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.