This dataset comprises inorganic carbon measurements from saltmarsh soils collected across 69 salt marshes in Great Britain during 2019. The sites were selected to represent a range of contrasting habitats, particularly with respect to sediment type, vegetation communities, and sea-level history. The dataset provides quantitative measurements of inorganic carbon concentrations within surface soils to a depth of 10 cm. A total of 310 soil samples were collected using modified syringe samplers as part of the CarbonQuest citizen science programme. Inorganic carbon content was quantified through elemental analysis. These data were collected to support the development of a detailed understanding of saltmarsh carbon storage across Great Britain and to improve estimates of the contribution of saltmarsh ecosystems to national blue carbon stocks. Full details about this dataset can be found at https://doi.org/10.5285/d137c080-e458-4427-8317-c62f24aa9433
