Yang Lizheng et al.(Chengdu University of Technology)

Base on the data of numerous hot springs in western China, the CO₂ contents of hot springs are calculated, the sources of CO₂ in hot water are discussed, the release process of CO₂ in hotwater is analyzed, and the release quantities are estimated.

1.The CO₂ content of hotsprings, which all occurred in carbonate rock strata, is relatively high. The maximum reaches 2575mg/L.The hot spring numbers, which CO₂ content exceeds 500mg/L, make up 35% of the totals, mean value of CO₂ content is 512.12mg/L in Tibet, 494.1mg/L in western Sichuan Province and 332.6mg/L in western Yunnan Province respectively. The statistical data indicate clearly that the CO₂ content in hotspring is abundant, and it is aproved that the hydrothermal activity zone in Tibet contains the richest CO₂ content in China.

2.The carbon stable isotope data explain that the CO₂ in hotspring mainly derived from inorganic CO₂ in the deep earth. The deep source CO₂ has three different causes: (1)mantle-derived CO₂, features a lower d¹³C value (-3‰~-6‰), Tengchong geothermal area in Yunnan Province is a typical CO₂ release area of this kind (Fig.1); (2) metamorphic CO₂, features a higher d¹³C value (around 0‰ or >0‰), typical region is located in Erdaoqiao, Kangding city, Sichuan Province (Fig.1); and (3) mixed origin CO₂, features a middle d¹³C value (0‰~ -3‰), the percentage of metamorphic CO₂ in Kangding hotsprings is very high, average value account for 49.3~86.9% of the whole, and mantle-derived CO₂ account for 13.1~50.7%.

3.In order to calculate the emission quantity of hotspring CO₂, the paper put forward the concept of discharge modulus, it means that the emitted CO₂ quantity of hotspring in unit time:

M_f = (C_h - C_o)Q

Where M_f - Modulus of discharge(mg/L)

Q - Discharge of hotspring water (L/s)

C_o- CO₂ content of shallow seepage water (mg/L)

Based on the statistic results from about 700 hot springs, the emission quantity of deep source CO₂ is 2.68´ 10⁵t/a according to the formula above, in which 1.681´ 10⁵t/a for Tibet, 0.616´ 10⁵t/a for West Yunnan, and 0.375´ 10⁵t/a for West Sichuan. The emission is uneven spatially, so it can be divided into 3 sections: First is from Yangbajin to Naqiu; Second is from Changdu to Kangding; Third is from Tengchong to Kunming(Fig.2).

4.According to the field measurement of Kangding hotspring, the release quantity of CO₂ decrease gradually with the increase of water flow distance, it presents an attenuated curve. The release quantity is about 38.3% of the total in a flow distance of 100m , that is , about 40% of deep source CO₂ transforms into gas, and immediately release into atmosphere. The other 60% enters into surface water with hot water.

The hydrothermal activities of hotspring in western China, especially in geothermal area of Himalayan Mountain , are an important way for deep source CO₂ to release, these deep source CO₂ is a major component part of the global carbon cycle.