Stable carbon isotopic composition (δ 13C) of the bulk POM (δ 13C POM) has long been used to evaluate proportions of marine-derived versus terrestrial POM fractions in coastal oceans 3, 4, 5, which are disproportionately important to ocean carbon cycles and budgets 6, 7, 8. Suspended particulate organic matter (POM) in seawater is a key component of the biological pump which transfers the upper ocean photosynthesized organic matter to the deep sea 1, 2. We suggest that temperature is the primary determinant of marine δ 13C POM due to temperature-dependent metabolism in phytoplankton, irrespective of inorganic carbon acquisition mode. We find that the relationship between temperature and δ 13C POM was independent of CO 2 concentration, whereas the relationship between δ 13C POM and CO 2 concentration was dependent on temperature also being correlated with CO 2 concentration. We find δ 13C POM values varied between stable and cyclonic gyre regions, but indicated autochthonous organic matter production and were more strongly correlated with temperature than dissolved CO 2 concentration throughout. Here we present δ 13C POM, hydrographic and carbonate system variables at the deep chlorophyll maxima of the southern Yellow Sea in late summer 2017. Dissolved carbon dioxide (CO 2) concentration is thought to influence δ 13C POM more than temperature, but this relationship is poorly constrained in marginal seas. The stable carbon isotopic composition of marine particulate organic matter (δ 13C POM) varies with source and environmental conditions.
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