海洋生态与环境科学重点实验室知识库

摘  要

溶解氧含量作为海洋生态系统重要的化学参数，对海洋生态环境具有重要影响，因此对海洋环境溶解氧状况的研究意义重大。海水中的氧化还原敏感元素（redox sensitive elements, RSE）如V、Cr、Mo、U等通常在氧化条件下呈溶解态，在还原性沉积环境中被还原成低价态转移至沉积物中富集积累，因此可以利用沉积物中氧化还原敏感元素的富集情况反演底层海水沉积环境的氧化还原状况。本文对2017年9月采自东海内陆架季节性低氧海区Zb7（29.0167°N，122.2750°E）沉积柱状样中氧化还原敏感元素V、Cr、Ni、Cu、Zn、Mo、U的垂直分布、富集特征以及比值进行了系统分析，并结合粒度、有机碳TOC、总氮TN、碳稳定同位素d¹³C等环境参数，探究氧化还原敏感元素对沉积环境的指示作用，主要结论如下：

1、Zb7沉积柱中不同层次RSE富集系数、RSE比值、RSE形态分析以及RSE/Al与Fe/Al、Mn/Al的相关性分析均指示该站底层海水呈氧化的沉积环境。Zb7沉积柱RSE所反映出的氧化沉积环境与该区域溶解氧历史数据反映的季节性低氧结果不一致，可能与夏季季节性低氧时在沉积物中富集的RSE由于秋冬季溶氧水平恢复再次向水体释放，使RSE富集程度降低有关。

Zb7沉积柱各层RSE富集系数均小于3，未见明显富集，RSE比值V/Cr<2、Ni/Co<5、U/Th<0.75、0.25<(Cu+Mo)/Zn<0.55以及Mo_EF/U_EF比值主要分布在0.08~0.3倍海水Mo/U比之间，均指示氧化的沉积环境。RSE/Al与Fe/Al、Mn/Al具有显著的相关性，表明RSE在剔除陆源碎屑输入后，主要通过与Fe/Mn氧化物或氢氧化物结合进入沉积物，也指示氧化的沉积环境。RSE各形态分析表明V、Cu、Ni在沉积物中主要以可还原态F2存在，说明这些RSE主要以与Fe/Mn氧化物或氢氧化物结合的形式在沉积物中富集，反映氧化的沉积环境，这与RSE/Al与Fe/Al、Mn/Al相关性分析结果一致。RSE最终指示氧化的沉积环境这一结果与该区域溶解氧历史数据反映的季节性低氧结果不一致，可能与秋冬季溶解氧水平恢复造成RSE在沉积物中的富集信号缺失有关。

2、RSE富集程度与通过d¹³C值和海源碳C_m所反映的初级生产力的变化趋势自20世纪90年代至今变化趋势一致。RSE富集程度、d¹³C值和海源碳C_m自20世纪90年代的增加以及2009年后的降低在一定程度上反映了该区域自20世纪90年代后季节性低氧程度加重，2009年后又有所缓解的变化趋势，也进一步说明了富营养化导致初级生产力和底层有机质矿化耗氧的增加是Zb7海域季节性低氧发生的主要诱因之一。

尽管RSE不能有效指示东海季节性低氧环境，但RSE指标在垂直方向的波动趋势，可反映沉积环境最终呈现的氧化还原信号的强弱，进而估测对应年份区间的季节性低氧强度的变化趋势。Zb7沉积柱RSE/Al和RSE富集系数在1978年之前变化不明显，自1978年后呈波动上升趋势，高值主要出现在3.5 cm（2009）、8.5 cm（2000）、12.5 cm（1995），但2009年后则呈现下降趋势。d¹³C值和海源碳C_m的变动趋势可以反映海洋初级生产力的变化趋势，发现20世纪90年代至2009年，d¹³C值、海源碳C_m的变化趋势与RSE/Al、RSE富集系数变化一致，均呈现上升趋势，可能与沿岸海域富营养化，赤潮频发有关，通常富营养化和底层水体低氧并发，也反映了由富营养化导致的季节性低氧程度加剧。2009年至今，d¹³C值、海源碳C_m与RSE/Al、RSE富集系数又呈现下降趋势，说明近几年富营养化引发的赤潮生态问题及季节性低氧有所缓解。这也进一步说明了富营养化导致初级生产力提高和底层有机质矿化耗氧的增加是Zb7所在海域季节性低氧发生的主要诱因之一。

Abstract

As an important chemical parameter of marine ecosystem, dissolved oxygen content has a momentous impact on marine ecological environment. Therefore, the study of dissolved oxygen status in marine environment is of great significance. Redox sensitive elements (RSE) such as V, Cr, Mo and U are usually dissolved under oxic conditions in seawater. They are reduced to low-valent state in reductive sedimentary environment and transferred to sediments for enrichment and accumulation. Therefore, the enrichment of redox sensitive elements in sediments can be used to invert the redox condition of sedimentary environment. This paper systematically studied and analyzed the Zb7 sediment core (29.0167°N, 122.2750°E) taken in September 2017 in the seasonal hypoxic sea area of the inner continental shelf of the East China Sea. The vertical distribution of the contents of redox sensitive elements V, Cr, Ni, Cu, Zn, Mo and U in the core sediment, RSE enrichment factors and RSE ratios were mainly studied to explore the changes of redox conditions in the sedimentary environment. Geochemical parameters such as grain size, TOC, TN, d¹³C of Zb7 sedimentary column were analyzed. The main conclusions were as follows:

1. RSE enrichment factors, RSE ratios, RSE morphology analysis and correlation analysis of RSE/Al with Fe/Al and Mn/Al in different layers of Zb7 sedimentary column indicated that the bottom seawater of the station was oxidative sedimentary environment. The oxidative sedimentary environment reflected by RSE of Zb7 sedimentary column was inconsistent with the seasonal hypoxia results reflected by the historical data of dissolved oxygen in this area, which may be related to the decrease of RSE enrichment during seasonal hypoxia in summer due to the recovery of dissolved oxygen and the re-release of RSE to seawater in autumn and winter.

RSE enrichment factors of all layers of Zb7 sedimentary column were less than 3, and no obvious enrichment was observed. RSE ratios V/Cr < 2, Ni/Co < 5, U/Th < 0.75, 0.25 <(Cu+Mo)/Zn < 0.55 and Mo_EF/U_EF ratio were mainly distributed between 0.08 and 0.3 times of seawater Mo/U ratio, indicating the oxidized sedimentary environment. RSE/Al were significantly correlated with Fe/Al and Mn/Al, which indicated that RSE entered sediments mainly by combining with Fe and Mn oxides or hydroxides after removing terrigenous debris inputs, and also indicated the oxidized sedimentary environment. RSE speciation analysis showed that V, Cu and Ni mainly existed in reducible state F2 in sediments, indicating that these RSE were enriched in sediments in the forms of binding with Fe, Mn oxides or hydroxides, reflecting the oxidized sedimentary environment, which was consistent with the results of correlation analysis between RSE/Al and Fe/Al, Mn/Al. RSE ultimately indicated the oxidized sedimentary environment, which was inconsistent with the seasonal hypoxia results reflected by the historical data of dissolved oxygen in the region. It may be related to the absence of RSE enrichment signals in sediments due to the recovery of dissolved oxygen levels in autumn and winter.

2. The change trends of RSE enrichment were consistent with that of primary productivity reflected by d¹³C value and marine-derived carbon(C_m) since 1990s to now. The increase in 1990s and the decrease after 2009 of RSE enrichment, d¹³C value and C_m reflected to a certain extent that seasonal hypoxia had increased in the region since 1990s, and it had been alleviated after 2009, which further explained the increase of primary productivity and oxygen consumption of bottom organic matter mineralization caused by eutrophication may be one of the main causes of seasonal hypoxia in the sea area of Zb7.

Although RSE could not effectively indicate the seasonal hypoxic environment in the East China Sea, the fluctuation trends of RSE indexes in the vertical distribution could reflect the intensity of redox signals eventually presented in the sedimentary environment, and then estimated the variation trends of seasonal hypoxic intensity in the range of corresponding years. The RSE/Al of Zb7 sedimentary column did not change significantly before the 1978. Since the 1978, the RSE/Al of Zb7 sedimentary column fluctuated upward. The high values mainly appeared in 3.5 cm (2009), 8.5 cm (2000), 12.5 cm (1995), but showed a downward trend from 2009. The variation trend of d¹³C value and C_m can reflect the change trend of marine primary productivity. From 1990s to 2009, the change trends of d¹³C value and C_m were consistent with that of RSE/Al and RSE enrichment factors, showing an upward trend, which may be related to coastal eutrophication and frequent occurrence of red tides, eutrophication and low oxygen in bottom waters were usually concurrent, which also reflected the intensification of seasonal hypoxia caused by eutrophication. From 2009 to now, the d¹³C value, C_m, RSE/Al and RSE enrichment factors had shown a downward trend, indicating that the ecological problems of red tide caused by eutrophication and seasonal hypoxia had been alleviated in recent years. This further indicated that the increase of primary productivity and oxygen consumption by mineralization of bottom organic matter caused by eutrophication may be one of the main causes of seasonal hypoxia in the sea area of Zb7.