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Permafrost stores an Amount of Mercury that is Globally Significant

Permafrost is thawing in northern regions as the climate changes to warmer conditions with major implications for the global mercury (Hg) cycle. Schuster et al. estimated, based on in situ measurements of the sediment total mercury (STHg), soil carbon (SOC), and mercury to carbon ration (R_HgC)­ mercury in regions of permafrost combined with maps of soil carbon. Schuster et al. measured a median (STHg) of 43 ± 30 ng Hg/g of soil and a median R_HgC of 1.6 ± 0.9 μg Hg/g organic carbon which is consistent with published results of STHg for tundra soils and 11,000 measurements from 4,926 temperate, nonpermafrost sites in North America and Eurasia. The permafrost regions of the Northern Hemisphere were estimated by Schuster et al. to contain 1,656 ± 962 Gg Hg, 793 ± 461 Gg Hg of which is frozen in permafrost. Almost twice as much Hg is stored in permafrost soils as all other soils, the ocean and the atmosphere combined, and as the permafrost thaws this Hg is vulnerable to release over the next century. The amount of mercury in permafrost soils has been greatly underestimated in existing estimates, which indicates there is a need to reevaluate the role in the global mercury cycle of the Arctic regions.

Put simply it is estimated by researchers that the amount of natural mercury stored in permafrost in the Northern Hemisphere is twice as much mercury as the rest of all soils, the oceans and the atmosphere combined.

Sedimentation has buried mercury (Hg) that is bound to organic material over thousands of years which has then been frozen in the permafrost (Obrist et al., 2017). Permafrost is soil that has remained at or below 0^oC for at least 2 consecutive years. The surface soil layer overlying the permafrost is the active layer that thaws in summer and refreezes every winter. Mercury bonds with the organic matter in the active layer as it is deposited from the atmosphere. When the organic matter is then consumed by microbial decay it releases the mercury (Smith-Downey et al., 2010). Simultaneously, sedimentation slowly increases the depth of the soil such that organic matter at the base of the active layer becomes frozen into permafrost. The organic matter is composed almost entirely of plant roots, and once it has frozen microbial decay practically stops, and the mercury is then locked into the permafrost. The permafrost has, however, begun thawing under the changing climatic conditions (Hinzman et al., 2005; Romanovsky et al., 2008; Smith et al., 2010). Once the permafrost and the organic matter that is associated with it thaws, microbial decay will resume and release mercury to the environment, which will them potentially impact the mercury balance in the Arctic, aquatic resources, as well as human health (Dunlap et al., 2007; Jonsson et al., 2017; Obrist et al., 2017; USGS Fact Sheet,  https://www2.usgs.gov/themes/factsheet/146-00/).  Schuster et al. took the novel approach of making the first ever estimate of the storage of mercury in the permafrost soils of the Northern Hemisphere using empirical relationships that were based on in situ measurements of sediment total mercury (STHg) combined with maps that had been published of soil organic carbon (Hugelius, Tamocai et al., 2013; Hugelius, Bockheim et al., 2103).

There are several atmospheric Hg sources that are unique to the high latitudes of the Northern Hemisphere which have significant spatial and temporal variability to explain STHg variability within and between cores (Fitzgerald & Lamborg, 2003). Mercury is released to the atmosphere by fires in the northern boreal forests which lead to spatial variability in the deposition of mercury (Homann et al., 2015; Rothenberg et al., 2010; Turetsky et al., 2006). Microbial respiration rates are changed by spatial variation in temperature and moisture. During summer peaks in atmospheric mercury resulting from atmospheric mixing with ozone enhance the deposition of mercury (Banic et al., 2003; Sonke & Heimbürger, 2012). Following the polar sunrise in springtime, atmospheric depletion of mercury to high latitudes may elevate the deposition of mercury (Berg et al., 2008; Fitzgerald et al., 2005; Lindberg et al., 2002). According to Schuster et al. they see no evidence of terrestrial geologic mercury sources within Alaska (Eberl, 2004; Williams, 1962) though there are geologic deposits of mercury in southeast Alaska (Gray et al., 2000). Mercury is released into the atmosphere by volcanic eruptions, which leads to mercury deposition being variable (Pirrone et al., 2010; Pyle & Mather, 2003; Schuster et al., 2002), though according to Schuster et al. they didn’t see much evidence of it in deposition of mercury from volcanic ash. The 13 sites that were cored were not the only sites where these depositional processes occur; rather it is inclusive to the high latitudes of the Northern Hemisphere.

The STHg measurements of Schuster et al. appear to be consistent with similar measurements from permafrost regions, in spite of these processes leading to depositional environments that are highly variable. STHg values and vertical profiles and deposits of peat in Tomsk Oblast, west Siberia (Lyapina et al., 2009) were found to be similar to those of Schuster et al. STHg of 40 (Rydberg et al., 2010) ng of mercury in soil below 25 cm depth, which is close to the median value of 43 ng of mercury of soil mercury found by Schuster et al.. In soils of the active layer along an approximately 970 kl north-south transect of Alaska ranged from 100 ng of mercury per gram of soil in the O horizon to 50 ng mercury per gram of soil in the A horizon (Wang et al. 2010). At a subantarctic site in Tiera del Fuego (Peña & Rodriguez et al., 2014), STHg ranged between 12 and 375 ng mercury per gram of soil, and R_HgC varied from 1 to 11.3 μg mercury per gram of soil.

The STHg measurements appear, moreover, to be consistent with data from nonpermafrost soils that have been published. Of the 11,000 data that have been published most fall in the temperate midlatitudes, with 2,088 being from boreal forests, and only 67 points in permafrost regions. It was shown in this study that a curve fit of the median STHg as a function of SOC, with the 90% envelope being defined as the 5^th to 95^th percentiles. When the data from this study are superimposed it indicates that 90% of the STHg measurements from this study fall within the 90% envelope of the published data, with a slight shift towards higher STHg.

STHg appears to shift from a regime that is receptor-limited to one that is flux-limited. Mercury enters plants through the roots or by dry deposition from the atmosphere onto the leaves (Obrist et al., 2017; Windham-Myers et al., 2009), where it attaches to appropriate receptors in organic molecules which would normally be attachment sites for nutrients such as iron or magnesium. The number of receptor sites limits the mercury that can be retained for mineral soils that have SOC less than 10%, and with SOC (R² = 0.98) the STHg increases strongly. Receptor sites appear unlimited for organic soils with SOC greater than 10%, and the flux of mercury from the atmosphere limits the STHg. The data become noisier and SOC appears almost independent of SOC, as the atmospheric deposition if highly variable over space and time. Schuster et al. suggest the 2 regimes might explain why some sites display correlations between STHg and SOC that are statistically significant, like data that has been published previously (Bargagli et al., 2007; Erikson, 2014), whereas others do not. Sites with low SOC in the regime that is that is receptor-limited tended to have correlations that are statistically significant while sites with organic soils in the flux-limited regime did not.

Schuster et al. estimated that in permafrost regions soils contain an estimated 1,656 ± 962 Gg of mercury, half or 793 ± 462 Gg of mercury is frozen in permafrost. They estimated the total mass of mercury in each layer by multiplying each individual pixel of by the grid cell area and summing across the permafrost domain. The average of 9 estimates of global soil mercury that been published previously is 454 ± 321, ranging from 325 to 1,000 Gg of mercury. These estimates, however, generally limit soil depth of 30 cm, which indicates that the 0-30 cm of soil mercury of Schuster et al. for permafrost regions (347 ± 196 Gg of mercury rivals the global soil estimates in previous estimates. The known link between microbial decay and the release of mercury are leveraged by these studies, often relying on biogeochemical models that tend to underestimate soil mercury in permafrost regions. In order to improve mercury estimates these models should account for large drops in microbial activity under freezing conditions, and sediment processes that bury and freeze and freeze organic matter into permafrost. The results of this study indicate that the largest reservoir of mercury on Earth is represented by the active layer alone. There is almost twice as much mercury in the active layer and the permafrost combined as all other soils, the atmosphere and the ocean combined. Mercury in permafrost soils represents an environmental risk as permafrost continues to thaw in the future. The mercury locked in permafrost is effectively stable on human rime scales as the turnover time associated with microbial decay of frozen organic matter is about 14,000 years. It is indicated by projections that there will be a 30-99% reduction, however, of near surface permafrost by 2100, and once it has thawed, the turnover for microbial decay drops to about 70 years (Koven et al., 2013; Schaefer et al., 2011). This makes the reservoir of mercury in permafrost soils vulnerable to release over the next century, with unknown environmental consequences.

In this study Schuster et al. measured a median STHg 43 ± 30 ng of Hg in the soil and median ­R_HgC of 1.6 ± 0.9 μg Hg per g of carbon based on 588 samples from 13 soil permafrost cores from the interior and the North Slope of Alaska. It appears these values are consistent with results of Hg concentrations for tundra soils and 11,000 nonpermafrost soil measurements from 4,296 different sites in North America and Eurasia that have been published.

1. Schuster, P. F., et al. "Permafrost Stores a Globally Significant Amount of Mercury." Geophysical Research Letters: n/a-n/a.