Holocene atmospheric mercury levels. isotopes

Supporting information for Holocene atmospheric mercury levels reconstructed from peat bog mercury stable isotopes Maxime Enrico*,,, Gaël Le Roux, Lars-Eric Heimbürger,#, Pieter Van Beek, Marc Souhaut, Jérome Chmeleff, Jeroen E. Sonke *, ECOLAB, Université de Toulouse, CNRS, INPT, UPS ; ENSAT, Avenue de l Agrobiopole, 31326 Castanet Tolosan, France. Laboratoire Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, CNRS/IRD/Université Paul Sabatier Toulouse III, 14 avenue Edouard Belin, 31400 Toulouse, France. Laboratoire d Etudes en Géophysiques et Océanographie, Observatoire Midi-Pyrénées, CNRS/IRD/Université Paul Sabatier Toulouse III, 14 avenue Edouard Belin, 31400 Toulouse, France. *Correspondence to: enrico@get.obs-mip.fr, sonke@get.obs-mip.fr This pdf includes: - Pages S1 S32 - Supplementary text - Supplementary figures S1-S10 - Supplementary Tables S1-S8. S1

Age models Pinet and Estibere peat cores were dated using the 210 Pb constant rate of supply (CRS) model 1. Profiles of excess 210 Pb are shown in Figure S2. The age models for Pinet cores were validated using independent markers. Profiles of 137 Cs activity in Pinet cores are shown in Figure S3. Only core B shows a well-defined peak in 137 Cs, which corresponds to a CRS model age range of 1981 1986, and is therefore highly consistent with 137 Cs emission during the Chernobyl accident in 1986. The activity of 241 Am was below the detection limits for all samples from core B. Significant 241 Am activity was detected in one sample from core A (CRS age of 1950 to 1972) and two samples from core C (CRS age of 1916 to 1972), which is consistent with 241 Am releases by nuclear weapon tests in the 60 s. Five samples from Pinet core A were dated using bomb pulse radiocarbon. The ages obtained are shown in Figure S4, and align well with the 210 Pb CRS age model. Although the 210 Pb CRS age models from the three Pinet peat cores agree well for the last 50-60 years, the chronology obtained for core B for the older peat layers differ significantly from the other two cores (Figure S4). Profiles of Pb stable isotope plotted against cumulative peat mass are very consistent between the three Pinet peat cores (Figure S5). This suggests that peat accumulation was similar for the three cores despite the differences observed in the CRS age models (Figure S4). This leads to differences in Pb stable isotope chronology for core B compared to cores A and C (Figure S6). This pattern is attributed to the significant excess 210 Pb activities found in the deeper peat layers of core B than for cores A and C (Figure S2). Ignoring the 210 Pb data from the deepest peat samples from core B (B26 to B34, see Table S2) consistently aligns the age model with cores A and C (Figure S4). This makes the age model more coherent with the first conventional radiocarbon date (Figure S4), and corrects well the Pb stable isotope chronology (Figure S6). We suggest that deep peat samples from core B suffered from cross-contamination during grinding using a blender. S2

In addition to 210 Pb CRS dating, conventional radiocarbon dates were obtained for three samples from Pinet core A and five from Pinet core B. One single radiocarbon date was used for Estibere age model. The age models obtained using the Bacon package for R software 2 are shown in Figure S7. All the radiocarbon dates (from cores A and B) with corresponding cumulative peat masses were combined to build the age model for core C (Figure S7). We observe constant peat accumulation in deeper peat layers corresponding to the first 6000 years of peat accumulation (10,000 4000 years BP), followed by a smooth transition to a period with lower peat accumulation rates. The low peat accumulation rate is attributed to higher peat decomposition, as shown by the increase in peat bulk density (Figure S7). Concerning Hg accumulation, this low peat accumulation rate is characterized by higher Hg concentrations (Figure S7), but consequently stable HgARs. Mastercore construction The chronologies of cumulative Hg inventories in the three Pinet peat cores are in very good agreement (Figure S8). In order to build one single record using these three peat cores, we smoothed cumulative Hg inventories from the three Pinet peat cores with ages (Figure S8). The mastercore HgAR is then calculated as the derivative of the smoothed curve. The uncertainties in HgAR are calculated as the difference between mastercore HgAR and individual core HgARs. Hg stable isotopes in peat Hg accumulation in peat occurs predominantly via Hg wet deposition and GEM uptake by vegetation (dry deposition) 3. The isotope signatures of Hg are helpful for quantifying the contribution of both deposition mechanisms. The peat MDF signature, δ 202 Hg, is shifted to S3

lower values due to the preferential uptake of light Hg isotopes during GEM uptake by sphagnum 3, 4. The mixing between GEM dry deposition and Hg wet deposition also contributes to variations in peat δ 202 Hg. Odd-Hg isotope anomalies ( 199 Hg, 201 Hg), driven by photochemical reactions during Hg cycling, are thought to be conservative during GEM uptake by vegetation and during Hg wet deposition. They are however dependent on the respective contribution of these two deposition pathways, and potentially on post-depositional photochemical Hg re-emission from sphagnum surfaces or cells. The variations in anthropogenic emissions and sources can affect the atmospheric 199 Hg (both wet deposition and GEM) as well. Even Hg isotope anomalies ( 200 Hg, 204 Hg) are the most useful signatures to estimate Hg deposition pathways. MIF of even Hg isotopes is even more restrictive than odd-isotope MIF, and is thought to occur only in the upper troposphere and stratosphere during GEM oxidation 5. Emission sources of Hg, both natural and anthropogenic, do not display significant even Hg isotope anomalies ( 200 Hg = 0) 6. Then, the peat 200 Hg signature only reflects the mixing between GEM uptake and Hg wet deposition, which have significantly different 200 Hg signatures (Figures 2, S9) 3. The Hg isotope signatures of GEM and wet deposition in the Pyrenees were determined previously 3. Peat 200 Hg (and 204 Hg) falls in between the signatures of GEM and wet deposition (Figure 2 in main text and Figure S9). An inverse relationship is found between 200 Hg and 204 Hg (Figure S9), similar to previously reported data on atmospheric samples 3, 4. Data of 204 Hg however suffer from higher analytical uncertainty, and peat 204 Hg was not measured in all samples. We therefore used 200 Hg only for the mass-balance between GEM uptake and Hg wet deposition. Hg isotope mass balance S4

Using a 200 Hg-based mass balance, we estimated the respective contributions of Hg wet deposition and GEM uptake to Hg accumulation in peat from Estibere and Pinet. = + (equation S1) + =1 (equation S2) This mass balance calculation assumes that 200 Hg signatures of GEM and Hg wet deposition remained constant over the Holocene. Based on the insignificant 200 Hg of all known primary Hg emission sources 6, 7 and the 200 Hg profile of Estibere showing constant peat 200 Hg (Figure 1), this assumption is thought to be reasonable. The evolution of Pinet peat 200 Hg shown in Figure 1 more likely reflects a change in the wet/dry deposition balance. The decrease in Pinet peat 200 Hg parallels the increase in HgAR since the 70 s. It is also reflected in peat 199 Hg which decreases during this period (Figure 1), and is consistent with a higher contribution from GEM uptake. As mentioned in the main text, the associated peak in HgAR at Pinet in the period 1971 2001 is late compared to Estibere and other European records (1945 1975, n = 13) 8. The combination of 200 Hg and HgAR argues for an increase in GEM uptake rate by plants rather than enhanced Hg wet deposition or GEM concentration. On average, we find that the GEM uptake contribution to HgAR was as high as 77 ± 8 % in Pinet bog and 57 ± 8 % in Estibere. The lower GEM uptake contribution at Estibere (and thus higher Hg wet deposition contribution) can indicate either that Hg wet deposition is higher and/or that the GEM uptake rate is lower. Both possibilities are compatible with the high altitude, higher wet deposition and shorter growing season at Estibere (2100m) compared to Pinet (880m). S5

Past Hg wet deposition and GEM dry deposition The GEM dry and Hg wet deposition contribution we found using the 200 Hg mass balance were applied to the calculated HgAR in order to reconstruct past Hg wet deposition (HgAR dry ) and past GEM dry deposition (HgAR wet ). = (equation S3) = (equation S4) The evolution of HgAR wet and HgAR dry for Estibere record are similar to HgAR total because the contributions of GEM uptake and Hg wet deposition did not vary with time. For Pinet record, we find that GEM dry deposition alone explains the late peak in HgAR beginning around 1970. While the uncertainty is relatively high, we observe that there is no increase in HgAR wet for this period (Figure S10). We can even distinguish a decrease in HgAR wet down to very low values, but this derives more likely from the uncertainties in the calculations of HgARs and 200 Hg isotope mass-balance. The increase in Pinet HgAR dry over the last 40 years probably results from an ecological disturbance. The peat bog was drained since the 1970 s by a central 10m wide ditch, located 75m from the coring sites, which likely affected the ecosystem. GEM dry deposition is dependent on two different parameters, which are atmospheric GEM concentration and GEM uptake velocity (also called GEM dry deposition velocity, V GEM in cm s -1 ). = ( ) (equation S5) While there is no indication of any large Hg emission point source nearby the Pinet bog during the last 40 years, the GEM dry deposition velocity could have been affected by the drainage since 1970. GEM deposition velocity is suspected to change with biomass S6

productivity. Any increase in productivity would increase leaf surface per ground surface unit, allowing increased gas exchange (i.e. GEM uptake) with the atmosphere. Other factors could also modify GEM deposition velocity but our knowledge on GEM uptake by vegetation is still limited. Reconstruction of past GEM concentration We used equation (5) to reconstruct past atmospheric GEM concentrations from the Estibere and Pinet peat cores. This requires the knowledge of present-day V GEM and the assumption of constant V GEM with time. Present-day V GEM can be estimated using recent HgAR dry reconstructed from the peat cores (31 ± 1 and 5.2 ± 2.0 µg m -2 y -1 for Pinet and Estibere respectively) and European GEM concentrations obtained from monitoring data (1.5 ± 0.3 ng m -3 ) 9. We find recent V GEM of 0.064 ± 0.005 and 0.011 ± 0.003 cm s -1 for Pinet and Estibere respectively. The next step consists in applying this V GEM to older reconstructed HgAR dry in order to calculate past GEM concentration, still using equation (5). The Pinet record does not however satisfy the requirement of constant V GEM, as we discussed above how the drainage of the peat bog led to higher V GEM during the last 40 years. Without the knowledge of V GEM, it is therefore impossible to reconstruct past GEM concentration using the Pinet record. In Estibere record, we found no evidence for such change in V GEM (no variation in peat 200 Hg and no delayed peak in HgAR). We therefore assume that V GEM was constant over time, and reconstructed past GEM concentrations (from 800 CE to 2011 CE) using Estibere record. In order to access to pre-anthropogenic times, we attempted to estimate V GEM in Pinet for peat layers pre-dating the ecological disturbance. As for recent times, this requires the S7

knowledge of GEM concentration and HgAR dry. We therefore used Pinet HgAR dry reconstructed for the period 800 1970 (pre-disturbance) and reconstructed GEM concentration from Estibere record for the same period. This gave a pre-disturbance V GEM of 0.012 ± 0.005 cm s -1 at Pinet. Applying this V GEM to pre-anthropogenic times HgAR dry (1.1 ± 0.4 µg m -2 y -1 for the period 8000 1000 BCE), for which peat 200 Hg (0.02 ± 0.04, 1σ, n = 76) is similar to 800 1970 peat (0.02 ± 0.03, 1σ, n = 36), gives an estimate of preanthropogenic atmospheric GEM concentration of 0.30 ± 0.13 ng m -3. S8

Supplementary figures Fig. S1. Location of the Pinet peat bog and Estibere peatland (map data: Google, Landsat / Copernicus) Fig. S2. 210 Pb profiles of Pinet and Estibere peat cores. Fig. S3. 137 Cs and 241 Am profiles in Pinet peat cores. Fig. S4. Comparison of 210 Pb CRS age model with radiocarbon dates in Pinet peat cores. The left panel shows the 210 Pb and post-bomb radiocarbon ages for the last 260 years while the right panel extends back to the first conventional radiocarbon date (from core B). Fig. S5. Profiles of Pb stable isotopes in Pinet peat cores as a function of cumulative peat mass. Fig. S6. Lead isotope chronology ( 206 Pb/ 207 Pb on the upper panels and 208 Pb/ 206 Pb on the lower panels) reconstructed using Pinet peat cores and 210 Pb CRS model (left panels) and after correcting the CRS age model for core B (right panels). Fig. S7. 210 Pb and radiocarbon age models computed using Bacon package for R software, density, Hg concentration. Fig. S8. Cumulative Hg inventories (upper panel) and HgAR (lower panel) vs. time at the Pinet peat bog. The red line is the smoothed, weighted average of the three Pinet cores, and the red shaded area represents 1σ uncertainty. Red circles, triangles and squares stand for samples from Pinet peat cores A, B and C respectively. Fig. S9. Even Hg isotope anomalies in peat samples from Pinet and Estibere, wet deposition and GEM from the Pyrenees. 200 Hg (A) and 204 Hg (B) plotted against δ 202 Hg, and the inverse relationship between 200 Hg and 204 Hg (C). Rainfall and GEM data were taken from Enrico et al. (2016) 3. Fig. S10. Reconstructed HgAR wet and HgAR dry for Estibere (blue) and Pinet (red) peat cores. S9

Supplementary Tables Table S1. Results of radionuclides measurements ( 210 Pb, 137 Cs, 241 Am), inferred CRS model ages, radiocarbon bomb pulse datings and Pb stable isotope composition of peat samples from Pinet core A. Table S2. Results of radionuclides measurements ( 210 Pb, 137 Cs, 241 Am), inferred CRS model ages, peat Pb stable isotope composition and corrected CRS model for core B (see supplementary text). Sample B40 was dated using conventional radiocarbon dating. Table S3. Results of radionuclides measurements ( 210 Pb, 137 Cs, 241 Am), inferred CRS model ages, and Pb stable isotope composition of peat samples from Pinet core C. Table S4. Results of radionuclides measurements ( 210 Pb, 137 Cs, 241 Am) and inferred CRS model ages for Estibere peat core. Table S5. Peat Hg concentrations and Hg stable isotope signatures of Pinet core A. Table S6. Peat Hg concentrations and Hg stable isotope signatures of Pinet core B. Table S7. Peat Hg concentrations and Hg stable isotope signatures of Pinet core C. Table S8. Peat Hg concentrations and Hg stable isotope signatures of Estibere peat core. S10

Fig. S1. S11

Fig. S2. S12

Fig. S3. S13

Fig. S4. S14

Fig. S5. S15

Fig. S6. S16

Fig. S7. S17

Fig. S8. S18

Fig. S9. S19

Fig. S10. S20

Table S1. sample top depth cumulative peat mass thickness bulk density 210 Pbex activity 137 Cs activity 241 Am activity CRS age (mid-point) radiocarbon age Mass AR 206 Pb/ 207 Pb cm kg m -2 cm g cm -3 Bq kg -1 ± Bq kg -1 ± Bq kg -1 ± yrs CE ± yrs CE kg m -2 yr -1 A0 0.0 0.0 4.2 0.05 391 16 242 4 2009 1 0.62 A1 4.2 2.3 1.1 0.05 413 58 62 7 2007 5 0.55 A2 5.4 2.9 1.0 0.08 373 54 57 6 2005 4 0.69 A3 6.4 3.7 1.0 0.07 315 45 46 5 2004 4 0.53 1.162 2.096 A4 7.4 4.5 1.1 0.08 456 46 47 5 2003 4 0.54 1.160 2.100 A5 8.6 5.4 1.4 0.07 492 48 54 5 2000 4 0.39 1.147 2.117 A6 10.0 6.3 1.5 0.07 375 39 23 3 1998 4 0.39 1.148 2.113 A7 11.6 7.4 1.4 0.09 537 36 39 3 1995 4 1997-2000 0.38 1.148 2.114 A8 13.0 8.7 1.4 0.08 175 29 44 4 1992 4 0.57 1.150 2.116 A9 14.4 9.9 1.4 0.10 359 32 39 3 1989 4 0.36 1.154 2.113 A10 15.9 11.3 1.4 0.12 486 36 48 4 1983 4 1992-1995 0.25 1.156 2.110 A11 17.3 13.1 1.3 0.12 457 36 48 4 1976 4 1984-1987 0.19 1.153 2.115 A12 18.6 14.7 1.4 0.13 450 25 71 3 43 11 1965 4 1956-1957 0.14 1.151 2.117 A13 20.1 16.5 1.3 0.12 431 36 48 4 1949 4 1955-1957 0.09 1.151 2.116 A14 21.4 18.2 1.5 0.11 312 28 47 3 1928 4 1954-1956 0.07 1.158 2.109 A15 22.9 19.9 1.4 0.15 207 22 47 3 1904 6 0.09 A16 24.4 22.0 1.5 0.17 69 13 47 3 1.174 2.099 A17 25.9 24.6 1.5 0.19 0 0 32 2 208 Pb/ 206 Pb S21

Table S2. sample top depth cumulative peat mass thickness bulk density 210 Pbex activity 137 Cs activity 241 Am activity CRS age (Mid-point) Mass AR 206 Pb/ 207 Pb 208 Pb/ 206 Pb corrected CRS age Mass AR cm kg m -2 cm g cm -3 Bq kg -1 ± Bq kg -1 ± Bq kg -1 ± yrs CE ± kg m -2 yr -1 yrs CE ± kg m -2 yr -1 B0 0.0 0.0 4.0 0.03 342 13 325 4 2010 1 0.60 2009 1 0.53 B1 4.0 1.4 1.0 0.03 234 23 190 7 2008 5 0.66 1.166 2.109 2008 5 0.61 B2 5.0 1.7 0.9 0.04 319 23 160 6 2008 4 0.68 1.156 2.106 2007 3 0.55 B3 5.9 2.0 0.9 0.05 268 20 155 6 2007 3 0.77 1.159 2.104 2007 3 0.49 B4 6.8 2.5 0.9 0.05 288 21 105 5 2007 4 0.80 1.167 2.096 2006 3 0.66 B5 7.7 3.0 1.0 0.04 248 23 69 4 2006 4 0.60 1.158 2.096 2005 3 0.56 B6 8.7 3.3 0.9 0.04 310 24 52 4 2005 4 0.54 1.155 2.108 2004 3 0.49 B7 9.6 3.7 0.8 0.04 373 25 44 4 2005 4 0.48 1.157 2.110 2004 3 0.34 B8 10.4 4.0 1.0 0.06 386 25 48 4 2004 4 0.46 2002 3 0.40 B9 11.4 4.6 1.0 0.06 298 21 36 3 2003 4 0.53 1.149 2.116 2001 3 0.46 B10 12.4 5.2 1.0 0.07 242 19 41 3 2001 4 0.67 1.152 2.111 2000 3 0.51 B11 13.4 5.9 1.0 0.07 219 16 44 3 2001 4 0.72 1.154 2.113 1998 3 0.56 B12 14.4 6.5 0.8 0.08 202 15 41 3 2000 4 0.79 1.157 2.111 1997 3 0.76 B13 15.2 7.1 1.0 0.06 190 13 47 3 1999 3 0.69 1.154 2.112 1996 3 0.56 B14 16.2 7.8 1.0 0.17 165 12 45 2 1997 4 0.77 1.150 2.110 1994 3 0.57 B15 17.2 9.5 0.9 0.14 392 15 103 3 1994 3 0.38 1.152 2.108 1991 3 0.29 B16 18.1 10.8 1.0 0.12 289 13 69 3 1991 4 0.35 1.149 2.113 1986 3 0.26 B17 19.1 12.0 1.0 0.11 342 12 93 3 1988 4 0.32 1.152 2.116 1982 3 0.23 B18 20.1 13.0 1.1 0.09 373 14 163 4 1984 4 0.24 1.143 2.126 1976 3 0.15 B19 21.2 14.1 1.1 0.11 359 14 126 3 1979 4 0.24 1.146 2.116 1968 3 0.15 B20 22.3 15.3 1.0 0.11 219 12 52 2 1975 4 0.28 1.152 2.115 1961 4 0.17 B21 23.3 16.4 1.1 0.10 246 8 1 1 1970 4 0.25 1.146 2.118 1953 3 0.13 B22 24.4 17.5 1.1 0.09 312 16 78 3 1965 4 0.16 1.154 2.110 1940 3 0.06 B23 25.5 18.5 1.0 0.12 252 12 69 2 1958 4 0.18 1.162 2.105 1918 4 0.05 B24 26.5 19.8 1.1 0.10 191 13 63 3 1952 5 0.16 1.165 2.105 1890 5 0.04 B25 27.6 20.9 1.1 0.12 168 12 55 3 1944 5 0.15 1.167 2.101 B26 28.7 22.2 1.1 0.17 133 10 51 2 1933 5 0.15 1.171 2.096 B27 29.8 24.1 1.1 0.17 107 7 52 2 1919 5 0.11 1.171 2.099 B28 30.9 26.0 1.1 0.21 57 5 49 2 1902 7 0.13 1.178 2.088 B29 32 28.2 1.1 0.20 35 4 39 1 1884 9 0.12 1.173 2.096 B30 33.1 30.4 1.1 0.22 21 3 27 1 1861 11 0.09 1.177 2.096 B31 34.2 32.8 1.1 0.21 14 3 17 1 1830 16 0.06 1.183 2.088 B32 35.3 35.2 1.1 0.22 5 3 16 1 1787 33 0.05 1.180 2.097 B33 36.4 37.6 1.1 0.17 1 2 15 1 1740 78 0.04 1.180 2.092 B34 37.5 39.4 1.1 0.22 7 3 12 1 B35 38.6 41.9 1.2 0.18 13 1 1.176 2.096 S22

B36 39.8 44.0 0.8 0.24 14 1 1.178 2.089 B37 40.6 45.9 1.3 0.13 11 1 1.178 2.093 B38 41.9 47.6 1.2 0.22 6 1 1.182 2.092 B39 43.1 50.2 1 0.21 6 1 B40 44.1 52.3 1.3 0.16 5 1 394 134 0.01 394 134 0.02 S23

Table S3. sample top depth cumulative peat mass thickness bulk density 210 Pbex activity 137 Cs activity 241 Am activity CRS age (mid-point) Mass AR 206 Pb/ 207 Pb 208 Pb/ 206 Pb cm kg m -2 cm g cm -3 Bq kg -1 ± Bq kg -1 ± Bq kg -1 ± yrs CE ± kg m -2 yr -1 C1 0.0 0.0 2.7 0.03 417 42 182 8 2010 1 0.82 C2 2.7 0.8 1.5 0.06 629 72 191 12 2009 5 0.55 C3 4.3 1.7 1.2 0.05 403 55 123 9 2008 4 0.74 C4 5.5 2.2 1.6 0.04 391 51 153 9 2007 4 1.02 C5 7.1 2.9 1.5 0.03 269 44 137 9 2007 4 0.90 C6 8.7 3.4 1.5 0.05 392 51 136 9 2006 5 0.85 1.161 2.107 C7 10.2 4.1 1.5 0.05 542 55 143 8 2005 4 0.64 C8 11.7 4.9 1.6 0.07 560 52 160 8 2003 4 0.54 1.159 2.107 C9 13.3 6.0 1.7 0.05 587 48 129 7 2001 5 0.44 1.158 2.117 C10 15.1 6.8 1.3 0.05 220 44 143 10 2000 4 0.83 1.155 2.108 C11 16.4 7.5 1.2 0.09 375 52 155 10 1999 4 1.13 C12 17.6 8.6 1.3 0.06 313 48 75 7 1998 4 0.54 1.153 2.114 C13 19.0 9.4 1.3 0.08 616 45 136 6 1995 4 0.35 1.152 2.115 C14 20.3 10.4 1.4 0.14 671 43 151 6 1991 4 0.39 1.150 2.113 C15 21.7 12.5 1.5 0.10 546 39 126 6 1986 4 0.26 1.151 2.117 C16 23.3 14.0 1.6 0.09 1979 5 0.19 1.152 2.114 C17 24.9 15.5 1.6 0.12 784 45 99 5 69 18 1967 5 0.13 1.151 2.115 C18 26.5 17.4 1.7 0.10 1950 5 0.08 1.158 2.108 C19 28.2 19.1 1.6 0.11 384 34 110 6 96 26 1927 6 0.07 1.163 2.107 C20 29.9 20.8 1.8 0.12 1896 7 0.06 1.165 2.105 C21 31.7 23.0 1.4 0.15 48 11 85 4 1864 10 0.08 1.172 2.101 C22 33.1 25.2 1.4 0.14 1835 18 0.07 1.171 2.101 C23 34.6 27.2 1.4 0.17 22 6 68 4 1.174 2.097 C24 36.0 29.7 1.4 0.16 1.177 2.093 C25 37.4 32.0 1.4 0.17 0 0 36 3 S24

Table S4. sample top depth cumulative peat mass thickness bulk density 210 Pbex activity 137 Cs activity 241 Am activity CRS age (mid-point) Mass AR cm kg m -2 cm g cm -3 Bq kg -1 ± Bq kg -1 ± Bq kg -1 ± yrs CE ± kg m -2 yr -1 EST1 0.0 0.0 2.2 0.03 495 74 39 6 2009 1 0.206 EST2 2.2 1.8 1.3 0.03 183 38 8 3 2008 5 0.488 EST3 3.6 2.1 1.3 0.03 272 23 31 2 2007 5 0.418 EST4 4.9 2.5 1.4 0.03 388 28 34 3 2006 5 0.357 EST5 6.3 2.9 1.4 0.02 417 26 30 2 2005 5 0.175 EST6 7.8 3.2 1.4 0.01 261 23 38 2 2004 5 0.249 EST7 9.2 3.4 1.4 0.02 382 30 31 2 2003 5 0.476 EST8 10.6 3.7 1.3 0.01 269 20 35 2 32 11 2002 5 0.247 EST9 12.0 3.9 1.4 0.02 2001 5 0.150 EST10 13.4 4.1 1.3 0.06 353 10 55 1 1999 5 0.263 EST11 14.7 4.9 1.4 0.06 1995 5 0.235 EST12 16.2 5.8 1.3 0.08 1991 5 0.276 EST13 17.5 6.9 1.4 0.07 1987 5 0.181 EST14 18.9 7.9 1.4 0.08 280 10 112 2 1980 5 0.170 EST15 20.4 9.0 1.5 0.09 1973 5 0.175 EST16 21.9 10.4 1.5 0.11 1965 6 0.181 EST17 23.4 12.0 1.5 0.13 1953 5 0.175 EST18 25.0 14.0 1.4 0.12 140 6 78 1 28 4 1937 5 0.107 EST19 26.4 15.7 1.4 0.13 1914 5 0.080 EST20 27.8 17.6 1.4 0.15 1886 7 EST21 29.3 19.7 1.4 0.15 EST22 30.7 21.8 1.4 0.15 0 0 15 0 S25

Table S5. core sample depth depth cumulative peat mass density c(hg) modeled age δ 202 Hg Δ 199 Hg Δ 200 Hg Δ 201 Hg Δ 204 Hg top, cm mid-point, cm top, kg m -2 g cm -3 ng g -1 yrs AD/BC A 0 0.0 2.1 0.0 0.05 50 2009 A 1 4.2 4.8 2.3 0.05 49 2007-1.57-0.20 0.00-0.23 0.00 A 2 5.4 5.9 2.9 0.08 48 2005 A 3 6.4 6.9 3.7 0.07 55 2004 A 4 7.4 8.0 4.5 0.08 52 2003-1.18-0.21-0.02-0.23 0.11 A 5 8.6 9.3 5.4 0.07 82 2000-1.51-0.30 0.00-0.29-0.02 A 6 10.0 10.8 6.3 0.07 122 1998-1.56-0.32-0.04-0.29 0.11 A 7 11.6 12.3 7.4 0.09 130 1995-1.43-0.25-0.05-0.28 0.06 A 8 13.0 13.7 8.7 0.08 116 1992-1.37-0.33-0.06-0.32 0.08 A 9 14.4 15.1 9.9 0.10 130 1989-1.59-0.31-0.05-0.34 0.03 A 10 15.9 16.6 11.3 0.12 147 1983-1.61-0.22-0.03-0.20 0.06 A 11 17.3 18.0 13.1 0.12 174 1976-1.54-0.19-0.01-0.23 0.06 A 12 18.6 19.4 14.7 0.13 209 1965-1.43-0.22 0.01-0.22 0.00 A 13 20.1 20.7 16.5 0.12 162 1949-1.58-0.14-0.05-0.27-0.01 A 14 21.4 22.2 18.2 0.11 149 1928-1.53-0.17 0.03-0.23-0.01 A 15 22.9 23.7 19.9 0.15 202 1904 A 16 24.4 25.1 22.0 0.17 218 1871-1.57-0.20 0.03-0.22 0.05 A 17 25.9 26.7 24.6 0.19 146 1766-1.94-0.26 0.01-0.31-0.01 A 18 27.5 28.2 27.5 0.25 128 1563-1.71-0.34 0.01-0.36-0.03 A 19 28.9 29.7 31.1 0.23 111 1329-1.91-0.31 0.03-0.35-0.05 A 20 30.4 31.2 34.7 0.28 110 1079-2.08-0.36-0.01-0.34 0.03 A 21 32.0 32.7 38.9 0.24 101 824 A 22 33.5 34.3 42.5 0.22 90 592-1.55-0.32 0.06-0.37-0.09 A 23 35.0 35.8 45.9 0.18 59 389 A 24 36.7 37.4 48.8 0.17 76 216 A 25 38.2 39.1 51.4 0.20 96 26 A 26 39.9 40.6 54.8 0.23 144-183 A 27 41.3 41.9 57.9 0.22 192-379 A 28 42.6 43.3 60.8 0.20 218-565 -1.30-0.48-0.02-0.50 0.08 S26

Table S6. core sample depth depth cumulative peat mass density c(hg) modeled age δ 202 Hg Δ 199 Hg Δ 200 Hg Δ 201 Hg Δ 204 Hg top, cm mid-point, cm top, kg m -2 g cm -3 ng g -1 yrs AD/BC B 0 0.0 2.0 0.0 0.03 49 2009 B 1 4.0 4.5 0.3 0.03 44 2008 B 2 5.0 5.5 0.7 0.04 34 2007 B 3 5.9 6.4 1.1 0.05 35 2007-1.34-0.19-0.01-0.25 0.00 B 4 6.8 7.3 1.6 0.05 39 2006 B 5 7.7 8.2 2.0 0.04 45 2005-1.31-0.25-0.04-0.27 0.10 B 6 8.7 9.2 2.3 0.04 37 2004 B 7 9.6 10.0 2.7 0.04 45 2004-1.25-0.26 0.01-0.27-0.04 B 8 10.4 10.9 3.3 0.06 49 2002 B 9 11.4 11.9 3.8 0.06 53 2001-1.20-0.24 0.01-0.19 0.02 B 10 12.4 12.9 4.5 0.07 53 2000-1.14-0.23-0.01-0.27 0.04 B 11 13.4 13.9 5.2 0.07 64 1998-1.23-0.20 0.04-0.28-0.05 B 12 14.4 14.8 5.8 0.08 107 1997-1.44-0.26-0.01-0.26 0.02 B 13 15.2 15.7 6.4 0.06 102 1996-1.38-0.23-0.02-0.32 B 14 16.2 16.7 8.1 0.17 117 1994-1.68-0.25-0.04-0.30 0.03 B 15 17.2 17.7 9.4 0.14 119 1991-1.47-0.26-0.01-0.26 0.04 B 16 18.1 18.6 10.6 0.12 139 1986-1.56-0.28-0.03-0.25 0.06 B 17 19.1 19.6 11.7 0.11 169 1982-1.25-0.12-0.04-0.17 B 18 20.1 20.7 12.7 0.09 158 1976-1.45-0.21-0.06-0.24 0.03 B 19 21.2 21.8 13.9 0.11 165 1968-1.49-0.21 0.01-0.23-0.02 B 20 22.3 22.8 15.1 0.11 176 1961-1.50-0.16-0.01-0.17-0.02 B 21 23.3 23.9 16.2 0.10 168 1953 B 22 24.4 25.0 17.2 0.09 184 1940-1.67-0.16-0.01-0.18 0.02 B 23 25.5 26.0 18.4 0.12 159 1918-1.71-0.13 0.01-0.15 0.04 B 24 26.5 27.1 19.5 0.10 220 1890-1.86-0.16 0.02-0.22 0.10 B 25 27.6 28.2 20.8 0.12 118 1849-1.71-0.22 0.01-0.22 0.00 B 26 28.7 29.3 22.7 0.17 128 1781-1.73-0.30 0.03-0.32 0.04 B 27 29.8 30.4 24.6 0.17 179 1689-1.63-0.29 0.02-0.28-0.13 B 28 30.9 31.5 26.9 0.21 108 1591-1.62-0.28 0.03-0.34-0.05 B 29 32.0 32.6 29.0 0.20 110 1489-1.70-0.37 0.02-0.35 0.01 B 30 33.1 33.7 31.5 0.22 104 1378-1.70-0.28 0.05-0.31-0.09 B 31 34.2 34.8 33.8 0.21 104 1266-1.99-0.29 0.00-0.31 0.00 B 32 35.3 35.9 36.3 0.22 107 1168-1.60-0.28 0.04-0.29 0.08 B 33 36.4 37.0 38.1 0.17 109 1071-1.65-0.22 0.03-0.30-0.08 B 34 37.5 38.1 40.5 0.22 114 963-1.58-0.28 0.01-0.29 B 35 38.6 39.2 42.6 0.18 92 864-1.49-0.28 0.07-0.31-0.10 S27

B 36 39.8 40.2 44.6 0.24 84 781-1.65-0.34 0.02-0.37 0.02 B 37 40.6 41.3 46.2 0.13 82 687-1.61-0.37 0.00-0.40-0.02 B 38 41.9 42.5 48.9 0.22 90 580 B 39 43.1 43.6 50.9 0.21 93 479 B 40 44.1 44.8 53.0 0.16 124 379 B 41 45.4 45.9 54.8 0.18 137 168-1.16-0.49-0.01-0.42 B 42 46.4 47.0 57.1 0.19 133-135 B 43 47.6 48.2 58.9 0.16 102-396 B 44 48.7 49.3 60.4 0.14 190-628 -0.64-0.31 0.00-0.37 S28

Table S7. core sample depth depth cumulative peat mass density c(hg) modeled age δ 202 Hg Δ 199 Hg Δ 200 Hg Δ 201 Hg Δ 204 Hg top, cm mid-point, cm top, kg m -2 g cm -3 ng g -1 yrs AD/BC C 1 0.0 1.4 0.0 0.03 52 2010 C 2 2.7 3.5 0.8 0.06 55 2009 C 3 4.3 4.9 1.7 0.05 55 2008 C 4 5.5 6.3 2.2 0.04 57 2007 C 5 7.1 7.9 2.9 0.03 56 2006 C 6 8.7 9.4 3.4 0.05 94 2006 C 7 10.2 10.9 4.1 0.05 61 2005 C 8 11.7 12.5 4.9 0.07 60 2003-1.36-0.19-0.02-0.21 0.00 C 9 13.3 14.2 6.0 0.05 56 2001-1.15-0.16 0.05-0.18-0.11 C 10 15.1 15.7 6.8 0.05 53 2000 C 11 16.4 17.0 7.5 0.09 56 1999-1.33-0.12-0.01-0.19-0.01 C 12 17.6 18.3 8.6 0.06 56 1997-1.90-0.08 0.04-0.15-0.02 C 13 19.0 19.6 9.4 0.08 98 1995-1.86-0.24-0.04-0.27-0.03 C 14 20.3 21.0 10.4 0.14 122 1991-1.20-0.18-0.05-0.28 0.02 C 15 21.7 22.5 12.5 0.10 173 1986-1.35-0.20-0.04-0.26 0.07 C 16 23.3 24.1 14.0 0.09 238 1980-1.80-0.25-0.02-0.27 0.07 C 17 24.9 25.7 15.5 0.12 183 1967-1.36-0.18-0.06-0.21 0.04 C 18 26.5 27.4 17.4 0.10 213 1950-1.59-0.17 0.02-0.21-0.02 C 19 28.2 29.1 19.1 0.11 205 1931-1.58-0.13 0.04-0.19-0.12 C 20 29.9 30.8 20.8 0.12 200 1902-1.81-0.17 0.03-0.20-0.01 C 21 31.7 32.4 23.0 0.15 187 1865-1.70-0.25-0.01-0.29-0.11 C 22 33.1 33.8 25.2 0.14 194 1827-1.83-0.20 0.03-0.24 0.01 C 23 34.6 35.3 27.2 0.17 140 1782-1.90-0.28 0.02-0.32-0.03 C 24 36.0 36.7 29.7 0.16 106 1699-2.00-0.30 0.05-0.30 0.01 C 25 37.4 38.1 32.0 0.17 93 1577-2.02-0.28 0.02-0.33 0.04 C 26 38.9 39.5 34.4 0.17 78 1449-1.62-0.34 0.01-0.38-0.13 C 27 40.2 40.8 36.7 0.17 76 1320-1.84-0.36-0.03-0.39-0.06 C 28 41.5 42.2 38.9 0.15 62 1190-1.80-0.38 0.01-0.36 0.00 C 29 42.9 43.6 41.1 0.19 59 1052 C 30 44.2 44.9 43.5 0.19 75 898 C 31 45.6 46.3 46.3 0.23 90 731 C 32 46.9 47.7 49.3 0.24 123 548 C 33 48.4 49.1 52.7 0.21 146 340 C 34 49.8 50.5 55.7 0.22 158 43 C 35 51.1 51.8 58.6 0.19 185-358 C 36 52.6 53.3 61.4 0.19 234-819 S29

Table S8. core sample depth depth cumulative peat mass density c(hg) modeled age δ 202 Hg Δ 199 Hg Δ 200 Hg Δ 201 Hg Δ 204 Hg top, cm mid-point, cm top, kg m -2 g cm -3 ng g -1 yrs AD/BC EST 1 0.0 1.1 0.00 0.03 34 2011 EST 2 2.2 2.9 0.57 0.03 32 2008 EST 3 3.6 4.2 2.12 0.03 29 2008 EST 4 4.9 5.6 2.51 0.03 31 2007 EST 5 6.3 7.1 2.93 0.02 34 2005-1.09-0.01 0.10-0.04-0.06 EST 6 7.8 8.5 3.17 0.01 41 2004 EST 7 9.2 9.9 3.37 0.02 33 2003 EST 8 10.6 11.3 3.66 0.01 42 2003 EST 9 12.0 12.7 3.86 0.02 46 2002 EST 10 13.4 14.1 4.13 0.06 53 2000 EST 11 14.7 15.5 4.90 0.06 64 1997-1.17-0.11 0.04-0.10-0.08 EST 12 16.2 16.8 5.83 0.08 77 1993 EST 13 17.5 18.2 6.89 0.07 66 1989 EST 14 18.9 19.7 7.89 0.08 74 1984-0.91-0.01 0.11-0.05-0.14 EST 15 20.4 21.1 9.02 0.09 116 1977-0.87 0.05 0.11-0.04-0.09 EST 16 21.9 22.7 10.38 0.11 128 1969-0.99-0.01 0.06-0.04-0.10 EST 17 23.4 24.2 12.05 0.13 152 1960-0.99-0.05 0.02-0.05-0.02 EST 18 25.0 25.7 14.00 0.12 129 1949-0.94-0.08 0.10-0.06-0.17 EST 19 26.4 27.1 15.72 0.13 102 1933-0.88-0.13 0.05-0.11-0.03 EST 20 27.8 28.6 17.56 0.15 100 1910-0.95-0.10 0.06-0.09-0.07 EST 21 29.3 30.0 19.71 0.15 83 1883-0.85-0.18 0.09-0.21-0.17 EST 22 30.7 31.4 21.80 0.15 73 1852-0.98-0.22 0.07-0.20-0.11 EST 23 32.1 32.9 24.00 0.18 70 1819-1.09-0.16 0.10-0.19-0.23 EST 24 33.6 34.3 26.59 0.13 58 1778-1.15-0.22 0.05-0.24-0.12 EST 25 35.0 35.7 28.48 0.17 75 1747-1.19-0.29 0.08-0.27-0.07 EST 26 36.5 37.2 30.96 0.18 62 1707-1.22-0.24 0.04-0.21-0.06 EST 27 37.9 38.6 33.56 0.13 50 1665-1.29-0.15 0.05-0.16-0.12 EST 28 39.3 40.0 35.48 0.20 52 1634 EST 29 40.8 41.5 38.32 0.16 50 1587-1.07-0.27 0.04-0.27-0.11 EST 30 42.2 42.9 40.63 0.19 43 1550-1.38-0.38 0.03-0.36-0.08 EST 31 43.6 44.3 43.39 0.16 44 1506-1.39-0.40 0.04-0.38-0.05 EST 32 45.1 45.8 45.65 0.17 48 1469-1.39-0.46 0.04-0.39-0.06 EST 33 46.5 47.2 48.03 0.13 52 1430-1.33-0.40 0.12-0.39-0.12 S30

EST 34 47.9 48.6 49.86 0.15 45 1400-1.33-0.43 0.04-0.44-0.10 EST 35 49.4 50.1 51.94 0.15 53 1366-1.31-0.40 0.07-0.39-0.10 EST 36 50.8 51.5 54.12 0.14 56 1331-1.49-0.46 0.06-0.39-0.05 EST 37 52.2 52.9 56.19 0.14 51 1299-1.24-0.45 0.04-0.45-0.14 EST 38 53.7 54.4 58.13 0.11 43 1268 EST 39 55.1 55.8 59.71 0.13 41 1244 EST 40 56.5 57.2 61.61 0.13 48 1213 EST 41 57.9 58.5 63.37 0.12 38 1184-1.34-0.42 0.03-0.41-0.13 EST 42 59.2 59.9 64.96 0.16 43 1159 EST 43 60.5 61.2 67.04 0.15 54 1125-1.19-0.56 0.08-0.57-0.13 EST 44 61.9 62.6 69.08 0.13 51 1092 EST 45 63.3 64.0 70.94 0.19 61 1062-1.29-0.54 0.06-0.52-0.10 EST 46 64.7 65.4 73.59 0.15 59 1020 EST 47 66.2 66.8 75.71 0.14 55 984 EST 48 67.5 68.3 77.59 0.16 56 949 EST 49 69.0 69.8 80.03 0.14 57 905-1.04-0.52 0.04-0.48-0.11 EST 50 70.6 71.3 82.12 0.15 52 864 EST 51 72.0 72.8 84.33 0.13 38 821 S31

REFERENCES 1. Appleby, P. G.; Nolan, P. J.; Oldfield, F.; Richardson, N.; Higgitt, S. R., 210Pb dating of lake sediments and ombrotrophic peats by gamma essay. Sci. Total Environ. 1988, 69, (0), 157-177. 2. Blaauw, M.; Christen, J. A., Flexible paleoclimate age-depth models using an autoregressive gamma process. Bayesian Analysis 2011, 6, (3), 457-474. 3. Enrico, M.; Roux, G. L.; Marusczak, N.; Heimburger, L. E.; Claustres, A.; Fu, X.; Sun, R.; Sonke, J. E., Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition. Environ. Sci. Technol. 2016, 50, (5), 2405-12. 4. Demers, J. D.; Blum, J. D.; Zak, D. R., Mercury isotopes in a forested ecosystem: Implications for airsurface exchange dynamics and the global mercury cycle. Global Biogeochem. Cycles 2013, 27, (1), 222-238. 5. Chen, J.; Hintelmann, H.; Feng, X.; Dimock, B., Unusual fractionation of both odd and even mercury isotopes in precipitation from Peterborough, ON, Canada. Geochim. Cosmochim. Acta 2012, 90, (0), 33-46. 6. Sun, R.; Streets, D. G.; Horowitz, H. M.; Amos, H. M.; Liu, G.; Perrot, V.; Toutain, J.-P.; Hintelmann, H.; Sunderland, E. M.; Sonke, J. E., Historical (1850 2010) mercury stable isotope inventory from anthropogenic sources to the atmosphere. Elementa: Science of the Anthropocene 2016, 4, 000091. 7. Zambardi, T.; Sonke, J. E.; Toutain, J. P.; Sortino, F.; Shinohara, H., Mercury emissions and stable isotopic compositions at Vulcano Island (Italy). Earth Planet. Sci. Lett. 2009, 277, (1-2), 236-243. 8. Amos, H. M.; Sonke, J. E.; Obrist, D.; Robins, N.; Hagan, N.; Horowitz, H. M.; Mason, R. P.; Witt, M.; Hedgecock, I. M.; Corbitt, E. S.; Sunderland, E. M., Observational and modeling constraints on global anthropogenic enrichment of mercury. Environ. Sci. Technol. 2015, 49, (7), 4036-47. 9. EMEP European Monitoring and Evaluation Programme. http://www.emep.int/ (accessed on 9/15/2016), S32