Response of redox sensitive elements to changes of sedimentary environment in core sediments of seasonal low-oxygen zone in East China Sea
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摘要: 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，在还原沉积环境中除Fe、Mn外，RSE被还原成低价态转移至沉积物中富集积累，因此可以利用氧化还原敏感元素在沉积物中的富集情况反演沉积环境的氧化还原状况。本文通过研究东海内陆架季节性低氧海区Zb7沉积柱中氧化还原敏感元素V、Cr、Ni、Cu、Zn、Mo、U的垂直分布、富集特征和比值，探究沉积环境氧化还原状况；发现RSE/Al和富集系数自1978年以来呈增加的趋势，但自2009年开始有所降低，整体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MoEF/UEF比值主要分布在0.08～0.3倍海水Mo/U值之间，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氧化物结合进入沉积物，也指示氧化的沉积环境。研究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RSE在夏季季节性低氧时，沉积物中的富集信号在秋冬季溶氧水平恢复后缺失有关。尽管RSE不能有效指示东海季节性低氧环境，但Zb7沉积柱RSE在1978年后富集程度的增加以及2011年后的降低，在一定程度上反映了该区域自1978年后季节性低氧程度加重，2009年后又有所缓解的变化趋势。Abstract: Redox sensitive elements (Redox Sensitive Elements, RSE) such as V, Cr, Mo and U, et al. are dissolved under oxidation conditions. They are reduced to low-valent state under reducing 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 status of sedimentary environment. In this paper, the vertical distribution, enrichment characteristics and ratios of redox sensitive elements V, Cr, Ni, Cu, Zn, Mo and U in the Zb7 core sediment in the seasonal low-oxygen sea area of East China Sea were studied. It was found that RSE/Al and enrichment coefficient had increased since 1978, but had decreased since 2009. The overall RSE enrichment coefficient was less than 3, and no obvious enrichment had been observed. RSE ratios V/Cr<2, Ni/Co<5, U/Th<0.75, 0.25<(Cu+Mo)/Zn<0.55 and MoEF/UEF ratios were mainly distributed between 0.08 and 0.3 times of seawater Mo/U ratios, indicating the oxidized sedimentary environment. RSE/Al was significantly correlated with Fe/Al and Mn/Al, which indicated that RSE entered sediments mainly by combining with Fe and Mn oxides after removing terrigenous debris inputs, and also indicated the oxidized sedimentary environment. The results were inconsistent with the results of seasonal hypoxia reflected by the historical data of dissolved oxygen in this region. It may be related to the absence of RSE enrichment signals in the sediments during seasonal hypoxia in summer after the recovery of dissolved oxygen levels in autumn and winter. Although RSE can not effectively indicate the seasonal hypoxic environment in the East China Sea, the increase of RSE enrichment in Zb7 after 1978 and the decrease after 2009 reflected to some extent the trend of seasonal hypoxia aggravating in the region since 1978 and easing after 2009.
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图 2 Zb7柱状沉积物210Pbex的垂直分布(a)和定年结果(b)
Fig. 2 Vertical distribution of 210Pbex (a) and 210Pb-derived chronology (b) of Zb7 core sediments
图 6 Zb7柱状沉积样品中RSE/Al垂直分布特征
Fig. 6 The vertical distribution of RSE/Al of Zb7 core sediments
图 7 Zb7柱状沉积样品中RSE富集系数的垂直分布特征
Fig. 7 The vertical distribution of RSE’s enrichment factor of Zb7 core sediments
图 8 Zb7柱状沉积样品RSE比值垂直分布
图中红色竖线分别代表氧化环境-次氧化环境的阈值，从左到右，依次代表V/Cr=2，N/Co=5，U/Th=0.75，(Cu+Mo)/Zn=0.25，0.55
Fig. 8 The vertical distribution diagram of RSE’ ratios of Zb7 core sediments
The red vertical lines in the figure represent the threshold values of the RSE ratio in the oxic-suboxic sedimentary environment respectively, which successively represent V/Cr=2，N/Co=5，U/Th=0.75，(Cu+Mo)/Zn=0.25，0.55 from left to right
图 9 Zb7柱状沉积样品MoEF-UEF共变图
SW表示现代海水Mo/U的数值
Fig. 9 MoEF-UEF diagram of Zb7 core sediments
SW represents MoEF /UEF in seawater
表 1 Zb7站位附近水域近年来低氧状况统计
Tab. 1 Statistics of low oxygen conditions in adjacent waters of Zb7 in recent years
时间 面积/km2 低氧位置 氧最小值/mg·L−1 氧平均值/mg·L−1 数据来源 1958年9月 16 400 29.23°N 122.47°E 0.73 2.51 参考文献[10] 1958年10月 2 370 29.73°N 122.76°E 1.83 2.51 参考文献[10] 1975年10月 2 330 30.88°N 123.50°E 2.07 2.72 参考文献[10] 1980年10月 528 29.01°N 123.50°E 2.59 2.85 参考文献[10] 1999年8月 13 700 30.83°N 122.75°E 1.00 2.51 参考文献[8] 2003年6月 8 370 30.95°N 122.71°E 1.00 2.56 参考文献[18] 2006年10月 11 000 29.07°N 123.67°E 1.91 2.88 参考文献[8] 2015年6月 − 30.0°N 123.0°E 3.06 − 参考文献[6] 2015年8月 − 29.14°N 122.47°E 1.92 − 参考文献[23] 
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表 2 微波消解程序
Tab. 2 Microwave digestion procedure
功率/W 升温时间/min 升至温度/℃ 保持时间/min 1 600 3 140 3 1 600 3 160 5 1 600 3 180 30
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表 3 RSE/Al与Fe/Al、Mn/Al和TOC的相对变化关系
Tab. 3 Relative variations of RSE/Al with respect to Fe/Al、Mn/Al and TOC
V/Al Cr/Al Ni/Al Cu/Al Zn/Al Mo/Al U/Al TOC 0.011 −0.008 0.164 0.209 0.154 0.341 −0.080 Fe/Al 0.826** 0.779** 0.808** 0.832** 0.863** 0.726* 0.644** Mn/Al 0.887** 0.856** 0.824** 0.847** 0.885** 0.694* 0.721** 注：*代表显著性水平p<0.05(双尾检验)；**代表显著性水平p<0.01(双尾检验)。
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表 4 Zb7柱状沉积物RSE与粒度的相关性分析
Tab. 4 Correlation analysis of RSE and grain sizes of Zb7 core sediments
<1 μm 1～2 μm 2～4 μm <4 μm 4～8 μm 8～16/μm 16～63 μm 4～63 μm D50 Dav Al 0.070 0.480** 0.284 0.381* 0.321 0.024 −0.270 −0.381* −0.331 −0.289 Fe −0.075 0.375* 0.357* 0.386* 0.503** −0.19 −0.205 −0.386* −0.425** −0.287 Mn −0.086 0.414* 0.156 0.255 0.325 0.053 −0.244 −0.255 −0.293 −0.267 V −0.178 0.145 0.155 0.152 0.367* −0.407* 0.072 −0.152 −0.259 −0.038 Cr −0.272 0.085 0.116 0.097 0.345* −0.394* 0.095 −0.097 −0.212 −0.005 Ni −0.305 −0.003 0.079 0.037 0.301 −0.391* 0.132 −0.037 −0.15 0.032 Cu −0.283 0.055 0.041 0.032 0.253 −0.299 0.092 −0.032 −0.153 0.002 Zn −0.304 0.032 0.105 0.068 0.326 −0.269 0.036 −0.068 −0.187 −0.037 Mo −0.092 0.120 0.217 0.193 0.397* −0.195 −0.086 −0.193 −0.285 −0.187 U −0.082 0.326 0.360* 0.370* 0.427** −0.192 −0.174 −0.370* −0.432** −0.276 Th −0.087 0.331* 0.353* 0.366* 0.493** −0.075 −0.265 −0.366* −0.457** −0.350* TOC −0.113 0.272 0.303 0.307 0.504** −0.333* −0.084 −0.307 −0.371* −0.192 注：*代表显著性水平p<0.05(双尾检验)；**代表显著性水平p<0.01(双尾检验)。
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表 6 Zb7沉积柱与长江口及其邻近海域表层沉积物中RSE/Al的对比
Tab. 6 Comparisons of RSE /Al between Zb7 core sediments and surface sediments of the Changjiang River Estuary and its adjacent waters
采样位置 (V/Al)/103 (Mo/Al)/106 (U/Al)/105 参考文献 长江口外低氧区8站（31.15°N，122.56°E） 1.43 47.53 3.18 [19] 长江口外低氧区9站（31.02°N，122.62°E） 1.50 51.79 3.10 [19] 长江口外低氧区16站（30.94°N，122.72°E） 2.36 10.32 2.90 [19] 长江口外溶解氧正常区26站（29.98°N，122.74°E） 1.38 10.26 3.18 [19] 长江口外溶解氧正常区29站（29.31°N，122.18°E） 1.27 9.39 3.50 [19] Zb7沉积柱RSE/Al最高值 1.72 18.51 5.31 本研究 Zb7沉积柱RSE/Al最低值 1.04 7.48 2.81 本研究
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