Mercury in Active‐Layer Tundra Soils of Alaska: Concentrations, Pools, Origins, and Spatial Distribution

Abstract

Tundra soils serve as major sources of mercury (Hg) input to the Arctic Ocean via river runoff and coastal erosion; yet little information is available on tundra soil Hg concentrations, pool sizes, origins, and dynamics. We present a detailed investigation of Hg in the active layer (upper ~100 cm subject to seasonal thaw) of tundra soils across 11 sites in Alaska. Soil Hg concentrations in organic horizons (151 ± 7 μg/kg) were in the upper range of temperate soil organic horizons, and concentrations in mineral horizons (98 ± 6 μg/kg) were much higher than in temperate soils. Soil Hg concentrations declined from inland to coastal sites, in contrast to a hypothesized northward increase expected because of proximity to coastal atmospheric mercury depletion events. Principle component analyses and elemental ratios results show that exogenic sources dominated over geogenic sources—in A‐horizons (66 ± 4%) and mineral B‐horizons (51 ± 1%). 14C age‐dating suggested recent origins of Hg in surface soils but showed that mineral soils (more than 7,300 years old) must have accumulated atmospheric inputs across millennia leading to high soil concentrations and pools. We estimated a total Northern Hemisphere active‐layer tundra soil Hg pool of 184 Gg (range of 136 to 274 Gg), suggesting a globally important Hg storage pool. Tundra soils are subject to seasonal thaw and freeze dynamics, thereby providing large inputs to rivers, lakes, and the Arctic Ocean. Understanding processes that mobilize Hg from tundra soils will be critical to understanding future Arctic wildlife and human Hg exposures.

An Assessment of Traditional Chemical Indicators of Atmospheric Pollution in Northern Saskatchewan Forest Soils

“…In the present study, even though Hg concentrations and SOM were generally much lower than the Alaskan tundra soils there was a strong linear relationship between soil (%) C and Hg with a slope (ratio) of 5.9, mid-way between values reported by Olson et al (2018). This relationship is explained by foliar uptake of Hg 0 , which then enters soils through litter fall over millennia (Olson et al, 2018), and is consistent with spatial patterns in soil Hg observed in this study. Friedli et al (2007) similarly reported that most (95%) of the Hg in upland boreal forests in Saskatchewan is associated with organic matter but highlighted concerns over potential Hg emissions following forest fires that may increase in frequency and severity with climate change.…”

“…While several studies have shown that a number of metals including V and Hg are released to the atmosphere as a result of industrial processes in the Athabasca Oil Sands Region, metal levels in lichen and moss tissue decrease exponentially with distance from the industrial center (Addison and Puckett, 1980; Kirk et al, 2014; Shotyk et al, 2014) and there is no evidence that oil sands emissions have led to elevated levels of these metals in surface soils in Saskatchewan. Instead, recent studies in tundra soils in Alaska have shown that Hg pools in soils are high (mean concentrations of Hg in upper mineral soil 110 ng g −1 ) and closely linked to soil C (average 12% C in the A horizon), with a mean Hg (ng g −1 ) to C (%) ratio of 4.0 in the O horizon and 9.2 in the A horizon (Olson et al, 2018). In the present study, even though Hg concentrations and SOM were generally much lower than the Alaskan tundra soils there was a strong linear relationship between soil (%) C and Hg with a slope (ratio) of 5.9, mid‐way between values reported by Olson et al (2018).…”

“…Instead, recent studies in tundra soils in Alaska have shown that Hg pools in soils are high (mean concentrations of Hg in upper mineral soil 110 ng g −1 ) and closely linked to soil C (average 12% C in the A horizon), with a mean Hg (ng g −1 ) to C (%) ratio of 4.0 in the O horizon and 9.2 in the A horizon (Olson et al, 2018). In the present study, even though Hg concentrations and SOM were generally much lower than the Alaskan tundra soils there was a strong linear relationship between soil (%) C and Hg with a slope (ratio) of 5.9, mid‐way between values reported by Olson et al (2018). This relationship is explained by foliar uptake of Hg 0 , which then enters soils through litter fall over millennia (Olson et al, 2018), and is consistent with spatial patterns in soil Hg observed in this study.…”

“…that were associated with metal deposition have been reported in other regions of Europe (Johansson et al, 2001; Heinrich and Mayer, 1989) as well as North America (Grigal and Ohmann, 1989). Mercury exhibits a different chemical behavior compared with other metals and due to the prevalence of Hg 0 ; it has a much longer residence time in the atmosphere resulting in almost global contamination (Olson et al, 2018). Nevertheless, some Hg deposition is associated with particulate Hg that tends to fall out nearer to emission sources and gradients in soil, moss and lichen Hg extending from pollution sources have been identified (Johansson et al, 2001; Kirk et al, 2014; Nater and Grigal, 1992).…”

Response to Reviewer 2

“…Throughout the manuscript, the authors refer to northern soil Hg pools calculated by Schuster et al (2018) et Olson et al (2018) for the upper 1 m: 755 Gg and 184 Gg, respectively. Olson et al (2018) actually showed that Arctic tundra soils store 184 Gg of Hg while boreal soils store additional 224 Gg. The authors therefore reported a pool of 408 Gg of Hg for northern tundra and boreal soils.…”