A revised pan-Arctic permafrost soil Hg pool based on Western Siberian peat Hg and carbon observations

Abstract

Abstract. Natural and anthropogenic mercury (Hg) emissions are sequestered in terrestrial soils over short, annual to long, millennial timescales before Hg mobilization and run-off impact wetland and coastal ocean ecosystems. Recent studies have used Hg-to-carbon (C) ratios (RHgC's) measured in Alaskan permafrost mineral and peat soils together with a northern circumpolar permafrost soil carbon inventory to estimate that these soils contain large amounts of Hg (between 184 and 755 Gg) in the upper 1 m. However, measurements of RHgC on Siberian permafrost peatlands are largely missing, leaving the size of the estimated northern soil Hg budget and its fate under Arctic warming scenarios uncertain. Here we present Hg and carbon data for six peat cores down to mineral horizons at 1.5–4 m depth, across a 1700 km latitudinal (56 to 67∘ N) permafrost gradient in the Western Siberian Lowland (WSL). Mercury concentrations increase from south to north in all soil horizons, reflecting a higher stability of sequestered Hg with respect to re-emission. The RHgC in the WSL peat horizons decreases with depth, from 0.38 Gg Pg−1 in the active layer to 0.23 Gg Pg−1 in continuously frozen peat of the WSL. We estimate the Hg pool (0–1 m) in the permafrost-affected part of the WSL peatlands to be 9.3±2.7 Gg. We review and estimate pan-Arctic organic and mineral soil RHgC to be 0.19 and 0.63 Gg Pg−1, respectively, and use a soil carbon budget to revise the pan-Arctic permafrost soil Hg pool to be 72 Gg (39–91 Gg; interquartile range, IQR) in the upper 30 cm, 240 Gg (110–336 Gg) in the upper 1 m, and 597 Gg (384–750 Gg) in the upper 3 m. Using the same RHgC approach, we revise the upper 30 cm of the global soil Hg pool to contain 1086 Gg of Hg (852–1265 Gg, IQR), of which 7 % (72 Gg) resides in northern permafrost soils. Additional soil and river studies in eastern and northern Siberia are needed to lower the uncertainty on these estimates and assess the timing of Hg release to the atmosphere and rivers.

Permafrost Thaw Increases Methylmercury Formation in Subarctic Fennoscandia

“…The range of R HgTC reported across systems and soil types (0.13 ± 0.12 μg HgT g –1 C for Western Siberian Lowland peatlands to 1.6 ± 0.9 μg HgT g –1 C for Alaskan tundra soils) underscores the need to incorporate the empirical relationship between HgT and % SOC for minerogenic and organic soils separately in order to derive R HgTC values that better represent the heterogeneity of Arctic and subarctic permafrost regions. With a median R HgTC of 0.09 ± 0.07 μg HgT g –1 C across investigated core classes and sites (14 cores, n = 178), our observations support the revised stock estimate of Lim et al 4 The lack of statistical differences between the five sites (one-way ANOVA, p > 0.05, Table S3 ), and between core classes ( p > 0.05 for both one and two-way ANOVA, Tables S5 and S6 ), suggests the calculated R HgTC to be representative for Fennoscandian permafrost peatlands and of high value for further stock estimates of Hg in the circumpolar north. 2 − 4 …”

“…Olson et al, 3 for instance, derived median R HgTC values ranging from 0.12 μg HgT g –1 C in organic soil (0–30 cm depth) to 0.62 μg HgT g –1 C in mineral soil (30–100 cm depth) by combining their data with HgT and SOC data from more than 30 published studies conducted in Arctic tundra and boreal regions. (Note that Olson et al 3 report the median organic soil R HgTC to be 0.27 μg HgT g –1 C, a value Lim et al 4 have identified as a typo; the corrected value of 0.12 μg HgT g –1 C is thus presented here). Of greater relevance to this study, due to the permafrost peatland nature of the study location, is the median R HgTC of 0.13 ± 0.12 μg HgT g –1 C for Western Siberian Lowland peat bogs measured recently by Lim et al 4 In their Western Siberian peat profiles, R HgTC values increased 5- to 10-fold from the organic (SOC >20%) to the mineral (SOC <20%) soil.…”

“…Although % SOC explained little of the HgT variability in the organic soil, R HgTC nonetheless warrants discussion, as this ratio has recently been used to estimate the soil HgT pool of the Arctic and subarctic permafrost regions. 2 − 4 In the first calculated estimate, Schuster et al 2 multiplied a median R HgTC of 1.6 ± 0.9 μg HgT g –1 C, derived from measurements of Alaskan tundra soils, with estimates of C storage in the Arctic. 1 , 40 Since then, additional work has demonstrated the need to account for regional differences and soil type in the R HgTC used for estimates of pan-Arctic Hg stocks.…”

“…Finally, there are likely also large regional differences in natural Hg sources. Lim et al 4 point out that HgT concentrations in the area Schuster et al 2 and Olson et al 3 sampled, along the Dalton Highway in Alaska, tend to be unusually high. This may be due to particularly high geogenic contributions of HgT from weathering bedrock or, perhaps, to a greater loss of C from mineral than organic soils in that region due to mineralization of C.…”

“…Schuster et al 2 have suggested that 863 ± 501 and 793 ± 461 Gg of HgT may be stored in the active layer and in the perennially frozen substrate, respectively, such that the top 3 m of northern permafrost soils contain 1656 ± 962 Gg HgT in total. Lim et al 4 have revised this estimate to 408 Gg (range of 319–584 Gg) 3 of Hg stored in the top 1 m, with 557 Gg (IQR: 371–699 Gg) of Hg stored in the top 3 m of northern permafrost soils. The range of R HgTC reported across systems and soil types (0.13 ± 0.12 μg HgT g –1 C for Western Siberian Lowland peatlands to 1.6 ± 0.9 μg HgT g –1 C for Alaskan tundra soils) underscores the need to incorporate the empirical relationship between HgT and % SOC for minerogenic and organic soils separately in order to derive R HgTC values that better represent the heterogeneity of Arctic and subarctic permafrost regions.…”