Mapping a large fossil alluvial peatland and modelling peat and C volume: first results from the Somme basin (northern France)

Peatlands represent a significant global carbon (C) stock, constituting 25% of soil carbon despite occupying 3% of the continental surface. Sometimes buried below the soil surface, peat is mainly composed of fossil plant debris and thus has considerable potential for paleoenvironmental reconstruction (e.g. pollens, macrofossils) and storing organic C. In the context of global change, peatlands face a range of anthropogenic and climate forcing that can disrupt their functioning and capacity to store C at the surface and at depth. Examples of such changes include drainage and peat extraction, which can encourage the decomposition of organic matter and the release of C into the atmosphere. Mapping the extent of (past-) peatland is essential for peat volume estimation and to protect and restore peatland functions. However, mapping large areas of fossil alluvial peatlands, using current remote sensing methods is not always straightforward. As is often observed in northwestern Europe, peatlands are frequently covered (fossilised) by loamy colluvium and alluvium resulting from soil erosion, especially in the upstream sections of small valleys.

This is the case of the Somme basin (northern France), which contains an extensive alluvial peatland, mostly fossil, where a large amount of peat accumulated during the Holocene (up to 11 m). The research, initiated in 2020, provides a precise reconstruction of the variations in the dynamics of peat accumulation in the Somme river basin over 15 ka (Garcia et al., 2024)ⁱ. The challenge for the current research is to quantify the volume of peat and C stocks based on morphostratigraphic and geochemical data.

Firstly, stratigraphic and geochemical data from three valley transects (Long, Morcourt, Tirancourt) were used to estimate current and past (pre-extraction) C stocks from peat, peaty silt, organic silt per site, locally representing a 38% loss such as in Long. Secondly, 750 punctual peat cores were digitalised through the basin (peat thickness GIS database) to define the potential fossil peat envelope (33,315 ha). Peat volume was modelled at the scale of the main valleys in the catchment using an interpolation method. The average C content per cubic meter of peat stratigraphic units (45.4%) was applied to the total volume per site and for the basin to estimate the C stocks. The preliminary results give a potential of 1,2 cubic kilometres of peat for 6,4 million tonnes of C. Thirdly, the volumes extracted in the modern period are calculated based on the current surface area of the valley ponds (former extraction pits, about 1/3 of the wetland area). The results will be available to environmental managers and institutions to raise awareness of carbon storage issues in fossil peat.

ⁱ Garcia C., Brasseur B., Bacon J., Saulnier-Copard S., Gauthier C., Mathieu L.-A., Gauthier A., Michaelis D., Mokadem F. & Antoine P. 2024. Lateglacial and Holocene palaeoenvironmental evolution of alkaline peatlands in the Somme valley (France): between climate and anthropogenic forcing. Boreas. 10.1111/bor.12676