Noble gas concentrations in fluid inclusions as tracer for the origin of coarse-crystalline cryogenic cave carbonates

摘要

The chemically inert behaviour of noble gases, theirwell-known solubility inwater and the long-termconstancy of atmospheric noble gas mixing ratios make them a unique tool for paleoclimate studies of groundwater. This concept has recently been extended to fluid inclusions in speleothems. Here we use noble gas concentrations of fluid inclusions contained in calcite crystals to constrain the formation history of coarse crystalline cryogenic cave calcites from Heilenbecker Cave (Germany). Cryogenic cave carbonates formunder conditions related to the freezing of cave pools and are a new archive for paleo-permafrost timing and extent. The cryogenic origin of these carbonates is typically deduced from their crystal habit and the carbon and oxygen isotope ratios, but this can be ambiguous in some cases. A geochemical method that provides clear-cut proof of the cryogenic origin of individual crystals has been lacking so far. Weexamined the formation process of cryogenic cave calcites usingwater and noble gases extracted from inclusions in these crystals. Based on their δ~(13)C and δ~(18)O values, these calcite crystals could have also formed as 'normal', i.e. non-cryogenic, speleothems. Noble gas concentrations derived from fluid inclusions in these cryogenic calcites, however, deviate significantly from commonly observed concentrations in atmospheric air, surface water, groundwater, and stalagmites. These concentrations cannot be explained simply by a solubility component nor by the addition of various amounts of excess-air to a solubility component. The data, however, are consistentwith the scenario of a slowly freezingwater body lacking exchangewith the cave atmosphere. The gradual freezing process leads to a partitioning of the noble gases between ice and remainingwater and thereby to a pronounced over-abundance of heavy noble gases in the liquid phase recorded in the fluid inclusions of the cryogenic calcite crystals.