Comparing the isotopic and molecular composition of dissolved organic carbon between the oligotrophic South China Sea and the adjacent North Pacific Ocean: Signals of biodegradation, conservative mixing, and terrestrial input

The cycling of oceanic dissolved organic carbon (DOC) is a crucial component of the global carbon cycle, yet the identification of sources and the mechanisms of its molecular transformation remain poorly understood. This study compared the isotopic and molecular composition of DOC between the oligotrophic South China Sea (SCS) and the adjacent North Pacific Ocean (NPO), and traced both its allochthonous and autochthonous sources as well as its dynamic cycling processes. DOC was collected through solid-phase extraction (SPE) from water samples of both the SCS and NPO. Carbon content, isotopic ratios, and high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) measurements revealed that SPE-DOC contained both labile and refractory fractions. According to our mass balance model, the labile fraction of SPE-DOC exhibited a decline from 11.5 to 12.6 μM in surface waters to a negligible concentration below 1000 m. Conversely, the refractory fraction of SPE-RDOC maintained a relatively consistent value, ranging from 12.7 to 19.0 μM across the entire water column. The vertical distribution patterns of the molecular composition and carbon isotopic ratios jointly indicated that the DOC distributions are shaped by distinct biological and physical processes within different biogeochemical realms of the water column. The production and transformation of the relatively labile DOC fractions were the dominant processes in the epipelagic and mesopelagic zones (upper 1000 m). The extent of diapycnal mixing between the SCS and NPO explained the different vertical distributions of refractory DOC molecules in the bathypelagic oceans. The molecular indices of polyphenol compounds, aromaticity, double bond saturation state, terrestrial mass peaks, and δ13C ratios of SPE-DOC indicated contributions from terrestrial sources, likely riverine input, in the SCS. This study sheds light on the molecular evidence of DOC sources, as well as their transformation and conservative mixing processes along the overturning circulation in marginal seas.