Compound class carbon isotope characterization of organic matter sources, transport, and degradation in sediments of large river-influenced coastal oceans

The Yangtze River Estuary and the adjacent inner shelf are significant areas for both the burial and degradation of terrestrial organic matter (OM). However, the impact of hydrodynamics on the degradation of different types of organic compound classes has not been fully elucidated. This study measured dual carbon isotopes (δ13C, Δ14C) of organic compound classes [total hydrolysable amino acids (THAAs), total lipids, humic acids (HAs) and acid-base insoluble OM] in both surface and core sediment samples to evaluate the influences of hydrodynamic processes on the fates of organic compound classes in the Yangtze River Estuary and the adjacent inner shelf. In surface sediments, the δ13C (−28.6 ± 0.6 ‰ in lipids; −21.8 ± 0.5 ‰ in THAAs) and Δ14C (−598 ± 41 ‰ in lipids; −143 ± 47 ‰ in THAAs) values of lipids and THAAs from the mobile-muds are lower than those from the offshore areas (δ13C: −27.9 ± 0.4 ‰ in lipids; −20.9 ± 0.8 ‰ in THAAs; Δ14C: −559 ± 20 ‰ in lipids; −64 ± 7 ‰ in THAAs), indicating replenishment of marine source may be the primary factor contributing to the rejuvenation of lipids and THAAs during seaward transport. In contrast, both HAs and acid-base insoluble OM have higher Δ14C values in mobile muds (−298 ± 28 ‰ and −322 ± 21 ‰, respectively) than those in offshore areas (−443 ± 14 ‰ and −533 ± 30 ‰, respectively), suggesting significant aging even for the refractory OM during transport processes. The three end-member model of organic compound classes shows that lipids and THAAs in mobile-muds have higher marine source than offshore areas, while HAs show opposite change. These results indicate organic compound classes undergo different transport and degradation mechanisms during seaward transport, then affecting the contributions of marine and terrestrial sources to marine sediments. The different correlations between Δ14C values of labile (THAAs) and refractory (HAs and acid-base insoluble OM) OM with specific mineral surface areas (SSA) also indicate different transport and degradation mechanisms of organic compound classes in surface sediments, and causing rejuvenation of THAAs and gradually aging of HAs and acid-base insoluble OM during seaward transport. In core QT1 samples, the younger 14C ages and large variations in the accumulation rates of labile THAAs since 2003 indicate that the construction of the TGD probably altered the hydrodynamic and depositional environment in these areas, and causing the degradation of THAAs in the sediment cores. In contrast, acid-base insoluble OM have stable 14C ages and accumulation rates in cores TQ1 and QT3, suggesting minor impacts from human activities and physical processes. These carbon isotopic results suggest that labile and refractory OM respond differently to hydrodynamics, with refractory OM playing a key role in the burial of terrestrial carbon in marine sediments.