流体渗漏强度变化对烟囱状冷泉碳酸盐岩中铁和磷循环的影响

高宇, 刘春阳, 胡钰, 陈琳莹, 梁前勇, 冯东, 陈多福

高宇,刘春阳,胡钰,等. 流体渗漏强度变化对烟囱状冷泉碳酸盐岩中铁和磷循环的影响[J]. 海洋地质与第四纪地质,2024,44(6): 96-104. DOI: 10.16562/j.cnki.0256-1492.2023100802
引用本文: 高宇,刘春阳,胡钰,等. 流体渗漏强度变化对烟囱状冷泉碳酸盐岩中铁和磷循环的影响[J]. 海洋地质与第四纪地质,2024,44(6): 96-104. DOI: 10.16562/j.cnki.0256-1492.2023100802
GAO Yu,LIU Chunyang,HU Yu,et al. The impact of change in fluid seepage intensity on iron and phosphorus cycling in chimney-like seep carbonates[J]. Marine Geology & Quaternary Geology,2024,44(6):96-104. DOI: 10.16562/j.cnki.0256-1492.2023100802
Citation: GAO Yu,LIU Chunyang,HU Yu,et al. The impact of change in fluid seepage intensity on iron and phosphorus cycling in chimney-like seep carbonates[J]. Marine Geology & Quaternary Geology,2024,44(6):96-104. DOI: 10.16562/j.cnki.0256-1492.2023100802

流体渗漏强度变化对烟囱状冷泉碳酸盐岩中铁和磷循环的影响

基金项目: 国家自然科学基金面上项目“南海海马冷泉区硫化环境中蓝铁矿的形成机制及其对海洋磷循环的指示”(42472145)
详细信息
    作者简介:

    高宇(1999—),男,硕士研究生,海洋地质专业,E-mail:M220200625@st.shou.edu.cn

    通讯作者:

    胡钰(1988—),男,博士,副教授,从事海底生物地球化学研究,E-mail:huyu@shou.edu.cn

  • 中图分类号: P736.3

The impact of change in fluid seepage intensity on iron and phosphorus cycling in chimney-like seep carbonates

  • 摘要:

    冷泉碳酸盐岩形成过程中渗漏流体特征的变化可以改变参与的碳、硫等元素循环的生物地球化学过程,但对铁(Fe)和磷(P)元素循环及相关生物地球化学过程的影响目前还不清楚。本文对南海东沙海域一个冷泉碳酸盐岩烟囱横截面进行了矿物学、碳和硫地球化学及Fe和P组分进行了分析。结果显示,生物成因甲烷的厌氧氧化作用导致了极端13C亏损的自生碳酸盐的形成(δ13CVPDB<−55.5‰),并且形成过程中由于水合物分解,加入了富18O 流体,使得烟囱碳酸盐岩表现出异常高的氧同位素组成(5.5‰~5.8‰)。烟囱外层矿物组成以方解石为主,内层以文石为主,内层烟囱样品形成于更强的甲烷渗漏强度条件。样品中最主要的含磷物质是自生磷酸盐(PAuth),并且最高含量出现在烟囱内层。相比外层烟囱样品,内层铁氧化物含量更低,但是铁结合态磷(PFe)含量更高,推测可能是有蓝铁矿的形成。综合上述结果表明,在形成冷泉碳酸盐岩的强渗漏条件下,甲烷渗漏强度的增强会影响PAuth和PFe的丰度,可能更有利于蓝铁矿和自生磷灰石的形成,从而对冷泉环境中的磷和铁循环产生影响。

    Abstract:

    During the formation of seep carbonates, variations in fluid seepage affect the biogeochemical processes that drive the cycling of various elements, such as carbon and sulfur. However, such an influence on iron (Fe) and phosphorus (P) cycling and related biogeochemical processes remains unclear. In this study, mineral compositions, carbon and sulfur geochemistry, Fe and P component were investigated on a cross section of a chimney-like seep carbonate sample collected from the Dongsha area of the South China Sea. Results reveal that the anaerobic oxidation of biogenic methane led to the formation of the extremely 13C-depleted authigenic carbonate rocks (δ13CVPDB<−55.5‰), and the addition of enriched 18O fluids from the dissociation of gas hydrates results in abnormally high oxygen isotope composition (5.5‰–5.8‰) observed in the chimney carbonates. Combined with the dominant minerals with calcite in the outer layer of the chimney carbonates and aragonite in the inner layer, we inferred that the carbonates in the inner layer were formed at the later stage under stronger methane seepage condition. The most significant P-bearing mineral in the samples is authigenic phosphate (PAuth), and the highest content is in the inner layer of the chimney carbonates. Compared with the outer chimney sample, the inner layer has lower Fe oxide content, but higher Fe-bound P (PFe) content, indicating the formation of vivianite. Therefore, we believe that under a stronger seepage condition during the formation of seep carbonates, an enhanced intensity of methane seepage can affect the abundance of PAuth and PFe, which may favor the formation of vivianite and authigenic phosphates, thereby influencing Fe and P cycling in cold-seep environments.

  • 海底冷泉活动中广泛发育自生碳酸盐岩[1], 其往往指示强度很高、同时伴随沉积环境和氧化还原条件剧烈变化的流体渗漏活动[1-2]。不同的沉积条件下形成的冷泉碳酸盐岩有着不同的形态学、矿物学和地球化学特征[3-5],可以很好地反映沉积时的冷泉流体类型、活动强度、流体来源及生物地球化学信息[6-11],是研究深海冷泉环境必不可少的研究对象。冷泉碳酸盐岩烟囱以其独特的流体管道和圆柱形的形貌成为冷泉流体渗漏活动最显著的证据之一,使其受到了特别关注,包括成岩环境、流体来源以及成岩模式的研究都较多[12-17]

    中国南海北部广泛发育冷泉区[18-30],其中不同海域的冷泉碳酸盐岩的碳、氧同位素组成变化范围较大, 反映了冷泉流体的来源、渗漏速率及沉积环境均有较大差异[2]。南海东沙东北陆坡发育大量冷泉碳酸盐岩烟囱[31-36],这些烟囱的形成除了与海底天然气水合物分解释放的流体有关外,其烟囱内外层具有明显不同的矿物学和地球化学特征,呈现由外向内的生长模式,表明冷泉中渗漏流体扩散作用的影响,这使得烟囱状冷泉碳酸盐岩成为了记录时间和空间上冷泉流体渗漏组成和强度等特征变化的良好载体[32-34]

    尽管冷泉碳酸盐岩烟囱样品中渗漏流体强度变化对碳、硫、氧化还原敏感的微量元素的影响已经有相关研究[32-35],但目前对这种强渗漏环境下,渗漏流体强度变化对铁(Fe)和磷(P)元素循环及相关生物地球化学过程的影响还不清楚。在海底甲烷渗漏环境中,碳-铁耦合的相关生物地球化学过程可能会驱动磷元素循环过程,即Fe驱动的甲烷厌氧氧化过程有可能导致了含磷矿物蓝铁矿的形成[37-44]。前人对南海块状冷泉碳酸盐岩的磷组分和磷循环进行了分析和研究[42],但块状冷泉碳酸盐岩样品相比烟囱状冷泉碳酸盐岩样品而言,无法提供甲烷渗漏流体强度变化信息,导致无法探讨渗漏流体强度变化对Fe和P元素循环和相关生物地球化学过程的影响。

    因此,南海东沙海域广泛发育的烟囱状冷泉碳酸盐岩样品是研究不同流体渗漏强度对不同磷组分和铁-磷耦合影响的绝佳材料。本文将以南海东沙海域典型的烟囱状冷泉碳酸盐岩为研究对象,进行相应的磷组分和铁组分的分步提取和其他相关基本地球化学参数分析,鉴别不同形态磷和铁的来源和特征,阐明不同形态磷和铁可能参与的生物地球化学循环过程,揭示不同甲烷渗漏强度环境中甲烷渗漏活动对不同磷组分和铁-磷耦合关系影响,从而加深甲烷渗漏活动对海底磷、铁循环影响的认识。

    本文将以由“海洋四号”科考船于2015年在九龙甲烷礁“Site 3”通过海底拖网采集获得的南海东沙海域典型的烟囱状冷泉碳酸盐岩为研究对象,将采集到的冷泉碳酸盐岩样品用切割机沿着烟囱横截面切割,总体上烟囱样品直径约10 cm,外层呈现的颜色要比内层颜色深,内层烟囱孔洞发育。沿横截面从外向内对冷泉碳酸盐岩烟囱用牙钻取10个样品(图1),进行相应的磷组分和铁组分的分步提取和其他相关地球化学分析。

    图  1  采样位置和样品情况
    a: 位于东沙海域的采样站位九龙甲烷礁site 3;b: 冷泉碳酸盐岩烟囱样品横切面和取样点S1到S10。冷泉碳酸盐岩烟囱的横截面直径约10 cm,用牙钻间隔约0.8 cm取一个点,共取10个点。
    Figure  1.  Sampling location and subsamples
    a: The sampling site located in the Dongsha sea area, known as Methane Reef Site 3; b: the subsample of seep carbonates chimneys from S1to S10. The cross-section diameter of the seep carbonates chimneys is about 10 cm, and one point was taken about 0.8 cm apart for 10 points in total.

    冷泉碳酸盐岩烟囱物相分析(XRD)在上海海洋大学完成。用德国 BRUKER D8 ADVANCE X 射线衍射仪进行分析,工作参数为铜靶 Kα射线,功率为 40 kV,30 mA,扫描范围 2θ = 3°~85°,入射狭缝为 1 mm,扫描速度为 4°/min。矿物百分含量以d[104]峰作为校准曲线计算方解石、镁方解石的比例,以d[111]峰高计算文石的相对含量。通过d[104]峰高计算碳酸盐岩矿物中镁的含量。

    冷泉碳酸盐岩烟囱中碳和硫含量在上海海洋大学Elementar型元素分析仪测定。用超精密天平取其中约 25 mg 粉末样品进行上机测试。得到的数据是冷泉碳酸盐岩烟囱中总碳(TC) 和总硫(TS)含量。采用正磷酸法测定碳酸盐岩中碳酸盐相的碳氧同位素组成,具体做法在 75 ℃ 条件下,利用正磷酸与样品反应释放的CO2,通过Thermo Finnigan Delta V Advantage稳定同位素质谱仪对释放的CO2进行分析获得碳氧同位素组成。以 VPDB 国际标准为参考标准,δ13C 和 δ18O分析精度均优于±0.1‰。

    参照经典文献和最新文献[45-47]中所示方法进行冷泉碳酸盐岩中不同铁组分化学连续提取。不同铁组分提取步骤和相应条件如下:①碳酸盐相的铁(FeCarb),约50 mg样品与1 M乙酸钠溶液反应24 h,pH=4.5;②易还原铁氧化物的铁(FeOx1),上一步离心后的残渣与溶于体积分数为25%乙酸的1 M 盐酸羟胺溶液反应 48 h;③可还原性铁氧化物的铁(FeOx2),上一步离心后的残渣与 50 g/L 连二亚硫酸钠溶液反应2 h,pH=4.8(用0.35 M乙酸/0.2 M乙酸钠调节pH值);④磁铁矿的铁(FeMag),上一步离心后的残渣与 0.2 M 草酸铵/0.17 M 草酸混合溶液反应 6 h,pH=3.2。上述操作均在室温下进行,每步提取完成后离心并用0.2 μm滤膜过滤取上清液待测。FeCarb、FeOx1 和 FeOx2含量测试按照Alcott 等[47]所示方法,使用分光光度法进行测试,FeMag的含量用电感耦合等离子体发射光谱仪(ICP-OES)测试。实验室内部标样和平行样用于整个提取实验的质量监控,结果显示不同铁组分分析的相对误差通常小于5%。

    冷泉碳酸盐样品中不同形态磷的提取及含量测定参照化学连续提取法[48](即SEDEX法)进行。约50 mg样品依次与相应提取液反应,主要步骤如下:①可交换态磷(PEx)提取。使用1 M MgCl2提取液(pH=8),提取0.5 h;②铁氧化物结合态磷(PFe)提取。使用0.3 M柠檬酸钠和25 g/L连二亚硫酸钠作为提取液(pH=7.5),反应8 h。过滤后的样品接着用1 M 氯化镁提取液(pH=8)清洗半小时并离心过滤取上清液;③自生磷酸盐的磷(PAuth)提取。使用1 M醋酸钠-醋酸缓冲液提取(醋酸调节pH至4),反应6 h。接着过滤的沉积物同样用1 M 氯化镁提取液(pH=8)清洗半小时并离心过滤取上清液;④陆源碎屑磷(PDetr)提取。使用1 M盐酸溶液提取,反应24 h;⑤有机磷(POrg)提取。为了防止化学连续提取过程中POrg的损失,采用差值法进行POrg的提取。具体步骤包括:(a)冷泉碳酸盐岩样品与1 M盐酸溶液反应24 h;(b)样品经过550 ℃灼烧2 h后,与1 M盐酸溶液反应24 h。上述步骤(b)与(a)磷含量的差值即POrg。上述所有提取实验均在室温下进行,其中,PEx和PFe的提取应在无氧的、充满氮气的手套箱中进行操作,以防止黄铁矿的氧化对PEx和PFe含量的干扰[43-44]。采用分光光度法测试提取液中PEx、PAuth、PDetr、POrg的含量。为了使得提取液中PFe含量能够用分光光度法测试、消除提取液对分光光度法的干扰,按照Huerta-Diaz等[49]前处理方法,对含PFe的提取液使用Mg(OH)2共沉淀法消除干扰,然后用弱碱性溶液清洗干净后用盐酸溶解,最后用分光光度法测试PFe含量。根据实验室内部标样和平行样结果,不同磷组分含量分析相对误差一般小于10%。

    冷泉碳酸盐岩烟囱的矿物成分主要有4种,包括石英、长石、方解石和文石(图2)。结合取样点,可以把样品分成内外两层,其中编号S1—S3和S8—S10为烟囱外层,编号S4—S7为烟囱内层。除内层的S5和S7碳酸盐岩矿物主要为文石外(文石含量在70%以上),其余编号的碳酸盐矿物主要由方解石组成,方解石含量为70.2%~84.1%。烟囱内外层都含有较高的陆源碎屑矿物,主要包括石英、长石,含量为 10.7%~29.8%,总体上来看,大致具有外层陆源碎屑矿物含量普遍高于内层的规律。

    图  2  冷泉碳酸盐岩烟囱的矿物组成
    Figure  2.  Mineral composition of seep carbonate chimneys

    冷泉碳酸盐岩烟囱样品总碳含量为 0.21%~11.70%,变化较大,平均含量约7.62%(图3a)。随外层向内层有微弱的减少趋势,S5点总碳含量最低,而S2出现了相对较高的总碳含量。样品中总硫(TS)含量变化较小,平均含量约0.11%,其中S1出现了极高的总硫含量,S5和S10 出现了相对较高的总硫含量,总体来看硫含量偏高。

    图  3  冷泉碳酸盐岩烟囱样品的碳、硫含量(a)以及碳和氧稳定同位素组成(b)
    Figure  3.  Carbon and sulfur content (a), and carbon and oxygen stable isotope compositions (b) of the seep carbonate chimney samples

    冷泉碳酸盐岩烟囱的碳氧同位素变化范围较大(图3b)。δ13C 为−61.73‰~−55.53‰,平均值约为−58.78‰,δ18O 为5.45‰~5.83‰,平均值约为5.61‰。其中编号S5、S6、S7的碳同位素组成较其他编号明显偏负,而氧同位素组成则呈波动性,编号S2、S6、S10的氧同位素组成明显偏正。

    碳酸盐岩烟囱样品各铁组分含量分析结果见图4a。结果显示,FeCarb含量总体变化不大,内层略低,其中S5处FeCarb含量仅有0.02 mg/g,而烟囱外层FeCarb平均含量为3.97 mg/g。FeMag含量从烟囱内层到外层逐渐升高,其变化范围为0.05~0.90 mg/g,平均为0.58 mg/g,在S5处含量最低。FeOx含量为FeOx1 和 FeOx2含量之和,其含量靠近烟囱内层同样逐渐减少,S5和S7处FeOx含量很低,其中S7处为0.80 mg/g,外层平均5.99 mg/g。由于冷泉碳酸盐岩样品中的硫一般由黄铁矿的硫构成,因此黄铁矿的铁(Fepy)含量可以根据样品中总硫含量进行粗略估算。Fepy含量除S1、S5和S10以外7个点含量大致相同,平均为1.01 mg/g。而S1处测出含量值8.94 mg/g,明显高于其他点位,S5和S10的含量分别为1.94 mg/g 和1.76 mg/g,略高于另外7个点位。整体来看,Fepy含量外层外围高于内层,向内含量逐渐降低,到内层中心又重新增加。

    图  4  冷泉碳酸盐岩烟囱的不同铁和磷组分含量
    Figure  4.  Contents of different iron and phosphorus component of seep carbonates chimneys

    碳酸盐岩烟囱样品各取样点铁组分相对含量分析结果见图5a。结果显示, FeCarb内层占比略高于外层,但是在S5 处出现极低值(0.995%);FeMag占比内外层相当,都较低;FeOx1内外层差异不大,S5未测出FeOx1,占比为0;FeOx2内层低于外层,向内占比逐渐降低,特别是S5和S7均为0;FePy内层占比高于外层,特别是S5处占比为96.5%,明显高于其他点位,S1(37.8%)和S10(34.9%)也较其他点位占比高。整体而言,S1、S5和S7三点FePy占比更高,其他点位FeOx1和FeCarb占比更高。以FePy与烟囱样品高活性铁FeHR(FeHR = FeCarb + FeOx1 + FeOx2 + FeMag + FePy)比值表征样品黄铁矿化程度,结果显示,烟囱外围黄铁矿化度较高,向内有所降低,内层又有所增高,特别是S5,FePy/FeHR值为0.964。

    图  5  冷泉碳酸盐岩烟囱样品各铁组分和磷组分相对百分占比
    a:铁组分相对百分占比,b:磷组分相对百分占比。
    Figure  5.  Relative proportions of iron and phosphorus component in the seep carbonate chimney samples
    a: Relative proportions of the iron component, b: relative proportions of the phosphorus component.

    烟囱样品中各取样点磷组分含量见图4b。POrg平均为0.57 µmol/g,内外层差异不大。PEx含量外层比内层稍高,S5和S7点PEx含量最低,为0.92 µmol/g。PDetr含量内层明显比外层低,外层平均含量为5.15 µmol/g,而S5处含量最低,仅为0.42 µmol/g。PFe和PAuth含量内层与外围含量相当,特别是S6处PAuth明显较其他点位大、出现极大值8.39 µmol/g。从样品的总无机磷(PInorg,PInorg = PEx + PFe + PAuth + PDetr)含量上看,外层含量相当,平均值为13.73 µmol/g。而内层含量则相对较少,特别是S5(8.87 µmol/g)和S7(9.31 µmol/g),含量明显较其他点位小。

    碳酸盐岩烟囱样品各取样点磷组分相对含量分析结果见图5b。结果显示,PEx和PFe内外层相当,占比均较低;PAuth占比内层高于外层,特别是S5和S6,PAuth占比约60%;而PDetr占比则外层高于内层,S5处PDetr占比最小,仅为4.33%。POrg占比均较低,个别点占比为0。整体而言,烟囱样品内外层磷组分占比中PAuth最高,其次为PDetr

    碳酸盐岩烟囱极端负偏的碳同位素组成(δ13CVPDB<−55.53‰,图3)表明甲烷的厌氧氧化作用导致了该碳酸盐的形成。由于样品的δ13C值均低于–50‰,可以推断形成烟囱的流体中甲烷来源为生物成因。根据前人在相同站位冷泉碳酸盐岩的年代学数据和当时的海底温度(3.3~4.1 ℃)和海水氧同位素组成(0.64‰~0.97‰)[32],假定本文冷泉碳酸盐岩样品也在同期形成,通过方解石/文石-水体系的氧同位素分馏方程计算它们与海水平衡时的氧同位素组成变化范围分别为2.5‰~2.7‰和3.2‰~3.4‰。对比发现冷泉碳酸盐岩烟囱表现出异常高的氧同位素组成(5.45‰~5.83‰),这说明其形成时有富18O 流体的加入。天然气水合物分解和黏土矿物脱水反应均有可能导致冷泉流体中的水富含18O。考虑到本研究区附近表层沉积物发育水合物[50],并且表层沉积物孔隙水中也没有黏土矿物脱水反应导致孔隙水浓度异常的报道[51-53],因此推测烟囱的18O富集主要来源于富含18O 水合物的分解。

    前人研究结果显示本研究区冷泉碳酸盐岩烟囱样品的生长是由外向内,即外层先形成,内层后形成[32-34]。冷泉碳酸盐岩烟囱外层具有较高含量的石英、长石等陆源碎屑物质以及铁氧化物以及碎屑磷(图24),说明其在早期形成过程中受到较大的背景沉积的影响,在形成过程中捕获了更多背景沉积物,这些陆源碎屑物质中的TOC可能会影响烟囱的δ13C值。另外,根据之前对本区冷泉碳酸盐岩烟囱的研究,烟囱由外层向内层形成的过程中,由于渗漏通道空间越来越小, 导致流体渗漏强度越来越大[31]。本文研究的碳酸盐岩烟囱内层S3和S5点位以文石为主,而其余样品均以方解石为主(图2),同时一般认为文石相比方解石形成时的环境中硫酸盐浓度更高[16, 54],因此推测本文碳酸盐岩烟囱形成过程中流体渗漏强度内层要高于外层。这种内层渗漏强度高的特征,会使得形成于内层的自生沉积矿物更容易受到甲烷流体的影响,导致本文和类似的烟囱状碳酸盐岩样品中δ13C组成普遍出现两边高、中心低的分布特征[34, 55]图3b)。

    在大陆边缘环境中,铁、磷的耦合循环是一个动态过程[39, 42-43, 56-61]。铁氧化物还原、有机质氧化或甲烷厌氧氧化等过程中铁氧化物伴生的磷会被释放出来。碎屑磷在沉积于海底之前无法被利用,在埋藏后也不会发生相变[40]。烟囱样品磷组分提取结果显示碎屑磷含量内层比外层低(图5b),表明烟囱形成前期陆源碎屑输入较多,受到陆源碎屑输入的影响,可能导致陆源输入的铁氧化物从外层向内层含量降低。随着流体强度变化,冷泉渗漏环境中较强的微生物硫酸盐还原作用,不仅导致更加还原的环境和更多活性铁(主要是铁氧化物)的消耗,影响了烟囱内外层黄铁矿化度,也抑制了Fe的异化还原[33]。这些叠加因素可能最终导致了碳酸盐岩烟囱样品中铁氧化物含量内层低于外层。同时,铁氧化物的还原也会伴随其吸附无机磷的释放[39],也有可能会影响样品中磷的分布。

    由冷泉碳酸盐岩烟囱磷组分占比(图5b)可知,PAuth所占比例最大,其次是PDetr、PFe、PEx和POrg。烟囱样品中磷的分配表明,流体中最主要的一部分磷以自生磷酸盐(PAuth)的形式沉淀。磷从不稳定形态转移到PAuth,被认为是导致磷在全球海洋沉积物中埋藏的主要过程[62]。PAuth的形成是海洋沉积物中普遍存在的现象,但其具体机制尚不清楚[42]。冷泉渗漏流体中的磷来源于硫酸盐-甲烷过渡带下方,并在自生碳酸盐岩矿物中被化学吸附[42]。硫酸盐驱动的甲烷厌氧氧化可通过增加冷泉渗环境中孔隙水的碱度,导致沉积物或海底自生钙碳酸盐岩的沉淀[39,63]。这些形成的碳酸钙可能是自生PAuth沉淀的成核位点[42]。冷泉碳酸盐岩烟囱中PFe/FeOx比值内层明显高于外层(图6),显示冷泉碳酸盐岩烟囱内层更富集PFe,表明内层有额外的磷加入。PFe的含量内层稍高,可能与铁氧化物吸附磷和铁(II)磷矿物有关,而铁(II)磷矿物主要为蓝铁矿类矿物[39],在铁的连续提取过程在第②步被提取,从而被包含在化学提取的PFe中。结合碳酸盐岩烟囱内层FeOx含量低和相应吸附磷含量也低的特征[39],推测烟囱内层可能有蓝铁矿形成。烟囱样品S6中铁结合态磷含量低可能与后期蓝铁矿发生蚀变转变成自生磷灰石有关。综上所述,通过对烟囱状冷泉碳酸盐岩不同的磷和铁组分分析,发现在强渗漏条件下甲烷渗漏强度的改变会影响样品中铁结合态磷和自生磷灰石含量,更强的甲烷渗漏强度更有利于蓝铁矿和自生磷灰石的形成,从而对冷泉环境中的磷和铁循环产生影响。

    图  6  冷泉碳酸盐岩烟囱样品的晶质铁氧化物(FeOx2)和非晶质氧化铁含量(FeOx1)分别与铁结合态磷(PFe)的投点图
    图中红色点为冷泉碳酸盐岩烟囱内层样品,黑色点为冷泉碳酸盐岩烟囱外层样品。
    Figure  6.  The cross-plot of crystalline iron oxides (FeOx2) and amorphous iron oxide content (FeOx1) from cold seep carbonate chimney samples vs iron-bound phosphorus (PFe), respectively
    The red dots are seep carbonate chimney samples. The black dots are samples from the outer layers of seep carbonates.

    通过对南海东沙海域九龙甲烷礁site 3站位的冷泉碳酸盐岩烟囱进行矿物组成、碳氧同位素、碳和硫占比,以及不同铁组分和磷形态分析,确定了碳酸盐岩烟囱从外向内沉积过程中曾发生过强烈的、可能由天然气水合物分解导致的甲烷渗漏活动。但该甲烷渗漏活动并不稳定,且随着流体通路变窄甲烷渗漏活动增强。较强的甲烷渗漏活动会导致烟囱碳同位素值大幅负偏,氧同位素值轻微偏正,并会导致大量沉淀碳酸钙和黄铁矿的生成。在甲烷强渗漏条件下,烟囱内层明显富集铁结合态磷和自生磷灰石,说明更强的甲烷渗漏强度和更接近海底的环境有利于蓝铁矿和自生磷灰石的形成,进而影响冷泉环境中的磷和铁循环,但对于其中可能形成的蓝铁矿机制和微环境等还有待未来进一步研究。

    致谢:“海洋四号”科考船组织者、船员和科考人员在样品采集过程中给予的帮助。感谢审稿人提出的宝贵意见。

  • 图  1   采样位置和样品情况

    a: 位于东沙海域的采样站位九龙甲烷礁site 3;b: 冷泉碳酸盐岩烟囱样品横切面和取样点S1到S10。冷泉碳酸盐岩烟囱的横截面直径约10 cm,用牙钻间隔约0.8 cm取一个点,共取10个点。

    Figure  1.   Sampling location and subsamples

    a: The sampling site located in the Dongsha sea area, known as Methane Reef Site 3; b: the subsample of seep carbonates chimneys from S1to S10. The cross-section diameter of the seep carbonates chimneys is about 10 cm, and one point was taken about 0.8 cm apart for 10 points in total.

    图  2   冷泉碳酸盐岩烟囱的矿物组成

    Figure  2.   Mineral composition of seep carbonate chimneys

    图  3   冷泉碳酸盐岩烟囱样品的碳、硫含量(a)以及碳和氧稳定同位素组成(b)

    Figure  3.   Carbon and sulfur content (a), and carbon and oxygen stable isotope compositions (b) of the seep carbonate chimney samples

    图  4   冷泉碳酸盐岩烟囱的不同铁和磷组分含量

    Figure  4.   Contents of different iron and phosphorus component of seep carbonates chimneys

    图  5   冷泉碳酸盐岩烟囱样品各铁组分和磷组分相对百分占比

    a:铁组分相对百分占比,b:磷组分相对百分占比。

    Figure  5.   Relative proportions of iron and phosphorus component in the seep carbonate chimney samples

    a: Relative proportions of the iron component, b: relative proportions of the phosphorus component.

    图  6   冷泉碳酸盐岩烟囱样品的晶质铁氧化物(FeOx2)和非晶质氧化铁含量(FeOx1)分别与铁结合态磷(PFe)的投点图

    图中红色点为冷泉碳酸盐岩烟囱内层样品,黑色点为冷泉碳酸盐岩烟囱外层样品。

    Figure  6.   The cross-plot of crystalline iron oxides (FeOx2) and amorphous iron oxide content (FeOx1) from cold seep carbonate chimney samples vs iron-bound phosphorus (PFe), respectively

    The red dots are seep carbonate chimney samples. The black dots are samples from the outer layers of seep carbonates.

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出版历程
  • 收稿日期:  2023-10-07
  • 修回日期:  2023-11-22
  • 刊出日期:  2024-12-27

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