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Effect of diatom growth and dissolution on silicon isotope fractionationin an estuarine system
Stockholm University, Faculty of Science, Department of Geological Sciences.
Linnéuniversitetet, Institutionen för naturvetenskap.
Naturhistoriska riksmuseet, Laboratoriet för isotopgeologi .
Linnéuniversitetet, Institutionen för naturvetenskap.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Si isotopes provide a powerful tool to reveal past and present patterns in diatom production. Most studies have focused on Si fractionation factors during diatom growth in open ocean systems and have found lower Si isotope values in diatom shells (biogenic silica). Recent findings indicate that even the fractionation of Si isotopes during the physicochemical dissolution of diatom shells in the opposite direction produces higher δ30Si values in the remaining biogenic silica (BSi), allowing for the interpretation of diatom production patterns over geological time scales. However, estuarine and coastal primary production represents approximately 30-50% of global marine production, and there are hardly any studies on Si isotope fractionation during either diatom growth or dissolution. In this study, Si isotope fractionation during diatom growth and the dissolution of the frustule were measured. Two species of diatoms from the Baltic Sea, one of the largest estuarine systems in the world, were selected for this study. The results show that both species of diatoms during growth yields an identical Si isotope fractionation factor of 0.99925 for 29Si and 0.9984 for 30Si. In contrast to findings from open ocean species, no Si isotope fractionation during dissolution was observed even after 90% of the diatoms dissolved. Whether there is isotope fractionation during dissolution or not will have profound implications for studies using Si isotopes to interpret the Si cycle in marine and estuarine systems. We propose that the small size of the diatoms living in estuarine systems with low salinity may explain the non-existence of Si isotope fractionation during dissolution. Therefore, we suggest that Si isotopes are an instrumental variable holding information about original environmental conditions of estuarine and even coastal systems. Finally, we tested the Si isotope fractionation patterns gained from the lab experiments on a sediment core, corroborating the observed dissolved silicates (DSi) uptake rates in the above water column during diatom growth.

National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-79184OAI: oai:DiVA.org:su-79184DiVA: diva2:547892
Funder
Swedish Research Council, 2007-4763
Available from: 2012-08-29 Created: 2012-08-29 Last updated: 2012-09-10Bibliographically approved
In thesis
1. Isotope-based reconstruction of the biogeochemical Si cycle: Implications for climate change and human perturbation
Open this publication in new window or tab >>Isotope-based reconstruction of the biogeochemical Si cycle: Implications for climate change and human perturbation
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The global silicon (Si) cycle is of fundamental importance for the global carbon cycle. Diatom growth in the oceans is a major sequestration pathway for carbon on a global scale (often referred to as the biological pump). Patterns of diatoms preserved in marine sediment records can reveal both natural and anthropogenic driven environmental change, which can be used to understand silicon dynamics and climate change. Si isotopes have been shown to have great potential in order to understand the Si cycle by revealing both past and present patterns of dissolved Si (DSi) utilization, primarily when diatoms form their siliceous frustules (noted as biogenic silica, BSi). However, studies using Si isotopes are still scarce and only a few studies exist where stable Si isotopes are used to investigate the biogeochemical Si cycle in aquatic systems. Therefore, this thesis focuses on developing analytical methods for studying BSi and DSi and also provides tools to understand the observed Si isotope distribution, which may help to understand impacts of climate change and human perturbations on marine ecosystems. The Baltic Sea, one of the biggest estuarine systems in the world, was chosen as the study site. BSi samples from a sediment core in Bothnian Bay, the most northern tip of the Baltic Sea, and diatom samples from the Oder River, draining into the southern Baltic Sea were measured and reported in Paper II and III, after establishing a method for Si isotope measurements (Paper I). Si isotope fractionation during diatom production and dissolution was also investigated in a laboratory-controlled experiment (Paper IV) to validate the observations from the field. The major result is that Si isotope signatures in BSi can be used as an historical archive for diatom growth and also related to changes in climate variables. There is isotopic evidence that the Si cycle has been significantly altered in the Baltic Sea catchment by human activities. 

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2012. 20 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 351
Keyword
diatoms, biogenic silica (BSi), dissolved Si (DSi), Si isotope fractionation, the Baltic Sea
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-79188 (URN)978-91-7447-559-3 (ISBN)
Public defence
2012-10-12, Ahlmansalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2007-4763
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2012-09-20 Created: 2012-08-29 Last updated: 2013-04-09Bibliographically approved

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