Time dependence of sediment accretion rates: Short- and long-term variability in an urban tidal marsh

Sediment accretion is vital for tidal marsh survival amid rising sea levels, yet its temporal variability remains poorly understood across different measurement scales. This study examines sediment dynamics in Tinicum Marsh, an urban tidal wetland in Philadelphia, using a comprehensive multi-method approach. We integrated sediment traps for event-scale measurements, Surface Elevation Table and Marker Horizons (SET-MH) for annual assessments, and radioisotope dating for decadal-scale analysis. Our findings reveal pronounced time-dependent variability in accretion rates consistent with the Sadler Effect, following a significant power-law relationship. Event-based deposition rates (median = 294.6 mm yr⁻¹) dramatically exceeded both annual (median = 11.3 mm yr⁻¹) and decadal (median = 5.8 mm yr⁻¹) measurements, with storm events delivering primarily mineral-rich sediments with a mean organic matter content of 15.7 % versus 21.7 %, respectively, compared to non-storm periods. Spatially, deposition was negatively correlated with marsh elevation and exhibited a non-linear relationship with distance from tidal channels, revealing complex hydrogeomorphic interactions. The discrepancy between surface accretion and net elevation change suggests substantial shallow subsidence, with subsidence accounting for as much as 70 % of total surface accretion at some sites. While current accretion rates exceed long-term relative sea-level rise, they barely keep pace with recent accelerated rates, raising concerns about future vulnerability. This research demonstrates that accurately assessing marsh resilience requires multi-scale temporal analysis and highlights the critical role of episodic deposition events in maintaining marsh elevation in urban coastal systems.