On the Stoichiometry of Degradable Fraction of Particulate Organic Matter Along a Diapycnal Mixing Path Using Dissolved Oxygen δ18O Approach

The stoichiometry of degradable fraction of particulate organic matter (POM) in the water column has long been one of the key fundamental parameters in chemical oceanography. In this work, we used a novel approach (δ¹⁸O of dissolved oxygen, DO) to verify the stoichiometry of degraded fraction of POM. A field survey was conducted in the northern South China Sea in 2024. Based on the observed DO and nutrients, we used an exhaustive inverse analysis to reveal all possible stoichiometric results of degradable POM in the 200–500 m zone, with the centroid (the most likely) result being C₁₀₄H_xO_yN_14.5P and the corresponding apparent oxygen utilization (AOU) as 140.5 O_2. Our result reveals that the H/O elemental ratio (x/y) is highly close to 2.8, which is closest to that of proteins. As for δ¹⁸O, it ranged from <0 at the surface to a maximum of 7.5‰ at 277 m, with an overall apparent fractionation factor (ε) of 6.84. Based on the negative deviation between δ¹⁸O and DO concentration coupling, we quantified the DO_min (AOU_max) in the water columns when there had been no mixing. The average AOU_max was estimated as 265 μM, and the average AOU_max/nitrate_max (AOU_max/DIP_max) for all four stations was 10.1 ± 1.9 (151 ± 15), which agrees with the previous centroid result. Our work not only reveals the stoichiometry of degraded POM in this dynamic region but also provides a new method for the study of degraded POM stoichiometry in the ocean.