Tracing solute sources and carbon dynamics under various hydrological conditions in a karst river in southwestern China

Abstract

Understanding the mechanisms that lead various hydrological conditions to influence solute and carbon dynamics in karst rivers is a crucial issue. In this study, high-frequency sampling and analyses of water chemistry and ẟ¹³C_DIC were conducted from October 2013 to September 2014 in a typical karst river, the Beipan River in southwestern China. The major ions (such as Ca²⁺, Mg²⁺, HCO₃⁻, K⁺, SO₄²⁻, Na⁺, and Cl⁻) in the river are mainly from the weathering of carbonates and silicates and present temporal hydrological variabilities. Sr and U are mainly derived from carbonate weathering and show chemostatic behaviors responding to increasing discharge, similar to carbonate-sourced ions Ca²⁺, Mg²⁺, and HCO₃⁻. Silicate weathering is the primary source of Al and Li, which show significant dilution effects similar to those of Na⁺ responding to high discharge. Meanwhile, most dissolved trace elements (such as Zn, Cu, Ba, Sb, Mn, Mo, and Pb) are strongly impacted by anthropogenic overprints and also exhibit a significant seasonal variability, which may be related with mining activities in the investigated area. A simultaneous increase of ẟ¹³C_DIC and decrease in ∆DIC contents and pCO₂ values suggests that photosynthesis is the primary control on riverine DIC variability during the high-flow season. Besides, the pCO₂ values display significant chemostatic behaviors owing to the influx of biological CO₂, which is produced by microbiological activities and ecological processes, and enhanced by monsoonal climatic conditions. A two-dimensional endmember mixing model demonstrates that carbonate weathering (averaging 62%) along with biological carbon (averaging 38%) are main sources to the riverine DIC, with temporal variability. Consequently, these results show that carbonate weathering and involved plant photosynthesis are the dominant processes controlling the riverine DIC contents under high discharge and temperature conditions. This work provides insight into the crucial influence of hydrological variability on solute sources and carbon dynamics under monsoonal climate for the karst rivers.

Appendix

Calculation of silicate and carbonate weathering fluxes:

The fluxes of silicate weathering (F_sil) and carbonate weathering (F_carb) were calculated as follows:

$$ {F}_{\mathrm{sil}}=\left({\left[\mathrm{K}\right]}_{\mathrm{sil}}\times {M}_{\mathrm{k}}+{\left[\mathrm{Na}\right]}_{\mathrm{sil}}\times {\mathrm{M}}_{\mathrm{Na}}+{\left[\mathrm{Ca}\right]}_{\mathrm{sil}}\times {\mathrm{M}}_{\mathrm{Ca}}+{\left[\mathrm{Mg}\right]}_{\mathrm{sil}}\times {\mathrm{M}}_{\mathrm{M}\mathrm{g}}+{\left[\mathrm{Si}\right]}_{\mathrm{sil}}\times {\mathrm{M}}_{\mathrm{SiO}2}\right)\times \mathrm{Discharge} $$

(7)

$$ {F}_{\mathrm{carb}}=\left({\left[\mathrm{Ca}\right]}_{\mathrm{carb}}\times {M}_{\mathrm{Ca}}+{\left[\mathrm{Mg}\right]}_{\mathrm{carb}}\times {M}_{\mathrm{Mg}}\right)\times \mathrm{Discharge} $$

(8)

where M is the molar mass of the element.