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Journal of Tropical Oceanography >

2023 , Vol. 42 >Issue 5: 144 - 153

DOI: https://doi.org/10.11978/2022224

Marine Geology

The impact of cold seepage on geochemical indices for redox conditions of marine sediments ―Site F active seep site in the northeastern South China Sea*

  • LI Niu , 1, 2 ,
  • DI Pengfei 1, 2 ,
  • FENG Dong 3 ,
  • CHEN Duofu 3
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  • 1. Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China
  • 2. Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
  • 3. Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China
LI Niu. email:

Received date: 2022-10-21

  Revised date: 2022-12-25

  Online published: 2023-03-14

Supported by

National Natural Science Foundation of China(41976061)

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Abstract

Redox-sensitive elements (Mo, U, V, Re, Ni, Co, Cr) have been widely used as geochemical indicators to infer the redox states of marine sediments at deposition, as well as oxygen concentrations in overlying water and atmosphere. However, the sulfidation environment in pore water formed by cold seepage due to microbial activity can result in alterations and ambiguities of redox signals indicated by these elements, which may challenge the effectiveness of the reconstructed redox state. In this paper, the contents of redox-sensitive elements of three push core sediments at the active seep site F of the South China Sea were studied. Compared with the oxic sediments, the seep sediments generally show higher Mo content, indicating the fixed Mo by hydrogen sulfide from the anaerobic oxidation of methane. U/Th, V/Cr, and Ni/Co indicate that the seep sediments are formed in the bottom water with high oxygen concentration, which is consistent with the measured results. However, V/(V+Ni) > 0.7 indicates anoxic conditions, which may be related to the lower Ni content in terrestrial debris. The Re/Mo ratio is similar to the modern seawater value, indicating a euxinic environment. The above analysis shows that Re and Mo in cold seep sediments are easily affected by methane seepage and possibly not used as geochemical indices for redox conditions in a methane-rich environment.

Key words： cold seep; redox conditions; geochemical index; anaerobic oxidation of methane; South China Sea

Cite this article

LI Niu , DI Pengfei , FENG Dong , CHEN Duofu . The impact of cold seepage on geochemical indices for redox conditions of marine sediments ―Site F active seep site in the northeastern South China Sea*[J]. Journal of Tropical Oceanography, 2023 , 42(5) : 144 -153 . DOI: 10.11978/2022224

钼(Mo)、铀(U)、钒(V)、铼(Re)、镍(Ni)、钴(Co)和铬(Cr)等对氧化还原敏感的痕量元素常被用作识别沉积环境氧化还原状态的代用指标(Brumsack, 2006; 林治家 等, 2008; 常华进 等, 2009; Tribovillard et al, 2012; 程猛 等, 2015; Scholz, 2018; 解兴伟 等, 2019; 吕荐阔 等, 2021)。这些元素在沉积物中的自生富集, 是由在不同氧化还原状态的溶解度差异和/或颗粒亲和力驱动, 这往往与沉积时或早期成岩过程中的氧化还原状态有关(Tribovillard et al, 2006)。然而导致这些元素富集的氧化还原条件和富集机制有很大的不同。此外, 其他因素如沉积时的开放与封闭环境、元素的生物富集程度、陆源碎屑通量的变化以及沉积后的再活化作用也会影响元素的含量(McKay et al, 2014)。
在氧化环境下, 海洋中的Mo主要以溶解态的钼酸盐形式存在, 可以被海水中的锰氢氧化物缓慢吸附(Tribovillard et al, 2006)。实验表明, Mo的积累是由于钼酸盐的还原和随后形成的硫代钼酸盐被铁硫化物和有机物清除(Helz et al, 1996; Vorlicek et al, 2004)。硫代钼酸盐的形成需要H2S的阈值浓度为~10μmol·L-1 (Erickson et al, 2000)。在极高的硫化物浓度时MoS2也能直接沉淀(Helz et al, 2011)。对现代海洋沉积环境的研究表明, 沉积物中Mo富集高达20μg·g-1, 同时伴有低的Mn浓度, 指示缺氧沉积物孔隙水中的硫化环境, 而极高的Mo浓度(>60μg·g-1)则指示硫化沉积环境(Scott et al, 2012)。在缺氧条件下(即底层海水中氧气浓度很低并没有硫化物), 即类似于Fe还原带, U和Re发生富集(U>5μg·g-1; Re>10ng·g-1)(Morford et al, 2012)。这种U和Re的富集被认为是Re和U还原以及随后沉淀的结果, 但确切的机制还不甚清楚(Morford et al, 2005, 2009, 2012)。虽然U和Re认为主要来自于海水, 但最近研究表明, 海洋中大量生物来源的颗粒(浮游植物残留物)明显富集U和Re (Zheng et al, 2002; Ownsworth et al, 2019)。因此, 生物来源颗粒可能是沉积物中自生U和Re的一种重要来源。Th在正常海洋环境中是一种相对惰性的元素, 主要来自于黏土碎屑。根据U和Th的地球化学差异, U/Th比值常被用来示踪沉积物形成时的氧化还原状态, U/Th < 0.75代表氧化环境, 0.75 < U/Th < 1.25代表次氧化环境, U/Th > 1.25代表缺氧环境(表1)(Jones et al, 1994; Wignall et al, 1996)。V在缺氧条件下被还原为VO22+, 并通过与有机物络合而在沉积物中富集; 随着沉积环境向硫化环境继续转变, V进一步被还原为难溶的V2O3和V(OH)3, 并在沉积物中进一步富集(Calvert et al, 2007)。在氧化条件下, 海洋中的Cr主要以可溶的铬酸盐形式存在, 在缺氧条件下, Cr会形成各种水合离子, 并被腐殖酸和铁锰氧化物吸附从而进入沉积物(Tribovillard et al, 2006)。但V的还原出现在反硝化作用界线的下部, Cr 的还原出现在界线的上部, 而且Cr对硫化环境并不敏感(Piper, 1994)。因此, V/Cr可以用来示踪沉积物形成时的氧化还原条件, 一般认为V/Cr < 2.0代表氧化环境, 2.0 < V/Cr < 4.25代表次氧化环境, V/Cr > 4.25代表缺氧环境(表1)(Jones et al, 1994)。在氧化水体中, Ni以溶解态或碳酸盐形式被有机物吸附(Calvert et al, 1993; Algeo et al, 2004), 在出现硫化氢的情况下形成不溶的硫化物, 并以固溶体的方式进入黄铁矿(Tribovillard et al, 2006)。但V较Ni优先在硫化环境中发生沉淀, 因此, V/(V+Ni)可用来作为氧化还原环境的判别指标(Hatch et al, 1992; Arthur et al, 1994)。Co在氧化水体中主要以溶解态或腐殖酸络合的形式存在, 有硫化氢存在时形成不溶的硫化物, 但Co在还原条件下较Ni优先活化, 造成沉积物中升高的Ni/Co比值升高(Algeo et al, 2004)。因此, Ni/Co比值可用来判断水体的氧化还原条件, 一般Ni/Co < 5代表氧化环境, Ni/Co在5~7之间代表次氧化环境, Ni/Co > 7代表缺氧环境(表1)(Jones et al, 1994)。
表1 氧化还原条件划分及海洋水体氧化还原环境的判识指标[据吕荐阔等(2021)修改]

Tab. 1 Division of redox conditions and identification indicators of redox conditions of marine waters, modified from LYU et al (2021)

指标 氧化环境 次氧化环境 缺氧环境(无H2S) 硫化环境
O2/(mL·L-1) >2.0 0.2~2.0 <0.2 0
H2S/(mL·L-1) 0 0 0 >0
Mo/(μg·g-1) >100
U/Th <0.75 0.75~1.25 >1.25
Ni/Co <5 5~7 >7
V/Cr <2 2~4.25 >4.25
V/(V+Ni) <0.45 0.45~0.60 0.54~0.82 >0.84
MoEF /UEF (0.1~0.3)×SW (1~3)×SW (3~10)×SW
Re/Mo <0.3×10-3 >0.77×10-3 接近 0.77×10-3

注: SW为现代大西洋海水的Mo/U摩尔比值, 为7.9, 引用自Algeo等(2009); MoEF 和UEF表示Mo和U相对上地壳值(Rudnick et al, 2013)的富集系数; 空白表示无数据

海底沉积物中微生物驱动的早期成岩过程可能会改变这些氧化还原敏感元素的地球化学特征, 从而使得对记录信息的解释复杂化。这些过程包括富有机质海洋沉积物中微生物驱动的硫酸盐还原作用: 2CH2O + SO42− → 2HCO3− + H2S, 以及冷泉活动环境在硫酸盐甲烷过渡区(sulfate methane transition zone, SMTZ)由硫酸盐还原菌和甲烷厌氧氧化古菌耦合的甲烷厌氧氧化作用(anaerobic oxidation methane, AOM)(Boetius et al, 2000): CH4 + SO42- → HCO3- + HS- + H2O。其他电子受体, 如铁锰氧化物等也能用来氧化甲烷(Beal et al, 2009)。以上两个作用均产生HS-, 从而可能影响对沉积环境氧化还原状态的判别(Liu et al, 2020a, 2022)。
本文通过对南海活动冷泉F站位3根插管沉积物样品的氧化还原敏感元素(Mo、U、V、Re、Ni、Co和Cr)的分析, 并与现代海洋硫化环境、缺氧环境和氧化环境中沉积物的氧化还原敏感元素特征进行对比, 研究活动冷泉相关的微生物作用对沉积物中的氧化还原敏感元素地球化学行为影响, 揭示冷泉活动对海洋沉积物氧化还原环境地球化学识别指标的影响。

1 样品采集与实验方法

研究样品采自南海北部活动冷泉F站位(图1), 水深1143~1163m (Zhao et al, 2020), 由加拿大水下机器人ROPOS号于2018年“嘉庚”号航次采集。其中PC2和PC3站位为细菌席区, PC1站位未见明显冷泉渗漏迹象(表2)。
图1 南海北部F站位冷泉位置图(a)和插管沉积物样品所在位置(b)[底图引自杨胜雄等(2015)]

图a审图号为JS(2015)02-107, 据阎贫等(2015)修改

Fig. 1 The location of active seep site F in the northern South China Sea (a), modified from Yan et al (2015) and the location of the push core sediment samples (b). The base map is from Yang et al (2015)

表2 采样位置和说明

Tab. 2 Sampling sites and description

站位 插管名称 岩芯长/cm 经度 纬度 水深/m 潜次名称 海底特征
F PC1 18 119°17′6.4″E 22°06′57.0″N 1143 Dive 2045 未见明显渗漏
F PC2 12 119°17′6.1″E 22°06′57.9″N 1151 Dive 2045 微生物席
F PC3 30 119°17′5.0″E 22°06′58.1″N 1163 Dive 2071 微生物席
所有插管岩芯切成2cm厚的沉积物样品后-20℃保存。沉积物经冷冻干燥且研磨至200目粉末。主量和微量元素分析在澳实矿物实验室——澳实分析检测(广州)有限公司完成, 取粉末样品40mg放入特氟龙杯并加入0.8mL浓HNO3、0.1mL浓HCl和0.2mL浓氢氟酸。把密封的特氟龙杯放入电烘箱中185℃维持36h。冷却后将特氟龙杯置于电热板上蒸干, 最后加入Rh内标及稀硝酸。主量元素(Al)在Agilent 5800 ICP-AES上测试, 微量元素(U、Mo、V、Re、Th、Cr、Ni和Co)在Agilent 7900 ICP-MS上测试。用于控制分析质量的标样为OU-6、BCR-1、GSD-11和GSD-12。Al、Mn、U、Mo、V、Re、Th、Cr、Ni和Co的分析精度优于±10%相对标准偏差(relative standard deviation, RSD)。

2 结果和讨论

2.1 冷泉沉积物氧化还原敏感元素富集特征

表3列出了分析样品的Al2O3、MnO、U、Mo、V、Re、Th、Cr、Ni和Co的含量。3个插管样品的Al2O3、MnO、U、Mo、V、Re、Th、Cr、Ni和Co的平均含量分别为(13.7±0.5)%、(0.04±0.01)%、(1.9±0.2)μg·g-1、(4.0±1.8)μg·g-1、(107±6)μg·g-1、(0.0030±0.0013)μg·g-1、(11.2±0.3)μg·g-1、(66±2)μg·g-1、(32.8±1.3)μg·g-1和(12.5±0.5)μg·g-1。相比于PC1插管样品, PC2和PC3插管样品具有较高的Mo含量, 而U、V、Re、Cr、Ni和Co的含量接近。
表3 南海北部F站位冷泉活动区3个插管沉积物样品元素含量和比值

Tab. 3 Major and trace element contents and ratios in seep site F sediments of the northern South China Sea

柱样名称 深度/cm Al2O3/% MnO/% Re/
(μg·g-1)		 Mo/
(μg·g-1)		 U/
(μg·g-1)		 V/
(μg·g-1)		 Co/
(μg·g-1)		 Cr/
(μg·g-1)		 Ni/
(μg·g-1)		 Th/
(μg·g-1)		 U/Th Ni/Co V/Cr V/(V+Ni) Re/Mo×103
PC1 0~2 13.4 0.07 0.0006 0.8 1.4 105 13.2 66 33.6 11.0 0.13 2.55 1.59 0.76 0.80
2~4 13.2 0.05 0.0012 0.9 1.6 103 12.4 63 32.2 10.9 0.15 2.60 1.63 0.76 1.40
4~6 13.6 0.05 0.0025 2.0 1.9 109 12.8 65 33.1 11.5 0.17 2.59 1.68 0.77 1.23
6~8 13.3 0.04 0.0021 3.5 1.7 106 11.9 63 32.0 10.9 0.16 2.69 1.68 0.77 0.61
8~10 13.8 0.04 0.0017 3.3 1.9 110 12.4 66 33.5 11.5 0.17 2.70 1.67 0.77 0.52
10~12 14.1 0.04 0.0022 1.5 1.8 112 12.4 67 32.9 11.6 0.16 2.65 1.67 0.77 1.46
12~14 13.5 0.04 0.0019 1.9 1.8 108 11.7 65 30.6 11.0 0.16 2.62 1.66 0.78 1.02
14~16 13.5 0.04 0.0033 1.0 2.3 105 11.8 65 31.9 10.7 0.22 2.70 1.62 0.77 3.24
16~18 14.1 0.04 0.0042 1.1 2.5 111 12.6 67 32.9 11.0 0.23 2.61 1.66 0.77 4.00
PC2 0~2 14.1 0.04 0.0056 4.1 1.9 112 13.0 68 33.5 11.3 0.17 2.58 1.65 0.77 1.37
2~4 14.1 0.04 0.0066 5.0 2.0 111 12.6 68 34.0 11.5 0.17 2.70 1.63 0.77 1.33
4~6 14.3 0.04 0.0055 5.3 2.1 114 12.8 68 34.3 11.6 0.18 2.68 1.68 0.77 1.04
6~8 14.5 0.05 0.0041 4.4 2.1 116 13.5 71 34.1 11.8 0.18 2.53 1.63 0.77 0.94
8~10 14.0 0.04 0.0037 4.0 2.2 111 12.5 67 33.9 11.3 0.20 2.71 1.66 0.77 0.93
10~12 14.5 0.04 0.0041 4.6 2.1 115 13.1 68 34.6 11.7 0.18 2.64 1.69 0.77 0.89
PC3 0~2 13.2 0.04 0.0019 5.1 2.0 102 12.0 63 31.4 11.4 0.18 2.62 1.62 0.76 0.37
2~4 12.7 0.04 0.0024 5.3 1.8 96 11.5 62 30.0 10.6 0.17 2.61 1.55 0.76 0.45
4~6 12.8 0.04 0.0039 7.1 2.2 95 12.0 63 31.4 11.5 0.19 2.62 1.51 0.75 0.55
6~8 12.5 0.04 0.0037 6.2 1.9 92 12.1 61 30.2 10.9 0.18 2.50 1.51 0.75 0.60
8~10 13.1 0.04 0.0040 6.3 2.0 96 12.5 64 35.9 11.3 0.18 2.87 1.50 0.73 0.64
10~12 13.0 0.04 0.0036 5.9 2.0 96 12.6 63 32.0 11.5 0.17 2.54 1.52 0.75 0.61
12~14 13.6 0.04 0.0031 5.1 2.0 103 12.9 66 32.5 11.2 0.18 2.52 1.56 0.76 0.60
14~16 14.2 0.04 0.0027 5.0 1.9 111 12.9 68 32.9 11.7 0.16 2.55 1.63 0.77 0.54
16~18 14.1 0.05 0.0028 5.3 1.9 110 12.7 68 32.9 11.4 0.17 2.59 1.62 0.77 0.53
18~20 14.1 0.05 0.0027 5.2 1.7 111 11.7 70 33.3 10.7 0.16 2.85 1.59 0.77 0.52
20~22 13.8 0.04 0.0025 6.1 1.9 110 13.0 68 34.3 11.4 0.17 2.64 1.62 0.76 0.41
22~24 13.3 0.04 0.0022 5.0 1.9 105 12.3 65 31.3 11.1 0.17 2.54 1.62 0.77 0.44
24~26 13.4 0.04 0.0021 3.9 1.9 105 12.6 65 32.3 11.3 0.17 2.56 1.62 0.76 0.54
26~28 13.7 0.04 0.0019 3.0 2.0 109 13.0 67 32.9 11.2 0.18 2.53 1.63 0.77 0.63
28~30 14.1 0.05 0.0018 3.0 2.0 111 13.2 68 34.1 11.7 0.17 2.58 1.63 0.76 0.59
沉积物中微量元素的富集可以通过与铝含量的比值(TM/Al)表示, 或者计算富集系数[EF=(TM/Al)/(TM背景/Al背景)], 背景值一般为当地陆源碎屑值, 但为了与全球其他海域沉积物做对比, 本研究的元素TM和Al背景值为上地壳值(Rudnick et al, 2013)。从图2可看出, 现代海洋上升流区(纳米比亚和秘鲁上升流)具有最高的Mo、Re、V和U 富集; 硫化环境(黑海和卡里亚科盆地)次之; 氧化环境中Mo/Al、V/Al和U/Al比值接近上地壳值, Re/Al比值高于上地壳值。图2结果显示了F站位的冷泉环境海底沉积物相比氧化环境具有较高的Mo富集, 但富集程度低于硫化和上升流环境, 没有明显V和U的富集。已有的研究显示PC2和PC3站位具有较浅的SMTZ (<10cm), PC1的SMTZ较深(未见底), 指示了PC2和PC3站位强烈的冷泉甲烷渗漏环境(Niu et al, 2022; Zhai et al, 2022)。但PC1站位相比上地壳, 也明显富集Mo (表3), 说明也受到了冷泉活动的影响(Chen et al, 2016)。在南海东沙海域水合物钻孔、南海海马冷泉、日本南开海槽冷泉区、日本海北部冷泉区和西北巴伦支海冷泉区沉积物中也发现了Mo的显著富集, 而没有其他氧化还原敏感元素的显著富集(Sato et al, 2012; Chen et al, 2016; Bazzaro et al, 2020; Liu et al, 2020b; Ota et al, 2022)。
图2 现代海洋硫化盆地、上升流区最小含氧带、氧化沉积物和冷泉沉积物中氧化还原敏感元素Mo (a)、Re (b)、V (c)、U (d)富集程度的变化

硫化盆地数据来自于Brumsack (1989)、Ravizza等(1991)、Piper等(2002)、Lüschen (2004); 上升流区最小含氧带数据来自于Calvert等(1983)、Nameroff等(2002)、Böning等(2004)、Borchers等(2005)、Scholz等(2011); 氧化沉积物数据来自于Morford等(1999)、Bennett等(2020)。方框代表四分位间距; 触须代表第5和第95百分位数; 超过第5和第95百分位数的数据以开放圆圈表示; 黑色实线为上地壳的平均值(Rudnick et al, 2013); 其中Mo、V和U的单位为μg·g-1, Re的单位为ng·g-1, Al的单位为%

Fig. 2 Trace metal enrichments (log10 scale) Mo (a)、Re (b)、V (c)、U (d) in a range of sediments from the modern euxinic basins (Brumsack, 1989; Ravizza et al, 1991; Piper et al, 2002; Lüschen, 2004), within perennial oxygen-minimum zones (Calvert et al, 1983; Nameroff et al, 2002; Böning et al, 2004; Borchers et al, 2005; Scholz et al, 2011), oxic (Morford et al, 1999; Bennett et al, 2020) and seep. The box represents the interquartile range; the whiskers represent the 5th and 95th percentiles. Data exceeding the 5th and 95th percentiles are represented as open circles. The crustal average value (Rudnick et al, 2013) is shown as a black solid line. The unit of Mo, V and U is μg·g-1, the unit of Re is ng·g-1, and the unit of Al is %

上升流环境海底沉积物Mo/Al×104的中位数为23.9, 显著高于硫化盆地的12.9和F站位冷泉环境的0.6。缺氧环境中Mo富集的一个主要驱动因素是游离的硫化物; 其富集方式是通过将钼酸盐转化为颗粒活性的硫代钼酸盐, 或通过将钼吸附/沉淀到自生铁硫化物中(Erickson et al, 2000; Algeo et al, 2006; Helz et al, 2019)。研究表明, 有机物也在Mo的清除作用中扮演了关键作用(Dahl et al, 2017; Wagner et al, 2017; Tessin et al, 2019)。沉积物中高的有机物含量能增强有机质驱动的硫酸盐还原作用, 从而在孔隙水形成高含量的硫化氢, 固定从海水中扩散的Mo (Wagner et al, 2017)。在上升流区海底的最小含氧带内, 由于生产力的勃发, 向沉积物中输入高活性有机物驱动了高的硫酸盐还原速率, 这导致沉积物孔隙水中出现高浓度的硫化氢(Brüchert et al, 2003)。同时, 对纳米比亚和秘鲁上升流的研究表明, 底层海水中出现的间歇性的硫化环境也是沉积物中Mo富集的关键因素(Scholz et al, 2017)。此外, Fe和Mn可以通过吸附钼酸盐到Fe或Mn羟基氧化物上, 最终沉降到沉积物表面, 在沉积物Mo富集中发挥作用(Scholz et al, 2017)。
与上升流区最小含氧带内的环境相反, 硫化盆地中底层海水高含量的硫化氢, 驱动了Mo在底层沉积物中的富集(Algeo et al, 2012)。然而, 由于硫化盆地如黑海的水文限制, 通常具有较低的底水更新速率, 限制了沉积物中可用于富集溶解钼的再补给(Algeo et al, 2006)。
相比正常海底的氧化环境, 冷泉环境出现的Mo富集主要与沉积物的AOM作用有关(Chen et al, 2016; 邬黛黛 等, 2020; Zhang et al, 2022a, 2022b)。沉积物孔隙水的硫化氢浓度的升高可以来自于有机质驱动的硫酸盐还原作用或冷泉渗漏甲烷的AOM作用。由于冷泉环境中的有机碳含量相对较低, 沉积物孔隙水中的硫化氢主要来自于AOM增强的硫酸盐还原作用。本文冷泉沉积物样品的有机碳含量为(0.56±0.09)%, 总硫为(0.41±0.21)% (Li et al, 2023), 有机碳含量和南海北部陆坡非冷泉区沉积物的值0.5%类似, 但总硫含量明显高于非冷泉区的0.1% (Chen et al, 2016), 说明冷泉沉积物中Mo的富集与有机质驱动的硫酸盐还原作用无关。同时, 本研究样品的MnO含量为0.04%~0.07% (表3), 排除了铁锰氧化物吸附作用对Mo富集的影响。
沉积物U和V的最大富集出现在上升流区的最小含氧带内的海底沉积层, 接着是硫化环境, 本研究冷泉环境3个插管沉积物样品中没有出现U和V的显著富集, 但沉积物中的V和Al之间具有很好的相关性(r2=0.9), 表明沉积物中的V主要来自于陆源碎屑。这主要是因为大陆边缘上升流中具有较高的底水更新速率, 而硫化盆地中受限的底水更新速率限制了U和V的富集。此外, 强上升流区具有较高的初级生产力, 因此向海底输送的有机碳通量较高, 这可以增强沉积物中U的自生富集作用, 同时带来更多的有机碳和铁锰氧化物吸附的V (McManus et al, 2006; Scholz et al, 2017)。

2.2 冷泉活动对氧化还原环境判别指标的影响

3个插管沉积物样品的氧化还原环境判别指标Ni/Co、U/Th和V/Cr均显示沉积物形成于氧化的沉积环境(图3)。这也和对PC1、PC2和PC3站位测得的底层海水氧气浓度高相一致(ROV现场测定的氧气浓度为103μmol·L-1), 说明Ni/Co、U/Th和V/Cr在富甲烷环境中仍然能判别环境的氧化还原状态。但南海东沙海域水合物钻孔中的样品受冷泉影响的样品中部分具有较高的U/Th比值, 指示次氧化甚至缺氧环境(图3)。我们发现这些U/Th比值高的样品大多具有高的碳酸钙含量, 并且二者之间具有很好的相关性(r2=0.7)。碳同位素偏负的沉积物总无机碳的结果表明, 这些碳酸钙主要为冷泉成因, 是AOM作用的产物, 矿物成分主要为文石和高镁方解石(Chen et al, 2016)。考虑到冷泉碳酸盐岩中具有高的Mo和U含量(Liang et al, 2017), 我们认为这些沉积物中升高的U/Th比值可能与冷泉碳酸盐岩的发育有关。但V/(V+Ni) > 0.7, 指示沉积物形成于缺氧的沉积环境, 与实际的沉积环境不一致。考虑到冷泉沉积物中没有明显的V的富集(图2), 过高的V/(V+Ni) 比值可能是由于沉积物中较低的Ni含量所致, 与输入物源中较低的Ni含量有关。因此, V/(V+Ni)用来判别氧化还原环境时应考虑到当地陆源背景值中Ni的变化(Tribovillard et al, 2006)。
图3 冷泉沉积物Ni/Co和U/Th之间的散点图(a)以及V/(V+Ni)和V/Cr之间的散点图(b)

图a中蓝色实心圆形为东沙水合物钻孔冷泉区样品, 黄色实心圆形为东沙水合物钻孔非冷泉区样品, 数据来自于南海北部东沙水合物钻孔(Chen et al, 2016)。图a和图b中蓝色空心圆形为PC1, 黑色空心圆形为PC2, 红色空心圆形为PC3。2条虚线表示U/Th比值分别为0.75和1.25; 黑色实线表示V/(V+Ni)比值为0.45

Fig. 3 The scatter plot between Ni/Co and U/Th in seep sediments (a); V/(V+Ni) and V/Cr (b). The data of sediment samples from cold seep and non-cold seep areas are from the Dongsha hydrate drilling in the northern South China Sea (Chen et al, 2016). The blue solid circle represents the samples from the Dongsha hydrate drilling seep area, the yellow solid circle are the samples from the Dongsha hydrate drilling non-seep, the blue hollow circle is PC1, the black hollow circle is PC2, and the red hollow circle is PC3. The two dashed lines indicated that the U/Th ratios were 0.75 and 1.25, respectively. The black solid line indicates that V/(V+Ni) ratio is 0.45

海洋沉积物中的Re/Mo比值可反映Mo的来源和氧化还原状态等方面的信息。因为在硫化环境中, 沉积物中的Re和Mo主要来自于海水。因此, 沉积物Re/Mo比值接近于海水值; 而在缺氧非硫化沉积物中, 由于锰羟基氧化物的溶解, 沉积物中没有锰羟基氧化物吸附Mo的贡献, 沉积物Re/Mo比值会高于海水值(Crusius et al, 1996; Crusius et al, 2000; Scholz et al, 2013; Häusler et al, 2018)。除了2个样品以外, F站位3个冷泉环境插管样品显示Re/Mo比值接近海水值(图4), 指示了硫化的沉积环境, 与南海海底实际沉积环境相反。这主要是在冷泉强烈甲烷渗漏条件下, 浅的SMTZ阻止了锰羟基氧化物对Mo吸附和累积(Ota et al, 2022), 推测冷泉沉积物中的Mo主要来自于海水, 通过扩散作用在硫化带中富集(Lin et al, 2021)。
图4 不同氧化还原条件下的通过海水值校正过的沉积物中的Re/Mo比值

硫化环境数据来自于Calvert等(2015); 非硫化和缺氧环境数据来自于van der Weijden等(2006); 方框代表四分位间距; 触须代表第5和第95百分位数; 超过第5和第95百分位数的数据以×表示

Fig. 4 The Re /Mo ratios in sediments under different redox conditions, euxinic environment data from Calvert et al (2015), and non-euxinic and anoxic environment data from van der Weijden et al (2006). The box represents the interquartile range; the whiskers represent the 5th and 95th percentiles. Data exceeding the 5th and 95th percentiles are represented as ×

由于Mo富集受底层海水和沉积物孔隙水中硫化氢浓度的控制, 且Mo容易受到铁锰氧化物吸附的影响(Scott et al, 2012)。而U是在铁还原带之上开始还原, U的沉淀早于Mo, 且不受硫化氢的影响(Algeo et al, 2009)。因此, Mo和U富集系数的变化能用来示踪沉积物形成时的氧化还原环境(Tribovillard et al, 2012)。F站位3个冷泉插管沉积物中的U的富集系数(UEF)接近于1, 而Mo的富集系数(MoEF)在2到10之间。从PC1、PC2到PC3孔显示逐渐增加的Mo富集系数, 可能反映了冷泉活动强度的增加和孔隙水中更加硫化的环境。这也与PC2和PC3孔发育细菌席和PC1孔未见明显甲烷渗漏的特征相一致。同时, 南海东沙海域水合物钻孔受冷泉影响的沉积物部分样品显示出受颗粒传输机制影响的特征, 说明铁锰氧化物在沉积物中Mo富集的作用(图5)。因此, Mo-U富集变化在富甲烷环境中的运用需要慎重。
图5 冷泉沉积物Mo-U 富集共变反映水体氧化还原状态(弱氧化、缺氧和硫化环境)和颗粒传输机制的出现[修改自Tribovillard等(2012)]

图中蓝色实心圆为东沙水合物钻孔冷泉区样品, 黄色实心圆为东沙水合物钻孔非冷泉区样品, 数据来自于南海北部东沙水合物钻孔(Chen et al, 2016)。蓝色空心圆形为PC1, 黑色空心圆形为PC2, 红色空心圆形为PC3。3×海水和0.3×海水是指现代大西洋海水的Mo/U 摩尔比值的3倍和0.3倍值; 颗粒传输指铁锰氧化物吸附对Mo富集的影响

Fig. 5 Seep sediments MoEF vs UEF diagram reflecting the redox state of water and the presence of particle transport, modified from Tribovillard et al (2012). The data of sediment samples from cold seep and non-cold seep areas are from the Dongsha hydrate drilling site in the northern South China Sea (Chen et al, 2016). The blue solid circle is the samples from the Dongsha hydrate drilling seep area, the yellow solid circle is the samples of Dongsha hydrate drilling non-seep, the blue hollow circle is PC1, the black hollow circle is PC2, and the red hollow circle is PC3. 3×seawater and 0.3×seawater are three times and 0.3 times of the Mo/U molar ratio of modern Atlantic seawater, respectively. Particle transport refers to the effect of iron-manganese oxide adsorption on Mo enrichment

3 结论和启示

对南海北部活动冷泉F站位的3根插管沉积物进行元素分析, 并通过与现代硫化、缺氧和正常海洋的氧化环境沉积物进行对比, 得出以下结论:
1) 南海北部F部位冷泉环境表层沉积物没有显著的V、U和Re的富集, 富集系数与接近于正常海洋的氧化环境沉积物的相似, 同时U/Th、V/Cr和Ni/Co指示了底层海水具有较高的氧气浓度, 与实测海水的氧浓度结果相一致;
2) F站位3个插管的冷泉沉积物与氧化环境沉积物相比, Mo富集主要与AOM作用增强的硫酸盐还原作用形成的硫化氢有关。因此利用元素Mo判别沉积古环境时应考虑是否存在古冷泉甲烷渗漏的影响;
3) F站位冷泉沉积物的V/(V+Ni) > 0.7指示缺氧环境, 可能与陆源碎屑中较低的Ni含量有关。Re/Mo比值接近于现代海水值, 与现代海洋硫化环境类似。同时, 冷泉沉积物的Mo-U富集共变显示出不同于弱氧化、缺氧、硫化和颗粒传输影响的特征。因此Mo、U和Re相关的氧化还原环境识别指标在富甲烷环境中需谨慎使用。

*感谢厦门大学“嘉庚”号科考船和加拿大“ROPOS”号ROV团队帮助采集研究样品。

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