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Aged organic carbon (OC) occurs widely in surface sediments deposited in shallow continental margin settings (Griffith et al., 2010;Inthorn et al., 2006;Tesi et al., 2016). Old terrestrial OC (OC terr ), including both petrogenetic OC (OC petro ) derived from bedrock erosion (Hemingway et al., 2018), as well as terrestrial biospheric pre-aged OC (OC pre-aged ) subject to storage in intermediate terrestrial reservoirs (e.g., soils;Eglinton et al., 2021;G. K. Li et al., 2021;Tao et al., 2015) prior to export may be much older than that produced in the marine environment. Together, these terrigenous inputs partly contribute to older ages of OC than would be predicted based on inputs exclusively derived from marine productivity. The influence of old terrigenous OC is prevalent in the Chinese marginal seas (CMS), for example, the OC pre-aged from the Yellow River (Tao et al., 2016;Xue et al., 2017) and the OC petro from the Taiwan Island (Dadson et al., 2003;Lin et al., 2020). Therefore, the presence of aged OC in marine sediments is often attributed to supply of OC terr from land (e.g., Hilton et al., 2015).Old surficial sediment OC ages are also observed in energetic coastal regions distal from terrigenous point sources (

<p>Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystem. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax <em>n</em>-alkanes and δ<sup>13</sup> C<sub>OM</sub> indicate that terrestrial non-mangrove plant OM accounted for 52-72% of the OM in the two estuaries, aquatic OM from phytoplankton and/or seagrass accounted for 8-29%, and OM from mangroves comprised 16-26% of the total. Terrestrial plants contributed 16-20% more of the OM to sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but aquatic OM (algae plus seagrass) fraction was 17% lower than that in Danzhou Bay sediments. In both estuaries, mangrove and aquatic OM fractions increased seaward while the terrestrial OM fraction decreased. Terrestrial fraction in BMB sediments is 12% higher in summer compared to autumn, which is offset by a comparable reduction in the mangrove OM fraction, as well as higher aquatic OM fractions in both estuaries. This may be caused by enhanced river discharge, more efficient mangrove leaf litter transport offshore, and/or higher aquatic productivity. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.</p>