Calcium isotopic fractionation during travertine deposition under different hydrodynamic conditions: Examples from Baishuitai (Yunnan, SW China)

Abstract

Revealing the potential of Ca isotopes in the research of travertine formation processes is the principal goal of this study. To achieve this, the hydrochemistry, travertine precipitation rates, and variations in Ca isotopic compositions (δ44/40Ca) of present-day endogenic (thermogenic) travertine in two different hydrodynamic systems (canal and pool) are studied at Baishuitai (Yunnan, SW China). In the canal, where the travertine precipitation is faster, δ44/40Ca values of both solution and travertine show a downstream increase, with Ca isotopic fractionation (Δ44/40CaCaCO3-aq) equal to ~−1.6‰. Compared to the canal, Ca isotopic fractionation between travertine and aqueous Ca2+ is smaller (Δ44/40CaCaCO3-aq≈−1.2‰) in the pools where travertine precipitation rates are lower. The spatial variations in δ44/40Ca values of solution and travertine are related to the amount of Ca removed from the solution when δ44/40Ca values of spring water remain stable. In addition, the results confirm the control of precipitation rates on Ca isotopic fractionation between calcite and parent aqueous Ca2+ using natural samples. The observed negative correlation between Δ44/40CaCaCO3-aq and travertine precipitation rates can be explained by the steady-state kinetic surface reaction model proposed by DePaolo (2011), with equilibrium and kinetic fractionation factors of αeq=1.0000±0.0001 and αf=0.9983±0.0002, respectively. An important consequence of this study is that Ca isotopes in travertine systems are good tools to have access to variations of δ44/40Caaq, which are directly linked to the amount of precipitated Ca. This could then be useful for reconstructing past hydrodynamic conditions when applied to travertine core data.