To comprehensively characterize the carbonate particles in the travertine system and elucidate their formation mechanisms and environmental implications, a multi - faceted analytical approach was employed. This included hydrodynamic and hydrochemical analysis to understand the physical and chemical properties of the water environment, petrographic and mineralogical analysis encompassing petrography, scanning electron microscopy - energy dispersive spectroscopy (SEM - EDS), Tescan Integrated Mineral Analyzer (TIMA), and cathodoluminescence (CL) techniques (Table 1). These methods were systematically applied to examine particle morphology, mineral composition, elemental distribution, and growth characteristics, providing a holistic understanding of the travertine carbonate particles from multiple perspectives.

Modern, sand-like particles are actively forming in the small depressions between rimstone dams within travertine pools along Huanglong Ravine. Notable deposition sites include the Colorful Pool, Horseshoe Sea, Charming Pool, and Mirror Reflecting Pool. These depressions exhibit water depths of 1–5 cm, where particles deposition occurs. Flow velocities in these areas range from 0.150 m/s to 0.500 m/s, with average water temperatures of 11.9 C and an average pH of 8.17. The water temperature initially increases and subsequently decreases along the flow path from the Colorful Pool to the Mirror Reflecting Pool, despite the decreasing altitude. This phenomenon is attributed to the significant diurnal temperature variation in the Huanglong area. Specifically, when measuring the water temperature of the Mirror Reflecting Pool, the readings were taken near dusk when ambient air temperatures had decreased. Ca²⁺ concentrations range from 3.00 mM to 4.59 mM, alkalinity ranges from 6.6 mM to 10 mM, and the average pCO₂ is 2987.92 ppm (Table 2).

TIMA results demonstrate that 97.31% ± 1.25% of the particles’ composition is calcite, with minor occurrences of quartz and anorthite (Table 3). Exogenous minerals such as mica, wollastonite, and ferro-actinolite are present within the interstitial spaces and growth suture lines, indicating a mix of endogenous and external influences during formation.

Energy-dispersive X-ray spectroscopy (EDS) identifies CaCO₃ as the primary component of the particles. In addition to C, O, and Ca, the particles also contained significant amounts of Fe, Si, K, Mg, Al (Table 4). Howe sand-like particles ver, spectra 3 and 7, which do not correspond to the particles, show significantly lower Ca content. At these points, elements like C, O, and Al dominate, likely due to the characteristics of the testing platform background.

The sand-like particles exhibit predominantly spherical to sub-spherical shapes, with occasional elongated spheroids (Figure 3). Their surfaces are irregularly rough, resembling aggregates of numerous minute fragments. Surface colors vary from yellow to light yellow, occasionally interspersed with black specks (Figure 3a). The particles are mechanically robust, resisting hand-crushing but yielding to knife cuts. Their sizes range from 0.5 mm to 3.0 mm, with some reaching up to 5 mm in long-axis diameter. Based solely on particle size, these deposits are a mixture of ooids and pisoids. Microscopic examination reveals the particle surfaces, characterized by irregular contours and protrusions (Figure 3b). These protrusions are calcite crystals displaying radial growth patterns (Figure 3c).

Biological processes significantly influence the deposition of CaCO_3, with microbes playing a pivotal role through their growth and metabolic activity (Al-Qayim and Ghafor, 2022; Christos et al., 2022; Ghafor et al., 2012; Ghafor and Mohialdeen, 2016; Ghafor and Najaflo, 2022; Sharbazheri et al., 2009; Tri et al., 2023). Calcifying algae contribute structural frameworks for travertine deposition, while additional mechanisms such as product induction and metabolic regulation have also been identified (Jones and Renaut, 2010; Gradziński, 2010; Shiraishi et al., 2010).

SEM reveals clear biological traces within the calcite crystal crevices, including algal filaments, gelatinous stalks (Figures 6a,b), and algal sheaths (Figures 6c,d), that connect calcite minerals at both ends of the crevices. Filamentous algae and their viscous secretions adhere to the surfaces of calcite crystals, promoting crystal growth. Radial crystalline growth is observed surrounding algae and their secretions, reinforcing their role in facilitating mineral deposition (Figures 6e,f).

Cathodoluminescence provides a more precise method for analyzing the developmental stages of cement structures and pore-filling cement than traditional approaches such as tracing, staining, and optical microscopy (Meyer, 1974; Fairchild, 1983). In this study, most particles exhibited weak blue luminescence (Figure 7a). At a scale of 100 μm, particles present localized areas of relatively more intense blue luminescence (Figure 7b). The luminescence in carbonate minerals is primarily influenced by impurities, with Mn serving as the activator and Fe acting as the quencher (Machel and Burton, 1991). While calcite typically exhibits yellow-orange to red luminescence, the blue luminescence observed in this study may be attributed to lattice defects rather than elemental activators (Amieux, 1982). Additionally, Luminescence patterns revealed distinct boundaries between luminescent and non-luminescent regions, highlighting the contact zones between particles and intergranular calcite cement. These patterns suggest that larger particles formed through the aggregation of smaller particles and mineral debris, supporting a staged cementation process (Figure 7c). Microscopic observations revealed biological traces and early rock debris sand-like particles materials exhibiting high rounding among particles (Figures 7e,f).

The Huanglong travertine system, characterized by its distinctive terrace landscapes, including colorful pools, slopes, and steep cliffs, provides a unique environment for the formation of the particles. Therefore, these particles exhibit distinct morphological, structural, and depositional characteristics (Table 5).

The hydrodynamic conditions within Huanglong Ravine vary significantly, ranging from turbulent waterfalls to slow-flowing slopes, creating a dynamic setting that influences pisoid development. The particles predominantly accumulate in small depressions along the slope flow system, particularly in gentle areas near rimstone dams surrounding travertine pools. These areas represent transitional hydrodynamic conditions between turbulent and laminar flow, with velocities ranging from 0.150 m/s to 0.500 m/s, influenced by seasonal variations. During the rainy season, increased water flow transports particles over short distances, while the thin water layer flowing over the rimstone dams enhances the water-air interface, facilitating CO₂ escape and promoting rapid calcium carbonate deposition (Braithwaite, 1979; Dreybrodt and Buhmann, 1991; Jones and Peng, 2016; Schelker et al., 2016). In the dry season, decreased water flow leads to the drying of pools, halting sedimentation and exposing particles to air. This likely explains the blackened particles observed within the deposits, potentially caused by such as exposure - induced oxidation or organic staining by microbial communities (Li et al., 2011; Zhang et al., 2023).

Morphologically, the particles exhibit dendritic structures with rough, sub-spherical outer edges, lacking distinct concentric banding. Instead, they show radial growth. Elemental analysis reveals that the heightened Si content in the particles likely arises from both biological activity and the incorporation of quartz, feldspar, and mica debris transported by surface runoff during the rainy season (Liu et al., 2009). The presence of Fe, Mg, Al, and K further supports this hypothesis. Additionally, localized strong blue luminescence could result from high - temperature quartz or specific orientations of orthoclase, anorthite, and potassium feldspar (Figure 7b) (Zinkernagel, 1978).

Hydrochemically, the unique conditions in Huanglong, including a Ca²⁺ concentration of 4.59 mM, play a critical role in sand-like particles formation. These high concentrations, coupled with low-temperature calcium-rich waters sourced from cold springs and surface runoff, create an environment conducive to rapid calcite deposition. The interplay of biological activity and inorganic chemical deposition is critical, with seasonal changes significantly influencing sedimentation processes and resulting in the unique morphological and structural characteristics of the particles (Liu et al., 1995). In summary, these particles develop under transitional hydrodynamic conditions, where both organic and inorganic processes interact with seasonal precipitation to shape their formation, highlighting the importance of high Ca²⁺ concentrations, low hydrodynamic conditions, and seasonal variations in water flow.

The genesis of travertine sand-like particles can be attributed to two main processes: first, the formation of submillimeter-scale detritus or loose deposits through weathering, erosion, or denudation of pre-existing geomorphological features; and second, the rapid crystallization of fine, submillimeter-sized calcite particles (Figure 4). In addition to calcite, the mineral composition includes exogenous materials such as quartz, feldspar, and mica, which are likely introduced from the surrounding terrain (Table 4). These materials are transported into the travertine system by flowing water following the denudation of nearby mountainous terrain.

The formation of these particles involves a process of “aggregation-cementation-accretion-compaction” of travertine clasts, exogenous debris, and calcite particles (Witten and Sander, 1981; Meakin, 1983; Jullien and Kolb, 1984). Based on field observations of sand-like particle accumulation (Figure 8a) and experimental results, a sedimentary model for the particles has been proposed (Figure 8). This model emphasizes the multifactorial nature of the sedimentation process.

Under favorable hydrodynamic conditions, detrital particles tend to agglomerate (stick together) and converge. In aquatic environments, four key forces act on particles:Effective gravity (W):Pulls particles downward; Horizontal shear force (Px):Pushes particles along the water flow direction; Vertical lift force (Pz):Lifts particles upward (reducing their weight in water); Resistance force (F):Slows particle movement (opposing Px/Pz). Px and Pz drive particles to move, while W and F act as counterforces (Figure 8c) (Liu et al., 2019). When slope flow hits debris/particles, a “push” on the upstream side causes upward + forward movement (Figure 8b). Meanwhile, eddies downstream create a pressure difference—like a “suction” that pulls particles forward gradually (Figure 8c) (Guha, 2008). Water currents combine Px (forward push) and Pz (upward lift) (Tai et al., 2020). Shear stress, turbulence, and differences in settling speed make particles bump into each other. Brownian motion (random particle jiggling) and weak forces from water films (bonding/van der Waals forces) help particles stick together (Zaichik et al., 2009).

Following agglomeration, as hydrodynamic conditions weaken, biological processes begin to influence CaCO₃ deposition. SEM imagery reveals the presence of organic entities, including diatoms, algal filaments, and extracellular polymers (Figure 6). Biological processes primarily impact travertine deposition through the following mechanisms (Figure 8f) (Wang et al., 2021):

Following agglomeration, as hydrodynamic conditions weaken, biological processes begin to influence CaCO₃ deposition. SEM imagery reveals the presence of organic entities, including diatoms, algal filaments, and extracellular polymers (Figure 6). Biological processes primarily impact travertine deposition through three key mechanisms (Figure 8f) (Wang et al., 2021):

1. Biological Assimilation–pH modulation via photosynthesis

Photosynthesis and respiration by aquatic flora induce changes in water pH, thereby influencing CaCO₃ precipitation (Merz-Preiß and Riding, 1999; Murray, 2003). During photosynthesis, algae release hydrogen ions (H⁺), raising the pH of the water. This higher pH enhances the bonding between carbonate and calcium ions, increasing the CaCO₃ saturation coefficient and facilitating supersaturation and precipitation.

2. Biological Structure Role–algae as scaffolds for CaCO₃ precipitation

Algae act as both a matrix and scaffold for CaCO₃ deposition. In addition to photosynthesis, calcifying algae precipitate CaCO₃ through physiological and ecological processes, providing a structural framework (Figures 6a,b) (Dupraz and Visscher, 2005; Dupraz et al., 2008). Filamentous algae and their secretions also influence the morphology of calcium carbonate precipitates (Figures 6e,f).

3. Adhesion Mechanism–EPS promoting particle aggregation

Algae promote the formation of carbonate particles by binding micrite calcite and quartz particles (Schneider, 1977; Schneider et al., 1983; Gomez et al., 2018). In high Ca²⁺ environments, extracellular polymeric substances (EPS) adsorb Ca²⁺ ions, promoting calcification (Wright et al., 1996). Settling detrital particles become bound by algae and other organisms, undergoing calcification through metabolic processes (Pentecost, 1995; Wu et al., 2014). Calcite particles can grow along microbial sheaths (Figures 6c,d).

While travertine structures formed solely through biological processes tend to be loose, poorly compacted, and vulnerable to weathering, the particles exhibit strength and resist manual crushing. This observation suggests that inorganic processes also play a role in their formation. Polarized light microscopy reveals distinct calcite growth zoning structures, appearing as alternating bright and dark bands on calcite minerals (Figures 5c,d). SEM examination further reveals a crystalline order within the calcite particles in the aggregates (Figures 5g,h). These findings underscore that the formation of the particles depends not only on microbial activity but also on the “accretion” of calcite. This accretion process, facilitated by algae, occurs along the boundaries of travertine detrital particles, resulting in pronounced growth and crystallization. The development of well-defined columnar crystals reduces void spaces between particles, thereby enhancing the consolidation and hardness of the aggregates.

The formation of travertine sand-like particles constitutes a complex process resulting from dynamic interactions between abiotic and biotic factors, characterized by multistage growth influenced by environmental conditions. Larger particles form through aggregation of smaller particles and mineral debris, supporting a staged cementation process. CL patterns reveal this progression: initial weak blue luminescence likely corresponds to early diagenetic phases with simple impurity incorporation, while more intense luminescence relates to later stages involving complex reactions and impurity enrichment. The observed particle boundaries and assemblages provide clear evidence for this growth and cementation sequence. Variations in cementation timing and degree result in differential interparticle void spaces. Furthermore, accretion and compaction processes progressively obscure individual calcite crystals near aggregate centers. The unique depositional environment of Huanglong facilitates initial development of small-scale microtopographic undulations (Figure 8d), with finer particles accumulating along flow directions (Figure 8e). Critically, seasonal precipitation affects particle growth by altering hydrodynamics, regulating hydrochemistry, and controlling biological activity. This climatic periodicity influences the distinct multi-stage growth patterns evident in both microscopic observations and CL analyses. However, more testing and detection matching work are still needed to clarify the specific control and regulation mechanisms.

The depositional evolution of different travertine landform types exhibits synergistic characteristics (Zhang et al., 2012a), with a positive feedback loop existing between particle deposition and microtopographic evolution. Taking travertine pools as an example, their life cycle encompasses the complete process from formation and development to eventual decline: During the pool formation stage, when particle-laden water overflows the dam, abrupt topographic changes cause kinetic energy attenuation, resulting in preferential deposition of coarse particles (with high inertia) at the dam crest while fine particles (such as micritic calcite) spread evenly across the pool bottom (Pedley, 1990). This differential deposition promotes continuous vertical growth and lateral expansion of rimstone dams. However, for mature travertine pools (e.g., abandoned pools in Yellowstone’s Mammoth Hot Springs), their vertical sequences display coarse travertine debris from high-energy environments at the base, gradually transitioning upward to fine-grained travertine containing plant debris, and capped by alluvial sand layers at the top (Fouke, 2011).

Field observations in Huanglong reveal distinct depositional differentiation phenomena in travertine dams during flood seasons (May-September; Figures 9a,b). The thickness of newly formed travertine layers in sandy particle accumulation areas is significantly smaller than that of exposed travertine bodies, with some dams even exhibiting growth stagnation. In contrast, the dry season is dominated by weathering and erosion of secondary travertine. Erosion products from travertine surfaces form detrital particles that, when hydraulic conditions permit, are either transported short distances to flat areas (Figure 9c) or intercepted and retained by surface litter. Comparative analysis demonstrates that secondary travertine bodies covered by detrital particles are more susceptible to erosion. Notably, under the combined action of flowing water and wind, particles around dams undergo continuous migration-deposition cycles, with this dynamic process further amplifying their negative effects on dam structures. It can thus be inferred that particle deposition around travertine dams not only inhibits vertical accretion but may also compromise structural stability. Furthermore, particle cementation and filling alter the internal hydraulic conductivity of dams, leading to degradation of their original water retention capacity. With Huanglong’s well-developed travertine terrace system and mature colored pools, when water flows forward from inclined beaches, particles are transported to the lower edges of rimstone dams or into colored pools, causing siltation and even pool disappearance (Figure 9f), indicating that particle accumulation predominantly exerts negative effects at this stage.

The travertine depositional system exhibits distinct phasic and cyclical characteristics, with its evolution controlled by multiple factors including hydrodynamic forces, biological activity, and detrital particle supply. Therefore, both landscape conservation practices and academic research must adopt systemic thinking to holistically consider the dominant processes and their interactions across different evolutionary stages. For instance, addressing particle accumulation issues in mature travertine pools requires integrated approaches combining hydrodynamic regulation and ecological restoration to delay landscape degradation. This understanding holds universal significance for the sustainable management of similar travertine landforms worldwide.