In the Arctic, the snowpack forms the major interface between atmospheric and
terrestrial cycling of mercury (Hg), a global pollutant. We investigated Hg
dynamics in an interior Arctic tundra snowpack in northern Alaska during two
winter seasons. Using a snow tower
system to monitor Hg trace gas exchange, we observed consistent concentration
declines of gaseous elemental Hg (
Mercury (Hg) is a neurotoxic pollutant of worldwide importance that is
transported over long distances in the atmosphere as gaseous elemental Hg
(
Representing about 26 % of the global land surface area, polar regions are unique environments with specific physical, chemical, and biological processes affecting pollutant cycles including that of Hg (Douglas et al., 2012). In particular, most of the northern latitudes are covered by a laterally continuous snowpack during long periods of the year. In the Alaskan tundra, the surface snow cover is present about two-thirds of the year (Cherry et al., 2014). The snowpack hence forms a critical interface between the Arctic atmosphere, tundra ecosystems, and underlying tundra soils. Trace gas exchanges between the atmosphere and the tundra are modulated by sinks and sources below and within snowpack, by snow diffusivity, snow height, and snow porosity (Dominé and Shepson, 2002; Lalonde et al., 2002; Monson et al., 2006). The snowpack accumulates nutrients, pollutants, and impurities that are deposited by snowfall and dry deposition processes, all of which can subsequently be transported to underlying ecosystems during snowmelt (Bergin et al., 1995; Uematsu et al., 2000).
The snowpack plays an important role for the cycling of Hg as well, including
for atmospheric deposition, photochemical redox reactions, and associated
phase changes between solid and gaseous Hg that can volatilize Hg to the
atmosphere (Douglas et al., 2008, 2012; Faïn et al., 2013; Mann et al.,
2014; Steffen et al., 2013). In particular, temperate and Arctic studies have
shown that the snowpack can serve as sink or source of
In the Arctic and Antarctic, Hg cycling is also affected by atmospheric Hg
depletion events (AMDEs), which are observed primarily in the springtime
along coastal locations (Angot et al., 2016a; Dommergue et al., 2010;
Schroeder et al., 1998; Steffen et al., 2008). During AMDEs, atmospheric
The objective of this study was to characterize Hg dynamics in the inland
Arctic snowpack at Toolik Field Station and along a 170 km transect between
this site and the Arctic coast. For the first time, we comprehensively linked
trace gas fluxes of
Study area in northern Alaska, including Toolik Field Station (orange bullet point) and the eight transect sites (yellow bullet points). Satellite images are true color images (Earthstar Geographics SIO, 2017).
Measurements were mainly performed at Toolik Field Station (Alaska, USA) over
two full snow cover seasons from October 2014 to May 2016. The research
station is located on the north slopes of the Brooks Range
(68
Snowpack sampling was also performed along a transect between Toolik and the Arctic Ocean in March 2016 (Fig. 1, yellow bullets). Detailed geographical characteristics of the sample sites are given in Table S1 in the Supplement. A total of eight study sites were sampled from south (500 m a.s.l.) to north (20 m a.s.l.). All the sampled sites were characterized by similar ecosystems and lithology (including undifferentiated volcanic Upper Cenozoic beds to the north) as described above for the Toolik area.
We continuously sampled and analyzed interstitial air of the tundra snowpack
at Toolik using a snow tower (Fig. S1 in the Supplement) as described in
detail by Seok et al. (2009) and Faïn et al. (2013). In summary, a snow
tower consists of an air inlet manifold placed in the snowpack, so sampling
of trace gases can be remotely alternated between various snow depths for
undisturbed sampling of interstitial snow air throughout an entire snow
season. The snow tower used at Toolik consisted of six 60 cm aluminum cross
arms mounted at heights of 0, 10, 20, 30, 40, and 110 cm above the ground
surface. Gas inlets were mounted to each cross arm, allowing vertical
sampling of snow interstitial air for analysis for multiple trace gases,
including
Gaseous
At Toolik, we characterized Hg in the snowpack both over the undisturbed
tundra and the adjacent frozen Toolik Lake (within 200 m of the tundra
location). Two snow pits were sampled on five dates between October and May
in the 2014–2015 season and on four dates between December and June in
2015–2016. For each pit, we vertically excavated snow samples using a
stainless-steel snow cutter (RIP 1 cutter 1000 cc), clean latex gloves, and
trace metal Nasco Whirl-Pak® (The Aristotle
Corporation, Stamford, CT, USA) HDPE plastic bags. We sampled at 10 cm layer
increments from the top to the bottom of the snowpack. Samples from two
perpendicular walls of the pit were each pooled together per layer for
analysis. Snow height, density, and temperature were measured for each layer,
and frozen snow samples were stored in a cooler before transferring to a
In the laboratory, we melted snow samples overnight in the Nasco
Whirl-Pak® bags at room temperature in the
dark, and melted snow samples were subsequently analyzed for Hg. A fraction
of snowmelt was directly transferred to 50 mL polypropylene tubes
(Falcon®, Corning Incorporated, Corning, NY,
USA) for analysis of
Major cation and anion concentrations were quantified at the US Army Cold
Regions Research and Engineering Laboratory's (CRREL) Alaska Geochemistry
Laboratory in Fort Wainwright, Alaska, with a Dionex ICS-3000 ion
chromatograph. An AS-19 anion column and a CS-12A cation column (Dionex
Corporation Sunnyvale, California) were used, each with a 10
Snowpack temperatures (red lines) and densities (blue lines) and
dissolved Hg concentrations (green bars, including mean values and standard
deviations) for five snow pits in the 2014–2015 season
Stable isotopes of oxygen and hydrogen were also measured at CRREL Alaska
using wavelength-scanned cavity ring-down spectroscopy on a Picarro L2120i
(Sunnyvale, California). Standards and samples were injected into the
analyzer for seven separate analyses. Results from the first four injections
were not used to calculate the stable isotope values to eliminate internal
system memory. The mean value from the final three sample injections was used
to calculate the mean and standard deviation value for each sample. Values
are reported in standard per mil notation. Repeated analyses of five internal
laboratory standards representing a range of values spanning the samples
analyzed and analyses of SMOW, GISP, and SLAP standards (International Atomic
Energy Agency) were used to calibrate the analytical results. Based on
thousands of these standards analyses and of sample duplicate analyses we
estimate the precision is
We performed all data processing and statistical analyses with RStudio
1.1.383 (RStudio Inc., Boston, Massachusetts, USA) using R 3.4.2 (R
Foundation for Statistical Computing, Vienna, Austria). Averaged data and
variance in figures and tables are shown as mean
Due to high wind conditions in the Arctic tundra (Cherry et al., 2014), the physical development of the snowpack and its depth and the thickness of wind slab layers at Toolik were subject to significant drifts and changes in snowpack height and were thus highly variable spatially and temporally throughout the winter season. The average snow height over the tundra site (shown in gray bars in Fig. 2) was continuously measured in both winters using a camera set to record daily pictures and using reference snow stakes placed in the snowpack. In the 2014–2015 season, the average snowpack height was 37 cm, with a standard deviation of 12 cm and a maximum depth of 60 cm. In the 2015–2016 season, the snowpack was almost half of that of the previous year, with an average snowpack height of 19 cm, a standard deviation of 7 cm, and a maximum depth of 35 cm.
Based on snow pit measurements in the 2014–2015 season, we observed an
increase of snow density with time, from an average of 0.18 g cm
Snowpack temperatures were highly variable throughout the seasons and also
strongly differed vertically within the snowpack (red lines in Fig. 2).
Temperatures ranged from
The snowpack over the adjacent frozen lake showed an average density of
0.23 g cm
The transect between Toolik and the Arctic Ocean performed in March 2016
showed snowpack height ranging between 30 and 66 cm. The maximum height was
observed at one site located 55 km from the Arctic Ocean where presence of
dense shrubs up to 40 cm height induced accumulation of local drifting snow
due to high roughness. Snow density (between 0.19 and 0.26 g cm
Gaseous
Gaseous
The
The top of the snowpack (ranging between 2 and 12 cm depth below the
atmosphere depending on snow depth) generally showed highest
During March and April, snowpack
Snow concentration profiles for
A key question pertaining to the wintertime snowpack
Since diffusivity is determined by both snowpack porosity and tortuosity –
both of which are poorly known and not directly measured – we used the flux
ratios between
We focused our analysis of
The constant and negative ratios between
Spatial pattern of dissolved Hg concentrations (Hg
Snow samples were analyzed at Toolik for
The snowpack sampled over the adjacent frozen lake showed
Most Arctic studies of snowpack Hg have been performed close to the coast
(i.e., Alert and Barrow), and few studies include inland sites such as Toolik
(Douglas and Sturm, 2004). In our study, measurements of
Temporal pattern of dissolved Hg (Hg
Surface snow that was collected throughout the season can serve as an
estimate for atmospheric wet deposition Hg concentrations and loads (Faïn
et al., 2011). Concentrations of
Little temporal variation in snowpack Hg concentrations was observed between
the early season snowpack evolving mainly under darkness and the late season
snowpack exposed to solar radiation (Figs. 2 and 6), although some temporal
differences were evident during March and April when AMDEs were present in
the region. Snowpack
Mean concentration (
Spearman's coefficient correlations (
Concentrations of
Major cations (
Ternary diagram of tundra surface snow (orange), tundra snowpack
(blue), and lake snowpack (green) samples from Toolik Field Station ordered
by dissolved Hg concentration between
Spearman correlation coefficient (
Dissolved Hg concentrations in surface snow samples for 2014 to 2016
in:
To further visualize the relationships between analytes, we plotted a ternary
diagram using three endmembers according to Garbarino et al. (2002), Krnavek
et al. (2012), Poulain et al. (2004), and Toom-Sauntry and Barrie (2002)
(Fig. 8). We considered
The lack of consistent statistically significant associations between major
ions and
Oxygen (
In this study, we investigated snow Hg dynamics in the interior Arctic tundra
at Toolik Field Station, Alaska, simultaneously analyzing Hg in (1) the
gas phase (
Snow chemistry data can be found in an Excel file in the Supplement.
The supplement related to this article is available online at:
The authors declare that they have no conflict of interest.
We thank Toolik Field Station staff for their support in this project over 2 years, especially Jeb Timm, Joe Franish, and Faye Ethridge for helping with snow collection. We also thank Martin Jiskra (Geosciences Environnement Toulouse) and Christine Olson (DRI) for their field support, Christopher Pearson, Olivia Dillon, and Jacob Hoberg (DRI) for their support with laboratory analyses, and Dominique Colegrove and Tim Molnar (University of Colorado) for helping with field work and data processing. We finally thank Alexandra Steffen for providing mercury snow data from Alert. Funding was provided by the US National Science Foundation (NSF) under award no. PLR 1304305 and cooperative agreement from National Aeronautics and Space Administration (NASA EPSCoR NNX14AN24A). Edited by: Becky Alexander Reviewed by: two anonymous referees