Thermal stability of sedimentary organic carbon in a large river dominated marginal sea.

The thermal stability of organic carbon (OC) in marine sediments is one of the critical factors that influences its burial efficiency in marine environments. However, the distribution patterns and influencing factors of the thermal stability of OC in marginal seas remain poorly understood. In this study, we conducted the thermal gravimetric analysis (TGA) of OC in surface sediments of the Changjiang Estuary (CE) and its adjacent East China Sea (ECS) shelf. Both labile and refractory organic matter (OM and OM) contents derived from the TGA were higher in the CE and Zhe-Min Coast (ZMC) mobile muds relative to those in the sandy areas. The average Carbon Reactivity Index (CRI) is 69.3 ± 4.2 %, ranging from 62.8 % to 85.1 %. Most of the stations in the CE and ZMC mobile muds were characterized by relatively low CRI values, while only some stations in the inner estuary and outer shelf had higher CRI values. As a result, the CRI values correlated reversely with the OC contents and positively with the median grain size, especially in sandy sediments. Despite being sandy sediments, there were significant differences in the thermal stability of OC among the three different sandy sediment areas, with the highest CRI value in the inner estuary, lower CRI values in the outer estuary and outer shelf sediments, possibly related to the sources and composition of OC in different regions as shown by the negative correlation between CRI and δC for sandy sediments. Compared with certain European marginal seas, the sedimentary OC (SOC) in the CE and ECS shelf exhibits greater thermal stability, which is probably linked to the reduced preservation efficiency of OC caused by the extensive sediment dynamics in this area. This study supports the notion that organo-mineral interactions and the sources are two major factors controlling the reactivity of OC.