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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_anthrois 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_anthroaccumulation changes (ΔC_anthro) consistent with water mass age and properties. From offshore to nearshore, ΔC_anthrodecreases with salinity to near zero in the subsurface, indicating no net increase in the export of C_anthrofrom estuaries and wetlands. Excesses over the conservative mixing baseline also reveal an uptake of C_anthrofrom the atmosphere within the shelf. Our analysis suggests that the continental shelf exports most of its absorbed C_anthrofrom the atmosphere to the open ocean and acts as an essential pathway for global ocean C_anthrostorage and acidification. 

Abstract Dissolved inorganic carbon (DIC) and its stable carbon isotope (δ¹³C‐DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed forδ¹³C‐DIC because the traditional isotope analysis is labor‐intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high‐precision method for ship‐based DIC andδ¹³C‐DIC analysis based on cavity ring‐down spectroscopy (CRDS). We also introduced a set of stable in‐house standards to ensure accurate and consistent DIC andδ¹³C‐DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40‐d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset ofδ¹³C‐DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg⁻¹for the DIC concentration and 0.03‰ for theδ¹³C‐DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg⁻¹between CRDS and the coulometry‐based results. The cross‐validation ofδ¹³C‐DIC in the deep‐ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed. 

The western Arctic Ocean is rapidly acidifying due to sea ice loss. 

Rapid climate warming and sea-ice loss have induced major changes in the sea surface partial pressure of CO2 ( pCO2I). However, the long-term trends in the western Arctic Ocean are unknown. Here we show that in 1994–2017, summer pCO2I in the Canada Basin increased at twice the rate of atmospheric increase. Warming and ice loss in the basin have strengthened the pCO2I seasonal amplitude, resulting in the rapid decadal increase. Consequently, the summer air–sea CO2 gradient has reduced rapidly, and may become near zero within two decades. In contrast, there was no significant pCO2I increase on the Chukchi Shelf, where strong and increasing biological uptake has held pCO2I low, and thus the CO2 sink has increased and may increase further due to the atmospheric CO2 increase. Our findings elucidate the contrasting physical and biological drivers controlling sea surface pCO2I variations and trends in response to climate change in the Arctic Ocean.