Anthropogenically mediated ocean acidification (OA) has negative impacts on many marine organisms, especially calcifiers. However, systematic measurements of OA have only been made over the past four decades. In order to improve future predictions and understand how ongoing OA compares to natural variability on longer timescales, it is critical to extend records beyond observational time series. In the Cariaco Basin, located in the tropical Atlantic, near-surface dissolved inorganic carbon reflects atmospheric carbon dioxide concentrations (CO 2 ) since the Industrial Revolution, making it an ideal site for examining longer-term variability. We extend the record of Cariaco Basin near-surface [CO 3 2−