Cascading tipping points of mercury dynamics in coastal sediments: Anthropogenic-enhanced deposition vs. warming-driven remobilization

Coastal sediments in rapidly urbanizing semi-enclosed bays act as critical yet vulnerable mercury (Hg) reservoirs, where converging anthropogenic and climatic pressures are pushing the Hg held within towards a critical tipping point, consequently threatening public and ecosystem health. This study deciphers the two-decadal sediment Hg dynamics in Jiaozhou Bay (JZB, Yellow Sea) through field sampling and historical data analysis (2002–2021), revealing that surficial sediment Hg in eastern nearshore hotspots decreased significantly between 2002 and 2018 (p < 0.05), followed by a rebound at a rate of 23 ± 5 ng/g/yr post-2018 (p < 0.05). Urbanization Impact Factor (UIF) and Hg/total organic carbon (TOC) ratios identified high-intensity urbanization as the primary enrichment driver, modulated by selenium (Se), organic matter (OM), redox conditions, and salinity. Crucially, nearshore Hg exhibited temperature sensitivity, negatively correlated with seasonal warming (winter: R² = 0.94, p < 0.05; spring: R² = 0.99, p < 0.001)—potentially reflecting a climate feedback that enhances legacy Hg remobilization. Projections that combine post-2018 accumulation rates with the negative thermal feedback suggest that, under human–climate pressures, nearshore “new Hg” could reach levels approximately one order of magnitude higher than those at present by 2100. Monte Carlo simulations predicted moderate contamination (I_geo) risk under extreme scenarios. This dual forcing mechanism—urbanization-driven sequestration versus climate-triggered re-release—highlights the need for integrated management strategies to prevent coastal sediment Hg from approaching cascading tipping points.