Calibration of speleothem δ 18 O records against hydroclimate instrumental records in Central Brazil
Jean‐Sébastien MoquetFrancisco W. CruzValdir F. NovelloNicolás M. StríkisMichael DeiningerIvo KarmannRoberto Ventura SantosChristian MilloJames ApaésteguiJean‐Loup GuyotA. SiffedineMathias VuilleHai ChengR. Lawrence EdwardsWilliam Santini
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Speleothem
δ18O
Stalagmite
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Paleoclimatology
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TIMS U-Th dating and high-resolution oxygen isotopic composition measurements were conducted on a stalagmite SI3 collected from Shizi cave, Nuoshuihe, northeastern Sichuan, Central China. The stalagmite SI3 was developed between 54-46 ka. Its δ18O record displays a high similarity with those of stalagmites MSL and MSD from Hulu cave, East China and D4 from Dongge cave, Southwest China, respectively. The trend of the δ18O record of SI3 also parallels with those of ice core GISP2 from polar area and of speleothem in Israel. These suggest that the δ18O record of SI3 is closely related with summer monsoon variation in East Asia. However, some evidences seem to suggest that carbonates in SI3 are more 18O enriched than those deposited under oxygen isotopic equilibrium condition. There was probably kinetic fractionation when SI3 was developed. This may be explained by the locus of SI3 that was collected near the entrance of Shizi cave. The possible mechanism between the δ18O record of SI3 and the variation in East Asian summer monsoon was speculated. It is suggested that in the East Asian summer monsoon domain, the δ18O record of some speleothem deposited even under nonequilibrium condition is still a potential proxy for summer monsoon variation.
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The Hendy Test is widely used for assessing whether isotopic equilibrium was reached in speleothems by examining the δ18O and δ13C correlation along a single growth layer. However, stalagmite micro-layers are typically only a few micrometers thick and taper off from the center towards the sides, making it challenging to sample within the same growth layer in practice. To address this, we selected three caves in the monsoon region of China (Shihua Cave in the north, Heshang Cave in the central, and Baojinggong Cave in the south) to verify whether the modern cave calcite has reached equilibrium fractionation with drip water. We examined the spatial variations in the δ18O and δ13C values of farmed calcite on glass plates, which are analogous to a single growth layer. The δ18O and δ13C correlations of farmed calcite from different cave sites are consistently strong, suggesting that kinetic fractionation effects are prevalent, especially at the drip sites with lower drip rates due to longer CO2 degassing. The δ18O–δ13C covariations can also occur along speleothem growth axes on short time scales, while isotopic variations over longer time scales are still in response to climate change. We propose that the Hendy Test criteria might not be prerequisites to isotopic equilibrium, and a Replication Test provides a more reliable indication of the integrity of isotopic proxies in paleoclimate research.
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