Spatiotemporal variability of dissolved carbon and sources of dissolved inorganic carbon influenced by freeze–thaw and subsurface flow in an alpine headwater catchment of the Qinghai-Tibetan Plateau

Abstract

The temporal freeze–thaw (FT) and the spatial rock distribution have great impacts on riverine dissolved carbon transport and dissolved inorganic carbon (DIC) sources. However, the effects of the subsurface flow (SSF) process during the FT process and spatial riverine transport on dissolved carbon transport and DIC sources are still unclear due to the lack of available spatiotemporal measurements. Herein, we designed and implemented a complete scheme of water sampling including stream water (from upper headwater to the middle and downward) and SSF (in the soil, saprolite, and weathered bedrock layers) in an alpine headwater catchment. The characteristics of the dissolved organic and inorganic carbon (DOC and DIC), and δ13C-DIC were analyzed, and the spatiotemporal variability of the DIC source was explored. Our findings reveal that the impact of the FT process on the dissolved carbon dynamics and DIC sources is revealed in the dissolved carbon transport process controlled by the SSF process. Specifically, the deepening of the active layer during the FT process enhances the movement of SSF into the deeper layer, thereby activating DIC in this deep layer. Concurrently, DOC from the shallow layer rapidly infiltrates deeper layers, leading to minimal differences in DOC concentrations across layers. This results in an enriched behavior of DOC (b = 0.21) and a depleted behavior of DIC (b = −0.27) during the FT process. In terms of influence on DIC sources, the deepening active layer promotes silicate weathering in the silicate-rich deep layers and strengthens organic carbon mineralization. On a spatial scale, the rock distribution, particularly the Si/Ca ratio, predominantly controls the chemical weathering process and, consequently, determines the DIC sources. In contrast, the influence of organic carbon mineralization on DIC sources becomes less significant. In this study, the impact mode of SSF on dissolved carbon transport and transformation is elucidated, and the spatial influence of rock distribution on riverine DIC sources is emphasized.