Formation process of travertine in Baishuitai

From the table 16. 1, it can be seen that the spring water and calcite are nearly balanced, and even slightly eroded, which is not conducive to the formation of travertine. However, with the downward flow of spring water, the CO2 partial pressure of air (less than 40Pa) is much lower than that of water, which makes CO2 released from water into the atmosphere. Therefore, the CO2 partial pressure of water decreases (from 14400 ~ 300 Pa), the pH value increases (from 6.60 ~ 8. 10), and the calcite saturation index increases from near zero in spring water to 60 downstream. The deposition of travertine along the way also greatly reduced the concentration of Ca2+ in water, from near192 mg L-1and 703 mg L-1in spring to downstream107 mg L-1and 48/kloc-respectively.

Fig. 16.2 evolution curve of hydrochemical parameters at Baishuitai observation point (see fig. 16. 1).

Curves 1, 2, 3 and 4 are the field measured values of concentration, Ca2+ concentration, pH and conductivity, respectively. Curves 5 and 6 are the indoor calculated values of calcite saturation index and CO2 partial pressure in water, respectively.

These changes are also reflected in the carbon stable isotope composition of travertine. According to table 16.2, the travertine in Baishuitai can be divided into three groups, in which group I is ancient travertine, group II is modern travertine and group III is modern travertine. From the modern No.5 geochemical survey (Ⅲ-5) to the downstream No.III ⅲ- 10/0 geochemical survey (the height difference is about 8m and the horizontal distance is 6m), the δ 13C has increased from 3.62‰ to 4.98‰. The increase of travertine δ 13C reflects the influence of water release into the atmosphere of CO2 gas rich in stable isotopes of light carbon. This phenomenon was also found in calcareous flowers deposited on the branches of a water diversion channel opened in June1999165438+1October. As shown in table 16.2, the δ 13C increased from 3.7 1‰ to 6.04‰ from the travertine hammer of ⅲ-b-0/branch to the travertine hammer of ⅲ-B-6 branch downstream (the runoff distance is about 2km).

Table 16.2 stable carbon and oxygen isotopic composition of travertine in Baishuitai

① Gu Quan Flower and Ancient Waterfall are located near the observation point ② in the figure 16. 1, and their branches are petrified.

② The modern waterfall is located near the observation point ③ in figure 16. 1, and there are still plant fossils such as branches in the waterfall.

③ travertine is deposited in travertine pool.

④ indicates that travertine is deposited on the dam of travertine pond, where ③, ④, ⑤ and ⑧ correspond to the observation points in figure 16. 1

⑤ travertine is deposited on the branches of a passage opened in June 1999 1 1, and the corresponding position is near the observation point ⑨ in figure 16. 1

In addition, after the spring water flows out, the system changes from relatively closed to open to the atmosphere and biosphere, and its hydrochemistry not only has the above spatial changes, but also has obvious temporal changes. As shown in figure 16.3, after the water flows out about 500 meters (monitoring point ⑨ in figure 16. 1), the pH, temperature and conductivity of the water show obvious daily changes, that is, the water temperature decreases at night, the CO2 in the water escapes slowly and the concentration increases, so its pH value decreases. The decrease of pH value is not conducive to the deposition of calcite, which will inevitably lead to the increase of ion concentration in the solution, so the conductivity of water will increase. In daytime, the situation is the opposite, that is, the water temperature increases, the pH value increases, and the conductivity decreases. It can be inferred that deposition mainly occurs during the day, especially in the weather with strong sunshine.