曼谷湾顶红树林区近百年来有机碳埋藏记录及影响因素

Organic Carbon Burial Records and Influencing Factors of Mangrove Forest at the Tip of Bangkok Bay Over the Past Century

通过对曼谷湾顶红树林区柱状沉积物D1的总有机碳(Total Organic Carbon, TOC)、总氮(Total Nitrogen, TN)、稳定碳同位素(δ^(13)C)及粒度等指标的分析,在^(210)Pb建立的年代地层框架的基础上,重建了该地区近百年来有机碳的埋藏记录,并对其影响因素进行了探讨。结果表明,TOC含量和δ^(13)C值变化范围分别为1.35%~2.26%,-24.18‰~-22.67‰,均自下而上表现为先增加后减少的趋势。研究区为河口型红树林,相比海洋型红树林,其沉积物中有机碳来源受红树林影响较小,这是由于受周期性涨落潮和河口径流的冲刷,较轻的红树林碳屑极易被分解或被带至外海,难以在沉积物中存储。综合研究区的环境因素,选择陆源、海源和人为源作为三端元,计算得出柱状沉积物中有机碳以人为源和海源为主,陆源贡献相对较低。1939年以来,人为源有机碳贡献不断增大;1980—2008年间,因入海河流上游修建大坝,导致入海物质通量减少,加之该时期区域气候模式发生变化,降雨量相对减少,陆源有机碳贡献相对降低。因围海造田、围海养殖等人类活动增强,人为源有机碳贡献相对增加。该时期红树林遭到大规模砍伐,红树林的海岸屏障作用削弱,导致沉积物中海源有机碳贡献相对增加。2008年以来,因红树林恢复和堤坝建设,陆源有机碳的贡献几乎消失。

Based on the chronostratigraphic framework established by 210Pb,the organic carbon and its burial records over the past century were reconstructed by analyzing the Total Organic Carbon(TOC),Total Nitrogen(TN),stable carbon isotope(δ^(13)C),and particle size of the core D1 sediments at the tip of Bangkok Bay,Thailand.The results showed that TOC andδ^(13)C contents increased first,and then decreased toward the top.The results showed that the TOC content andδ^(13)C values of the core D1 sediments varied from 1.35%to 2.26%and-24.18‰to-22.67‰,respectively,both of which took on a bottom-up trend of increasing and then decreasing.The mangrove forests in the sampling area are estuary type.Compared with marine type mangrove forests,the source of organic carbon in sediments in the study area is less affected by mangroves,which is due to the fact that the light mangrove carbon debris is easy to be decomposed or carried to the open sea by the periodic high and low tides and the scouring of the runoff from the estuaries,making it difficult to be stored in sediments.Through a comprehensive analysis of environmental factors in the study area,terrigenous,marine and anthropogenic sources were selected as the three end members.The calculation results showed that the organic carbons in the core D1 sediments are mainly from anthropogenic and marine sources,and the contribution of terrestrial sources is relatively low.Since 1939,the organic carbon contribution of anthropogenic source has been increasing.From 1980 to 2008,the material flux into the sea decreased due to the construction of dams upstream of the rivers.In addition,the regional climate pattern was changed during this period,the rainfall was correspondingly decreased,and the contribution of terrestrial organic carbon was relatively reduced.Moreover,due to the enhancement of human activities such as reclamation and aquaculture,the contribution of anthropogenic organic carbon has increased relatively.During the period,mangrove forests were cut down on a large scale,weakening its role of coastal barrier at the sampling locations and increasing the contribution of marine organic carbon in sediments.Since 2008,the organic carbon contribution of terrigenous source has almost disappeared due to the restoration of mangrove forests and the construction of dykes.