The Hudson Bay Lowlands are a globally significant peatland, housing around 30 petagrams of carbon (Packalen at al., 2014). Thanks to postglacial rebound, these peatlands continue to expand laterally, colonizing newly exposed seabed on the retreating shoreline. However, little research has focused on the carbon dynamics of these coastal margins. The coastlines are also the site of a proposed National Marine Conservation Area, making their biogeochemistry a relevant conservation issue. As such, our study aims to evaluate coastal carbon from the shoreline to incipient peatland. We collected surface soil samples and cores along ~1 kilometre coast-perpendicular transects at two James Bay coastal sites and one Hudson Bay coastal site. Core samples were dated with 210Pb and analyzed for bulk density and carbon to nitrogen ratio, while surface samples were analyzed for carbon, nitrogen, and sulfur to determine carbon stores and marine influence. Organic matter content was significantly positively correlated with distance to shore on the Hudson Bay transect (Spearman correlation, ρ = 0.89, p = 0.012), while no such correlation was found on the shorter James Bay transects. Average carbon density from the inlandmost James Bay soil core (NP14) was ~31 kgC/m3. Notably, that core possessed a 12-centimetre-thick layer of peat despite being less than a kilometre inland. These results reflect the potential for rapid peatland development on the shoreline, as well as its status as an overlooked carbon pool. They also support the need for protection of the area in light of increasing threats from climate change.