Mercury isotopic fractionation during pedogenesis in a tropical forest

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Mercury isotopic fractionation during pedogenesis in a tropical forest soil catena (French Guiana): deciphering the impact of historical goldmining Stéphane Guédron, David Amouroux, Emmanuel Tessier, Catherine Grimaldi, Julien Pierre, Gilbert Barre, Sylvain Berail, Vincent Perrot, and Michel Grimaldi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02186 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Environmental Science & Technology

202 Hg (‰) -4

-3

-2

-1

0

anthropogenic

goldmined pristine

Litterfall Palm tree leaves

Litter

Mineral horizon

Hg0 (reduction / evaporation)

Hg(II)

Hg0liq

mixing (litter – mineral)

Hg(II)

Complexation (OM & oxide)

Oxic (-) MDF (+) Anoxic (Fe oxides)

Hg (II)

(Fe reduction)

ACS Paragon Plus Environment

Hg(II)-DOM

Soil

Organo -mineral horizon

Hg0

Atmosphere

background

Foliar uptake Hg(II)

1


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Mercury isotopic fractionation during pedogenesis in a tropical forest soil catena (French Guiana): deciphering the impact of historical goldmining S. Guédron*1, D. Amouroux2, E. Tessier2, C. Grimaldi3, J. Barre2, S. Berail2, V. Perrot1 and M. Grimaldi4,5.

1

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000

Grenoble, France. 2

CNRS / Univ. Pau & Pays Adour / E2S UPPA, Institut des Sciences Analytiques et de

Physico-chimie pour l’Environnement et les Matériaux – IPREM, UMR5254, 64000, PAU, France. 3

UMR SAS, INRA, Agrocampus Ouest, 35000 Rennes, France

4

Sorbonne Universities, Science Faculty, Paris 06, IRD, CNRS, INRA, UPEC, Univ Paris

Diderot, Institute of Ecology and Environmental Sciences, iEES Paris, 75005 Paris, France; 5

Institut de Recherche pour le Développement, Centre IRD de Cayenne, 97323 Cayenne

cedex, France

*corresponding author: S. Guédron Email: [email protected] Keywords: Mercury, stable isotopes, amazon region, goldmining, pedogenesis, soil catena

1 ACS Paragon Plus Environment

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1

Abstract:

2

We used mercury (Hg) stable isotopes to investigate the Hg cycle in a rainforest soil catena

3

(French Guiana) partially goldmined during the early 1950´s. Litterfall showed homogeneous

4

∆199Hg values (-0.18 ± 0.05 ‰, i.e. modern gaseous elemental Hg (GEM) isotopic signature).

5

After litter decomposition, Hg bound to organic matter (OM) is mixed with Hg from pristine

6

(-0.55 ± 0.22 ‰) or goldmined (-0.09 ± 0.16 ‰) mineral materials. Negative ∆199Hg values in

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deep pristine mineral horizons (-0.60 ± 0.16 ‰) suggest the transfer of Hg bound to dissolved

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OM depleted in odd isotopes due to mass independent fractionation during Hg abiotic

9

reduction. Perennial palm tree leaves collected above goldmined and pristine soil recorded

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contrasted ∆199Hg signatures likely resulting from GEM re-emission processes from soils and

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leaf surfaces. Upslope, soil δ202Hg signatures showed a negative shift (ε= ~ -1 ‰) with depth

12

attributed to mass dependent fractionation during Hg sorption and complexation onto iron

13

oxides and dissolved OM. Downslope, higher δ202Hg values in soils resulted from

14

hydromorphy (lower humification, higher Hg(II) reduction…). The unique Hg isotopic

15

signatures of Amazonian soils probably results in multistep fractionation processes during

16

pedogenesis (millions of years) and in potentially different Hg isotopic signature of pre-

17

anthropogenic background GEM.



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Introduction

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Tropical soils of the Amazon region have accumulated atmospheric mercury (Hg) during their

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development that lasted millions of years 1. Their inherited Hg geochemical background is

21

higher than those reported in boreal and temperate climate soils 2-6. In French Guiana, highest

22

Hg concentrations were reported in upslope oxic ferralsols associated to the clay-size iron

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oxides and sulfur-bearing functional groups of the organic matter (OM)

24

associated into aggregates 7-9. In the lowlands, where the water table comes up to the top soils,

25

gleysols are depleted in Hg because of prevailing reducing conditions that lead to the

26

dissolution of iron oxides. To this natural geochemical Hg background adds past and current

27

artisanal small-scale goldmining (ASGM) Hg contamination. Indeed, ASGM activities in the

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Amazonian region use Hg to extract eluvial gold concentrated in soil and sediment, and emit

29

large amounts of Hg in the atmosphere (i.e., burning of amalgams) or in soils and rivers

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through direct discharges of elemental and particulate Hg 10-12.

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Natural Hg stable isotopes show large mass dependent (MDF) and mass independent (MIF)

32

fractionation ranges (~ 7 ‰ for δ202Hg and ~ 10 ‰ for ∆199Hg) which allow tracing natural or

33

anthropogenic Hg sources and identifying biogeochemical Hg pathways 13. In particular, Hg

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MIF signature is reported to be efficient tracer of specific Hg sources since it is not altered

35

during MDF processes, but can be changed significantly by photochemical reduction, the

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abiotic reduction of inorganic Hg(II) by dissolved organic matter (DOM)14 and the

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evaporation of liquid elemental Hg15 causing anomalies of odd-mass relative to even-mass

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isotopes 16, 17. For example, a recent study of ASGM impacted soils in Amapá (Brazil) showed

39

that ∆199Hg values were higher in mine tailings (~ 0 to −0.01 ‰) than in pristine soils (~ -

40

0.55 ‰)18. Besides, MDF signatures can be used as tracer of biogeochemical reactions since

41

they result from numerous processes including redox reactions, volatilization, complexation

42

and OM binding … 16, 19-21. Fractionation of Hg isotopes in soil during pedogenesis has been

3

which are closely


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increasingly investigated during the last decade. Uptake of atmospheric Hg by foliage was

44

reported to result in a large shift of δ202Hg values (up to -2.9 ‰) suggestively caused by

45

kinetic MDF during foliar uptake of gaseous elemental Hg (GEM) while little fractionation

46

was observed during the incorporation of deciduous material in the soil 22-25. Downward in the

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soil profile, Hg isotopic compositions can be linked to geogenic Hg mixed with atmospheric

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Hg and biogeochemical processes controlled by hydro-pedological conditions (redox)

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affecting Hg isotopic fractionation mostly by reduction and complexation processes 22, 25.

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In this work, we studied Hg isotopic fractionation during pedogenesis in a soil catena located

51

in a small catchment area of French Guiana covered by tropical rain forest combining upslope

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undisturbed soils and downstream reworked soils contaminated with Hg during ancient

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goldmining activities dating the early 1950´s. Perennial palm tree (PT) leaves collected at

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various distances from the goldmined site were used as bio-indicators of Hg atmospheric

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signature below the canopy. We used Hg isotopes to track the dissemination of anthropogenic

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Hg in the catchment and studied the Hg isotopic composition along with the downward Hg

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incorporation in soil (from litter to deep mineral horizons) with consideration of geochemical

58

and hydro-pedological processes.

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Material and methods

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Research areas

61

Soils were sampled in October 2012 on the Combat Creek watershed (52°23’W, 4°35’N), a

62

small catchment of ~1 km² covered by tropical rain forest in French Guiana. Details on the

63

geology and morphology of the watershed are given in Guédron et al. 3. Four soil profiles

64

were collected along a toposequence (Fig. 1) including: (i) a ferralsol (IT4) located upslope

65

and having a high clay (< 2 µm size fraction) content and ferruginous nodules throughout the

66

entire profile with a micro-aggregated structure extending over 1 m depth allowing a good

67

vertical water drainage, (ii) a midslope acrisol (IT1) with a massive weathered schist (C) 4 ACS Paragon Plus Environment


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horizon having high fine silt content at shallow depth (< 1 m), (iii) a downslope acrisol (IT2)

69

with an hydromorphic weathered schist (Ch) horizon and (iv) a gleysol (IT3) typical of

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hydromorphic conditions of the low lands with a permanent shallow groundwater table

71

imposing reducing conditions. In these soils, very common in French Guiana3, 9, quartz,

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kaolinite, and Fe-Al-(oxyhydr)oxides dominate mineralogy, quartz being the sole remnant

73

from the primary rock-forming minerals.

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Samples from the 3 soil profiles of the ancient gold mined “flat” (i.e.; III-0, SL3 and SL6)

75

were collected in 20053. Gold from the former flat was extracted using Hg for amalgamation

76

according to the ancient ‘Long Tom’ sluices process dating from the early 1950´s. The

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resulting soils (initially gleysols) are strongly disorganized with gravel, sand and silt as

78

dominant grain-size fractions, the finest fraction being lost during mining operations. Hg

79

droplets and Au-Hg amalgams were identified in these gold-mined gleysols 3.The area where

80

SL6 and III-0 were collected has been deforested and re-exploited (2008-2010) without Hg

81

use (gravimetrical gold extraction).

82

Sampling collection and analysis

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Soil and leaves collection. Soil samples were collected every 10 cm with an auger down to

84

120 cm depth when possible, homogenized and stored in clean polyethylene bags. Litter

85

samples were collected according to different layers of the organic horizon (O) with Oi the

86

recent or slightly humified litter and Oe the humified organic matter. Perennial palm tree

87

(Oenocarpus Bataua Mart.) leaves were collected in the vicinity of each soil profile along the

88

toposequence (IT4, IT1 and IT3), on a upslope ferralsol of the Creek Mouche (CM) located

89

upstream in the catchment, on the former goldmined flat (SL6) and at its edge (FP) (Fig. 1

90

and S.I.1). All samples were kept in a fridge at 4°C before being freeze-dried and crushed to

91

obtain a powder of d < 63 µm. More details for sample collection and conditioning are given

92

in S.I.1. 5 ACS Paragon Plus Environment

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Determination of soil, litter, and leaves chemical composition. Total Hg concentration

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([THg]) in solids was determined by atomic absorption spectroscopy after dry mineralization

95

and gold amalgamation (Altec, Model AMA 254). Triplicates were performed for each

96

analysis (S.I.2). Concentrations obtained for certified reference material (CRM MESS-3 –

97

National Research Council Canada) were in the published range of concentration (0.091 ±

98

0.008 µg g−1).

99

Total carbon ([C]), sulfur ([S]), and nitrogen ([N]) concentrations in samples were determined

100

from the dry combustion of soil sample aliquots using a Fisons® (model 1500CHNS) infra-

101

red analyzer. Granulometry was determined with standard sieving and sedimentation

102

procedures

103

sample aliquot in aqua regia solution (HCl/HNO3; 3:1 v/v ratio) for 10 h at 70 °C.

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Mercury isotopes analysis (see details in S.I.2). Hg isotope ratios were determined after

105

digestion of 0.2 to 0.5 gram of the soil sample in 50 mL PP vials with addition 4 mL of

106

HNO3/HCl/H2O2 mixture and heated at 85°C for 4 hours according to the procedure

107

established by Foucher and Hintelmann

108

leaves, high pressure acid digestion has been achieved following the protocol validated by

109

Estrade et al.

110

according to the total Hg content) was digested in 4 mL suprapur HNO3 at 300°C and 130

111

bars for 3 hours (S.I.2). Then, all sample extracts were centrifuged at 1400 rpm for 10 min

112

and the supernatant was removed and diluted for isotopic measurement to about 10% of acid

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mixture. Hg isotope analyses were performed according previous works

114

vapor generation with SnCl2 reduction coupled to a MC-ICP-MS (Nu Plasma, Nu

115

Instruments). To correct the instrumental mass bias, internal standard of Tl (NIST 997,

116

205

26, 27

. Fe and Al contents were determined by AAS after the digestion of a soil

30

28

and used previously in our lab29. For palm tree

for lichen specimen. Homogenized sample matrix (between 0.5 to 1.2 g,

16, 31, 32

using cold

Tl/203Tl = 2.38714) and sample standard bracketing with NIST 3133 (prepared in the same



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acid matrix than samples) standard solution were used. MDF of Hg are reported relative to the

118

NIST 3133 Hg solution16:

119

δXXXHg = (XXX/198Hgsample / XXX/198HgNIST3133 -1) * 1000 in ‰ 199

Hg

200

Hg,

201

Hg and

204

120

where xxx is the studied isotopes. For

Hg isotopes, MIF of Hg is

121

reported as the difference between the theoretical value predicted by MDF of δXXXHg and the

122

measured values noted ∆XXXHg in ‰ 16:

123

∆XXXHg = δXXXHg - δ202Hg * βXXX

124

where βXXX are equal to 0.2520, 0.5024, 0.7520 and 1.493 for the studied isotopes (xxx) 199,

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200, 201 and 204 isotopes, respectively. Enrichment factor ε is used to compare fractionation

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or mixing of Hg isotopes for MDF between two pools (i.e., a sink or product (B) relative to a

127

source or reactant (A)) and expressed as follow: ε202HgB-A = δ202Hg B − δ202Hg A.

128

Analytical uncertainty was evaluated by multiple measurements of certified reference material

129

for Hg concentrations. All details for the method, Quality Assurance/Quality Control and

130

tables of Hg analysis data are given in S.I. 2.

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Statistical treatment and data presentation. Linear regressions were performed only when

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normality test, constant variance test and alpha power of the performed regression (α test, p >

133

0.8) passed. Correlation coefficient (R) and p values are reported for the computed linear

134

regressions. In the manuscript, mean concentrations (arithmetic means) are always presented

135

with the associated standard deviation (mean ± SD) and isotopic compositions with their

136

associated 2SD. When the range (2SD) of measured values of a group of samples is higher

137

than the 2SD values of the analytical measurement, mean values are presented with this 2SD

138

range and the number of corresponding measurements (N).

139

Results and discussion

140

Discrimination of isotopic Hg pools in a tropical forest soil catena.


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The composition of Hg even isotopes generally follows the mass dependent fractionation

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(MDF) theoretical line and exhibits a large range of delta values for both δ202Hg (~ 3 ‰ - Fig.

143

2) and δ200Hg (~ 1.5 ‰ - S.I.3.a). Mass independent fractionation of even isotopes (MIFeven;

144

∆200Hg or ∆204Hg) was weak and generally not significant in the samples investigated (S.I.3),

145

even if larger dispersion for ∆204Hg was observed resulting probably from larger analytical

146

uncertainty. Significant Hg mass independent fractionation of odd isotopes (MIFodd) is

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observed within a 0.7 ‰ range for both ∆199Hg and ∆201Hg (Fig. 2 and S.I.3.b). They both

148

plot on the 1:1 empirically derived relationship resulting of fractionation during

149

photochemical reduction of inorganic mercury from aqueous solutions in the presence of

150

DOM 16 (S.I. 3). Owing to the contrasted signatures of both odd and even isotopes, our plot of

151

∆199Hg and δ202Hg allows identifying three major pools: (i) former goldmining soils with the

152

highest δ202Hg and ∆199Hg values of our dataset; (ii) foliage and soil litter that have

153

intermediate values and (iii) undisturbed soil horizons (A, B and C) enriched in lighter Hg

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isotopes together with lower ∆199Hg values (Fig. 2).

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Hg contaminated soils of the former goldmined flat ([THg] = 2360 ± 2533 ng g-1) have

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isotopic composition for both MIFodd and MDF (∆199Hg = -0.09 ± 0.16 ‰; δ 202Hg = -0.65 ±

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0.65‰, N=12) in agreement with other Amazonian goldmining tailings18 and cinnabar or Hg

158

ores

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isolated elemental Hg (Hg0) droplets or Hg0 amalgamated with micrometric gold particles

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inherited from goldmining operations dating from the early 1950´s in the Combat watershed 4.

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During ancient mining operations, gleysols of the lowland were suspended with creek water

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forming a slurry to remove fine particles and concentrate heavy gold-rich particles before

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their onsite amalgamation with Hg 3. Thus, in opposition to undisturbed gleysols that belong

164

to the same lowland area, mined soils have ∆199Hg values negatively correlated with coarse

23, 33-35

. These near-zero ∆199Hg values are also consistent with previous observations of 3



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sand fraction (R = -0.88, p 2mm) contents (bottom right panels). In upslope and midslope soils of the undisturbed toposequence (IT1, IT2 and IT3), the coarse fraction is essentially composed of ferruginous nodules. Vertical profiles of total Hg in soils impacted by former goldmining activities (right corner panel) are from Guédron et al. 3. Total carbon is equivalent to organic carbon since no carbonates are present in this rain forest setting. Soil of the toposequence are defined undisturbed because of their neighboring position to the flat. The recently reworked goldmined flat (grey area) is an open field area (deforested without organic horizon). The 3 soil profiles of this ancient gold mined “flat” (i.e.; III-0, SL3 and SL6) were collected in 2005 before its rework while palm tree leaves were collected in 2012. Figure 2. Mass-independent fractionation (Δ199Hg) vs mass-dependent fractionation (δ202Hg) in palm tree (PT) leaves and soil horizons and their comparison with literature data. The shaded areas correspond approximately to i.e., precipitation (blue)22, 24 ,47, 61, ores and mine tailings (orange) 18, 23, 33, 35, atmospheric TGM and GEM (grey)24, 46, 47, 60, and litterfall, foliage and lichens (green) 22, 24, 25, 36, 47, 48 data from the literature. A bidirectional error bar represent 2σ of replicated isotopic measurements of NIST RM 8610 (UM Almaden). Figure 3. Mass-independent fractionation (Δ199Hg) vs mass-dependent fractionation (δ202Hg) in palm tree (PT) leaves collected over forested ancient goldmined soils (SL6), at the edge of the reworked goldmined flat (FP), over undisturbed soils (IT4, IT1 and IT3) of the toposequence IT and over pristine remote soils (CM). Are also presented the mean value with 2SD of THg isotopic composition for the four litter samples of the undisturbed toposequence IT (dark green diamond), the soil horizons (A, B and C) below litter (red diamond, N = 22) collected in the oxic undisturbed toposequence soil profiles and the soil horizons (A and B) below the litter of the goldmined soils (blue diamond, N = 12). The colored arrows illustrate the isotopic trends in the products of the reaction for abiotic reduction (abiotic red.)55, 56 (red arrow), the foliar uptake processes22, 25, 47 (grey arrow) and the Hg photochemical reduction on foliage and forest floor 16,25 (green arrow). The blue arrow illustrates the isotopic trends in the substrate of the reaction for Hg(0) evaporation (Hg(0) evapo.)15. Blue and gray shaded rectangles refer to available literature data for the mean with 2SD isotopic extent of atmospheric GEM in pristine background areas (δ202Hg= 0.5 ± 0.5 ‰ and ∆199Hg= -0.25 ± 0.15 ‰) 22, 25, 46 , 47 and anthropogenic contaminated areas (δ202Hg= -0.65 ± 0.37 ‰ and ∆199Hg= -0.05 ± 0.15 ‰)24, 46 which is likely similar to re-emission over goldmined soils. Figure 4. Depth profiles for Δ199Hg (left panel) and δ202Hg (right panel) isotopes in pristine and contaminated goldmined soils. Blue dotted lines refer to the mean position of the water table recorded along the year (S.I.6) for acrisol IT1 (~ 65cm depth) and IT2 (~ 75cm depth).



Undisturbed soil profiles

60

Palm tree leaves

Undisturbed soils Ferralsol IT4 Acrisol IT1 Acrisol IT2 Nodule acrisol IT2 Gleysol IT3 Goldmined soils III-0 SL-3 SL6

40

30

20

Combat creek watershed 10

IT4

400

sandy clayey hydromorphic weathered schist horizon (Ch) IT2

300

IT3

200

100

0

Distance from the flat (m) [THg] (ng g-1)

IT1 0

IT3

200

400

600 0

-20 Depth (cm)

CM

SL6

-40 -60 -80

III-0

-100

0 100m

-120


3

6

Log[THg] (ng g-1)

Coarse fraction > 2 mm (%)

[C] (%)

0

IT2

FP

bright-brown clayey compact mineral horizon (B)

Soil horizons Fresh Litter (Oi) Humified Litter (Oe) Organo-mineral (A) Mineral (B) Weathered schist (C) Weathered schist hydromorphic (Ch)

I2

SL3

Reddish-brown macroporous mineral horizon (B)

reddish brown micro-aggregated horizon (B) IT1

50

Elevation (m)

N

litter, root mat (Oi, Oe) and organo-mineral horizon (A)

IT4

Goldmined soil profiles Goldmined flat (Open field)

red compact weathered schist horizon (C)

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40

80

0

20

40

60

80

100

1000

10000

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0.6

Soils

0.4 0.2

Undisturbed soils Ferralsol IT4 Acrisol IT1 Acrisol IT2 Nodule acrisol IT2 Gleysol IT3 Goldmined soils III-0 SL-3 SL6

Precipitation

Ores and

0.0

199 Hg (‰)

Soil litter & palm tree leaves

mine tailings Goldmined soils atmospheric TGM & GEM

-0.2 Young palm tree leaves

-0.4

Soil horizons Fresh Litter (Oi) Humified Litter (Oe) Organo-mineral (A) Mineral (B) Weathered schist (C) Weathered schist hydromorphic (Ch)

Litterfall, foliage and lichens

Old palm tree leaves Undisturbed soil horizons

Palm tree leaves 2SD

SL6 forested ancient mine flat CM over Ferralsol (young) FP edge of the mine flat (young) IT4 over Ferralsol IT1 over Acrisol IT3 over Gleysol CM over Ferralsol FP edge of the mine flat

-0.6

-0.8 -3

-2

-1

0

202 Hg (‰)ACS Paragon Plus Environment

1


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Palm tree leaves SL6 forested ancient mine flat CM over Ferralsol (young) FP edge of the mine flat (young) IT4 over Ferralsol IT1 over Acrisol IT3 over Gleysol CM over Ferralsol FP edge of the mine flat

'199 Hg (‰) 0.4

R

=

0.9

, p

0.0

Mercury isotopic fractionation during pedogenesis in a tropical forest

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