Sea-level change, carbon storage and greenhouse gas fluxes in a Northumberland (UK) salt marsh

Salt marshes are currently receiving increased attention because of their ability to act as climate change mitigators by sequestering carbon at high rates, while also providing multiple co-benefits. However, questions still exist concerning the precise functioning of these ecosystems which need to be addressed before any climate offsetting benefit is quantified. Using separate but linked elements, this study addresses two main questions aimed at providing more certainty about natural salt marsh functioning: first, the impact of sea-level rise on carbon accumulation rate and, second, the extent to which greenhouse gas fluxes – carbon dioxide, methane and nitrous oxide – detract from carbon sequestration. To this end, a proxy based sea-level reconstruction was developed for a salt marsh at Lindisfarne, Northumberland (UK). Complementary tide-gauge data were compared with high-resolution carbon and sediment accretion rates calculated for each centimetre down a core of sediment. Further, greenhouse gas fluxes were measured using static chambers across the marsh surface and converted to carbon dioxide equivalents from which estimates of net radiative balance were calculated. Rates of relative sea-level change over a ~60-year period were found to explain a significant proportion of variation in carbon accumulation rate, with high rates of sea-level rise associated with increased rates of carbon accumulation. The net radiative balance of the high marsh zone was -6.03 ± 2.20 SD t CO2 eq ha-1 yr-1, meaning that a beneficial net impact is still achieved when greenhouse gas fluxes are included in calculations. These findings support the assertion that salt marshes have a role to play as nature-based climate solutions. If the results of this study are reflected in other locations, it will be possible to state that where marshes keep pace with sea-level rise, increased carbon sequestration is likely and will not be significantly negated by greenhouse gas emissions from the marsh.