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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(6): 2020-2030

WANG Ping, WANG Jian, DU YuanSheng, ZHOU Qi, YU WenChao, XU Yuan, YUAN LiangJun, PAN Wen, XU YaJun. Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2020-2030. doi: 10.14027/j.issn.1000-0550.2023.140
Citation: WANG Ping, WANG Jian, DU YuanSheng, ZHOU Qi, YU WenChao, XU Yuan, YUAN LiangJun, PAN Wen, XU YaJun. Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2020-2030. doi: 10.14027/j.issn.1000-0550.2023.140
shu

Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance

doi: 10.14027/j.issn.1000-0550.2023.140
  • 1.

    School of Resource and Environment, Henan Polytechnic University, Jiaozuo, Henan 454003, China

  • 2.

    School of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China

  • 3.

    Engineering Technology Innovation Center of Mineral Resources Explorations in Bedrock Zones, Ministry of Natural Resources, Guiyang 550081, China

  • 4.

    103 Geological Party, Guizhou Bureau of Geology and Mineral Exploration and Development, Tongren, Guizhou 554300, China

Funds:

National Natural Science Foundation of China 42202111

Open Foundation of State Key Laboratory of Biogeology and Environmental Geology GBL22107

Postdoctoral Foundation of Henan Province HN2022027

Key Scientific Research Project of Colleges and Universities in Henan Province 24A170019

  • Received Date: 2023-07-28
  • Accepted Date: 2024-01-08
  • Rev Recd Date: 2023-12-21
    • Available Online: 2024-01-08
  • Publish Date: 2024-12-10
  • Objective The Neoproterozoic Struttin Glaciation （~717⁃660 Ma） developed widely across the world and was well recorded in the Nanhua Basin, but the study of carbon cycling during this period is lacking. To investigate the carbon cycle process during the Sturtian Glaciation, the carbon isotope compositions of the Tiesi'ao Formation precipitated during the glacial interval were systematically studied. Methods This study focuses on drillcore ZK2115, located in Gaodi Manganese Deposit, Songtao area, eastern Guizhou province, China. We analyzed high-resolution organic and inorganic carbon isotopes （δ13Corg and δ13Ccarb） from the syn-Sturtian Tiesi'ao Formation. Results The results show that the Tiesi'ao Formation is characterized by negative carbon isotope excursions, ranging between -9.29‰ and -3.37‰ （mean -7.24‰）. The δ13Corg values vary between -33.63‰ and -23.35‰ （mean -29.29‰）. There is a positive correlation between the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by diagenesis and can be used to reflect the original carbon isotope signals of the Dissolved Inorganic Carbon （DIC） reservoir in the Nanhua Basin. Organic matter preserved in the Tiesi'ao Formation was generated through photosynthesis, and the carbon fixed in the organic matter was sourced from the DIC reservoir in seawater. Conclusions Combined with the low Total Organic Carbon （TOC） content （mean 0.2%） of the Tiesi'ao Formation, our results indicate that photosynthesis still occurred under the extreme glacial conditions at a low rate, although only a small amount of organic matter was generated. The study of carbon isotope compositions in glacial sediments can be used to explore carbon cycling during the Sturtian Glaciation, and to provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic conditions.
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Publishing history
  • Received:  2023-07-28
  • Revised:  2023-12-21
  • Accepted:  2024-01-08
  • Published:  2024-12-10

Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance

doi: 10.14027/j.issn.1000-0550.2023.140
  • 1. School of Resource and Environment, Henan Polytechnic University, Jiaozuo, Henan 454003, China
  • 2. School of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China
  • 3. Engineering Technology Innovation Center of Mineral Resources Explorations in Bedrock Zones, Ministry of Natural Resources, Guiyang 550081, China
  • 4. 103 Geological Party, Guizhou Bureau of Geology and Mineral Exploration and Development, Tongren, Guizhou 554300, China
Funds:

National Natural Science Foundation of China 42202111

Open Foundation of State Key Laboratory of Biogeology and Environmental Geology GBL22107

Postdoctoral Foundation of Henan Province HN2022027

Key Scientific Research Project of Colleges and Universities in Henan Province 24A170019

  • Received Date: 2023-07-28
  • Accepted Date: 2024-01-08
  • Rev Recd Date: 2023-12-21
  • Available Online: 2024-01-08
  • Publish Date: 2024-12-10

Abstract: Objective The Neoproterozoic Struttin Glaciation （~717⁃660 Ma） developed widely across the world and was well recorded in the Nanhua Basin, but the study of carbon cycling during this period is lacking. To investigate the carbon cycle process during the Sturtian Glaciation, the carbon isotope compositions of the Tiesi'ao Formation precipitated during the glacial interval were systematically studied. Methods This study focuses on drillcore ZK2115, located in Gaodi Manganese Deposit, Songtao area, eastern Guizhou province, China. We analyzed high-resolution organic and inorganic carbon isotopes （δ13Corg and δ13Ccarb） from the syn-Sturtian Tiesi'ao Formation. Results The results show that the Tiesi'ao Formation is characterized by negative carbon isotope excursions, ranging between -9.29‰ and -3.37‰ （mean -7.24‰）. The δ13Corg values vary between -33.63‰ and -23.35‰ （mean -29.29‰）. There is a positive correlation between the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by diagenesis and can be used to reflect the original carbon isotope signals of the Dissolved Inorganic Carbon （DIC） reservoir in the Nanhua Basin. Organic matter preserved in the Tiesi'ao Formation was generated through photosynthesis, and the carbon fixed in the organic matter was sourced from the DIC reservoir in seawater. Conclusions Combined with the low Total Organic Carbon （TOC） content （mean 0.2%） of the Tiesi'ao Formation, our results indicate that photosynthesis still occurred under the extreme glacial conditions at a low rate, although only a small amount of organic matter was generated. The study of carbon isotope compositions in glacial sediments can be used to explore carbon cycling during the Sturtian Glaciation, and to provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic conditions.

WANG Ping, WANG Jian, DU YuanSheng, ZHOU Qi, YU WenChao, XU Yuan, YUAN LiangJun, PAN Wen, XU YaJun. Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2020-2030. doi: 10.14027/j.issn.1000-0550.2023.140
Citation: WANG Ping, WANG Jian, DU YuanSheng, ZHOU Qi, YU WenChao, XU Yuan, YUAN LiangJun, PAN Wen, XU YaJun. Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2020-2030. doi: 10.14027/j.issn.1000-0550.2023.140
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0.   引言

  • 新元古代记录了两次全球性的冰期事件,即Sturtian冰期（~717~660 Ma）和Marinoan冰期（~654~635 Ma）[1⁃3]。“雪球地球”假说[4⁃6]认为在这两次冰期事件过程中,地球表面完全被冰川覆盖,生态系统崩塌,水循环停滞,并且大陆风化作用基本停止[5⁃6]。随着对雪球地球的古气候、生物标志化合物以及古海洋氧化还原条件等研究的深入[7⁃9]以及气候模拟[10],有些学者认为在冰期期间,水文循环和大陆风化仍然活跃,地球并未被冰川完全覆盖,尤其是赤道附近存在一定的开放水域,这就是著名的冰雪地球（hard Snowball Earth）和冰水地球（Slushball Earth）之争[11]。目前对Marinoan冰期的研究比较深入,包括冰期之间小的冰期—间冰期旋回[12],冰期海洋的氧化还原状态及碳—氮循环过程[9,13⁃14],生物标志化合物及初级生产力[15⁃16]等,但是对Sturtian冰期的古海洋环境研究相对较少,尤其缺乏生物地球化学循环的研究。

    地质历史时期出现多次无机碳同位素的正偏和负偏现象,但是目前对其偏移机制仍然存在不同的认识,可能与成岩作用、生物生产力的增加/降低、有机质埋藏比例等因素有关[17⁃18]。目前已有很多关于Sturtian冰期之前和冰期之后地层中碳同位素的报道[17⁃18],但是对于Sturtian冰期期间碳同位素的组成以及碳循环过程仍然缺乏系统的研究。南华盆地完整地记录了南华系的地层序列,包括Sturtian冰期、Marinoan冰期以及两个冰期之间的间冰期地层,是研究雪球地球的理想区域。选取南华盆地中黔东松桃地区高地锰矿区ZK2115钻孔为研究对象,对Sturtian冰期沉积的铁丝坳组进行详细的有机碳和无机碳同位素研究,结合地层中的TOC含量,旨在研究Sturtian冰期期间海洋中的碳同位素组成特征,并探讨极端气候条件下海洋中的碳循环过程。

1.   地质背景

  • 中元古代末期—新元古代早期（~1.3~0.9 Ga）形成一个全球性的超大陆—Rodinia超大陆[19⁃21]。在Rodinia超大陆形成过程中,扬子板块与华夏板块碰撞拼合,最终形成华南板块（~820 Ma）[22]。新元古代中期,伴随着Rodinia超大陆的裂解,华南板块在~ 820 Ma开始发生裂谷作用[23],在扬子板块和华夏板块之间形成南华裂谷盆地,在扬子西缘形成康滇裂谷盆地[23]。

    南华裂谷盆地沿北东东向展布[24⁃25],盆地中发育武陵次级裂谷盆地和雪峰次级裂谷盆地,二者被天柱—怀化隆起隔开（图1）,在武陵次级裂谷盆地中包含一系列更次级的地垒和地堑盆地,在这些小的地堑盆地中发育一系列大型—超大型的锰矿床,被称为“大塘坡式”锰矿床[24⁃25]。在南华盆地中发育的地堑盆地完整地记录了南华系的地层序列,地层厚度变化较大。

    Figure 1.  (a) Structure of the Neoproterozoic Nanhua rift basin, located in southeastern margin of the Yangtze Block, South China[25]; (b) geological map of Wuling secondary rift basin during the early Nanhua period in the Guizhou⁃Hunan⁃Chongqing adjacent area[25]

    研究区位于南华盆地的雪峰次级裂谷盆地,区内南华纪地层出露较完整,自下而上分别为两界河组、铁丝坳组、大塘坡组和南沱组。两界河组以中厚层岩屑砂岩和石英砂岩为主,含白云岩透镜体,地层厚度变化大,在区域上呈零星分布,主要分布在贵州东北部的松桃—印江地区,属于陆源碎屑沉积。一般认为铁丝坳组是Sturtian冰期冰海沉积的产物,主要由冰碛岩组成,冰碛岩中砾石的分选较差,粒径大小不一,在0.1 cm×0.2 cm~1 cm×3 cm,磨圆也较差,并且砾石的含量自下而上逐渐变少。铁丝坳组与下伏两界河组整合接触。大塘坡组代表Sturtian冰期和Marinoan冰期之间的间冰期沉积,下部由黑色页岩组成,页岩底部包含一套富锰页岩层,是“大塘坡式”锰矿的富集层位,上部主要由灰绿色粉砂岩组成。锰矿层中的锰主要以菱锰矿（MnCO3）的形式存在,从矿床的中心到边缘,矿床品位逐渐降低,块状矿石逐渐变为条带状矿石。大塘坡组与下伏铁丝坳组和上覆南沱组呈整合接触。南沱组代表Marinoan冰期期间的冰海沉积,主要由砂泥质冰碛岩组成。选取黔东松桃高地锰矿区ZK2115钻孔为研究对象,该钻孔到铁丝坳组底部终止,与大塘坡组（约231 m）和南沱组（约284 m）相比,铁丝坳组较薄（约10 m）（图2）。ZK2115钻孔铁丝坳组冰碛岩中可见大小不等的砾石,并且在冰碛岩基质中发现很多细小的岩石碎屑,例如石英、长石等（图3）。

    Figure 2.  Features and description of Cryogenian strata from the drillcore ZK2115 in Gaodi manganese deposit, Songtao area, eastern Guizhou province

    Figure 3.  Rock photo (a) and matrix microscope photo (b) of diamictites in the Tiesi'ao Formation

    Sturtian冰期的开始和结束在全球范围内是同步的[3,26],精确的年代学研究将Sturtian冰期的初始时间限定在717 Ma左右[1⁃2,27]。目前普遍认为Sturtian冰期包含两幕冰期事件,在华南地区分别对应年龄较老的长安冰期和较新的古城冰期[28],并且越来越多的研究表明Sturtian冰期的第二幕冰期在全球范围内也是同步的,其开始的年龄在690 Ma左右[28⁃29],结束的年龄对应整个Sturtian冰期结束的年龄,即660 Ma左右[30⁃33]。在扬子东南缘南华裂谷盆地中两界河组底部最年轻的单颗粒锆石U-Pb年龄为708±15 Ma[34],区域上相当层位的渫水河组顶部凝灰质粉砂岩中的锆石SIMS U-Pb年龄为691.9±8.0 Ma[28]。这些年代学资料限定两界河组的沉积时代介于708~690 Ma。除此之外,两界河组的化学风化指标——CIA（Chemical Index of Alteration）整体上介于65~75[35],代表了当时温暖湿润的古气候,同时也说明两界河组属于Sturtian冰期两次幕式冰期之间的间冰期沉积,铁丝坳组代表了Sturtian冰期的第二幕冰期沉积。

2.   样品测试和结果

  • 研究的样品均来自黔东松桃地区高地锰矿区ZK2115钻孔,该钻孔中铁丝坳组地层厚度约为10 m,采集样品的间隔为0.3~0.6 m,共采集铁丝坳组样品24件。由于铁丝坳组样品主要为Sturtian冰期期间沉积的冰碛岩,岩石中含有大小不等的砾石,因此将固体的岩石样品研磨成粉末（200目）之前,需要剔除掉样品中的砾石,地球化学测试的对象是冰碛岩的基质部分。

    δ13Ccarb和δ18O的测试：首先称量适量样品置于反应瓶中,连接至真空系统,然后与高纯磷酸在70 °C反应,将收集到的CO2进行C-O同位素测试,该过程在kiel IV-MAT253连用系统上进行,采用的国际标样为GBW04416（δ13C=+1.61‰,δ18O=-11.59‰）和GBW04417（δ13C=-6.06‰,δ18O=-24.12‰）,分析结果以Vienna Pee Dee Belemnite（V-PDB）为标准,分析精度小于0.1‰。

    在测试δ13Corg之前需要去除掉样品中的无机碳部分,具体方法如下：首先取3~5 g样品放到50 mL离心管中,然后少量多次加入4 mol/L的盐酸,搅拌均匀,并静置2 h,然后用离心机离心,去除上清液,重复上述过程2~3次,确保除掉样品中的无机碳被完全去除。向残余物中加入超纯水,摇晃均匀,然后离心,去掉上清液,重复上述过程5~6次,直至离心之后的上清液呈中性。最后将水洗之后的残余物放在65 °C的烘箱中,待样品烘干后磨成粉末备用。称量适量样品,用锡纸杯包裹紧密之后在EA+MAT253上进行测试。采用的国际标样为GBW04407（-22.43±0.07‰）和GBW04408（-36.91±0.10‰）,国内标样ACET（-26.33‰）分析结果以V-PDB为标准,分析精度小于0.06‰。

    研究中涉及的无机碳—氧同位素和有机碳同位素的测试均在中国地质大学（武汉）地质过程与矿产资源国家重点实验室完成,测试结果如表1所示。

    样品编号深度/mδ13Ccarb/‰δ18O/‰δ13Corg/‰△13Ccarb-org/‰TOC/%
    ZK2115-H511 637.98-30.220.2
    ZK2115-H521 638.28-29.500.1
    ZK2115-H531 638.58-7.45-14.06-33.1825.730.7
    ZK2115-H541 638.88-7.31-15.89-33.2125.900.7
    ZK2115-H551 639.18-33.362.7
    ZK2115-H561 639.48-8.05-11.78-30.4922.440.1
    ZK2115-H571 639.78-8.21-13.58-30.8622.650.1
    ZK2115-H581 640.08-8.18-11.72-30.1721.990.1
    ZK2115-H591 640.38-29.930.1
    ZK2115-H611 640.98-8.16-10.53-30.0721.910.1
    ZK2115-H631 641.58-9.29-11.91-30.3021.010.1
    ZK2115-H641 641.88-30.070.1
    ZK2115-H651 642.48-29.400.1
    ZK2115-H661 642.78-27.950.1
    ZK2115-H681 643.38-4.91-7.84-26.8721.960
    ZK2115-H691 643.68-29.000.1
    ZK2115-H711 644.28-6.61-15.14-27.5720.960.1
    ZK2115-H721 644.58-6.80-14.86-27.1520.350
    ZK2115-H741 645.18-3.37-7.37-23.3519.980.1
    ZK2115-H751 645.48-7.11-11.42-27.8920.780.1
    ZK2115-H771 646.08-6.91-14.14-27.4320.520.1
    ZK2115-H781 646.38-7.43-14.57-27.0119.580.1
    ZK2115-H791 646.68-8.86-15.91-26.7017.840
    ZK2115-H821 647.58-31.210.1
    注：TOC数据来自文献[36]。

    Table 1.  Geochemical results of the Tiesi’ao Formation in drillcore ZK2115, Gaodi manganese deposit, Songtao area, eastern Guizhou province

    铁丝坳组δ13Ccarb介于-9.29‰~-3.37‰,平均值为-7.24‰,δ18O介于-15.91‰~-7.37‰,平均值为-12.71‰；δ13Corg介于-33.36‰~-23.35‰,平均值为-29.29‰。此外,铁丝坳组的碳同位素分馏（△13C；δ13Ccarb-δ13Corg）介于17.84‰~25.90‰。研究样品整体上TOC含量相对较低（0.2±0.6%）,但是在铁丝坳组顶部个别样品的TOC含量相对较高,最高可达2.7%[36]（图4）。

    Figure 4.  The δ13Ccarb, δ18O, δ13Corg, △13C and TOC profiles of the Tiesi’ao Formation in drillcore ZK2115, Gaodi manganese deposit, Songtao area, eastern Guizhou province (TOC data are from reference [36])

    在本研究的测试数据中,δ13Ccarb和δ13Corg从铁丝坳组底部开始存在一个明显的上升趋势,随后又呈下降的趋势,△13C从铁丝坳组的底部到顶部具有明显的上升趋势（图4）。δ13Ccarb和δ18O显示正相关关系（r=+0.55,p（α）<0.02,n=15；图5a）,δ13Ccarb和δ13Corg也显示正相关关系（r=+0.61,p（α）<0.01,n=15；图5b）,δ13Ccarb和TOC没有相关性（r=-0.12,p（α）<0.1,n=15；图5c）,δ13Corg和TOC显示微弱的负相关性（r=-0.55,p(α)<0.01,n=24；图5d）。δ13Ccarb和△13C不显示相关性（r=-0.06,p(α)<0.1,n=15；图5e）,但是δ13Corg和△13C显示很好的负相关关系（r=-0.83,p(α)<0.001,n=15；图5f）。

    Figure 5.  Crossplots of 13Ccarb⁃18O, 13Ccarb⁃13Corg, δ13Ccarb⁃TOC, δ13Corg⁃TOC, δ13Ccarb⁃△13C, δ13Corg⁃△13C in the Tiesi’ao Formation, drillcore ZK2115

3.   讨论

    3.1.   碳同位素数据的可用性分析

  • 沉积碳酸盐岩记录了当时的古海洋环境信息,但是碳酸盐岩在后期成岩过程中容易受到成岩作用的影响,导致其微量元素（Mn、Fe、Ca和Sr）含量以及δ13Ccarb和δ18O值的变化[37⁃38]。因此,在利用碳酸盐岩进行古海洋环境研究之前,需要考虑成岩作用是否影响碳酸盐岩中记录的古海水初始信号。

    在成岩过程中,沉积碳酸盐岩晶格中的Ca和Sr会被Fe和Mn取代,导致Fe和Mn含量的增加,Ca和Sr含量的降低,因此Mn/Sr、Fe/Sr比值可以作为沉积碳酸盐岩成岩蚀变的指标[37,39]。一般认为Mn/Sr<2,Fe/Sr<50时,碳酸盐岩仍然保存了原始的碳同位素组成特征[39⁃40],但是在前寒武纪时期,原始沉积的碳酸盐岩矿物中铁、锰含量本来就比较高,并不是后期成岩作用导致的[41]。因此,对于本研究中的样品,Mn/Sr和Fe/Sr不适合作为成岩作用的指标。

    成岩作用会导致碳酸盐岩δ13Ccarb和δ18O的降低,并且δ18O比δ13Ccarb更容易受到成岩作用的影响[37,39⁃40]。碳酸盐岩δ18O值可以作为成岩作用的判别指标[40],一般认为当δ18O>-10‰时,碳酸盐岩受到的成岩作用比较微弱[39]。此外,δ13Ccarb和δ18O的相关性也被用来评估成岩作用对碳酸盐岩的影响[37⁃39],当成岩作用较强烈时,会同时降低δ13Ccarb和δ18O的值,δ13Ccarb和δ18O显示正相关关系。当地层中出现耦合的δ13Ccarb和δ13Corg时,也可以判定δ13Ccarb记录了原始的碳同位素信号,其碳同位素组成不受成岩作用的影响[42]。总而言之,判断碳酸盐岩是否经历了成岩作用,需要基于各种指标的综合分析,而不是使用某一项指标来进行判断。

    南华盆地Sturtian冰期期间沉积的铁丝坳组样品中δ18O值介于-15.91‰~-7.37‰（平均值为-12.71‰）,大部分样品的δ18O都小于-10‰,并且δ13Ccarb-δ18O呈现中等程度的正相关关系（图5a）,似乎预示着成岩作用对初始古海洋信号的改变。但是铁丝坳组冰碛岩基质δ13Ccarb（-9.29‰~-3.37‰,平均值为-7.24‰）和δ13Corg（-33.36‰~-23.35‰,平均值为-29.29‰）显示明显的正相关关系（图5b）,这与成岩作用相悖,因为在前寒武纪时期没有任何一种机制可能同时改变δ13Ccarb和δ13Corg值[43]。因此,我们认为铁丝坳组记录了原始的碳同位素组成信号,这与地质历史时期其他地层记录一致[42⁃43],但是δ18O可能受到了成岩作用的影响。

    • 3.2.   Sturtian冰期期间的光合作用

  • 在新元古代雪球地球期间,最开始认为该时期整个地球几乎完全被冰雪覆盖,陆地、海洋和大气之间的物质循环受到阻碍,生态系统崩塌,光合作用停滞,海洋处于缺氧状态[4⁃6],并且认为海洋中的DIC库δ13C值在-5‰左右[5],与地幔δ13C值接近[44]。随着对冰期沉积地层越来越深入的研究以及气候模型模拟,一些学者对雪球地球的冰川范围产生了不同的意见,认为在冰期期间,地球并未完全被冰川覆盖,部分地区仍然存在开放水域[9⁃10]。化学风化指标（CIA）表明冰期期间化学风化作用仍在进行[8,45],并且冰期期间水文循环并未停止[45],陆地和海洋之间仍然存在物质交换。

    在雪球地球期间,全球生态系统并未完全崩溃,生命主要以微观和软体的形式存在[46]。有机分子证据以及生物标志化合物的证据表明,在冰期期间存在微弱的透光带,光合作用比较微弱,虽然生物生产力很低,但是从未停止,并且沉积有机质主要来源于海洋中的光合自养生物[47⁃48]。在澳大利亚和斯瓦尔巴群岛Sturtian冰期沉积的冰碛岩中已经发现有机壁微化石,尽管其多样性很低,但是其分布广泛且相当丰富[49]。在巴西东南部Sturtian冰期期间沉积地层中发育横向上广泛分布的黑色页岩层（TOC可达3.0%）,远远高于上下冰碛岩层的TOC含量（< 0.5%）,地层中提取出的生物标志化合物表明Sturtian冰期期间,该地区存在一个复杂而多产的微生物生态系统,包括光合细菌和真核生物,光合作用仍在进行,表层海水处于氧化状态,当时海洋中可能存在很薄的冰或者处于无冰状态[7]。阿曼地区Marinaon冰期沉积物中发现了24-异丙基胆甾烷,该物质被认为是海洋中海绵动物产生的C30甾醇的碳氢化合物残留物,代表了化石记录中最古老的动物证据[15]；华南Marinoan冰期冰碛岩中的页岩层中首次发现了底栖大型藻类[16]。这些化石数据都表明真核生物在Marinaon冰期幸存下来,并未由于冰期恶劣的环境而消失。纳米比亚Marinoan冰期沉积的Ghaub组和同时期的华南南沱组的铁组分以及C-N同位素等地球化学数据的研究表明,冰期期间存在显著的开阔水域,生物地球化学循环较为活跃,生物生产力以及产氧作用存在,并且表层海水含氧,深部水体缺氧[9,14]。

    在新元古代南华盆地中,Sturtian冰期期间沉积的铁丝坳组TOC含量很低,除了一个样品（黑色页岩）达到2.7%,其余样品都小于1.0%。铁丝坳组的低TOC含量也与其他地区Sturtian冰期期间沉积冰碛岩中的TOC含量一致,例如华南地区江口组[48]和澳大利亚Wilyerpa组[50]。Marinaon冰期期间,纳米比亚地区沉积的Ghaub组[14]和华南地区沉积的南沱组[9,47⁃48,51]冰碛岩中TOC含量也很低（<0.2%）,与Sturtian冰期类似。沉积物中有机质的来源一般分为两种,一种是光合作用产生的有机质,另外一种是陆源输入的有机质。铁丝坳组δ13Ccarb和δ13Corg具有明显的正相关关系（图5b）,表明地层中的有机质主要是水体中光合作用的产物,有机质中的碳来源于海洋中的DIC库。以上证据均表明Sturtian冰期期间的生态系统仍然存在,光合作用仍在进行。有机质的矿化作用是影响地层中有机质保存的重要因素,在新元古代海水中硫酸盐的浓度很低[52],通过硫酸盐还原等过程消耗的有机质的量特别少,有机质矿化作用比较微弱,地层中TOC的含量主要受到光合作用产生的有机质的影响。Sturtian冰川沉积物中TOC的含量比间冰期低1~2个数量级[36,47⁃48],尽管冰川沉积物的沉积速率相对较高,但是仍然可以说明冰室气候条件下光合作用的速率比较缓慢,初级生产力水平较低,通过光合作用产生的有机质十分有限。

    • 3.3.   Sturtian冰期碳同位素的负偏移

  • 黔东松桃地区高地ZK2115钻孔铁丝坳组冰碛岩基质部分δ13Ccarb值介于-9.29‰~-3.37‰（平均值为-7.24‰）,与研究区临近的ZK405钻孔同一层位冰碛岩基质的δ13Ccarb值（-11.3‰到-8.3‰,平均值为-9.6‰）[53]一致,存在明显的碳同位素负偏。铁丝坳组无机碳同位素代表原始的碳同位素组成信号,并且在冰期沉积物沉积过程中矿化的有机质很少,因此该过程产生的无机碳对海洋DIC库的影响很小,铁丝坳组δ13Ccarb可以代表当时水体中DIC库的碳同位素组成。桂西地区Sturtian冰期沉积的富禄组发育一套碳酸盐岩夹层,可能代表了冰期期间一次小的间冰期沉积,碳酸盐岩夹层δ13Ccarb介于-5.0‰~-3.0‰（平均值为-3.5‰）[54],与Sturtian之后的盖帽白云岩类似[30],并且δ13Ccarb与δ13Corg（-27.8‰~-22.2‰）呈现明显的解耦现象[54],这可能与环境变化导致的碳循环扰动有关。此外,世界其他地区也报道了Sturtian冰期沉积的冰碛岩中无机碳同位素数据。例如,在澳大利亚Sturtian冰期沉积的Wilyerpa组,沉积过程中形成的白云石δ13Ccarb介于-5.3‰~+2.0‰（平均值为-1.7‰）[50,55]；在斯瓦尔巴特群岛,Sturtian冰期沉积的Petrovbreen段冰碛岩基质δ13Ccarb介于-5.0‰~+2.0‰ [56]；在纳米比亚,Sturtian冰期沉积的Chuos组,其泥晶白云岩和灰岩层δ13Ccarb介于-1.9‰~-9.2‰（平均值为-7.3‰）[57]。这些证据表明Sturtian冰期期间,海洋中的DIC库存在负偏,其δ13C值低于现代海洋的δ13Ccarb（0左右）。此外,南华盆地Sturtian冰川沉积物中δ13Ccarb值低于Sturtian冰期之前（0~-5.0‰）[56⁃57]和冰期之后（-4.0‰~+4.0‰）[30,57]的地层（图6）。南华盆地在Sturtian冰期期间海水DIC库可能受到大气CO2的溶解,火山/热液活动的输入以及有机质矿化的影响,但是大气中CO2在水中溶解的δ13Ccarb（-4.0‰~-5.0‰）[5],火山或者热液活动δ13Ccarb（-5.0‰~-7.0‰）[5]均高于水体中DIC库的δ13Ccarb,并且冰期期间有机质矿化作用较微弱,可以忽略,因此冰期DIC库碳同位素的负偏需要其他13C缺乏的碳源的输入。Sturtian冰期期间δ13Ccarb的负偏可能与DIC库碳同位素的大规模扰动有关,受到环境因素的制约,也可能受到全球碳循环过程的影响[43,58],对于其负偏机制目前仍然无法确定,缺乏实质性的证据。

    Figure 6.  The δ13Ccarb variation trend from the Neoproterozoic to the early Cambrian (1 000⁃500 Ma) (modified from reference [17])

    • 3.4.   Sturtian冰期碳循环及对海洋状态的启示

  • Sturtian冰期沉积的铁丝坳组δ13Ccarb代表了当时水体DIC库的碳同位素组成信息,在-7‰左右,具有明显的负偏。在冰期期间,全球处于冰室气候状态,生态系统并未完全崩塌,光合作用仍在进行,但是光合作用的速率极低。此外,有机质的矿化作用比较微弱,光合作用产生的有机质大部分都在地层中保存下来,由于初级生产力很低,产生的有机质有限,在地层中保存下来的TOC含量很低（0.2%）。随着冰室气候逐渐向温室气候转变[8],温度的升高可能导致有机质和DIC库之间的碳同位素分馏逐渐增大,△13C值从铁丝坳组底部到顶部具有明显的上升趋势。

    在Sturtian冰期期间,有机碳和无机碳之间存在比较活跃的碳循环过程。此外,铁丝坳组古气候指标——CIA介于45~67（平均值为58）,并且从铁丝坳组的底部到顶部整体上显示上升的趋势,表明在冰室气候环境下,化学风化作用并未完全停止[8]。铁丝坳组δ13Ccarb的升高—降低趋势也反映了当时的水体不是封闭的环境,而是开放的环境,海洋中的碳循环受到扰动。如果是封闭的环境,那么随着海水中的无机碳通过光合作用固定在有机质中,那么水体中将会越来越富集13C,导致DIC库的δ13Ccarb值逐渐升高,这与实际情况相悖。以上证据均支持“冰水地球”假说[10],认为在Sturtian冰期这种极端气候条件下,部分地区仍然存在开放水域,陆地和海洋之间的物质交换仍然存在,生物地球化学循环仍然比较活跃。

4.   结论

  • （1） Sturtian冰期期间,南华盆地中的DIC库明显缺乏13C,具有较低的δ13C值（-7‰左右）,铁丝坳组具有明显的δ13Ccarb负偏现象。

    （2） Sturtian冰期期间,虽然光合作用在极端条件下仍在进行,但光合速率极低,铁丝坳组中保存下来的有机质主要是通过光合作用产生的,并且有机质中的碳主要来源于海洋中的DIC库。

    （3） Sturtian冰期期间,地球并未被冰川完全覆盖,存在开放水域,生物地球化学循环过程仍然比较活跃。

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