Sediment source-to-sink process variations of sandy-muddy transitional beaches and their morphological indications

Abstract

The clay mineralogy of 28 sandy-muddy transitional beach (SMT-Beach) sediments and surrounding mountain river sediments along the coasts of southeastern China was systematically investigated to reveal the sediment source-to-sink process variations of such beaches and their morphological indications. The results show that the clay mineral assemblages of these SMT-Beaches mainly comprise of almost equal illite (∼30%), kaolinite (∼28%), chlorite (∼22%), and smectite (∼20%) contents. From the surrounding mountain rivers to the SMT-Beaches, clay mineral assemblages show distinct spatial changes characterized by a large decrease (∼40%) in kaolinite, whereas the other three clay minerals present relative increases, especially clear for smectite. The muddy sediment sources of SMT-Beaches inferred from the clay mineralogy are mainly derived from nearby mountain rivers coupled with long-distance transport and penetration of the Changjiang River. The sandy sediments of these beaches are predominantly sourced from nearby mountain rivers, the weathering products of surrounding rocks in both mainland and island environments, and erosion of the “Old Red Sand” and “Red Soil Platform”. However, the sandy sediment sources of the SMT-Beaches are largely reduced because of the remarkable decrease in the river fluvial supply associated with intensive human activities such as dam construction and coastal reclamation. Subsequently, the sandy sections of SMT-Beaches present clear erosion and have revealed by both time series remote sensing images and a compilation of published literature. In contrast, the muddy sediment supply of SMT-Beaches is temporarily stable and relatively constant, resulting in the landward migration of the mudflats with relative transgression or accumulation. These findings highlight that the natural evolution processes of SMT-Beaches have been greatly reshaped by intensive human activities.