AbstractThe stable carbon isotopic composition of marine particulate organic matter (δ¹³C_POM) varies with source and environmental conditions. Dissolved carbon dioxide (CO₂) concentration is thought to influence δ¹³C_POM more than temperature, but this relationship is poorly constrained in marginal seas. Here we present δ¹³C_POM, hydrographic and carbonate system variables at the deep chlorophyll maxima of the southern Yellow Sea in late summer 2017. We find δ¹³C_POM values varied between stable and cyclonic gyre regions, but indicated autochthonous organic matter production and were more strongly correlated with temperature than dissolved CO₂ concentration throughout. We find that the relationship between temperature and δ¹³C_POM was independent of CO₂ concentration, whereas the relationship between δ¹³C_POM and CO₂ concentration was dependent on temperature also being correlated with CO₂ concentration. We suggest that temperature is the primary determinant of marine δ¹³C_POM due to temperature-dependent metabolism in phytoplankton, irrespective of inorganic carbon acquisition mode.