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Carbon stable isotope (δ13C) and elemental (TOC, TN) geochemistry in saltmarsh surface sediments (Western Brittany, France): a useful tool for reconstructing Holocene relative sea‐level

Providing a dense network of Holocene relative sea‐level (RSL) records still faces some challenges, such as the paucity of microfossil assemblages within some Holocene sedimentary sequences. Recently, the isotopic (δ13C) and elemental geochemistry [total organic carbon (TOC), total nitrogen (TN) and C/N ratios] of saltmarsh sediments has showed potential for RSL reconstruction. This study presents the results of new δ13C, TOC and TN measurements obtained on surface sediments of two marshes of the Brittany region (France). Overall, the distribution of δ13C, TOC, TN and C/N values shows clear dependencies with elevation. Sediments from above and below the high tide level show contrasting TN contents. Clustering analyses identified three elevation‐dependent groups, characterized by specific values of δ13C, TOC and TN. The use of this geochemical reference for RSL reconstruction is tested on a core retrieved from one of the marshes. The sea‐level index points obtained plot in good accordance with the microfauna‐based RSL positions previously obtained in the region. Some diagenetic losses in TN are observed between modern and Holocene sediments. This does not appear to be harmful to the accuracy of the RSL reconstructions but suggests that C/N ratios may be unreliable palaeoenvironmental proxies in this specific context.