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| Titel |
Comparison of climate proxies from two “Siamese twin” stalagmites: Some keys for evaluating paleoclimate reconstructions based on speleothems |
| VerfasserIn |
María Belén Muñoz-García, Carlos Rossi, María Jesús Turrero, Javier Martín-Chivelet |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043352
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| Zusammenfassung |
| Siamese twin stalagmites are those that grow so close to each other that they develop
continuous growth laminae between their apexes. Therefore, the resulting composite
stalagmite is deposited by two coeval drips, instead of by a shifting single drip. In polished
cross sections, optical correlation of growth layers allows the confident determination of
coeval sections in both stalagmites. In these coeval sections, paleoenvironmental proxies
should be identical if controlled by paleoclimate. Otherwise the proxies may be influenced by
local factors within each drip (the so called “piping effects”) or by diagenetic alteration. The
study of two Holocene Siamese stalagmites collected in Cueva del Cobre (N Spain) allows a
direct calibration of records of growth rates and stable isotopes (C and O), and their
suitability for paleoclimate reconstructions.
The composite stalagmite (~19 cm wide in the base and ~28 cm high) is formed by two
cone-shaped Siamese stalagmites with apexes separated ~7 cm from each other. The
stalagmites grew in an isolated relict canyon situated ~1 km away from the entrance of the
cave, 51 m above the resurgence elevation, and more than 100 m below the ground surface.
Both stalagmites appeared to be growing when collected. Present-day temperature in the
sampling site remains around 5.5±0.3ºC and the measured relative humidity is
98±2%.
The study of 35 thin sections revealed that both stalagmites are composed of columnar
calcite with undulose extinction. Growth banding mainly results from variations in the
abundance of primary fluid inclusions. Absolute ages of both stalagmites are constrained by
ten ICP-MS Th-230 datings. The results further confirm that the Siamese sections grew
synchronously for about 5000 years, from ~5400 to ~600 years BP. These dates,
integrated with microscopic optical correlation of continuous growth layers using
thin sections, allowed us to determine a robust age model. The carbon and oxygen
isotopic records are composed of 281 samples drilled along the growth axes of both
stalagmites.
According to the proposed age model, the two stalagmites grew at very different rates,
despite their proximity and their internal and external resemblance. The differences are very
important in both magnitude and trends, suggesting that piping and dripping effects
were strong enough to mask any paleoclimate information. On the contrary, the
δ18O records of the Siamese sections of the two stalagmites are virtually identical,
suggesting that this parameter is independent of stalagmite growth, being mostly
controlled by external factors to the dripping system, probably climate variables (e.g.,
rainfall composition). Finally, important differences were found when comparing the
δ13C time-series. These differences concern general patterns and trends, as well
as the average values of each series. This could indicate that δ13C is much more
influenced by piping effects than δ18O, and thus more difficult to interpret in terms of
paleoclimate.
In summary, similar δ18O records have been obtained in both stalagmites, but
important differences were found in growth rates and δ13C values. A single record
from only one of these stalagmites would have not been representative of the whole
environmental system. These results suggest that the paleoclimate interpretations based on
growth rates and δ13C records obtained in a single speleothem should be treated with
caution.
Contribution to project CGL2007-60618-BTE (MCI, Spain), research grants
PR-2007-0111 and PR-2007-0197, and the Paleoclimatology and Global Change
Research Group (UCM-CM-910198). L.R. Edwards and X. Wang are thanked for his
advisory help and support during stays of JMC and MJT in the Univ. of Minnesota. |
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