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Titel |
High-resolution analysis of Quaternary calcretes: a coupled stable isotope and micromorphological approach |
VerfasserIn |
Kathryn Adamson, Ian Candy, Liz Whitfield |
Konferenz |
EGU General Assembly 2015
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 17 (2015) |
Datensatznummer |
250112113
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Publikation (Nr.) |
EGU/EGU2015-12267.pdf |
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Zusammenfassung |
Pedogenic calcretes are abundant in arid and semi-arid regions, and they are widely used
as proxy records of palaeoclimatic change. Calcrete oxygen (δ18O) and carbon
(δ13C) isotopic signatures are indicative of temperature, aridity, or vegetation at the
time of calcrete formation. Their microfabrics also reflect carbonate formation
mechanisms in response to the prevailing environmental conditions. Many studies
have explored calcrete micromorphology or stable isotope composition, but these
techniques have not yet been applied simultaneously. This co-analysis is important as it
allows us to establish whether calcrete morphology directly reflects environmental
change. This study tests the potential of combining these analyses to examine the
relationships between calcrete microfabrics, their isotopic signals, and Quaternary climate
change.
Calcretes from four river terraces of the Rio Alias in southeast Spain have been
analysed in detail. On the basis of morphostratigraphic correlation (Maher et al., 2007)
and Uranium-series ages (Candy et al., 2005), these span the period from 304 ±
26 ka (MIS 9) to the Holocene. The oldest profiles have therefore been exposed
to multiple glacial-interglacial cycles. A total of 37 micromorphological profiles
have been used to extract stable oxygen and carbon isotopic indicators from 77
microfacies.
The morphological and isotopic complexity of the calcrete profiles increases with
progressive age. The oldest samples display multiple calcretisation phases, and their
microfabrics have a larger isotopic range than the younger samples. Alpha (non-biogenic)
fabrics have higher δ13C and δ18O values than beta (biogenic) fabrics. Strong positive
covariance between δ13C and δ18O within all profiles suggests that both isotopes are
responding to the same environmental parameter. We suggest that this is relative aridity. The
study demonstrates that the detailed co-analysis of calcrete micromorphology and
stable isotope signatures allows calcrete formation patterns to be placed into a wider
palaeoclimatic context. Importantly, this technique provides a level of detail that is not
possible through bulk isotope sampling alone. It demonstrates the potential of this
technique to more reliably constrain the palaeoenvironmental significance of secondary
carbonates in dryland settings where other proxy records may be poorly preserved. |
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