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The 4th meeting of the Isotopes in Biogenic Silica Working Group was held 8-9 February 2010 in Swansea, Wales.

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Meeting Programme

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Monday 8th February 

15.00 – 15.30 Registration and Refreshments in the Rhossili Room

15.30 – 15.40 Welcome talk in the Port Eynon Room

 

Oxygen isotopes

Chaired by Martin Hodson

15.40 – 16.20 Keynote: Anne Alexandre Oxygen isotopic composition of phytoliths from tropical rainforests (Queensland, Australia): a new palaeo-environmental tool

16.20 – 16.45 Talk 2: Florence Sylvestre Re-examination of the thermo-dependent relationship between δ18Odiatoms and δ18Olake water and implications for palaeoclimatic applications

16.45 – 17.10 Talk 3: Jonathan Tyler Oxygen isotope exchange during silica condensation under laboratory conditions

17.10 – 17.35 Talk 4: Bernhard Chapligin Preliminary results from the on-going inter-laboratory comparison of the oxygen isotope composition of biogenic silica

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17.35 – 18.00 Poster session

Posters by: Helen Cockerton, Maria Lehtimäki, Frauke Müller, Virpi Siipola, Petra Tallberg and Floor Vandevenne

 

19.30 Workshop Dinner at the Marriott Hotel in the Port Eynon Room

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21.30 Meeting to discuss the inter-laboratory comparison in the Port Eynon Room

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Tuesday 9th February

09.00 - 09.15 Registration and refreshments in the Rhossili Room

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Oxygen isotope contamination correction techniques

Chaired by Phil Barker

09.15 – 09.40 Talk 5: Katy Wilson Investigating the effect of contamination on δ18Odiatom palaeoclimate records

09.40 – 10.05 Talk 6: Matthew Jones New techniques for δ18Osilica contamination calculations

10.05 – 10.30 Talk 7: Andrea Snelling A novel technique in diatom cleaning and preparation

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10.30 -10.50 Refreshments in the Rhossili Room

 

Oxygen isotopes and other isotopes

Chaired by Sarah Davies

10.50 – 11.15 Talk 8: Melanie Leng Reconstructing past climate in Scandinavia using diatom oxygen isotopes in lake sediments

11.15 – 11.40 Talk 9: Philip Barker Oxygen and carbon isotope analysis of diatom silica from East African mountain lakes

11.40 – 12.05 Talk 10: Matt Horstwood Silicon isotope analysis by MC-ICP-MS

12.05 – 12.30 Talk 11: Morten Andersen The zinc isotopic composition of diatom frustules, a proxy for zinc availability in ocean surface seawater

 

12.30 – 13.30 Lunch in Aberbethy’s Restaurant Biological regulation of the silicon cycle chaired by Alayne Street-Perrott

 

13.30 – 13.55 Talk 12: Eric Struyf Interactions between climate change, land use and the biological silica buffer in wetlands and forests

13.55 – 14.20 Talk 13: Jonas Schoelynck Silica uptake in aquatic and wetland macrophytes: a strategic choice between silica, lignin and cellulose?

14.20 – 14.45 Talk 14: Sander Jacobs Amorphous silica contents in tidal marsh sediments: biological or physicochemical control?

14.45 – 15.05 Refreshments in the Rhossili Room

 

Long-term changes in the biogeochemical cycle of silicon

Chaired by Melanie Leng

 

15.05 - 15.30  Talk 15: George Swann Changes in the North Pacific Ocean biological pump from MIS 5e to MIS 3

15.30 – 15.55 Talk 16: Laetitia Pichevin Diatom-bound trace metals and silicon isotopes in sediment from the Gulf of California: implication for the significance of sedimentary opal records

 

Annual to decadal variability in the biogeochemical cycle of silicon

Chaired by Melanie Leng

15.55 – 16.20 Talk 17: Sophie Opfergelt Variations of δ30Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

16.20 – 16.45 Talk 18: Jennifer Pike Late Quaternary diatom silica oxygen isotope records from the Antarctic Margin

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Oral Presentation Abstracts (in chronological order by presentation)

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Oxygen isotopic composition of phytoliths from tropical rainforests (Queensland, Australia): a new palaeo-environmental tool

A. Alexandre1, J. Crespin1, C. Sonzogni1, F. Sylvestre1, D. Hilbert2

1CEREGE, CNRS UMR6635, UPC-III, IRD, Europôle de l'Arbois, BP 80, 13545, Aix-en- Provence Cedex 04, France

2CSIRO Tropical Forest Research Centre, PO Box 780, Atherton, QLD 4883 Australia 

Records of oxygen isotopic composition of precipitation (δ18Oprecipitation) from fossil biogenic and authigenic minerals sequences would provide unique information on both local and global past climatic changes. However tracers abundant enough for producing long and continuous terrestrial δ18O records are scarce. Moreover the relationship between oxygen isotopic composition of minerals that form in isotopic equilibrium and oxygen isotopic composition of forming water is temperature-dependant and multiproxies approaches are required for addressing one of the two controls. At least, from precipitation to water from which biogenic and authigenic minerals form, several fractionating processes may occur, which often prevent direct reconstructions of δ18Oprecipitation. Phytoliths may help to fill this lack. Here, for the first time, a thermodependant relationship between oxygen isotopic composition of wood phytoliths (δ18Owood phytolith) and δ18Oprecipitation is demonstrated. Phytolith assemblages were extracted from soil humic horizons collected along four altitude, temperature and precipitation gradients in the rainforests of North-east Queensland. Oxygen isotopic analyses were performed on phytoliths (δ18Ophytolith), after a controlled isotopic exchange (CIE), using the IR Laser-Heating Fluorination Technique. Long-term mean annual precipitation and temperature at the sampled sites were obtained from a regional database. δ18Oprecipitation values were estimated. δ18Osoil water under the rainforest canopy was assumed to be close to δ18Oprecipitation. Obtained δ18Owood phytolith values reflect the variability of amount weighted mean annual δ18Oprecipitation and mean annual temperature. The relationship is expressed for windward samples by [δ18Owood phytolith -

δ18Oprecipitation] (‰ vs VSMOW) = -0.38t (°C) (±0.22) + 47.7 (±4.61) (R 2=0.46, p=0.01; n=13). The obtained coefficient of -0.38‰/°C is clo se to fractionation coefficients previously estimated for phytolith-water, diatom-water and quartz-water couples. This consistency supports existence of a unique silica/water equilibrium fractionation coefficient and the reliability of the relationship as a transfer function for reconstructing past δ18Osoil water (and δ18Oprecipitation when both isotopic compositions are assumed to be close) and/or temperatures changes. Effects of forest fires on phytoliths dehydration measured by FT-IR and δ18Ophytolith are tested through heating experiments. While rainforests dynamics can be revealed by morphological phytolith indexes, δ18O analysis of rainforest phytolith assemblages from continuous sedimentary sequences, especially from the Western Australo-Pacific area, should simultaneously provide unique insight into terrestrial climate changes.

 

Re-examination of the thermo-dependent relationship between δ18Odiatoms and δ18Olake water and implications for palaeoclimatic applications

F. Sylvestre1, J. Crespin1, A. Alexandre1, C. Sonzogni1, C. Paillès1, M-E. Perga2

1CEREGE, Aix-Marseille Université, CNRS, IRD, Europôle de l’Arbois, BP 80, 13545 Aix en Provence cedex 04, France.

2Station d’Hydrobiologie lacustre, INRA, BP 511, 74203 Thonon-les-Bains cedex, France.

The oxygen isotopic composition (δ18O) of diatoms is commonly used for palaeoclimatic reconstructions. However, there is no consensus regarding the equilibrium isotopic fractionation factor between diatom silica and the water in which they precipitated. We re-examined the thermo-dependent relationship between δ18Odiatoms and δ18Olake water from Lake Annecy (France). A temperature coefficient of -0.16‰/°C (R 2=0.51) was determined, supporting previous calibrations. However, regression lines obtained from different calibration studies were shifted. In this manuscript, four hypotheses are discussed regarding selected variables (temperature, δ18Olake water), analytical techniques, the impact of diagenetic processes, and biological effects. The rather similar magnitude of the diatom-temperature coefficients determined is promising, supporting its use as a valuable tool for interpreting variations in δ18O values from fossil lacustrine diatoms in temperate lakes. In view of palaeoclimatic applications, the thermo-dependent relationship yielded uncertainties of ± 3°C on reconstructed temperatures and ± 0.5‰ on δ18Olake water.

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Oxygen isotope exchange during silica condensation under laboratory conditions

J.J. Tyler1, M.J. Leng2, G.E.A. Swann2, H. Sloane2, S.V. Patwardhan3

1Department of Earth Sciences, University of Oxford.

2NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham.

3School of Science and Technology, Nottingham Trent University, Nottingham.

Biogenic silica is structurally hetrogenous, consisting of tetrahedrally bonded silica (Si- (O-Si)4) and silica of varying degrees of hydration, whereby one to three O-Si bonds are replaced by hydroxyl units (e.g. HO-Si-(O-Si)3). Although the oxygen within tetrahedral silica is securely bound and resistant to isotope exchange, hydroxyl silica freely exchanges oxygen with water and has an isotopic composition which varies with to ambient conditions. Consequently, modern techniques for analysing the oxygen isotopic composition of biogenic silica include a preparatory step which accounts for this exchangeable silica, for example controlled isotope exchange (CIE) or stepped fluorination. However, following the death of a diatom, silica condensation begins to occur whereby hydroxyls are replaced by O-Si units. Thus the relative amount of tetrahedral biogenic silica is greater in fossil biogenic silica than in samples taken from the modern environment, having accumulated new tetrahedral silica during early diagensis. This implies that the oxygen bound within the newly condensed tetrahedral silica will to some extent reflect the conditions during sinking and sedimentation in nature, or during preparation for isotope analysis within the laboratory. Although little is yet known regarding natural rates of silica condensation, it is likely that heating and drying are influential in accelerating this process, with important implications for preparatory techniques. In order to better understand this issue, were are conducting experiments using two types of hydrous silica: fresh silica frustules of the diatom Stephanodiscus hantzschii and synthetic silica anospherules, which are chemically analogous to biogenic silica. Initial results indicate marked shifts in the isotopic composition of biogenic silica associated to heating and drying procedures which may warrant a revision of existing preparatory methods. These results will be presented and their significance for the laboratory treatment of biogenic silica and the interpretation of natural δ18Osilica signals will be discussed.

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Preliminary results from the on-going inter-laboratory comparison of the oxygen isotope composition of biogenic silica

B. Chapligin1, A. Alexandre2, J. Dodd3, A. Ijiri4, M. Leng5, A. Lücke6, A.Shemesh7, E. Webb8, A. Abelmann1, F. Longstaffe8, H. Meyer1, R.

Moschen6, Y. Okazaki4, Z. Sharp3, H.J. Sloane5, C. Sonzogni2, G. Swann5, F. Sylvestre2, J. Tyler5

1Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43, D-14473 Potsdam, Germany. E-Mail: bernhard.chapligin@awi.de, hanno.meyer@awi.de, andrea.abelmann@awi.de

2IRD-CEREGE, Europôle de l'Arbois, BP 80, 13545 Aix-en-Provence cedex 4, FRANCE. E-maill: alexandre@cerege.fr, sylvestre@cerege.fr, sonzogni@cerege.fr

3Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, New Mexico 87131. E-mail: jpd829@unm.edu, zsharp@unm.edu

4Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061 Japan, E-Mail: ijiri@jamstec.go.jp, okazakiy@jamstec.go.jp

5NERC Isotope Geosciences Laboratory (NIGL), British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK. E-Mail: mjl@nigl.nerc.ac.uk, gean@bgs.ac.uk, jonathanjtyler@googlemail.com, hjs@nigl.nerc.ac.uk

6Institute of Chemistry and Dynamics of the Geosphere 4: Agrosphere, Energy & Environment Research Center Jülich, 52425 Jülich, GERMANY. E-mail: a.luecke@fzjuelich.de, r.moschen@fz-juelich.de

7Department of Environmental Sciences, The Weizmann Institute of Science, Rehovot, 76100, ISRAEL.E-mail: Aldo.Shemesh@weizmann.ac.il

8Laboratory for Stable Isotope Science, Department of Earth Sciences, The University of Western Ontario, London, Ontario, CANADA, N6A 5B7. Email: ewebb5@uwo.ca, flongsta@uwo.ca

In 2009 the Alfred-Wegener-Institute organised an inter-laboratory comparison for analysing the oxygen isotope compositions of biogenic silica. The relationship of the oxygen isotope compositions of carbonate to climate-relevant parameters is widely utilized. However, challenges still exist in the use of biogenic silica. Problems arise during the sample preparation stage and during the isotopic analysis itself. The problems are commonly related to the removal of loosely bound exchangeable oxygen from the silica. Amorphous silica contains OH groups within the SiO2 skeleton as well as chemically combined water. The OH groups and chemically combined water must be removed prior to analysis as their oxygen is easily exchanged following silica formation and thus does not reflect the original isotopic composition of the water. Various methods have been established in the past 20 years for the dehydration and dehydroxylation of amorphous silica, including Controlled Isotopic Exchange (CIE) followed by fluorination, Stepwise Fluorination (SWF) and inductive High-Temperature carbon reduction (iHTR). A new method under consideration is Helium Flow Dehydration (HFD) followed by fluorination. These methods have never been compared in a comprehensive interlaboratory comparison.

This inter-laboratory comparison should function as a method performance study as well as a material certification study. The study’s goals are:

1. To evaluate the agreement of δ18O results in terms of accuracy and reproducibility among different methods in different laboratories, and

2. To provide the δ18OSi community with well-calibrated biogenic standards covering a large range of δ18O values.

The second goal is related to the current existence of only one quartz standard (NBS- 28) made available by IAEA as a reference material for analysis of oxygen isotopes from SiO2. There is a need for natural biogenic reference materials with oxygen isotopic compositions in the range of naturally occurring lacustrine and marine sediments. Six different samples – so far only used as internal standards in the participating laboratories – have been analyzed by eight different laboratories using their typical analytical methods. The samples cover a wide-range of δ18O values (23 to 43 ‰) and originate from lacustrine and marine sediment deposits as well as from chemically precipitated amorphous silica. The samples consist of diatoms, phytoliths or synthetical nano-spheres. Each laboratory has analyzed each sample at least ten times on various days to evaluate reproducibility.

X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses have also been performed to verify the purity and the structure of the proposed standards. The XRF analysis of the six standards indicates SiO2 contents ranging from 94 to 99%. The XRD analyses showed that five of the standards have a broad, pure amorphous silica (opal-A) peak whereas one (Kieselguhr) is crystalline, with crystobalite “bands” resulting from pre-calcination performed by the supplying company.

At the time of this abstract submission the laboratories were completing their analyses. Preliminary results will be presented at the meeting. Based on these results, we hope to recommend up to six biogenic silica standards for δ18OSi values with a reproducibility of better than ±0.3 ‰. We will also evaluate the various methods in terms of the analytical uncertainties accompanying each approach.

 

Investigating the Effect of Contamination on δ18Odiatom Palaeoclimate Records​

K.E.Wilson1, M.J. Leng2, R.K. Edgar3, G.E.A. Swann2, M.A Maslin1, A.W. Mackay1

1Environmental Change Research Centre, University College London, Gower Street, London, WC1E 6BT, UK.

2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG.

3Farlow Herbarium of Cryptogamic Botany, Harvard University, Cambridge, Massachusetts 02318.

Oxygen isotope measurements from diatom silica have been increasingly used as indicators of palaeoclimate especially in archives where no reliable carbonate proxies are available, such as in terrestrial lacustrine sequences or non-carbonate marine environments. We investigate potential climate signals recorded in a series of freshwater diatomite deposits exposed in the Baringo-Bogoria basin in the Central Kenyan Rift Valley. The diatomites represent the regular cycling of a large freshwater lake system during the Late Pliocene coincident with the intensification of Northern Hemisphere glaciation. The timing of diatomite formation during wet periods closely approximates maxima in the 30°N (June) insolation c urve implying that regional precipitation at this time was controlled by precessionally-forced monsoonal circulation. Geochemical analyses of the Baringo diatomites using X-ray fluorescence, infra-red spectroscopy and SEM microprobe analysis revealed varying amounts of clay and tephra remaining after cleaning. A recent study by Brewer et al. (2008) demonstrated that, in spite of extensive chemical and physical cleaning techniques employed both prior to and during isotope analysis, ‘purified’ diatom material can still contain significant amounts of contamination which can compromise the precision of δ18Odiatom values. We investigate the limits to which it is possible to use mass balance techniques to model corrections to δ18Odiatom values and to what degree the presence of contaminant material within samples can alter the δ18Odiatom measurements and mask the underlying climate signal.

Brewer, T.S., Leng, M.J., Mackay, A.W., Lamb, A.L., Tyler, J.J., Marsh, N.G. 2008. Unravelling contamination signals in biogenic silica oxygen isotope composition: the role of major and trace element geochemistry. Journal of Quaternary Science 23: 321-330.

 

New techniques for δ18Osilica contamination calculations

M.D. Jones1, G. Swann2, M. Leng1,2, N. Roberts3, J. Woodbridge3, H. Sloane2, A. Mackay4

1School of Geography, University of Nottingham

2NERC Isotope Geosciences Laboratory, BGS, Keyworth, Nottingham

3School of Geography, Earth and Environmental Sciences, University of Plymouth

4Department of Geography, University College London

Cleaning samples prior to δ18O analysis of diatoms is time consuming and “pure” diatom samples are rarely produced. XRF analysis of samples has therefore been used to mass balance out the effect of any remaining contaminant. 

However, XRF requires large amounts of sample, relative to the sample sizes required for isotope analysis, and can be prohibitively expensive for representative analysis from large data sets. Here we investigate the use of infrared spectroscopy and elemental sample compositions derived from EDAX measurements from an SEM to measure contamination amounts in diatom samples for δ18O analysis.

Firstly we analysed samples created from mixed standards of BFC (the NIGL laboratory diatom standard), Mica, Montmorillonite, Kaolinite and Chlorite, i.e. samples with a known composition. Samples measured with EDAX explained 80% of the theoretical composition of each mixed sample. This figure increases to 90% if samples containing Kaolinite are removed from the data set, suggesting the type of contaminant may be important in the success of this technique. Results from infrared analysis also show absorption bands in accordance with the expected spectra.

Secondly we use these techniques to calculate contamination in samples taken from cores from 2 lakes, Lake Baikal (Russia), from which we have standard XRF data with which to compare the results, and a new δ18Osilica record from Nar Gölü (Turkey).

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A novel technique in diatom cleaning and preparation

A.M. Snelling1, M.J. Leng1, G.E.A. Swann1, J. Pike2

1NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5AA, UK

2School of Earth and Ocean Sciences, Cardiff University, Main building, Park Place, Cardiff CF10 3YE, UK

Diatoms are known to be excellent palaeoindicators and it has been shown that δ18Odiatom shows great potential for understanding climatic and oceanographic changes in areas depleted in carbonates. δ18Odiatom is usually carried out on bulk samples due to the large amount of material required for isotope analysis (5 mg), which will likely contain a variety of species of various size fractions. It has previously been shown that size-related species effects may exist but the mechanisms underlying such effects are poorly understood. In order to minimise these problems it has been suggested that samples should be as size and sample specific as possible. The ability to isolate individual species is also important for measurement of seasonal variation in δ18Odiatom through laminated sediments, where particular species are known to bloom in the spring or autumn and will likely directly reflect seasonal conditions.

Sieving enables large and small fractions to be separated but isolating individual diatom species within small sample fractions (<75 μm) has often proved to be difficult given the increased numbers of diatom species present as well as there often being fragments of larger frustules present or other contaminant material. A novel technique using a micromanipulator allows unwanted debris or species of a similar size to those required for isotope analysis to be removed from the sample to leave a virtually mono-specific, ultra-clean sample.

 

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Figure 1: Diatom samples from Palmer Deep, Antarctica. A (left). is an uncleaned sample with a variety of fragments, including clay particles. B (right). is a cleaned sample almost entirely made up of Chaetoceros sp.

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Reconstructing past climate in Scandinavia using diatom oxygen isotopes in lake sediments

M.J. Leng1, G.C. Rosqvist2, C.E. Jonsson2, H.J. Sloane1

1NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham NG12 5GG, UK

2Department of Physical Geography and Quaternary Geology, Stockholm University, 106 91 Stockholm, Sweden

Lake sediment records from Sweden illustrate how oxygen isotopes from diatoms, located in different hydrological settings, can provide information about climate change. In particular changes in precipitation, atmospheric circulation and water balance. We show the importance of understanding the present lake hydrology, and the relationship between climate variables and the oxygen isotopic composition of precipitation and lake waters for interpretation of diatom based oxygen isotope records. Both precipitation reconstructions from northern Sweden and water balance reconstructions from south and central Sweden show that the atmospheric circulation changed from zonal to a more meridional airflow over the Holocene. Superimposed on this Holocene trend are δ18O precipitation minima resembling intervals of the negative phase of the North Atlantic Oscillation (NAO), thus suggesting that the climate of Northern Europe is strongly influenced by atmospheric and oceanic circulation changes over the North Atlantic.

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Figure showing the variable lake water δ18Olake water versus δ2Hlake water for lakes in Sweden highlighting the various hydrological controls on diatom δ18O

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Oxygen and carbon isotope analysis of diatom silica from East African mountain lakes

P. Barker and E. Hurrell

Department of Geography, Lancaster University, Lancaster, LA1 4YB

Interpreting isotope records from biogenic silica is frequently difficult because of the wide range of controls on isotope fractionation and the diverse nature of lake catchments. Multiproxy studies offer a way of distinguishing climate forcing from catchment sensitivity. Here we present oxygen and carbon isotope data from lake sediments and the ice cores of Mt. Kilimanjaro and compare these to similar biogenic silica based isotope data from Mt Kenya. Results suggest that diatom-based oxygen isotopes from Kilimanjaro (Lake Challa) for the last 25,000 years track closely to equatorial insolation and are negatively related to ice core data covering the last 11,700 years (Thompson et al. 2002). Carbon isotope data from diatom frustule proteins from the same lake show some underlying relations with regional hydrological changes but only limited correspondence with century to millennial scale changes in the oxygen isotopes. Paradoxically, stronger similarities between the Kilimanjaro diatom carbon isotopes and the oxygen isotopes from Mt Kenya are found than with oxygen isotope data from the same lake. Carbon isotopes from the diatoms of Mt Kenya have also been investigated and may help with understanding these data. This presentation seeks to reconcile these apparently contradictory datasets with a consideration of catchment sensitivities. It is hoped that this approach may help understandings of other sites where biogenic silica based isotope data has been dismissed as not conforming to expected patterns.

Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Henderson, K.A., Brecher, H.H., Zagorodnov, V.S. Mashiotta, T.A., Lin, P-N., Mikhalenko, V.N., Hardy, D.R., Beer, J. 2002. Kilimanjaro ice core records: evidence of Holocene climate change in tropical Africa. Science 298: 589-593.

 

Silicon isotope analysis by MC-ICP-MS

V. Pashley and M. Horstwood

NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG

Si isotope determination is an increasingly popular approach for establishing productivity rates in the worlds lakes and oceans and the role this plays in carbon sequestration and environmental and climate change. Most recently multi-collector (MC-)ICP-MS has been utilised to determine Si isotope compositions, providing more rapid sample throughput and higher analytical precisions than conventional gas source methods.

Since initial development (De La Rocha, 2002), methodologies have improved and become more robust with reported precisions now in the range of 0.12-0.16 permille (2SD) for δ 29Si (Georg et al., 2009; Sun et al., 2010). For accurate isotope analyses by MC-ICP-MS it is essential that samples are prepared using clean chemical purification techniques. Failure to purify the samples appropriately will readily result in inaccurate and fractionated data. A number of preparation methodologies have so far been forwarded including dissolution by conventional acid attack as well as alkali fusion.

Equally, a number of analytical protocols have been proposed, each offering differing capabilities, potential benefits as well as pitfalls. In establishing the technique anew, the capability and practicality of all of these approaches requires assessment so that the most appropriate protocols can be established at the new laboratory to achieve the desired throughput and data quality levels required. NIGL has recently begun applying its established MC-ICP-MS facilities to Si isotope diatom research to compliment its unique single-sample combined Si-O gas-source capabilities. A review of the current state-of-the-art in MC-ICP-MS Si isotope methodologies, precision and resolution with a forward look considering upcoming instrumentation advancements will be given to highlight future potential capabilities in MC-ICP-MS Si isotope analysis.

De La Rocha, C.L. 2002. Measurement of silicon stable isotope natural abundances via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Geochemistry, Geophysics, Geosystems 3(8).

Georg, R.B., Zhu, C., Reynolds, B.C., Halliday, A.N. 2009. Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA. Geochimica et Cosmochimica Acta 73: 2229-2241.

Sun, X., Andersson, P., Land, M., Humborg, C., Mörth, C-M. 2010. Stable silicon isotope analysis on nanomole quantities using MC-ICP-MS with a hexapole gas-collision cell. Journal of Analytical Atomic Spectrometry 25: 156-162.

 

The Zn isotopic composition of diatom frustules, a proxy for Zn availability in ocean surface seawater

M.B. Andersen1*, D. Vance1, C. Archer1, M. Ellwood2, C. Allen3, C.D. Hillenbrand3, R.F. Anderson4

1Bristol Isotope Group, Department of Earth Sciences, University of Bristol, United Kingdom (*correspondence: morten.andersen@bris.ac.uk)

2Department of Earth and Marine Sciences, Australian National University, Australia

3British Antarctic Survey, Cambridge, United Kingdom

4Lamont-Doherty Earth Observatory of Columbia University, USA

Zinc is among the essential trace-metal micronutrients for phytoplankton. In common with some other bio-active trace metals, Zn concentrations are highly depleted in those parts of the surface ocean that are replete in the major nutrients (so-called High Nutrient-Low chlorophyll, or HNLC, zones), including the Southern Ocean. The release of these HNLC zones from trace metal limitation may be key for explaining lower atmospheric CO2 during glacial periods. The preferential incorporation of light Zn isotopes into phytoplankton organic material is expected to leave residual surface seawater Zn isotopically heavy. Thus the degree of trace metal depletion in surface oceans in the past could be tested with a suitable archive of surface seawater Zn isotopes.

We are investigating the reliability of diatom opal as a recorder of the Zn isotopic composition of surface seawater, and have measured Zn isotopic compositions in cleaned diatom frustules from a sequence of core-top samples across the Southern Ocean [1]. All diatom opal exhibits heavy Zn isotopic compositions, as expected from surface waters in highly trace metal-depleted HNLC zones, and the Zn isotope composition tracks decreasing diatom opal burial rates with progressively heavier Zn isotope compositions. Furthermore the measured Zn isotope and Zn/Si ratios, a potential proxy for the free Zn2+ content in surface water [2], are consistent with a model of Zn isotope evolution of the surface ocean in response to the fractionation of Zn isotopes into phytoplankton organic material as measured in experiments [3]. These results suggest that Zn isotopes in diatom frustules record trace metal availability in HNLC zones. Our initial down-core results also show a clear correlation between Zn isotopes and opal burial rates.

[1] Chase et al. (2003) Deep Sea Research II, 799-832

[2] Ellwood & Hunter (2000) Limnol Oceanogr. 45, 1517-1524.

[3] John, S.G. et al. (2007) Limnol Oceanogr. 52, 2710-2714.

 

Interactions between climate change, land use and the biological silica buffer in wetlands and forests.

E. Struyf1, U. Kokfelt3, A. Smis1, D.J. Conley2, C. Humborg4, C-M. Mörth3, F. Vandevenne1, P. Meire1

1University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1c, 2610 Wilrijk, Belgium

2Geobiosphere Science Centre, Department of Geology, Quaternary Sciences, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden

3Stockholm University, Department of Geology and Geochemistry, 10691 Stockholm, Sweden

4Stockholm University, Department of Applied Environmental Science, 10691 Stockholm, Sweden

We have studied the storage and recycling of amorphous biogenic silica in boreal wetlands and temperate forests and croplands. From both studies it is clear that climate, hydrology and land use changes can impact strongly on the biological silica buffer in these ecosystems.

In boreal wetland ecosystems, factors such as hydrosere, permafrost, climate and human interference may disturb the prevailing mire vegetation, whereby a new dominant assemblage can develop. At the transition from one vegetation type to another, the old vegetation may be suppressed, die out or start to decay, and some time may pass until a new mire vegetation is fully established We observed that diatoms thrive during periods associated with major vegetation transitions, creating isolated and shallow peat layers with significantly elevated amorphous Si content. We also observed high amorphous Si storage associated with sedge vegetations. Modelling indicates that the biological Si buffer impacts strongly on watershed Si fluxes.

In the temperate Scheldt river basin, we observed that biological, reactive Si storage in forests strongly exceeds storage in cropland ecosystems. In cropland dominated watersheds, strong erosion is associated with significant transport of amorphous Si in rivers during precipitation peaks. At base-flow, we observe significantly lower dissolved Si fluxes from old agricultural watersheds compared to forested watersheds. These results have inspired the development of a new conceptual model for Si fluxes after deforestation: initially, fast recycling and erosion of amorphous Si will enhance Si fluxes from deforested watersheds. However, on long-term, loss of biological control on Si fluxes may lead to lower Si fluxes from agricultural watersheds. 

 

Silica uptake in aquatic and wetland macrophytes: a strategic choice between silica, lignin and cellulose?

J. Schoelynck1*, K. Bal1, H. Backx1, T. Okruszko2, P. Meire1, E. Struyf1

1University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1C, B-2610 Wilrijk, Belgium (*Corresponding author: jonas.schoelynck@ua.ac.be (Tel +32 3 265 22 52, Fax +32 3 265 22 71))

2Warsaw Agricultural University, Department of Hydraulic Engineering and Environmental Reclamation, ul. Nowoursynowska 166, 02-787 Warszawa, Poland

Although silica is not an essential element for plant growth in the classical sense, evidence points towards its functionality for a better resistance against (a)biotic stress. Recently it was shown that wetland vegetation has a considerable impact on silica biogeochemistry. However, detailed information on Si uptake in aquatic macrophytes is lacking.

We investigated the biogenic silica (BSi), cellulose and lignin content of 16 aquatic/wetland species along the Biebrza river (Poland) in June 2006 and 2007. BSi data were correlated with cellulose and lignin concentrations.

Our results show that macrophytes contain significant amounts of BSi: between 2 and 28 mg g-1 DM. This is in the same order of magnitude as wetland species (especially grasses). Significant antagonistic correlations were found between lignin, cellulose and BSi content. Interestingly, observed patterns were opposite for wetland macrophytes and true aquatic macrophytes.

We conclude that macrophytes have an overlooked but potentially vast storage capacity for Si. Studying their role as temporal silica sinks along the land-ocean continuum is needed. This will move forward understanding of the role of ecosystems and the impact of human activities on land ocean transport of this essential nutrient. 

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Amorphous silica contents in tidal marsh sediments: biological or physicochemical control?

S. Jacobs

University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1c, 2610 Wilrijk, Belgium (Sander.jacobs@ua.ac.be)

Tidal marshes are amorphous silica sinks. They accumulate silica from dead and living diatom communities and silica-accumulation vegetations. This silica is partly dissolved in the porewater, where very high DSi-concentrations can be encountered. Transformation of ASi to DSi was hypothesised to enhance resilience of estuaries and coastal zones against eutrophication events, since DSi-fluxes from these systems to the pelagic waters are observed, and this DSi-source could buffer silica-depletion events which occur in the estuary due to anthropogenic N and P input.

In this presentation, a dataset of ASi contents in tidal marsh sediments of different sites, habitats, and depths over 13 seasonal samplings are presented. These data were first explored by means of PCA and ANOVA-posthoc. A more detailed analysis is now being performed using hierarchical modelling to explore the independent contribution of a wide range of biological, chemical and physical parameters, which were registered simultaneously, on the ASi content. This was done for biological, chemical and physical parameters separately and for the different depth layers, to be able to see changes in contributions with depth.

We hope discussion of the data and analyses will shed light on the factors controlling the complex and peculiar dynamics of silica in tidal marsh sediments. This knowledge will be useful to design experiments and develop mechanistic models of silica cycling in tidal marsh sediments.

 

Changes in the North Pacific Ocean biological pump from MIS 5e to MIS 3

G.E.A. Swann

NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK (gean@bgs.ac.uk)

Diatom isotope records from the North Pacific Ocean provide an important means by which to further investigate past climatic and oceanographic changes in a region marked by depleted concentrations of foraminifera and other carbonates within the sedimentary record. Combined measurements of diatom oxygen (δ18Odiatom) and silicon (δ30Sidiatom) from ODP Site 882 indicate significant changes in both freshwater input to the region and the biological pump between MIS 5e and MIS 3. Unchanging values of δ18Odiatom and low/moderate values of δ30Sidiatom suggest relatively stable oceanographic conditions during MIS 5e until a marked, freshwater induced, decrease in δ18Odiatom at the MIS 5e/d boundary. Increases in opal concentrations and δ30Sidiatom to values of 1.2- 1.3‰ between MIS 5d and MIS 5b indicate a subsequent increase in nutrient utilisation and export production. With these changes coinciding with a progressive long-term decrease in atmospheric pCO2, the North West Pacific Ocean may have assisted in lowering pCO2 and driving the climatic system into the glacial conditions that prevailed during the last glacial. Subsequent changes in δ30Sidiatom/nutrient utilisation from MIS 5a to MIS 3 covary with δ18Odiatom inferred changes in freshwater input to the region, indicating a long-term regulation of the regional biological pump via a strengthening/weakening of the halocline stratification.

 

Diatom-bound Trace Metals and Silicon Isotopes in Sediment from the Gulf of California: Implication for the Significance of Sedimentary Opal Records

L.E. Pichevin1, R.S. Ganeshram1, B.C. Reynolds2,W. Geibert1

1School of Geosciences, Grant Institute, University of Edinburgh, West Main Road, EH10 3JW, Edinburgh, UK, Laetitia.pichevin@ed.ac.uk, Tel +44 131 650 8547, Fax +44 131 668 3184

2IGMR, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland

Here we investigate the significance of sedimentary opal records as a palaeoproductivity proxy commonly used to reconstruct past changes in the Si and C cycles. We report opal and organic carbon records and, for the first time, diatom-bound trace metal and δ30Si measurements in a well-dated, millennially-resolved sediment core from the Guaymas Basin. This basin is a well-known setting of high opal sedimentation driven by seasonal upwelling.

Our δ30Si data show that silicic acid utilisation is nearly complete during periods of intense upwelling. However, during Heinrich events silicic acid is not limiting in the surface water whereas opal production declines drastically in relation to reduced upwelling intensity. This implies that silicic acid availability is not the ultimate control on opal production and that other factors such as Fe availability or grazing may play an equally important role on silicic acid uptake and diatom growth during these events.

Although productivity is dominated by diatoms in the Guaymas Basin, our records reveal a strong decoupling between opal and organic carbon export. Such decoupling may be due to the effect of (1) intermittent Fe limitation, which alters the relative uptake of Si and C by biota, and/or (2) the limited efficiency of opaline material to export organic carbon to the sea floor. Preliminary measurements of diatom-bound trace metal concentrations suggest that occasional Fe limitation might have triggered high relative Si utilisation during periods of intense upwelling and altered the Si:C uptake and export ratios.

This work further demonstrates that opal records alone cannot be adequately used to reconstruct past change in the Silicon and Carbon cycles, even in setting dominated by diatom production.

 

Variations of δ30Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

S. Opfergelt1*, D. Cardinal2, L. André2, C. Delvigne2, L. Bremond3, B. Delvaux1

1Soil Science Unit, Université catholique de Louvain, Croix du Sud 2/10, B-1348 Louvain-la-Neuve, Belgium (sophie.opfergelt@uclouvain.be)

2Dept. of Geology and Mineralogy, Musée Royal de l'Afrique Centrale, Leuvensesteenweg 13, B-3080 Tervuren, Belgium

3Center of Bio-Archaeology and Ecology (UMR5059 CNRS, EPHE, Université Montpellier 2), Institut de Botanique, 163 rue Broussonet, F-34090 Montpellier, France

*Present Address: University of Oxford, Department of Earth Sciences, Parks Road OX1 3PR, UK

Silicon released by mineral weathering in soils can be retrieved from soil solution through plant uptake, clay formation, and Si adsorption onto secondary oxides, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers.

These proxies were used here to assess the contribution of biogenic Si (BSi) in a soilplant system involving basaltic ash soils differing in weathering degree under intensive banana cropping (Cameroon). δ30Si (MC-ICP-MS Nu Plasma) and Ge/Si ratios (HRICP- MS and ICP-AES) were determined in bulk soils (<2mm), sand (50-2000μm), silt (2-50μm), amorphous Si (ASi, 2-50μm), and clay (<2μm) fractions. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes).

Compared to fresh ash (δ30Si = -0.38‰; Ge/Si = 2.21 μmol.mol-1), clay fractions were enriched in light Si isotopes and Ge: with increasing weathering degree, δ30Si decreased from -1.19 to -2.37‰, and Ge/Si increased from 4.10 to 5.25μmol.mol-1.

Sand and silt fractions displayed δ30Si values close to fresh ash or higher due to saharan dust quartz deposition. The bulk soil Si-isotopic budget could be closed indicating that all phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ30Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution available for Si sequestration in surface clay-sized secondary minerals, which is confirmed by a mass balance calculation.

This study is a major contribution to the assessment of (1) soils processes governing δ30Si and Ge/Si signatures of soils components and (2) efficiency of δ30Si and Ge/Si signatures for tracing the plant DSi input from phytolith dissolution for secondary minerals formation at the soil-plant system scale. 

 

Late Quaternary Diatom Silica Oxygen Isotope Records from the Antarctic Margin

J.Pike1, M.J. Leng2, G.E.A. Swann2, A. Snelling2

1School of Earth and Ocean Sciences, Cardiff University, Main building, Park Place, Cardiff, CF10 3YE, UK

2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5AA, UK

Antarctica and its ice sheets have played, and continue to play, a major role in the global ocean-atmosphere system, hence, it is crucial that we have a sound understanding of the past behaviour of Antarctica and its ice sheets with a view to understanding their variability under a warming climate. Ice cores and ocean sediments provide intriguing insights into the timing and nature of the rapid climate transition that occurred at the last deglaciation (~13-11 kyr BP) and the Southern Ocean is key to mechanisms proposed for the bipolar seesaw of global thermohaline circulation that drives such glacialinterglacial cycles and rapid climate transitions. In order to further investigate these processes that originate in Antarctica and lead to changes in global climatic conditions it is necessary to understand the transfer mechanisms of ocean-climate signals from the Antarctic ice sheets, across the continental margin, into the Southern Ocean. The environment of the Antarctic continental margin is dominated by the seasonal advance and retreat of the sea ice. As such, the seasonal variability in sea ice coverage around Antarctica is one of the most significant factors regulating the energy balance of the Southern Hemisphere atmosphere and oceans. Exceptional, high resolution Antarctic margin sediment cores recovered during the last decade contain an excellent archive of these ice-ocean-climate interactions, often on seasonal timescales, from the last deglaciation and throughout the Holocene. Indeed, many of these cores from the continental shelf contain exceptionally well-preserved laminated sequences through the last deglaciation and partially laminated sequences through the Holocene. These laminations are dominated by exceptionally well-preserved fossil planktonic diatom assemblages; individual species of which are sensitive to sea surface conditions including sea ice concentration, fresh water influx, and open ocean influence upon the margin. The deglacial laminations have been shown to be seasonal in origin with spring laminae dominated by diatoms associated with the spring sea ice melt and the formation of a freshwater cap trapping nutrients in the surface waters (e.g. Hyalochaete Chaetoceros spp.); summer laminae associated with diatoms that favour slightly more oligotrophic, open-water conditions (e.g. Corethron criphilum and Rhizosolenia antennata); and autumn laminations being associated with diatoms that form resting spores in response to lowering light levels and sea ice re-advance (e.g. Thalassiosira antarctica). We intend to analyse the δ18Odiatom record from the deglacial and Holocene Antarctic sediments in order to investigate proposed mechanisms of formation for the deglacial laminated unit. Further, because the deglacial laminations contain diatom assemblages/species related to specific seasons, we hope to construct δ18Odiatom records through the Holocene to look at the long term evolution of seasonality. The first results of this research will be presented.

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Poster Presentation Abstracts (in alphabetical order by first author)

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Late Quaternary variations in the nutrient flux from the River Nile to the Mediterranean: silicon-isotope evidence from lacustrine diatoms

H.E. Cockerton1*, F.A. Street-Perrott1, K.J. Ficken1, M.J. Leng2, P.A. Barker3

1School of the Environment and Society, Swansea University, Swansea, SA2 8PP

2NERC Isotope Geosciences Laboratory, Nottingham, NG12 5GG

3Department of Geography, Lancaster University, Lancaster, LA1 4YB

*H.E. Cockerton, 292399@swan.ac.uk, +44 (0)1792 295531

Until recently, the importance of the continental silicon cycle at Quaternary (decadal to million-year) time scales has been relatively ignored with regard to its contribution to the global carbonate-silicate cycle. Focus has been placed on the longer-term processes of silicate-rock weathering and the resulting drawdown of CO2 rather than on shorter-term processes occurring along the land-ocean continuum (continental Si cycle).

Si accumulating plants (e.g. tropical rainforest trees, savanna and wetland grasses, papyrus) and aquatic organisms (i.e. diatoms and sponges in lakes, rivers and swamps) on the continents have the potential to modify the flux of silica to the oceans, and therefore affect the productivity of siliceous marine organisms and consequently the rate of CO2 drawdown. The ability of some terrestrial plants and aquatic organisms to take up, store and recycle significant amounts of Si is becoming increasingly recognised, although their impact on the continental Si cycle and the export of Si to the oceans under different climatic regimes remains unidentified.

The project looks at how variations in the type and extent of terrestrial and aquatic ecosystems within the Nile basin, driven by Late Quaternary climate changes, have altered the continental Si cycle. Utilising existing sediment cores from lakes situated along the Nile system, silicon and oxygen isotopes will be measured on lacustrine diatoms to determine downstream changes in the degree of biotic Si cycling on the continent and to provide an indication of palaeoclimate, respectively. The abundance and carbon isotope values of lipids (n-alkanes) extracted from the sediments will be used to track major changes in terrestrial and aquatic vegetation. The main aim of the project is to create spatial and temporal reconstructions of Si cycling along the Nile system for the period since the LGM.

 

Distribution of silicon in soils, sediments and watercourses in a small catchment area in southern Finland

L. Maria, P. Tallberg, V. Siipola

Department of Food and Environmental Sciences, Soil Science Section, P.O. Box 27, FIN-00014 University of Helsinki, Finland

Silicon (Si) has an important role in many biogeochemical processes, for example as a nutrient for large part of the world's primary production and in the regulation of atmospheric carbon dioxide content due to phytoplankton. A slow decline in the riverine supply of Si to many water bodies has been evident for a few decades, both absolutely but especially in proportion to the supply of N and P. The occurrence and short-term runoff of potentially bio-available Si from drainage areas of contrasting soil types may influence this phenomenon and consequently to the phytoplankton composition and ecological state of the receiving water bodies. The distribution of biogenic Si (BSi) in soil, seston and sediment and the content of dissolved silica (DSi) in the watercourses from a small watershed in Southern Finland were studied and the different compartments compared to each other and to environmental variables. Preliminary results demonstrate that the speciation of Si change along the watershed-river-lake-sea continuum which may affect the biological availability of Si. These preliminary results also indicate that the storage of BSi in soils may be significant and may hence affect the output of silica to the receiving water bodies.

 

Silica cycling – Which role do salt marshes and salt marsh management play?

F. Müller1*, J. Hartmann2, K. Jensen1

1University of Hamburg, Department of Biology, Applied Plant Ecology

2University of Hamburg, Department of Geosciences, Chemistry of Natural Aqueous Solutions

*E-mail: frauke.mueller@botanik.uni-hamburg.de

Silica (Si) is an essential nutrient for diatoms and thus has a key function in the food web of coastal waters. A depletion of dissolved silica (DSi) in early summer can lead to a limited diatom production accompanied by (partly toxic) algae blooms of non-diatomalgae.

Altered nutrient ratios in coastal waters during the last decades have deteriorated this effect. N- and P-concentrations increased due to anthropogenic deposition. At the same time constructional changes at coasts and rivers (e.g. embankments, building of barrages) led to a reduced resolution of Si, respective to enhanced Si-sedimentation.

Freshwater marshes are known to increase the supply of dissolved silica in the water column in times of limitation. The tissue of many marsh plant species contains a large amount of biogenic silica (BSi). After decomposition of the plant material, DSi leaves the marsh during ebb tide with the seeping interstitial water and can be assimilated by benthic and planktic diatoms. As BSi it then reenters the marsh with the inundation water, settles down, dissolves partly and can be taken up by plants again. Freshwater marshes can consequently be considered as silica recyclers.

In our study we investigate the so far unknown relevance of salt marshes and salt marsh management at the North Sea coast in the silica cycling. Due to their higher salinity and larger extent, salt marshes are suspected to contribute even more to the supply of DSi then freshwater marshes. In the ongoing project BASSIA (Biodiversity, management and ecosystem functions of salt marshes in the Wadden Sea National Park of Schleswig-Holstein) the fluxes of silica in and between different compartments of the ecosystem are analyzed. The content of DSi and BSi in the inundation water and the seepage water will be determined in 12-h-sampling campaigns. Pore water samplers will help to get insight in the concentration of DSi in the interstitial water.

Further, the BSi content in different soil horizons and in the vegetation will be an objective of investigations. To clarify the influence of management on the silica cycling, research is conducted on intensively grazed and on abandoned sites within the National Park of Schleswig-Holstein.

Preliminary results from August 2009 show a mean concentration of 0.64 mg DSi/l in the inundation water. This low value stands in contrast to throughout high values in the 25 seepage water (on average 7.07 mg DSi/l). Concentrations in the seepage water vary depending on land use. Creek water of the ungrazed sites contained on average 8.57 mg DSi/l, whereas concentrations in creek water of the grazed sites averaged out at only 5.57 mg DSi/l.

These first results indicate that different conservation management measures in the National Park differ in their impact on the silica cycle. The next step will be to quantify and to explain the differences. 

 

Competitive adsorption of silicon and phosphorus on goethite: influence on adsorption rates

V. Siipola and P. Tallberg

Department of Food and Environmental Sciences, Soil Science Section, P.O. Box 27, FIN-00014 University of Helsinki, Finland

This experiment is part of a larger research focused on the adsorption reactions of silicon (Si) and phosphorus (P) in sediments. In the short term cycling of silicon the biological incorporation of dissolved silicate into the cell walls of diatom algae is much faster than the chemical weathering of silicate minerals. As diatoms dissolve in the sediment, biogenic silicon is released. The dissolved silicon can move to the water column or adsorb on the sediment particles’ oxide surfaces. Si and P have similar adsorption behaviours and they compete for the same adsorption sites. Si can replace P in the sediment. Therefore silicon adsorption affects the nutrient amounts in the water due to the release of adsorbed phosphorus to the water column.

Adsorption rates are affected by pH, temperature and the concentration of the adsorbing species. In this experiment three different pH values, two different concentrations and room temperature are used. Adsorption rates of Si and P are determined separately and compared to rates determined when there is competition for adsorption sites. Synthetic goethite is used as the adsorbent because it is very common natural mineral and its chemical properties including surface properties, are well known. The aim of this experiment is to determine Si and P adsorption rate’s sensitivity to interference caused by competing species in the solution. Results will be presented in the poster.

 

Silicate release from sand-manipulated sediment cores: biogenic or adsorbed Si?

P. Tallberg1, S. Hietanen2, J. Lehtoranta3, M. Lehtimäki1, V.Siipola1

1Department of Food and Environmental Sciences, Soil Science Section, P.O. Box 27, FIN-00014 University of Helsinki, Finland

2Department of Biological and Environmental Sciences, P.O. Box 65, FIN-00014 University of Helsinki, Finland

3Marine Research Centre, Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finland

When studying the short-term cycling of Si in water ecosystems, it is often assumed that any Si released from the sediment to the water is of biogenic origin, i.e. released through the dissolution of diatom frustules deposited relatively recently at the sediment surface. An experiment where either sand, spring-bloom algae or the two together were added to surface sediment cores did, however, show that the sand addition significantly increased the outflux of Si from the sediment cores, while the algal addition did not. The outfluxes of the other main nutrients (P, N) remained relatively unaffected by all the additions. A follow-up bottle experiment was made to clarify whether (1) the Si outflux was caused by release of (adsorbed) Si from the sand grains or (2) the sand addition increased the dissolution of biogenic Si present in the sediment through some other mechanism. Preliminary results indicate that the release of Si from sand was, while not negligible, too low to account for the entire Si flux observed from the cores, and that some other mechanism was involved. 

 

Impact of land use on biological control of silica fluxes: an ecosystem signature study

F. Vandevenne1, E. Struyf1, D. Cardinal3, G. Govers2, W. Clymans2, O. Batelaan2, P. Meire1

1University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1c, 2610 Wilrijk, Belgium

2Catholic University of Leuven, Department of Geology, Physical and Regional Geography Research Group. Celestijnenlaan 200 E, B-3001 Heverlee, Belgium

3Royal Museum for Central Africa Tervuren, Geology en Mineralogy / Petrography, Leuvensesteenweg 13, 3080 Tervuren - Belgium

It is well known that anthropogenic land use changes have strongly influenced the occurrence of biota and soil formation over the last millennia. Land use changes can have a strong effect on the export of carbon, nitrogen and weathering products. Poor knowledge of the biological component in silica biogeochemistry challenges our ability to predict the effects of these land use changes on the silica cycle.

We have recently started a project that will identify and quantify Si (dissolved Si and amorphous Si) fluxes from forest, grassland and cropland ecosystems, using isotope techniques and trace-element/Si ratios. We want to establish “ecosystem Si-signatures” for these land use types. In a second phase, we will track the transport of the different forms of silica within the aquatic continuum. In order to do so, a basin wide sampling will be set up that will allow to measure Si isotope signature as well as trace-element/Si ratios through the aquatic continuum. As such, land use signatures will be qualitatively linked to the Si transport through the aquatic system, using ecosystem signatures.

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