Fractionation of stable isotopes and the carbon-water cycle in Yangtze River

doi:10.13745/j.esf.sf.2024.2.8

Abstract

Abstract:

The karst carbon cycle exhibits rapid dynamic responses and is sensitive to environmental changes. The water cycle can influence the two driving factors of the karst carbon cycle (water and CO₂), making it an important influencing factor. This study analyzes samples collected in June and October 2016 from the main stream, major tributaries, reservoirs, and lakes of the Yangtze River. The study examines δD, δ¹⁸O, inorganic carbon content, and isotope fractionation characteristics and controlling factors, revealing the impact of the water cycle on the karst carbon cycle in the Yangtze River Basin. The results show that the spatial variation in δD and δ¹⁸O compositions in the Yangtze River Basin’s water bodies reflects continental, latitudinal, and altitudinal effects, and varies with the seasonal changes in rainfall. Inorganic carbon mainly originates from the weathering of carbonate rocks, and δ¹³C_DIC values are primarily controlled by the relative contributions of carbonic acid weathering and sulfuric/nitric acid weathering of carbonate rocks to $\mathrm{HCO}_{3}^{-}$. Hydrological processes in the Yangtze River Basin significantly impact the karst carbon cycle. In the upstream permafrost region, soil freezing results in mantle-derived and atmospheric CO₂ participating in the weathering of carbonate rocks, significantly increasing δ¹³C_DIC values. During the summer and autumn monsoon rains, the rapid decline in precipitation δD and δ¹⁸O, coupled with soil CO₂ accumulation and the decrease in carbon isotope values, leads to a decrease in δD, δ¹⁸O, and δ¹³C_DIC values in the water. Hydrological processes also affect the “biological carbon pump” effect, which is stronger during normal flow periods, and the stratification effect in reservoirs becomes more pronounced.

Key words: δD and δ¹⁸O, δ¹³C_DIC, carbon-water cycle, reservoir, Yangtze River

CLC Number:

YUAN Yaqiong, SUN Ping’an, YU Shi, HE Shiyi. Fractionation of stable isotopes and the carbon-water cycle in Yangtze River[J]. Earth Science Frontiers, 2024, 31(5): 409-420.

Figures/Tables 6

Fig.1 Schematic map of the sampling locations in the Yangtze River

Table 1 The δD, δ18O, HCO 3 - concentration and δ13CDIC values of sampling sites in the Yangtze River

Fig.2 Distribution of δ18O-δD values in the Yangtze River (a) and hydrochemical characteristics of Dongting Lake (b), Ertan Reservoir (c), Geheyan Reservoir (d)

Fig.3 Relationshhips between δD and altitude (a), δ18O and altitude (b) in Yangtze River

Fig.4 Variations of δD (a), δ18O (b), HCO 3 - concentration (c) and δ13CDIC (d) along the flow in the Yangtze River

Fig.5 Relationship between [Ca2++Mg2+]/[ HCO 3 -] and δ13CDIC during wet season (a) and normal season (b) in the Yangtze River

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