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Polar Desert Chronologies Through Quantitative Measurements of Salt Accumulation

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Polar Desert Chronologies Through Quantitative Measurements of Salt Accumulation Polar desert chronologies through quantitative measurements of salt accumulation Joseph A. Graly1, Kathy J. Licht1, Gregory K. Druschel1, and Michael R. Kaplan2 1Department of Earth Sciences, Indiana University–Purdue University Indianapolis, Indianapolis, Indiana 46202, USA 2Lamont–Doherty Earth Observatory, Palisades, New York 10964, USA ABSTRACT isotopes suggest a primarily atmospheric ori- We measured salt concentration and speciation in the top horizons of moraine sediments gin, with some trace mineral boron (Leslie et from the Transantarctic Mountains (Antarctica) and compared the salt data to cosmogenic- al., 2014). However, the Dry Valleys boron iso- nuclide exposure ages on the same moraine. Because the salts are primarily of atmospheric tope values that suggest a trace mineral compo- origin, and their delivery to the sediment is constant over relevant time scales, a linear rate of nent could also form from Rayleigh distillation accumulation is expected. When salts are measured in a consistent grain-size fraction and at during the precipitation of atmospheric boron a consistent position within the soil column, a linear correlation between salt concentration (Rose-Koga et al., 2006). and exposure age is evident. This correlation is strongest for boron-containing salts (R2 > 0.99), Atmospheric aerosols exist either as acid 2 but is also strong (R ≈ 0.9) for most other water-extracted salt species. The relative mobility vapors and gases (e.g., HNO3, N2O5, etc.) or of salts in the soil column does not correspond to species solubility (borate is highly soluble). as anion-cation pairs, commonly adhering to Instead, the highly consistent behavior of boron within the soil column is best explained by particulate matter. Upon deposition through a the extremely low vapor pressure of boric acid at cold temperatures. The environment is suf- range of dry and precipitation-driven processes, iciently dry that mobility of salt species within the soil column is controlled by vapor phase acidic aerosols will encounter earth materials effects. In other cold desert settings, topsoil salts, speciically boron, may be employed as a and react to form salts. Though halides (i.e., proxy for relative sediment exposure age. chloride) source primarily from sea spray aero- sols, atmospheric pathways exist by which they INTRODUCTION Currently, cosmogenic-nuclide exposure ages are oxidized and incorporated into acidic aero- Terrestrial sediment is a direct record of the are most widely employed to assess Antarctic sol gases (Andreae and Crutzen, 1997). While history of Antarctic glaciation and geochemi- terrestrial chronologies. However, the cost and atmospheric boron exists mostly as a vapor of cal processes on the landscape. The temporal labor involved in cosmogenic isotope measure- boric acid, this vapor is not stable at low temper- context of sediment transport and moraine for- ments can make it impractical to employ the atures. At ground surface temperatures <−25 °C mation constrains past ice-sheet size and low approach on a large number of samples. Ant- (as commonly prevail in Antarctica), boric acid patterns. The extremely arid environment that arctic exposure ages can sometimes be hetero- vapors adhere to atmospheric particles and are covers most of Antarctica allows for the reten- geneous, with adjacent rocks in moraines and deposited in the particulate fraction (Anderson tion within soils of a wide range of atmospheri- sedimentary sequences displaying widely vary- et al., 1994). cally sourced salts that would be lushed from ing ages (e.g., Brook et al., 1995; Bromley et In ice cores relective of contemporary cli- soil by liquid water in higher-precipitation envi- al., 2010). A complementary approach using mate at Vostok Station (1000 km inland), sea salts ronments. If we can accurately quantify the rate salt concentrations may therefore expand the make up 36% of aerosol deposition (Legrand of salt accumulation in hyperarid environments, applications of surface exposure dating in Ant- and Mayewski, 1997). During glacial periods, salt concentrations can act as a chronometer of arctica and help resolve heterogenous cosmo- which have higher winds, salt lux increases to surface exposure age. genic-nuclide data sets. While salt chronometers 54% of total deposition; nitrate lux increases The progressive development of salts in Ant- are unlikely to replace cosmogenic dating, we likewise. In contrast, ice core records show arctic soils is well documented, but has typically make a case that they can provide a parallel and atmosphere-sourced (non-sea salt) sulfate depo- been assessed through qualitative descriptions technically simpler approach across a range of sition has relatively little variation between the of salt stages (Ugolini and Bockheim, 2008). Antarctic cold desert settings. Holocene and Pleistocene periods (e.g., Iizuka Where the total salt content of a soil proile has et al., 2012). Acid lux is similarly constant in been measured, a linear correlation between BACKGROUND the Vostok core, with ~2 µeq L–1 of H+ deposi- salt content and age emerges (Bockheim, 1997). The dominant salt species in most Antarctic tion (Legrand and Mayewski, 1997). Studies of Except in near-coastal regions, where rainfall soils are sulfates, chlorides, and nitrates (Bock- borate in glacial ice are presently absent. allows for leaching of salts, soil age has been heim, 1997), though trace halides (F, Br) and identiied as the primary factor controlling salt borate are also considered. In the McMurdo Dry FIELD SITES AND METHODS content in the McMurdo Dry Valleys (Toner et Valleys, atmospheric sources of sulfate domi- Our ield site is Mount Achernar moraine, al., 2013). Here, we present a systematic attempt nate inland, and sea spray dominates near the located in the central Transantarctic Mountains to identify salt accumulation tracers whose coast, though in both settings a non-negligible (84.2°S, 161°E), ~1800 m above sea level and behavior is suficiently uniform that they can terrestrial weathering component is also inferred ~750 km from seasonally open ocean water. be employed as reliable geochronometers in (Bao and Marchant, 2006). Chloride also shows Sediment emerges onto the moraine via sub- a setting without recorded seasonal melt. We diminishing inluence of sea spray with distance limation of Law Glacier—an outlet that lows also employ a simple modeling approach to help inland (Bao et al., 2008). Nitrates have isotopic off of the polar plateau (~20 km inland) toward explain why a salt-accumulation chronometer signatures consistent with an exclusively atmo- the Ross Ice Shelf (~100 km downstream). works in this setting. spheric origin (Michalski et al., 2005). Boron Progressively greater sediment thicknesses are GEOLOGY, April 2018; v. 46; no. 4; p. 1–4 | GSA Data Repository item 2018105 | https://doi.org/10.1130/G39650.1 | Published online XX Month 2018 ©GEOLOGY 2018 Geological | Volume Society 46 | ofNumber America. 4 For| www.gsapubs.org permission to copy, contact [email protected]. 1 found with distance from the active ice mar- salts with depth in both these pits and others higher-ionic-strength samples tending to have gin; these accumulations rest above stagnant sampled by Scarrow et al. (2014), we modeled greater Na relative to Ca. glacial ice (Bader et al., 2017; Scarrow et al., the solubility of the principal salts at measured The largest deviation from the distance–salt 2014). Moraine sediment also emerges from ratios and freezing-point temperatures. The content correlation occurs between 4200 and much smaller, secondary ice bodies that low model was implemented with Geochemist’s 5500 m from the ice margin, where ionic con- over the mountains lanking the moraine: Lewis Workbench software (see details in the GSA centrations consistently fall and then rise over Cliff Ice Tongue and a few small, unnamed ice Data Repository1). several samples. This region is near where sur- tongues. Though Lewis Cliff Ice Tongue is the The soil samples were collected along the face morphology suggests a complex boundary site of evaporative features associated with dis- same transect where boulder samples had been between sediment supplied laterally from Law charging subglacial waters (Fitzpatrick et al., previously analyzed for cosmogenic isotope Glacier and sediment supplied terminally from 1990), the rest of the moraine does not have abundance (Kaplan et al., 2017). Eleven of Lewis Cliff Ice Tongue. A smaller but signii- evaporite features, and soil salts found at Mount the cosmogenic isotope samples were located cant deviation also occurs between 250 and 500 Achernar are comparable to those elsewhere in within 100 m of soil samples collected along the m from the ice margin, where ionic concentra- Antarctica. Unlike in the McMurdo Dry Val- same ridge crest, allowing for direct compari- tions are elevated compared to the surrounding leys, summer temperatures do not reach 0 °C son between 10Be-26Al-3He ages and the chemi- regions. This corresponds to a region where the at Mount Achernar moraine, melt is restricted cal properties of surrounding soil (Fig. 1). The parallel-ridge topography of the region gives to very low-albedo surfaces, fallen snow is lost cosmogenic nuclide exposure ages generally way to a 500-m-wide lat-lying zone. The soils to sublimation, and ground ice is not observed increase with distance from the margin. How- in this zone are more lightly colored than in (Scarrow et al., 2014). This absence of above- ever, some secondary discrepancies in the rela- surrounding areas, suggesting greater oxidation. freezing temperatures differs importantly from tionship between distance and age are observed, Highly signiicant, linear correlations are sites of previous studies on the transport of salts as discussed below. found between the concentrations of all detect- to depth in polar settings (e.g., Amundson et al., Sediment samples were freeze-dried and dry able ions and the cosmogenic-nuclide expo- 2012; Hagedorn et al., 2010). sieved. In order to standardize approximately for sure age on boulders from the same moraine We collected 38 sediment samples from a grain-size variation, analyses were performed on crest (Fig.
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