Ca isotope constraints on chemical weathering processes: Evidence from headwater in the Changjiang River, China

Abstract

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and to better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream is dominated by evaporite weathering, which has a low 87 Sr/ 86 Sr (0.7098 to 0.7108) and wide range of Sr contents (2.70 to 9.35 μmol/L). In contrast, tributaries of the Jinsha River have higher 87 Sr/ 86 Sr (0.7090 to 0.7157) and lower Sr contents (~1μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87-1.11‰) are heavier than the tributaries (0.68-0.88 ‰) and could not attribute to the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of dissolved Ca is removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ~35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry.

New estimate of chemical weathering rate in Xijiang River Basin based on multi-model

“…When calculating the carbon flux, this part of the HCO 3 − produced by the H 2 SO 4 weathered carbonate rock must be deducted 18 . While the research on the chemical weathering intensity of the marine sedimentary carbonate rock formation is relatively few 19 , the actual scenario may be more complex. In addition, chemical weathering is also affected by geomorphological units 20 , 21 .…”

The Calcium Isotope (δ44/40Ca) Record Through Environmental Changes: Insights From the Late Triassic

“…However, in most natural evaporitic settings, basins recharge occurs, providing additional Ca that prevents the precipitation of evaporites with extremely high δ 44/40 Ca isotopic values (Hensley, 2006). The range of δ 44/40 Ca values of continental evaporites described so far is between −1.91‰ and 1.21‰ (Chen et al., 2020; Hensley, 2006). The investigation of marine evaporites is restricted to the Messinian, which yields similar or heavier values than reported for the continental evaporites, ranging between −1.72‰ and −0.28‰ (Hensley, 2006).…”

Inverse Isotopic Altitude Effect of Surface Water across the Upper Reaches of the Jinsha River, China: A Consideration of Moisture Sources

“…The Tongtian River accepts the Batang River near the Zhimenda hydrological station in Yushu County, Qinghai Province, and becomes the Jinsha River as it travels downstream (Figure ). The Jinsha River has a length of ∼2300 km with a total drainage area of 473 640 km 2 and descends from northwest to southeast through deep valleys in the mountains east of the Tibetan Plateau. , The main stream of the Jinsha River is divided into the upper, middle, and lower reaches. The upper reaches were selected as the study area with little anthropogenic impact .…”