Cryogenic opal-A deposition from Yellowstone hot springs
Introduction
Yellowstone's geothermal basins are utilized as analogues for ancient [1], [2] and possible extraterrestrial counterparts [3], [4], [5] in research to establish criteria for recognizing biogenic signatures in hot spring deposits [6]. Interpretations of silica deposition, sinter formation and microbial fossilization in and around the hot springs and geysers of Yellowstone have been based on processes observed during the late-spring, summer and early autumn ([7], [8] but see [9]). Here we describe silica deposition during the Yellowstone winter, when geothermal fluids flow into a sub-zero environment where erupted-waters freeze. We describe cryogenic silica precipitation that produces unconsolidated opal-A sediment within and beneath geothermal water-ice, which illustrates a dichotomy between winter and summer opal-A precipitation pathways.
Yellowstone National Park, Wyoming, USA, has Earth's largest concentration of terrestrial hot springs and geysers, which are associated with the ∼ 600 Ka Yellowstone Caldera and the Norris–Mammoth Corridor graben system [10]. Major areas of geothermal activity occur at Norris, Lower, Midway, Black Sand and Upper Geyser Basins (Supplementary data Fig. 1), located on a series of plateaus at an altitude of ∼ 2500 m. Local climate is characterized by long, cold winters and short, cool summers (Supplementary data Table 1a). Daily maximum air temperatures generally remain below freezing between November and March (Supplementary data Table 1b).
The reduced atmospheric pressure of the Yellowstone Plateau dictates that water boils at 93 °C, whilst during extreme cold conditions air temperatures are as low as − 35 °C. Geothermal gradients at the margins of a hot spring pool and its sinter apron can span 128 °C over < 1 m. Almost half of annual precipitation falls during the winter as snow curtailing hydrodynamic sediment transport until the spring thaw.
Fluids emanating from the geothermal features at Yellowstone generally have an alkali-chloride (ca. 470 ppm Na+(aq); ca. 450 ppm Cl−(aq)), high silica (up to 750 ppm SiO2(aq)), character and are derived at depth in the geothermal system by the interaction of meteoric water with acid-intermediate volcanic rocks [10]. Geothermal waters up-welling into and erupting from hot spring vent pools cool and become supersaturated with respect to opaline silica, which promotes opal-A nucleation. Where high pH and low salt concentrations prevent aggregation, nuclei grow via Ostwald ripening into nanospheres and microspheres (diameter ca. 10 nm − 1 μm). If pH/electrolyte concentrations remain favourable and/or when fluid turbulence maintains particle-dispersion, colloidally-stable particle-suspensions (sols) form. As microspheres grow to exceed colloidal dimensions sol stability collapses and they sediment from suspension [11]. Hot spring pools containing opal-A sols appear milky-blue as sunlight is scattered by suspended microspheres [12]. Whilst within colloidal dimensions, deposition of opal-A microspheres from a sol is effected as chemical and physical conditions in the enclosing medium change. A lowering of pH or an increase in electrolyte concentration encourages particle aggregation via flocculation (where microspheres are strongly attracted) and coagulation (where microspheres remain weakly repelling) [13]. Interparticle collisions may be encouraged where microspheres are forced together via physical removal of the aqueous medium (e.g. by evaporation) in the process of gelation. Particle aggregation progresses to a point where sedimentation occurs [13]. In sub-aqueous environments such as vent pools or sinter terrace pools particle growth and particle aggregation are the dominant sedimentation processes, whilst gelation is predominant in drying sub-aerial environments such as sinter aprons.
Silica deposited in summer conditions forms well documented sub-aqueous and sub-aerial sinter deposits that are hard, structurally robust and relatively immobile. Silica precipitation from erupted fluid commonly occurs in situ on microbes, creating microbially influenced mineral fabrics including columnar and stratiform stromatolites, botryoids, spicules, shrubs, streamers and palisades [6], [7], [8], [14], [15]. The durability of these precipitates is evidenced by their presence in the fossil record where they provide morphological marker fabrics of former hot spring and biological activity [1], [2]. Silica deposited during the Yellowstone summer is rich in distinctive biological silica precipitates such as diatom tests. Despite the dominance of published studies of summer deposition, the climate regime at Yellowstone dictates that a great proportion of the yearly output of geothermal fluid is erupted into a freezing environment.
Section snippets
Natural silica materials
Natural cryogenic sediment was collected from Opalescent Hot Spring, Porkchop Geyser and Big Blue Hot Spring at Norris Geyser Basin and Ear Spring and Sawmill Geyser at Upper Geyser Basin. At all springs, except Sawmill Geyser, sediment was collected from sinter surfaces below or at the margins of melting geothermal water-ice. At Sawmill sediment was collected from an ice cone forming on a boardwalk elevated above the sinter apron.
Mineral phases present were identified with a Phillips PW1710
Environments of opal-A sediment accumulation
Unconsolidated cryogenic opal-A sediments are common and widespread in the geyser basins of Yellowstone during winter and accumulate in a variety of environments, e.g. on sinter apron (Fig. 1a–j, Supplementary data Fig. 2c–g) and geothermally influenced wetland (Fig. 1d) surfaces and in dormant vent pools (Supplementary data Fig. 2a–b). A visual inventory of springs in the Norris and Upper Geyser Basins in February 2000 revealed opal-A sediments associated with the majority of alkali-chloride
Discussion
The particle morphologies produced experimentally by freezing of silica solutions replicate closely the shapes and morphological diversity observed in natural sediments. We propose that the observed morphologies are the product of cryogenic processes analogous to those reported from sea-ice formation and from investigations of freeze-gelation.
Conclusions
Sub-zero winter temperatures on the Yellowstone plateau promote rapid freezing of geothermal fluids and alter opal-A precipitation pathways resulting in cryogenic opal-A precipitation. This produces a range of unusual opal-A particle morphologies moulded within brine channels within ice. Sub-zero mean air temperatures between November and March on the Yellowstone Plateau mean that a significant proportion of the annual silica budget may be precipitated via a cryogenic mechanism. Despite
Acknowledgements
We thank Carolyn Davies, Jen Whipple, Smokey Sturtevant, Bill Wise and the staff of Yellowstone National Park. This research was supported through a NERC studentship grant and by the Leverhulme Trust, Grant F/00 407/S (A.C.) and Special Research Fellowship SRF/2000/0246 (I.B.).
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2019, Chemical GeologyCitation Excerpt :The brine veins in between forming ice crystals become super-saturated with respect to silica and high in ionic strength, which promotes silica nucleation, polymerization, and particle growth (Channing and Butler, 2007; Fox-Powell et al., 2018). When the COA-containing ice melts, unconsolidated COA particles are washed away from the location of formation and are deposited downstream (Channing and Butler, 2007). COA takes the shape of the brine veins in which they form and are commonly tube- or funnel-shaped (Channing and Butler, 2007; Fox-Powell et al., 2018).
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ECOSSE (Edinburgh Collaborative of Subsurface Science and Engineering). A Joint Research Institute of the Edinburgh Research Partnership in Engineering and Mathematics.