Characteristics of Spatial–Temporal Variations and Controlling Factors of Chemical Weathering in the Han River Basin

Watershed weathering provides a critical pathway for understanding the feedback mechanisms between continental rock chemical weathering and global climate change. As the longest tributary of the Yangtze River, the Han River plays a crucial role, where samples were collected from the mainstem and tributaries in spring, summer, and autumn to analyze major ion compositions and calculate chemical weathering rates using graphical methods and a forward model. Results show carbonate weathering dominated solute sources (75.7%), followed by silicates (14.8%), with minimal atmospheric and anthropogenic inputs. Spatially, carbonate weathering rate (CWR) and CO2 consumption rate (ΦCO2car) increase downstream with lithological variations, while silicate weathering rate (SWR) and CO2 consumption rate (ΦCO2sil) exhibit the opposite trend. Basin-wide averages were 9.4 ± 1.2 t/km2/yr (CWR) and 1.3 ± 0.3 t/km2/yr (SWR), with CO2 consumption rates of 262.6 × 103 and 55.5 × 103 mol/km2/yr for carbonates and silicates, respectively. Seasonally, CWR and ΦCO2car peaked in summer, while SWR and ΦCO2sil were lower in summer than in spring and autumn. This seasonal pattern suggests that cooler temperatures limit weathering in spring and autumn, while increased summer runoff favors carbonate dissolution. The findings highlight the need for seasonal sampling to accurately assess weathering rates and CO2 drawdown.