Enhanced debris flow activities in the subtropical mountainous catchment of South China during early Marine Isotope Stage 3

Abstract

Geomorphic processes and their responses to climatic change in subtropical mountainous catchments are not yet well understood. In this study, a unique sequence of alluvial sediments (>33 m thick) from the Nanxu karst valley, Guilin, South China, was investigated to characterize the nature of erosion–deposition processes in humid, subtropical catchments and decipher any climatic constraints on the earth surface dynamics and landscape evolution. Erosional outcrops show that the Nanxu valley fill sequence mainly comprises m-scale sheets of matrix-supported gravel (a debris flow facies), which, in the distal part of the valley, are intercalated with beds of fine-grained sediments (facies of horizontally interbedded sand and mud or massive mud). The fine-grained sediments were likely deposited in a subaqueous environment, which was probably formed when the karst sinkhole was contemporarily blocked by a large volume of debris flows. Optically stimulated luminescence (OSL) as well as radiocarbon dating constrained a chorology framework of ca. 62–42 ka for the valley fill sequence. Comparisons of the sedimentation phase with local and regional climate proxy records revealed that enhanced debris flow activities and sediment aggradation occurred under generally warm and wet but unstable climatic conditions during early Marine Isotope Stage 3. Under these climatic conditions, there was likely a subtropical landscape with high vegetation cover in the catchment at the time of sedimentation, as supported by our palynological data. Thus, it is suspected that enhanced debris flow activities were triggered by the recurrence of extreme monsoonal rainfall events. Since ∼ 42 ka, the stream-channel process dominated the valley, and the valley fill was deeply dissected, which might be related to changes in the overall intensity and internal variability of monsoonal rainfall over a period. These findings identify the role of high-intensity monsoonal precipitation in controlling geomorphological activities in humid, subtropical landscapes of South China.