A ground-breaking method to map peat thickness and detect the carbon pool of Indonesian peatlands.

Despite the key role that peatlands play in carbon (C) storage at the global scale, our estimate of C volume is still approximate. Even though remote sensing combined with field surveys allows a reasonably accurate determination of peatland areas over vast and often inaccessible territories, the major uncertainty in the quantification of C is due to the inability of remote sensors to measure peat thickness. Ground-based geophysical methods (such as Ground Penetrating Radar - GPR - or electrical resistivity imaging) have been shown to be highly effective in detecting peat thickness at high resolution; however, these methods are difficult to implement beyond the local scale. In this study, we successfully apply the Airborne Electromagnetic method (AEM) to infer the 3D distribution of a peatland at the regional scale without need of calibrating the inversion process with field measurements. Our results come from a helicopter AEM survey performed over an area of about 23,500 hectares in West Kalimantan, Indonesia. The analysis of the electrical resistivity of the subsurface, extracted from the inversion of the AEM data, shows a clear contrast between the resistivity of the basinal peat (50-120 ohm-m), formed in ancient swamp forested environments, and the thick layer of clay underneath, characterized by a resistivity lower than 15 ohm-m. In between these two layers, we detect a transition zone, probably due to the presence of peat mixed with marine sediment that commonly has pyrite (or other sulphuric substances). We found that it is the difference in the rate at which resistivity changes with depth - the vertical gradient in electrical resistivity - that allows for the accurate estimation of the separation surface between peat and the mineral substrate, allowing us to infer the peat thickness. The 3D model of the peat retrieved from AEM data was combined with organic C content analysed from peat samples collected at several locations across the study area, allowing for an accurate evaluation of the total C stored in the peatland.