环渤海地区河流河口及海洋表层沉积物有机质特征和来源

PROVENANCE OF ORGANIC MATTER IN ESTUARINE AND MARINE SURFACE SEDIMENTS AROUND THE BOHAI SEA

2013年8月采集了环渤海地区35条主要河流河口表层沉积物样品,12月采集了渤海与北黄海24个表层沉积物样品,分析了其生物地球化学指标:总有机碳(TOC)、总氮(TN)、有机碳同位素(δ13C)和氮同位素(δ15N),探讨该区域表层沉积物有机质特征及组成。研究表明:河流河口表层沉积物有机碳同位素(δ13C)值在–26.4‰—–21.8‰,平均值为–24.5‰;渤海表层沉积物有机碳同位素(δ13C)值在–23.8‰—–21.7‰,平均值为–22.3‰。河口表层沉积物TOC含量在0.06%—3.87%,平均值为1.31%;渤海表层沉积物TOC含量在0.52%—2.09%,平均值为1.08%。河流δ13C富集较轻,偏向陆源;海洋δ13C富集较重,偏向水生有机质来源。河流河口表层沉积物的δ13C值差异较明显,最大值与最小值相差4.6‰,但是流域地理位置距离近的河流δ13C值差异不大。河流河口表层沉积物δ15N在1.5‰—10.2‰,平均值为5.5‰;渤海表层沉积物δ15N在4.4‰—5.6‰,平均值为5.0‰。河流表层沉积物δ15N范围比渤海表层沉积物δ15N范围广,原因是河流受陆源有机物影响,且陆源有机物来源差异大。海洋表层沉积物δ15N相对均一,说明海洋表层沉积物δ15N受物源影响较小,体现了水体中有机质的转化和微生物活动对氮同位素的影响。本研究中表层沉积物的δ13C与δ15N没有明显的相关性,也体现了陆源有机质输入的影响。根据经典的二元模式计算,35条河流陆源有机质的贡献比例范围为10%—90%,平均值为60%;渤海陆源贡献比例范围为10%—50%,平均值为20%。河流有机质的来源以陆源有机质为主,水生有机质为辅。渤海有机质的来源以水生有机质为主,环渤海河流的陆源输入也有重要贡献。需要指出的是,有机碳同位素(δ13C)、氮同位素(δ15N)和Corg/Ntotal对有机质来源判别有一定局限性,虽然稳定同位素有示踪性,然而其成分仍然不可避免地受到生物地球化学等过程的改造,在使用稳定同位素技术示踪物源时,须小心谨慎。

Thirty-five samples of estuarine surface sediments were collected in major feeding rivers of the Bohai Sea in August 2013, and 24 marine surface sediments were sampled in the Bohai Sea and the north Yellow Sea in December 2013. Total organic carbon(TOC), total nitrogen(TN), organic carbon isotope(δ13C) and nitrogen isotope(δ15N) were investigated to trace the organic matter sources. The results show that δ13C values in the estuarine surface sediments ranged from –26.4‰ to –21.8‰ in average of –24.5‰ and those in marine surface sediments were from –23.8‰ to –21.7‰ in average of –22.3‰. TOC in the estuarine surface sediments ranged 0.04%—3.70% in average of 1.24% and TOC in the marine surface sediments ranged from 0.49% to 2.01% in average of 1.03%. The values of δ13C in the estuarine surface sediments were smaller and originated mainly from continental shelf, while those in the marine surface sediments were greater and originated mainly from aquatic sources of organic matter. The range of δ13C varied largely among rivers with a 4.6‰ difference between the maximum and minimum. However, it varied very little among rivers which are geographically close. The δ15N values in the estuarine surface sediments ranged from 1.5‰ to 10.2‰ in average of 5.5‰ and those in marine surface sediments ranged from 4.4‰ to 5.6‰ in average of 5.0‰. Compared to marine surface sediments, the range of δ15N values in the estuarine surface sediments was wider, which may reflect complex origin of terrestrial organic matter. Values of δ15N in marine surface sediments were relatively uniform, indicating that the original δ15N source signature might have been lost or overprinted in biochemical alteration. The δ13C—δ15N relationship implies the mixing sources of terrestrial and aquatic organic matter. Using the classic binary mode, the proportion of terrestrial organic matter ranged from 10% to 90% in average of 60% for the 35 rivers, and from 10% to 50% in average of 20% for the Bohai Sea. Organic matters in the surface sediments of 35 rivers were originated mainly from terrestrial organic matter, while those in the surface sediments of the Bohai Sea came mainly from marine planktonic microalgae. Terrestrial input originated from rivers was also an important source. To be clear, stable organic carbon and nitrogen isotopes and elemental Corg/Ntotal ratios could provide limited information on the source because the original source signature may be lost or overprinted by biochemical alteration.