Optically stimulated luminescence (OSL) dating of Qin-Han ancient city moat sediments in South China and its implications for geomorphic evolution

• Human activities in the upper Min River during the late Holocene intensified soil erosion. • The SAR protocol and SGC method proved more effective for De determination in younger archaeological samples. • The geomorphic evolution of southeastern China's mountainous streams contrasts sharply with that of alluvial plain rivers. As a hallmark of early urbanism worldwide, moat systems represent sophisticated human adaptations to diverse hydrological environments. This study investigates the Chengcun Han City Site(late Qin-early Han period, 206BCE), a remarkably preserved vassal capital of the Minyue Kingdom in subtropical South China, to elucidate the dynamic interplay between anthropogenic modifications and fluvial processes in mountainous terrain. Combining cutting-edge optically stimulated luminescence (OSL) dating (employing an optimized SAR-SGC protocol for young sediments) with Landlab geomorphic modeling, we reconstruct the sedimentary evolution of this strategic moat system and its environmental context from the Pre-Qin to Han periods.Our multi-proxy analysis reveals three significant findings with global implications: First, the refined SAR-SGC dating method establishes a robust chronological framework for humid subtropical archaeological sites, addressing persistent challenges in dating young fluvial deposits across tropical regions. Second, sedimentological evidence demonstrates a Holocene transition from climate-controlled (early-mid Holocene) to anthropogenically-dominated (late Holocene) depositional regimes − a pattern echoing contemporaneous urban centers from Southeast Asian moated settlements to Roman frontier cities. Third, Landlab simulations quantitatively demonstrate that post-abandonment landscape changes (∼2.0 ka) triggered accelerated siltation and floodplain expansion, mirroring the “legacy sediment” phenomena observed in post-urban landscapes globally, from Mesoamerica to the Mediterranean. This research advances our understanding of early urban resilience by: (1) providing the first quantitative assessment of human-induced geomorphic changes in a Chinese mountainous vassal state; (2) developing transferable methodologies for studying ancient hydraulic systems in humid environments; and (3) establishing comparative frameworks to examine how emergent states worldwide negotiated environmental constraints through landscape engineering. The findings particularly illuminate the critical yet understudied role of subtropical highland zones in global narratives of early state formation and decline.