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Isotope-based reconstruction of the biogeochemical Si cycle: Implications for climate change and human perturbation
Stockholm University, Faculty of Science, Department of Geological Sciences.
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 [en]
diatoms, biogenic silica (BSi), dissolved Si (DSi), Si isotope fractionation, the Baltic Sea
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-79188ISBN: 978-91-7447-559-3 (print)OAI: oai:DiVA.org:su-79188DiVA: diva2:550506
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
List of papers
1. Stable silicon isotope analysis on nanomole quantities using MC-ICP-MS with a hexapole gas-collision cell
Open this publication in new window or tab >>Stable silicon isotope analysis on nanomole quantities using MC-ICP-MS with a hexapole gas-collision cell
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2010 (English)In: Journal of Analytical Atomic Spectrometry, ISSN 0267-9477, E-ISSN 1364-5544, Vol. 25, no 2, 156-162 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate in this study that a single focusing multiple collector inductively coupled plasma massspectrometer (MC-ICP-MS) equipped with a hexapole gas-collision cell (GV-instrument Isoprobe) canprecisely determine the d29Si (2S.D., 0.2&) using a total Si consumption of less than 14 nmole (390 ngSi). Testing and evaluation of background, rinse time, and major matrix effects have been performed ina systematic way to establish a procedure to measure d29Si in small quantities. Chemical purificationprior to analysis is required to remove potential interferences. For data collected during a four-yearperiod, the average d29Si value of IRMM-018 relative to NBS-28 was found to be 0.95& (n ¼ 23,2S.D. 0.16&) with a 95% confidence interval (0.95 0.028&). The mean d29Si value of the Big-Batchstandard was found to be 5.50& (n ¼ 6, 2S.D. 0.26&). Although determination of the d30Simeasurements is not possible, with our current instrument we demonstrate that this system providesa fast and long-term reliable method for the analysis of d29Si in purified samples with low Siconcentration (18 mM Si).

Keyword
silicon isotopes, MC-ICP-MS
National Category
Analytical Chemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-37447 (URN)10.1039/b911113a (DOI)000273962800006 ()
Projects
Silicon isotope-based reconstruction of silicon cycle and diatom production in the Baltic Sea; implications for climate change and eutrophication
Available from: 2011-01-05 Created: 2010-03-04 Last updated: 2017-12-12Bibliographically approved
2. Climate Dependent Diatom Production is Preserved in Biogenic Si Isotope Signatures
Open this publication in new window or tab >>Climate Dependent Diatom Production is Preserved in Biogenic Si Isotope Signatures
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2011 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 8, no 11, 3491-3499 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study was to reconstruct diatom production in the subarctic northern tip of the Baltic Sea, Bothnian Bay, based on down-core analysis of Si isotopes in biogenic silica (BSi). Dating of the sediment showed that the samples covered the period 1820 to 2000. The sediment core record can be divided into two periods, an unperturbed period from 1820 to 1950 and a second period affected by human activities (from 1950 to 2000). This has been observed elsewhere in the Baltic Sea. The shift in the sediment core record after 1950 is likely caused by large scale damming of rivers. Diatom production was inferred from the Si isotope composition which ranged between δ30Si −0.18‰ and +0.58‰ in BSi, and assuming fractionation patterns due to the Raleigh distillation, the production was shown to be correlated with air and water temperature, which in turn were correlated with the mixed layer (ML) depth. The sedimentary record showed that the deeper ML depth observed in colder years resulted in less production of diatoms. Pelagic investigations in the 1990's have clearly shown that diatom production in the Baltic Sea is controlled by the ML depth. Especially after cold winters and deep water mixing, diatom production was limited and dissolved silicate (DSi) concentrations were not depleted in the water column after the spring bloom. Our method corroborates these findings and offers a new method to estimate diatom production over much longer periods of time in diatom dominated aquatic systems, i.e. a large part of the world's ocean and coastal seas.

National Category
Geosciences, Multidisciplinary Geochemistry Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-65184 (URN)10.5194/bg-8-3491-2011 (DOI)000298132200024 ()
Funder
Swedish Research Council, 2007-4763
Available from: 2011-12-05 Created: 2011-12-05 Last updated: 2017-12-08Bibliographically approved
3. Silicon isotope enrichment in diatoms during nutrient-limited bloomsin a eutrophied river system
Open this publication in new window or tab >>Silicon isotope enrichment in diatoms during nutrient-limited bloomsin a eutrophied river system
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We examined the Si isotope fractionation in diatoms by following a massive nutrient limited diatom bloom from a eutrophied natural system. We hypothesized that the Si isotope fractionation should be larger in comparison to observations in less nutrient rich environments. The Oder River, which is a eutrophied river draining the western half of Poland and entering the southern Baltic Sea, shows that a diatom bloom may cause extreme Si isotope fractionation. The rapid nutrient depletion and fast biogenic silica (BSi) increase observed during the spring bloom suggests a Rayleigh behavior for a closed system for dissolved Si (DSi) and BSi in the river at certain time scales. An enrichment factor (ε) of up to -1.6‰ is found based on observations between April and June, 2004. A very high δ30Si value of up to +3.05‰ is measured in diatoms. This is about 2 times higher than previously recorded δ30Si in freshwater diatoms. The Rayleigh model used to predict the δ30Si values of DSi suggests that the initial value before the start of the diatom bloom is close to +2‰. This indicates that there is a biological control of the Si isotope compositions entering the river, probably caused by Si isotope fractionation during uptake of Si in phytoliths. Clearly, eutrophied rivers with enhanced diatom blooms deliver 30Si-enriched DSi and BSi to the coastal ocean, which can be used to trace the biogeochemistry of DSi/BSi in estuaries.

National Category
Geochemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-79186 (URN)
Funder
Swedish Research Council, 2007-4763
Available from: 2012-08-29 Created: 2012-08-29 Last updated: 2012-09-10Bibliographically approved
4. Effect of diatom growth and dissolution on silicon isotope fractionationin an estuarine system
Open this publication in new window or tab >>Effect of diatom growth and dissolution on silicon isotope fractionationin an estuarine system
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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:nbn:se:su:diva-79184 (URN)
Funder
Swedish Research Council, 2007-4763
Available from: 2012-08-29 Created: 2012-08-29 Last updated: 2012-09-10Bibliographically approved

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