Distribution of organic carbon abundance and carbon-nitrogen isotope composition of surface sediment in the Beibu Gulf

As a semi-enclosed sea, the Beibu Gulf is an important site for the deposition and burial of organic carbon. The spatial distribution and sources of sedimentary organic carbon are of great significance for estimating regional sedimentary organic carbon sinks. Analyses on the contents and isotopic compositions of total organic carbon (TOC) and total nitrogen (TN) in 16 surface sediment samples taken from the northern Beibu Gulf revealed that the TOC content in the surface sediments ranged from 0.07 to 0.84 wt%, on average of 0.38 wt%, while the TN content ranges from 0.02 to 0.15 wt%, on average of 0.07 wt%. A significant positive linear correlation between TOC and TN indicates that nitrogen existed mainly as organic nitrogen bound to organic carbon. The distribution range of the organic carbon isotope composition was from −21.9‰ to −20.7‰, on average of −21.2‰, suggesting that the organic matter in the surface sediments was predominantly sourced from autochthonous marine inputs, although the mixed input of terrestrial and marine organic matter was also evident. Calculations showed that the proportion of terrestrial organic matter input ranged from 17.4% to 47.6%, on average of 29.0%. The nitrogen isotope composition ranged from 7.6‰ to 13.9‰, on average of 11.0‰; and the abnormally high δ¹⁵N_bulk values of sediments near the coastal zone and Weizhou Island might be related to anthropogenic nitrogen inputs. This study provided a scientific support for analyzing the intrinsic relationship between carbon and nitrogen distribution of sediments in the Beibu Gulf, for estimating sedimentary organic carbon sinks, and for regulating marine sedimentary organic carbon sinks.