Holocene atmospheric mercury levels. isotopes

Transcription

1 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, Castanet Tolosan, France. Laboratoire Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, CNRS/IRD/Université Paul Sabatier Toulouse III, 14 avenue Edouard Belin, Toulouse, France. Laboratoire d Etudes en Géophysiques et Océanographie, Observatoire Midi-Pyrénées, CNRS/IRD/Université Paul Sabatier Toulouse III, 14 avenue Edouard Belin, 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

2 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 , and is therefore highly consistent with 137 Cs emission during the Chernobyl accident in 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 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

3 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, 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

4 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

5 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 is late compared to Estibere and other European records ( , 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

6 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 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 GEM deposition velocity is suspected to change with biomass S6

7 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 ± and ± 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

8 knowledge of GEM concentration and HgAR dry. We therefore used Pinet HgAR dry reconstructed for the period (pre-disturbance) and reconstructed GEM concentration from Estibere record for the same period. This gave a pre-disturbance V GEM of ± 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 BCE), for which peat 200 Hg (0.02 ± 0.04, 1σ, n = 76) is similar to 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

9 Supplementary figures Fig. S1. Location of the Pinet peat bog and Estibere peatland (map data: Google, Landsat / Copernicus) Fig. S Pb profiles of Pinet and Estibere peat cores. Fig. S 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. S 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

10 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

11 Fig. S1. S11

12 Fig. S2. S12

13 Fig. S3. S13

14 Fig. S4. S14

15 Fig. S5. S15

16 Fig. S6. S16

17 Fig. S7. S17

18 Fig. S8. S18

19 Fig. S9. S19

20 Fig. S10. S20

21 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 A A A A A A A A A A A A A A A A A A Pb/ 206 Pb S21

22 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 B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B S22

23 B B B B B S23

24 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 C C C C C C C C C C C C C C C C C C C C C C C C C S24

25 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 EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST S25

26 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 A A A A A A A A A A A A A A A A A A A A A A A A A A A A S26

27 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 B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B S27

28 B B B B B B B B B S28

29 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 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S29

30 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 EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST S30

31 EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST EST S31

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