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Journal of Tropical Oceanography

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Distribution Characteristics and Sources of Organic Matter Composition in Surface Sediments of Mangroves in the Beibu Gulf

NONG Lanxin1,2, CAO Xinxing2,*, YAO Lipeng2, CHEN Juan2, SONG Zhiguang2,*,HUANG Shengmin2 ,HUANG  Huamei1,2    

  1. 1. School of Geographic Sciences and Planning, Nanning Normal University, Nanning, Guangxi 530001, China

    2. Key Laboratory of Environment Change and Resource Use in Beibu Gulf, Ministry of Education, Nanning Normal University, Nanning, Guangxi 530001, China




  • Received:2026-06-28 Revised:2026-09-07 Accepted:2026-09-17
  • Supported by:

     Guangxi Maritime Economy Talents Training Support Special Project(2026XHRC30); Beibu Gulf Environmental Evolution and Resource Utilization Key Experimental Open Project Funded by the Ministry of Education ( GTEU-KLOP-X1803 ); Guangxi Higher Education Middle-aged and Young Teachers Research Foundation Capacity Enhancement Project (KY26ZQN004); 2025 "Bagui Light" Visiting Scholar and Training Program (Special Program for Scientific and Technological Talents)

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Abstract: This study investigated the sources and regional distribution characteristics of organic matter in mangrove surface sediments from Maowei Sea and Yingluo Bay in the Beibu Gulf by analyzing total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), the ratio of organic carbon to total nitrogen (C/N), carbon, nitrogen, and sulfur isotope composition (δ13C, δ 15N, δ 34S), and the n-alkane composition of organic matter. The results showed that the total organic carbon (TOC) and total nitrogen (TN) contents in the surface sediments of mangroves in Maowei Sea were 0.40% ~ 1.30% and 0.05% ~ 0.16%, respectively; while those in the mangrove sediments of Yingluo Bay were 0.16% ~ 0.84% and 0.03% ~ 0.08%, respectively. The TOC and TN contents in Maowei Sea sediments were generally higher than those in Yingluo Bay, and this difference may be influenced by topographic and hydrodynamic conditions. The δ¹³C values of mangrove sediments in Maowei Sea range from-26.0‰ to -24.4‰, while those in Yingluo Bay range from -26.5‰ to -21.3‰, indicating a mixed input of terrestrial and marine sources. However, the marine contribution is higher in the Yingluo Bay mangrove sediments. Simultaneously, both mangrove sediments are generally enriched in δ¹⁵N (10~14‰), reflecting significant anthropogenic nitrogen source influences such as aquaculture discharge and wastewater pollution. The total sulfur (TS) contents in Maowei Sea and Yingluo Bay are 0.15% ~ 0.38% and 0.08% ~ 0.23%, respectively, consistent with the TOC trend. The average TS/TOC values in both regions are higher than the global average for oxidative environments, with δ³⁴S ranging from -36.72‰ to -17.25‰, indicating that sediment sulfur burial is dominated by microbial sulfate reduction and influenced by both organic matter input and hydrodynamic disturbances. The carbon number distribution of n-alkane in sedimentary organic matter ranges from C₁₁ to C₃₃, with the main peak carbon being C₂₅ ~ C₂₉. Combined with relevant parameters such as TAR, ACL, Paq, and CPI, it is shown that the organic matter in Maowei Bay sediments is mainly inputted by terrestrial higher plants, accounting for ~69%; while the organic matter in Yingluo Bay sediments shows a mixed input of terrestrial and aquatic plants, with terrestrial plants contributing ~52% and aquatic plants contributing up to ~38%. Some samples show signs of petroleum hydrocarbon pollution. Analysis using the MixSIAR model shows that the overall source of organic matter in mangrove sediments follows a pattern of "terrestrial (79.6%) > marine (19.3%) > mangrove endogenous (1.1%)".

Key words: mangrove sediments, organic carbon, n-alkane, carbon, nitrogen, and sulfur isotopes, Beibu Gulf