Restoration and Sedimentation in Managed Realignment Saltmarshes: A Geochemical Perspective

Saltmarshes provide critical ecosystem services, including carbon sequestration, biodiversity, and coastal protection, underscoring the need to understand how quickly and effectively the benefits recover following managed realignment. Realigned saltmarshes often differ from natural systems, even decades after restoration, and are highly influenced by hydrological and sedimentary changes. Accurately measuring carbon sequestration rates in these areas remains a challenge, given variable sediment accumulation, limited methodological approaches, and delays in carbon sink establishment post-restoration.

This study investigates the sediment structure and geochemical characteristics of both managed realigned and natural saltmarshes in the Ribble Estuary, UK. Sediment cores were collected along transects in three areas: Hesketh Out Marsh West (HOMW), realigned after 27 years of agricultural use; Hesketh Out Marsh East (HOME), realigned after 37 years under agriculture; and the adjacent natural Bank Marsh. Non-destructive ITRAX core scanning was employed to analyse sediment structure and geochemical evolution, supplemented by radiometric dating to determine sedimentation rates.

The findings reveal higher sediment densities at the realigned sites (HOMW and HOME), indicative of extended drainage and compaction. In contrast, natural marsh areas displayed enhanced lamination, reflecting regular tidal inundation and storm events. Following embankment breaches greater sediment dynamics were observed at the realigned sites, with elevated Ca/K ratios at HOME indicating the influx of marine sediments, and historical creek modifications contributing to erosion at HOMW. Notably, trace metal analysis revealed a significant reduction in heavy metal contaminants (Zn, Cu, Pb) at the realigned sites, reflecting a decline in historical industrial pollutants The marked decrease in these metals in realigned sites provided a means to estimate sedimentation rates, which ranged from 1.13–1.80 cm yr⁻¹ in HOMW and 0.87–1.50 cm yr⁻¹ in HOME. In contrast, radiometric dating highlighted spatial heterogeneity in sediment deposition across the estuary. Accretion rates at the natural Bank Marsh varied by elevation, from 0.21 cm yr⁻¹ in the upper marsh to 1.02 cm yr⁻¹ in the lower marsh—rates similar to those in the managed realignment sites, particularly HOME. Surprisingly, the more recently realigned HOME did not show higher accretion rates compared to HOMW or Bank Marsh, emphasizing the need for a deeper understanding of saltmarsh development under managed realignment. Overall, our findings demonstrate the value of non-destructive core scanning for assessing sedimentation rates where radiometric dating may be limited. They also highlight the complex, dynamic processes influencing sedimentation and carbon accumulation in saltmarsh ecosystems, emphasizing the need for continued, site-specific investigations into managed realignment outcomes.