The permafrost carbon (C) pool is a major storage component of the terrestrial C cycle and it is vulnerable in a warming climate. Permafrost carbon is mobilized by different processes of thaw and erosion, including thermokarst and thermo-erosion. For example, thermokarst lagoons in the Arctic form along ice-rich permafrost coasts of Siberia, Alaska, and Canada by thaw subsidence, lake formation, and subsequent breaching by coastal erosion and marine inundation of lakes or drained lake basins. Thermokarst lagoon formation is an important step in the process of mobilizing terrestrial permafrost C pools along rapidly changing Arctic coasts. In addition, they affect the temperature and salinity of former thermokarst lake taliks during their transition to the marine environment. During current and future climate change in the Arctic, sea-level rise, accelerated permafrost thaw, intensified coastal erosion and changing sea ice regimes likely will increase the rate of thermokarst lagoon formation. Given the potentially increasing frequency of thermokarst lagoon formation and their rapid effect on permafrost degradation during the transition from a terrestrial to a marine system, it is important to understand how sedimentation regime, permafrost warming, and organic C stocks are affected during this transition. The objective of this master thesis is to asses (1) the sediment and pore water characteristics, (2) the C inventory, and (3) the spatial coverage of such thermokarst lagoon features with a multidisciplinary approach using sedimentological, hydrochemical, biogeochemical and remote sensing techniques. Samples of 30 m long sediment cores from two thermokarst lagoons on the Bykovsky Peninsula (Laptev Sea, Siberia) were analysed to characterise and quantify the C-pools as well as the sediment and pore water properties. The lagoons are examples for two different lagoon systems, an open and a semi-closed lagoon system. GIS and remote sensing tools were used to identify, map, and characterise thermokarst lagoons on a panarctic context along coasts of Siberia, Alaska, and NW Canada. The results showed that salt intrusion into sediments is higher in the open lagoon with electrical conductivities of up to 108mS/cm leading to cryotic talik formation. The total organic C density varies between 2 and 85 kg/m3 for the chosen sites, with higher values found in the class “open system lagoons”. To evaluate the larger-scale spatial relevance of this data, I identified eight lagoons along the southern Laptev Sea coast covering an area of about 18.2 x 106m2 and extrapolated my measured data on C storage to a regional level. I measured 16.5 kg/m3 as the mean for lagoon C density, which is well within the range of the terrestrial yedoma (8 kg/m³) and thermokarst (24 kg/m3) deposits in the yedoma region. Using this lagoon C density mean and the spatial coverage, I calculated 9.4 Mt C in the first 30m of southern Laptev Sea lagoon sediments, which makes it a substantial inventory of formerly frozen but now unfrozen C that has become available for microbial degradation. Along the pan-arctic coast between Taimyr Peninsula in North Siberia and Tuktoyaktuk Peninsula in Northwest Canada I mapped 690 lagoons of which 292 (42%) originated from thermokarst basins indicating the broader relevance of my findings to many regions along the Arctic coast. Thermokarst lagoons along the southern Laptev Sea coast were on average five times larger than non-thermokarst lagoons. The case study on Bykovsky provides an initial estimate of the potential contribution of this highly dynamic degradation process that combines permafrost degradation in both the marine and the terrestrial system.