Distribution and sources of organic matter in surface sediment near the Hong Kong-Zhuhai-Macao Bridge

To study the distribution and sources of sedimentary organic matter (SOM) near the Hong Kong-Zhuhai-Macao Bridge (HZMB), 58 surface sediment samples were collected in the waters of the HZMB in the Lingdingyang Bay (LDB) in May 2021. The grain size, total organic carbon (TOC), total nitrogen (TN) contents, and organic carbon stable isotope (δ¹³C_org) composition of the samples were analyzed. The spatial distribution and sources of SOM were studied. Results show that the TOC content ranged from 0.17% to 1.19%, TOC/TN ratio was between 8 and 18, and δ¹³C_org value was −31‰～−22‰. The SOM could be divided into three types based on the TOC/TN ratio and δ¹³C_org value. Type I SOM belonged to the recent sediment in LDB, derived from soil organic matter, freshwater algae, and marine algae in the proportion of 61%, 11%, and 28%, respectively. Type II and type III SOM were likely derived from the Quaternary terrestrial strata. As the foundation pit of the undersea tunnel for immersed tube of the bridge was excavated, types II and III SOM were transported to the northern side and the central-western parts of the bridge, showing a patchy distribution. Correlation analysis indicates that grain size was the controlling factor on the enrichment of SOM. However, there were spatial differences for SOM of different sources in Type I sediment. The construction of HZMB and other human activities have changed the SOM types from marine to terrestrial sources. It is suggested that the impact of surface sediment changes on the marine ecological environment in the LDB should be evaluated in future studies.