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Studies of an elusive element: processes that influence the net retention of mercury in lake sediments and peatlands
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Because of its toxic nature mercury is a threat to both wildlife and human health, and thus, it is an element of concern in the environment. Currently much of the mercury emitted to the atmosphere is derived from anthropogenic sources – both direct emissions and re-emission of already deposited anthropogenic mercury. Following deposition mercury is affected by a long array of processes, and this thesis has focused on trying to increase our knowledge on the net retention of mercury in lake sediments and peatlands. This information is vital in order to understand how mercury behaves in the environment and where mercury is at risk of becoming a problem. Knowledge about the retention of mercury is also important when using lake sediments and peat records as environmental archives over past mercury deposition.

By using varved, annually laminated, sediments I have determined that lake sediments are reliable archives for inorganic mercury, but not for methylmercury. A study of the spatial distribution of mercury in a whole-lake basin shows that inorganic- and methylmercury are controlled by different sediment properties. Inorganic mercury is controlled by combination of fine-grained mineral matter and organic matter concentrations, whereas methylmercury is controlled by water depth and sulfur concentration. This study also shows that especially methylmercury have a very heterogeneous spatial distribution across the lake basin, something that might be of large importance when using lake sediments to calculate whole-lake burdens of mercury. In a study regarding the effects of vegetation on the net retention of mercury in a peatland I showed that there are considerable differences in both plant- and peat-mercury concentrations depending on vegetation type. This might have implications for the use of peat records as archives over atmospheric mercury deposition. Finally I have used a combination of a peat and a lake sediment record to study how past and recent climatic changes affects the stability of a peatland currently underlain by permafrost. Here we are able to show that destabilization of peatlands, as a result of permafrost melt, can cause a significant release of organically bound mercury from the mire to the surrounding aquatic environment. Considering the currently warming climate there is a risk of sub-arctic peatlands turning into mercury sources, which might be important to recognize when assessing current mercury pollution pathways.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, Institutionen för ekologi, miljö och geovetenskap , 2010. , 20 p.
Keyword [en]
Mercury, retention, lake sediments, peatlands, environmental archives, sedimentation, diagenesis, vegetation, permafrost
Research subject
biology, Environmental Science
Identifiers
URN: urn:nbn:se:umu:diva-38424ISBN: 978-91-7459-113-2 (print)OAI: oai:DiVA.org:umu-38424DiVA: diva2:377707
Public defence
2011-01-14, KBC-huset, KB3A9 (lilla hörsalen), Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2010-12-21 Created: 2010-12-14 Last updated: 2010-12-21Bibliographically approved
List of papers
1. Assessing the stability of mercury and methylmercury in a varved lake sediment deposit
Open this publication in new window or tab >>Assessing the stability of mercury and methylmercury in a varved lake sediment deposit
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2008 (English)In: Environmental Science & Technology, ISSN 0013-936X, Vol. 42, no 12, 4391-4396 p.Article in journal (Refereed) Published
Abstract [en]

Using lake sediments to infer past total mercury and methylmercury loading to the environment requires that diagenetic processes within the sediment do not significantly affect the concentrations or net accumulation rates of the mercury species. Because carbon is lost during early sediment diagenesis, the close link between carbon and mercury raises the question of how reliable lake sediments are as archives of total mercury and methylmercury loading. In this study we used a series of freeze cores taken in a lake with varved (annually laminated) sediment to assess the stability of total mercury and methylmercury over time. By tracking material deposited in specific years in cores collected in different years, we found that despite a 20–25% loss of carbon in the first 10–15 years, there was no apparent loss of total mercury over time; hence, lake sediments can be considered as reliable archives. However, over the first 5–8 years after sedimentation, about 30–40% of the methylmercury was lost (a decrease of 0.025–0.030 μg MeHg m−2 yr−1), suggesting that sediment profiles showing increasing methylmercury concentrations toward the sediment surface are in large part an artifact of diagenetic processes (net demethylation), rather than a record of changes in methylmercury loading.

Place, publisher, year, edition, pages
American Chemical Society, 2008
Identifiers
urn:nbn:se:umu:diva-10271 (URN)10.1021/es7031955 (DOI)18605560 (PubMedID)
Available from: 2008-08-06 Created: 2008-08-06 Last updated: 2013-01-17Bibliographically approved
2. A whole-lake basin analysis of the spatial distribution of total- and methylmercury in relation to the sediment matrix using WD-XRF and FT-IRS
Open this publication in new window or tab >>A whole-lake basin analysis of the spatial distribution of total- and methylmercury in relation to the sediment matrix using WD-XRF and FT-IRS
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The aim of this study was to try and identify some of the factors controlling the spatial distributions of total- and methyl-Hg across a single whole-lake basin. In order to achieve this we analyzed the geochemical composition of 83 surface sediment samples from the lake Stor-Strömsjön using WD-XRF and FT-IRS. Our results show that in the north-western basin (i.e., the main recipient of the dominant inlet) total-Hg distribution is controlled by fine-grained mineral matter, while the eastern basin (which lacks major inlets) the main controlling factor is organic matter. There is only a weak correlations between methyl-Hg concentrations and either total-Hg (r2=0.12) or organic matter (r2=0.15) concentrations. Instead the distribution of methyl-Hg is largely controlled by water depth and sulfur (59 %). The spatial patterns for total-Hg concentrations vs. inventories (mass per unit area) are rather different. Even if total-Hg concentrations are about the same in locations controlled by organic matter and fine-grained mineral matter, the total-Hg inventories are much higher in the latter. For methyl-Hg the spatial distribution is patchy both for concentrations and inventories, and the highest methyl-Hg inventory was found in a shallow location with coarse-grained minerogenic sediment. The large variations in the controlling factors, even within a single whole-lake basin, suggest that lake type and surrounding catchment have an effect on the behavior of Hg. This needs to be recognized both when studying Hg loading on ecosystems, and when using lake sediments to reconstruct atmospheric Hg deposition.

Keyword
Mercury, Methylmercury, Lake sediment, Spatial distribution, Sediment composition, WD-XRF, FT-IRS
National Category
Environmental Sciences
Research subject
biology, Environmental Science
Identifiers
urn:nbn:se:umu:diva-38423 (URN)
Available from: 2010-12-14 Created: 2010-12-14 Last updated: 2010-12-21Bibliographically approved
3. Importance of vegetation type for mercury sequestration in the northern Swedish mire, Rödmossamyran:  
Open this publication in new window or tab >>Importance of vegetation type for mercury sequestration in the northern Swedish mire, Rödmossamyran:  
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2010 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 74, no 24, 7116-7126 p.Article in journal (Refereed) Published
Abstract [en]

Even if mires have proven to be relatively reliable archives over the temporal trends in atmospheric mercury deposition, there are large discrepancies between sites regarding the magnitude of the anthropogenic contribution to the global mercury cycle. A number of studies have also revealed significant differences in mercury accumulation within the same mire area. This raises the question of which factors, other than mercury deposition, affect the sequestration of this element in peat. One such factor could be vegetation type, which has the potential to affect both interception and retention of mercury. In order to assess how small-scale differences in vegetation type can affect mercury sequestration we sampled peat and living plants along three transects on a northern Swedish mire. The mire has two distinctly different vegetation types, the central part consists of an open area dominated by Sphagnum whereas the surrounding fen, in addition to Sphagnum mosses, has an understory of ericaceous shrubs and a sparse pine cover. A few main patterns can be observed in our data; (1) Both peat and Sphagnum-mosses have higher mercury content (both concentration and inventory) in the pine-covered fen compared to the open Sphagnum area (100% and 71% higher for peat and plants, respectively). These differences clearly exceed the 33% difference observed for lead-210, which is considered as a good analogue for atmospheric mercury deposition. (2) The differences in mercury concentration between peat profiles within a single vegetation type can largely be attributed to differences in peat decomposition. (3) When growing side by side in the open Sphagnum area, the moss species Sphagnum subsecundum has significantly higher mercury concentrations compared to S. centrale (24 ± 3 and 18 ± 2 ng Hg g−1, respectively). Based on these observations we suggest that species composition, vegetation type and decomposition can affect the mercury sequestration in a peat record, and that any changes in these properties over time, or space, have the potential to modify the mercury deposition signal recorded in the peat.

Place, publisher, year, edition, pages
Elsevier Ltd, 2010
Identifiers
urn:nbn:se:umu:diva-38044 (URN)10.1016/j.gca.2010.09.026 (DOI)000285076600012 ()
Available from: 2010-11-23 Created: 2010-11-23 Last updated: 2017-12-12Bibliographically approved
4. Climate driven release of carbon and mercury from permafrost mires increases mercury loading to sub-arctic lakes
Open this publication in new window or tab >>Climate driven release of carbon and mercury from permafrost mires increases mercury loading to sub-arctic lakes
2010 (English)In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 408, no 20, 4778-4783 p.Article in journal (Refereed) Published
Abstract [en]

In sub-arctic and arctic regions mercury is an element of concern for both wildlife and humans. Over thousands of years large amounts of atmospherically deposited mercury, both from natural and anthropogenic sources, have been sequestered together with carbon in northern peatlands. Many of these peatlands are currently underlain by permafrost, which controls mire stability and hydrology. With the ongoing climate change there is concern that permafrost thawing will turn large areas of these northern peatlands from carbon/mercury-sinks into much wetter carbon/mercury-sources. Here we can show that such a change in mire structure in the sub-arctic Stordalen mire in northern Sweden actually is responsible for an increased export of mercury to the adjacent lake Inre Harrsjön. We also show that sediment mercury accumulation rates during a warm period in the pre-industrial past were higher than in the 1970s when atmospheric input peaked, indicating that in areas with permafrost, climate can have an effect on mercury loading to lakes as large as anthropogenic emissions. Thawing of permafrost and the subsequent export of carbon is a widespread phenomenon, and the projection is that it will increase even more in the near future. Together with our observations from Stordalen, this makes northern peatlands into a substantial source of mercury, at risk of being released into sensitive arctic freshwater and marine systems.

Keyword
mercury, permafrost dynamics, sediment, peat; NIRS
Identifiers
urn:nbn:se:umu:diva-35574 (URN)10.1016/j.scitotenv.2010.06.056 (DOI)000281931500058 ()20674959 (PubMedID)
Available from: 2010-08-24 Created: 2010-08-24 Last updated: 2017-12-12Bibliographically approved

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