Mercury isotope fractionation during natural travertine formation

Abstract

Natural precipitation of travertine provides high-resolution information about paleoclimate change. Although significant progress has been made to trace paleoenvironments using metal stable isotopes in travertine precipitation, research on their systematics and fractionation is still lacking, limiting their application in paleoclimate reconstruction. In this study, Hg isotope variations in two natural travertine precipitation systems (canal and pools) under different hydrodynamic conditions, together with hydrochemical parameters and mineral phase in water and freshly-formed minerals were investigated during travertine precipitation to determine if the resulting Hg isotope signatures could be used in paleoenvironmental reconstruction. Our results clearly demonstrated significant Hg isotope fractionation during calcite precipitation, with light Hg isotopes being preferentially incorporated into the solid phase relative to the coexisting aqueous solutions in both systems. The magnitude of Hg isotope fractionation between travertine and the solution, Δ²⁰²Hg_{travertine-water}, varied between −0.87 and − 3.29 ‰ and between −2.10 and − 3.77 ‰ for the canal and the pool systems, respectively, and decreased with increasing precipitation rate and water temperature. Hg isotope fractionation was likely triggered by the transformation from organic matter-bound Hg (e.g. fulvic acid) in solution to CO₃-Hg complex on solid surface during calcite precipitation, which was controlled, to a certain extent, by both temperature and calcite precipitation kinetics. Our study implies that Hg isotope signatures in natural travertine could provide useful information for evaluating the paleoenvironmental changes.