Anthropogenic activities have drastically perturbated carbon distribution and transformation in global coastal systems mostly by eutrophication, industrial effluents and climate change. They have led to acceleration of anthropogenic organic carbon (OC) deposition onto coastal sediments and subsequently a shoaling of sediment biogeochemical zonation, which in turn results in a formation of CH4 legacy pools in sediments. This project aims to use the Baltic Sea to test the hypothesis whether the sediment CH4 legacy pool, especially derived from the OC legacy pool with large proportion of terrestrial OC, will be a long-lasting source of CH4 to the atmosphere from years to centuries. In the next three years, we plan to train and educate a Posdoc to develop a unique methodological approach by combining a coupled sediment reactive-transport model and a Baltic-scale physical-biogeochemical model with biogeochemical analysis including stable isotopes, geophysical methods and a non-invasive approach of measuring CH4 distribution continuously in water column. This will allow us to identify CH4 hotspots and its evolvement in response to future changes throughout the entire Baltic Sea with bearing for global coastal CH4 budgets. The findings will hold important implications on quantifying how much and how long a CH4 legacy pool will offset the carbon sequestration potential of coastal systems that have recently been often-quoted as a powerhouse of climate mitigation.