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G2 Mercury stable isotope biogeochemistry

Monday, 25 July, 2011

MG2-P1 — 11:00-12:00 and 17:30-18:30
TEMPORAL VARIATIONS IN THE STABLE ISOTOPIC COMPOSITION OF HG IN SEDIMENTS OF THE CENTRAL PORTUGUESE MARGIN
Authors: MIL-HOMENS, Mário1, BLUM, Joel D.2, CANÁRIO, João3, CAETANO, Miguel3, COSTA, Ana1, LEBREIRO, Susana4, BRANCO, Vasco3, JOHNSON, Marcus W.2, TRANCOSO, Maria A.5, DE STIGTER, Henko6, RICHTER, Thomas6, BOER, Wim6
(1) UGM-LNEG, mario.milhomens@lneg.pt; (2) University of Michigan; (3) IPIMAR-INRB; (4) IGME; (5) LAQ-LNEG; (6) NIOZ;

Three short marine sediment cores from the Cascais canyon (PE252-32 at 38.36307N; 9.50690W; 2100 m water depth; and PE252-35 at 38.49345N; 9.47880W; 445 m water depth) and the Estremadura Spur (PE252-16 –at 39.17670N; 10.66612W; 2084 m water depth) on the Portuguese Margin, collected during cruise Canyons 2006 onboard the RV Pelagia, were analysed for Hg, Pb, Al, and Pb (206Pb/207Pb) and Hg (d202Hg, ?199Hg, ?201Hg) isotope stable ratios in order to reconstruct Hg and Pb metal contamination. Geochronology is based on 210Pb. The studied cores reveal increasing Hg and Pb, independent of grain-size variations, since the middle of the nineteen century towards the Present-day. At the same time we have observed a decreasing trend of 206Pb/207Pb to less radiogenic values towards the surface, indicating anthropogenic contamination with different intensities. Down-core d202Hg values do not show a clear pattern implying either multiple sources, or varying amounts of microbial Hg reduction and loss, or a combination of both. The lowest values of d202Hg are shown in general in the older sediments encompassed by low Hg and Pb contents, suggesting a low input of the anthropogenic component. Core PE252-16 (located in an area of predominant hemipelagic sedimentation) has an average of d202Hg of -0.69‰±0.24‰ in sediments older than AD 1800, similar to values obtained by Gehrke et al. (2009) for Mediterranean sapropel samples. In this core, both odd isotopes deviate from the theoretical mass-dependent fractionation line (?199Hg, ?201Hg), showing that sediments were subject to a small degree of mass-independent fractionation (MIF) with ?199Hg =+0.09‰±0.04‰ and ?201Hg=+0.04‰±0.04‰. These slightly positive values indicate that the cause of MIF could be photochemical reduction of Hg2+. The 2 cores from the Cascais Canyon located closer to the Tagus estuary (an area highly influenced by Hg contamination derived from pyrite roasting and chloralkali plants) do not reveal significant MIF, which is consistent with the addition of anthropogenic Hg sources. This trend has also been observed by Estrade et al. (2010) in lichens where anthropogenic Hg is added near an industrial point source. In synthesis, the integration of stable Hg and Pb isotopic ratios, 210Pb chronology, and Hg and Pb concentrations, were useful in reconstructing the Hg and Pb contamination history in sediments of the Central Portuguese Margin.

MG2-P2 — 11:00-12:00 and 17:30-18:30
A SPATIAL ANALYSIS OF MERCURY CONCENTRATION AND ISOTOPIC RATIO IN LICHENS FROM SOUTH WESTERN FRANCE (PYRÉNÉES-ATLANTIQUES)
Authors: BARRE, Julien P.G.1, AMOUROUX, David1, DELETRAZ, Gaëlle2, BÉRAIL, Sylvain1, DONARD, Olivier F.X.1, TRAN, Thao2
(1) IPREM/LCABIE UMR 5254, Pau, France; (2) Laboratoire Société, Environnement et Territoire (SET), UMR 5603, Pau, France;

A recent study demonstrated that analysis of the dispersion of mercury isotopes in epiphytic lichens can distinguish local anthropogenic sources of contamination and background atmospheric inputs (variable deficit in odd isotopes). The project proposes to study the variability of mercury atmospheric contamination in lichens of the Pyrénées-Atlantiques (South-West of France), a mesoscale area exhibiting urban, rural, mountain range and coastal land type. To this end, a purposive sampling was conducted in the area including geographic information system data output to take into account the different characteristics of such as population density, water balance and distance to the ocean. Different geographical areas were identified as urban, industrial, agricultural and forest where various species of lichens were collected such as Usnea sp., Evernia Prunastri, Parmelia Sulcata et Lobaria pulmonaria, and mixed in order to be representative of the biological diversity of each sampling site. Selected sample collection was also carried out during two different seasons (spring and fall). Comparison of mercury concentrations in lichens indicates relatively low levels over the Pyrénées-Atlantiques indicating no significant sources of contamination (from 0.038 to 0.160µg/g). Nevertheless, the isotopic composition of lichens determined by MC-ICP-MS may reveal a contribution of local sources for lichens collected in urban and industrial zones. Instead lichens taken away from these sources (agricultural and forestry areas) exhibit a background pollution linked to global atmospheric mercury inputs. This study demonstrates that mercury isotopic composition, in opposition to mercury concentration, measured in lichens from less polluted area may provide a better discrimination of mercury atmospheric contamination pathways. The possibility of identifying sources of contamination despite similar concentrations of mercury in the investigated area will be discussed taking into account various geographical parameters available from each sampling sites.

MG2-P3 — 11:00-12:00 and 17:30-18:30
VARIATIONS IN STABLE ISOTOPE FRACTIONATION OF HG IN FOOD WEBS OF ARCTIC LAKES
Authors: GANTNER, Nikolaus1, HINTELMANN, Holger 2, ZHENG, Wang2, MUIR, Derek3
(1) W-CIRC, Environment Canada, U. of Victoria, Victoria, BC, Canada, gantnern@uvic.ca; (2) Department of Chemistry, Trent University, Peterborough, ON, Canada; (3) Water Science and Technology Directorate, Environment Canada, Burlington, ON, Canada.

Biotic and abiotic fractionation of mercury (Hg) isotopes has recently been shown to occur in aquatic environments. We determined isotope ratios (IRs) of Hg in food webs (zooplankton, chironomids, Arctic char) and sediments of 10 Arctic lakes from four regions and investigated the extent of Hg isotope fractionation. Hg IRs were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP/MS). Hg mass independent fractionation (MIF; ?199Hg) and mass dependent fractionation (MDF;d202Hg) were calculated and compared among samples. IRs of Hg in sediment were characterized mainly by MDF and low MIF (?199Hg -0.37 to 0.74‰). However, all biota showed evidence of MIF, most pronounced in zooplankton (?199Hg up to 3.40 ‰) and char (?199Hg up to 4.87‰). Zooplankton takes up highly fractionated MeHg directly from the water column, while benthic organisms are exposed to sedimentary Hg, which contains less fractionated Hg. As evidenced by d13C measurements, benthic chironomids make up a large proportion of char diet, explaining in part why MIFchar<MIFzooplankton in lakes, where both samples were measured. Hg IRs in char varied among regions, while char from lakes from each region showed similar degrees of MIF. A MIF-offset was derived representing the mean MIF difference between sediment and fish, and indicated that fish in two regions retain sediment signatures altered by a consistent offset. Due to its minimal lake to-catchment area and very high water retention time (~330 years), the meteor impact crater lake (Pingualuk) reflects a “pure” atmospheric Hg signature, which is modified only by aqueous in-lake processes. All other lakes are also affected by terrestrial Hg inputs and sediment processes. We will be discussing our recent findings and discuss future applications of IRs in the context of the Arctic environment.

MG2-P4 — 11:00-12:00 and 17:30-18:30
EVOLUTION OF ATMOSPHERIC MERCURY DEPOSITION AND EMISSION SOURCES RECORDED IN AN OMBROTROPHIC PEAT BOG IN THE FRENCH PYRENEES OVER THE PAST 10.000 YEARS
Authors: HEIMBÜRGER, Lars-Eric1, ENRICO, Maxime 1, DE VLEESCHOUWER , Francois2, LE ROUX, Gael2, SONKE, Jeroen E.1
(1)CNRS-GET, heimburger@get.obs-mip.fr; (2) ECOLAB;

Ombrotrophic peat bogs integrate atmospheric deposition and can therefore be used as archives to interpret variations of emission sources, e.g. addressing anthropogenic impacts. Coal combustion is nowadays the major anthropogenic source for Hg in the atmosphere. The wide range of Hg isotopic compositions observed in coal deposits suggests that Hg isotopes signatures may offer a new tool to track mercury pollution. The isotopic signature of Hg accumulated in peat bogs may give new insights about its biogeochemical cycling and changes in emission sources. In September 2010, 3 peat cores were collected in the Pinet peat bog (French Pyrenees, 880m a.s.l.). This peat bog is relatively sheltered from direct emission sources and integrates therefore mostly Hg transported over long distances. The cores, that received atmospheric inputs during the past 10.000 years, are about 5m long and have been dissected in slices of 1cm allowing a temporal resolution of 20 years. We present a comparison of the 3 cores to address the local variability of Hg records, which are backed-up by auxiliary parameters such as loss on ignition, water content and density. Furthermore, a combustion and trapping method was used to isolate and purify Hg from the organic matrix (Biswas et al., 2008) before analysis of Hg isotopes using a MC-ICPMS “Neptune” with cold vapor generation. Total Hg concentrations are around 30ng/g before times of significant industrial emissions, consisted with other ombrotrophic peat records, and increase significantly at the top of the cores. However, several minor peaks are detected before industrial times. Hg stable isotopes were used to investigate variations in Hg deposition, in particular the increased impact due to coal combustion.

Monday, 25 July, 2011