of rodent fecal pellets from the ice beneath the surface lag of CCC yielded radiocarbon ages that calibrate to between AD 1600 and 1850 33 using the IntCal20 calibration curve 34 . Together this evidence strongly supports the conclusion that CCC formation in WWC occurred in the late Holocene, and is still occurring today.
Winter Wonderland Cave as a Unique Natural Laboratory for the study of CCC.
However, there are only a few reports of CCC from locations outside Eurasia 19,37 , and all of these are categorized isotopically as CCC ne . Winter Wonderland Cave is, therefore, the rst location in the Western Hemisphere where the unique paleoclimate indicator CCC coarse has been identi ed.
Furthermore, nearly all previous observations of CCC describe these minerals from locations where they are present as loose concentrations of mineral grains on an ice-free cave oor 21,26 . Only twice has CCC coarse been reported in association with modern, perennial ice 31,36 , and a recent comprehensive review noted that "Despite increasing evidence for Holocene CCC coarse , actively forming sites have not yet been observed" 13 . In WWC, CCC coarse with late Holocene to modern ages is present in association with perennial ice, making this cave an exceptional natural laboratory in which to study these precipitates and their genesis.  (Fig. 4) formed as a result of sublimation 38 . Short-term studies in an ice cave in Alberta, Canada suggest ice sublimation rates on the order of 3 mm per year 39 , although rates up to 10× higher have also been reported 13 . Either way, extrapolation from these benchmarks suggests that the relief observed on the ice surface in 2016 re ects 10 1 to 10 2 years of sublimation without the addition of new water. In contrast, in summer 2018, and again in 2019, liquid water entered the cave, lling many of the furrows, and freezing to create a new ice surface (Fig. 4). Deeper pools of water had lids of ice from 1 to 5 cm thick ( Fig. 2b), precisely the mechanism proposed for the formation of CCC coarse 15,21,23 . This water ranged from clear to a deep yellow color, and CCC was observed on the oor of each pool (Table S1). From the available data we cannot determine if this CCC precipitated from the water, or whether it was present as a lag on the ice surface before the pools formed. However, the concentration of solutes in this pool water was very high, with the highest values (K, Mg, and Ca >200 mg/L) corresponding to the yellow color (Fig.   S5), consistent with conditions necessary for mineral precipitation through freezing-induced saturation 14 .
The change from 2016 to 2018 suggests that the thermal state of the epikarst has recently shifted, at least locally, to allow liquid water to penetrate to the level of the cave (~100 m below the ground surface), or to produce meltwater from ice farther back in inaccessible parts of the cave system. Under extensive permafrost conditions, caves are unlikely to contain ice sourced from dripwater because the permafrost inhibits the downward movement of liquid water 40 . Only when permafrost is degrading is it possible for liquid water to reach a cave where temperatures remain consistently subzero. The available evidence supports the interpretation that WWC and its surrounding host rock comprise a sporadic permafrost body that is currently undergoing this transition. Numerical modeling suggests that the time window between thawing and ultimate loss of permafrost is relatively short 41 , and studies have noted that cave ice is rapidly ablating in various locations around the world 42 . Future investigations in WWC should take advantage of this singular opportunity to observe the formation of different types of CCC in (near) real time, with the goal of improving our ability to use the presence of these features in currently ice-free caves as a dateable indicator of past permafrost thaw 21 .

Conclusion
Winter Wonderland Cave in the Uinta Mountains of Utah contains cryogenic cave carbonate (CCC) associated with perennial ice. Two types of CCC with different genesis are present and can be distinguished on the basis of O and C isotope values. CCC ne is produced through open system freezing as a thin lm of water ows over the ice surface. In contrast, CCC coarse is produced by (semi) closedsystem freezing in deeper pools of water beneath thickening lids of ice. These conditions arise during permafrost thaw, thus CCC coarse records past episodes of permafrost degradation. Available age control suggests that CCC formation occurred in this cave during the late Holocene and may be a contemporary process. This cave is the rst location in the world where such young CCC coarse has been found, and is the rst location in the Western Hemisphere where CCC coarse has been identi ed. Records of past cryosphere transitions are critical context for assessing contemporary changes in Arctic and alpine environments. Winter Wonderland Cave provides a singular opportunity to test and improve theories of CCC genesis that will ultimately allow better insight into how permafrost responded to past climatic transitions.

Methods
The methods employed in sample analysis are brie y described here; full details are presented in the supporting information. At Middlebury College, CCC samples were wet sieved into >250, 250-75, and <75 µm size fractions. The morphology of all fractions was examined with a Tescan Vega 3 LMU scanning electron microscope (SEM). Energy dispersive x-ray spectroscopy (EDS) was used to evaluate whether contrasting grain morphologies observed in samples YS-3, YS-6, TS-2, CP, YP, and TF correspond to different elemental compositions.
The grain size distribution of the <75-µm fraction of each sample was investigated with laser scattering in a Horiba LA-950 particle size analyzer. Samples were dispersed in distilled water, and soni ed before analysis.
CCC mineralogy, along with representative samples of the Madison Limestone collected from the cave, was investigated with a Bruker D8 Advance x-ray diffractometer. The mineralogy of several samples was compared across different size fractions (<75 µm and 250-75 µm) to detect any compositional differences. Because none were apparent, the <75-µm and 250-75-µm fractions were used interchangeably for subsequent analyses depending on the amount of sample remaining.
The abundance of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in the samples was investigated with x-ray uorescence on a Thermo Scienti c ARL QuantX energy dispersive XRF. Trace elements were measured in the <75-µm fraction of samples YS-3, YS-4, YS-5, YS-6, TS-1, TS-2, in a representative bedrock sample, and in the 250-75-µm fraction of sample YS-4 with inductively coupled plasma mass spectrometry on a Thermo Scienti c iCAP Q ICP-MS after dissolution in HNO 3

Supplementary Files
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