Clumped isotope thermometry of cryogenic cave carbonates

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• 作者：Tobias Kluge ; Hagit P. Affek ; Yi Ge Zhang ; Yuri Dublyansky ; Christoph Sp枚tl ; Adrian Immenhauser ; Detlev K. Richter
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：1 February, 2014
• 年：2014
• 卷：126
• 期：Complete
• 页码：541-554
• 全文大小：1336 K

文摘

Freezing of cave pool water that is increasingly oversaturated with dissolved carbonate leads to precipitation of a very specific type of speleothems known as cryogenic cave carbonates (CCC). At present, two different environments for their formation have been proposed, based on their characteristic carbon and oxygen isotope ratios. Rapidly freezing thin water films result in the fast precipitation of fine-grained carbonate powder (CCC_fine). This leads to rapid physicochemical changes including CO₂ degassing and CaCO₃ precipitation, resulting in significantly p>13p>C-enriched carbonates. Alternatively, slow carbonate precipitation in ice-covered cave pools results in coarse crystalline CCC (CCC_coarse) yielding strongly p>18p>O-depleted carbonate. This is due to the formation of relatively p>18p>O-enriched ice causing the gradual depletion of p>18p>O in the water from which the CCC precipitates.<p>Cryogenic carbonates from Central European caves were found to have been formed primarily during the last glacial period, specifically during times of permafrost thawing, based on the oxygen isotope ratios and U-Th dating. Information about the precise conditions of CCC_coarse formation, i.e. whether these crystals formed under equilibrium or disequilibrium conditions with the parent fluid, however, is lacking. An improved understanding of CCC_coarse formation will increase the predictive value of this paleo-permafrost archive.<p>Here we apply clumped isotopes to investigate the formation conditions of cryogenic carbonates using well-studied CCC_coarse from five different cave systems in western Germany. Carbonate clumped isotope measurements yielded apparent temperatures between 3 and 18 掳C and thus exhibit clear evidence of isotopic disequilibrium. Although the very negative carbonate 未p>18p>O values can only be explained by gradual freezing of pool water accompanied by preferential incorporation of p>18p>O into the ice, clumped isotope-derived temperatures significantly above expected freezing temperatures indicate incomplete isotopic equilibration during precipitation of CCC.