Factors controlling the growth rate, carbon and oxygen isotope variation in modern calcite precipitation in a subtropical cave, Southwest China

• δ18O-VPDB and δ13C-VPDB in modern calcite precipitates shows clear seasonal variation.
• Isotope kinetic fractionation occurs between parent drip water and depositing calcite.
• Kinetic fractionation amplifies the equilibrium fractionation value of calcite δ18O (by ∼1.5‰) and δ13C (by ∼1.7‰).

A prerequisite for using cave speleothems to reconstruct palaeoenvironmental conditions is an accurate understanding of specific factors controlling calcite growth, in particular the isotopic partitioning of oxygen (δ18O) and carbon (δ13C) which are the most commonly used proxies. An in situ monitoring study from April 2008 to September 2009 at Xueyu Cave, Chongqing, SW China, provides insight into the controls on calcite growth rates, drip water composition, cave air parameters and δ18O and δ13C isotopic values of modern calcite precipitation. Both cave air PCO2 and drip water hydrochemical characteristics show obvious seasonality driven by seasonal changes in the external environment. Calcite growth rates also display clear intra-annual variation, with the lowest values occurring during wet season and peak values during the dry season. Seasonal variations of calcite growth rate are primarily controlled by variations of cave air PCO2 and drip water rate. Seasonal δ18O-VPDB and δ13C-VPDB in modern calcite precipitates vary, with more negative values in the wet season than in the dry season. Strong positive correlation of δ18O-VPDB vs. δ13C-VPDB is due to simultaneous enrichment of both isotopes in the calcite. This correlation indicates that kinetic fractionation occurs between parent drip water and depositing calcite, likely caused by the variations of cave air PCO2 and drip rate influenced by seasonal cave ventilation. Kinetic fractionation amplifies the equilibrium fractionation value of calcite δ18O (by ∼1.5‰) and δ13C (by ∼1.7‰), which quantitatively reflects surface conditions during the cave ventilation season. These results indicate that the cave monitoring of growth rate and δ18O and δ13C of modern calcite precipitation are necessary in order to use a speleothem to reconstruct palaeoenvironment.