The impact of climate change on mercury in permafrost: insights from the ORCHIDEE-MICT-PEAT-LEAK model 

Arctic permafrost, with inherent low microbial activity, has historically immobilized soil organic matter (OM) and other substances such as mercury (Hg). Derived from both natural sources (e.g., forest fires, volcanism) and human activities, Hg is a highly toxic contaminant. As permafrost thaws, microbial activity reactivates, leading to the degradation of soil OM. Simultaneously, Hg, once sequestered with OM, is released into the environment. However, the extent of Hg remobilization and its subsequent fate remain uncertain. To address this knowledge gap, we are developing a continental model that focuses on the fate of Hg, particularly from permafrost, within the context of Arctic climate change. Given the strong affinity of Hg to OM, their cycles within the terrestrial biosphere are intricately interconnected. Leveraging the foundational framework of the ORCHIDEE land surface model, which mechanistically represents the production, transport, and transformation of organic carbon in soils and permafrost, we are integrating the Hg cycle. This model will be evaluated using available observational data, including soil cores with vertical and latitudinal gradients, as well as measurements of Hg riverine exports on a pan-Arctic scale. Then, the model will be used to estimate the quantities of Hg emitted into the atmosphere and rivers during permafrost thawing, along with the associated timing. Different climate change scenarios from CMIP6 will be used to assess the sensitivity of permafrost thaw and Hg emissions to varying climatic conditions.