Clumped isotope thermometry of cryogenic cave carbonates
Abstract 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 chara...
| Main Authors: | , , , , , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1074.7058 http://people.earth.yale.edu/sites/default/files/files/Zhang%20Y/YZKlugeT_Cryogenic%20Calcite_Clumped%20Isotope_GCA.pdf |
| Summary: | Abstract 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 CO 2 degassing and CaCO 3 precipitation, resulting in significantly 13 C-enriched carbonates. Alternatively, slow carbonate precipitation in ice-covered cave pools results in coarse crystalline CCC (CCC coarse ) yielding strongly 18 O-depleted carbonate. This is due to the formation of relatively 18 O-enriched ice causing the gradual depletion of 18 O in the water from which the CCC precipitates. 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. 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°C and thus exhibit clear evidence of isotopic disequilibrium. Although the very negative carbonate d 18 O values can only be explained by gradual freezing of pool water accompanied by preferential incorporation of 18 O into the ice, clumped isotope-derived temperatures significantly above expected freezing temperatures indicate ... |
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