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The 3rd meeting of the Isotopes in Biogenic Silica Working Group was held on 16 May 2008 at Oxford Brookes University, UK.

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Meeting Programme

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Papers

• 09.30 Registration and Coffee/Tea

• 10.10 Dr. Adrian G. Parker (Oxford Brookes University) Welcome and Introduction

• 10.20 Dr. Eric Struyf (Lund University) Silica: an essential nutrient in wetland biogeochemistry

• 11.00 Liz. Turner (Lancaster University) A 25,000 year record of carbon isotope variation from biogenic silica Lake Challa, Kilimanjaro.

• 11.30 Prof. Alayne Street-Perrott (Swansea University) 30Si and 18O analyses of biogenic silica in a multi-isotope palaeolimnological context:  application to Lake Rutundu, Mt Kenya

• 12.00 Morten Andersen (University of Bristol) The Zn isotopic composition of diatom frustules

• Lunch

• 1.30 Prof. Melanie J. Leng (NIGL and University of Nottingham) Progress towards obtaining diatom oxygen isotopes records from Scandinavian lakes 

• 2.00 Dr. Andrew Henderson (University of Glasgow) Holocene climate change in southern Alaska: what does the diatom isotope record tell us?

• 2.30 Dr. George Swann (NIGL)  A diatom oxygen and silicon isotope record from Lake El'gygytgyn, North East Siberia: evidence of solar forcing and abrupt palaeoenvironmental change over the last 18,000 years.

• 3.00 Dr. Martin J. Hodson (Oxford Brookes University) Silicon isotopes in plants: where are we now?

• 3.30 Tea/Coffee

• 3.45 Discussion session and future directions

• 4.45 Close

 

Posters

• Katy Wilson (University College London) Plans for a high-resolution reconstruction of Pliocene Palaeoclimatology in the East African Rift Valley using terrestrial diatomites.

• Tom Stephens (University of Auckland) Inter-Hemispheric Climate Variability: A novel multi-proxy application of oxygen and silicon isotopes to a Southern Hemisphere maar crater lake (ca. 30, 000 cal. yr BP to Present

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Abstracts

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Silica: an essential nutrient in wetland biogeochemistry

Eric Struyf 1,2, Daniel J Conley 1
1Department of Geology, University of Lund, Lund, Sweden
2Department of Biology, Ecosystem Management Research Group, University of Antwerp, Antwerp, Belgium

Recent research has emphasized the importance of terrestrial ecosystems in the global biogeochemical cycle of silica. The production, retention and dissolution of amorphous silica of biological origin in soils and vegetation effectively control terrestrial Si fluxes. In this context, surprisingly little is known about the role of wetlands. Wetlands are known hot-spots in both nitrogen and phosphorus cycling, and research on both these nutrients has been conducted in countless studies worldwide, and numerous reviews have been published. By bringing together previously scattered results, this review shows that the potential impact of wetland ecosystems on Si transport and processing potentially rivals their impact on other important biogeochemical cycles. Yet, the range of studied systems is small and incomplete. This is an important shortcoming in our understanding of both coastal eutrophication and climate change, issues strongly linked to Si biogeochemistry. We feel addressing this essential gap warrants attention of ecosystem scientists and wetland biogeochemists worldwide.

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A 25 000 year record of carbon isotope variation from biogenic silica.

Elizabeth Turner1, Philip Barker1, Melanie Leng2,3 and Dirk Verschuren4
1Department of Geography, Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ
2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG
3School of Geography, University of Nottingham, Nottingham, NG7 2RD, UK
4 Department of Biology, Ghent University, Ghent. Belgium

Tropical mountain lake ecosystems are sensitive barometers of CO2 change on glacial – interglacial timescales due to the slow diffusion rates of CO2 in water and decreasing atmospheric CO2 concentrations with elevation. Previous analysis of lake sediments from East Africa have focused on stable carbon isotopes of bulk organic matter (δ13Cbulk) whose interpretation is complicated by source effects. To refine these existing analyses, a methodology has been developed utilising organic inclusions within fossil diatoms as hosts for carbon isotope analysis. Diatoms respire using dissolved carbon within lakes, thus stable carbon isotopes of diatoms (δ13Cdiatom) should reflect changes in biogenic dissolved carbon flux in response to climate or environmental change. A 25ka (21m) sediment core taken from Lake Challa, 880m altitude on the foothills of Kilimanjaro, forms the backdrop to this study. Presented here are the results of δ13Cdiatom and the oxygen isotopes of diatoms (δ18Odiatom) alongside other palaeoenvironmental proxies collected on Lake Challa in connection with the ESF funded CHALLACEA project. Initial results show a clear negative correlation between δ18Odiatom and δ13Cdiatom and a suggestion that δ13Cdiatom could be a new palaeoproductivity indicator. A positive correlation is found with δ13Cbulk, reflecting the diatomaceous nature of these sediments, although δ13Cdiatom is 3 per mille more negative suggesting the frustule proteins are enriched in 12C. We also observe a correlation with δ13Ccarb (C.Wolff pers.com), although the lack of carbonate in sediments older than 6ka and diagenetic effects limit the use of this isotopic host. In developing a new proxy we add to the arsenal of isotopic techniques provided by biogenic silica and enable new insights into lacustrine biogeochemical cycling to be made.

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The Zn isotopic composition of marine diatom frustules 

Morten Andersen 
Department of Earth Sciences, Wills Memorial Building, University of Bristol BS8 1RJ, Bristol, UK

Zinc is among the essential trace-metal micronutrients for phytoplankton and Zn concentrations are highly depleted in those parts of the surface ocean that are replete in the major nutrients, (so-called High Nitrate-Low chlorophyll, or HNLC, zones). The proposed release of these HNLC zones from trace metal limitation is one of the key hypotheses to explain lower atmospheric CO2 during glacial periods. The preferential incorporation of light Zn isotopes into phytoplankton organic material leaves residual seawater Zn isotopically heavy. The isotopic heaviness of the residual seawater Zn could track the degree of trace metal depletion in the past and we investigate diatom opal as a record of the Zn isotopic composition of surface seawater. We have measured Zn isotopic compositions in cleaned diatom frustules from a sequence of core-top samples across the Southern Ocean. Diatom opal exhibits heavy Zn isotopic compositions, centred at δ66ZnLyons-JMC compositions around +1‰, which is significantly heavier than the continental input of +0.3 ‰. These results suggest that Zn isotopes in diatom frustules have potential as a record of past trace metal depletion in HNLC zones.

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δ30Si and δ18O analyses of biogenic silica in a multi-isotope palaeolimnological context: application to Lake Rutundu, Mt Kenya.

F. Alayne Street-Perrott1, Philip A. Barker2 and Melanie J. Leng3,4
1School of the Environment and Society (SOTEAS), Swansea University, Singleton Park, Swansea SA2 8PP
2Department of Geography, Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ
3NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG
4School of Geography, University of Nottingham, Nottingham, NG7 2RD, UK

Few studies have so far considered how continental, biogeochemical Si fluxes varied on an orbital time scale. We have reconstructed changes in Si cycling by the catchment-lake ecosystem of the crater lake L. Rutundu, Mt. Kenya (3078 m a.s.l.), over the last ~38 ka, using a novel combination of sediment fluxes and stable-isotope (δ18O, δ30Si, δ15N, molecular δ13C) data (Street-Perrott et al., in press). Under glacial conditions (38.3 – 14.3 cal. ka BP), high diatom productivity was maintained by substantial losses of dissolved SiO2 (DSi) and soil nutrients from a sparse, leaky, terrestrial ecosystem. Very high δ18Odiatom values (+29 to +41‰) are consistent with significantly reduced monsoon precipitation, increased evaporation and lower water temperatures. δ30Sidiatom varied by almost 1.9‰, from –1.3 to +0.5‰, showing that Lake Rutundu did not behave as a closed system during periods of biological Si uptake (Varela et al., 2004). The most negative values of δ30Sidiatom suggest a plentiful silica supply. They coincided with high δ18Odiatom values and diatom assemblages dominated by Aulacoseira distans sp. gr. On Mt. Kenya today, this taxon is found in very shallow, spring-fed, crater lakes with dense floating mats of aquatic vegetation. We infer that during its water-level minima, (~33 and 23 ka BP), Lake Rutundu was maintained by drawdown of Si-rich groundwater issuing directly from the crater-wall trachytes. During the following period of enhanced monsoon rainfall and seasonality (14.3 to 9.5 ka BP), δ18Odiatomdecreased to +27‰. Rapid biotic Si cycling by fire-prone, mesic grassland was associated with substantial aeolian transport of opal phytoliths by smoke plumes, but greatly reduced nutrient losses in runoff. Invasion of tall, subalpine shrubs after 9.5 ka BP further enhanced landscape stability, leading to very low sediment fluxes of both phytoliths and diatoms.  This case study offers insights into processes that may have operated at biome to continental scales during the Late Quaternary.

References:
Street-Perrott FA, Barker PA, Leng MJ, Sloane HJ, Wooller MJ, Ficken KJ, Swain DL. in press. Towards an understanding of Late Quaternary variations in the continental biogeochemical cycle of silicon: multi-isotope and sediment-flux data for Lake Rutundu, Mt. Kenya, East Africa, since 38 ka BP. Journal of Quaternary Science.
Varela DE, Pride CJ, Brzezinski MA. 2004. Biological fractionations of silicon isotopes in southern ocean surface waters. Global Biogeochemical Cycles 18: GB1047, doi:10.1029/2003GB002140.

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Progress towards obtaining diatom oxygen isotopes records from Scandinavian lakes 

Melanie J. Leng 1&2, Ninis Rosqvist3, Christina Jonsson3, Hilary Sloane1
1NERC Isotope Geoscience Laboratory, British Geological Survey, Nottingham. NG12 5GG, UK. 
2School of Geography, University of Nottingham, Nottingham NG7 2RD, UK.
3 Department of Physical Geography and Quaternary Geology, Stockholm University.

The oxygen isotope composition of diatom silica is being increasingly used as a palaeoclimate proxy from lake sediments. This talk reviews the controls (including contamination) on the isotope composition of lacustrine deposits and illustrates how stable isotope studies contribute to an understanding of changes in precipitation patterns in lakes in Scandinavia. Four examples of δ18Odiatom will be presented, all are small and medium sized open (non evaporating) lakes which have a through-flow and typically contain sediments with δ18O values that vary by no more than a few ‰. These variations are generally ascribed to variations in annual and winter precipitation (δp). The important conclusion is that the interpretation of isotope data from lacustrine successions requires knowledge of the local processes (lake water residence time, period of diatom bloom) that control the signals. The relationship between δ18Odiatom and climate however is complex.

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A diatom oxygen and silicon isotope record from Lake El´gygytgyn, North East Siberia: evidence of solar forcing and abrupt palaeoenvironmental change over the last 18 000 years

George E.A. Swann1, Melanie J. Leng 1,2, Hilary J. Sloane1
1NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
2School of Geography, University of Nottingham, Nottingham, NG7 2RD, UK

Lake El´gygytgyn, a crater lake in North East Siberia, Russia, is uniquely positioned within the Arctic Circle to provide detailed records of high latitude palaeoenvironmental conditions over the last c. 3 Ma. Results are presented here from Lake El´gygytgyn showing large variations in δ18Odiatom and δ30Sidiatom through the last glacial and the Holocene, reflecting significant alterations in the isotopic balance of the lake and rates of nutrient utilisation. Based on a preliminary radiocarbon age-model, a large proportion of the variations in δ18Odiatom over the last 18,000 years appear related to changes in solar variability.

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Silicon isotopes in plants- where are we now?

Martin J. Hodson1 & Adrian G. Parker2
1School of Life Sciences, Oxford Brookes University, Gipsy Lane, Headington, Oxford, OX3 0BP, UK. Email: mjhodson@brookes.ac.uk
2Department of Anthropology and Geography, Oxford Brookes University, Gipsy Lane, Headington, Oxford, OX3 0BP, UK.

The aim of this paper will be to assess the present state of knowledge concerning stable silicon isotopes in plants, and to suggest areas for future study. Recent work on silicon isotopes in four plant species has revealed a number of significant findings:

1. The isotopic composition of the substrate affects the composition of the plants.

2. Once inside the plant Rayleigh fractionation occurs, with the lighter isotopes being deposited first and the heavier ones further along the transpiration stream.

3. Fractionation appears to be greater in plants that accumulate higher levels of silicon, but this needs to be confirmed with future work.

As yet the fractionation within plants remains an interesting finding with no obvious application to palaeoecological, archaeological or biogeochemical work. Some topics for future work include: the investigation of more plant species, particularly trees; determining whether fractionation also occurs at a biochemical level; assessing the effects of environmental conditions on fractionation; and investigating the partitioning of isotopes in various parts of the terrestrial silicon biogeochemical cycle.

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Plans for a high-resolution reconstruction of Pliocene Palaeoclimatology in the East African Rift Valley using terrestrial diatomites.

Wilson, K.E.1, Leng, M.J.2&3, Mackay, A.W.1, Maslin, M.A.1, Kingston, J.D.4
1Department of Geography, University College London, Pearson Building, Gower Street, London, WC1E 6BT.
2NERC Isotope Geoscience Laboratory, British Geological Survey, Nottingham. NG12 5GG, UK.
3School of Geography, University of Nottingham, Nottingham NG7 2RD, UK 
4Department of Anthropology, Emory University, 1557 Dickey Dr., Atlanta, GA 30322. 

The East African Rift Valley offers many opportunities to explore and understand interactions between environmental change and faunal speciation events throughout the Plio-Pleistocene. A series of 5 diatomite units exposed near the Barsemoi river in the Kenyan Rift Valley offer a unique opportunity to study Pliocene (2.68-2.55 Ma) climate change at high resolution. This will enable an evaluation of the effects of extreme short-term climate variability on faunal, including hominin, evolution. It is believed that sinusoidal precessional forcing resulted in the rapid development of a deep, aerially-extensive freshwater lake system in the Central Kenyan Rift Valley through changes in African monsoonal circulation. The proposed study will conduct initial investigations into one of the diatomite units, measuring stable isotope proxies (δ18O; δ30Si) on diatoms in conjunction with SEM and XRF analyses to assess sample purity. 

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Inter-Hemispheric Climate Variability: A novel multi-proxy application of oxygen and silicon isotopes to a Southern Hemisphere maar crater lake (ca. 30 000 cal. yr BP to Present)

Tom Stephens 
School of Geography, Geology and Environmental Science, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Lake Pupuke, a maar crater lake formed ~260 cal. kyr. BP within the Auckland Volcanic Field, Auckland, New Zealand, represents one of the few uninterrupted, high-resolution Late Quaternary (~30 kyr. BP to Present) palaeolimnologic archives within the Southern Hemisphere. Climatological and ecological variability is to be reconstructed from numerous geochemical and biological markers preserved within the finely laminated sediments, most notably stable isotopes of O and Si. Novel approaches to the dual analysis of O and Si isotopes within biogenic silica and of O in aquatic lake cellulose should allow reconstruction of a detailed record of continuous palaeohydrological change. This project intends to meet the aims of the INQUA Paleoclimate Commission (INTIMATE program) by providing a high-resolution (<500 yr. interval) palaeoclimatic record from the Southern Hemisphere mid-latitudes so as to better assess the nature, timing and regional to global extent of palaeoenvironmental changes associated with the Last Glacial Termination. In this poster we present details of the study rationale and methods we are employing to achieve our objectives followed by a preliminary analysis f lipid data over the period ~7.0 to ~8.2 cal. kyr. BP that demonstrate marked variations in functional lipid class abundance.