Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Domain: earth_science
Published: 2024-06-26
Editorial outcome: minor_revision

Abstract
Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.

Introduction
Mercury (Hg) is a neurotoxic heavy metal, emitted to the atmosphere by both anthropogenic activities and natural sources and dispersed globally through atmospheric transport1. Forest ecosystems are the largest global sink of atmospheric Hg due to foliar uptake of gaseous elemental mercury (GEM) from the atmosphere2. Hg is subsequently cycled to underlying soils by litterfall (LF), canopy throughfall (TF), and additional deposition of GEM directly to the forest floor and other above-ground surfaces2–6. 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 erosive transport to aquatic ecosystems2,4,7–10. In a biogeochemical twist, soil Hg is also partially re-emitted back to the atmosphere. Such 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 sequestration11–16. The scale of terrestrial GEM re-emission remains highly uncertain, however, with global estimates spanning 1000–3000 Mg y−1, or roughly 30–100% of gross terrestrial Hg deposition4,7,12,17. This uncertainty raises fundamental questions about the long-term efficacy of soils in sequestering legacy and contemporary Hg, the lifetimes of Hg in the surface environment, and as a result about the fate of Hg in the context of a changing global environment1,3,4,6.

Contemporary measurements of GEM fluxes at the soil surface reveal complex bi-directional exchanges between soils and the atmosphere, with evidence for both net deposition and net emission (Fig. 1)3,13,14,18,19. Further complicating Hg dynamics, ecosystem components can play opposing roles in Hg cycling; for example, foliage acts as a strong and unambiguous net sink4,20, but forest floors (leaf litter or soil Oi horizon) may act as either source or sink19,21,22. 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 the decomposition of organic matter, which we term legacy emission if derived from past pollution. In deeper soils, GEM concentrations are lower than atmospheric levels indicating mineral soils are a stable Hg sink14,21,23–25. The balance of these 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 due to national and international legislative actions26.

Recent advancements in soil chronometry with atmospheric fallout radionuclides (FRNs) provide an 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 210Pb (half-life 22.3 years) is a vital tool for reconstructing historical Hg deposition using both lake sediment and peat environmental archives27–29. To extend this approach to terrestrial ecosystems, we employed and validated 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 7Be:210Pb, the nuclear bomb-pulse 241Am, and the novel 228Th:228Ra chronometer, yield concordant SOM ages spanning subannual to centennial timescales30–32. The concordance of the 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 210Pb in soils is identical to that of atmospheric 14C33, 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 matter33–38. The FRN chronometers accurately measure soil organometallic dynamics not possible with more conventional Δ14C chronometry due to uncertain lag times between CO2 assimilation and SOM production in the forest carbon cycle39–43.

With their ability to accurately date SOM, the chronometer metals provide a powerful hypothesis for predicting Hg dynamics in soils, and thus an emerging tool for assessing the extent to which Hg behaves as a conservative particle-reactive metal similarly entrained in the pedogenic cycle with SOM versus a fraction that may be re-emitted or otherwise mobilized as GEM. Here we employ LRC and 241Am age models to measure Hg accumulation rates in a range of soils spanning Arctic, boreal, temperate, and tropical ecosystems. A subset of soils for which Hg is reported here were also previously dated by the 228Th:228Ra method32. We then directly compare rates of soil Hg accumulation over annual to centennial timescales with measured whole-ecosystem atmospheric depositional fluxes. This approach thus provides a new basis for constraining the fate of past soil Hg accumulation, for understanding key processes that regulate Hg accumulation in soils, and for assessing the susceptibility of soil Hg to legacy emission and redistribution in response to environmental and climate perturbation.

Results
Soil ages and Hg accumulation rates
FRN age estimates provide a critical and objective basis for comparing Hg accumulation in foliage and organic soil O-horizons across contrasting ecosystems (Fig. 2, Table S1). We define 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 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 in Oa horizons. Ages of Arctic foliage and soil horizons are significantly older than corresponding temperate, boreal, or tropical horizons which we attribute 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. Note that data in Fig. 2 represent sampled depth intervals and not entire horizons.

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. 2b; Table S1). There were no significant differences in foliar concentrations among ecosystems [p<0.05]. Only Arctic Oi and temperate Oa 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, g m−2 cm−1) (Fig. 2c). 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, but these studies lacked the ability to comprehensively date soil layer by layer44. For reference, total atmospheric Hg stocks in full soil profiles increased significantly across Arctic, temperate, and tropical 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 [p<0.05; Table S2]. Hg inventories were summed through mineral soils to depths of up to 50 cm corresponding in each profile to calendar years >1900 and were corrected for geogenic contribution (see “Methods”).

Rates of foliar and soil Hg accumulation are converted from Hg inventories using LRC soil ages. Accumulation rates increase substantially and statistically significantly across ecosystems: Arctic < temperate < tropical [p<0.05; Fig. 2d]. Temperate foliar Hg accumulation estimates of 12±3 µg m−2 y−1 (mean±SE, n=4) are in good agreement with adjacent LF data for three forest sites (average of Harvard, Howland, and Downer Forests = 11.8±0.8 µg m−2 y−1), confirming that the LRC approach accurately reconstructs foliar Hg deposition rates. Arctic foliar fluxes average 7.5±2.9 µg m−2 y−1 (n=4), comparable with prior estimates and consistent with lower foliar-derived Hg deposition in the high latitudes45–47. 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 LF estimate of 29 µg m−2 y−1 for the surrounding Luquillo Experimental Forest (LEF)48. The substantial increase in tropical foliar fluxes when understory vegetation is included emphasizes the importance of tropical ecosystem complexity in driving high rates of Hg deposition49. Our data thus support the notion of tropical forests as hotspots of Hg deposition driven by vegetative uptake and high net primary productivity4,48. Similarly, tropical Oe and Oa horizon accumulation rates are by far highest among all measured soils, further supporting high Hg deposition as a key characteristic of low-latitude forests [p<0.05].

Hg accumulation and atmospheric flux in temperate forests
Another salient feature of soil Hg accumulation evident in Fig. 2 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, first for temperate forests where total ecosystem Hg deposition data are available for the northeastern United States (Fig. 2a); two sites, Harvard Forest (site HaF, deciduous) and Howland Forest (site HoF, coniferous), include micrometeorological GEM flux quantification3,22, and a third Downer Forest (site DoF, mixed)50,51. The primary pathways of atmospheric Hg deposition to soil systems include LF originating largely from GEM uptake; deposition by rain and canopy wash-off originating largely from oxidized HgII (throughfall or TF); 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 (Fig. 1)5. The non-foliar GEM component is calculated here as the above-canopy GEM flux minus the foliar component, and the latter is approximated as LF flux. Together these sum to total ecosystem flux (EF). Deposition studies at the three temperate sites thus yield the following flux estimates (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.

Hg accumulation rates for temperate forest soils are compared with measured LF, TF, and EF atmospheric fluxes in Fig. 3 for calendar years 2019–2022 (measured 1–5 years post soil collection). Both deciduous and coniferous soils follow a pattern of strongly increasing Hg accumulation rates with depth (Fig. 3a; we call this typical pattern “accumulating”; see discussion below). Hg accumulation in foliage with ages <1–3 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]. Ongoing TF and non-foliar GEM deposition can increase Hg deposition to the forest floor following LF22,52–54. Oi horizon accumulation rates are considerably lower than total EF measured by flux-gradient at both the HaF and HoFsites, 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 the atmosphere, or more likely as discussed below, preferential translocation of Hg from the forest floor deeper into mineral soils.

In contrast to a missing Hg sink in Oi horizons, Hg accumulation rates in Oe soil horizons converge with contemporary EF [mean age 7.4±2.8 years, flux 26.4±2.2 µg m−2 y−1]. Hg accumulation rates in Oa horizons are higher still, nearly twice the contemporary ecosystem atmospheric Hg flux in soils aged 20–30 years [45.9±3.3 µg m−2 y−1]. Hg accumulation rates continue to increase in underlying mineral soil and peak at depths corresponding to calendar years 1975–2000 (Fig. 4). At these depths about half of our temperate soils (6 of 11) record robust histories of atmospheric deposition (Supporting Information, Table S2, Figs. S1–S3). Here, accumulation rates average 73±5 µg m−2 y−1 which is ~2.5 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 present27,55,56.

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 two15,56–58. If peak soil accumulation of 73±5 µg m−2 y−1 for the years 1975–2000 is representative of the total Hg EF in that era, it would be consistent with a similar decrease by a factor of 2 to contemporary terrestrial deposition for this region (Fig. 4). 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 199026,59.

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 direct constraints on historical terrestrial Hg fluxes. This opens unprecedented opportunities to assess historic Hg deposition across much larger spatial scales given the ubiquity and accessibility 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, and that, while individual surface horizons are missing some fraction of ongoing Hg atmospheric deposition, this Hg is retained within the deeper soil profile. Our soil mass balance estimates hence indicate that substantial legacy emission of Hg from soils back to the atmosphere is unlikely across most soils (see discussion of low-accumulating soils below in the “Mercury low-accumulating soils and coniferous ecosystems” section), and instead suggest that typical temperate forest soils quantitatively retain atmospheric Hg ecosystem deposition over annual to decadal timescales.

Latitudinal trends in forest soil Hg accumulation
Across contrasting forest ecosystems, reconstructed soil Hg accumulation rates demonstrate a strong latitudinal gradient in atmospheric Hg deposition, on the order of 10 µg m−2 y−1 for Arctic, 50 µg m−2 y−1 for temperate, and 150 µg m−2 y−1 for tropical Luquillo sites (Fig. 5a–c). Despite wide-ranging fluxes, however, similar temporal patterns in Hg accumulation spanning ecosystems suggest 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. 5d–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 Gelisol, 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 with higher 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 well2. We note, too, that the tundra Oi horizon is sufficiently old (up to 40 years) that it records higher rates of Hg deposition that predates 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 anthropogenic emissions of the global north, rising steeply through the twentieth century before declining around 1990–2000 in line with the implementation of international emissions controls (Fig. 5d–f). The precise magnitude and timing of peak soil accumulation may be influenced by variable SOM decomposition and colloid migration rates with soil depths among diverse soil orders (Fig. 5d–f). Shifts in peak Hg accumulation from that of mid-latitudes in Arctic (later) and tropical soils (earlier) might also reflect preferential affinity of Hg for humic matter in Oa horizons. Yet, within each ecosystem, the consistency of Hg accumulation patterns suggests that our Hg accumulation reconstructions 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 both hydric Inceptisols (site LeF03, e.g., 80% organic matter by weight) and Ultisols (site LeF01, e.g., 50% Al- and Fe-oxides by weight). Peak deposition in tropical soils appearing later (ca. 2000) than in temperate latitudes may also 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 floor60, 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. Overall, the paucity of both FRN and Hg data for high- and low-latitude sites should be a focus for future work.

Forest Hg dynamics and mechanisms of soil sequestration
All surface soil horizons (forest floor Oi horizons) show a 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. 5). 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 HgII, and that this potentially yields a highly mobile GEM pool that could be subject to re-emission6,14,18,19. 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 quantitatively in agreement with historical changes from other independent deposition records. The mass balance thus requires that 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 precipitation events (Fig. S4), and how TF is enriched in total Hg by factors of two or more over incident precipitation by washing Hg from foliar and canopy surfaces5,61–64. This is surprising since Hg is deposited to soils primarily through LF and direct deposition of GEM to the forest floor rather than by wet deposition. We suggest that Hg may be translocated as GEM preferentially to FRNs by rainfall and snowmelt during organic matter decomposition. To any extent that GEM may be re-emitted from surface horizons back to atmosphere, our mass balance suggests that this GEM is recycled back to soils, rather than lost to the atmosphere. Although enigmatic, such internal GEM recycling could occur via TF or LF of fine and coarse debris and may occur over decadal timescales. For example, on the order of 16% of Hg in TF may represent GEM that is re-deposited within the canopy from soil re-emission of prior Hg deposition22. At the same time, the 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 reservoirs19,65. Direct re-emission of GEM from foliar surfaces may also be important for recycling wet-deposited Hg to the atmosphere before it has the opportunity to enter underlying soils14, with an especially strong impact on the mass balance of isotope spike experiments such as METAALICUS. There, 50% of wet-deposited Hg was recycled back to the atmosphere from the forest canopy over a period of months66.

In contrast to forest floor, underlying organic and mineral horizons are closed systems with respect to Hg. Pore-gas GEM concentrations there are typically lower than atmospheric levels, suggesting that in deep soils any available GEM becomes firmly bound or oxidized to particle-reactive HgII14,21,24. Hg thus seems to be translocated from the forest floor in a gaseous form or deposited directly to mineral soil by percolation and thereafter continues its inexorable downward migration as organometallic colloid. Critically, the retention of Hg in deep soils locks in its historical pattern of atmospheric deposition, like that of the 241Am nuclear bomb pulse (Table S2)31,32. As such, the fate of Hg in soils is also 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 that prevent subsequent reduction and emission67–70.

Mercury low-accumulating soils and 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. 3b). These soils are all coniferous, yet we stress that there is no generic difference between deciduous and coniferous soils across 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 the 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. 3b), suggesting that in some cases podzolization (downward mobilization of iron and aluminum oxides along with dissolved organic carbon) may preferentially mobilize Hg relative to the FRN tracers or bulk carbon. 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 flowpaths (Fig. S2). While Hg may be decoupled from both FRNs and carbon in these cases and cannot be dated per se, here the FRNs provide a means for identifying the mass transfer of Hg even if quantifying its time of deposition and rate of redistribution will require additional new approaches.

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 watershed soils by runoff. 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 this minority of soils, the Hg ecosystem loss based on our mass balance would equate to only about 2% of annual EF. This scale is compatible with the re-emission of 0.9 µg m−2 y−1 measured by micrometeorology from the HoF coniferous Podsol forest floor22. At the watershed scale, isotope tracer experiments similarly suggest that about 1% of new deposition may be exported from upland forests in streamflow9,71, although the export of cumulative legacy deposition may approach 10% of contemporary watershed input72–74. Weaker retention of Hg in coniferous forest organic soils and the 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 and impact boreal and Arctic soil Hg retention under global warming75, with implications for downstream food webs76,77.

Typical uncertainties on soil Hg accumulation rates due to errors in the LRC age models are estimated to be on the order of 10% for soil ages <60 years31. High precision of the age models coupled with typically congruent behaviors between soil Hg and the FRN tracers allow us to reconstruct confident estimates of Hg deposition, peaking at 73±5 µg m−2 y−1 in northeastern forests. Where Hg diverges from historical patterns of deposition, however, for example in low-accumulating soils, the concordance of multiple radionuclide tracers in the studied soil systems demonstrates that Hg mass loss can be confidently attributed to the specific physicochemical behavior of Hg. Given this, the considerable variation in Hg accumulation rates among northeastern soils is apparent in Figs. 3 and 4 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 210Pb and 241Am (100% versus 20%; Table S1) and thus indicates the potential for high patchiness of Hg accumulation and hotspots of Hg losses.

Insights into Hg biogeochemical cycling in soils
We demonstrate the direct estimate of soil Hg accumulation rates in upland soils using a “bottom–up” mass balance based on FRN chronometry, thereby establishing both reference inventories and accumulation rates for Arctic, temperate, boreal, and tropical soils impacted by global atmospheric Hg. Reconstructed rates support the magnitude of deposition measured in 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 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 opportunity to expand conventional archives such as lake sediments, peat, or ice records to include soils as a large scale, widespread, and easily accessible medium to quantify Hg depositional processes and historical fluxes28,29,75,78,79. A key distinction is that soil accumulation rates accurately represent terrestrial deposition where inputs from vegetation uptake of GEM dominate (>70%) and result in terrestrial fluxes 4–5 times higher than measured in conventional archives. Flux reconstructions based on lake sediment records may underestimate total ecosystem Hg loads, in contrast, because they are biased to wet deposition, dependent on watershed connectivity, and are convoluted by impacts of land use that impact sedimentary Hg flux80.

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 HgII in the upper troposphere through deep convection15,48. Our measurements demonstrate that very high Hg fluxes in the tropics are not dependent on nearby pollution sources63. 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 ecosystems17,25,48.

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 LF and TF measurements overestimate net Hg fluxes by including a recycled component. Soil reconstructions across larger scales now 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 the soil becomes increasingly important, whether certain ecosystems such as boreal peatlands shift to become net sources that offset reductions in industrial emissions13,14,16. Our data strongly suggest that, with exceptions of surface litter layers and a subset of coniferous soils possibly linked to podzolization, most forest soils across latitudinal gradients are strong net sinks and act as a closed system with respect to atmospheric Hg.

The potential of FRN chronometry for predicting the fate of atmospheric Hg is twofold. First, in most soils, Hg behaves congruently with a suite of FRN radionuclide chronometers and this allows us to reconstruct robust decadal histories of Hg EF. It must be emphasized that critical physicochemical processes including reduction of Hg2+ to GEM, podzolization, and possibly preference of Hg for thiol ligands81–83, may fractionate Hg from FRN tracers in certain environments. In these cases, the second potential of FRN chronometry is an opportunity to quantify the extent to which Hg may deviate from historical expectation and thereby provide insight into drivers of Hg environmental behavior such as the mobilization of GEM from organic to mineral soil. This may be pursued with future work that couples Hg stable isotope dynamics with the FRN tracer 210Pb in organometallic colloid formation and transport, toward show these may act to sequester Hg in mineral horizons, or alternatively to preferentially export Hg 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 populations.

Methods
We measured Hg accumulation rates in foliage and soils using high-precision gamma spectrometry and high-resolution quantitative soil pits (Table S2). 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 forests 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 EF measured by micrometeorological flux-gradient methods in addition to TF and LF estimates3,22. The DoF site in Vermont has new multi-year records of Hg LF and TF 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)84 and Sisimuit, Greenland (GR) with shrub cover of black crowberry (Empetrum nigrum) over Sphagnum. Tropical sites include an elevational gradient (~350–1000 m a.s.l) in the LEF, Puerto Rico, that spans subtropical wet, lower montane, and rainforest life zones85. 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), respectively86,87.

Soils were collected from 30×30 cm or 50×50 cm quantitative pits, at hilltop, ridgeline, and locally flat locations selected to isolate vertical flow paths31. For 16 short soil profiles consisting of uppermost organic soils (max. depth 5 cm), samples of thickness 0.5–1 cm were excavated by hand corresponding to Oi, Oe, and Oa organic horizons32. Eighteen additional long profiles in 1–5 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)88. Foliage and litter for Hg analysis were homogenized using a stainless-steel Wiley Mini-Mill (Thomas Scientific), and soils were disaggregated by a gate mortar and pulverized in a zirconia ball mill.

Atmospheric Hg in soils was resolved from geogenic contributions using Al as a reference element2,89, with total atmospheric Hg calculated as the sum of [Hg_i − Al_i × (Hg/Al)_mineral] for layers with ages <225 years, and the mineral component taken for soils older than the 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 HNO3–HCl–HF–H2O2 and microwave heating with hot plate evaporation in Teflon vessels, and 150 mg samples/15 mL acid. Major/trace elements were measured by ICP-OES (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 and leaf mass area, 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 every ten samples. Recoveries averaged 103.4±4.8% (mean±SD, n=98) using standards of ~5 ng total Hg, and 99.3±8.6% with standards of ~1 ng total Hg (n=98), respectively.

Radionuclides were measured by high-precision gamma spectrometry (Mirion BeGE3830 gamma detectors)90. The FRN LRC model based on the 7Be:210Pb ratio was implemented as described by Landis et al.31. LRC soil ages were confirmed with the 241Am bomb-pulse chronometer in deep soil pits (depths of 30–50 cm; Table S2)31. For shallow soil pits that included only organic horizons (FLH pits, Table S2), the 228Th:228Ra method was previously used to verify LRC ages in the 0–5-year age range32. (Table S2). We also report the more familiar bomb-pulse radionuclide 137Cs, but we stress that it is unreliable in soil systems due to the strong biological cycling of cesium as a potassium analog31,91.

Statistical analyses were performed in JMP 16 Pro, using analysis of variance (ANOVA) to compare Hg concentrations, inventories, and accumulation rates among contrasting soil horizons and across contrasting ecosystems.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript demonstrates a novel means by which to characterize the history of atmospheric\ndeposifion of Hg to soil through the applicafion of independent fallout radionuclide chronometers. The\nresults obtained by this study are significant to the field of Hg geochemistry in parficular, and to the\ngeneral issue of using soils as an archive of recent environmental change in the annual to decadal scale.\nThe chronometry of Hg deposifional fluxes obtained by this method are consistent with other published\nrecords, and the manuscript cites numerous published studies to validate this consistency. The work\nsupports the claim that the fallout radionuclide approach can be used widely to invesfigate Hg\ndeposifion history across the globe. Although soils are complex and heterogeneous, the authors have\ntaken a viable approach in sampling and analysis that deals well with the heterogeneity, and they used a\nconcordance criterion (cumulafive 241Am/210Pb) to weed out disturbed soil profiles. The analyfical\nmethodology used by the authors is sounds and sufficiently described to allow reproducfion of the\nresults by others. The first author is one of the leading experts in the applicafion of fallout radionuclides\nto invesfigafions of soil accumulafion rates and the physical and chemical behavior of soils.\nOverall the manuscript is well-wriften and deals well with uncertainfies in the data. The presentafion and\nstafisfical treatment of the data is of high quality. In this reviewer's opinion, this is an important\nmanuscript that can be published nearly in its present form, with only minor revision in response to the\npoints noted below. It is certainly fimely, of broad interest, and suitable for publicafion in NATURE\nCOMMUNICATIONS.\nSpecific comments on text:\n- In the Introducfion, the authors refer to the 228Th/228Ra chronometer, and the reader might assume\nthat this was applied in the present study. However, these isotopes do not appear to have been\nmeasured as part of this study (no data for 228Th or 228Ra appear in Table S1). For clarity up front, the\nauthors should indicate which measurements were made in this study (e.g. somewhere in the paragraph\nbeginning at line 97).\n- Table S1 includes measurements of the 137Cs inventory in the soil profiles, but these data are not\ndiscussed in the manuscript. It would be worth menfioning 137Cs in the Introducfion (paragraph\nbeginning at line 97), and also worth including a brief sentence or two discussion about what is learned\nfrom the 137Cs inventory data somewhere later in the text or in the SI; if not, perhaps these data should\nbe deleted from Table S1?\n- The capfion to Figure 3 needs some edifing. The two panels are not labeled “a” and “b”, even though\nthere is reference to (b) in the capfion. Also, in the right side of panel (b), there are different colored\ncircles that indicate Students’ t-test results of each pair, but it is unclear what the colors indicate or\nwhich pairs are being compared. A bit more explanafion of this in the capfion would be helpful for the\nreader to understand this part of the figure.\n- In the last sentence of the Figure 4 capfion, “throughfall (LF)” should be corrected to “throughfall (TF)”.", "reviewer_2": "The manuscript enfitled, “Quanfifying soil accumulafion of atmospheric mercury using fallout\nradionuclide chronometry” by Landis et al. uses a novel soil chronometer to esfimate soil accumulafion\nrates for soil of varying age. The complexity of soil processes and development have made elucidafing\nterrestrial mercury cycling difficult to tease apart, a major problem given the importance of soil within\nthe broader mercury cycle. Thus, the new chronometer holds great promise for the soil community\nbroadly and the mercury community, specifically. The study specifically looks at how mercury\naccumulafion rates very across lafitude, ecosystem type, soil horizon, and soil age. Several interesfing\npafterns are documented through this approach, exemplifying the ufility of the chronometer. Perhaps\nthe two most salient findings are the dafing of peak mercury accumulafion to the 1950-1990 period and\nthe implicafions from the mass- balance, which suggest relafively liftle soil mercury is lost to the\natmosphere. It is the lafter point which gives me pause, considering some of the possible limitafions of\nthe approach used here (expanded on below). Overall, I found the manuscript well wriften, fimely,\nimpacfful, and generally quite interesfing.\nMajor Comments:\n1. The LRC model produces an esfimate of average SOC age and the authors have provided convincing\nevidence of its ability to do so. Although this approach has advantages over other methods for soil\ndafing (e.g., the authors point out limitafions with 14C), I would think there are sfill many of the same\nproblems with connecfing soil mercury processes with SOC age. In fact, the authors do an excellent job of\nhighlighfing some of these in their accompanying paper.\nA) any Hg process independent of SOC could bias results. For instance, if the mineralizafion of older SOM\nwas accompanied by a release of mercury and that mercury was subsequently bound by other (perhaps\nnewer) SOM, the age model would no longer accurately reflect the deposifion age of mercury.\nB) the esfimated age of the soil is really the average of a mixture of both young and new carbon and thus\ncould be young and new mercury (which the authors point out in their companion paper, See Figure 2).\nC) A number of physical processes may affect the movement of soil SOC and thus affect reservoir masses\n(and therefore accumulafion rates). The accompanying paper presents several examples (lateral flow,\nverfical flow, etc.).\nI think that addressing some of these limitafions is necessary. Alternafively, highlighfing that the other\npaper discusses them at length would be helpful. While I do not think they are parficularly problemafic\nfor most of the data interpretafion, I think they become an issue for the mass balance approach. The\nabove three points could all result in biases of the actual accumulafion at any given point in fime. In fact,\nthe isotopic mass balance in the accompanying paper makes this point quite clearly and, notably, that is\nwith a single pulse (i.e., no previous deposifion). Here, the mass balance reflects mercury deposifion\nsince soil formafion.\nIn summary, I think the uncertainty in the mass balance should at the very least be commented on, given\nthe importance of the finding. Perhaps you can report the ELA mass balance findings and some of the\npossible sources of variability. I think this point is especially important as readers may not be working\nthrough both documents concurrently.\n2. In the boftom row of figure 4, the peak accumulafion rates all roughly occur at the Oa/mineral\ninterface, but the peak year gets younger as lafitude decreases (~1950 for arcfic, ~1970 for temperate,\nand ~2000 for tropical). It seems like these pafterns could also be explained by the different turnover\nfimes of SOM, with the largest amount of mercury associated with the largest, slowest horizon (i.e., the\nOa). Do the authors have thoughts on this?\nGeneral Comments:\nI think the abstract art looks very nice.\nI menfion this under figure 1, but are the concentrafion values presented in text and in Figure 1\naluminum normalized/atm?\n59 – 61: It might be nice to add specific citafions to the high and low esfimates, for the convenience of\nreaders.\n127: Consider adding measurements of variability to horizonal averages and in the reporfing of\nsubsequent values.\n191: Addifionally, could the lower accumulafion rate in lifter reflect the fact that not all deposifion is\nbeing taken up by the Oi horizon? If direct deposifion or throughfall passed largely through the Oi to, say,\nthe Oe, then there wouldn’t be missing mercury.\n328: Assuming that the DOF peak in the deep mineral horizon was due to increased verfical migrafion of\nmercury, wouldn’t the carbon signature associated with it sfill be younger than the observed ~100 years?\nThe “missing mercury” was deposited in the last 20 years, no? I may have misunderstood.\nFigures\nFigure 1 Comments:\nIs mercury accumulafion on a per cm basis or are you scaling up the inventory by horizon depth prior the\naccumulafion calculafion?\nAre the mercury values in panel B total values or do they represent atmospheric values (i.e., Al\nnormalized)? If it is the lafter, I think it would be good to somehow denote that in the figure.\nFigure 3:\nThis plot has a “(b)” but no “(a)” and no demarcafion on the plots.\nDoes the left-most (a) plot consist of both organic and mineral soils? Could these be disfinguished\nsomehow?\nI also would be interested in seeing a plot (a) with the concentrafion values. Perhaps as an SI inclusion.\nIf my interpretafion is correct, the right-most plot uses red for two different purposes: highlighfing the\n“increasing accumulafion” zone and for the different comparison circles. I would suggest using two\ndifferent color schemes to improve clarity.\nLine edits:\n16: This sentence is worded somewhat awkwardly, and I would suggest reworking it. “Accumulated”\nalone reads odd. Addifionally, this sentence neglects on-going emissions of mercury and direct releases\nto soil.\n17: I might specify that “typical soils” refers here to “typical forested and arcfic soils”, unless you feel that\nthe forested soils here are representafive of other soil types (e.g., grasslands). The majority of US soils\nare not forested or arcfic soils.\n28: This sentence has four clauses, which make it a bit cumbersome to read and may lead to confusion.\nConsider reworking.\n65: Consider cufting “...to soils”.\n183: Should this read “...1 to 5 years post soil collecfion” instead? 470: “every” instead of “each”", "reviewer_3": "Key Results\nThis study ufilizes novel FRN chronometry to compare Hg accumulafion in upland soil horizons at\ndifferent lafitudes encompassing 3 disfinct ecosystems: arfic, temperate, and tropical.\nThe study reports the highest Hg accumulafion rates in tropical ecosystems, consistent with higher\natmospheric deposifion in the tropics. Considering the small number of tropical and arcfic sites, these\nresults need further verificafion, but they are indeed the first of their kind reported.\nThe study also reports increasing Hg accumulafion with soil depth, in general. The authors note that\nfoliage flux is lower than total ecosystem flux (EF), but that Hg accumulafion in deeper soil layers exceeds\nEF, except in a few temperate soils dominated by conifers. Temperate soils cores make up the majority of\nthe samples and it is noteworthy that excepfions arise here. Arcfic and tropical have n = 3-4, which is\nconcerning. Further studies could confirm or deny these results.\nValidity\nThe authors do an excellent job describing how their data fits with previous work in lake sediments,\natmospheric deposifion, recycling of GEM, implicafions for atmospheric Hg mass balances, etc. I have\nsome concerns about migrafion of Pb and other isotopes and how that impacts dafing of the cores that\nmay need to be addressed.\nSignificance\nThe work described in this manuscript is highly significant. Following up on this work will lead to befter\nconstraints on Hg mass balances in the atmosphere and a befter understanding of how ecosystems may\nrespond to decreases in Hg deposifion. The work is certainly novel. Relying on 3 or 4 cores from the\narcfic and tropical ecosystems is the primary troubling factor. However, even without the comparisons\nacross ecosystems the results for the temperate soils are significant in the field of Hg research.\nData and methodology\nThe authors do an excellent job framing the findings and implicafions of the soil cores, parficularly\nrelafive to reported atmospheric fluxes of Hg. Quesfions that I had after examining the figures were\naddressed clearly and impressively in the arficle. The examinafion of EF relafive to the accumulafion\nrates in the soil cores was quanfitafive and thorough.\nThe finding of some cores being low-accumulafing is difficult to explain and fies into concerns regarding\nthe dafing of these soils. The authors offer plausible explanafions. However, if there is significant\ndownward transport of Hg via dissolved organic mafter (DOM), which is offered as potenfially explaining\nhigher Hg accumulafion in deeper soil horizons, how can the dafing sfill be valid? Would not the 210Pb\nalso be transported downward and impact the dafing? Also, if the Pb and Hg (and C) did NOT behave the\nsame with colloidal transport, wouldn't this invalidate the dafing of the Hg accumulafion?\nAnd furthermore, if there is downward movement of Hg there would also be lateral movement. Is this\nlateral movement insignificant, i.e. how much dissolved organic mafter is transported out of each\nwatershed? If DOM is being transported out there will be Hg associated with it. Is this transport\nnegligible at the sites, especially those low-accumulafing sites?\nAnalyfical Approach\nThe study is very strong in this regard. More cores would have been preferred, but it is not very pracfical\nto get a large value of n for this type of field work.\nSuggested improvements\nThe authors should address the limitafions of the dafing methods and the implicafions of DOM transport\nof Pb and other isotopes on the dafing methods.\nClarity and context\nThis manuscript was excepfionally well-wriften and clearly states the implicafions of using this new\nmethod of dafing soil cores. I really enjoyed reading this arficle and am excited by the results presented.\nReferences\nAppropriate\nMy experfise\nLake and peat coring are areas of experfise for me. I have always used 210Pb dafing and know some\nshortcomings of that method. The newer methods described here are not as familiar to me."}, "author_response": "Response to Reviewers, NCOMMS-24-07443\nThe Reviewer comments are reproduced in green text, and our responses are in black. Unless specified\notherwise, line references refer to the revised manuscript.\nWith respect to the Editors comments requiring aftenfion to limitafions and uncertainfies in the mass balance\nmodel, these are addressed throughout the revised manuscript as detailed below in responses to Reviewer\ncomments.\nReviewer #1 (Remarks to the Author):\nThis manuscript demonstrates a novel means by which to characterize the history of atmospheric deposifion\nof Hg to soil through the applicafion of independent fallout radionuclide chronometers. The results obtained\nby this study are significant to the field of Hg geochemistry in parficular, and to the general issue of using soils\nas an archive of recent environmental change in the annual to decadal scale. The chronometry of Hg\ndeposifional fluxes obtained by this method are consistent with other published records, and the manuscript\ncites numerous published studies to validate this consistency. The work supports the claim that the fallout\nradionuclide approach can be used widely to invesfigate Hg deposifion history across the globe. Although\nsoils are complex and heterogeneous, the authors have taken a viable approach in sampling and analysis that\ndeals well with the heterogeneity, and they used a concordance criterion (cumulafive 241Am/210Pb) to weed\nout disturbed soil profiles. The analyfical methodology used by the authors is sounds and sufficiently\ndescribed to allow reproducfion of the results by others. The first author is one of the leading experts in the\napplicafion of fallout radionuclides to invesfigafions of soil accumulafion rates and the physical and chemical\nbehavior of soils.\nOverall the manuscript is well-wriften and deals well with uncertainfies in the data. The presentafion and\nstafisfical treatment of the data is of high quality. In this reviewer's opinion, this is an important manuscript\nthat can be published nearly in its present form, with only minor revision in response to the points noted\nbelow. It is certainly fimely, of broad interest, and suitable for publicafion inNATURECOMMUNICATIONS.\nSpecific comments on text:\n- In the Introducfion, the authors refer to the 228Th/228Ra chronometer, and the reader might assume that\nthis was applied in the present study. However, these isotopes do not appear to have been measured as part\nof this study (no data for 228Th or 228Ra appear in Table S1). For clarity up front, the authors should indicate\nwhich measurements were made in this study (e.g. somewhere in the paragraph beginning at line 97).\nApologies for misleading. The 228Th:228Ra data were reported in a previous publicafion, but for many of the\nsame samples now reported here for Hg (the \"FLH\" code we use in Table S1). We feel it is important to\nhighlight the 228Th:228Ra data because they give extra confidence in the Hg fluxes we report, since the dafing\nwas done by mulfiple concordant methods. We have added text to clarify this at line 97 and referenced at line\n509.\n- Table S1 includes measurements of the 137Cs inventory in the soil profiles, but these data are not discussed\nin the manuscript. It would be worth menfioning 137Cs in the Introducfion (paragraph beginning at line 97),\nand also worth including a brief sentence or two discussion about what is learned from the 137Cs inventory\ndata somewhere later in the text or in the SI; if not, perhaps these data should be deleted from Table S1?\nWe believe the 137Cs data are important to include because readers familiar with fallout radionuclide dafing in\nother systems (peat, sediment) will expect this corroborafing bomb-pulse marker. In soils we use 241Am\nbecause Cs is strongly biologically cycled and thus unreliable. We have included a new reference to this effect\nin line 510 (Methods) where have introduced discussion of 137Cs (Kaste et al. 2021 in our reference list). We\nchose not to discuss at line 97 in order to maintain readability of the Introducfion, since there we are\ndescribing more broadly the advances in FRN chronometry that have made possible the present study of Hg.\n- The capfion to Figure 3 needs some edifing. The two panels are not labeled “a” and “b”, even though there\nis reference to (b) in the capfion. Also, in the right side of panel (b), there are different colored circles that\nindicate Students’ t-test results of each pair, but it is unclear what the colors indicate or which pairs are being\ncompared. A bit more explanafion of this in the capfion would be helpful for the reader to understand this\npart of the figure.\nWe have labeled panels. We have changed colors of the circles for befter clarity, and now include a\ndescripfion in the capfion.\n- In the last sentence of the Figure 4 capfion, “throughfall (LF)” should be corrected to “throughfall (TF)”.\nSo changed, thank you for the careful read!\nReviewer #2 (Remarks to the Author):\nThe manuscript enfitled, “Quanfifying soil accumulafion of atmospheric mercury using fallout radionuclide\nchronometry” by Landis et al. uses a novel soil chronometer to esfimate soil accumulafion rates for soil of\nvarying age. The complexity of soil processes and development have made elucidafing terrestrial mercury\ncycling difficult to tease apart, a major problem given the importance of soil within the broader mercury\ncycle. Thus, the new chronometer holds great promise for the soil community broadly and the mercury\ncommunity, specifically. The study specifically looks at how mercury accumulafion rates very across lafitude,\necosystem type, soil horizon, and soil age. Several interesfing pafterns are documented through this\napproach, exemplifying the ufility of the chronometer. Perhaps the two most salient findings are the dafing of\npeak mercury accumulafion to the 1950-1990 period and the implicafions from the mass- balance, which\nsuggest relafively liftle soil mercury is lost to the atmosphere. It is the lafter point which gives me pause,\nconsidering some of the possible limitafions of the approach used here (expanded on below). Overall, I found\nthe manuscript well wriften, fimely, impacfful, and generally quite interesfing.\nMajor Comments:\n1. The LRC model produces an esfimate of average SOC age and the authors have provided convincing\nevidence of its ability to do so. Although this approach has advantages over other methods for soil dafing\n(e.g., the authors point out limitafions with 14C), I would think there are sfill many of the same problems with\nconnecfing soil mercury processes with SOC age. In fact, the authors do an excellent job of highlighfing some\nof these in their accompanying paper.\nWe clarified (line 118) that a crifical difference is that radionuclides accurately date exposure age of SOM\nwhereas 14C is heavily biased to include carbon recycling fime. We agree that there is sfill a conceptual\nproblem of dafing SOM versus dafing the Hg itself (more below).\nA) any Hg process independent of SOC could bias results. For instance, if the mineralizafion of older SOM was\naccompanied by a release of mercury and that mercury was subsequently bound by other (perhaps newer) SOM,\nthe age model would no longer accurately reflect the deposifion age of mercury.\nWe agree and clarified (lines 487-493, 512-514, 590-598) that it is likely that Hg is somefimes decoupled and\nconvoluted with SOM in ways not fully understood. However, while we are dafing organic mafter, the age\nmodel itself is based on (primarily) 210Pb, and the central thesis of the manuscript becomes, whether Hg and\nPb follow the same pathways during SOC decomposifion. Our central hypothesis (lines 111-115) is that the\nFRNs (210Pb) provide a normalizing term that isolates Hg flux from various biogeochemical transformafions,\nand that Hg and FRNs should follow the same pathways during decomposifion, absent the influence of Hg\nreducfion to GEM and possible subsequent mobilizafion. Where Hg quanfitafively follows Pb, we can truly\ndate it (this appears to be the case in most soils studied given their converging accumulafion histories).\nWhere it does not, the model provides a way of idenfifying rates of losses (Hg mobilizafion).\nB) the esfimated age of the soil is really the average of a mixture of both young and new carbon and thus\ncould be young and new mercury (which the authors point out in their companion paper, See Figure 2).\nWe agree, this is certainly true. The FRN ages represent averages as well (same as in sediment/peat records).\nTo the extent that Pb traces Hg behavior, the LRC age represents average age of the deposited Hg. It is only\nwhen Hg behavior diverges from the tracers that the Hg \"age\" is substanfially different than that of SOC to\nwhich it is bound. We added a short statement (line 650) to clarify this.\nWe also note that we referred to Hg deviafion from tracers due to GEM \"mobilizafion\", \"translocafion\", or\n\"percolafion\" throughout the original manuscript at lines 97, 204, 263, 280, 306, 348, 362, 369, and 385. We\nbelieve these are calling out the very processes that Reviewer has cited, although perhaps not as explicitly as\npossible describing how these processes relate to the age model and mass balance. We have added text to\nemphasize that Hg can deviate from expectafion based on FRNs, at line 417. We have also taken this\nopportunity to menfion that Hg has ligand preferences that may also contribute to its fracfionafion from FRNs\n(now cifing Jiskra et al. 2014, Chen et al. 2016, Zhang et al. 2018).\nC) A number of physical processes may affect the movement of soil SOC and thus affect reservoir masses\n(and therefore accumulafion rates). The accompanying paper presents several examples (lateral flow,\nverfical flow, etc.).\nI think that addressing some of these limitafions is necessary. Alternafively, highlighfing that the other paper\ndiscusses them at length would be helpful. While I do not think they are parficularly problemafic for most of\nthe data interpretafion, I think they become an issue for the mass balance approach. The above three points\ncould all result in biases of the actual accumulafion at any given point in fime. In fact, the isotopic mass\nbalance in the accompanying paper makes this point quite clearly and, notably, that is with a single pulse (i.e.,\nno previous deposifion). Here, the mass balance reflects mercury deposifion since soil formafion.\nThe Reviewer has done an excellent job of summarizing the conceptual challenge to interprefing soil\nchronometry to Hg. They key to the method is appreciafing that the LRC model, by way of dafing SOC,\nprovides a hypothesis for behavior of Hg. This is our key statement, at line 93. We have restructured this\nparagraph to emphasize this as it should guide the reader through the paper. Unfortunately, our companion\npaper is sfill in Review and we believe best to publish the present manuscript as quickly as possible, but note\nthat the changes we have made, listed above, will make this clear to the reader.\nIn summary, I think the uncertainty in the mass balance should at the very least be commented on, given\nthe importance of the finding. Perhaps you can report the ELA mass balance findings and some of the\npossible sources of variability. I think this point is especially important as readers may not be working\nthrough both documents concurrently.\nWe agree, per comments above. We also cite references to METAALICUS at line 460.\n2. In the boftom row of figure 4, the peak accumulafion rates all roughly occur at the Oa/mineral interface,\nbut the peak year gets younger as lafitude decreases (~1950 for arcfic, ~1970 for temperate, and ~2000 for\ntropical). It seems like these pafterns could also be explained by the different turnover fimes of SOM, with the\nlargest amount of mercury associated with the largest, slowest horizon (i.e., the Oa). Do the authors have\nthoughts on this?\nIt is a good observafion. We had already suggested that the SOM decay might regulate the extent to which\nOi/Oe horizon retain Hg, with higher rates of mobilizafion in tropical soils. However, we added a sentence to\nrefer to this possibility (line 422), i.e., that if the shift in apparent peak accumulafion across biomes is related\nto SOM dynamics and preference for Oa humic mafter, this would require that Hg migrate more quickly\ndownward than Pb in the arcfic soils, but more slowly in the tropics. It is unclear why this should be. We also\nemphasize that the tropical soils all show the same trend, despite LeF3 being 80% organic mafter and the\nothers 50% oxyhydroxide, about as different as soils can get! It is unclear why Hg would behave idenfically in\nthese disparate tropical soils, and yet differ from temperate or arcfic soils. Substanfially more work will be\nrequired to resolve this, beyond present scope.\nFor the arcfic soils we suggested that the late peak is very likely due to uncertainfies in both age model and\natmospheric Hg correcfion (lines 420-425) since it is very difficult to resolve both atmospheric 210Pb and Hg\nfrom their geogenic counterparts. So, these combined, we believe more likely that Hg and Pb are sfill\nfollowing similar behaviors across biomes, but that soils of each biome soil have specific challenges to the age\ndafing itself that requires further work, and the histories in both Arcfic and tropics require further study.\nGeneral Comments:\nI think the abstract art looks very nice.\nThank you!\nI menfion this under figure 1, but are the concentrafion values presented in text and in Figure 1 aluminum\nnormalized/atm?\nGood catch. They are not, which we believe is important for comparability with other studies. In our data\nrepository we include total Hg and total atmospheric Hg.\n59 – 61: It might be nice to add specific citafions to the high and low esfimates, for the convenience of\nreaders.\nThese were added at the sentence following the esfimates but we have moved forward for clarity (Smith-\nDowney et al. 2020, Zhu et al. 206, Zhou et al. 2020, Zhang et al. 2023).\n127: Consider adding measurements of variability to horizonal averages and in the reporfing of subsequent\nvalues.\nWe have omifted them since the averages are informafional to provide a frame of reference, but variance\nwould reflect cross-ecosystem range which is less straighfforward to interpret. These are now added in a new\nSupporfing Table.\n191: Addifionally, could the lower accumulafion rate in lifter reflect the fact that not all deposifion is\nbeing taken up by the Oi horizon? If direct deposifion or throughfall passed largely through the Oi to, say,\nthe Oe, then there wouldn’t be missing mercury.\nYes, we think it just so, and we clarified this in lines 430-436. This is the reason we discuss percolafion as a\ndominant process for translocafing Hg (line 308 and elsewhere) and described the depth distribufion of 7Be\n(Supporfing Informafion, Figure S4) since this short-lived isotope (half-life 54 days) records primarily\npercolafion and does not survive long enough to record subsequent advecfion/diffusion. This is surprising for\nHg, however, since most Hg enters soil as GEM either with lifterfall or directly to forest floor, rather than by\nrainfall as for 7Be. In this sense the Hg is missing from surface Organic horizons where we should expect it\nbased on both foliar and non-foliar GEM deposifion, but found deeper in the soil profile than we expect if\nGEM were to remain bound to this bulk organic mafter.\n328: Assuming that the DOF peak in the deep mineral horizon was due to increased verfical migrafion of\nmercury, wouldn’t the carbon signatureassociated with it sfill be younger than the observed ~100 years? The\n“missing mercury” was deposited in the last 20 years, no? I may have misunderstood.\nWe would not expect the carbon age to be different than expectafion, since in this case we expect that the Hg\nhas moved deeper relafive to either the FRN tracers or bulk carbon itself that the FRNs are dafing. We believe\nmore likely that Hg is mobilized by GEM or instead some highly mobile Hg-DOC complex, but in this case, yes,\nthe apparent age of the Hg would be much older than reality. We have clarified at line 341 that the process\nwould require preferenfial mobilizafion of Hg. We also caufion over-interpretafion of individual soil profiles\nsince their local histories are difficult to constrain with certainty.\nFigures\nFigure 1 Comments:\nIs mercury accumulafion on a per cm basis or are you scaling up the inventory by horizon depth prior the\naccumulafion calculafion?\nThe figure shows Hg inventories on a cm-1 basis for panel C. For the accumulafion calculafion, the inventories\n[ug m-2] are instead normalized by the fime durafion of each cm interval, which is actually one of the most\npowerful uses of the chronometry since depth is an arbitrary axis with respect to atmospheric fluxes. This is\nwhat allows use to reconstruct high-resolufion histories, but it does inflate the variance of flux values we\nobserve within an individual soil horizon due to changing fluxes through depth/fime. We have added\nclarificafion to the figure capfion.\nAre the mercury values in panel B total values or do they represent atmospheric values (i.e., Al normalized)? If\nit is the lafter, I think it would be good to somehow denote that in the figure.\nThese are not normalized to befter compare with literature data. This is now clarified in the figure and legend.\nFigure 3:\nThis plot has a “(b)” but no “(a)” and no demarcafion on the plots.\nDoes the left-most (a) plot consist of both organic and mineral soils? Could these be disfinguished somehow?\nWe now use the same shape and color scheme as previous figures to show both ecotype and soil horizon.\nI also would be interested in seeing a plot (a) with the concentrafion values. Perhaps as an SI inclusion.\nWe have added this as requested. It is reassuring to see the same trends in both concentrafions and fluxes,\nthis is due to the preponderance of atmospheric Hg residing in organic soils with low geogenic contribufions,\nand liftle in mineral soil where bulk density is much higher.\nIf my interpretafion is correct, the right-most plot uses red for two different purposes: highlighfing the\n“increasing accumulafion” zone and for the different comparison circles. I would suggest using two different\ncolor schemes to improve clarity.\nWe now use the same shape and color scheme as previous figures and show both ecotype and soil horizon.\nLine edits:\n16: This sentence is worded somewhat awkwardly, and I would suggest reworking it. “Accumulated” alone\nreads odd. Addifionally, this sentence neglects on-going emissions of mercury and direct releases to soil.\nWe have reworded, thank you.\n17: I might specify that “typical soils” refers here to “typical forested and arcfic soils”, unless you feel that the\nforested soils here are representafive of other soil types (e.g., grasslands). The majority of US soils are not\nforested or arcfic soils.\nWe have changed to 'forest and tundra soils'. Grasslands should be a priority for future work.\n28: This sentence has four clauses, which make it a bit cumbersome to read and may lead to confusion.\nConsider reworking.\nWe changed this accordingly.\n65: Consider cufting “...to soils”.\nWe changed this accordingly.\n183: Should this read “...1 to 5 years post soil collecfion” instead?\nYes, changed accordingly, thank you!\n470: “every” instead of “each”\nSo changed.\nReviewer #3 (Remarks to the Author):\nKey Results\nThis study ufilizes novel FRN chronometry to compare Hg accumulafion in upland soil horizons at different\nlafitudes encompassing 3 disfinct ecosystems: arfic, temperate, and tropical.\nThe study reports the highest Hg accumulafion rates in tropical ecosystems, consistent with higher\natmospheric deposifion in the tropics. Considering the small number of tropical and arcfic sites, these results\nneed further verificafion, but they are indeed the first of their kind reported.\nWe agree that applying our method to addifional sites should be a high priority. The excellent consistency of\nthe results at the two arcfic sites (Alaska and Greenland), and within the one tropical site (Puerto Rico) gives\ngood confidence that the method is reliable, but we need broader spafial coverage to understand global Hg\ndynamics.\nThe study also reports increasing Hg accumulafion with soil depth, in general. The authors note that foliage\nflux is lower than total ecosystem flux (EF), but that Hg accumulafion in deeper soil layers exceeds EF, except\nin a few temperate soils dominated by conifers. Temperate soils cores make up the majority of the samples\nand it is noteworthy that excepfions arise here. Arcfic and tropical have n = 3-4, which is concerning. Further\nstudies could confirm or deny these results.\nWe have added a comment about the low replicafion and pointed out the need for more site (lines 285-286).\nHowever, we point out that the soils within the Arcfic (both Greenland and Alaska) and tropics (only Puerto\nRico) are very consistent internally. This provides a very strong lafitudinal gradient that is of great interest and\nshould be explored further.\nValidity\nThe authors do an excellent job describing how their data fits with previous work in lake sediments,\natmospheric deposifion, recycling of GEM, implicafions for atmospheric Hg mass balances, etc. I have some\nconcerns about migrafion of Pb and other isotopes and how that impacts dafing of the cores that may need to\nbe addressed.\nThe corroborafion of mulfiple age models including 5 different metals as well as 14C gives us confidence that,\nto the first order and to such an extent to be an extremely useful method, we are measuring fundamental soil\nprocesses that very similarly impact a range of metals and carbon. That Hg is an outlier does in fact show that,\nin some fashion, it is preferenfially mobilized, but that is of course the crux of our paper... that we can use the\nage models to reconstruct Hg fluxes in some cases, and in others to highlight very important aspects of Hg\nenvironmental chemistry. We have added clarity to this point in responses to Reviewer #2 above.\nSignificance\nThe work described in this manuscript is highly significant. Following up on this work will lead to befter\nconstraints on Hg mass balances in the atmosphere and a befter understanding of how ecosystems may\nrespond to decreases in Hg deposifion. The work is certainly novel. Relying on 3 or 4 cores from the arcfic and\ntropical ecosystems is the primary troubling factor. However, even without the comparisons across\necosystems the results for the temperate soils are significant in the field of Hg research.\nWe agree that temperate soils are the focus, and the arcfic and tropical data, while limited, provide a strong\ngradient and important global perspecfive that at once is consistent with available deposifion data for these\nregions, but also highlights the need for new work in these locafions.\nData and methodology\nThe authors do an excellent job framing the findings and implicafions of the soil cores, parficularly relafive to\nreported atmospheric fluxes of Hg. Quesfions that I had after examining the figures were addressed clearly\nand impressively in the arficle. The examinafion of EF relafive to the accumulafion rates in the soil cores was\nquanfitafive and thorough.\nThe finding of some cores being low-accumulafing is difficult to explain and fies into concerns regarding the\ndafing of these soils. The authors offer plausible explanafions. However, if there is significant downward\ntransport of Hg via dissolved organic mafter (DOM), which is offered as potenfially explaining higher Hg\naccumulafion in deeper soil horizons, how can the dafing sfill be valid? Would not the 210Pb also be\ntransported downward and impact the dafing? Also, if the Pb and Hg (and C) did NOT behave the same with\ncolloidal transport, wouldn't this invalidate the dafing of the Hg accumulafion?\nThe fundamental challenge to using FRN for soil chronometer is understanding what it is that we are dafing.\nSoils are generally not aggradafional in the sense of sediment or peat, except perhaps in deep O horizons of\ncold climates, so the advecfion that we measure must be interpreted differently. The model demonstrates\nthat 210Pb migrates through soil at rates of a one to few mm y-1, which is an order of magnitude higher than\nsoil producfion, so must represent some DOM or colloidal process (described more in Landis et al. 2016).\nHowever, the concordance of mulfiple chronometers including 14C that we discuss in lines 80-90 is crifical for\nappreciafing that this migrafion process incorporates both parficle-reacfive metals and carbon itself. The\ncorrelafion between 210Pb and 14C first described by Dorr and Munich (1989) and now in a coming manuscript\nof ours for which data was provided in our Cover Lefter, confirm that many metals and carbon are migrafing\ntogether. Thus, the migrafion of 210Pb does not invalidate the method, it is in fact the basis of the method,\ninsofar as 210Pb provides a tracer of this fundamental soil process(es).\nThe real quesfion is, we believe, whether Hg and Pb will behave the same way. This is our crifical hypothesis,\nas posed in lines 93-96: we hypothesize that Hg and Pb will behave the same, since C, Pb, Be, Ra, and Th\ntracers appear to behave the same insofar as their age models are concordant. If, however, there is evidence\nHg does not, this becomes the point of acute interest and the secondary value of our method (the first being\nflux reconstrucfion). Our ability to see that in some coniferous forests Hg appears to be preferenfially\nmobilized is a key finding that points to a crifical need for more work in these environments. We address\nthese same points following the recommendafions of Reviewer 2 above.\nAnd furthermore, if there is downward movement of Hg there would also be lateral movement. Is this lateral\nmovement insignificant, i.e. how much dissolved organic mafter is transported out of each watershed? If\nDOM is being transported out there will be Hg associated with it. Is this transport negligible at the sites,\nespecially those low-accumulafing sites?\nWe agree that this is true. Our soils are collected in suitable reference sites where to every extent possible we\ntry to isolate atmospheric deposifion and minimize potenfial for lateral transport by sampling at hilltops and\nridgelines in locally flat locafions (now clarified in line 532 and more in Supporfing Informafion).\nNonetheless, we cannot rule it out, and this is now specified at lines 342-348 and shown in Figure S2. We also\ncite rates of Hg export that may be expected in associafion with DOM and how these compare with our Hg\nesfimates at lines 356-363.\nLateral subsurface flow is most likely the cause of high subsurface concentrafions in soils ELA302-2 (bedrock\ndepression, confined) and HoF (irregular fill and boulder confined). But we do have some tools such as\nconcordance of the LRC and 241Am models and Am/Pb inventories and rafios (Figure S1), and now, expected\nHg deposifion inventories, to help rule out these situafions. Truly it is extraordinary the degree to which these\nage models agree in dafing the bomb-pulse horizon (befter than 10%!).\nRather than a weakness of the dafing, as we have argued to Reviewer 2, this really is a key importance of it. In\nlocalifies where Hg inventories and fluxes do not conform to expectafion based on histories of Hg deposifion\nand regional soil inventories (smaller or larger), we can gain insight into processes that may be responsible.\nThis is the approach that highlights the need for befter understanding of Hg mobilizafion in coniferous forests\nsince they are likely at greater risk of Hg accumulafion in food webs (e.g., lines 355-358 and lines 367-369,\n426-428).\nAnalyfical Approach\nThe study is very strong in this regard. More cores would have been preferred, but it is not very pracfical to\nget a large value of n for this type of field work.\nThe Reviewer's understanding is much appreciated. The analyfical demands for soil dafing are extreme and\nthere simply is not enough instrumentafion available (anywhere). We have, to our knowledge, the largest\nacademic facility in the US in terms of the number of appropriate state-of-the-art gamma detectors (large,\nBroad-Energy style), and we sfill must count each sample for 4 days to provide reasonable uncertainfies for\nthe FRNs. A single soil pit requires over one month of 24/7 lab fime! We are trying very hard to double our\ncapacity, but this requires about $1.5M. Hopefully this publicafion will help sway the funding agencies!\nSuggested improvements\nThe authors should address the limitafions of the dafing methods and the implicafions of DOM transport of\nPb and other isotopes on the dafing methods.\nWe have addressed this per comments to Reviewer #2 above. We note too that we have described the\ninfluence of podzolizafion (DOM transport) at mulfiple places throughout the manuscript.\nClarity and context\nThis manuscript was excepfionally well-wriften and clearly states the implicafions of using this new method of\ndafing soil cores. I really enjoyed reading this arficle and am excited by the results presented.\nThank you for the careful read and the encouragement!\nReferences\nAppropriate\nMy experfise\nLake and peat coring are areas of experfise for me. I have always used 210Pb dafing and know some\nshortcomings of that method. The newer methods described here are not as familiar to me.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThis review is of the revised version of NCOMMS-24-07443A. The authors have done a good job of\nresponding to my previous review comments and suggesfions, as well as those of the other two\nreviewers. In my opinion, the manuscript is now ready for publicafion.\nReviewer #2 (Remarks to the Author):\nThe authors of the manuscript enfitled, “Quanfifying soil accumulafion of atmospheric mercury using\nfallout radionuclide chronometry” have responded to all of my comments and have made a clear effort\nto address all of the concerns raised. Although I think there are some quesfions sfill surrounding the\nmass balance aspect of their paper, I believe they now more clearly highlight areas of uncertainty in the\nuse of their novel tool. As menfioned previously, I think this work is of high caliber and will be an\nimportant addifion to the mercury literature. I thank the authors for their work and suggest acceptance\nof their manuscript for publicafion.\nBelow I have included a few minor edits the authors may choose to implement.\nIn many instances the citafions occur out of order numerically.\nLine 63: You may menfion here that the extent to which re-emissions occur have an important effect on\nthe surface lifefimes of mercury.\nLine 130: I’d suggest removing “horizons” for readability.\nLine 193: You might add the annualized value in brackets too, since up unfil now that has been the main\nformat of discussion.\nLine 339: add “the” to “…over past…”\nReviewer #3 (Remarks to the Author):\nMy concerns were fully met by the responses to the inifial review\nResponse to Revision, NCOMMS-24-07443\nWe are pleased to safisfy all remaining suggesfions of Reviewers and the Editor.\nReviewer #2 (Remarks to the Author):\nThe authors of the manuscript enfitled, “Quanfifying soil accumulafion of atmospheric mercury using\nfallout radionuclide chronometry” have responded to all of my comments and have made a clear effort\nto address all of the concerns raised. Although I think there are some quesfions sfill surrounding the\nmass balance aspect of their paper, I believe they now more clearly highlight areas of uncertainty in the\nuse of their novel tool. As menfioned previously, I think this work is of high caliber and will be an\nimportant addifion to the mercury literature. I thank the authors for their work and suggest acceptance\nof their manuscript for publicafion.\nBelow I have included a few minor edits the authors may choose to implement.\nIn many instances the citafions occur out of order numerically.\nFinal citafions have been corrected.\nLine 63: You may menfion here that the extent to which re-emissions occur have an important effect on\nthe surface lifefimes of mercury.\nWe have added this statement, \"This uncertainty raises fundamental quesfions about the long-\nterm efficacy of soils in sequestering legacy and contemporary Hg, the lifefimes of Hg in the\nsurface environment, and as a result about the fate of Hg in a context of a changing global\nenvironment 1,3,4,6.\nLine 130: I’d suggest removing “horizons” for readability.\nRevised accordingly.\nLine 193: You might add the annualized value in brackets too, since up unfil now that has been the main\nformat of discussion.\nValues are annualized.\nLine 339: add “the” to “…over past…”\nCorrected.\nReviewer #3 (Remarks to the Author):\nMy concerns were fully met by the responses to the inifial review"}]

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