Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry Joshua Landis, Daniel Obrist, Jun Zhou, Carl Renshaw, William McDowell, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3937465/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 25 Jun, 2024 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant accumulated through a history of anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg deposition in soils remains fundamentally uncertain, however, particularly to what degree Hg is quantitatively retained versus re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we introduce a new bottom-up soil mass balance based on fallout radionuclide (FRN) chronometry that allows direct quantification of historical Hg soil accumulation rates and comparison with measured contemporary atmospheric deposition. We show that soils spanning Arctic, boreal, temperate, and tropical ecosystems are strong and long-term sinks for atmospheric Hg, and that the soil sink strength decreases with latitude. Peak deposition reconstructed for years 1950-2000 strongly exceeds contemporary deposition fluxes by factors of approximately two. In the northeastern USA, trends in soil-derived Hg accumulation rates agree in timing and magnitude with records derived from regional lake sediments and atmospheric measurements. We show that typical soils are quantitatively efficient at retaining atmospheric Hg deposition, with exception of a subset of soils (about 20%, all temperate and boreal coniferous), where approximately 10% of Hg deposition is unaccounted for, suggesting that up to 2% of soil Hg may be lost by legacy emission of GEM back to the atmosphere when scaled across the landscape. The observation that most soil Hg is effectively sequestered long-term calls into question global model and mass balance studies that assume strong and continued re-cycling of legacy Hg pollution in the environment that prolongs the impacts of past Hg emissions. Availability of FRN chronometry to reconstruct soil Hg accumulation rates poses a powerful new tool to quantify Hg deposition and trends across much larger spatial scales than previously possible, and should advance the understanding of Hg deposition, accumulation, and fate in the context of changing global environment. Earth and environmental sciences/Biogeochemistry/Element cycles Earth and environmental sciences/Environmental sciences/Environmental chemistry/Atmospheric chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Synopsis The fallout radionuclide (FRN) chronometry technique newly employed in soils shows Hg accumulation rates that are consistent in time and magnitude with other deposition records, thus providing a powerful new tool to quantify historical Hg deposition fluxes across much larger spatial scales than previously possible. 1. Introduction Mercury (Hg) is a neurotoxic heavy metal, emitted to the atmosphere by both anthropogenic activities and natural sources and dispersed globally through atmospheric transport 1 . Forest ecosystems are the largest global sink of atmospheric Hg due to foliar uptake of gaseous elemental mercury (GEM) from the atmosphere 2 . Mercury is subsequently cycled to underlying soils by litterfall, canopy throughfall, and additional deposition of GEM directly to soils 2 3 4 5 6 . After centuries of anthropogenic Hg emissions, soils represent the largest global reservoir of anthropogenic Hg and pose an ongoing risk of exposure to terrestrial and aquatic food webs through uptake, leaching, and erosion to aquatic ecosystems 7 8 4 9 10 11 . In a biogeochemical twist, soil Hg is also partially re-emitted back to the atmosphere. Such soil re-emission of GEM counteracts the migration of Hg from surface to deep mineral soils through pedogenic processes, and thus prevents Hg from entering long-term sequestration 12 13 14 8 15 16 . The scale of terrestrial GEM re-emission remains highly uncertain, however, with global estimates spanning 1000 to 3000 Mg y − 1 , or roughly 30 to 100% of gross terrestrial Hg deposition. This uncertainty raises fundamental questions about the efficacy of soils in sequestering legacy and contemporary Hg, and as a result about the fate of Hg in a context of a changing global environment 7 1 17 6 18 . Contemporary measurements of Hg fluxes at the soil surface reveal complex bi-directional exchanges between soils and the atmosphere, with evidence for both net deposition and net emission to soils 19 14 3 20 16 . Further complicating Hg dynamics, ecosystem components can play opposing roles in Hg cycling; for example, foliage acts as a strong and unambiguous net sink 2 4 , but forest floors (leaf litter or soil Oi horizon) may act as either source or sink 21 20 22 . Within the soil profile, high pore-gas GEM concentrations in some surface soils point to leaf litter and soil humus as net emitters of GEM during decomposition of organic matter (termed legacy emission if derived from past pollution), while elsewhere GEM concentrations are lower than atmospheric levels indicating deeper soils to be stable Hg sinks 23 22 24 25 16 . The balance of the two pathways has profound influences on the global Hg cycle. Processes that favor Hg retention in soils effectively remove Hg pollution from continued circulation in the environment, while legacy emissions extend exposures to pollution and globally delay recovery from Hg pollution even as primary anthropogenic Hg emissions continue to decrease 26 driven by national and global legislative actions 26 . Recent advancements in soil chronometry with atmospheric fallout radionuclides (FRNs) provide a new and exciting opportunity to directly measure Hg soil accumulation rates using a 'bottom up' mass balance approach, thereby providing a critical missing view of terrestrial Hg biogeochemistry. Already the FRN 210 Pb (half-life 22.3 years) is a vital tool for reconstructing historical Hg deposition using both lake sediment and peat environmental archive 27 28 29 . To extend this approach to terrestrial ecosystems, we employed and validated new FRN age models that accurately date foliar and soil organic matter (SOM). Three independent radionuclide age models, the Linked Radionuclide aCcumulation (LRC) model based on 7 Be: 210 Pb, the nuclear bomb-pulse 241 Am, and the novel 228 Th: 228 Ra chronometer, yield concordant SOM ages spanning subannual to centennial timescales 30 31 32 . Concordance of these models is predicated on the strong affinity of these five particle-reactive metals for SOM through the formation of stable organometallic complexes. The migration rate of 210 Pb in soils is identical to that of atmospheric 14 C 33 , confirming that sorption of the tracer metals to SOM persists from uptake in vegetation through incorporation in soil, subsequent SOM decomposition, and hydrologic transport in association with natural organic matter (NOM) 33 34 35 10 36 37 38 . The FRN chronometers accurately measure soil organometallic dynamics not possible with more conventional Δ 14 C chronometry due to uncertain lag times between CO 2 assimilation and foliar production in the forest carbon cycle 39 40 41 42 43 . In a similar way, the chronometer metals provide a powerful comparator for constraining Hg dynamics in soils, and a novel tool for assessing the extent to which Hg behaves as a conservative particle-reactive metal entrained in the pedogenic cycle with SOM versus a fraction that may be re-emitted or translocated through gaseous pathways as GEM (Landis et al., in submission). Here we employ FRN age models to measure Hg accumulation rates in a range of soils spanning Arctic, boreal, temperate, and tropical ecosystems. We then directly compare rates of soil Hg accumulation over annual to centennial timescales with measured whole-ecosystem atmospheric depositional fluxes. This novel approach thus provides a new basis for understanding key processes that regulate Hg accumulation in soils; for constraining the fate of past soil Hg accumulation; and for assessing the susceptibility of soil Hg to legacy emission and redistribution in response to environmental and climate perturbation. 2. Results and Discussion 2.1 Soil ages and Hg accumulation rates Fallout radionuclide age estimates provide a critical and objective basis for comparing Hg accumulation in foliage and organic soil O-horizons across contrasting ecosystems (Fig. 1 ). We here use the definition of soil Oi horizon as senesced leaves on the forest floor; Oe horizon as partially decomposed leaf fragments and fibers; and Oa horizon as humified organic matter lacking identifiable leaf fragments. Within each ecosystem, ages increase from foliage, Oi, Oe, and Oa horizons to reflect clear differentiation of horizons as they were sampled in the field [p < 0.05]. Ages pooled across ecosystems increase significantly from 1.5 years in foliage, to 3.3 years in Oi horizons, 7.5 years in Oe horizons, and 29 years Oa horizons 42 32 . Ages of Arctic foliage and soil horizons are significantly older than corresponding temperate, boreal, or tropical horizons which is attributed to slower SOM decomposition at high latitudes [p < 0.05]. Similarly, tropical soil horizons are significantly younger than corresponding temperate ones [p < 0.05]. There were no significant differences between temperate ( n = 23 sites) and boreal forests ( n = 4 sites), and these are hereafter analyzed together. Total Hg concentrations pooled across ecosystems increase significantly from 41 ng g − 1 in foliage to 74 ng g − 1 in Oi, 140 ng g − 1 in Oe, and 192 ng g − 1 in Oa horizons (Fig. 1 b). There were no significant differences in foliar concentrations among ecosystems [p < 0.05]. Only Arctic Oi and temperate Oa horizons Hg concentrations are significantly higher than contrasting ecosystems [p < 0.05], but concentration data are poor predictors of Hg accumulation rates. Soil Hg concentrations were converted to total inventories by multiplication with total soil mass recovered from the defined area of quantitative soil pits (per cm thickness, in g m − 2 cm − 1 ) (Fig. 1 c). Oa horizons store the largest soil Hg inventories in each ecosystem, increasing significantly from 140 µg m − 2 cm − 1 in tropical soils, 440 µg m − 2 cm − 1 in temperate deciduous soils, 470 µg m − 2 cm − 1 in temperate coniferous soils, and 460 µg m − 2 cm − 1 in Arctic soils [p < 0.05]. Previously, high Hg concentrations and inventories in Oa horizons have been attributed to their older ages and longer accumulation times of this horizon 52 , but these studies lacked the ability to comprehensively date soils by layer 52 . Total atmospheric Hg stocks in full soil profiles (including mineral soils to depths of 10–50 cm corresponding in each profile to calendar years < 1900) increase significantly across ecosystems from 2.4 ± 0.4 mg m − 2 (mean ± SD, n = 4), to 6.9 ± 3.4 mg m − 2 ( n = 10), and 11.2 ± 2.4 mg m − 2 ( n = 3), respectively, in Arctic, temperate, and tropical soils [p < 0.05; Table SI 1]. Rates of foliar and soil Hg accumulation are converted from Hg inventories using LRC soil ages. Foliar and soil Hg accumulation rates increase substantially and statistically significantly across ecosystems in the following order: Arctic < temperate < tropical [p < 0.05; Fig. 1 d]. Temperate foliar Hg accumulation estimates of 12 ± 3 µg m − 2 y − 1 (mean ± SE, n = 4) are in good agreement with adjacent litterfall data for three forest sites (average of Harvard, Howland, and Downer Forests = 11.8 ± 0.8 µg m − 2 y − 1 ), suggesting that the LRC approach accurately reconstructs foliar Hg deposition rates. Arctic foliar flux estimates average 7.5 ± 2.9 µg m − 2 y − 1 ( n = 4), which is comparable with prior estimates and consistent with lower foliar-derived Hg deposition in the high latitudes 53 54 55 . Tropical foliar fluxes average 28 ± 3 µg m − 2 y − 1 when considering only the dominant canopy but increase to 49 ± 9 µg m − 2 y − 1 when significant understory vegetation is also included ( n = 3). Foliar Hg deposition fluxes for Caribbean tropics are not well constrained, but our canopy average is comparable to one prior litterfall estimate of 29 µg m − 2 y − 1 for the surrounding Luquillo Experimental Forest 56 . The substantial increase in tropical foliar fluxes when understory vegetation is included emphasizes the importance of tropical ecosystem complexity to rates of Hg deposition 57 . High foliar Hg fluxes in tropical forests are consistent with high atmospheric deposition across tropical forests, supporting the notion of tropical forests as hotspots of Hg deposition driven by vegetative uptake and high net primary productivity 56 4 . Similarly, tropical Oe and Oa horizon accumulation rates are by far highest among all measured soils, in further support that high Hg deposition is a key characteristic of low-latitude forests [p < 0.05]. 2.2. Hg accumulation and atmospheric flux in temperate soils Another salient feature of soil Hg accumulation evident in Fig. 1 is a pronounced and significant increase in accumulation rates with soil depth [p < 0.05]. To contextualize this pattern, we compare soil Hg accumulation rates to independent atmospheric Hg flux estimates for each ecosystem type, beginning with temperate forests where total ecosystem Hg deposition data are available for the Northeastern United States (Fig. 2 a); two sites, Harvard Forest (site HaF, deciduous) and Howland Forest (site HoF, coniferous) 3 21 include micrometeorological GEM flux quantification, and a third Downer Forest (site DoF, mixed). The primary pathways of atmospheric Hg deposition to soil systems include litterfall (LF) originating largely from GEM uptake, deposition by rain and canopy wash-off originating largely from oxidized Hg II , and non-foliar deposition of GEM directly to both forest floor (O horizon soil) as well as to long-lived above-ground tissues such as mosses, lichen, and tree bark 5 . The non-foliar GEM component is calculated as above-canopy GEM flux minus the foliar component, that latter approximated as LF flux. Together these sum to total ecosystem flux (EF). Deposition studies at the three temperate sites yield (mean ± SD): LF = 11.8 ± 0.6, TF = 7.8 ± 0.9, non-foliar GEM = 7.5 ± 7.5, and EF = 28.2 ± 6.0, µg m − 2 y − 1 . Soil Hg accumulation rates for temperate forests are compared with measured LF, TF and EF atmospheric fluxes in calendar years 2019–2022 (i.e., measured 1–5 years prior to soil collection) (Fig. 2 ). Both deciduous and coniferous soils follow a pattern of strongly increasing Hg accumulation rates with depth (Fig. 2 a; we call these typical pattern soils "accumulating"; see discussion below). Hg accumulation in foliage with ages < 1 to 3 years is consistent with local litterfall flux (LF) measurement as noted above. Accumulation rates in Oi horizon [ages 2.5 ± 3.3 years, mean ± SE] are consistently higher than LF fluxes and show additional Hg accumulation in the forest floor [18.0 ± 1.8 µg m − 2 y − 1 ]. Both TF and additional non-foliar GEM deposition can increase Hg deposition to the forest floor following litterfall 58 59 60 21 . Oi horizon accumulation rates are considerably lower than total EF measured by flux-gradient at both the HaF and HoF sites, however, with a cumulative shortfall averaging 45 µg m − 2 , or 36% of total deposition. This "missing Hg" could indicate an overestimation of EF at these sites, re-emission of Hg as GEM back to atmosphere, or more likely as discussed below, translocation of Hg from forest floor deeper into mineral soils. In contrast to a missing Hg sink in Oi horizons, Hg accumulation rates in Oe soil horizons [mean age 7.4 ± 2.8 years] converge with contemporary EF [26.4 ± 2.2 µg m − 2 y − 1 ]. Hg accumulation rates in Oa horizons are higher still [45.9 ± 3.3 years], nearly twice the contemporary ecosystem atmospheric Hg flux in soils aged 20–30 years. Mercury accumulation rates continue to increase in underlying mineral soil and peak at depths corresponding to calendar years 1975–2000 (Fig. 3 ). At these depths about half of our temperate soils (6 of 11) record reliable histories of Hg atmospheric deposition ( Supporting Information ). Here, accumulation rates average 67 ± 9 µg m − 2 y − 1 which is approximately 2.4 times higher than measured contemporary atmospheric EF. Higher Hg accumulation in Oa and mineral soil horizons versus contemporary atmospheric fluxes suggests that some Hg may be preferentially translocated from overlying organic soil horizons. Most importantly, however, our measurements support evidence that historical Hg forest deposition fluxes in the northeastern United States have much been higher than at present 61 27 62 . The timing of peak soil Hg accumulation (1950–2000) is notably consistent with regional lake sediment archives, where deposition of atmospheric Hg peaked during 1970–1990 and has subsequently declined to present by about a factor of two 8 63 62 64 . If peak soil accumulation 67 ± 9 µg m − 2 y − 1 for years 1975–2000 is representative of the total Hg ecosystem flux in that era, it would be consistent with a similar decrease by a factor of 2.4 to contemporary terrestrial deposition for this region. Hence, soil Hg reconstructions agree well with lake sediment records. Both archives are also consistent with measured declines in atmospheric GEM concentrations in North America that have averaged 1–2% per year since 1990 26 65 . The consistency among soil, lake and atmospheric records provides strong evidence that, despite the complexity of pedogenic processes and Hg biogeochemical dynamics, soil FRN chronometry provides new and direct constraints on historical terrestrial Hg fluxes. This opens unprecedented opportunities to assess historic Hg deposition across much larger spatial scales given the ubiquity of soils. This line of reasoning further concludes that typical northeastern US soils are quantitatively efficient at retaining past atmospheric deposition of Hg from combined depositional sources. Our soil mass balance estimates hence indicate that substantial legacy emission of Hg from soils back to atmosphere is unlikely across most soils (see discussion of low-accumulating soils below in Section 2.5 ), and instead suggest that typical temperate and boreal forest soils quantitatively retain atmospheric Hg ecosystem deposition over annual to decadal timescales. 2.3. Latitudinal trends in soil Hg accumulation across ecosystems Reconstructed soil Hg accumulation rates across ecosystems demonstrate a strong latitudinal gradient in atmospheric Hg deposition, on the order of 10 µg m − 2 y − 1 for Arctic soils, 50 µg m − 2 y − 1 for temperate sites, and 150 µg m − 2 y − 1 for tropical Luquillo (Fig. 4 a to 4 c). Despite wide-ranging fluxes, however, similar soil Hg accumulation patterns occur across ecosystems, suggesting that the processes impacting soil Hg sequestration and overall mass balance are global. Specifically, like their temperate counterparts, both tropical and Arctic soils show Hg accumulation rates that fall short of respective contemporary deposition estimates in surface soil horizons, and that eclipse ecosystem Hg fluxes in deeper soils of multi-decadal ages (Fig. 4 d to 4 f). The convergence between soil accumulation and atmospheric deposition rates occurs at corresponding soil ages of 5–10 years across highly variable soil orders spanning Gellisol, Inceptisol, Spodosol, and Ultisol. The magnitude of this effect also varies in accordance with ecosystem properties; for example, in the Luquillo tropical forests, soil Oi horizons accumulate Hg at just about 20% of estimated EF, versus 70% in temperate forests. We interpret this difference as a decreasing ability of surface organic soils to sequester Hg under increasing mean annual temperatures and SOM mineralization rates. In Arctic samples the Oi horizon Hg fluxes average 120% of contemporary EF flux, which is likely within reasonable uncertainty of estimated atmospheric deposition and indicates a strong retention of Hg by Arctic soils, as well 50 . We note, too, that the tundra Oi horizon is sufficiently old (up to 40 years) that it records higher rates of Hg deposition that predate successive emissions reductions implemented by the Clean Air Act (1970), Clean Air Act Amendments (1990), and Minimata Convention (2017). Centennial histories of Hg accumulation in both Arctic and tropical soils mirror those of temperate forests and are similarly consistent with global anthropogenic emissions, rising steeply through 20th century before declining around 1990–2000 in line with implementation of global emissions controls (Fig. 4 d to Fig. 4 f). The precise magnitude and timing of peak soil accumulations may be influenced by variable SOM decomposition and colloid migration rates with soil depths among diverse profiles, but within each ecosystem the consistency of Hg accumulation patterns among contrasting soils suggests that our reconstructed peak Hg accumulations accurately reflect the dynamics of atmospheric Hg deposition and accumulation across latitudes. For example, we observe good correspondence in temporal trends among highly different tropical soils, which include hydric Inceptisols (site LeF03, e.g., 80% organic matter by weight) to Ultisols (site LeF01, e.g., 50% Al- and Fe-oxides by weight). Peak deposition in tropical soils appears later (ca. 2000) than in temperate latitudes, a shift that may be influenced by the unique disturbance history of Puerto Rico. Here, hurricane Maria (2017) caused massive defoliation of the Luquillo forest canopy that delivered 60% of above-ground forest biomass to the forest floor 67 , and along with the large pool of Hg contained therein. In Arctic soils, the period of peak Hg deposition is less clear due to large uncertainties in soil Hg reconstructions for years prior to 1950, which is a result of low atmospheric deposition of both FRNs and Hg, combined with slow SOM decomposition and colloid migration rates. 3.4. Forest Hg dynamics and mechanisms of soil sequestration All surface soil horizons (forest floor Oi horizons) we have dated indicate substantial shortfall of Hg relative to atmospheric fluxes, at rates of 10 µg m − 2 y − 1 for temperate systems or even approaching 100 µg m − 2 y − 1 for tropical forests of Puerto Rico (Fig. 4 ). A large body of work supports the proposition that Hg in the forest floor is reduced to GEM, both during SOM decomposition and following wet deposition of Hg II , and that this potentially yields a highly mobile GEM pool that could be subject to re-emission 19 6 20 16 . In contrast to the notion of significant net GEM emission, however, soil Hg accumulation rates increase in deeper soils far exceeding contemporary depositional fluxes, and as discussed above, in Oa to upper mineral horizons are quantitively in agreement with historical changes from other independent deposition records. Mass balance thus requires that the balance of any Hg lost from surface soils must be redistributed in a net downward direction that augments long-term sequestration. We propose that percolation is likely key to facilitating mass transport of Hg from surface organic horizons, analogous to how FRNs reach soil depths of 5–10 cm or more during throughfall events (Supporting Fig. 1 ), and how throughfall is in enriched in total Hg by factors of 2 or more over incident precipitation during transit through the canopy 68 69 5 70 . To any extent that GEM may be re-emitted from surface horizons back to atmosphere, our mass balance would require that this GEM is recycled back to soils, rather than lost to the atmosphere. Although enigmatic, such internal GEM recycling could occur via throughfall or litterfall of fine and coarse debris and may occur over decadal timescales. Studies show that on the order of 16% of Hg in throughfall may represent GEM that is re-deposited within the canopy from soil re-emission of prior Hg deposition 21 . At the same time, stable isotope composition of GEM emitted from soils appears to be identical to that of atmospheric GEM and dissimilar from soil legacy Hg, which suggests that measured soil GEM re-emission reflects the transient turnover of atmospheric GEM rather than net emission from legacy soil reservoirs 71 20 . Direct re-emission of GEM from foliar surfaces may also be important for recycling wet-deposited Hg to the atmosphere before entering underlying soils 72 16 . In contrast to forest floor, underlying organic and mineral horizons are closed systems with respect to Hg. Pore gas GEM concentrations here are typically lower than atmospheric levels, suggesting that in deep soils any available GEM becomes firmly bound or oxidized to particle-reactive Hg II 24 22 16 . Mercury thus seems to be translocated from forest floor in gaseous form or deposited directly to mineral soil by percolation and thereafter continues its inexorable downward migration as organometallic colloid at migration rates quantified by the LRC age model and corroborated by bomb-pulse 14 C and 241 Am, and novel 228 Th: 228 Ra chronometry 31 32 . The retention of Hg in deep soils locks in its historical pattern of atmospheric deposition, like that of the 241 Am nuclear bomb-pulse 31 32 (Supporting Table 1). As such, the fate of Hg in soils is closely aligned with that of soil carbon insofar as long-term storage of both Hg and C in soils is contingent on their mobilization from organic to mineral soils and stabilization by complexation with secondary soil minerals including authigenic clays and Al,Fe-oxides 73 74 75 . 2.5. Mercury in low-accumulating soils and implications for coniferous ecosystems In contrast to typical Hg-accumulating soils, a subset of northeastern temperate and boreal soils (20% of sites) shows decreasing Hg accumulation rates with soil depth, which we call "low-accumulating" soils (Fig. 2 b). These soils are all coniferous, yet we stress that there is no generic difference between deciduous and coniferous soils at Hg typical accumulating sites, and that ecotype alone is therefore insufficient to explain these patterns. We highlight results from the Woody Adams conservation forest in Vermont (site WoA) where deciduous maple and oak soils display typical Hg accumulating behavior, yet adjacent pine and hemlock soils are low-accumulating. Although the pine and hemlock organic soils have total Hg loads that are higher per horizon than the hardwoods by factors of two to three, this is attributable largely to their older ages, slower decomposition rates, and greater accumulation of SOM. In terms of mass balance, however, the low-accumulating coniferous soils are ‘missing’ 250 µg m − 2 or about 10% of accumulated Hg over past 20 years relative to the deciduous sites. The nearby Downer Forest (DoF) coniferous pine soil, also low-accumulating and for which we have collected a full soil profile, shows a strong peak of Hg accumulation in the underlying mineral soil (Fig. 2 b), suggesting that podzolization (downward mobilization of iron and aluminum oxides along with dissolved organic carbon) could explain low-accumulating behavior in some coniferous forest floors due to preferential leaching of Hg to underlying mineral soil. The coniferous 302-2 and HoF01 soils similarly show strong and atypic Hg enrichment in mineral horizons which we attribute to lateral podzolization along horizontal flow paths (Fig. S2 ). More research is needed to confirm whether Hg in low-accumulating surface soils is lost to the atmosphere via re-emission, leached to mineral horizons, or exported from watersheds by runoff (Landis et al. in submission). The Hg shortfall from individual surface soils approaches 20 µg m − 2 y − 1 , which is comparable to typical contemporary atmospheric flux, and therefore would be large enough to be readily measured by atmospheric or watershed studies if such losses occurred at large scales. On the other hand, if the frequency of low-accumulating soils (20%) is representative of northeastern forests and such losses are limited to these particular soils, the Hg ecosystem loss based on our mass balance would equate to only about 2% of annual EF. This scale is compatible with re-emission of 0.9 µg m − 2 y − 1 measured by micrometeorology from the HoF coniferous Podsol forest floor 21 . At the watershed scale, isotope tracer experiments similarly suggest that about 1% of new deposition may be exported from upland forests in streamflow 76 10 , although export of cumulative legacy deposition may approach 10% of contemporary watershed input 66 77 78 . Weaker retention of Hg in coniferous forest organic soils and possible influence of podzolization on mineral horizons described here demands further study as it may potentially be linked with high riverine export in boreal rivers 79 and impact boreal and Arctic soil Hg retention under global warming, with implications for downstream food webs 79 80 . Typical uncertainties on soil Hg accumulation rates due to error in the LRC age models are estimated to be on the order of 10% for soil ages less than 60 years 31 32 , so the Hg mass losses in low-accumulating soils can be confidently attributed to physicochemical behavior of Hg. Given this, the considerable variation in Hg accumulation rates among northeastern soils apparent in Fig. 2 and Fig. 3 reflects spatial variation in both how and where Hg is deposited and sequestered across the terrestrial landscape. Variable accumulation of Hg is underscored by a factor of five higher coefficient of variation among soil Hg inventories in comparison to 210 Pb and 241 Am (100% versus 20%; Supporting Table 1) and is thus consistent with the potential for high patchiness of Hg accumulation, redistribution, and hotspots of Hg losses by either re-emission or runoff. 3. Conclusions We demonstrate the first direct estimates of soil Hg accumulation rates in upland soils using a ‘bottom up’ mass balance based on FRN chronometry, thereby establishing reference accumulation rates in Arctic, temperate, boreal, and tropical soils. The reconstructed rates in surface soils support the magnitude of deposition measurements available by micrometeorological studies, which together affirm that GEM-derived atmospheric deposition is dominant across forest ecosystems. Our soil mass balance measurements show that surface soil horizons under-accumulate Hg while deeper organic and mineral horizons over-accumulate Hg compared to modern atmospheric deposition estimates. Superimposed on recent declines in rates of Hg atmospheric deposition, we propose that this is facilitated by vertical translocation of Hg, possibly mediated by GEM dynamics, percolation, and/or colloid transport, from surface to deeper soil horizons. In deeper organic and mineral soils, our data provide evidence of a closed system for Hg wherein Hg is firmly sequestered and thereafter regulated by colloidal transport at migration rates that are accurately measured by FRN chronometry. This application of FRN chronometry to terrestrial Hg dynamics thus establishes an exciting and novel opportunity to expand conventional archives such as lake sediments, peat, or ice records to include soils as a largescale, widespread, and easily accessible medium to quantify Hg depositional processes and historical fluxes, 81 82 28 83 29 . A key distinction is that soil accumulation rates serve as accurate measures of terrestrial deposition where inputs from vegetation uptake of GEM dominate (> 70%) and result in terrestrial fluxes 3–4 times higher than measured in conventional archives. Flux reconstructions based on lake sediment records may underestimate total ecosystem Hg loads, for example, because they are biased to wet deposition, depend on watershed connectivity, and are convoluted by impacts of land use and watershed erosion that impact sedimentary Hg flux 84 . Reconstructed accumulation rates for contrasting ecosystems confirm enormous gradients in Hg deposition across global biomes previously reported in atmospheric measurements, ranging from 10 µg m − 2 y − 1 in the Arctic to ~ 30 µg m − 2 y − 1 in northeastern US forests and over 100 µg m − 2 y − 1 in tropical forests of Luquillo. Importantly, extraordinary rates of Hg accumulation observed in the "clean air" site of Puerto Rico (exceeding 300 µg m − 2 y − 1 at one location) demonstrate that low-latitude tropical forests may be inherently strong Hg sinks due to a combination of high net primary productivity, complex disturbance histories, and high rainfall totals with access to a global pool of oxidized Hg II in the upper troposphere through deep convection 56 8 . Our measurements demonstrate that very high Hg fluxes in the tropics are not dependent on nearby pollution sources 70 . Few direct measures of atmospheric Hg deposition are available in tropical forests, however, and given their likely critical role in global Hg cycling, high priority should be placed on improving our understanding of Hg depositional pathways and accumulation in these critical ecosystems 56 25 18 . Our data contradict reports of widespread soil Hg losses by GEM re-emission and indicate that if such losses occur, they might efficiently re-cycle within ecosystems and back to soils rather than being exported to the global atmosphere. This might further imply that conventional litterfall and throughfall measurements overestimate net Hg fluxes by including a recycled component. Soil reconstructions across larger scales have the potential to resolve this uncertainty in contemporary and historical Hg fluxes to terrestrial ecosystems, as well as to better understand the role of soil processes in Hg global mass balance. Atmospheric Hg concentrations are now slowly declining in North America, Europe, and East Asia due to emission controls, although emissions continue to increase in South America. Under declining Hg emission scenarios, the fate of legacy Hg stored in soil becomes increasingly important, in particular whether certain ecosystems such as boreal peatlands shift to become net sources that offset reductions in industrial emissions 15 14 16 . Our data strongly suggest that, with exceptions of surface litter layers and a subset of coniferous soils possibly linked to podzolization, most forest soil horizons across latitudinal gradients are strong net sinks and act as a closed system with respect to Hg. Additional work is needed to understand and couple Hg dynamics with the FRN tracer and chronometer 210 Pb in organometallic colloid formation, and processes and rates of podzolization, and how colloid transport may work to sequester Hg in mineral horizons, or alternatively to export Hg with DOC through hydrologically connected watershed elements to discharge in aquatic and marine ecosystems where it ultimately bioaccumulates in food webs to cause negative health outcomes for both wildlife and human population. 4. Methods We measured Hg accumulation rates in foliage and soils using high-precision gamma spectrometry and high-resolution quantitative soil pits (Table S1 ). Our experimental design includes (1) contrasting canopy tree species within sites, (2) contrasting sites with identical species, and (3) contrasting ecosystems across climate and elevational gradients. Selected sites represent soil orders that include Inceptisol, Spodosol, Gellisol, and Ultisol. We focus on temperate forests in northeastern US states (Vermont, VT; New Hampshire, NH, Massachusetts, MA; and Maine, ME) and boreal forest in Ontario, Canada. Sites in Vermont include multiple forest types with the same lithology, elevation, and climate, with forest stands dominated by maple ( Acer saccharum ) and beech ( Fagus grandifolia ); red oak ( Quercus rubra ); white pine ( Pinus strobus ); or eastern hemlock ( Tsuga canadensis ). Sites in Massachusetts ( Quercus rubra , Harvard Forest) and in Maine ( Tsuga canadensis and Pinus strobus , Howland Forest) have published records of total Hg ecosystem flux measured by micrometeorological flux-gradient methods in addition to throughfall and litterfall estimates 3 21 . The Downer Forest site in Vermont has new multi-year records of Hg litterfall and throughfall that are reported here. One boreal site in Ontario, Canada, was also sampled where canopy is dominated by black spruce ( Picea mariana ). Temperate and boreal sites were contrasted with tundra sites on the Seward Peninsula, Alaska (AK) dominated by moss ( Sphagnum ) 44 and Sisimuit, Greenland (GR) with shrub cover of black crowberry ( Empetrum nigrum ) over Sphagnum . Tropical sites include an elevational gradient (approximately 350-1,000 m a.s.l) in the Luquillo Experimental Forest (LEF), Puerto Rico, that spans subtropical wet, lower montane, and rain forest life zones 45 . The LEF sites include distinct forest communities of tabonuco ( Dacroydes excelsa - Manilkara bidentata ), palo colorado ( Cyrilla racemiflora - Micropholis garciniifolia) , and elfin woodland ( Tabebuia rigida - Eugenia borinquensis ), respectively 46 47 . Soils were collected from 30 x 30 cm or 50 x 50 cm quantitative pits following methods previously described 31 . For 16 short soil profiles consisting of uppermost organic soils (max. depth 5 cm), samples of thickness 0.5 to 1 cm were collected corresponding to Oi, Oe, and Oa organic horizons 32 . Eighteen additional long profiles in 1 to 5 cm increments to soil depths of 30–50 cm are also reported here. A total of 379 samples were analyzed and dated. Due to the large sample masses required for FRN analysis soils and foliage were oven-dried at 50°C rather than by lyophilization, which previously has shown non-detectable to minor losses during drying (max. 4% of total Hg) 48 . Foliage and litter for Hg analysis were homogenized using a stainless-steel Wiley Mini-Mill (Thomas Scientific), and soils were disaggregated by agate mortar and pulverized in a zirconia ball mill. Atmospheric Hg in soils was resolved from geogenic contributions using Al as a reference element 49 50 , with total atmospheric Hg calculated as the sum of [Hg i – Al i ×(Hg/Al) mineral ] for i layers with ages < 225 years, and the mineral component taken for soils older than reference year 1800 which is the practical limit for reliable FRN chronometry. This procedure provides a conservative estimate of anthropogenic Hg accumulation in soils, although we recognize that earlier emissions occurred and could be significant near regional sources. For measurements of Al and major/trace elements, samples were digested with a mixed acid approach using HNO 3 -HCl-HF-H 2 O 2 and microwave heating with hot plate evaporation in Teflon vessels, and 150 mg samples per 15 mL acid. Major/trace elements were measured by ICPOES (Spectro ARCOS) with reference materials NIST 1547 (peach leaves), NIST 2706 (New Jersey soil), NIST 2710 (Montana Soil II), USGS G3 (granite), CCRMP TILL 1 and 2 (glacial till) and STSD 1 and 2 (stream sediment). Recoveries for Al averaged 104 + 3%, 95 ± 2%, 104 ± 4%, and 97 ± 3%, 96 ± 3%, 99 ± 3%, 93 ± 2% and 98 ± 6% (mean ± SE), respectively. Canopy foliar Hg inventories were estimated by multiplication of leaf Hg concentration by leaf area index (LAI) and leaf mass area (LMA), or in one case by whole-tree harvest. Soil Hg inventories for each depth increment were estimated by multiplication of soil Hg concentrations by total recovered mass per sampling area of the quantitative pit (kg m − 2 ) rather than with an estimate of soil bulk density. Hg concentrations were measured by DMA (Milestone DMA-80) with reference materials NIST 2706 (New Jersey soil), and NIST 1547 (peach leaves), run each 10 samples. Recoveries averaged 103.4 ± 4.8% (mean ± SD, n = 98) using standards of approximately 5 ng total Hg, and 99.3 ± 8.6% with standards of approximately 1 ng total Hg ( n = 98), respectively. Radionuclides were measured by high-precision gamma spectroscopy (Mirion BeGE 3830 gamma detectors 51 . The FRN linked radionuclide accumulation (LRC) model based on the 7 Be: 210 Pb ratio was implemented as described by Landis et al. 31 . LRC soil ages for a subset of soils reported here have been confirmed by concordance with both 228 Th: 228 Ra and 241 Am chronometers with typical uncertainties < 10% for ages < 60 years 31 32 (Table S1 ). Statistical analyses were performed in JMP16 Pro, using analysis of variance (ANOVA) to compare Hg concentrations, inventories, and accumulation rates among contrasting soil horizons and across contrasting ecosystems. Declarations Acknowledgements. This project was funded by the Dept. Earth Sciences at Dartmouth College through the Fallout Radionuclide Analytics facility (FRNA). Visit us at sites.dartmouth.edu/frna/. Special thanks to Tatiana Barreto, Lee Hrenchuk, and Ken Sandilands for assistance at field sites. Samples from Seward Peninsula, AK were obtained with funding to MP (NSF OPP-ANS-2116471). Samples from Harvard and Howland Forests were obtained with funding to DO (NSF EAR-1848212). Contributions from WM and TB were supported by NSF EAR 2012403 and NSF DEB 1831592. Samples from Luquillo Experimental Forest were obtained by permission of US Forest Service, El Yunque Special Use permit #YNF3019. Samples from Downer State Forest were obtained by permission of Vermont Dept. Natural Resources, Special Use permit #22857. Author contributions. JDL conceived and designed the project. JDL and DO wrote the manuscript. JDL, VFT, JZ, JDV, and FM collected samples in the field. JDL and VFT performed analytical and laboratory procedures. 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B., Engstrom, D. R., Brigham, M. E., Henning, T. A. & Brezonik, P. L. Increasing Rates of Atmospheric Mercury Deposition in Midcontinental North America. Science (1979) 257 , 784–787 (1992). Schütze, M., Tserendorj, G., Pérez-Rodríguez, M., Rösch, M. & Biester, H. Prediction of holocene mercury accumulation trends by combining palynological and geochemical records of lake sediments (Black Forest, Germany). Geosciences (Switzerland) 8 , (2018). Additional Declarations There is NO Competing Interest. Supplementary Files HgFLHSupportingInformation020724.pdf Supporting Information abstractart.docx Cite Share Download PDF Status: Published Journal Publication published 25 Jun, 2024 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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(a) organic matter age (LRC age model), (b) soil Hg concentrations, (c) total Hg inventories, and (d) Hg accumulation rates versus horizon. Box plots indicate 25%, 50% and 75% quartile ranges. Asterisk (*) marks ecosystem comparisons that are significantly different across horizons [ANOVA, p\u0026lt;0.05]. Blue lines connect horizon mean values across ecosystems. Symbols are colored by horizon for foliage (green), Oi horizon (yellow), Oe horizon (orange), and Oa horizon (brown). Symbol shape corresponds to ecotype for Arctic (square), temperate/boreal coniferous (triangles), temperate deciduous (circle), and tropical (diamond).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/ae10c4c581c13b5b7b68c0d2.png"},{"id":50983155,"identity":"e3e5b0c7-96c4-464c-89b1-b6257a83194f","added_by":"auto","created_at":"2024-02-12 06:50:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":324121,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Hg soil accumulation rate versus soil age in temperate/boreal forest soils. Foliage in green, Oi horizon in yellow, Oe horizon in orange, Oa horizon in brown, E horizon in gray and A horizon in black; deciduous soils (circles), coniferous soils (triangles), soils mantled in moss (squares). (b) low-accumulating soils, all six are coniferous, one is boreal. *Contemporary atmospheric fluxes for years 2019 to 2022 are shown as indicated, including litterfall (LF), throughfall (TF), and ecosystem flux, which is the sum of LF, TF, and non-foliar GEM deposition.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/3e4f116e3cd7488c57a37d00.png"},{"id":50983156,"identity":"592f5ef6-4942-4de5-bcf5-1a47a0514629","added_by":"auto","created_at":"2024-02-12 06:50:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":354566,"visible":true,"origin":"","legend":"\u003cp\u003eDecadal history of atmospheric Hg accumulation in six soil profiles of northeastern USA with robust chronometry (ELA302-1, PiP01, PiP02, HaF01, HuB01, HuB02; Supporting Information). Colored symbols contrast two distinct depth zones in soil Hg accumulation, with red symbols highlighting surface soils where the pattern of increasing Hg accumulation with soil depth is influenced by Hg depositional processes, and black symbols showing historical Hg accumulation in deeper soil horizons. Contemporary total ecosystem Hg flux (EF) measured by micrometeorology is indicated. Dashed lines show respective best fits with shaded 95% confidence intervals. Solid green line shows spline fit. (b) ANOVA for Hg accumulation by period, R\u003csup\u003e2\u003c/sup\u003e=0.26, d.f.=6, \u003cem\u003em\u003c/em\u003e=6, \u003cem\u003en\u003c/em\u003e=93, p\u0026lt;0.0001]\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/389a452f0628715233caec1e.png"},{"id":50983010,"identity":"3db76ff6-99c2-4f4f-a275-f70fa01d7278","added_by":"auto","created_at":"2024-02-12 06:42:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":443721,"visible":true,"origin":"","legend":"\u003cp\u003esoil Hg accumulation rates in (a,d) Arctic tundra (AK, GR), (b,e) northeastern temperate (VT, NH, ME, MA) and boreal (ON) forests, and (c,f) tropical forest (PR). Temperate data are reproduced from Fig. 2a. Points are colored by soil horizon for foliage (green), Oi horizon (yellow) Oe horizon (orange), Oa horizon (brown), and mineral soil (black). Soils highlighted in discussion are colored white. Puerto Rico litterfall (LF), throughfall (LF), and ecosystem flux (EF) estimates are based on \u003cem\u003eShanley et al. \u003c/em\u003e\u003csup\u003e66 56\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/ee6a80bf5473bac4c31aede6.png"},{"id":66346021,"identity":"f64962f7-58a4-4241-87ee-d7fce48b34ea","added_by":"auto","created_at":"2024-10-10 16:58:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1712085,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/4c2bcd16-56ca-4350-84b6-83b7e7f22c88.pdf"},{"id":50983009,"identity":"7bdef924-ec58-4546-b110-bbe05a50d1cf","added_by":"auto","created_at":"2024-02-12 06:42:55","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1081213,"visible":true,"origin":"","legend":"\u003cp\u003eSupporting Information\u003c/p\u003e","description":"","filename":"HgFLHSupportingInformation020724.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/e5954cc740bfbdbc94fa1b63.pdf"},{"id":50983008,"identity":"571e221f-4e41-4905-8506-005fe62f12fc","added_by":"auto","created_at":"2024-02-12 06:42:55","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":302439,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"abstractart.docx","url":"https://assets-eu.researchsquare.com/files/rs-3937465/v1/b008a89db5552604d9960497.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry","fulltext":[{"header":"Synopsis","content":"\u003cp\u003eThe fallout radionuclide (FRN) chronometry technique newly employed in soils shows Hg accumulation rates that are consistent in time and magnitude with other deposition records, thus providing a powerful new tool to quantify historical Hg deposition fluxes across much larger spatial scales than previously possible.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eMercury (Hg) is a neurotoxic heavy metal, emitted to the atmosphere by both anthropogenic activities and natural sources and dispersed globally through atmospheric transport\u003csup\u003e1\u003c/sup\u003e. Forest ecosystems are the largest global sink of atmospheric Hg due to foliar uptake of gaseous elemental mercury (GEM) from the atmosphere\u003csup\u003e2\u003c/sup\u003e. Mercury is subsequently cycled to underlying soils by litterfall, canopy throughfall, and additional deposition of GEM directly to soils\u003csup\u003e2 3 4 5 6\u003c/sup\u003e. After centuries of anthropogenic Hg emissions, soils represent the largest global reservoir of anthropogenic Hg and pose an ongoing risk of exposure to terrestrial and aquatic food webs through uptake, leaching, and erosion to aquatic ecosystems\u003csup\u003e7 8 4 9 10 11\u003c/sup\u003e. In a biogeochemical twist, soil Hg is also partially re-emitted back to the atmosphere. Such soil re-emission of GEM counteracts the migration of Hg from surface to deep mineral soils through pedogenic processes, and thus prevents Hg from entering long-term sequestration\u003csup\u003e12 13 14 8 15 16\u003c/sup\u003e. The scale of terrestrial GEM re-emission remains highly uncertain, however, with global estimates spanning 1000 to 3000 Mg y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, or roughly 30 to 100% of gross terrestrial Hg deposition. This uncertainty raises fundamental questions about the efficacy of soils in sequestering legacy and contemporary Hg, and as a result about the fate of Hg in a context of a changing global environment\u003csup\u003e7 1 17 6 18\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eContemporary measurements of Hg fluxes at the soil surface reveal complex bi-directional exchanges between soils and the atmosphere, with evidence for both net deposition and net emission to soils\u003csup\u003e19 14 3 20 16\u003c/sup\u003e. Further complicating Hg dynamics, ecosystem components can play opposing roles in Hg cycling; for example, foliage acts as a strong and unambiguous net sink\u003csup\u003e2 4\u003c/sup\u003e, but forest floors (leaf litter or soil Oi horizon) may act as either source or sink\u003csup\u003e21 20 22\u003c/sup\u003e. Within the soil profile, high pore-gas GEM concentrations in some surface soils point to leaf litter and soil humus as net emitters of GEM during decomposition of organic matter (termed legacy emission if derived from past pollution), while elsewhere GEM concentrations are lower than atmospheric levels indicating deeper soils to be stable Hg sinks\u003csup\u003e23 22 24 25 16\u003c/sup\u003e. The balance of the two pathways has profound influences on the global Hg cycle. Processes that favor Hg retention in soils effectively remove Hg pollution from continued circulation in the environment, while legacy emissions extend exposures to pollution and globally delay recovery from Hg pollution even as primary anthropogenic Hg emissions continue to decrease\u003csup\u003e26\u003c/sup\u003e driven by national and global legislative actions\u003csup\u003e26\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eRecent advancements in soil chronometry with atmospheric fallout radionuclides (FRNs) provide a new and exciting opportunity to directly measure Hg soil accumulation rates using a 'bottom up' mass balance approach, thereby providing a critical missing view of terrestrial Hg biogeochemistry. Already the FRN \u003csup\u003e210\u003c/sup\u003ePb (half-life 22.3 years) is a vital tool for reconstructing historical Hg deposition using both lake sediment and peat environmental archive \u003csup\u003e27 28 29\u003c/sup\u003e. To extend this approach to terrestrial ecosystems, we employed and validated new FRN age models that accurately date foliar and soil organic matter (SOM). Three independent radionuclide age models, the Linked Radionuclide aCcumulation (LRC) model based on \u003csup\u003e7\u003c/sup\u003eBe:\u003csup\u003e210\u003c/sup\u003ePb, the nuclear bomb-pulse \u003csup\u003e241\u003c/sup\u003eAm, and the novel \u003csup\u003e228\u003c/sup\u003eTh:\u003csup\u003e228\u003c/sup\u003eRa chronometer, yield concordant SOM ages spanning subannual to centennial timescales\u003csup\u003e30 31 32\u003c/sup\u003e. Concordance of these models is predicated on the strong affinity of these five particle-reactive metals for SOM through the formation of stable organometallic complexes. The migration rate of \u003csup\u003e210\u003c/sup\u003ePb in soils is identical to that of atmospheric \u003csup\u003e14\u003c/sup\u003eC \u003csup\u003e33\u003c/sup\u003e, confirming that sorption of the tracer metals to SOM persists from uptake in vegetation through incorporation in soil, subsequent SOM decomposition, and hydrologic transport in association with natural organic matter (NOM) \u003csup\u003e33 34 35 10 36 37 38\u003c/sup\u003e. The FRN chronometers accurately measure soil organometallic dynamics not possible with more conventional Δ\u003csup\u003e14\u003c/sup\u003eC chronometry due to uncertain lag times between CO\u003csub\u003e2\u003c/sub\u003e assimilation and foliar production in the forest carbon cycle \u003csup\u003e39 40 41 42 43\u003c/sup\u003e. In a similar way, the chronometer metals provide a powerful comparator for constraining Hg dynamics in soils, and a novel tool for assessing the extent to which Hg behaves as a conservative particle-reactive metal entrained in the pedogenic cycle with SOM versus a fraction that may be re-emitted or translocated through gaseous pathways as GEM (Landis et al., in submission).\u003c/p\u003e \u003cp\u003eHere we employ FRN age models to measure Hg accumulation rates in a range of soils spanning Arctic, boreal, temperate, and tropical ecosystems. We then directly compare rates of soil Hg accumulation over annual to centennial timescales with measured whole-ecosystem atmospheric depositional fluxes. This novel approach thus provides a new basis for understanding key processes that regulate Hg accumulation in soils; for constraining the fate of past soil Hg accumulation; and for assessing the susceptibility of soil Hg to legacy emission and redistribution in response to environmental and climate perturbation.\u003c/p\u003e"},{"header":"2. Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1 Soil ages and Hg accumulation rates\u003c/h2\u003e\n\u003cp\u003eFallout radionuclide age estimates provide a critical and objective basis for comparing Hg accumulation in foliage and organic soil O-horizons across contrasting ecosystems (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). We here use the definition of soil Oi horizon as senesced leaves on the forest floor; Oe horizon as partially decomposed leaf fragments and fibers; and Oa horizon as humified organic matter lacking identifiable leaf fragments. Within each ecosystem, ages increase from foliage, Oi, Oe, and Oa horizons to reflect clear differentiation of horizons as they were sampled in the field [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05]. Ages pooled across ecosystems increase significantly from 1.5 years in foliage, to 3.3 years in Oi horizons, 7.5 years in Oe horizons, and 29 years Oa horizons\u003csup\u003e42 32\u003c/sup\u003e. Ages of Arctic foliage and soil horizons are significantly older than corresponding temperate, boreal, or tropical horizons which is attributed to slower SOM decomposition at high latitudes [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05].\u003c/p\u003e\n\u003cp\u003eSimilarly, tropical soil horizons are significantly younger than corresponding temperate ones [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05]. There were no significant differences between temperate (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;23 sites) and boreal forests (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4 sites), and these are hereafter analyzed together.\u003c/p\u003e\n\u003cp\u003eTotal Hg concentrations pooled across ecosystems increase significantly from 41 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in foliage to 74 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Oi, 140 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Oe, and 192 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Oa horizons (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb). There were no significant differences in foliar concentrations among ecosystems [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05]. Only Arctic Oi and temperate Oa horizons Hg concentrations are significantly higher than contrasting ecosystems [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05], but concentration data are poor predictors of Hg accumulation rates. Soil Hg concentrations were converted to total inventories by multiplication with total soil mass recovered from the defined area of quantitative soil pits (per cm thickness, in g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec). Oa horizons store the largest soil Hg inventories in each ecosystem, increasing significantly from 140 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in tropical soils, 440 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in temperate deciduous soils, 470 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in temperate coniferous soils, and 460 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Arctic soils [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05]. Previously, high Hg concentrations and inventories in Oa horizons have been attributed to their older ages and longer accumulation times of this horizon\u003csup\u003e52\u003c/sup\u003e, but these studies lacked the ability to comprehensively date soils by layer \u003csup\u003e52\u003c/sup\u003e. Total atmospheric Hg stocks in full soil profiles (including mineral soils to depths of 10\u0026ndash;50 cm corresponding in each profile to calendar years\u0026thinsp;\u0026lt;\u0026thinsp;1900) increase significantly across ecosystems from 2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mg m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4), to 6.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4 mg m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;10), and 11.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.4 mg m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3), respectively, in Arctic, temperate, and tropical soils [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; Table SI 1].\u003c/p\u003e\n\u003cp\u003eRates of foliar and soil Hg accumulation are converted from Hg inventories using LRC soil ages. Foliar and soil Hg accumulation rates increase substantially and statistically significantly across ecosystems in the following order: Arctic\u0026thinsp;\u0026lt;\u0026thinsp;temperate\u0026thinsp;\u0026lt;\u0026thinsp;tropical [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed]. Temperate foliar Hg accumulation estimates of 12\u0026thinsp;\u0026plusmn;\u0026thinsp;3 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SE, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4) are in good agreement with adjacent litterfall data for three forest sites (average of Harvard, Howland, and Downer Forests\u0026thinsp;=\u0026thinsp;11.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), suggesting that the LRC approach accurately reconstructs foliar Hg deposition rates. Arctic foliar flux estimates average 7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4), which is comparable with prior estimates and consistent with lower foliar-derived Hg deposition in the high latitudes \u003csup\u003e53 54 55\u003c/sup\u003e. Tropical foliar fluxes average 28\u0026thinsp;\u0026plusmn;\u0026thinsp;3 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e when considering only the dominant canopy but increase to 49\u0026thinsp;\u0026plusmn;\u0026thinsp;9 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e when significant understory vegetation is also included (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3). Foliar Hg deposition fluxes for Caribbean tropics are not well constrained, but our canopy average is comparable to one prior litterfall estimate of 29 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the surrounding Luquillo Experimental Forest \u003csup\u003e56\u003c/sup\u003e. The substantial increase in tropical foliar fluxes when understory vegetation is included emphasizes the importance of tropical ecosystem complexity to rates of Hg deposition \u003csup\u003e57\u003c/sup\u003e. High foliar Hg fluxes in tropical forests are consistent with high atmospheric deposition across tropical forests, supporting the notion of tropical forests as hotspots of Hg deposition driven by vegetative uptake and high net primary productivity \u003csup\u003e56 4\u003c/sup\u003e. Similarly, tropical Oe and Oa horizon accumulation rates are by far highest among all measured soils, in further support that high Hg deposition is a key characteristic of low-latitude forests [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2. Hg accumulation and atmospheric flux in temperate soils\u003c/h2\u003e\n\u003cp\u003eAnother salient feature of soil Hg accumulation evident in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e is a pronounced and significant increase in accumulation rates with soil depth [p\u0026thinsp;\u0026lt;\u0026thinsp;0.05]. To contextualize this pattern, we compare soil Hg accumulation rates to independent atmospheric Hg flux estimates for each ecosystem type, beginning with temperate forests where total ecosystem Hg deposition data are available for the Northeastern United States (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea); two sites, Harvard Forest (site HaF, deciduous) and Howland Forest (site HoF, coniferous) \u003csup\u003e3 21\u003c/sup\u003e include micrometeorological GEM flux quantification, and a third Downer Forest (site DoF, mixed). The primary pathways of atmospheric Hg deposition to soil systems include litterfall (LF) originating largely from GEM uptake, deposition by rain and canopy wash-off originating largely from oxidized Hg\u003csup\u003eII\u003c/sup\u003e, and non-foliar deposition of GEM directly to both forest floor (O horizon soil) as well as to long-lived above-ground tissues such as mosses, lichen, and tree bark \u003csup\u003e5\u003c/sup\u003e. The non-foliar GEM component is calculated as above-canopy GEM flux minus the foliar component, that latter approximated as LF flux. Together these sum to total ecosystem flux (EF). Deposition studies at the three temperate sites yield (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD): LF\u0026thinsp;=\u0026thinsp;11.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6, TF\u0026thinsp;=\u0026thinsp;7.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9, non-foliar GEM\u0026thinsp;=\u0026thinsp;7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;7.5, and EF\u0026thinsp;=\u0026thinsp;28.2\u0026thinsp;\u0026plusmn;\u0026thinsp;6.0, \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eSoil Hg accumulation rates for temperate forests are compared with measured LF, TF and EF atmospheric fluxes in calendar years 2019\u0026ndash;2022 (i.e., measured 1\u0026ndash;5 years prior to soil collection) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). Both deciduous and coniferous soils follow a pattern of strongly increasing Hg accumulation rates with depth (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea; we call these typical pattern soils \"accumulating\"; see discussion below). Hg accumulation in foliage with ages\u0026thinsp;\u0026lt;\u0026thinsp;1 to 3 years is consistent with local litterfall flux (LF) measurement as noted above. Accumulation rates in Oi horizon [ages 2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3 years, mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SE] are consistently higher than LF fluxes and show additional Hg accumulation in the forest floor [18.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]. Both TF and additional non-foliar GEM deposition can increase Hg deposition to the forest floor following litterfall \u003csup\u003e58 59 60 21\u003c/sup\u003e. Oi horizon accumulation rates are considerably lower than total EF measured by flux-gradient at both the HaF and HoF sites, however, with a cumulative shortfall averaging 45 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, or 36% of total deposition. This \"missing Hg\" could indicate an overestimation of EF at these sites, re-emission of Hg as GEM back to atmosphere, or more likely as discussed below, translocation of Hg from forest floor deeper into mineral soils.\u003c/p\u003e\n\u003cp\u003eIn contrast to a missing Hg sink in Oi horizons, Hg accumulation rates in Oe soil horizons [mean age 7.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8 years] converge with contemporary EF [26.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e]. Hg accumulation rates in Oa horizons are higher still [45.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3 years], nearly twice the contemporary ecosystem atmospheric Hg flux in soils aged 20\u0026ndash;30 years. Mercury accumulation rates continue to increase in underlying mineral soil and peak at depths corresponding to calendar years 1975\u0026ndash;2000 (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). At these depths about half of our temperate soils (6 of 11) record reliable histories of Hg atmospheric deposition (\u003cem\u003eSupporting Information\u003c/em\u003e). Here, accumulation rates average 67\u0026thinsp;\u0026plusmn;\u0026thinsp;9 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which is approximately 2.4 times higher than measured contemporary atmospheric EF. Higher Hg accumulation in Oa and mineral soil horizons versus contemporary atmospheric fluxes suggests that some Hg may be preferentially translocated from overlying organic soil horizons. Most importantly, however, our measurements support evidence that historical Hg forest deposition fluxes in the northeastern United States have much been higher than at present \u003csup\u003e61 27 62\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe timing of peak soil Hg accumulation (1950\u0026ndash;2000) is notably consistent with regional lake sediment archives, where deposition of atmospheric Hg peaked during 1970\u0026ndash;1990 and has subsequently declined to present by about a factor of two \u003csup\u003e8 63 62 64\u003c/sup\u003e. If peak soil accumulation 67\u0026thinsp;\u0026plusmn;\u0026thinsp;9 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for years 1975\u0026ndash;2000 is representative of the total Hg ecosystem flux in that era, it would be consistent with a similar decrease by a factor of 2.4 to contemporary terrestrial deposition for this region. Hence, soil Hg reconstructions agree well with lake sediment records. Both archives are also consistent with measured declines in atmospheric GEM concentrations in North America that have averaged 1\u0026ndash;2% per year since 1990 \u003csup\u003e26 65\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe consistency among soil, lake and atmospheric records provides strong evidence that, despite the complexity of pedogenic processes and Hg biogeochemical dynamics, soil FRN chronometry provides new and direct constraints on historical terrestrial Hg fluxes. This opens unprecedented opportunities to assess historic Hg deposition across much larger spatial scales given the ubiquity of soils. This line of reasoning further concludes that typical northeastern US soils are quantitatively efficient at retaining past atmospheric deposition of Hg from combined depositional sources. Our soil mass balance estimates hence indicate that substantial legacy emission of Hg from soils back to atmosphere is unlikely across most soils (see discussion of low-accumulating soils below in Section \u003cspan class=\"InternalRef\"\u003e2.5\u003c/span\u003e), and instead suggest that typical temperate and boreal forest soils quantitatively retain atmospheric Hg ecosystem deposition over annual to decadal timescales.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e2.3. Latitudinal trends in soil Hg accumulation across ecosystems\u003c/h2\u003e\n\u003cp\u003eReconstructed soil Hg accumulation rates across ecosystems demonstrate a strong latitudinal gradient in atmospheric Hg deposition, on the order of 10 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for Arctic soils, 50 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for temperate sites, and 150 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for tropical Luquillo (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea to \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). Despite wide-ranging fluxes, however, similar soil Hg accumulation patterns occur across ecosystems, suggesting that the processes impacting soil Hg sequestration and overall mass balance are global. Specifically, like their temperate counterparts, both tropical and Arctic soils show Hg accumulation rates that fall short of respective contemporary deposition estimates in surface soil horizons, and that eclipse ecosystem Hg fluxes in deeper soils of multi-decadal ages (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed to \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef). The convergence between soil accumulation and atmospheric deposition rates occurs at corresponding soil ages of 5\u0026ndash;10 years across highly variable soil orders spanning Gellisol, Inceptisol, Spodosol, and Ultisol. The magnitude of this effect also varies in accordance with ecosystem properties; for example, in the Luquillo tropical forests, soil Oi horizons accumulate Hg at just about 20% of estimated EF, versus 70% in temperate forests. We interpret this difference as a decreasing ability of surface organic soils to sequester Hg under increasing mean annual temperatures and SOM mineralization rates. In Arctic samples the Oi horizon Hg fluxes average 120% of contemporary EF flux, which is likely within reasonable uncertainty of estimated atmospheric deposition and indicates a strong retention of Hg by Arctic soils, as well \u003csup\u003e50\u003c/sup\u003e. We note, too, that the tundra Oi horizon is sufficiently old (up to 40 years) that it records higher rates of Hg deposition that predate successive emissions reductions implemented by the Clean Air Act (1970), Clean Air Act Amendments (1990), and Minimata Convention (2017).\u003c/p\u003e\n\u003cp\u003eCentennial histories of Hg accumulation in both Arctic and tropical soils mirror those of temperate forests and are similarly consistent with global anthropogenic emissions, rising steeply through 20th century before declining around 1990\u0026ndash;2000 in line with implementation of global emissions controls (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed to Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef). The precise magnitude and timing of peak soil accumulations may be influenced by variable SOM decomposition and colloid migration rates with soil depths among diverse profiles, but within each ecosystem the consistency of Hg accumulation patterns among contrasting soils suggests that our reconstructed peak Hg accumulations accurately reflect the dynamics of atmospheric Hg deposition and accumulation across latitudes. For example, we observe good correspondence in temporal trends among highly different tropical soils, which include hydric Inceptisols (site LeF03, e.g., 80% organic matter by weight) to Ultisols (site LeF01, e.g., 50% Al- and Fe-oxides by weight). Peak deposition in tropical soils appears later (ca. 2000) than in temperate latitudes, a shift that may be influenced by the unique disturbance history of Puerto Rico. Here, hurricane Maria (2017) caused massive defoliation of the Luquillo forest canopy that delivered 60% of above-ground forest biomass to the forest floor \u003csup\u003e67\u003c/sup\u003e, and along with the large pool of Hg contained therein. In Arctic soils, the period of peak Hg deposition is less clear due to large uncertainties in soil Hg reconstructions for years prior to 1950, which is a result of low atmospheric deposition of both FRNs and Hg, combined with slow SOM decomposition and colloid migration rates.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4. Forest Hg dynamics and mechanisms of soil sequestration\u003c/h2\u003e\n\u003cp\u003eAll surface soil horizons (forest floor Oi horizons) we have dated indicate substantial shortfall of Hg relative to atmospheric fluxes, at rates of 10 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for temperate systems or even approaching 100 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for tropical forests of Puerto Rico (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). A large body of work supports the proposition that Hg in the forest floor is reduced to GEM, both during SOM decomposition and following wet deposition of Hg\u003csup\u003eII\u003c/sup\u003e, and that this potentially yields a highly mobile GEM pool that could be subject to re-emission \u003csup\u003e19 6 20 16\u003c/sup\u003e. In contrast to the notion of significant net GEM emission, however, soil Hg accumulation rates increase in deeper soils far exceeding contemporary depositional fluxes, and as discussed above, in Oa to upper mineral horizons are quantitively in agreement with historical changes from other independent deposition records. Mass balance thus requires that the balance of any Hg lost from surface soils must be redistributed in a net downward direction that augments long-term sequestration.\u003c/p\u003e\n\u003cp\u003eWe propose that percolation is likely key to facilitating mass transport of Hg from surface organic horizons, analogous to how FRNs reach soil depths of 5\u0026ndash;10 cm or more during throughfall events (Supporting Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), and how throughfall is in enriched in total Hg by factors of 2 or more over incident precipitation during transit through the canopy \u003csup\u003e68 69 5 70\u003c/sup\u003e. To any extent that GEM may be re-emitted from surface horizons back to atmosphere, our mass balance would require that this GEM is recycled back to soils, rather than lost to the atmosphere. Although enigmatic, such internal GEM recycling could occur via throughfall or litterfall of fine and coarse debris and may occur over decadal timescales. Studies show that on the order of 16% of Hg in throughfall may represent GEM that is re-deposited within the canopy from soil re-emission of prior Hg deposition \u003csup\u003e21\u003c/sup\u003e. At the same time, stable isotope composition of GEM emitted from soils appears to be identical to that of atmospheric GEM and dissimilar from soil legacy Hg, which suggests that measured soil GEM re-emission reflects the transient turnover of atmospheric GEM rather than net emission from legacy soil reservoirs \u003csup\u003e71 20\u003c/sup\u003e. Direct re-emission of GEM from foliar surfaces may also be important for recycling wet-deposited Hg to the atmosphere before entering underlying soils \u003csup\u003e72 16\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn contrast to forest floor, underlying organic and mineral horizons are closed systems with respect to Hg. Pore gas GEM concentrations here are typically lower than atmospheric levels, suggesting that in deep soils any available GEM becomes firmly bound or oxidized to particle-reactive Hg\u003csup\u003eII 24 22 16\u003c/sup\u003e. Mercury thus seems to be translocated from forest floor in gaseous form or deposited directly to mineral soil by percolation and thereafter continues its inexorable downward migration as organometallic colloid at migration rates quantified by the LRC age model and corroborated by bomb-pulse \u003csup\u003e14\u003c/sup\u003eC and \u003csup\u003e241\u003c/sup\u003eAm, and novel \u003csup\u003e228\u003c/sup\u003eTh:\u003csup\u003e228\u003c/sup\u003eRa chronometry\u003csup\u003e31 32\u003c/sup\u003e. The retention of Hg in deep soils locks in its historical pattern of atmospheric deposition, like that of the \u003csup\u003e241\u003c/sup\u003eAm nuclear bomb-pulse \u003csup\u003e31 32\u003c/sup\u003e (Supporting Table\u0026nbsp;1). As such, the fate of Hg in soils is closely aligned with that of soil carbon insofar as long-term storage of both Hg and C in soils is contingent on their mobilization from organic to mineral soils and stabilization by complexation with secondary soil minerals including authigenic clays and Al,Fe-oxides \u003csup\u003e73 74 75\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003e2.5. Mercury in low-accumulating soils and implications for coniferous ecosystems\u003c/h2\u003e\n\u003cp\u003eIn contrast to typical Hg-accumulating soils, a subset of northeastern temperate and boreal soils (20% of sites) shows decreasing Hg accumulation rates with soil depth, which we call \"low-accumulating\" soils (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). These soils are all coniferous, yet we stress that there is no generic difference between deciduous and coniferous soils at Hg typical accumulating sites, and that ecotype alone is therefore insufficient to explain these patterns. We highlight results from the Woody Adams conservation forest in Vermont (site WoA) where deciduous maple and oak soils display typical Hg accumulating behavior, yet adjacent pine and hemlock soils are low-accumulating. Although the pine and hemlock organic soils have total Hg loads that are higher per horizon than the hardwoods by factors of two to three, this is attributable largely to their older ages, slower decomposition rates, and greater accumulation of SOM. In terms of mass balance, however, the low-accumulating coniferous soils are \u0026lsquo;missing\u0026rsquo; 250 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e or about 10% of accumulated Hg over past 20 years relative to the deciduous sites. The nearby Downer Forest (DoF) coniferous pine soil, also low-accumulating and for which we have collected a full soil profile, shows a strong peak of Hg accumulation in the underlying mineral soil (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb), suggesting that podzolization (downward mobilization of iron and aluminum oxides along with dissolved organic carbon) could explain low-accumulating behavior in some coniferous forest floors due to preferential leaching of Hg to underlying mineral soil. The coniferous 302-2 and HoF01 soils similarly show strong and atypic Hg enrichment in mineral horizons which we attribute to lateral podzolization along horizontal flow paths (Fig. \u003cspan class=\"InternalRef\"\u003eS2\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eMore research is needed to confirm whether Hg in low-accumulating surface soils is lost to the atmosphere via re-emission, leached to mineral horizons, or exported from watersheds by runoff (Landis et al. in submission). The Hg shortfall from individual surface soils approaches 20 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is comparable to typical contemporary atmospheric flux, and therefore would be large enough to be readily measured by atmospheric or watershed studies if such losses occurred at large scales. On the other hand, if the frequency of low-accumulating soils (20%) is representative of northeastern forests and such losses are limited to these particular soils, the Hg ecosystem loss based on our mass balance would equate to only about 2% of annual EF. This scale is compatible with re-emission of 0.9 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e measured by micrometeorology from the HoF coniferous Podsol forest floor \u003csup\u003e21\u003c/sup\u003e. At the watershed scale, isotope tracer experiments similarly suggest that about 1% of new deposition may be exported from upland forests in streamflow \u003csup\u003e76 10\u003c/sup\u003e, although export of cumulative legacy deposition may approach 10% of contemporary watershed input \u003csup\u003e66 77 78\u003c/sup\u003e. Weaker retention of Hg in coniferous forest organic soils and possible influence of podzolization on mineral horizons described here demands further study as it may potentially be linked with high riverine export in boreal rivers\u003csup\u003e79\u003c/sup\u003e and impact boreal and Arctic soil Hg retention under global warming, with implications for downstream food webs \u003csup\u003e79 80\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTypical uncertainties on soil Hg accumulation rates due to error in the LRC age models are estimated to be on the order of 10% for soil ages less than 60 years\u003csup\u003e31 32\u003c/sup\u003e, so the Hg mass losses in low-accumulating soils can be confidently attributed to physicochemical behavior of Hg. Given this, the considerable variation in Hg accumulation rates among northeastern soils apparent in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e reflects spatial variation in both how and where Hg is deposited and sequestered across the terrestrial landscape. Variable accumulation of Hg is underscored by a factor of five higher coefficient of variation among soil Hg inventories in comparison to \u003csup\u003e210\u003c/sup\u003ePb and \u003csup\u003e241\u003c/sup\u003eAm (100% versus 20%; Supporting Table\u0026nbsp;1) and is thus consistent with the potential for high patchiness of Hg accumulation, redistribution, and hotspots of Hg losses by either re-emission or runoff.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Conclusions","content":"\u003cp\u003eWe demonstrate the first direct estimates of soil Hg accumulation rates in upland soils using a \u0026lsquo;bottom up\u0026rsquo; mass balance based on FRN chronometry, thereby establishing reference accumulation rates in Arctic, temperate, boreal, and tropical soils. The reconstructed rates in surface soils support the magnitude of deposition measurements available by micrometeorological studies, which together affirm that GEM-derived atmospheric deposition is dominant across forest ecosystems. Our soil mass balance measurements show that surface soil horizons under-accumulate Hg while deeper organic and mineral horizons over-accumulate Hg compared to modern atmospheric deposition estimates. Superimposed on recent declines in rates of Hg atmospheric deposition, we propose that this is facilitated by vertical translocation of Hg, possibly mediated by GEM dynamics, percolation, and/or colloid transport, from surface to deeper soil horizons. In deeper organic and mineral soils, our data provide evidence of a closed system for Hg wherein Hg is firmly sequestered and thereafter regulated by colloidal transport at migration rates that are accurately measured by FRN chronometry. This application of FRN chronometry to terrestrial Hg dynamics thus establishes an exciting and novel opportunity to expand conventional archives such as lake sediments, peat, or ice records to include soils as a largescale, widespread, and easily accessible medium to quantify Hg depositional processes and historical fluxes, \u003csup\u003e81 82 28 83 29\u003c/sup\u003e. A key distinction is that soil accumulation rates serve as accurate measures of terrestrial deposition where inputs from vegetation uptake of GEM dominate (\u0026gt;\u0026thinsp;70%) and result in terrestrial fluxes 3\u0026ndash;4 times higher than measured in conventional archives. Flux reconstructions based on lake sediment records may underestimate total ecosystem Hg loads, for example, because they are biased to wet deposition, depend on watershed connectivity, and are convoluted by impacts of land use and watershed erosion that impact sedimentary Hg flux \u003csup\u003e84\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eReconstructed accumulation rates for contrasting ecosystems confirm enormous gradients in Hg deposition across global biomes previously reported in atmospheric measurements, ranging from 10 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the Arctic to ~\u0026thinsp;30 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in northeastern US forests and over 100 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in tropical forests of Luquillo. Importantly, extraordinary rates of Hg accumulation observed in the \"clean air\" site of Puerto Rico (exceeding 300 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at one location) demonstrate that low-latitude tropical forests may be inherently strong Hg sinks due to a combination of high net primary productivity, complex disturbance histories, and high rainfall totals with access to a global pool of oxidized Hg\u003csup\u003eII\u003c/sup\u003e in the upper troposphere through deep convection \u003csup\u003e56 8\u003c/sup\u003e. Our measurements demonstrate that very high Hg fluxes in the tropics are not dependent on nearby pollution sources \u003csup\u003e70\u003c/sup\u003e. Few direct measures of atmospheric Hg deposition are available in tropical forests, however, and given their likely critical role in global Hg cycling, high priority should be placed on improving our understanding of Hg depositional pathways and accumulation in these critical ecosystems \u003csup\u003e56 25 18\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOur data contradict reports of widespread soil Hg losses by GEM re-emission and indicate that if such losses occur, they might efficiently re-cycle within ecosystems and back to soils rather than being exported to the global atmosphere. This might further imply that conventional litterfall and throughfall measurements overestimate net Hg fluxes by including a recycled component. Soil reconstructions across larger scales have the potential to resolve this uncertainty in contemporary and historical Hg fluxes to terrestrial ecosystems, as well as to better understand the role of soil processes in Hg global mass balance. Atmospheric Hg concentrations are now slowly declining in North America, Europe, and East Asia due to emission controls, although emissions continue to increase in South America. Under declining Hg emission scenarios, the fate of legacy Hg stored in soil becomes increasingly important, in particular whether certain ecosystems such as boreal peatlands shift to become net sources that offset reductions in industrial emissions \u003csup\u003e15 14 16\u003c/sup\u003e. Our data strongly suggest that, with exceptions of surface litter layers and a subset of coniferous soils possibly linked to podzolization, most forest soil horizons across latitudinal gradients are strong net sinks and act as a closed system with respect to Hg.\u003c/p\u003e \u003cp\u003eAdditional work is needed to understand and couple Hg dynamics with the FRN tracer and chronometer \u003csup\u003e210\u003c/sup\u003ePb in organometallic colloid formation, and processes and rates of podzolization, and how colloid transport may work to sequester Hg in mineral horizons, or alternatively to export Hg with DOC through hydrologically connected watershed elements to discharge in aquatic and marine ecosystems where it ultimately bioaccumulates in food webs to cause negative health outcomes for both wildlife and human population.\u003c/p\u003e"},{"header":"4. Methods","content":"\u003cp\u003eWe measured Hg accumulation rates in foliage and soils using high-precision gamma spectrometry and high-resolution quantitative soil pits (Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e). Our experimental design includes (1) contrasting canopy tree species within sites, (2) contrasting sites with identical species, and (3) contrasting ecosystems across climate and elevational gradients. Selected sites represent soil orders that include Inceptisol, Spodosol, Gellisol, and Ultisol. We focus on temperate forests in northeastern US states (Vermont, VT; New Hampshire, NH, Massachusetts, MA; and Maine, ME) and boreal forest in Ontario, Canada. Sites in Vermont include multiple forest types with the same lithology, elevation, and climate, with forest stands dominated by maple (\u003cem\u003eAcer saccharum\u003c/em\u003e) and beech (\u003cem\u003eFagus grandifolia\u003c/em\u003e); red oak (\u003cem\u003eQuercus rubra\u003c/em\u003e); white pine (\u003cem\u003ePinus strobus\u003c/em\u003e); or eastern hemlock (\u003cem\u003eTsuga canadensis\u003c/em\u003e). Sites in Massachusetts (\u003cem\u003eQuercus rubra\u003c/em\u003e, Harvard Forest) and in Maine (\u003cem\u003eTsuga canadensis\u003c/em\u003e and \u003cem\u003ePinus strobus\u003c/em\u003e, Howland Forest) have published records of total Hg ecosystem flux measured by micrometeorological flux-gradient methods in addition to throughfall and litterfall estimates \u003csup\u003e3 21\u003c/sup\u003e. The Downer Forest site in Vermont has new multi-year records of Hg litterfall and throughfall that are reported here. One boreal site in Ontario, Canada, was also sampled where canopy is dominated by black spruce (\u003cem\u003ePicea mariana\u003c/em\u003e).\u003c/p\u003e\n\u003cp\u003eTemperate and boreal sites were contrasted with tundra sites on the Seward Peninsula, Alaska (AK) dominated by moss (\u003cem\u003eSphagnum\u003c/em\u003e) \u003csup\u003e44\u003c/sup\u003e and Sisimuit, Greenland (GR) with shrub cover of black crowberry (\u003cem\u003eEmpetrum nigrum\u003c/em\u003e) over \u003cem\u003eSphagnum\u003c/em\u003e. Tropical sites include an elevational gradient (approximately 350-1,000 m a.s.l) in the Luquillo Experimental Forest (LEF), Puerto Rico, that spans subtropical wet, lower montane, and rain forest life zones \u003csup\u003e45\u003c/sup\u003e. The LEF sites include distinct forest communities of tabonuco (\u003cem\u003eDacroydes excelsa\u003c/em\u003e-\u003cem\u003eManilkara bidentata\u003c/em\u003e), palo colorado (\u003cem\u003eCyrilla racemiflora\u003c/em\u003e-\u003cem\u003eMicropholis garciniifolia)\u003c/em\u003e, and elfin woodland (\u003cem\u003eTabebuia rigida\u003c/em\u003e-\u003cem\u003eEugenia borinquensis\u003c/em\u003e), respectively \u003csup\u003e46 47\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eSoils were collected from 30 x 30 cm or 50 x 50 cm quantitative pits following methods previously described \u003csup\u003e31\u003c/sup\u003e. For 16 short soil profiles consisting of uppermost organic soils (max. depth 5 cm), samples of thickness 0.5 to 1 cm were collected corresponding to Oi, Oe, and Oa organic horizons \u003csup\u003e32\u003c/sup\u003e. Eighteen additional long profiles in 1 to 5 cm increments to soil depths of 30\u0026ndash;50 cm are also reported here. A total of 379 samples were analyzed and dated. Due to the large sample masses required for FRN analysis soils and foliage were oven-dried at 50\u0026deg;C rather than by lyophilization, which previously has shown non-detectable to minor losses during drying (max. 4% of total Hg) \u003csup\u003e48\u003c/sup\u003e. Foliage and litter for Hg analysis were homogenized using a stainless-steel Wiley Mini-Mill (Thomas Scientific), and soils were disaggregated by agate mortar and pulverized in a zirconia ball mill.\u003c/p\u003e\n\u003cp\u003eAtmospheric Hg in soils was resolved from geogenic contributions using Al as a reference element \u003csup\u003e49 50\u003c/sup\u003e, with total atmospheric Hg calculated as the sum of [Hg\u003csub\u003ei\u003c/sub\u003e \u0026ndash; Al\u003csub\u003ei\u003c/sub\u003e\u0026times;(Hg/Al)\u003csub\u003emineral\u003c/sub\u003e] for \u003cem\u003ei\u003c/em\u003e layers with ages\u0026thinsp;\u0026lt;\u0026thinsp;225 years, and the mineral component taken for soils older than reference year 1800 which is the practical limit for reliable FRN chronometry. This procedure provides a conservative estimate of anthropogenic Hg accumulation in soils, although we recognize that earlier emissions occurred and could be significant near regional sources. For measurements of Al and major/trace elements, samples were digested with a mixed acid approach using HNO\u003csub\u003e3\u003c/sub\u003e-HCl-HF-H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and microwave heating with hot plate evaporation in Teflon vessels, and 150 mg samples per 15 mL acid. Major/trace elements were measured by ICPOES (Spectro ARCOS) with reference materials NIST 1547 (peach leaves), NIST 2706 (New Jersey soil), NIST 2710 (Montana Soil II), USGS G3 (granite), CCRMP TILL 1 and 2 (glacial till) and STSD 1 and 2 (stream sediment). Recoveries for Al averaged 104\u0026thinsp;+\u0026thinsp;3%, 95\u0026thinsp;\u0026plusmn;\u0026thinsp;2%, 104\u0026thinsp;\u0026plusmn;\u0026thinsp;4%, and 97\u0026thinsp;\u0026plusmn;\u0026thinsp;3%, 96\u0026thinsp;\u0026plusmn;\u0026thinsp;3%, 99\u0026thinsp;\u0026plusmn;\u0026thinsp;3%, 93\u0026thinsp;\u0026plusmn;\u0026thinsp;2% and 98\u0026thinsp;\u0026plusmn;\u0026thinsp;6% (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SE), respectively.\u003c/p\u003e\n\u003cp\u003eCanopy foliar Hg inventories were estimated by multiplication of leaf Hg concentration by leaf area index (LAI) and leaf mass area (LMA), or in one case by whole-tree harvest. Soil Hg inventories for each depth increment were estimated by multiplication of soil Hg concentrations by total recovered mass per sampling area of the quantitative pit (kg m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) rather than with an estimate of soil bulk density. Hg concentrations were measured by DMA (Milestone DMA-80) with reference materials NIST 2706 (New Jersey soil), and NIST 1547 (peach leaves), run each 10 samples. Recoveries averaged 103.4\u0026thinsp;\u0026plusmn;\u0026thinsp;4.8% (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;98) using standards of approximately 5 ng total Hg, and 99.3\u0026thinsp;\u0026plusmn;\u0026thinsp;8.6% with standards of approximately 1 ng total Hg (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;98), respectively.\u003c/p\u003e\n\u003cp\u003eRadionuclides were measured by high-precision gamma spectroscopy (Mirion BeGE 3830 gamma detectors \u003csup\u003e51\u003c/sup\u003e. The FRN linked radionuclide accumulation (LRC) model based on the \u003csup\u003e7\u003c/sup\u003eBe:\u003csup\u003e210\u003c/sup\u003ePb ratio was implemented as described by Landis et al. \u003csup\u003e31\u003c/sup\u003e. LRC soil ages for a subset of soils reported here have been confirmed by concordance with both \u003csup\u003e228\u003c/sup\u003eTh:\u003csup\u003e228\u003c/sup\u003eRa and \u003csup\u003e241\u003c/sup\u003eAm chronometers with typical uncertainties\u0026thinsp;\u0026lt;\u0026thinsp;10% for ages\u0026thinsp;\u0026lt;\u0026thinsp;60 years \u003csup\u003e31 32\u003c/sup\u003e (Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eStatistical analyses were performed in JMP16 Pro, using analysis of variance (ANOVA) to compare Hg concentrations, inventories, and accumulation rates among contrasting soil horizons and across contrasting ecosystems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements.\u003c/h2\u003e \u003cp\u003eThis project was funded by the Dept. Earth Sciences at Dartmouth College through the Fallout Radionuclide Analytics facility (FRNA). Visit us at sites.dartmouth.edu/frna/. Special thanks to Tatiana Barreto, Lee Hrenchuk, and Ken Sandilands for assistance at field sites. Samples from Seward Peninsula, AK were obtained with funding to MP (NSF OPP-ANS-2116471). Samples from Harvard and Howland Forests were obtained with funding to DO (NSF EAR-1848212). Contributions from WM and TB were supported by NSF EAR 2012403 and NSF DEB 1831592. Samples from Luquillo Experimental Forest were obtained by permission of US Forest Service, El Yunque Special Use permit #YNF3019. Samples from Downer State Forest were obtained by permission of Vermont Dept. Natural Resources, Special Use permit #22857.\u003c/p\u003e\n\u003ch2\u003eAuthor contributions.\u003c/h2\u003e \u003cp\u003eJDL conceived and designed the project. JDL and DO wrote the manuscript. JDL, VFT, JZ, JDV, and FM collected samples in the field. JDL and VFT performed analytical and laboratory procedures. JDL, DO, JZ, and CER analyzed data. All authors contributed to manuscript edits. DO, WM, CN and MP provided material support essential to project success.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eDriscoll, C. T., Mason, R. P., Chan, H. M., Jacob, D. J. \u0026amp; Pirrone, N. 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Prediction of holocene mercury accumulation trends by combining palynological and geochemical records of lake sediments (Black Forest, Germany). \u003cem\u003eGeosciences (Switzerland)\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, (2018).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3937465/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3937465/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSoils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant accumulated through a history of anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg deposition in soils remains fundamentally uncertain, however, particularly to what degree Hg is quantitatively retained versus re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we introduce a new bottom-up soil mass balance based on fallout radionuclide (FRN) chronometry that allows direct quantification of historical Hg soil accumulation rates and comparison with measured contemporary atmospheric deposition. We show that soils spanning Arctic, boreal, temperate, and tropical ecosystems are strong and long-term sinks for atmospheric Hg, and that the soil sink strength decreases with latitude. Peak deposition reconstructed for years 1950-2000 strongly exceeds contemporary deposition fluxes by factors of approximately two. In the northeastern USA, trends in soil-derived Hg accumulation rates agree in timing and magnitude with records derived from regional lake sediments and atmospheric measurements. We show that typical soils are quantitatively efficient at retaining atmospheric Hg deposition, with exception of a subset of soils (about 20%, all temperate and boreal coniferous), where approximately 10% of Hg deposition is unaccounted for, suggesting that up to 2% of soil Hg may be lost by legacy emission of GEM back to the atmosphere when scaled across the landscape. The observation that most soil Hg is effectively sequestered long-term calls into question global model and mass balance studies that assume strong and continued re-cycling of legacy Hg pollution in the environment that prolongs the impacts of past Hg emissions. Availability of FRN chronometry to reconstruct soil Hg accumulation rates poses a powerful new tool to quantify Hg deposition and trends across much larger spatial scales than previously possible, and should advance the understanding of Hg deposition, accumulation, and fate in the context of changing global environment.\u003c/p\u003e","manuscriptTitle":"Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-12 06:42:50","doi":"10.21203/rs.3.rs-3937465/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fa095e26-fb18-4391-8b07-ac95e5cd8794","owner":[],"postedDate":"February 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":28681895,"name":"Earth and environmental sciences/Biogeochemistry/Element cycles"},{"id":28681896,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry/Atmospheric chemistry"}],"tags":[],"updatedAt":"2024-06-27T00:34:17+00:00","versionOfRecord":{"articleIdentity":"rs-3937465","link":"https://doi.org/10.1038/s41467-024-49789-7","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2024-06-26 00:34:17","publishedOnDateReadable":"June 26th, 2024"},"versionCreatedAt":"2024-02-12 06:42:50","video":"","vorDoi":"10.1038/s41467-024-49789-7","vorDoiUrl":"https://doi.org/10.1038/s41467-024-49789-7","workflowStages":[]},"version":"v1","identity":"rs-3937465","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3937465","identity":"rs-3937465","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}