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Redox-dependent changes in manganese speciation in Baltic Sea sediments from the Holocene Thermal Maximum: An EXAFS, XANES and LA-ICP-MS study

Lenz, Conny LU ; Behrends, Thilo; Jilbert, Tom; Silveira, Miguel and Slomp, Caroline P. (2014) In Chemical Geology 370. p.49-57
Abstract
Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca-Mn-carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e. g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000-4000 cal. yr BP) at a site in the northern Gotland Basin. Analyses were performed on... (More)
Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca-Mn-carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e. g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000-4000 cal. yr BP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum(Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7 mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe-S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters. (C) 2014 Elsevier B.V. All rights reserved. (Less)
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organization
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Contribution to journal
publication status
published
subject
keywords
Manganese speciation, EXAFS spectrometry, Sediment, Anoxic basin, Baltic, Sea
in
Chemical Geology
volume
370
pages
49 - 57
publisher
Elsevier
external identifiers
  • wos:000332530000005
  • scopus:84894069380
ISSN
0009-2541
DOI
10.1016/j.chemgeo.2014.01.013
language
English
LU publication?
yes
id
edc45092-3937-4a70-9b14-61b7d750da1f (old id 4410963)
date added to LUP
2014-04-29 11:24:48
date last changed
2017-10-01 03:30:11
@article{edc45092-3937-4a70-9b14-61b7d750da1f,
  abstract     = {Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca-Mn-carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e. g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000-4000 cal. yr BP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum(Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7 mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe-S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters. (C) 2014 Elsevier B.V. All rights reserved.},
  author       = {Lenz, Conny and Behrends, Thilo and Jilbert, Tom and Silveira, Miguel and Slomp, Caroline P.},
  issn         = {0009-2541},
  keyword      = {Manganese speciation,EXAFS spectrometry,Sediment,Anoxic basin,Baltic,Sea},
  language     = {eng},
  pages        = {49--57},
  publisher    = {Elsevier},
  series       = {Chemical Geology},
  title        = {Redox-dependent changes in manganese speciation in Baltic Sea sediments from the Holocene Thermal Maximum: An EXAFS, XANES and LA-ICP-MS study},
  url          = {http://dx.doi.org/10.1016/j.chemgeo.2014.01.013},
  volume       = {370},
  year         = {2014},
}