The ocean has absorbed anthropogenic carbon dioxide (Canthro) from the atmosphere and played an important role in mitigating global warming. However, how much Canthro is accumulated in coastal oceans and where it comes from have rarely been addressed with observational data. Here, we use a high-quality carbonate dataset (1996-2018) in the U.S. East Coast to address these questions. Our work shows that the offshore slope waters have the highest Canthro accumulation changes (ΔCanthro) consistent with water mass age and properties. From offshore to nearshore, ΔCanthro decreases with salinity to near zero in the subsurface, indicating no net increase in the export of Canthro from estuaries and wetlands. Excesses over the conservative mixing baseline also reveal an uptake of Canthro from the atmosphere within the shelf. Our analysis suggests that the continental shelf exports most of its absorbed Canthro from the atmosphere to the open ocean and acts as an essential pathway for global ocean Canthro storage and acidification.