UC Santa Cruz

Abstract There is currently no explanation or carbon cycle model scenario for severe radiocarbon ( 14 C) depletion reconstructed in the intermediate depth eastern tropical North Pacific Ocean (ETNP) at the end of the last ice age. Here, we develop new constraints on hypothesized geologic carbon release based on the boron isotope pH proxy and regional refinement of a global carbon cycle model. We find that pH‐neutral release of ∼800 Pg C over 5,000 years can produce the radiocarbon anomalies in the ETNP while inducing neither a large pH anomaly nor CaCO 3 dissolution (as observed) and causing minimal atmospheric CO 2 rise. Mass‐balance considerations dictate that the anomalous carbon was concentrated near the geologic source in the ETNP without significantly affecting radiocarbon‐based reconstructions of global ocean circulation change. Plain Language Summary At the end of the last ice age, unusually low levels of radiocarbon were found in ocean waters off the west coast of Mexico. These values are too extreme to be explained by normal ocean circulation. To understand what caused them, we used computer models and chemical records from ancient marine organisms. Our results show that a release of carbon from the seafloor could explain the unusual radiocarbon signals. This release occurred in a part of the ocean that is naturally isolated, which helped trap the carbon and its signal. The carbon was released in a neutralized form, so it didn't cause ocean acidification—consistent with what scientists observe in seafloor fossils. Our findings suggest that these radiocarbon anomalies were caused by a local carbon release into an already isolated region, not by changes in global ocean circulation. This supports the continued use of radiocarbon as a tool for studying past ocean changes. Key Points A regional carbon cycle model shows that geologic carbon release can explain extreme radiocarbon depletion in the eastern tropical Pacific Neutralized carbon release matches observations without significantly lowering pH or causing calcium carbonate dissolution Localized geologic carbon inputs can produce strong regional radiocarbon anomalies without requiring changes in large‐scale ocean circulation