海南岛万泉河口沉积物有机碳、 氮同位素的特征及其环境意义*

对海南岛万泉河口沙美内海柱样K3和K4沉积物进行生物地球化学指标总有机碳(TOC)、总氮(TN)、有机碳同位素(δ¹³ C)、氮同位素(δ¹⁵ N)的分析,以及²¹⁰ Pb定年了解其近代变化特征,追踪沉积物有机质来源的变化,提取沉积环境演变及人类活动的信息。K3中TOC的变化介于0.51 ％ ~1.12 ％ ,K4变化介于0.43 ％ ~1.49 ％ ,而其随时间的变化反映了流域内人类活动的历史。17世纪TOC含量明显增加记录了明末清初开始流域内广泛种植农作物的活动,19世纪以来TOC含量逐渐增加,至现代达最高值,对应了工业革命以来人类活动的增强。K3和K4的δ¹³ C值变化分别在-24.38 ‰ ~-21.02 ‰ 之间和-24.74 ‰ ~-20.17 ‰ 之间,δ¹⁵ N值变化分别在2.20 ‰ ~4.62 ‰ 之间和2.39 ‰ ~5.02 ‰ 之间,映证了本区沉积物有机质是陆源物质和海洋物质混合的特点。明末清初开始流域内广泛种植玉米等农作物造成δ¹³ C值增加和δ¹⁵ N值减少; 自19世纪以来至现在,该区的经济活动增加,人口的持续增长,尤其是近50年来化肥农药的大量使用,造成陆源输入的有机碳含量大幅增加,使δ¹³ C值减少和δ¹⁵ N比值增加。 δ¹³ C值自19世纪以来就逐渐变轻,说明陆源和海洋有机碳的比例发生了变化,较轻的陆源有机碳比例相对增加,较重的海洋有机碳比例相对减少,反映了玉带滩沙坝的增长、沙美内海封闭、海水交换作用减少的状况。C/N比值分别从底部的16.19和17.36到顶部减小为11左右,自19世纪以来呈明显减小的趋势也反映了沙美内海泻湖自生的藻类有机质供应越来越多,也说明了其逐渐封闭、水动力条件越来越弱,与外海的交换作用逐渐减少的状况。

VARIABILITY OF ORGANIC CARBON ISOTOPE, NITROGEN ISOTOPE, AND C/N IN THE WANQUAN RIVER ESTUARY, EASTERN HAINAN ISLAND, CHINA AND ITS ENVIRONMENTAL IMPLICATIONS

²¹⁰ Pb geochronology of two cores K3 and K4 and sediment core profiles of multiple biogeochemical indicators including total organic carbon, total nitrogen, organic carbon isotope(δ¹³ C)values, and nitrogen isotope(δ¹⁵ N)values were used to trace the organic matter sources and reconstruct the local sedimentary history and anthropogenic impacts on the Shamei Lagoon located in the Wanquan River estuary, Eastern Hainan Island, China. Total organic carbon contents of Cores K3 and K4 vary in the ranges of 0.51 ％ ~1.12 ％ and 0.43 ％ ~1.49 ％ respectively. Their profiles with time record the history of human activities in the drainage area. The obvious increase of TOC content since about 1600A.D. reflect the crop planting expansion began during the late Ming Dynasty and early Qing Dynasty. TOC contents increased gradually since about 1800A.D. , reaching the highest at present, which is corresponding to the increase of anthropogenic impacts since industry revolution. δ¹³ C values of Cores K3 and K4 are -24.38 ‰ ~-21.02 ‰ and -24.74 ‰ ~-20.17 ‰ , and δ¹⁵ N values are 2.20 ‰ ~4.62 ‰ and 2.39 ‰ ~5.12 ‰ respectively, which indicate the organic matters have typical characteristics of mixed terrigenous and marine provenance. Expansion of corn planting in the drainage area since about 1600A.D. resulted in an increase of δ¹³ C value and a decrease of δ¹⁵ N value. Since about 1800A.D. , δ¹³ C values have decreased and δ¹⁵ N values have increased due to the more terrestrial organic carbon input with the population growth and the development of the lower catchment. In particular, an increased use of fertilizers over the last 50 years has caused a marked increase in terrestrial organic carbon input. δ¹³ C values have become lighter gradually since about 1800A.D. This implies an upward increase in the relative proportion of terrigenous C₃ organic carbon in the deposits and a corresponding decrease in the marine organic matter content. It may reflect the narrowing of the mouth of Wangquan River estuary and the increasingly restricted entry of seawater into the lagoon with the gradual growth of the Yudai Spit towards the north. C/N ratios decreased gradually from 16.19 and 17.36 respectively to about 11 at present, but have had an obvious downward trend since about 1800A.D. , which may result from more supply of autogenious algae in the Lagoon under the much slower currents due to the decrease entry of seawater into the Lagoon. C/N variation since about 1800A.D. implies the same sedimentary environmental change as the δ¹³ C values do.