Magnesium isotope fractionation during natural travertine deposition from Baishuitai, SW China

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Travertine samples deposited in Earth's surface environments can be used as an effective archive for paleo-climatic reconstruction. As a common element in carbonates, magnesium (Mg) and its isotopic composition in travertine could provide useful information for evaluating paleo-environment changes. In this study, we investigate the Mg isotope systematics in both endogenic travertines (mainly calcite) and spring/stream waters at Baishuitai, Yunnan, SW China. Our results show a systematic increase in δ26Mg value from −1.37 to −1.26‰ for water samples downstream, but varied δ26Mg values between −4.12 and −3.95‰ (average −4.02‰) for solid carbonates, thus a corresponding fractionation Δ26Mgcalcite-water between −2.76 to −2.59‰ (mean value of −2.69‰). Therefore, the solid carbonates preferentially incorporate light Mg isotopes during travertine formation. More interestingly, the Mg distribution coefficient (KMg/Ca) between travertine and water exhibits two variation trends with the calcite deposition rate (Rp) along the canal, which can be explained by the change of calcite formation mechanism from direct nucleation to precipitation via amorphous calcium carbonate (ACC) intermediate. In the upper-stream, the direct nucleation of calcite results in the rapid incorporation of Mg ions into crystal lattice, while a relatively slow precipitation of calcite downstream would incorporate Mg via ACC formation pathway in a quasi-equilibrium pattern. This is consistent with the grain size distribution and crystal morphology observed under SEM. Our results show the important control of water Mg/Ca ratios on the calcite precipitation during travertine formation, and imply the potential and complexity of using Mg isotopes of travertine deposits to reconstruct paleo-environments.