Three-dimensional distribution of organic matter in coastal-deltaic peat: Implications for subsidence and carbon dioxide emissions by human-induced peat oxidation

Human-induced groundwater level lowering in the Holocene coastal-deltaic plain of the Netherlands causes oxidation of peat organic matter, resulting in land subsidence and carbon dioxide (CO₂) emissions. Here, a three-dimensional (3D) analysis of the distribution of the remaining peat organic matter is presented, to quantify the potential of this area to further subsidence and CO₂ emissions by oxidation. Hereto, we established relations between dry mass ratios of organic matter and sediment in peat formed in different environmental settings. This was combined with a high-resolution 3D geological model of the subsurface of the Netherlands to map the proportions of organic matter, clastic sediment and void space in peat. The 3D model indicates that c. 15 km³ of Holocene peat is embedded in the coastal-deltaic plain subsurface, of which c. 1.5 km³ consists of organic matter, 0.4 km³ of sediment, and 13.1 km³ of void space. During future human-induced oxidation, this peat has a volumetric loss potential of 14.6 km³, responsible for locally 0.4–6.0 m of subsidence, and a CO₂ emission of 2.0 Gton. The 3D modelling revealed that the amount of peat organic matter varies considerably between regions. Especially the subsurface of urban areas overlying back-barrier peat were identified as hot-spots accommodating the highest quantities of peat organic matter. The peat in agricultural areas contains less organic matter but is more prone to oxidation than peat underlying urban areas, because in the latter settings anthropogenic brought-up soil restricts oxidation. Future mitigation strategies should therefore focus on restricting peat oxidation in the agricultural areas of the coastal-deltaic plain.