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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(6): 2054-2065

WANG ZeTang, WANG ChaoYong, DONG ZaiTian, ZHU YuXuan. A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2054-2065. doi: 10.14027/j.issn.1000-0550.2023.036
Citation: WANG ZeTang, WANG ChaoYong, DONG ZaiTian, ZHU YuXuan. A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2054-2065. doi: 10.14027/j.issn.1000-0550.2023.036
shu

A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China

doi: 10.14027/j.issn.1000-0550.2023.036
  • 1.

    Xi’an University of Science and Technology, Collage of Geology and Environment, Xi’an, 710054, China

  • 2.

    School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China

  • 3.

    School of Mining, Liaoning Technical University, Fuxin, Liaoning 123000, China

Funds:

National Natural Science Foundation of China 41772129

  • Received Date: 2023-01-26
  • Accepted Date: 2023-06-09
  • Rev Recd Date: 2023-04-28
    • Available Online: 2023-06-09
  • Publish Date: 2024-12-10
  • Methods A total of 47 shale samples from the Tianlin and Shaba sections, which were deposited in different regions of the Sichuan Basin during this period （23 samples from the Tianlin section, and 24 samples from the Shaba section）, were selected for organic carbon （δ13Corg） and pyrite sulfur isotope （δ34Spy） analysis. The system dynamics software Vensim7.3.5 was used to reconstruct the model and analyze the dynamics. Results The main results are as follows： （1） Both δ13Corg and δ34Spy exhibited synchronous positive shifts before the Hirnantian glacial period, reached the peak value in the Hirnantian glacial period, and then decreased. The synchronous positive deviation of C and S isotopes can occur when the burial fluxes of organic carbon were 1.5 to 2.0 times, and pyrite 1.5 to 3.0 times, that of the pre-glacial period. This indicates that the high primary productivity and widely developed anoxic water column of iron were beneficial to the burial of sedimentary organic matter and pyrite. During the Hirnantian glacial period, with the decreased primary productivity and increased oxygen content in water column, the burial flux of organic carbon and pyrite decreased gradually, and the carbon and sulfur isotopes were slightly negative. The intensity of volcanic activity mainly affects the migration of δ13C, and the weakened volcanic activity was beneficial to the positive deviation of δ13C, while the enhanced volcanic activity after the glacial period was the necessary condition for the significant negative deviation of δ13C over a short time （0.5-1.0 Myr）. （2） The lower seawater sulfate concentration （approximately 5 mM） in the early stage of the Hirnantian glacial period was the basis of the positive shift of δ34S, with an increase of pyrite burial flux, the ocean sulfate concentration gradually decreased to 3 mM and reached the lowest during the Hirnantian glacial period, while the increase of terrestrial sulfate input flux in the post-glacial period was the main factor leading to the negative deviation of δ34S, making the ocean sulfate concentration gradually return to 5 mM. Conclusions The results quantitatively evaluate the main influencing factors of δ13C and δ34S synchronous migration. A new idea and quantitative prediction model are provided for the study of the kinetics of C and S isotope fractionation throughout geological history. [Obective] The significant migration events of carbon and sulfur isotopes （δ13C, δ34S） in the global transition period from the Late Ordovician to the Early Silurian have been widely recognized, but the causes of these isotopes are still controversial. Quantitative numerical simulation is one of the ways to solve this problem.
  • [1] Poulton S W, Canfield D E. Ferruginous conditions: A dominant feature of the ocean through Earth's history J]. Elements, 2011, 7(2): 107-112.
    [2] Gomez-Saez G V, Dittmar T, Holtappels M, et al. Sulfurization of dissolved organic matter in the anoxic water column of the Black Sea[J]. Science Advances, 2021, 7(25): eabf6199.
    [3] Wang C Y, Dong Z T, Fu X H, et al. Origin and paleoenvironment of organic matter in the Wufeng–Longmaxi shales in the northeastern Sichuan Basin[J]. Energy Exploration & Exploitation, 2021, 39(1): 134-155.
    [4] 何龙,王云鹏,陈多福. 四川盆地晚奥陶世有机碳、氮同位素异常及其古环境意义[J]. 地球化学,2021,50（6）：623-634.

    He Long, Wang Yunpeng, Chen Duofu. Organic carbon and nitrogen isotopic anomalies during Late Ordovician in Sichuan Basin, and their implications for the paleoenvironment[J]. Geochemistry, 2021, 50(6): 623-634.
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    Chen Xu, Rong Jiayu, Fan Junxuan, et al. A global correlation of biozones across the Ordovician-Silurian boundary[J]. Acta Palaeontologica Sinica, 2000, 39(1): 100-114.
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    [15] Cooper R A, Sadler P M, Contributors, et al. Chapter 20–the Ordovician period[M]//Gradstein F M, Ogg J G, Schmitz M D, et al. The geologic time scale. Amsterdam: Elsevier, 2012: 489-523.
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    [18] Kozik N P, Gill B C, Owens J D, et al. Geochemical records reveal protracted and differential marine redox change associated with Late Ordovician climate and mass extinctions[J]. AGU Advances, 2022, 3(1): e2021AV000563.
    [19] Dong Z T, Wang Z T, Zhang W L, et al. Dynamic sulfur and carbon cycles related to microbial sulfate reduction and volcanic activity during the Hirnantian glaciation in the Upper Yangtze Basin, South China[J]. Frontiers in Earth Science, 2022, 10: 971031.
    [20] Dong Z T, Wang Z T, Zhang W L, et al. Distribution characteristics and genesis of marine anoxic conditions in the southwest of the Upper Yangtze Basin during the Late Ordovician-Early Silurian, South China[J]. Frontiers in Earth Science, 2022, 10: 934488.
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  • Received:  2023-01-26
  • Revised:  2023-04-28
  • Accepted:  2023-06-09
  • Published:  2024-12-10

A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China

doi: 10.14027/j.issn.1000-0550.2023.036
  • 1. Xi’an University of Science and Technology, Collage of Geology and Environment, Xi’an, 710054, China
  • 2. School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China
  • 3. School of Mining, Liaoning Technical University, Fuxin, Liaoning 123000, China
Funds:

National Natural Science Foundation of China 41772129

  • Received Date: 2023-01-26
  • Accepted Date: 2023-06-09
  • Rev Recd Date: 2023-04-28
  • Available Online: 2023-06-09
  • Publish Date: 2024-12-10

Abstract: Methods A total of 47 shale samples from the Tianlin and Shaba sections, which were deposited in different regions of the Sichuan Basin during this period （23 samples from the Tianlin section, and 24 samples from the Shaba section）, were selected for organic carbon （δ13Corg） and pyrite sulfur isotope （δ34Spy） analysis. The system dynamics software Vensim7.3.5 was used to reconstruct the model and analyze the dynamics. Results The main results are as follows： （1） Both δ13Corg and δ34Spy exhibited synchronous positive shifts before the Hirnantian glacial period, reached the peak value in the Hirnantian glacial period, and then decreased. The synchronous positive deviation of C and S isotopes can occur when the burial fluxes of organic carbon were 1.5 to 2.0 times, and pyrite 1.5 to 3.0 times, that of the pre-glacial period. This indicates that the high primary productivity and widely developed anoxic water column of iron were beneficial to the burial of sedimentary organic matter and pyrite. During the Hirnantian glacial period, with the decreased primary productivity and increased oxygen content in water column, the burial flux of organic carbon and pyrite decreased gradually, and the carbon and sulfur isotopes were slightly negative. The intensity of volcanic activity mainly affects the migration of δ13C, and the weakened volcanic activity was beneficial to the positive deviation of δ13C, while the enhanced volcanic activity after the glacial period was the necessary condition for the significant negative deviation of δ13C over a short time （0.5-1.0 Myr）. （2） The lower seawater sulfate concentration （approximately 5 mM） in the early stage of the Hirnantian glacial period was the basis of the positive shift of δ34S, with an increase of pyrite burial flux, the ocean sulfate concentration gradually decreased to 3 mM and reached the lowest during the Hirnantian glacial period, while the increase of terrestrial sulfate input flux in the post-glacial period was the main factor leading to the negative deviation of δ34S, making the ocean sulfate concentration gradually return to 5 mM. Conclusions The results quantitatively evaluate the main influencing factors of δ13C and δ34S synchronous migration. A new idea and quantitative prediction model are provided for the study of the kinetics of C and S isotope fractionation throughout geological history. [Obective] The significant migration events of carbon and sulfur isotopes （δ13C, δ34S） in the global transition period from the Late Ordovician to the Early Silurian have been widely recognized, but the causes of these isotopes are still controversial. Quantitative numerical simulation is one of the ways to solve this problem.

WANG ZeTang, WANG ChaoYong, DONG ZaiTian, ZHU YuXuan. A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2054-2065. doi: 10.14027/j.issn.1000-0550.2023.036
Citation: WANG ZeTang, WANG ChaoYong, DONG ZaiTian, ZHU YuXuan. A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin, China[J]. Acta Sedimentologica Sinica, 2024, 42(6): 2054-2065. doi: 10.14027/j.issn.1000-0550.2023.036
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0.   引言

  • 海洋水体或沉积物孔隙水中C、S元素的生物地球化学循环,是控制沉积环境演化和有机质富集的主要影响因素之一。古水体的氧化还原条件控制了C、S元素循环和同位素的分馏过程[1⁃3]。奥陶纪—志留纪过渡时期（缩写为OST,下同）出现了显著的碳同位素（δ13C）、硫同位素（δ34S）偏移现象,其中在赫南特冰期出现的δ13C的正偏被称为赫南特碳同位素偏移（Hirnantian Isotopic Carbon Excursion,HICE）[4],与该时期盛行的冰川作用和奥陶纪末期的生物大规模灭绝事件相吻合[5⁃7]。对于耦合的δ13C、δ34S正偏,常认为是在缺氧条件下有机碳、黄铁矿的共同强化埋藏造成的[8⁃9]。然而,对该时期古海洋C、S同位素的分馏过程及其动力学研究尚未形成定论,研究区C、S同位素分馏过程及其模拟研究也鲜见报道。鉴于此,选取四川盆地南部OST时期沉积于不同区域的田林剖面、沙坝剖面为研究对象（图1）,在δ13C、δ34S分析测试的基础上,运用Vensim 7.3.5系统动力学软件模拟和讨论C、S同位素分馏过程与动力学机制,定量评价有机碳埋藏通量和硫酸盐输入量对C、S同位素分馏的影响。

    Figure 1.  Paleogeographic background and location of the study area (modified from references [10⁃11])

1.   研究方法与结果

    1.1.   研究方法

  • 在中国地质大学生物地质与环境地质国家重点实验室对田林剖面（23个）与沙坝剖面（24个）页岩样品分别完成黄铁矿硫同位素（δ34Spy）和有机碳同位素（δ13Corg）测试工作。其中δ34Spy采用Canfield et al.[12]提出的铬还原法,在隔氧、密闭条件下,将粉末状样品与酸化的CrCl2溶液反应,黄铁矿硫反应生成为H2S后,被硝酸银捕获而沉淀为Ag2S。随后对生成的Ag2S使用Thermo Finnigan MAT 253气体稳定同位素比值质谱仪进行测量,测量校正利用IAEA S1（δ34S=-0.3‰）、IAEA S2（δ34S=22.65‰）和IAEA S3（δ34S=-32.5‰）,分析误差小于0.2‰。有机碳同位素（δ13Corg）使用Thermo Finnigan MAT 253对除酸和漂洗后的样品进行测定,分析误差小于0.1‰。

    • 1.2.   结果

  • 田林剖面的δ13Corg介于-30.80‰~-29.60‰,平均为-30.30‰,最大变化幅度为1.20‰（表1、图2）。从D. complexus段到P. pacificus段,δ13Corg值呈现一定的波动,介于-30.80‰~-30.20‰,平均为-30.50‰；在赫南特期,δ13Corg值从M. extraordinariu段逐渐增加,并在Himantia Fauna段（即观音桥组）达到最大值,出现明显正偏,介于-30.50‰~-29.60‰,平均为-30.10‰,正偏幅度为0.90‰；到M. persulptus段时正偏开始回落,并持续到A. ascensus段,δ13Corg介于-30.60‰~-30.30‰,平均为-30.50‰。

    笔石带样品编号累计厚度/mδ34Spy/‰δ13Corg/‰笔石带样品编号累计厚度/mδ34Spy/‰δ13Corg/‰
    田林剖面沙坝剖面
    A. ascensusTL-2312.10-5.5-30.6A. ascensusSB-248.89-0.78-30.3
    TL-2211.85-6.4-30.6SB-238.24-1.20-29.0
    TL-2111.33-1.5-30.3SB-227.93-3.70-29.7
    M. persulptusTL-2010.88-7.4-30.5SB-217.75-10.20-30.9
    TL-1910.45-4.4-30.4SB-207.37-8.20-30.2
    H.FTL-1810.3611.7-29.6M. persulptusSB-197.22-9.80-29.6
    TL-1710.256.8-29.8SB-186.94-3.40-28.9
    TL-1610.105.9-29.8SB-176.43-9.60-29.4
    M. extra.TL-159.942.3-30.4H.FSB-166.023.10-27.2
    TL-149.281.1-30.5SB-155.942.40-28.5
    TL-138.95-0.9-29.8M. extra.SB-145.88-10.60-30.9
    TL-127.54-4.2-30.2SB-135.64-7.30-30.2
    TL-116.401.1-30.4P. pacificusSB-125.58-5.10-29.5
    P. pacificusTL-105.88-2.9-30.6SB-114.29-10.60-30.9
    TL-095.32-3.8-30.5SB-103.63-9.10-30.3
    TL-084.83-5.9-30.8SB-93.47-6.60-30.1
    TL-074.051.7-30.4SB-83.01-9.80-30.5
    TL-063.30-3.1-30.2SB-72.69-6.40-29.7
    D. complexusTL-052.45-5.5-30.6SB-62.28-9.20-30.4
    TL-041.901.8-30.5SB-51.88-6.40-29.8
    TL-031.352.4-30.3SB-41.64-10.40-30.8
    TL-020.84-9.6-30.8SB-31.02-12.20-31.0
    TL-010.30-5.4-30.2SB-20.76-18.60-31.1
    SB-10.15-10.20-30.6

    Table 1.  Results of δ13Corg and δ34Spy in the Tianlin and Shaba shale samples

    Figure 2.  Variation characteristics of δ13Corg and δ34Spy in the Tianlin and Shaba sections during the OST period

    与田林剖面相比,沙坝剖面的δ13Corg变化范围稍大,为-31.10‰~-27.20‰,平均为-29.98‰,最大变化幅度达3.90‰（表1、图2）。δ13Corg在各地层的变化与田林剖面具有一定相似性:从D. complexus段到P.pacificus段,δ13Corg值呈现一定波动,介于-31.10‰~-29.50‰,平均为-30.39‰；赫南特期从M. extraordinariu段逐渐增加,并在Himantia Fauna段达到最大值,出现明显正偏,其值介于-30.90‰~-27.20‰,平均为-29.20‰,正偏幅度达3.70‰；到M. persulptus段时正偏开始回落,并持续到A. ascensus段,其值介于-30.90‰~-28.90‰,平均为-29.75‰。

    与有机碳同位素呈现的变化特征类似,研究区田林剖面和沙坝剖面的δ34Spy也呈现出在赫南特时期正偏的特征（图2）。对于田林剖面,δ34Spy介于-9.60‰~11.70‰,平均为-1.38‰,最大变化幅度为21.30‰（表1、图2）。从D. complexus段到P. pacificus段,δ34Spy值呈现一定的波动,介于-9.60‰~2.40‰,平均为-3.03‰；在赫南特期,从M. extraordinariu段逐渐增加,并在Himantia Fauna段达到最大值,并出现明显正偏,其值介于-4.20‰~11.70‰,平均为2.98‰,正偏幅度达15.90‰；到M. persulptus段时正偏开始回落,并持续到A. ascensus段,其值介于-7.40‰~-1.50‰,平均为-5.04‰。

    与田林剖面相比,沙坝剖面的δ34Spy值较低,介于-18.60‰~3.10‰,平均为-7.24‰,最大变化幅度达21.70‰（表1、图2）。δ34Spy在各地层的变化与田林剖面具有一定相似性：从D. complexus段到P. pacificus段,δ34Spy值呈现一定的波动,且具有缓慢升高的趋势,其值介于-18.60‰~-5.10‰,平均为-9.55‰；在赫南特期,从M. extraordinariu段逐渐增加,并在Himantia Fauna段达到最大值,出现明显正偏,其值介于-10.60‰~3.10‰,平均为-3.10‰,正偏幅度达13.70‰；到M. persulptus段时正偏开始回落,并持续到A. ascensus段,其值介于-10.20‰~-0.78‰,平均为-5.86‰。

    田林剖面和沙坝剖面在赫南特期所展示的这种δ34Spy正偏的变化特征与晚寒武世发生的C、S同位素正漂移类似（Steptoean Positive Carbon Isotope Excursion,SPICE）,且在同时期的丹麦Billegrav[9]、苏格兰Dob’s Linn[9]、Anticosti岛[7]相似的硫同位素正偏。

2.   模型的构建

  • 海洋中的C、S元素的储库是一个动态的变化过程,当输入和输出量不相等时,会造成溶解在水体中的无机碳和硫酸盐的通量发生一定变化。为便于对比不同输入量状况下碳、硫同位素含量波动的影响因素,采用传统的同位素质量平衡方程进行讨论,即单一储层模型[4]。海洋储库中C、S通量相对于时间（t）的变化,可使用下列偏微分方程表示：

    ∂M0C/∂t=（FWC+FMC）-（Fcarb+Forg） (1)
    ∂M0S/∂t=（FWS+FMS）-（Fevap+Fpy） (2)

    式中：M0C和M0S分别表示海洋水体中溶解的无机碳（DIC）和硫酸盐；FWC、FMC和FWS、FMS分别表示由陆源风化和火山活动输入至海洋的C、S通量；Fcarb和Forg分别表示碳酸盐和有机质的埋藏通量；Fevap和Fpy分别表示蒸发岩和黄铁矿的埋藏通量。

    将方程（1）、（2）与同位素数值相结合,可进一步表示出C、S同位素随通量变化的扰动情况,得出：

    ∂（M0Cδ0C）/∂t=F’WCδWC-Fcarbδ0C+Forg（δ0C-ΔB） (3)
    ∂（M0Sδ0S）/∂t=F’WSδWS-Fevapδ0S+Fpy（δ0S-εpy） (4)

    该方程忽略了C、S同位素之间的质量分馏。式中δ0C和δ0S分别表示海洋储库中的C、S同位素组成,为本次模拟的观测对象；δWC和δWS分别代表了陆源风化和火山活动输入总和的C、S同位素组成,按照现代海洋输入情况,此处初始赋值分别为-4.0‰和+5‰[13],当火山活动频繁时,可适当降低两数值；F’WC和F’WS分别表示陆源风化和火山活动输入的碳、硫的总通量；ΔB和εpy分别表示C、S同位素分馏,其数值在显生宙时期分别为-28‰和-35‰[13]。

    方程（3）、（4）与方程（5）、（6）结合,进一步求商和简化海洋C、S同位素的变化情况,可得其方程为：

    dδ0C/dt=[F’WC（δWC-δ0C）-ForgΔB]/M0C (5)
    dδ0S/dt=[F’WS（δWS-δ0S）-Fpyεpy]/M0S (6)

    方程（5）、（6）的推导结果可以用作C、S元素分馏动力学模型。

3.   参数选择与优化

  • 参照已知的数据给予同位素分馏动力学模型中的各项参数进行赋值（表2）。模型的时间设定为：将赫南特阶的持续时间限定在1.5 Myr以内,为该时期硫同位素持续正偏时间的上限[7]。整个模型的测试时间限定于5 Myr以内,在该时间内各模型曲线若能恢复到基线位置附近,即认为该模型有效。

    变量显生宙典型范围值HICE模型估算值
    M0S28 mM SO42-3~10 mM SO42-
    M0C2 mM DIC2 mM DIC
    FWS1.5×1018 mol/Myr1.5~4.5 ×1018 mol/Myr
    FWC25×1018 mol/Myr25×1018 mol/Myr
    δWC-4.0‰-4.0‰
    δWS8.0‰5.0‰
    △B-28.0‰-28.0‰
    εpy-3.05‰0~-40‰
    Fcarb20×1018 mol/Myr21×1018 mol/Myr
    Forg5×1018 mol/Myr4×1018 mol/Myr
    Fpy0.6×1018 mol/Myr0~1.1×1018 mol/Myr
    Fevap0.8×1018 mol/Myr0.4×1018 mol/Myr
    ts26 Myr0.3~4.6 Myr
    tc0.2 Myr0.2 Myr
    注：显生宙典型数值据文献[14⁃15]。

    Table 2.  Value of boundary parameters in the C, S cyclic coupling model

    为了模拟C、S循环过程中的通量变化情况,将海洋内溶解的无机碳（DIC）和硫酸盐的基础浓度分别设定为2 mM和2~12 mM（硫酸盐浓度按照3 mM、5 mM和10 mM三个梯度分别进行讨论）,均处于古生代海洋水体硫酸盐浓度界限内[15⁃17]。随后使用OST时期已发表的剖面数据对其进行校准（表3）。

    剖面位置晚凯田阶赫南特阶早鲁丹阶数据来源
    δ13Corgδ34Spyδ13Corgδ34Spyδ13Corgδ34Spy
    华南王家湾-30.8-27.0-27.67.0-30.3-23.0[8]
    华南南坝子-30.4-19.0-26.624.0-29.2-20.0
    华南双河-30.8-1.0-29.017.0-30.4-16.0[10]
    华南大田坝-31.1-11.0-27.613.0-30.7-26.0
    Billegrav-36.0-20.0-27.840.0-33.5-40.0[13]
    Dob’s Linn-33.5-36.0-29.47.0-35.6-40.0
    Anticosti Island-29.5-17.0-25.04.0-29.4-21.0[14]
    Monitor Range———23.0—0.0[15]
    Kardla Core—-28.0—37.0—-34.0
    华南田林-30.8-9.6-29.611.7-30.6-6.4本研究
    华南沙坝-31.1-18.6-27.23.1-30.9-10.2
    注：δ13Corg和δ34Spy分别在凯田阶晚期和鲁丹阶早期取最小值、赫南特阶取最大值。

    Table 3.  Numerical distribution of δ13Corg and δ34Spy for known sections

    在对硫同位素扰动分析过程中,所使用的边界数值均为多值函数,而不是单因素变量,即至少需要两个及两个以上参数的变化,才能在不超过1.50 Myr的时间内实现埋藏的黄铁矿硫同位素达到~30‰的偏移幅度[18]。该方案虽不能将所有可能存在的因素考虑在内,但也为不同变量提供了一个合理的一阶量化过程。在模拟中,除定量参数外主要对FWS、Fpy和εpy三个参数进行调整,三参数扰动过程模拟的起止时间为0.5~1.25 Myr（图3）。

    Figure 3.  Assignment changes of FWS, Fpy, and εpy for different initial sulfate concentrations

4.   模拟结果分析

    4.1.   碳循环特征及其定量分析

  • 在模拟的碳同位素偏移过程中,为呈现δ13Corg在赫南特冰期期间出现约+4‰的偏移量,从0.5 Myr开始将有机碳的埋藏通量（Forg）从初始的4.0×1018 mol/Myr增加至8.0×1018 mol/Myr,观察到δ13CDIC能够在1.0~1.5 Myr内即出现显著正偏移。同样的,减少Forg（从最高的8.0×1018 mol/Myr降低至4.0×1018 mol/Myr时）也可使δ13CDIC在对应时间内降低4‰。此时若增加火山活动输入的δ13C通量（即FWC）,δ13CDIC负偏所用时间则会更短（图4）。

    Figure 4.  Comparison of carbon isotope excursion before and after the Hirnantian glaciation

    结合研究区两剖面OST时期的地球化学数据推测,在赫南特冰期早期（即M. extraordinariu段）,较高的初级生产力和铁质缺氧水体的存在使得水体中的有机碳能够在短时间内大量埋藏,并逐渐引起δ13Corg的正偏移；到观音桥组沉积时期,随着海平面的不断下降,水体含氧量逐渐上升,有机碳埋藏速率大幅降低,此时δ13Corg达最大值并逐渐负偏。赫南特末期增强的火山作用,促进了δ13Corg偏移幅度,并缩短了其恢复至初始状态所需的时间。

    当然,此次模拟并没有考虑碳同位素生物分馏（即ΔB）的影响（由于该时期有机质为腐泥型有机质,可以看成无生物分馏）,因为仅通过调整Forg即可实现碳同位素的大幅扰动,ΔB是否参与已显得无关紧要。

    • 4.2.   硫循环特征及其定量分析

  • OST时期发现的硫同位素的最大偏移量远高于碳同位素,意味着硫酸盐储库更易受到沉积环境改变的影响。现代海洋中硫酸盐的浓度相对较为稳定,其值为28 mM,海水的硫酸盐硫同位素稳定在21‰左右。而OST时期多变的硫同位素组成说明海水硫酸盐在海洋中的滞留时间比现代海洋要短很多,且在不同区域有着不同的浓度变化。

    为研究该时期海洋硫同位素分馏的规律,对海水硫酸盐的初始浓度分别设定为3 mM、5 mM和10 mM,并尝试通过分别改变FWS、Fpy和εpy三个参数（图5）,定量判断和评价硫酸盐硫同位素的偏移特征（δ34Ssulfate）。

    Figure 5.  Effects of Fpy, FWS and εpy on sulfur isotope fractionation and SO42-concentration when the initial concentration of sulfate was 3 mM

    在海水硫酸盐初始浓度为3 mM时,通过无限增大Fpy的值来实现δ34Ssulfate~30‰的正偏,但也严重削弱了海水硫酸盐的浓度（图5a,e）,甚至使δ34Ssulfate达到负值,这显然是不合理的。同样的,仅通过改变FWS或εpy也很难达到δ34Ssulfate的正偏（图5b,c,f,g）。当将三个变化参数两两结合时,则达到了这种正偏幅度,特别是在25%FWS和250%Fpy状态时（图5d,h）,δ34Ssulfate值从30‰升高至58‰,偏移幅度最接近,但这种组合下硫酸根离子的浓度从3 mM降至-0.09 mM,其合理性受到质疑（图5d,h）。

    相比之下,50%FWS和200%Fpy组合模拟结果更接近于实际海洋环境的变化情况（图5d,h）：赫南特冰期前段,火山活动减弱、温度急剧下降[19⁃20],FWS逐渐降低,低纬度水域底部水体的缺氧条件暂未发生明显波动,Fpy相对较高,引起了δ34Ssulfate的正偏,并在观音桥组沉积时期达到峰值。需要注意的是,该组合下δ34Ssulfate曲线从最大值恢复到基线位置所用的时间远短于地层记录中所用的1.50 Myr,说明冰期后FWS和Fpy恢复至初始值（FWS升高1倍,Fpy降低1/2）即可实现δ34S的负偏移。当然,-35‰ εpy和150%Fpy组合下δ34S正偏值也接近于30‰,但仅反映Fpy单一参数的变化对δ34S偏移的影响[13]。

    在海水硫酸盐初始浓度为5 mM时,通过单一调整FWS、Fpy和εpy三个参数也很难达到δ34S的正偏幅度（图6a~c）。而将三个参数两两结合也未发现接近30‰的偏移量（图6d）。同样的问题也出现在海水硫酸盐浓度为10 mM的环境下（图7）。且假设在10 mM及以上浓度时,只有不合理的调整才会出现高达30‰的偏移,如黄铁矿的埋藏通量高于硫酸盐输入通量两个数量级,或硫酸盐输入通量成为负值（未列出）。

    Figure 6.  Effects of Fpy, FWSand εpy on sulfur isotope fractionation and SO42-concentrations when the initial concentration of sulfate was 5 mM

    Figure 7.  The effects of Fpy, FWS and εpy on sulfur isotope fractionation and SO42-concentration when the initial sulfate concentration was 10 mM

    为进一步验证FWS、Fpy和εpy三个参数对δ34S分馏的综合影响,在3 mM和5 mM海水硫酸盐浓度下将εpy的赋值从开始的-40‰调整为-10‰（调整区间从0.5 Myr至1.25 Myr）,并对应调整FWS、Fpy的赋值,此时两个浓度下均发现δ34S高达30‰的偏移（图8）。

    Figure 8.  Influences of the covariant relationship of Fpy and FWS on sulfur isotope fractionation and SO42- concentration under different sulfate concentrations

    相比之下,3 mM硫酸盐浓度仅需要较小的FWS、Fpy调整即可实现30‰的正偏移,且在短时间内偏移量即可复位至基线附近（图8a）,意味着赫南特冰期期间海水硫酸盐的浓度可能为3 mM,与基于流体包裹体和晚奥陶世硫同位素模拟的最低值相近[9]。但这似乎不能解释整个OST时期海水硫酸盐的浓度,因为上述现象是在εpy从-40‰调整至-10‰的状态下发生的,说明在赫南特冰期之前可能就已发生过或持续发生着黄铁矿相对大量埋藏的情况[17]。

    结合硫酸盐在5 mM条件下时的各参数变化情况,模拟结果显示非冰期期间水体硫酸根离子浓度应为5 mM,且在凯迪阶晚期随着缺氧水体的发育和相对较强的Fpy使得海水硫酸盐浓度逐渐由5 mM下降至3 mM（图8d）。赫南特冰期前段受Fpy持续增加的影响,硫酸盐浓度再次下降,硫同位素分馏逐渐正偏,并在观音桥组沉积时期达到峰值；赫南特冰期后段,随着温度的升高,海平面迅速上升,陆源风化输入增强,硫同位素分馏减弱,硫酸盐浓度逐渐恢复到非冰期的5 mM。

    C、S同位素循环模型表明,在0.5~1.5 Myr时间内,当有机碳的埋藏通量增加至原来的1.5~2.0倍,黄铁矿埋藏通量增加至原来的1.5~3.0倍,叠加于输入通量的降低（0.9~0.75倍的FWS）,即可出现C、S同位素同步正偏的现象；当黄铁矿埋藏通量降低至原来的0.50~0.75倍,火山作用或陆源风化作用参与的FWS增加1.10~1.25倍的前提下,则会在短时间内（0.75 Myr）出现δ34S负偏现象。

    为了检验模型的是否符合当时的古化学环境,选取了距离田林剖面与沙坝剖面较远位置处于开阔陆棚比邻秦岭海的WC1数据进行验证[3]。该剖面的有机碳含量平均值4.74%,有机碳的埋藏通量比上述剖面高1.5~2.0倍,有机碳积累速率最高达到109.31 mg/cm2/kyr,总硫含量为1.15%,总铁含量为2.91%,同时数据显示DOP（黄铁矿矿化度）数值较低为缺氧环境,反映了水体硫含量处于较低的水平造成硫酸盐浓度的下降,测试结果显示该剖面的硫同位素大多数处于正偏状态,δ34S的平均值达到10.6‰,检验了模型是有效的。

5.   结论

  • （1） 以田林剖面与沙坝剖面测试数据为约束条件以及利用前人研究结果进行检验,建立了C、S分馏动力学模型。

    （2） C、S同位素循环模型表明,在0.5~1.5 Myr时间内,当有机碳的埋藏通量增加至原来的1.5~2.0倍,黄铁矿埋藏通量增加至原来的1.5~3.0倍,叠加于输入通量的少量降低（0.9~0.75倍的FWS）,即可出现C、S同位素同步正偏的现象；较高的初级生产力和铁质缺氧水体的存在使得水体中的有机碳能够在短时间内大量埋藏,并逐渐引起δ13Corg的正偏移；观音桥组沉积时期,随着海平面的不断下降,水体含氧量逐渐上升,有机碳埋藏速率大幅降低,此时δ13Corg达最大值并逐渐负偏。赫南特末期增强的火山作用,促进了δ13Corg偏移幅度,并缩短了其恢复至初始状态所需的时间。

    （3） 结合硫酸盐在5 mM条件下时的各参数变化情况发现,非冰期期间水体硫酸根离子浓度应为5 mM,且在凯迪阶晚期随着缺氧水体的发育和相对较强的Fpy使得海水硫酸盐浓度逐渐由5 mM下降至3 mM。赫南特冰期前段受Fpy持续增加的影响,硫酸盐浓度再次下降,硫同位素分馏逐渐正偏,并在观音桥组沉积时期达到峰值；赫南特冰期后段,随着温度的升高,海平面迅速上升,陆源风化输入增强,硫同位素分馏减弱,硫酸盐浓度逐渐恢复到非冰期的5 mM。研究表明硫酸盐输入量对硫同位素分馏具有重要的影响,是造成不同区域硫同位素差异的重要原因之一。

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