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A new approach on the oxygen isotope micro analysis of diatom silica with a laser-fluorination based mass spectrometry unit

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Chapligin, B. , Meyer, H. , Marent, A. , Hubberten, H. W. and Friedrichsen, H. (2007): A new approach on the oxygen isotope micro analysis of diatom silica with a laser-fluorination based mass spectrometry unit , El'gygytgyn Lake Workshop, Cologne, November 02-03 .
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Abstract:

The analysis of oxygen isotopes from diatom silica in sediment cores has reached importance for reconstructing the paleoclimate and is especially valuable in non-carbonate lakes of cold regions, where no other bioindicators such as ostracods and foraminifera are available. Analyses so far were performed in marine (e.g. Shemesh et al. 1992) as well as in lacustrine environments (e.g. Leng et al. 2004). A new approach for samples in sub-mg range has been developed to provide a better chronological resolution and to expand the method to periods where less biogenic silica is available (e. g. samples corresponding to MIS 2-4). After extracting the diatoms from sediment cores with various preparation steps including wet chemistry, sieving and heavy liquid separation a minimum of ~700µg fine material from 5 g of wet sample is obtained. The pure sample (degree of purity: &#8805; 99%) is melted into a bead to eliminate the water vapour and reduce the surface to avoid an explosion of the material. The used periphery of the mass spectrometer (PDZ Europa 20-20) consists of a laser fluorination system operated under vacuum. The bead is reacted with a focused CO2 laser under BrF5 atmosphere within a stainless steel-chamber. The non-oxygen gas components are trapped in a -150°C cold trap, whereas oxygen passes on to a molecular sieve cooled with liquid nitrogen. It is then transferred to the mass spectrometer and compared with a reference standard of known isotopic composition.Specially designed software and a video camera are used to survey and record the process in the reaction chamber and allow an automised, remote operation. This guarantees maximum safety as the instrument is installed in two different rooms, with the reagent (BrF5), reaction chamber and the laser unit arranged under a hood. Tests on standard material (NBS 28, Campolungo) showed a standard deviation <0.2. Sample material from Lake Elgygytgyn (core LZ 1024) will be analysed and a &#948;18O curve of the last 280.000 years will be generated to add a strong climate proxy to the various analysis performed so far. The Lake lies inside a meteorite impact crater formed approximately 3.6 million years ago and hence offers a unique option to fill the spatial gap of locations in the Arctic where paleoclimate reconstructions are sparse. Former drilling operations show that the lake could contain the longest, most continuous terrestrial record of past climate change in the entire Arctic back to the time of impact (Brigham-Grette, 2006).The analysis aims on the planktonic Cyclotella ocellata-complex which is persistent through a variety of climate conditions and present throughout the core (Cherapanova, 2006). Another goal is to examine a possible species-dependent fractionation by comparing the mentioned complex with Pliocaenicus costatus var. Sibiricus in the holocene.The expected results will be the base for studying the climate history using stable isotopes in lacustrine diatoms of the whole 300 m sedimentary sequence on sediment cores at Lake Elgygygtgyn to be drilled within the frame of the ICDP in 2009.Keywords: paleoclimate; laser; fluorination; mass spectrometry; oxygen isotopes; diatom silica;

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