Enlargement of Pyrite Framboid Size in Sulfate-Methane Transition Zone of Marine Sediments and Its Implying of Marine Methane Event
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摘要: 海洋沉积物或海相地层中莓球状黄铁矿的莓球粒径大小和偏差被广泛应用于推测水体的氧化还原沉积环境.然而，在现代海洋天然气水合物赋存海区沉积物中硫酸盐‒甲烷转换带（SMTZ）内，甲烷厌氧氧化作用（AOM）成因的黄铁矿莓球粒径出现异常增大现象.当天然气水合物失稳引起异常多数量甲烷释放时，SMTZ位置可能跃升至沉积柱浅表层、海底甚至海水中，可能引起水体的缺氧、酸化和分层等变化；与此同时，增强的AOM仍会促进沉积物中黄铁矿的莓球粒径异常增大，因此有必要重新评估前人依据黄铁矿莓球粒径推测水体氧化还原环境的判别标准.莓球粒径 > 20 μm或莓球内核粒径平均 > 12 μm、标准偏差 > 3 μm的莓球状黄铁矿可能是海洋“甲烷事件”发生时的产物.Abstract: Single framboid size and deviation of pyrites in marine sediments or strata have been widely used as a useful proxy to indicate the seawater redox environment. However, our recent data about pyrite framboid size from the modern sediments bearing nature gas hydrate show a tremendous increasing trend in pyrite framboid size within the sulfate-methane transition zone (SMTZ), indicating the anaerobic oxidation of methane (AOM) dominated in SMTZ might play a key role to enhance the enlargement of pyrite framboid size. In case of large methane release caused by dissociation of gas hydrate (so called methane event), the rising SMTZ position would move up to shallow sediment or near seafloor and even into bottom seawater, most likely resulting into some anaerobic and acid environmental changes in bottom seawater. Meanwhile, the enhancing AOM coupled with the methane event will still greatly enlarge the pyrite framboid size in sediments. So in this situation, the traditional critical relationship between the pyrite framboid size and seawater redox environment will be no longer functional and need to be modified. It is proposed that the coupling of average framboid size > 20 μm or core size > 12 μm and deviation > 3 μm might be used as a critical proxy to indicate the environment of marine methane event.
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图 1 海底附近流体交换过程、沉积柱地球化学分带和化学反应示意
图据林杞（2016）；SRZ. sulfate reduction zone，硫酸盐还原带；SMTZ. sulfate-methane transition zone，硫酸盐‒甲烷转换带；SMI. sulfate- methane interface，硫酸盐‒甲烷界面
Fig. 1. Schematic map showing fluids convection, interaction and chemical zone near seafloor
图 2 南海北部陆坡沉积物中莓球状黄铁矿的莓球粒径、含量和硫稳定同位素组成
Site 2A和Site 973-4站位的资料引自Lin et al.（2016a）；Site GMGS4-W02B站位的资料引自许力源（2022）.莓球粒径采用合须图表示.阴影部分为（古）SMTZ位置
Fig. 2. Framboid size, concentration and S isotope of pyrite in cored sediments from northern South China Sea
图 3 IODP311航次U1329站位沉积物中黄铁矿莓球粒径大小、黄铁矿含量、硫同位素组成和海平面变化对比
a.铬还原硫（CRS）同位素组成；b、c.全球底栖有孔虫壳体氧同位素组成和推测的海平面变化趋势；d.黄铁矿的莓球内核粒径平均值；e.莓球粒径平均值；f.莓球粒径加大边的大小；g.黄铁矿的相对含量.灰色阴暗部分代表现代SMTZ位置；蓝色阴影部分代表古SMTZ位置.最左侧代表U1329站位的岩性柱和年代框架
Fig. 3. Comparing of framboid size, concentration, S isotope of pyrite in Site U1329 of IODP 311 and correlated with global sea level changes
图 4 南海北部陆坡水合物赋存海域沉积物中莓球状黄铁矿的莓球平均粒径‒标准偏差图
图据许力源（2022）；Site 2A等代表南海北部陆坡水合物赋存海域的不同站位，*代表具有加大边的草莓状黄铁矿；黑色斜虚线代表传统的缺氧‒硫化和氧化‒次氧化底层水沉积环境的分界线，据Wilkin et al.（1996）；黑色横直和红色竖直虚线代表Lin et al.（2016a）给出的草莓状黄铁矿粒径异常的判断标准（平均粒径 > 20 μm，标准偏差 > 3 μm）；红色横直和红色竖直虚线代表莓球内核粒径大小的判断标准（平均粒径 > 12 μm，标准偏差 > 3 μm）
Fig. 4. Framboid size and deviation of pyrites in sediments from northern South China Sea
图 5 草莓状黄铁矿莓球粒径特征与SMTZ位置演化关系示意
图据许力源（2022）；a.上涌流体的甲烷浓度较小，SMTZ位于沉积物较深处，AOM较弱，莓球粒径通常较小；b.上涌流体的甲烷浓度增大，SMTZ移至沉积物浅表层，AOM增强，莓球粒径异常增大；c.上涌流体的甲烷浓度增大，SMTZ移至水‒沉积物界面，AOM增强，莓球粒径异常增大；d.上涌流体的甲烷浓度增大，SMTZ移至底层海水，AOM增强，莓球粒径大小难以判断
Fig. 5. Schematic relationship between SMTZ evolution and pyrite framboid size
图 6 华南宜昌秭归青林口新元古代陡山沱组的莓球状黄铁矿莓球平均粒径变化
图据Chen et al.（2025）；左侧为陡山沱组地层柱状图；中间为莓球粒径平均值的合须图分布；右侧为统计的莓球粒径平均值和数量.在陡一段（M1）和陡四段（M4）层位中莓球粒径平均值存在明显增大趋势（> 20 μm）；在陡二段（M2）中下部和陡二段\陡三段交界层位也存在一些莓球粒径增大的趋势，据此推测陡山沱组沉积时期至少存在3次古海洋甲烷事件
Fig. 6. Framboid pyrite size in Neoproterozoic Doushantuo Fm. in Qinglinkou section at Zigui, Yichang, South China
表 1 莓球状黄铁矿的莓球迷粒径特征与水体氧化还原环境之间关系（常晓琳等, 2020；许力源，2022）
Table 1. The critical relationship between the pyrite framboid size and the mariner redox environment (Chang et al., 2020; Xu, 2022)
Wilkin et al. (1996) Bond et al. (2010) 水体环境 平均粒径 最大粒径 水体
环境平均粒径 最大粒径及其特征 沉积特征 氧化‒次氧
化环境(7.7$ \pm $4.1) μm 10 %~50 %颗粒粒径超过10 μm，MFD会
大于20 μm氧化
环境无草莓状黄铁矿，黄铁矿晶体亦少见 细水平纹层发育 上贫氧环境 极少草莓状黄铁矿，且粒径范围极大，鲜有粒径 < 5 μm的分子出现 细水平纹层发育 下贫氧环境 6~10 μm 少数粒径较大，并有少量自形晶黄铁矿出现 细水平纹层发育，
出现少量生物扰动缺氧‒硫化
环境(5.0$ \pm $1.7) μm 大于10 μm的占比通常小于4%，MFD一般
小于20 μm缺氧
环境4~6 μm 数量较多，少数个体较大(> 10 μm)，且以草莓状黄铁矿为主 可见小潜穴, 纹层被
生物扰动破坏硫化
环境3~5 μm 数量丰富，粒径分布窄，且以草莓状黄铁矿为主 潜穴发育或块状构造 
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