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Tracing anthropogenic Hg and Pb input using stable Hg and Pb isotope ratios insediments of the central Portuguese Margin

Mário Mil-Homens a,⁎, Joel Blum b, João Canário c, Miguel Caetano c, Ana M. Costa a,1, Susana M. Lebreiro d,Maria Trancoso e, Thomas Richter f, Henko de Stigter f, Marcus Johnson b, Vasco Branco c, Rute Cesário c,Filomena Mouro e, Manuela Mateus e, Wim Boer f, Zenaida Melo e

a Laboratório Nacional de Energia e Geologia, I.P., Unidade de Geologia Marinha, Estrada da Portela, Apartado 7586, 2610-999 Alfragide, Portugalb University of Michigan, Department of Earth and Environmental Sciences, 1100 N. University Avenue, Ann Arbor, MI 48109, USAc IPMA, Portuguese Institute of Sea and Atmosphere, Av. Brasília, 1449-006 Lisboa, Portugald Geological and Mining Institute of Spain, OPI, Dept. of Geosciences Research and Prediction, Global Change, C/ Ríos Rosas, 23, 28003, Madrid, Spaine Laboratório Nacional de Energia e Geologia, I.P., Laboratório de Biocombustiveis e Ambiente, Azinhaga dos Lameiros, 1649-038 Lisboa, Portugalf NIOZ, Royal Netherlands Institute for Sea Research, Department of Marine Geology, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

a b s t r a c ta r t i c l e i n f o

Article history:Accepted 23 February 2012Available online 1 March 2012

Keywords:Portuguese MarginMarine sedimentsTotal Hg and Pb concentrationsStable Hg isotopesStable Pb isotopes

Three short marine sediment cores from the Cascais Submarine Canyon (CSC; cores 252-32 and 252-35) andthe Estremadura Spur (core 252-16) on the central Portuguese Margin were analysed for Hg, Pb, Al, and Mnconcentrations, and both Pb and Hg stable isotope compositions, in order to reconstruct trends in Hg and Pbsources and accumulation. Sediment ages were determined based on 210Pb measurements. The studied coresreveal increasing Hg and Pb concentration, independent of grain-size variations, since the middle of the 19thcentury towards the Present. Concomitantly, a decreasing trend of 206Pb/207Pb to less radiogenic values to-wards the surface was found, indicating increasing anthropogenic inputs. The lowest values of δ202Hg weregenerally observed in the older sediments characterized by low Hg and Pb concentrations, suggesting alow input of anthropogenic metals. Odd mass number (199Hg and 201Hg) Hg isotopes in sediment samplesfrom core 252-16 were characterized by positive mass independent fractionation (MIF), while recent sedi-ments from cores 252-32 and 252-35 did not reveal significant MIF, probably reflecting both the proximityto the source of anthropogenic Hg contamination (Tagus Estuary) and the importance of the CSC as a particlecarrier. The multi-tracer approach, based on both stable Hg and Pb isotopic signatures, confirms anthropo-genic Hg and Pb enrichment in recent marine sediments and also allows us to distinguish between areasdominated by detrital (e.g. CSC) versus hemipelagic (Estremadura Spur) sedimentation.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Toxic heavy metals (e.g. Hg, Pb) are released into the environmentby both natural processes and anthropogenic activities (Selin, 2009),and are transported through (and stored in) environmental reser-voirs, including the atmosphere, water, soils and sediments. Humansin their activities have been using Hg and Pb for thousands of years invarious applications (e.g. manufacturing of materials, health applica-tions, fossil fuel burning and mining; Patterson, 1972; Nriagu, 1993).Although anthropogenic impacts started during Greek and Romantimes, the Industrial Revolution corresponds to the historical periodwhere large increments in the use of heavy metals resulted in signif-icant release into the environment (Nriagu, 1996). Due to their

toxicity these metals represent a serious threat to human health(Jarup, 2003), especially when bioaccumulated in the tissues of or-ganisms and biomagnified through the food web (Harada, 1995).Technological improvements in the treatment of industrial and do-mestic effluents together with the decreasing use of Hg in industry(e.g. chlor-alkali production using Hg cells) and the worldwide reduc-tion in use of alkyl-leaded gasoline since the late twentieth century,have led to a reduction of anthropogenic emissions (Nriagu, 1996;Pacyna et al., 2001). In order to implement remediation strategiesfor contaminated sites and policies for regulating contaminant emis-sions it is necessary to increase the level of knowledge regardingcontamination effects of toxic metals and their preservation in the en-vironment, including distinction between natural and anthropogeniccontributions.

Stable Pb isotopic ratios have been used for decades for investigat-ing biogeochemical cycling of trace metals and for tracing natural andanthropogenic sources of Pb in the environment (Gobeil et al., 1995;Ferrand et al., 1999; Ritson et al., 1999; Weiss et al., 2008). Lead has

Chemical Geology 336 (2013) 62–71

⁎ Corresponding author.E-mail address: mario.milhomens@lneg.pt (M. Mil-Homens).

1 Present address: Laboratório de Arqueociências, IGESPAR, I.P. and LARC/CIBIO/INBIO, Rua da Bica do Marquês, n°2, 1300-087 Lisboa, Portugal.

0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2012.02.018

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four stable isotopes: 204Pb, 206Pb, 207Pb and 208Pb, of which the lastthree are final end-members of the 238U, 235U and 232Th decay chains,respectively (Dickin, 1995). Anthropogenic Pb emissions are general-ly characterized by lower 206Pb/207Pb and higher 208Pb/206Pb ratiosthan those from natural background sources, depending on the varie-ty of lead ores used in nearby industries (Sangster et al., 2000).

Improved analytical methodologies, particularly in multicollectorinductively coupled plasma mass spectrometry (MC-ICP-MS), haveonly recently allowed the accurate measurement of Hg isotope ra-tios. Stable Hg isotope ratios have been found to be useful for inves-tigation of the sources and biogeochemical cycling of Hg in theenvironment (Bergquist and Blum, 2007; Sonke et al., 2010). Mercu-ry has seven stable isotopes ranging in mass from 196Hg to 204Hg. Isoto-pic variation is caused by fractionation during both natural andindustrial processes (Stetson et al., 2009; Estrade et al., 2010; Gehrkeet al., 2011). Hg isotopes in the environment display both mass-dependent fractionation (MDF reported as δ202Hg) and mass-independent fractionation (MIF) of odd isotopes (reported as Δ199Hgand Δ201Hg; Blum and Bergquist, 2007). MDF is known to occur duringredox transformations, biological cycling and volatilization of Hg, whileMIF is largely produced by photochemical reactions and remainsunchanged by most dark chemical and biological transformations(Bergquist and Blum, 2009).

Fine-grained marine sediments, having a relatively large adsorp-tion capacity for trace metals, constitute important repositories forcontaminants. If environmental conditions allow, such sedimentshave the potential to record temporal changes in contaminationthat may be used for historical reconstructions (Valette-Silver,1993). Estuarine and coastal areas close to major cities have been ofparticular interest for studies of contamination (Frignani et al.,1997; Palanques et al., 1998; Mecray and ten Brink Buchholtz,2000). Despite the recognized importance of submarine canyons asthe main conduits of sediments to deeper parts of the ocean(Mullenbach and Nittrouer, 2000; Puig et al., 2003; Martín et al.,2006; de Stigter et al., 2007), less attention has been paid to contam-ination of these sensitive environments, especially in terms of heavymetal contents (Roussiez et al., 2005; Palanques et al., 2008).

The central Portuguese Margin receives contaminant input fromthe Lisboa metropolitan area, as well as the Tagus River, which drainslarge parts of the Iberian Peninsula. Contaminants are initially con-centrated in the Tagus Estuary, then in the fine-grained shelfdepocenter (Tagus Prodelta), and subsequently further dispersedthrough the marine environment with focused transport throughsubmarine canyons. This study reports the Hg and Pb isotopic compo-sition of three short marine sediment cores, two of them collectedfrom the Cascais Submarine Canyon where lateral sediment transportis assumed to play an important role, and another collected from theEstremadura Spur, an area dominated by hemipelagic sedimentation.The main goal of the work is to explore how δ202Hg and Δ199Hgvalues of these sediments, combined with more commonly measuredstable Pb isotopic ratios, can be used for differentiating between con-tamination sources.

2. Materials and methods

2.1. Study area

The western Portuguese Margin is characterized by a relativelynarrow shelf (5 to 60 km; Dias, 1987)) and steep irregular slope. Itis intersected by a series of submarine canyons (e.g. Porto, Aveiro,Nazaré, Cascais, Lisboa, Setúbal and São Vicente) that are the prefer-ential conduits through which shelf sediments are transported to-ward the abyssal plain (Lebreiro et al., 1997; de Stigter et al., 2007,2011; Lastras et al., 2009). The Estremadura Spur, a protruding seg-ment of continental margin located north of the Cascais SubmarineCanyon, represents an area with limited terrigenous sediment supply.

The Tagus River contributes the bulk of the terrigenous sedimentssupplied to the shelf. Other sources of sediments are associatedwith small streams located in the northern part of the area, cliff ero-sion, and re-working of shelf sediments. Seasonal coastal upwellingduring spring and summer induced by northerly winds (Fiúza,1983) and riverine input of nutrients also lead to enhanced produc-tion of biogenic sediments (Rodrigues et al., 2009). Extending alongthe margins of the Tagus Estuary is, the highly industrialized Lisboametropolitan area (with chloralkali plant, pyrite roasting plant,smelter, shipyards) with approximately 2.5 million inhabitants. Lis-boa is responsible for the high level of contamination documentedin previous studies (Figuères et al., 1985; Vale, 1990; Canário et al.,2005; Vale et al., 2008; Cobelo-García et al., 2011). In the most con-taminated estuarine areas, located in the vicinity of the two majorhistorical anthropogenic sources identified by Figuères et al. (1985),Hg and Pb concentrations reach 49 μg g−1 and 2000 μg g−1, respec-tively (Canário et al., 2005; Vale et al., 2008). On the adjacent shelfarea the combination of high river sediment input and water supplywith favorable hydrodynamic and environmental conditions hasresulted in the development of fine-grained shelf deposits (theTagus Prodelta) where trace element enrichment has been identified(Jouanneau et al., 1998; Mil-Homens et al., 2009). Enriched sedimentsare not constrained to this preferential accumulation area, but arealso transferred through the submarine canyons to deeper parts ofthe Portuguese Margin (Richter et al., 2009; Jesus et al., 2010; Costaet al., 2011).

2.2. Sample collection

The three multi-cores investigated in this study were collectedin 2006 during cruise 64PE252 onboard RV Pelagia. Two of themwere collected from the Cascais Submarine Canyon (252-32 at38.36307 N; 9.50690 W; 2100 m water depth; and 252-35 at38.49345 N; 9.47880 W; 445 m water depth) and one from theEstremadura Spur (252-16 at 39.17670 N; 10.66612 W; 2084 mwater depth) (Fig. 1). Onboard, the sediment cores were stored re-frigerated. In the laboratory each core was split, described, photo-graphed, X-radiographed and sub-sectioned at 1 cm intervals. Thesediment slices were subsequently freeze-dried and ground.

2.3. Sample analyses

Major elements (Al and Mn) were determined on fusion beads byXRF using a sequential wavelength dispersive spectrometer (modelAXIOS). A mixture of about 0.1 g of sample with 9.0 g of lithium tetra-borate flux (Li2B4O7) and 20 mg of ammonium iodide (NH4I) wasfused in Pt95-Au5 crucibles at 1150 °C for 20 min and then pouredinto Pt-Au dishes. For laboratory quality control the calibrationswere validated by daily analysis of a certified reference material. Mer-cury concentrations were determined by thermal vapor AAS (LECOAMA 254) after pyrolysis of each sample in a combustion tube at750 °C under an oxygen-rich atmosphere and collection on a goldamalgamator (Costley et al., 2000). Lead was determined by GF-AAS, following a total decomposition of 0.25 g of freeze-dried andground sample with 2.5 ml of HNO3 (Suprapur 65%), 2.5 ml ofHClO4 (Suprapur 70%) and 5 ml of HF (Suprapur 40%). Proceduralblanks and certified reference materials MESS-3 (National ResearchCouncil of Canada) were analyzed for QA/QC throughout the analyses.Analytical uncertainties, expressed as the relative standard deviationof 61 replicate samples for Hg and 25 for Pb, were less than 2% and 3%(pb0.05), for Hg and Pb respectively. The obtained results indicateaverage recoveries of 105% and 99% for Hg and Pb, respectively. All el-emental concentrations used were previously published in Costa et al.(2011).

Organic carbon (Corg) contents were calculated based on the dif-ference between total carbon and inorganic carbon. Inorganic carbon

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was measured after combustion of the sediment organic fraction for3 h at 400 °C. Both total and inorganic carbon contents were deter-mined on 2 mg of freeze-dried and homogenized sediment using aLeco CHNS 932. The precision of two repeated measurements ofboth samples and standards was lower than 0.03 wt.%.

Stable Pb isotopes were determined with a quadrupole ICP-MS(Thermo Elemental, X-series) equipped with a Peltier Impact beadspray chamber and a concentric Meinhard nebulizer following theprocedure described by Caetano et al. (2007). Briefly, 0.1 g sub-samples of sediment were totally digested with a mixture of HF andAqua regia (6:1, HNO3:HCl), in closed Teflon vessels at 100 °C for1 h. Sample solutions were evaporated until near dryness in Teflonvials (DigiPrep HotBlock—SCP Science), redissolved with 1 ml ofdouble-distilled HNO3 and 5 ml of Milli-Q water (18.2 MΩ), heatedfor 20 min at 75 °C and diluted to 50 ml with Milli-Q water. Two pro-cedural blanks were prepared using the same analytical procedureand reagents, and included within each batch of 20 samples. An inter-national standard reference material (NIST SRM 981) was measuredafter every two samples to correct for instrumental mass fraction-ation. The internal reproducibility of NIST SRM 981 measured, as thecoefficient of variation, was 0.10% for 206Pb/207Pb. The coefficient ofvariation obtained in the external quality control between batcheswas 0.37% for 206Pb/207Pb ratios. The Pb isotopic composition of pro-cedural blanks did not influence significantly the 206Pb/207Pb ratiosmeasured in sediment samples.

Mercury isotope analyses were carried out according to methodol-ogies previously described (Blum and Bergquist, 2007; Gehrke et al.,2009). Briefly, sediment samples were combusted in a two-stage fur-nace at 750 °C to separate Hg from the sediment matrix, and volatil-ized Hg0 and other combustion products were passed through asecond furnace at 1000 °C to assure complete combustion. VolatilizedHg0 was transported in a stream of air, and then trapped as Hg2+ bybubbling through an oxidizing 1% KMnO4 solution. Sample recoverieswere determined for each sample and standard and proceduralblanks were run every ten samples. Following combustion, Hg

isotopic compositions were determined using a Nu InstrumentsMC-ICP-MS. Instrumental mass-bias was corrected using a Tl internalstandard (NIST 997) and sample-standard bracketing using NIST3133solution at the same concentration and in the same sample matrix.Additionally, on-peak zero corrections were applied. Mass-dependentHg isotope compositions are reported as δ202Hg in permil (‰) relativeto the NIST 3133 Hg standard (Blum and Bergquist, 2007) accordingto the equation:

δ202Hg ‰ð Þ¼ 202Hg=198Hg� �

sample=

202Hg=198Hg� �

SRM3133

� �–1Þ×1000

Mass-independent Hg isotope fractionation is reported as Δ199Hgand Δ201Hg in permil (‰), and is defined as the difference betweenmeasured δ199Hg and δ201Hg values and the predicted MDF valuesaccording to the equations described by Blum and Bergquist (2007):

Δ199Hg ¼ δ199Hg–ðδ202Hg×0:2520Þ

Δ201Hg ¼ δ201Hg–ðδ202Hg×0:7520Þ

Analytical uncertainty was evaluated using replicate analyses ofthe UM-Almadén standard and replicate analysis of certified refer-ence materials, indicating an average analytical precision (2SD) of0.06‰ for δ202Hg, 0.05‰ for Δ201Hg and 0.06‰ (2SD) for Δ199Hg.Replicate analysis of core samples yielded δ202Hg values within±0.25‰ (2SD), and Δ201Hg and Δ199Hg values±0.16‰ (2SD) and±0.13‰ (2SD), respectively (Table 3).

Sediment chronology was based on down-core distribution of 210Pb(half-life 22.3 years) determined by α-spectrometry indirectly via itsgranddaughter 210Po in 12 samples per core as previously reported(Costa et al., 2011). Sediment accumulation rateswere determined by ap-plying amodel of Constant Flux and Constant Sedimentation Rate includ-ing a surface mixed layer (SML) on the top (CF/CSSML; Carpenter et al.(1982)) for core 252-35 and a model of Constant Flux and Constant

Fig. 1. Location of short sediment cores at the Central Portuguese Margin. Shading denotes the Tagus Prodelta, characterized by sediment with more than 90% mud (silt+clay) andless than 30% CaCO3 (data extracted from the Chart of Surface Sediments of the Portuguese shelf (sheet number 5), published by the Portuguese Hydrographic Institute (2005)).

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Sedimentation Rate (CF/CS; Appleby and Oldfield (1992)) for cores 252-16 and 252-32 (see more details in Costa et al., 2011). Average sedimentaccumulation rates (and average sedimentation rates in parentheses)were estimated to be 0.40 g cm−2 y−1 (0.67 cm y−1), 0.06 g cm−2 y−1

(0.1 cm y−1), and 0.02 g cm−2 y−1 (0.03 cm y−1) for cores 252-35,252-32 and252-16, respectively (Costa et al., 2011; de Stigter et al., 2011).

3. Results and discussion

3.1. Temporal trends of Hg and Pb content in sediments

Samples from the base of cores 252-32 and 252-16 did not displayexcess 210Pb. Therefore, the age of sediments older than 1800 AD was

Table 1Organic carbon (Corg), Al, Mn, Hg and Pb concentrations and Pb isotopic composition ofsediments in the studied cores.

Core (depth) Age Corg

(%)Al(%)

Mn(%)

Hg(μg g−1)

Pb(μg g−1)

206Pb/207Pb

252-16 (0–1) 1996 1.72 6.7 0.08 0.133 26 1.1863±0.0013252-16 (1–2) 1973 1.76 6.9 0.08 0.110 26 1.1893±0.0013252-16 (2–3) 1949 1.28 6.8 0.08 0.095 20 1.1899±0.0013252-16 (3–4) 1922 1.30 6.7 0.09 0.082 17 1.1918±0.0013252-16 (5–6) 1863 1.40 6.6 0.09 0.081 15 1.1940±0.0013252-16 (6–7) 1832 1.29 6.6 0.10 0.074 13 1.1932±0.0013252-16 (7–8) 1800 1.22 6.4 0.11 0.070 14 1.1945±0.0013252-16 (8–9) 1768 1.02 5.7 0.18 0.061 13 1.1933±0.0013252-16 (9–10) 1735 1.34 6.3 0.21 0.059 16 1.1927±0.0013252-16 (12–13) 1633 1.11 6.3 0.28 0.055 17 1.1924±0.0013252-16 (15–16) 1529 1.32 6.0 0.04 0.055 14 1.1936±0.0013252-16 (18–19) 1421 1.23 6.0 0.03 0.049 16 1.1929±0.0013252-16 (21–22) 1312 1.01 5.9 0.03 0.047 16 1.1937±0.0013252-16 (24–25) 1202 0.83 5.7 0.04 0.042 13 1.1925±0.0013252-16 (27–28) 1090 1.05 5.4 0.03 0.058 12 1.1942±0.0013252-16 (30–31) 973 0.99 4.8 0.03 0.045 13 1.1936±0.0013252-32 (0–1) 2003 1.99 7.4 0.09 0.217 47 1.1855±0.0013252-32 (1–2) 1994 2.07 7.7 0.39 0.239 52 1.1854±0.0013252-32 (2–3) 1986 1.69 7.7 0.14 0.244 52 1.1864±0.0013252-32 (3–4) 1978 2.21 7.8 0.04 0.235 48 1.1851±0.0013252-32 (4–5) 1970 1.85 7.8 0.03 0.209 45 1.1851±0.0013252-32 (5–6) 1961 2.13 7.7 0.03 0.174 41 1.1883±0.0013252-32 (6–7) 1952 2.20 7.8 0.03 0.165 43 1.1869±0.0013252-32 (7–8) 1944 1.73 7.8 0.03 0.149 39 1.1861±0.0013252-32 (8–9) 1935 1.64 8.0 0.03 0.147 39 1.1876±0.0013252-32 (9–10) 1927 2.04 8.0 0.03 0.140 39 1.1877±0.0013252-32 (12–13) 1901 1.50 8.2 0.03 0.099 31 1.1905±0.0013252-32 (15–16) 1875 1.54 8.0 0.03 0.088 25 1.1954±0.0013252-32 (18–19) 1846 1.56 7.8 0.03 0.086 22 1.1965±0.0013252-32 (21–22) 1816 1.78 7.7 0.03 0.075 25 1.1950±0.0013252-32 (24–25) 1784 1.60 7.2 0.03 0.047 17 1.1984±0.0013252-32 (27–28) 1750 1.87 6.9 0.02 0.049 18 1.1993±0.0013252-32 (30–31) 1713 1.48 6.4 0.02 0.046 18 1.1969±0.0013252-32 (33–34) 1671 1.34 6.7 0.02 0.057 20 1.1928±0.0013252-32 (36–37) 1622 1.05 6.5 0.02 0.018 16 1.2040±0.0013252-35 (0–1) 2006 n.d. n.d. n.d. 0.431 45 1.1828±0.0013252-35 (1–2) 2005 2.26 n.d. n.d. 0.404 44 1.1837±0.0013252-35 (2–3) 2004 2.12 n.d. n.d. 0.435 39 1.1827±0.0013252-35 (3–4) 2003 n.d. 7.7 0.02 0.461 63 1.1803±0.0013252-35 (4–5) 2002 2.29 7.8 0.02 0.421 55 1.1825±0.0013252-35 (5–6) 2000 2.67 7.9 0.02 0.492 55 1.1814±0.0013252-35 (6–7) 1999 2.74 8.0 0.02 0.542 43 1.1819±0.0013252-35 (7–8) 1998 2.73 8.0 0.02 0.594 48 1.1833±0.0013252-35 (8–9) 1996 2.51 8.0 0.02 0.549 47 1.1814±0.0013252-35 (9–10) 1995 2.70 8.0 0.02 0.496 53 1.1805±0.0013252-35 (12–13) 1991 2.72 8.2 0.03 0.569 57 1.1805±0.0013252-35 (15–16) 1987 2.55 8.2 0.03 0.514 53 1.1809±0.0013252-35 (18–19) 1982 2.27 8.3 0.03 0.433 46 1.1817±0.0013252-35 (21–22) 1977 2.44 8.1 0.02 0.431 41 1.1811±0.0013252-35 (24–25) 1972 2.44 8.2 0.03 0.348 41 1.1865±0.0013252-35 (27–28) 1967 2.35 8.3 0.03 0.355 35 1.1848±0.0013252-35 (30–31) 1962 2.19 8.4 0.03 0.263 33 1.1852±0.0013252-35 (33–34) 1957 2.41 8.5 0.03 0.268 45 1.1863±0.0013252-35 (36–37) 1953 2.49 8.6 0.03 0.404 54 1.1846±0.0013252-35 (39–40) 1948 2.26 8.2 0.03 0.195 35 1.1900±0.0013

n.d. – not determined.

Fig. 2. Down-core variations of organic carbon (Corg), Al, Mn, Hg and Pb concentrations.Dashed line represents the base of excess 210Pb (ca. 1850 AD).

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extrapolated based on the assumption of constant accumulationrates. The highest Hg and Pb concentrations were found in bothcores of the Cascais Submarine Canyon, probably reflecting the prox-imity to the Tagus Estuary (Table 1, Fig. 2). Nevertheless, sedimentsyounger than 1990 are characterized by Pb and Hg concentrationsthat are approximately 2 and 3-fold, respectively, lower than the av-erage Pb and Hg concentrations (93 μg g−1 and 0.95 μg g−1, respec-tively) obtained in sediment samples from the Tagus Prodelta of thesame age (Mil-Homens et al., 2009), which were characterized byHg and Pb concentrations one order magnitude lower than thatobtained inside of the Tagus Estuary (Canário et al., 2005; Vale etal., 2008). This suggests dilution with non-contaminated sedimentduring transport and deposition. Additionally, Costa et al. (2011)encountered high median Ca and low median Si concentrations incore 252-16 (Ca=20%; Si=15%) in comparison to cores 252-35(Ca=7%, Si=21%) and 252-32 (Ca=10%, Si=19%). The high me-dian concentrations of Si obtained in core 252-35 reflect the lateraltransport of detrital materials derived from the Tagus Estuary,whereas high Ca concentrations in core 252-16 indicate verticalsettling through the water column or reduced terrestrial contribu-tions. Furthermore, the 3 cores have nearly uniform grain-size dis-tributions throughout, with median fine fraction contents of about95%.

Temporal trends in Hg and Pb distribution generally show an in-crease towards the Present in all cores, but there is a slight decreasein concentrations in the top samples from both canyon cores. Inorder to compensate for the effect of sediment composition andgrain size, Pb concentrations were normalized to the Al content (e.g.Windom et al., 1989). Mercury concentrations were normalized tothe Corg content because Hg strongly complexes with organic carbonphases and this can act as a Hg concentrator in sediments (e.g.Daskalakis and O'Connor, 1995; Outridge et al., 2007). Trends of Hg/Corg and Pb/Al ratios are quite similar to those obtained for total con-centrations, suggesting factors other than grain-size or organic matter

content are responsible for the variations of metal concentrations(not shown due to similar patterns as the metal enrichments plottedin Fig. 3). While both total Hg content and Hg/Corg ratios show a de-crease with depth along the CSC, Pb has similar concentrations incores 252-32 and 252-35. However, over longer distances in theCSC, Jesus et al. (2010) noticed a distinct decreasing trend in Pbconcentrations.

In cores 252-32 and 252-16 color changes from yellowish gray/olive brown to grayish at distinct depths (2 and 12 cm, respectively)located in close proximity to Fe (not shown) and Mn peaks (Fig. 2),suggesting the occurrence of marked oxidation–reduction (redox)boundaries associated with post-depositional sedimentary processeswhich may have redistributed metals. Changes in sediment colorabove and below the redox boundaries could reflect variable Corg con-tent in the sediment and/or redox changes in the lattice Fe content ofclay minerals (Wilson et al., 1985). The redox profile with depth isdriven by the oxidation of Corg which leads to successively morereducing conditions with depth (e.g. Thomson et al., 1993). Thevariation in depth of the redox boundary (precipitation of Mn- andFe-oxides coupled with Corg oxidation) is linked to changes in sedi-mentation rates (e.g. Epping et al., 2002), with major redox bound-aries associated with low sedimentation rates such as observed incore 252-16. However, temporal Hg and Pb variations appear unrelatedto diagenetic enrichment of redox-sensitive elements represented byMn peaks (see Fig. 2). Moreover, the diagenetic remobilization of Hgfrom deeper layers would be expected to represent an importantcomponent in the magnitude of total Hg concentrations only inareas characterized by sedimentation rates lower than 0.0005 cmy−1 (Gobeil and Cossa, 1993), which is considerably less than in ourstudy. Both arguments support the anthropogenic origin of Hg andPb. Nevertheless, early diagenetic remobilization may have occurredin core 252-16 (Estremadura Spur) which is characterized by a sedi-mentation rate of 0.03 cm y−1 and relatively low Hg concentrations(~0.13 μg g−1).

Fig. 3. Temporal trends of enrichment factors for Hg and Pb. Samples older than 1750 AD for cores 252-16 and 252-32 are not shown. The age for each core is derived from 210Pbdating and was extrapolated for sediments older than 1800 based on the assumption of similar deposition conditions through time.

Table 2Pre-industrial values of Al (%), Hg and Pb (μg g−1) for the three studied cores, based on the average of samples older than 1800 AD (Costa et al., 2011). The pre-industrial values forcore 252-35, of which the base is considerably younger than 1800 AD, was estimated as the average of values in adjacent cores 252-30 (core collected in the CSC at 3914 m waterdepth, not considered in the present work) and 252-32. Pre-industrial values obtained by Jesus et al. (2010) in sediment cores from the Central Portuguese Margin are also shown.Data is presented as follows: average±standard deviation (SD).

Estremadura Spur Cascais Canyon

Core 252-16 (Costaet al., 2011)

Pre-industrial values(Jesus et al., 2010)

Core 252-32 (Costaet al., 2011)

Core 252-35 (Costaet al., 2011)

Pre-industrial values(Jesus et al., 2010)

Corg (%) 1.10±0.17 – 1.47±0.30 1.41±0.26 –

Al (%) 5.9±0.5 6.5±2.0 6.7±0.3 6.7±0.2 6.3±1.5Pb (μg g−1) 15±2 23±6 18±1 16±3 20±5Hg (μg g−1) 0.052±0.07 – 0.043±0.015 0.059±0.021 –

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The degree of Hg and Pb enrichment relative to pre-industrialvalues is defined as the metal enrichment factor (EF) according tothe equation:

EFmetal¼ metal=normalizing factorð Þsample= metal=normalizing factorð Þpre�industrialvalues

where enrichment is assessed relative to pre-industrial values(Table 2) taken as the average metal concentrations in sedimentsolder than 1800 AD (Costa et al., 2011). These values are in agreementwith those obtained by Jesus et al. (2010) in cores collected from thecentral Portuguese Margin (more specifically at Lisboa and Setubalsubmarine canyons, see Fig. 1 for locations of these physiographic fea-tures) for sediment samples representing sediments deposited justprior to 1850 AD. Since diagenetic enrichment is unimportant, theEF trends are interpreted as reflecting an increase of anthropogenicinfluence since the 1900's (Fig. 3). The highest Hg and Pb enrichmentin core 252-35, and the lowest enrichment in core 252-16, may beexplained by the influence of the Tagus River as well as the degreeto which contaminants were mixed with uncontaminated marinesediments. The steady enrichment trend of metal profiles in core252-32 probably reflects low sedimentation rates where enrichmentevents, such as observed in 1950 AD for core 252-35, are not pre-served. This event was also identified in the 3 studied sedimentcores of the Tagus Prodelta in agreement with the delay in the Portu-guese industrial development that occurred during World War II(Mil-Homens et al., 2009).

3.2. Temporal trends of stable Pb isotopic ratios in sediments

Stable Pb isotope profiles in the three cores are characterized by acontinuous decrease of 206Pb/207Pb towards the top of the cores(Fig. 4a, Table 1), reflecting an increase of anthropogenic Pb withlow 206Pb/207Pb ratios. The 206Pb/207Pb ratios are relatively constant(around 1.1955) in sediments older than 1850 AD. This value isslightly lower than the ratio of 1.2050 reported by Richter et al.(2009) for sediment dated ~1000 AD from a core collected from theLisboa Submarine Canyon (38.36667 N; 9.24167 W; 1710 m waterdepth). Age differences among cores indicate that basal levels ofshort cores have not reached the natural background 206Pb/207Pband suggest that inputs of Pb-enriched particles in the marine envi-ronment started to increase well before the Industrial Revolution.Several studies provide evidence of earlier input of anthropogenicPb during Greek and Roman times (e.g. Shotyk et al., 1998;Kylander et al., 2005). The 206Pb/207Pb ratios encountered in theupper centimeters of the Tagus short sediment cores collected atRosário saltmarsh (Sundby et al., 2005; Caetano et al., 2007) andnear a motorway bridge (Vasco da Gama Bridge opened to the traffic

Fig. 4. Variation of Pb isotopic ratios (206Pb/207Pb) versus approximate year of sedimentation. Pre-industrial values obtained in this study were included in grey boxes representingsediments older than 1875 AD. Error bars represent the uncertainty. Samples older than 1750 AD for cores 252-16 and 252-32 are not represented (a). 206Pb/207Pb ratios versus Pbenrichment factors (EFPb) (b).

Fig. 5. The down-core variations of δ202Hg values (‰) in studied sediment cores. Agelabels for samples older than 1800 AD are not presented (a). (b) δ202Hg values (‰)versus Hg enrichment factors (EFHg), and (c) δ202Hg values (‰) versus 1/Hg (μg g−1).

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in 1998; Cobelo-García et al., 2011) indicate an enhanced anthropo-genic influence with 206Pb/207Pb varying between 1.166 and 1.170,which is lower than that obtained in cores 252-35 and 252-32. Inpost-1970 portions of the two CSC cores the 206Pb/207Pb ratios areconsistent with the hypothesis that the Tagus Estuary is the principalsource of anthropogenic Pb. In the more proximal, shallower watercore (252-35) 206Pb/207Pb averages 1.1817, whereas in the more dis-tal, deeper water core (252-32) the average ratio is 1.1855, whichprobably reflects increased dilution by uncontaminated marine sedi-ments. Lead enrichment factors (EFPb) exhibit a good correlationwith Pb isotopic composition (206Pb/207Pb ratios), with the highestenrichment factors found in recent sediments and associated withless radiogenic (more contaminated) values of 206Pb/207Pb ratios(Fig. 4b).

3.3. Temporal trends of Hg isotopic composition in sediments

Down-core δ202Hg variability in the three cores does not revealany simple pattern with depth (Fig. 5a, Table 3) implying either mul-tiple sources, or varying amounts of isotope fractionation during Hgtransformation (e.g. biotic and abiotic reduction and volatilization)and mobilization in sediment, or a combination of both. Nevertheless,sediments dated after 1900 AD in each core location (except level1–2 cm (dated as 1973 AD) of core 252-16) are in general character-ized by high Hg enrichments (EFHg>1.5) and Hg concentrations(>0.1 μg g−1), associated with high δ202Hg values (Fig. 5b and c).This suggests a recent Hg source with higher δ202Hg values that isnot present in the oldest sediments, and is consistent with other pre-vious studies (Feng et al., 2010; Sonke et al., 2010). In Fig. 5c, thetrend of increasing δ202Hg versus 1/Hg for cores 252-16 and 252-32seems to also indicate a change in Hg composition through time,with increasing δ202Hg values. These results reflect the dominanceof anthropogenic Hg contamination in surface sediments, corroborat-ing stable Pb isotope results, which indicate a change in Pb sourcesreflecting a greater proportion associated with anthropogenic activi-ties (Fig. 4c). Gehrke et al. (2009) reported δ202Hg values for pre-anthropogenic Mediterranean background sediments (−0.76‰±0.16‰) and sapropel samples (−0.91‰±0.15‰) similar to those incore 252-16 for sediments older than 1750 AD (δ202Hg=−0.76‰±0.21‰), which clearly have lower δ202Hg values compared to recent“contaminated” sediments.

The three-isotope plots δ199Hg versus δ202Hg and δ201Hg versusδ202Hg (Fig. 6a & b) show that recent sediment samples from cores252-35 and 252-32 (CSC), which are characterized by higher δ202Hgvalues, do not show significant mass-independent fractionation(MIF). This is consistent with their proximity to the source of anthro-pogenic Hg contamination (the Tagus Estuary) and is in agreementwith results obtained from other contaminated sediments (Foucheret al., 2009; Feng et al., 2010) and also in lichens from industrialareas (Carignan et al., 2009). The only samples that reveal a slight de-viation from mass-dependent fractionation (MDF) of odd mercuryisotopes (199Hg and 201Hg) are from core 252-16, indicating that Hgin the sediments was subject to MIF (Fig. 6a & b). Odd isotope MIFanomalies are slightly positive (Δ199Hg=+0.09‰ and Δ201Hg=+0.04‰) in agreement with results reported for sediments fromthe Mediterranean and from San Francisco Bay (Gehrke et al., 2009,2011). In these studies the small deviation from mass-dependencewas interpreted to suggest photochemical reduction of Hg in the ma-rine system. These results are in contrast with negative values ofΔ199Hg measured for some continental sediments in previous studies(Laffont et al., 2009; Sonke et al., 2010; Liu et al., 2011).

The different degree ofMDF andMIF distinguishes core 252-16 fromthe other 2 cores (Fig. 7a & b). As shown in Fig. 7b, independent of theincrease of Hg concentrations (probably derived from different sourcesthrough the period of deposition) in each core location, sediment fromthe Estremadura Spur has a distinctly different isotopic signature (core252-16, Δ199Hg=0.03 to 0.17) than sediments from Cascais submarinecanyon (core 252-32, Δ199Hg=−0.01 to 0.04; core 252-35, Δ199Hg=−0.03 to 0.04). The overlap between Δ199Hg values in the CSC coressuggests a predominance of Hg from sediment contaminated with an-thropogenic Hg and transported through the canyon, over Hg fromothermarine particulate contributions to the sediment. The consistentlyhigher MIF (Δ199Hg) values (Fig. 7b) together with nearly constant Hgenrichment values (Figs. 3 & 5b) through time in core 252-16 suggestthat Hg in this core is less influenced by anthropogenic additions ofHg. The distinct isotopic signature measured in core 252-16 may reflectcontributions of Hg from different sources and/or different fractionationprocesses that affected Hg during transport to the seafloor.

3.4. Stable Hg isotopes versus stable Pb isotopes

The trends of increasing Pb and Hg enrichment towards the Pre-sent are in general characterized by less radiogenic 206Pb/207Pb ratios

Table 3Mercury isotopic composition of sediments in the studied cores. Mercury isotopic compositions (δ202Hg, δ201Hg, δ200Hg, δ199Hg, Δ199Hg, Δ200Hg and Δ201Hg) are reported in deltaand capital delta notation (in permil). 2 SD (standard deviation) uncertainties are based on n measurements.

Core (depth) n δ202Hg 2SD δ201Hg 2SD δ200Hg 2SD δ199Hg 2SD Δ201Hg 2SD Δ200Hg 2SD Δ199Hg 2SD

252-16 (0–1) 2 −0.24 0.04 −0.19 0.12 −0.12 0.02 0.02 0.10 −0.01 0.15 −0.01 0.01 0.08 0.11252-16 (1–2) 2 −0.94 0.04 −0.66 0.09 −0.48 0.04 −0.16 0.14 0.05 0.06 0.00 0.02 0.08 0.13252-16 (3–4) 1 −0.38 – −0.21 – −0.18 – −0.01 – 0.07 – 0.01 – 0.08 –

252-16 (6–7) 1 −0.21 – −0.10 – −0.03 – 0.08 – 0.06 – 0.08 – 0.13 –

252-16 (8–9) 1 −0.40 – −0.20 – −0.17 – 0.07 – 0.10 – 0.03 – 0.17 –

252-16 (12–13) 1 −0.63 – −0.50 – −0.30 – −0.07 – −0.02 – 0.01 – 0.09 –

252-16 (15–16) 3 −1.04 0.12 −0.74 0.07 −0.50 0.02 −0.17 0.04 0.04 0.10 0.02 0.05 0.09 0.02252-16 (24–25) 1 −0.56 – −0.33 – −0.24 – −0.06 – 0.09 – 0.04 – 0.08 –

252-16 (30–31) 1 −0.81 – −0.59 – −0.39 – −0.17 – 0.02 – 0.02 – 0.03 –

252-32 (0–1) 2 −0.33 0.13 −0.20 0.01 −0.15 0.11 −0.04 0.04 0.05 0.09 0.02 0.05 0.04 0.00252-32 (3–4) 2 −0.18 0.11 −0.15 0.16 −0.07 0.09 −0.01 0.08 −0.01 0.07 0.02 0.03 0.03 0.05252-32 (9–10) 1 −0.26 – −0.17 – −0.10 – −0.03 – 0.02 – 0.03 – 0.03 –

252-32 (15–16) 2 −0.17 0.09 −0.15 0.18 −0.10 0.05 −0.06 0.07 −0.02 0.11 −0.01 0.01 −0.01 0.05252-32 (21–22) 2 −0.23 0.19 −0.15 0.17 −0.12 0.18 −0.06 0.06 0.02 0.03 0.00 0.08 −0.01 0.01252-32 (33–34) 1 −0.47 – −0.37 – −0.21 – −0.12 – −0.02 – 0.03 – 0.00 –

252-35 (4–5) 2 −0.26 0.04 −0.18 0.05 −0.12 0.10 −0.05 0.04 0.02 0.08 0.01 0.08 0.02 0.03252-35 (9–10) 2 −0.50 0.03 −0.41 0.19 −0.27 0.12 −0.09 0.09 −0.03 0.16 −0.02 0.10 0.04 0.08252-35 (21–22) 2 −0.36 0.25 −0.32 0.17 −0.22 0.08 −0.11 0.01 −0.05 0.02 −0.04 0.05 −0.02 0.07252-35 (27–28) 2 −0.18 0.15 −0.15 0.19 −0.12 0.12 −0.06 0.01 −0.02 0.09 −0.04 0.04 −0.02 0.02252-35 (35–36) 2 −0.35 0.03 −0.29 0.01 −0.20 0.00 −0.12 0.07 −0.03 0.03 −0.03 0.02 −0.03 0.08252-35 (39–40) 2 −0.39 0.15 −0.30 0.03 −0.20 0.04 −0.12 0.01 0.00 0.15 0.00 0.03 −0.02 0.03

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and higher δ202Hg values that are indicative of increasing anthropo-genic influences (Figs. 4, 5, 7, 8a & b). The absence of significant MIF(non-zero Δ199Hg and Δ201Hg values) in sediments of the CSC togeth-er with reduced values of 206Pb/207Pb are interpreted as reflecting theinfluence of anthropogenic contributions derived from the Tagus es-tuary (Fig. 8b). The difference in Δ199Hg and Δ201Hg values in CSCcores (252-32 and 252-35), versus core 252-16 from EstremaduraSpur, suggests the existence of different contaminant sources and/ordifferent fractionation processes affected the Hg isotopic composi-tion of these sediments. This contrast may potentially be used fordistinguishing between environments receiving contaminated fine-grained sediment particles from the adjacent estuary and shelf versushemipelagic areas. In sediments derived from hemipelagic areas, atrend of decreasing 206Pb/207Pb values together with increasing Pband Hg enrichment towards the Present may be associated with in-creasing contaminant transport through the atmosphere. The compo-sition of atmospheric particles has changed with time, and there hasbeen a significant increase in heavy metal contents reflecting the in-crease of human activities (e.g. heavy industry, coal burning, smeltersand automotive emissions; Nriagu (1996)). The slightly more radio-genic 206Pb/207Pb values for pre-industrial sediments in core 252-32compared to core 252-16 (Fig. 4a) may suggest an atmospheric com-position marked by less radiogenic Pb than the pre-industrial Pb iso-topic composition derived from the Tagus area. Since 1875 AD, thistrend was inverted in response to the increase of anthropogenic Pbcontributions derived from urban and industrial developments thatreached a maximum during the second half of the twentieth century.Although data on Hg isotopes in atmospheric deposition are scarce,Gratz et al. (2010) and Sherman et al. (2011) measured negativeδ202Hg and positive Δ199Hg values in atmospheric precipitation sam-ples in the Great Lakes Region and Florida (North America) and these

data indicate that atmospheric depositionmay potentially provide an ex-planation for theHg isotope trendswe observe in hemipelagic sediments.However, the processes that affect Hg isotope fractionation in the atmo-sphere, during transport through themarinewater column, and followingdeposition need to be further studied and better understood.

Although some industries have decreased or stopped heavymetals emissions during the last 2–3 decades, their legacy of contam-ination still lingers in the Tagus estuarine sediments. Re-suspensionby waves, tidal currents and dredging operations together with bio-turbation are responsible for exposing older anoxic contaminatedsediments to sunlight. Canário and Vale (2004) showed that the dailyexposure of intertidal sediments from the Tagus to direct solar radiationinduces photochemical Hg-reduction and consequent release of Hg0 tothe atmosphere, slightly reducing Hg concentrations in sediments andpore waters by altering its partitioning and speciation. Photochemicalreduction of Hg2+ and methylmercury species, the main processthought to cause MIF in the environment (e.g. Bergquist and Blum,2009), occurs in the Tagus Estuary. The modification of the MIF valuesfrom the original Hg source signature by photochemical Hg-reductionmay thus affect the Tagus estuarine sediments prior to mixture withless contaminated sediments and transfer to the seafloor. Differencesbetween estuarine and sea sediment dynamics may in part explainthe relatively low Δ199Hg values for the surface sediments in bothcores of CSC when compared with the Δ199Hg values obtained byGehrke et al. (2011) for San Francisco Bay wetland (Δ199Hg=0.13±0.03‰) and intertidal (Δ199Hg=0.08±0.03‰) sediments.

4. Conclusions

The Hg (δ202Hg, Δ201Hg, Δ199Hg) and Pb (206Pb/207Pb) isotopiccompositions of sediments through time contribute to reconstruction

Fig. 6. Three-isotope plot of δ199Hg (‰) versus δ202Hg (‰) (a) and δ201Hg (‰) versus δ202Hg (‰) (b). The solid lines represent the theoretical mass-dependent fractionation line(MDF) based on the exponential law (Blum and Bergquist, 2007): δ199Hg=0.2520×δ202Hg and δ201Hg=0.7520×δ202Hg.

Fig. 7. Plots of Δ199Hg (‰) versus δ202Hg (‰) (a), and Δ199Hg values (‰) versus 1/Hg (μg g−1) (b).

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of the history of Hg and Pb contamination in the central PortugueseMargin. Through the use of stable Hg and Pb isotopic signatures it ispossible to confirm an anthropogenic origin for Hg and Pb enrich-ments in the three sediment cores and also to distinguish betweennatural and anthropogenic Hg and Pb contributions in the sediments.Mercury and Pb enrichments in both cores of the CSC are associatedwith the transfer of contaminated sediments from the Tagus area,confirming the role of the CSC as a contaminant carrier to deeperparts of the Portuguese Margin. In an area characterized by hemipela-gic sedimentation with no direct continental inputs, such as the Estre-madura Spur, the slight Hg and Pb enrichments together with adecreasing trend of 206Pb/207Pb ratios to less radiogenic values to-wards the Present suggests an increase in contaminant transportthrough the atmosphere. Additionally, the distinct Hg isotopic signalmarked by positive Δ199Hg values in the Estremadura Spur coremay reflect an influence from atmospheric sources and/or isotopicfractionation processes that affect Hg in the environment from thetime it is released until the time it is deposited in hemipelagicsediments.

Acknowledgements

This study was financially supported by the Portuguese ScienceFoundation through the CANYONS project (PDCT/56781/2004). Allcores were recovered by Royal Netherlands Institute for Sea Research(NIOZ) within the projects “Lead in Canyons” (contract 814.01.018 ofthe Netherlands Organization for Scientific Research (NWO)) andHERMES – Hotspot Ecosystem Research on the Margins of EuropeanSeas (contract GOCE-CT-2005-511234). We would also like to thankall the technical support given at NIOZ, University of Michigan, IPIMAR,LNEG-UGM and LNEG-LBA. The manuscript benefitted from helpfulcomments and suggestions by two anonymous reviewers.

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