What Affects the Weathering Rate of Granite Mountains? From the Perspectives of Geology and Vegetation in Small Watersheds: A Case Study in Jiuhuashan

Abstract

Granite mountainous regions not only exhibit stunning natural landscapes but also play a vital role in the terrestrial carbon cycle. The Silicate Weathering Rates (SWR) serves as a critical factor influencing both landscape formation and carbon consumption. The common method of estimating the watershed SWR by combining the ion flux at watershed outlets with average annual runoff depth does not consider geological-vegetation effects and cannot accurately reflect the actual weathering rate in small watersheds. This study, based on the typical granite geomorphic landscape of Jiuhuashan in eastern China, refines the classification of granite types and vegetation distributions in the region. The chemical composition and instantaneous flow rate of river water in 27 small granite watersheds in Jiuhuashan are systematically measured, thereby the Instantaneous Silicate Weathering Rate (I_SWR) is estimated. The results revealed that the river water at Jiuhuashan is primarily of the HCO₃-Ca type and that the dissolved substances are controlled mainly by rock weathering. The average contribution of silicate weathering is 66.41%, which is significantly greater than that of carbonate rocks (17.64%) and atmospheric precipitation (15.44%), with minimal impact from human activities. The average I_SWR in small watersheds in the region is 486.10 mg·km⁻²·s⁻¹, and the corresponding instantaneous CO₂ consumption rate (I_CCR) averages 23.95 × 10⁻³ mol·km⁻²·s⁻¹. Among the watersheds, the I_SWR of the Jiuhua River Basin (average of 682.18 mg·km⁻²·s⁻¹) is significantly greater than that of the Qingtong River Basin (453.59 mg·km⁻²·s⁻¹) and the Qingyi River Basin (256.99 mg·km⁻²·s⁻¹). Differences in mineral composition lead to significant variations in weathering rates among different granite types. The I-type granite has higher weathering rates than A-type granite does, with the weathering rate of granodiorite (which is representative of I-type granite) being approximately 1.6 times greater than that of alkali-feldspar granite (which is representative of A-type granite). Additionally, fault structures and vegetation cover significantly influence weathering rates. Increased bamboo forest coverage notably enhances silicate weathering and increases atmospheric CO₂ consumption. This study provides scientific evidence for a deeper understanding of silicate weathering mechanisms within watersheds and their role in the carbon cycle.

Keywords

granite watersheds / instantaneous silicate weathering rate / Jiuhuashan UNESCO Global Geopark / subtropical region / carbon cycle / surface morphology

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