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| Titel |
POTENTIAL OF GDGTS AS TEMPERATURE PROXIES ALONG ALTITUDINAL TRANSECTS IN EAST AFRICA |
| VerfasserIn |
Sarah Coffinet, Arnaud Huguet, Christine Omuombo, David Williamson, Céline Fosse, Christine Anquetil, Sylvie Derenne |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086372
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| Publikation (Nr.) |
 EGU/EGU2014-225.pdf |
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| Zusammenfassung |
| Glycerol dialkyl glycerol tetraethers (GDGTs) are lipids of high molecular weight and
include the isoprenoid GDGTs (iGDGTs) produced by Archaea and the branched GDGTs
(brGDGTs) produced by unknown bacteria. Several indices were developed to
describe the relationship between GDGT distribution and environmental parameters:
the TEX86 (tetraether index of tetraethers consisting of 86 carbons), based on the
relative abundances of iGDGTs in sediments, and the MBT (methylation index of
branched tetraethers) and CBT (cyclisation ratio of branched tetraethers), based on
the relative abundance of brGDGTs in soils. The TEX86 was shown to correlate
well with water surface temperature, and the MBT and CBT with mean annual air
temperature (MAAT) and soil pH. The GDGTs are increasingly used as temperature
proxies.
In this study, 41 surface soils were sampled along two altitudinal transects, from 500 to
2800 meters in Mount Rungwe (South western, Tanzania) and from 1897 to 3268 meters in
Mount Kenya (Central Kenya). MAAT was reconstructed along the two transects
using the MBT/CBT proxies. A linear correlation between the MBT/CBT-derived
temperatures and the altitude (R2=0.83) was obtained by combining results of the two
transects. The reconstructed temperature lapse rate (0.5 °C/100 m) was consistent
with the one determined from temperature measurements at six altitudes. These
results show that the MBT/CBT is a suitable and robust temperature proxy in East
Africa.
In Mt. Rungwe soil samples, the TEX86 index, which was mainly used to reconstruct
water surface temperatures until now, was found to vary linearly with altitude (R2=0.50).
Such a relationship between TEX86 and altitude in organic soils has also been recently
noticed in Mt. Xiangpi, China (Liu et al., 2013; R2=0.68). The adiabatic cooling of air
with altitude could explain the TEX86 variation with altitude. If such a relationship
is confirmed, the use of the TEX86 as a temperature proxy could be extended to
soil environments. However, a lower correlation between TEX86 and altitude was
observed for Mt. Kenya samples, implying that the environmental factors affecting
the TEX86 values should be further investigated. Moreover, a given TEX86 value
was shown to correspond to a much higher altitude (ca. 1800 m higher) for Mt.
Xiangpi soils (Liu et al., 2013) than for Mt. Rungwe samples, suggesting that the
geographical origin of the soils could also impact the TEX86 values. Therefore, a better
understanding of the environmental mechanisms controlling the iGDGTs distribution in
soils is needed prior any application of the TEX86 as a temperature proxy in these
environments.
REFERENCES
Liu, W., Wang, H., Zhang, C.L., Liu, Z., He, Y., 2013. Organic Geochemistry 57, 76–83. |
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