Holocene environmental evolution of the Pinqing Lagoon: insights from multiproxy sediment analysis

Abstract

To address the environmental changes in the South China coastal region and to investigate the interplay among sea-level fluctuations, monsoon variability, and sediment dynamics, a sediment core from the Pinqing Lagoon was extracted, covering the last 8.5 ka. Furthermore, multiple proxies were analyzed in the core, including grain size end-members (EM1, EM2, and EM3), magnetic susceptibility and S-ratio, the carbon (C) isotopic composition of organic matter, its carbon and nitrogen (N) contents, the resulting C/N ratio, and Itrax XRF-derived elemental ratios such as Mn/Ti, Si/Ti, K/Ti, and Fe/Ti. The results reveal that changes in sea level play a primary role in shaping the lagoon sedimentary and geochemical evolution, with EASM-driven runoff acting as a secondary control on terrestrial sediment supply, especially during low sea-level phases. During the 8.5-6.8 ka, low water levels, strong EASM-driven runoff, and dominant terrestrial C-4 plant input resulted in coarse detrital sedimentation (high EM2 and low S-ratio) and poor bottom water oxygenation (low Mn/Ti). Between 6.8 and 5.8 ka, despite already high sea levels, the lagoon underwent rapid deepening, with a shift toward in-situ aquatic productivity, improved oxygenation, and finer sedimentation (EM1 dominance and high S-ratio), likely reflecting local geomorphological changes. From 5.8 to 4.2 ka, as sea level stabilized, the lagoon became stratified and marine-influenced, with low oxygenation, minimal terrestrial input, and background fine-grained sedimentation. After 4.2 ka, stable high water levels and low runoff persisted. A prominent EM3 peak between 0.4 and 0.2 ka, coinciding with the Late Little Ice Age (LIA), reflects frequent typhoon-induced high-energy deposition, supported by coarse grain size, elevated MS, and increased Si/Ti, K/Ti, and Fe/Ti ratios. Overall, the results highlight that long-term sea-level fluctuations primarily controlled lagoonal sedimentation and oxygenation, while EASM variability shaped runoff-driven detrital input.