Abstract The marine carbonate system plays a crucial role in regulating the global carbon cycle. In the North Pacific, the carbonate saturation depth (CSD), where biogenic calcium carbonate (CaCO3) starts to dissolve rapidly beneath, is suggested to be above ∼1,000 mbsl, contradicting with pervasive CaCO3 preservation in deep‐sea sediments. To resolve this paradox, we re‐visit the sedimentary CaCO3 distribution and carbonate chemistry across the western Pacific with a carbonate accumulation model. Our results show that CSD shoals along the route of Lower Circumpolar Deep Water, resulted from respiration‐driven declining [CO32−]. In contrast, the development of mid‐depth carbonate undersaturation, caused by enhanced particulate organic carbon (POC) remineralization driven by high productivity, sharply raises the CSD in northern Northwest Pacific and substantially lowers the regional CaCO3 preservation. The carbonate system in intermediate‐deep NWP is sensitive to increased CO2 mostly due to POC remineralization, which may exacerbate widespread CaCO3 dissolution in the future.

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