The source and accumulation of anthropogenic carbon in the U.S. East Coast

The ocean has absorbed anthropogenic carbon dioxide (C_anthro) from the atmosphere and played an important role in mitigating global warming. However, how much C_anthro 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 C_anthro accumulation changes (ΔC_anthro) consistent with water mass age and properties. From offshore to nearshore, ΔC_anthro decreases with salinity to near zero in the subsurface, indicating no net increase in the export of C_anthro from estuaries and wetlands. Excesses over the conservative mixing baseline also reveal an uptake of C_anthro from the atmosphere within the shelf. Our analysis suggests that the continental shelf exports most of its absorbed C_anthro from the atmosphere to the open ocean and acts as an essential pathway for global ocean C_anthro storage and acidification.