Organic and inorganic carbon and their stable isotopes in surface sediments of the Yellow River Estuary

Abstract. Estuarine sediment is an important carbon reservoir, and thus may play an important role in the global carbon cycle. The Yellow River Estuary is a large estuary in northern China, having implications for the Bohai Sea's carbon cycle. However, little is known about carbon dynamics in the sediment of the transitional zone near the river mouth. In this study, we collected 15 short sediment cores from the Yellow River Estuary, and measured grain size, total nitrogen (TN), total organic carbon (TOC) and inorganic carbon (TIC) and the isotopic compositions of TOC (δ¹³C_org) and carbonate (δ¹³C_carb and δ¹⁸O_carb). We found that TIC concentration (6.3–20.1 g kg⁻¹) was much higher than TOC (0.2–4.4 g kg⁻¹) in the surface sediment. Both TOC and TIC were higher to the north (2.6 and 14.5 g kg⁻¹) than to the south (1.6 and 12.2 g kg⁻¹), except in the southern bay where TOC and TIC reached 2.7 and 15.4 g kg⁻¹, respectively. The δ¹³C_org value ranged narrowly from −24.26 ‰ to −22.66 ‰, indicating that TOC might be mainly autochthonous. However, C : N ratio varied from 2.1 to 10.1, with higher ratio found in the southern bay. We estimated that 60.8 % of TOC might be from terrigenous OC in the southern bay. The lower TOC values in the south section were due to relatively higher kinetic energy level whereas the higher values in the bay was attributable to terrigenous matters accumulation and lower kinetic energy level. There was a significantly positive correlation between TIC and TOC, indicating that TIC was primarily from autogenic carbonate. However, the southern bay revealed the most negative δ¹³C_org and δ¹³C_carb, suggesting that there might exist some transfer of OC to IC in the section. Our study points out that the dynamics of sedimentary carbon in the Yellow River Estuary is influenced by multiple and complex processes, and highlights the importance of carbonate in carbon sequestration.