The accretionary balance of saltmarshes in Great Britain since 1900

The ability of saltmarshes to accrete sediments and keep pace with sea-level rise is key to their multifaceted role as nature-based solutions to current environmental challenges, including their capacity to accumulate and store ‘blue’ carbon. While saltmarshes can gain elevation through in-situ organic production and trapping of organic and minerogenic sediments, thresholds exist above which rates of sea-level rise outstrip saltmarshes’ vertical accretion capability. Current and future anthropogenically enhanced rates of sea-level rise may therefore pose a significant threat to saltmarsh resilience. A negative accretionary balance (i.e. sea-level rates exceeding sediment accumulation) may result in transgression and potentially erosion, impacting on a range of ecosystem services and threatening stored carbon. Consequently, understanding the relationship between sea-level rise and saltmarsh accretion is critical for projecting future changes to saltmarsh ecosystems. Here, we use age-depth models based on Bayesian analysis of ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am activities to quantify sediment accumulation rates for 34 cores from 21 saltmarshes distributed around the coastline of England, Scotland, and Wales. These sites were selected to encompass the range of different marsh types found in Great Britain, including large open-coast systems, back barrier, estuarine-fringing, and loch-head marshes. Site average sedimentation rates vary between 0.12 and 1.28 cm yr^-1, with a mean of 0.41 ± 0.16 cm yr^-1. We compare sedimentation rates at 1 cm depth increments with corresponding site- and time-specific rates of sea-level rise, modelled using estimates of barystatic, sterodynamic and inverse barometric contributions that we benchmark against long tide-gauge records. This comparison enables us to determine the accretionary balance and its development since the start of the 20^th century at each core location. We discuss these results in the context of spatially explicit projections of accelerated future sea-level rise around the coast of Great Britain.