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Long-term (~57 ka) controls on mercury accumulation in the Southern Hemisphere reconstructed using a peat record from Pinheiro mire (Minas Gerais, Brazil) Marta Pérez-Rodríguez Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on January 5, 2015
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Long-term (~57 ka) controls on mercury accumulation in the Southern Hemisphere reconstructed using a peat record from Pinheiro mire (Minas Gerais, Brazil)
Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:
Environmental Science & Technology es-2014-04826d.R2 Article 29-Dec-2014 Pérez-Rodríguez, Marta; Universidade de Santiago de Compostela, Edafoloxía e Química Agrícola Horák-Terra, Ingrid; ESALQ/ Universidade de São Paulo, Departamento de Ciência do Solo Rodríguez-Lado, Luis Manuel; University of Santiago de Compostela, Departamento : Edafología y Química Agrícola. Fac. Bioloxia Aboal, Jesus; Univ. Santiago de Compostela, Biologia Celular y Ecologia Martínez-Cortizas, Antonio; Faculty of Biology, University of Santiago de Compostela, Soil Science and agricultural chemistry
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Long-term (∼57ka) controls on mercury accumulation in the Souther Hemisphere reconstructed using a peat record from Pinheiro mire (Minas Gerais, Brazil) Marta P´erez-Rodr´ıguez,∗,† Ingrid Hor´ak-Terra,‡ Luis Rodr´ıguez-Lado,† Jes´us R. Aboal,¶ and Antonio Mart´ınez Cortizas† Departamento de Edafolog´ıa y Qu´ımica Agr´ıcola,Universidad de Santiago de Compostela, Spain, Departamento de Ciˆencia do Solo, Escola Superior de Agricultura Luiz de Queiroz ESALQ/ USP,Piracicaba-SP, Brazil, and Ecolog´ıa,Universidad de Santiago de Compostela, Spain E-mail:
[email protected] Abstract 1
Natural archives have been used to reconstruct mercury atmospheric deposition at
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different spatial and temporal scales during the Holocene in the Northern Hemisphere.
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In this study we present the results from a Brazilian mountain mire (Pinheiro mire,
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Minas Gerais, SE Brazil), extending back to ∼57 ka. The core was analyzed for mercury ∗
To whom correspondence should be addressed Departamento de Edafolog´ıa y Qu´ımica Agr´ıcola, Facultad de Biolog´ıa, Campus Sur s/n, Santiago de Compostela, Spain ‡ Departamento de Ciˆencia do Solo, Escola Superior de Agricultura Luiz de Queiroz ESALQ/ USP, Av. P´ adua Dias, 11, 13418-900, Piracicaba-SP, Brazil ¶ Ecolog´ıa, Facultad de Biolog´ıa, Campus Sur s/ n, Santiago de Compostela, Spain †
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concentration, organic matter content, organic carbon isotopic composition and tracers
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of mineral matter flux. Principal components analysis followed by principal components
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regression enabled us to determine the evolution of the weight of the latent processes
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governing the accumulation of mercury through time. We show that climate change
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was the main driver for the variations of mercury concentrations, either indirectly by
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i) enhancing soil erosion in the mire’s catchment, which led to a decrease in mercury
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concentration due to dilution by low mercury-containing mineral matter, ii) increasing
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regional dust deposition, which resulted in increased concentrations; or directly, by
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long-term changes in atmospheric wet deposition (arid vs humid periods). Internal
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peat processes (i.e. decomposition and mass loss) had a minor influence at the time
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scale represented by the core.
Introduction 16
Mercury is a metal of environmental concern because of its high volatility, long atmospheric
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residence time and the toxicity of its methylated forms. 1 Elemental mercury (Hg0 ) is the
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dominant species in the atmosphere, 2,3 with a mean residence time of 1-2 years. This means
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that a substantial fraction of mercury can be transported over long distances from the emis-
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sion sources. Eventually, elemental mercury is oxidized into mercury ions, Hg(II), in the
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atmosphere, which are washed out by rainfall. 4 Hg(II) is the dominant species in precipita-
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tion. 5 Particulate mercury represents a minor fraction of the total mercury in the atmosphere
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and can be dispersed over tens to hundreds of kilometers. 3 Methylated forms of mercury are
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most toxic, and bioaccumulate and become enriched at the higher trophic levels. Hammer-
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schmidt and Fitzgerald 6 found that, at continental scale, mercury deposition was correlated
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to the concentrations of mercury in fish.
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The use of natural archives such as peatlands, 7–29 ice-cores 30–32 and lake-sediments 22,23,33–49
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to reconstruct mercury atmospheric deposition trends on a local, regional and global scale
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has increased over the last two decades. However, large differences in the estimated back2
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ground fluxes exist between lake-sediments and peat cores, 44 hampering our understanding
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of emission sources, atmospheric processes, rates of exchange between terrestrial and ocean
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and atmospheric pools. 50–52
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It has been argued that different factors, such as organic matter degradation and mass loss
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associated to the evolution of the peat, 13,44,53,54 control mercury accumulation in peatlands.
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Climate may also play a significant role, either by controlling the re-emission of part of the
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accumulated mercury 8 or by inducing changes in peat decomposition. 54 These considerations
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are almost exclusively based on Holocene records and only a few studies extended their
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conclusions back to the Pleistocene. For example, Roos-Barraclough et al. 12 studied a 14500-
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year peat record from the Swiss Jura Mountains and found a 5 fold increase in mercury
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concentration during the Younger Dryas (9550-11050 BP). This increase was attributed either
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to higher particle scavenging or to an increase in the oceanic productivity, which led to higher
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mercury emissions from the oceans under cold climatic conditions. Jitaru et al. 55 also found
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elevated mercury concentrations in sections of Antarctic ice cores corresponding to glacial
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periods over the last 672,000 years BP, attributing them to mercury depletion phenomena
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in the polar atmosphere and increased atmospheric dust loads. More long-term records of
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mercury deposition are needed in order to fully understand the various processes that can
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influence emission, deposition and accumulation of mercury in continental ecosystems.
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Additionally, most of the studies which have investigated the processes controlling mer-
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cury concentrations in peat and in most other natural records are located in the Northern
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Hemisphere, while there is only a limited amount of quite heterogeneous information avail-
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able for the Southern Hemisphere. 10,13,16,30,35,36,46,48,49,56
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In this study, we analyzed a peat core sampled in a Brazilian mountain peatland (Pinheiro
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mire, Minas Gerais, SE Brazil), extending back to ∼57 ka. We analyzed it for mercury
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concentration, organic matter content (C, N), degree of decomposition (C/N ratios) isotopic
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composition (δ 13 C) of the organic matter, and tracers of mineral dust deposition (Si, Ti, Zr,
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Y, Th). The objectives of the research were i) to determine the main factors that controled
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peat mercury concentrations over long time scales; ii) to quantify their relative effect; and
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iii) to establish a chronology of the changes in mercury fluxes.
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Material and Methods
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Location and sampling
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Pinheiro (18◦ 03’44.42” S 43◦ 39’42.37” W) is a minerogenic, valley mire located in Serra do
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Espinha¸co Meridional, state of Minas Gerais (Brazil) (Figure 1 and in supporting informa-
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tion Figure S1). Higher elevations (1300-1320 m a.s.l.) are found at the northwestern and
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southern slopes of the catchment and the mire’s valley, which drains from SW to NE, is sit-
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uated between 1270 and 1230 m a.s.l. Although minerogenic, the location of the mire on the
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mountain summit and the rather small catchment area (∼140 ha) makes it quite sensitive to
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variations in rainfall. The background lithology corresponds to the Galho do Miguel Forma-
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tion, and comprises pure and thin quartzites (∼ 90%), thin micaceous quartzites and gray
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or greenish metargilites (∼ 5 a 10%). 57 According to the K¨oppen classification, the present
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climate is tropical mountainous, with mean annual temperature and annual accumulated
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precipitation values of 18.7◦ C 58 and 1500 mm respectively. 59 Rainfall is controlled by the
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South Atlantic Convergence Zone (SACZ), which is one of the most prominent features of
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the South Atlantic Monsoon System (SAMS) during the austral summer and it is associated
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with intense convective activity in the Amazonian region. 60 The SACZ extends in a south-
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eastern direction from the interior of the continent to the South Atlantic Ocean. 61 May to
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October are characterized by extremely low rainfall coinciding with the end of the monsoon
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period. Natural vegetation is similar to a typical Cerrado biome (savanna), but with patches
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of trees (seasonal semi-deciduous forest and Cerrad˜ao), known locally as “Cap˜oes”, which
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appear as small islands among grassland formations within the mire.
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The core was sampled in 2010 using a vibracore 62 to a depth of 324 cm. It is composed of 4
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quartzitic sands below 220 cm, sapric peat layers from 220 to 8 cm (with strongly decomposed
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peat at 212-220, 190-198, 136-157 and 8-58 cm), and an upper (
