Hydrodynamic controls on differential travertine deposition: A case study of Huanglong Scenic Area, Sichuan, China
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摘要: 钙华作为重要的陆相碳酸盐沉积物，其形成过程受水动力条件显著控制，但不同地貌单元的水动力差异如何控制沉积过程仍需深入研究。文章以四川黄龙风景区不同地貌单元（滩流、微丘）的钙华沉积系统为研究对象，通过水动力数值模拟、水化学监测和原位沉积实验，探讨地形控制下的水动力过程对钙华沉积差异的影响。结果表明：不同地貌单元的水动力特征差异显著，滩流区（坡度<15°）水流平缓，以局部微地形扰动为主；微丘区（坡度>20°）则因其弧形外凸结构，形成独特的强紊流流态。水化学数据显示，微丘区水流落点至跳跃点间${\rm{HCO}}_3^{-}$浓度显著下降，指示快速CO2脱气过程；滩流区沿流程水化学特征呈平缓变化趋势。原位实验揭示，微丘区方解石沉积致密，以无机化学沉淀为主导；滩流区沉积量较少，方解石与藻类呈团聚共沉积，微生物参与程度显著增大。Abstract:
Travertine deposition is governed by a complex interplay of physical, chemical, and biological factors, among which hydrodynamic conditions are widely recognized as a key control fatcor on CO2 degassing efficiency and calcite precipitation rate. However, a systematic understanding of how hydrodynamic differences across distinct geomorphological units regulate the entire chain from CO2 degassing to calcite deposition characteristics remains insufficient. To address this issue, the Huanglong World Natural Heritage Site in Sichuan Province, southwestern China, at elevations of 3,000 to 3,600 m above sea level, was selected as the study area. Huanglong is renowned for its actively depositing travertine landscape, which encompasses diverse geomorphological units-sheet flows, rimstone dams, pools, mounds, and waterfalls-distributed along a valley approximately 3.6 km in length. CO2-rich groundwater emerges from springs at the valley head and flows downstream over the travertine surface, undergoing progressive CO2 degassing and CaCO3 precipitation. Two representative and contrasting geomorphological units-gentle slopes with sheet flow characteristics (slope<15°) and curved convex features on steep slopes (slope>20°)-were selected as research objects. An integrated approach combining Computational Fluid Dynamics (CFD) simulation, systematic hydrochemical monitoring along flow paths, and in-situ deposition experiments was employed. Three-dimensional terrain models of typical gentle slopes and steep slopes were constructed from high-resolution topographic survey data and served as the geometric basis for CFD simulations, validated against field-measured flow velocities. Water samples were collected at 21 stations along a 500 m gentle slope transect and at paired landing-point and jumping-point locations across successive steep slopes. Major hydrochemical parameters including Ca2+, ${\rm{HCO}}_3^{-}$, pH, Ec, pCO2, and the calcite Saturation Index (SIc) were determined. Additionally, glass deposition slides were deployed in situ at representative positions within both areas for five days and examined by Scanning Electron Microscopy (SEM) to characterize the mineralogy, morphology, and biological content of newly formed precipitates. Hydrodynamic simulations revealed pronounced contrasts between the two units. The gentle slope area exhibited flow velocities of 0.52 to 3.07 m·s−1 with a thin water layer; turbulence was primarily triggered by micro-topographic features on the bed surface formed by earlier travertine deposition. The steep slope area displayed higher velocities of 1.32 to 4.15 m·s−1, where the distinctive semi-circular arcuate convex morphology of successive crests drove continuous hydraulic jumps. Hydraulic head, velocity, and turbulent kinetic energy at landing points significantly exceeded those at jumping points, indicating that the most intense hydrodynamic action concentrates at landing points. Topographic gradient constitutes the fundamental cause of hydrodynamic differentiation between the two geomorphological units. Hydrochemical monitoring demonstrated that contrasting hydrodynamic conditions directly control CO2 degassing efficiency and the spatial pattern of water chemistry evolution. In the gentle slope area, Ca2+ concentration decreased by 43.5 mg·L−1 over the 500 m transect, accompanied by a gradual pH increase of 0.26 and progressive pCO2 decline; all SIc values exceeded 1, indicating sustained calcite oversaturation and a mild, gradual degassing process. Regression analysis revealed that the hydraulic gradient exhibited a strong positive correlation with Ca2+ concentration, far exceeding that of elevation alone, identifying it as the key hydrodynamic parameter governing travertine deposition intensity. In the steep slope area, ${\rm{HCO}}_3^{-}$ concentration displayed a characteristic zigzag decline: at each successive crest, the jumping-point concentration was consistently lower than that at the upstream landing point, directly evidencing rapid, pulsed CO2 degassing driven by hydraulic jumps across very short distances. Comparison across different slope gradients confirmed that steeper terrain corresponds to higher flow velocity, more vigorous degassing, and greater reductions in Ca2+ and ${\rm{HCO}}_3^{-}$ per unit distance. In-situ deposition experiments provided direct evidence linking hydrodynamic intensity to depositional characteristics. Steep slope area slides exhibited the most abundant calcite precipitation, with crystals densely packed into thick compact layers, reflecting high-intensity inorganic chemical precipitation driven by rapid CO2 degassing. Gentle slope flow slides showed comparatively sparse precipitation, with notable aggregated co-precipitation of calcite with diatoms and filamentous algae, indicating significant microbial participation under weaker hydrodynamic conditions. Downstream gentle slope flow slides displayed minimal precipitation, consistent with progressive ion consumption. This study establishes a complete control pathway, "topography-hydrodynamic conditions-CO2 degassing efficiency-calcite precipitation rate and deposition characteristics". Stronger hydrodynamic conditions enhance CO2 degassing and inorganic chemical precipitation while suppressing microbial colonization through continuous scouring, producing dense, inorganically dominated deposits. Conversely, weaker hydrodynamic conditions reduce chemical driving forces while providing stable environments favorable for microbial participation, yielding biologically influenced, less compact deposits. A positive feedback loop between deposition and topography continuously reinforces the distinctive characteristics of each unit. These findings provide a process-based framework for interpreting travertine spatial heterogeneity and a theoretical foundation for utilizing depositional density and structure as sedimentological indicators of paleo-hydrodynamic environments. -
图 1 研究区域概况
注：a.黄龙钙华形成的水文地质条件剖面图，据参考文献[14]修改；b.黄龙沟钙华平面图；c.钙华微丘地貌。
Figure 1. Overview of the study area
Note: a.Cross section showing hydrogeological settings for the formation of Huanglong travertine(modified from reference [14]), b.Plan view of travertine deposits plan in Huanglong, c.Travertine curved convex landform.
图 2 采样点位置（a.滩流采样点位置；b.微丘采样点位置）
Figure 2. Sampling point location (a.The sampling points for the gentle slope samples; b.The sampling points for the curved convex samples)
图 3 数值模拟得到的流速与实测值的流速比较
Figure 3. Comparison of the flow velocity obtained from numerical simulation (black line) and the measured flow velocity (pink squares)
图 4 钙华滩流和微丘的水动力数值模拟[滩流和微丘的野外照片（a,d）、三维地形模型（b,e）和对应的水动力模拟结果（c,f）]
Figure 4. Hydrodynamic numerical simulation of travertine gentle slope and travertine curved convex[field photos of gentle slope and curved convex (a, d), 3D terrain models (b, e), and corresponding hydrodynamic simulation results (c, f)]
图 5 钙华滩流（500 m）沿流程水化学变化特征
Figure 5. Hydrochemical characteristics of travertine gentle slope (500 m) along the flow
图 7 微丘和滩流样品宏观剖面特征与原位沉积片微观形貌特征
Figure 7. Macroscopic section characteristics of gentle slopes and steep slopes travertine samples and microscopic morphology characteristics of in-situ deposited glass slides
表 1 不同钙华地貌单元的地球化学指标变化
Table 1. Variation of geochemical indicators in travertine landforms with different slopes
点位 平均坡度/
°高差/
m距离/
m平均流速/
（m·s−1）∆pH ∆Ec/
（μS·cm−1）∆[Ca2+]/
（mg·L−1）∆[${\rm{HCO}}_3^{-}$]/
（mg·L−1）∆SIc ∆lgPco2 钙华滩流区 7.92 27.55 200.00 1.32 0.19 34 26.02 19.34 0.06 0.19 钙华微丘区 54.79 1.53 1.87 3.36 0.14 25 1.25 2.29 0.12 0.16 
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