Controlling processes in a CaCO 3 precipitating stream in Huanglong Natural Scenic District, Sichuan, China

Abstract

Huanglong Scenic District is well known for its unusual and diversified landforms such as travertine pools, travertine falls and travertine flows. These landforms, resulting from high-altitude surface cold-water CaCO 3 precipitation, were chosen by UNESCO in 1994 as an entry in The World's Nature Heritage. Huanglong is a pristine region where there are limited human activities. Water analyses and thin section (glass slide) precipitation experiments were conducted to determine the aqueous processes controlling CaCO 3 precipitation and travertine landform formation. Results from the travertine flow indicate that the concentrations of HCO 3 −, Ca +2, and H + decrease regularly along the flow paths. Chemical equilibrium modeling results demonstrate the importance of CO 2 out-gassing and CaCO 3 precipitation processes. CO 2 out-gassing and CaCO 3 precipitation increase with increasing flow velocities. In the pool area, varying hydrodynamics are the primary factors which determine the extent of processes such as advection and diffusion, and hence also control CaCO 3 precipitation and CO 2 out-gassing. When the pool water circulation is very slow, the pH of water flowing over the travertine dams increases significantly (approximately 0.15 pH units) downstream. When the circulation is relatively fast, the pH of stream water initially decreases followed by an increase of approximately 0.21 pH units as it flows past the travertine pool dams. In both cases, the pH rise is caused by sudden changes in the hydrodynamics of the pools, despite the different initial flow conditions. Pool development is a consequence of spatial variations in pH which provide different conditions for CaCO 3 precipitation inside the travertine dam, where less precipitation or even dissolution occurs, compared to conditions at the top and downstream side of the dams. Precipitation experiments demonstrate that the top and downstream side of travertine dams are the locations of the most active precipitation, particularly for pools having faster circulation. Precipitation experiments also reveal that vaterite, a rare polymorph of CaCO 3, co-precipitates with calcite in milky opalescent water near the upstream input portion of the pool groups. Thin sections covered by algae at the bottom of pools have 40% less CaCO 3 precipitation than those not covered by algae. SEM photographs of the surface of natural travertine deposits show that biofilms with diatom minimize CaCO 3 precipitation and that diatom-adhered calcite surfaces show signs of etching, suggesting that calcite dissolution may be aided by diatoms.