Shanahan Timothy M.

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Shanahan
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Timothy M.
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  • Article
    Scanning micro-X-ray fluorescence elemental mapping : a new tool for the study of laminated sediment records
    (American Geophysical Union, 2008-02-27) Shanahan, Timothy M. ; Overpeck, Jonathan T. ; Hubeny, J. B. ; King, J. ; Hu, Feng Sheng ; Hughen, Konrad A. ; Miller, G. ; Black, J.
    The utility of elemental mapping by scanning X-ray fluorescence (XRF) in the study of annual laminated sedimentary records was investigated on eight annually laminated sediment types. The examples were chosen to illustrate the potential of this approach in environments dominated by terrigenous, biological and chemical deposition. Individual laminae were identifiable in elemental maps of all sediment types and were enhanced through the use of data reduction techniques (e.g., principal components transformation). Laminae were least apparent in clastic dominated systems with no seasonal changes in sediment sources. In biologically dominated systems, element maps provided insights into the composition of the varve subcomponents, related to alternating terrigenous and biologically dominated seasonal periods of deposition. Chemically precipitated structures were more prevalent than expected from visual investigations alone and may provide an underutilized paleoenvironmental signature of changing limnological conditions. Elemental mapping offers a valuable tool for the study of laminated records that complements existing techniques (e.g., SEM, digital image analysis).
  • Article
    An interlaboratory study of TEX86 and BIT analysis using high-performance liquid chromatography–mass spectrometry
    (American Geophysical Union, 2009-03-20) Schouten, Stefan ; Hopmans, Ellen C. ; van der Meer, Jaap ; Mets, Anchelique ; Bard, Edouard ; Bianchi, Thomas S. ; Diefendorf, Aaron ; Escala, Marina ; Freeman, Katharine H. ; Furukawa, Yoshihiro ; Huguet, Carme ; Ingalls, Anitra ; Menot, Guillemette ; Nederbragt, Alexandra J. ; Oba, Masahiro ; Pearson, Ann ; Pearson, Emma J. ; Rosell-Mele, Antoni ; Schaeffer, Philippe ; Shah, Sunita R. ; Shanahan, Timothy M. ; Smith, Richard W. ; Smittenberg, Rienk ; Talbot, Helen M. ; Uchida, Masao ; Van Mooy, Benjamin A. S. ; Yamamoto, Masanobu ; Zhang, Zhaohui ; Sinninghe Damste, Jaap S.
    Recently, two new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were proposed, i.e., the TEX86 proxy for sea surface temperature reconstructions and the BIT index for reconstructing soil organic matter input to the ocean. In this study, fifteen laboratories participated in a round robin study of two sediment extracts with a range of TEX86 and BIT values to test the analytical reproducibility and repeatability in analyzing these proxies. For TEX86 the repeatability, indicating intra-laboratory variation, was 0.028 and 0.017 for the two sediment extracts or ±1–2°C when translated to temperature. The reproducibility, indicating among-laboratory variation, of TEX86 measurements was substantially higher, i.e., 0.050 and 0.067 or ±3–4°C when translated to temperature. The latter values are higher than those obtained in round robin studies of Mg/Ca and U37 k′ paleothermometers, suggesting the need to primarily improve compatibility between labs. The repeatability of BIT measurements for the sediment with substantial amounts of soil organic matter input was relatively small, 0.029, but reproducibility was large, 0.410. This large variance could not be attributed to specific equipment used or a particular data treatment. We suggest that this may be caused by the large difference in the molecular weight in the GDGTs used in the BIT index, i.e., crenarchaeol versus the branched GDGTs. Potentially, this difference gives rise to variable responses in the different mass spectrometers used. Calibration using authentic standards is needed to establish compatibility between labs performing BIT measurements.