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Volume 42 Issue 4
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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(4): 1212-1228

CHEN YuChao, JIN Xin, DU YiXing, ZHANG YunWang, LI BinBing, SHI ZhiQiang. Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1212-1228. doi: 10.14027/j.issn.1000-0550.2022.147
Citation: CHEN YuChao, JIN Xin, DU YiXing, ZHANG YunWang, LI BinBing, SHI ZhiQiang. Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1212-1228. doi: 10.14027/j.issn.1000-0550.2022.147
shu

Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin

doi: 10.14027/j.issn.1000-0550.2022.147
cstr: 32268.14.cjxb.62-1038.2022.147
  • 1.

    Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China

  • 2.

    State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China

  • 3.

    State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China

Funds:

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences SKLLQGZR2005

  • Received Date: 2022-08-24
  • Accepted Date: 2022-11-24
  • Rev Recd Date: 2022-11-16
    • Available Online: 2022-11-24
  • Publish Date: 2024-08-10

  • Objective    The paleoclimate and environment have changed significantly across the Norian-Rhaetian transition （NRT, Late Triassic）, as demonstrated by carbon-isotope fluctuations and biological extinction events. However, the causes of climate perturbations and biotic crises during the NRT remain controversial. It is believed that the eruption of contemporaneous volcanisms （e.g., the Angayucham Large Igneous Province） was the main cause of paleoenvironmental changes during the NRT. The large-scale volcanic activity released a large amount of greenhouse gases, which resulted in global temperature rise, carbon-isotope perturbation, and biological crises during the NRT. At present, the majority of NRT studies have focused on the shallow marine strata in the Tethys region, but knowledge on the changes in terrestrial paleoclimate, paleoenvironment, and their driving mechanism during the NRT is extremely limited. Studies have shown that terrestrial strata can faithfully record paleoclimatic and paleo-environmental changes during geological events, such as the end-Permian mass extinction. However, previous studies mainly focused on paleobotany, sedimentology, organic carbon isotopes, and wildfires during the NRT in the Sichuan Basin, but lacked element geochemical evidence, limiting the accurate understanding of the climatic changes in this time interval and the comparison between different research methods.     Methods    To tackle this scientific question, we examined the Norian-Rhaetian section （Xujiahe section） located 4.5 km NE of Guangyuan city, northwest Sichuan Basin. Thirty-four samples were collected at a resolution of 10 cm to 2 m in the Xujiahe section for major and trace element compositions. The surface dust and weathered portions of samples were removed with a rasper and then washed with deionized water. After 8 hours of oven drying at 50 °C, samples were ground into powder using agate mortars. Sample preparations were completed in the School of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology. Analysis of major and trace elements in samples was completed at the Institute of Earth Environment, Chinese Academy of Sciences, Xi 'an. A glass bead was created by fusing 0.6 g of the powdered sample with 6 g of dry lithium tetraborate （Li2B4O7） for 5 minutes at 1 000 °C. The glass bead was further scanned by an X-ray fluorescence spectrometer （WD-XRF； PANalytical, Ea Almelo, The Netherlands）. The analytical accuracy was better than 2%.     Results and Discussions    The analyzed samples have high Chemical Index of Alteration （CIA） values and Rb/Sr at 105.5, 110.2-122.5, 119, 123.4-123.45, and 137.7 m in the Xujiahe section, and low CIA values and Rb/Sr at 107, 109.5, 115.5-116.5, 121.5, and 127-135m. The uppermost part of Upper Norian successions （105.5-129.5m） have CIA values ranging from 59 to 82, with a mean of 73； Rb/Sr values ranging from 0.2 to 2.6, with a mean of 1.2； and R values ranging from 3.8 to 16.9, with a mean of 8.2. For the Norian-Rhaetian boundary interval （NRB, 129.5-135 m）, CIA values range from 59 to 63, with a mean of 60； Rb/Sr values range from 0.5 to 0.6, with a mean of 0.5； and R values range from 13.1 to 13.9, with a mean of 13.5.     Conclusions    Results show that the climate fluctuated frequently during the NRT of Xujiahe section in the Sichuan Basin. The Late Norian was dominated by warm and humid climate, which was interrupted by a short-term cooling event close to the NRB. The prevailing mega-monsoon in Pangaea during the Late Triassic may be the main trigger for the frequent climate change and NRB cooling events in the Xujiahe section, but the influence of volcanic activity and wildfire events on the paleoclimate system cannot be eliminated in this time period. To determine the precise timing of volcanic eruptions and wildfires during the NRT and how they contributed to climate change, more research is needed.

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  • Received:  2022-08-24
  • Revised:  2022-11-16
  • Accepted:  2022-11-24
  • Published:  2024-08-10

Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin

doi: 10.14027/j.issn.1000-0550.2022.147
  • 1. Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
  • 2. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
  • 3. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
Funds:

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences SKLLQGZR2005

  • Received Date: 2022-08-24
  • Accepted Date: 2022-11-24
  • Rev Recd Date: 2022-11-16
  • Available Online: 2022-11-24
  • Publish Date: 2024-08-10

Abstract: 

Objective    The paleoclimate and environment have changed significantly across the Norian-Rhaetian transition （NRT, Late Triassic）, as demonstrated by carbon-isotope fluctuations and biological extinction events. However, the causes of climate perturbations and biotic crises during the NRT remain controversial. It is believed that the eruption of contemporaneous volcanisms （e.g., the Angayucham Large Igneous Province） was the main cause of paleoenvironmental changes during the NRT. The large-scale volcanic activity released a large amount of greenhouse gases, which resulted in global temperature rise, carbon-isotope perturbation, and biological crises during the NRT. At present, the majority of NRT studies have focused on the shallow marine strata in the Tethys region, but knowledge on the changes in terrestrial paleoclimate, paleoenvironment, and their driving mechanism during the NRT is extremely limited. Studies have shown that terrestrial strata can faithfully record paleoclimatic and paleo-environmental changes during geological events, such as the end-Permian mass extinction. However, previous studies mainly focused on paleobotany, sedimentology, organic carbon isotopes, and wildfires during the NRT in the Sichuan Basin, but lacked element geochemical evidence, limiting the accurate understanding of the climatic changes in this time interval and the comparison between different research methods.     Methods    To tackle this scientific question, we examined the Norian-Rhaetian section （Xujiahe section） located 4.5 km NE of Guangyuan city, northwest Sichuan Basin. Thirty-four samples were collected at a resolution of 10 cm to 2 m in the Xujiahe section for major and trace element compositions. The surface dust and weathered portions of samples were removed with a rasper and then washed with deionized water. After 8 hours of oven drying at 50 °C, samples were ground into powder using agate mortars. Sample preparations were completed in the School of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology. Analysis of major and trace elements in samples was completed at the Institute of Earth Environment, Chinese Academy of Sciences, Xi 'an. A glass bead was created by fusing 0.6 g of the powdered sample with 6 g of dry lithium tetraborate （Li2B4O7） for 5 minutes at 1 000 °C. The glass bead was further scanned by an X-ray fluorescence spectrometer （WD-XRF； PANalytical, Ea Almelo, The Netherlands）. The analytical accuracy was better than 2%.     Results and Discussions    The analyzed samples have high Chemical Index of Alteration （CIA） values and Rb/Sr at 105.5, 110.2-122.5, 119, 123.4-123.45, and 137.7 m in the Xujiahe section, and low CIA values and Rb/Sr at 107, 109.5, 115.5-116.5, 121.5, and 127-135m. The uppermost part of Upper Norian successions （105.5-129.5m） have CIA values ranging from 59 to 82, with a mean of 73； Rb/Sr values ranging from 0.2 to 2.6, with a mean of 1.2； and R values ranging from 3.8 to 16.9, with a mean of 8.2. For the Norian-Rhaetian boundary interval （NRB, 129.5-135 m）, CIA values range from 59 to 63, with a mean of 60； Rb/Sr values range from 0.5 to 0.6, with a mean of 0.5； and R values range from 13.1 to 13.9, with a mean of 13.5.     Conclusions    Results show that the climate fluctuated frequently during the NRT of Xujiahe section in the Sichuan Basin. The Late Norian was dominated by warm and humid climate, which was interrupted by a short-term cooling event close to the NRB. The prevailing mega-monsoon in Pangaea during the Late Triassic may be the main trigger for the frequent climate change and NRB cooling events in the Xujiahe section, but the influence of volcanic activity and wildfire events on the paleoclimate system cannot be eliminated in this time period. To determine the precise timing of volcanic eruptions and wildfires during the NRT and how they contributed to climate change, more research is needed.

CHEN YuChao, JIN Xin, DU YiXing, ZHANG YunWang, LI BinBing, SHI ZhiQiang. Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1212-1228. doi: 10.14027/j.issn.1000-0550.2022.147
Citation: CHEN YuChao, JIN Xin, DU YiXing, ZHANG YunWang, LI BinBing, SHI ZhiQiang. Paleoclimate Perturbation and Its Driving Mechanism Across Norian-Rhaetian Transition (Late Triassic) in the Xujiahe Formation, Sichuan Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1212-1228. doi: 10.14027/j.issn.1000-0550.2022.147
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0.   引言

  • 在晚三叠世诺利—瑞替期之交（Norian-Rhaetian transition,NRT）,古环境发生了显著变化[1⁃2],同时生物灭绝和更替也更加频繁[3⁃4]。然而,NRT时期生物灭绝和更替的原因仍然具有争议[4⁃6]。同时期的火山活动（如Angayucham大火成岩省爆发）被认为是导致NRT时期生物灭绝的主要原因[2,4],大规模的火山活动释放的大量温室气体,会导致全球温度上升,甚至出现极端气候,最终可能导致NRT时期的生物危机[7⁃10]。

    上述观点主要来自于NRT时期的海相地层记录,却极少涉及陆相地层。研究表明陆相地层也能够有效记录地质事件过程中的古气候—环境变迁,如二叠纪末生物大灭绝事件[11]。四川盆地位于中国西南地区,发育了上三叠统—第四系陆相地层[12],其中上三叠统须家河组发育连续的诺利—瑞替阶地层[2,13]。此外,四川盆地记录到了诺利—瑞替期有机碳同位素的多期次波动和汞（Hg）元素的浓度异常,表明四川盆地在此时期可能受到火山作用影响,并导致古气候—环境发生了明显变化[2]。然而,前人对四川盆地须家河组诺利—瑞替期的古气候重建工作主要集中于古植物、沉积学研究和碳同位素研究[2⁃3,14⁃16],而缺乏元素地球化学证据,从而限制了对四川盆地NRT时期气候变化的精确认识以及不同研究方法之间的相互比较。

    本文以四川盆地须家河剖面为研究对象,对研究剖面的须家河组岩石样品进行了主微量元素分析,并利用相关元素地球化学指标,重建了四川盆地NRT时期的古气候记录,并讨论了其可能的驱动机制,为探讨NRT时期生物灭绝和更替的原因提供启示。

1.   地质背景

  • 四川盆地位于华南板块西北部,是大型的陆相含油气盆地[17⁃18]。盆地内发育一套元古代至中三叠世的海相碳酸盐岩[19],上覆上三叠统至第四系陆相地层[12]。中三叠统与上三叠统之间的不整合,标志着华南板块西缘逐步由被动大陆边缘向前陆盆地转换[19],地势由西高东低变为东高西低,海水从盆地西缘退出,碳酸盐岩台地阶段结束[20⁃21]。四川盆地在晚三叠世时期处于特提斯东缘[22],而现在盆地东面和南面为滇黔川鄂台褶带,西面为松潘—甘孜褶皱带和龙门山断褶带,北面为米仓山隆起和秦岭断褶系[23]（图1）。

    Figure 1.  Map showing the tectonic background of the Sichuan Basin and general paleogeography of the Norian

    四川盆地上三叠统发育马鞍塘组、小塘子组和须家河组。有学者将小塘子组等同于须家河组一段[17,24],主要由砂岩和泥岩夹层为主的河流相、湖泊相和沼泽相地层组成[17]。前人对须家河组的划分提出过多种不同的方案,最少的将其划分为四段,最多的则将其划分为七段[25⁃27]。本文使用夏宗实等[28]采用的地层划分方案,自下而上将须家河组分为须一段至须六段,其中须一段、须三段、须五段以灰色泥岩为主,伴有丰富的植物化石和煤层,沉积相主要为湖泊相或沼泽相；须二段、须四段、须六段以粗粒砂岩为主,沉积相主要为河流相[18]。

    此次研究的须家河剖面,位于广元市东北方向4.5 km处的须家河村（图1）。剖面的~110.8 m至~115 m、~118.6 m至~120.6 m为泥灰质粉砂岩,~104.4 m至~106.5 m、~114.9 m至~118.6 m为细砂岩,块状粗砂岩主要位于~124.2 m至~137.5 m[2],见图2[2,4,13⁃14,29]。同时剖面中识别出两套煤层,分别为M1、M2；其中M1位于~104.4 m处,M2位于~123.5 m处,两者厚度约为10 cm。基于黑碳研究,发现M2记录有低温成因的古野火事件[2]（图2）。Jin et al.[2]还识别出至少四次有机碳同位素负漂并能够进行全球对比以及三次汞元素富集的层位（图2）。Jin et al.[2]认为须家河剖面中碳同位素负漂和汞元素富集可能与同时期的Angayucham大火成岩省爆发有关,但不排除区域长英质火山活动的影响。

    Figure 2.  Change of geochemical proxies in stratigraphic order across the Norian⁃Rhaetian transition in the Xujiahe Formation, Sichuan Basin

2.   诺利—瑞替阶界线（Norian⁃Rhaetian Boundary,NRB）研究

  • 瑞替期持续的时间一直存在争议,有长瑞替期和短瑞替期两种观点[30⁃32],这种差异在很大程度上是由NRB的生物地层标准的不同所引起的[32]。短瑞替期持续时间为~4 Myr,其底界以牙形石Mi. posthernsteini sensu stricto首现为标志[30⁃32]。诺利—瑞替阶的生物地层标准可与Newark盆地磁极性序列（国际标准磁极性序列）相对应,得出瑞替阶底界为205.7 Ma[30⁃31]（即短瑞替期）。此外,Maron et al.[29]还在意大利Pignola-Abriola剖面中提出了一个化学地层标准（即NRB处存在一个δ13Corg负漂,偏移幅度约6‰）且该标准可与Newark磁极性序列E20r（处于205.7 Ma位置）相对应。长瑞替期持续时间为~8 Myr,其底界以牙形石Mi. posthernsteini sensu lato首现为标志[32⁃34]。在奥地利Steinbergkogel剖面,其磁极性序列E3r和诺利—瑞替阶的生物地层标准可与Newark盆地磁极性序列E16r相对应,从而得出瑞替阶底界为209.5 Ma[34]（即长瑞替期）。Li et al.[13]在四川盆地须家河剖面进行了旋回地层学和磁性地层学研究,其能够与Newark磁极性序列和意大利Pignola-Abriola剖面的磁极性序列相对应[13,29,31]。基于此,Li et al.[13]将NRB限制在~135 m处。而根据海相诺利—瑞替期碳同位素波动特征和古生物化石组合[4],Jin et al.[2]将NRB限制在~129.5 m处,并依据Li et al.[13]的数据,计算出研究剖面的沉积时间跨度为~206 Ma至~205.7 Ma（图2）。

3.   样品采集和分析方法

  • 在须家河剖面,以不超过2 m的间隔距离在诺利—瑞替阶采集了34件样品,用于主微量元素测试。首先用锉刀处理样品,去除表面灰尘和风化部分,然后用去离子水冲洗,再将样品放在50 ℃的烤箱中烘干8 h,最后用玛瑙钵研磨成粉至200目以下。样品制备在成都理工大学材料与化学化工学院完成。

    34件样品的主、微量元素分析在中国科学院地球环境研究所完成。首先将大约0.6克样品粉末与6克干式四硼酸锂（Li2B4O7）充分混合,在1 000 ℃下熔融5 min,融成玻璃微珠,然后使用X射线荧光光谱仪（WD-XRF; PANalytical, Ea Almelo, The Netherlands）扫描玻璃微珠,分析精度优于2％。使用了两个国际标准（GSS-8和GSD-12）。样品主量元素、微量元素的测试分析结果分别见表1、2,其中K、Al、Zn、Ni、Rb元素含量引自文献[2]。

    高度/mSiO2Al2O3Fe2O3MgOCaONa2OK2OTiO2MnOP2O5
    105.5055.1718.226.522.503.530.684.170.760.040.14
    107.0056.1813.074.533.827.080.932.670.690.030.15
    107.4052.0912.284.793.999.940.902.600.640.040.15
    109.0562.9116.426.461.900.750.733.240.890.030.15
    109.5080.868.142.840.840.350.841.490.630.030.09
    110.2070.2713.254.401.510.540.802.430.980.030.10
    111.0058.2918.176.222.511.230.673.530.870.030.14
    112.5056.7317.587.702.611.800.713.900.810.060.17
    114.5055.5414.125.303.236.990.933.040.710.060.17
    115.0055.9811.764.602.399.580.982.420.630.070.15
    115.5061.1511.285.571.826.891.052.190.640.130.15
    116.5062.2910.774.061.468.110.972.180.550.050.14
    117.0058.3914.605.372.445.570.932.990.810.060.15
    119.0053.7016.355.862.256.610.663.680.620.050.16
    119.5048.0211.734.722.6914.090.852.450.600.100.14
    120.0040.129.993.982.7222.160.842.100.530.100.14
    120.5037.938.593.633.7323.030.791.750.450.090.12
    121.0038.539.795.034.2119.850.732.010.500.080.11
    121.5043.315.492.352.2924.690.901.130.320.060.09
    122.2048.7210.624.282.8515.100.842.210.540.040.12
    122.5038.839.333.604.4520.690.781.960.480.050.10
    123.4044.5919.953.051.323.660.623.140.610.010.05
    123.4557.5518.991.921.600.860.623.760.660.010.04
    123.7575.6512.451.670.620.270.982.520.710.010.05
    123.8573.4113.332.660.950.371.012.730.800.010.09
    124.0070.9413.753.671.270.530.992.730.770.020.12
    127.0064.047.833.141.558.771.071.800.420.050.11
    128.0064.107.903.162.318.121.241.820.420.050.11
    130.2066.458.123.221.487.351.181.920.420.050.12
    132.5061.477.983.182.448.771.201.890.430.050.12
    133.5066.068.533.551.257.741.042.000.430.040.12
    134.0062.197.742.832.339.111.211.840.400.060.11
    135.0062.917.792.851.8410.131.141.860.410.060.11
    137.7058.4513.674.521.846.380.783.160.680.030.15
    注：主量元素单位为%；表中K、Al和Zn元素含量引自文献[2]。

    Table 1.  Major element variations in the bulk rocks of the Norian⁃Rhaetian transition from the Xujiahe Formation

    高度/mVCrNiZnGaRbSrYZrNbBaPb
    105.50144.69123.7950.31108.1625.31173.94100.1824.73145.9120.07672.3837.00
    107.00135.5686.7745.8367.3717.49115.68120.9421.90204.9516.28393.4827.91
    107.4095.6087.3035.2975.4415.24114.18159.4825.92169.6311.57401.7823.12
    109.05160.32112.4760.2285.0920.49148.6074.8828.97229.0718.97460.2128.11
    109.5064.9848.0521.0162.9611.4961.7039.6225.05310.9313.22252.35—
    110.20103.3686.4555.2259.5018.88104.8063.2331.04376.8821.33387.7019.55
    111.00146.37104.7547.18119.1725.10177.3384.5130.27182.4418.60450.9634.84
    112.50138.19109.8376.49123.8921.78171.5586.9528.97156.5220.65577.0797.96
    114.50124.5587.9342.4991.2819.53129.44120.6230.60179.7318.68494.2637.55
    115.0087.7477.1441.5574.8118.45101.6194.1527.77187.8415.43382.9721.85
    115.5086.1679.8942.2865.6916.9694.1393.1923.97211.3614.16464.9430.56
    116.5072.0069.9537.5973.2414.2886.7479.3221.57176.2311.49407.9837.71
    117.00110.92105.9249.6992.7419.63126.4593.7225.38207.4521.01506.4540.88
    119.00129.5992.5944.7995.5821.45157.58113.1025.92128.9013.80603.5535.78
    119.50113.7573.0236.7576.6015.35106.20182.4527.23147.4115.91414.5040.01
    120.0077.8874.9249.1775.1312.4693.73237.1924.95128.4012.62359.1237.66
    120.5078.1962.0243.7456.5712.0379.76249.7919.94105.988.07293.3334.72
    121.0099.2766.6730.8164.0110.96101.61241.4320.60102.8811.08367.9442.51
    121.5049.1435.3618.5029.308.7146.84304.7519.51153.226.40212.9418.19
    122.20104.5270.8036.6556.3613.5399.12181.3918.53185.349.14328.9625.22
    122.5090.3658.8439.2656.888.1799.32275.5220.70127.009.07341.3628.45
    123.40182.4590.0522.054.7723.49231.51100.9261.59179.5325.24429.21104.19
    123.45124.3492.389.3216.2027.02191.0073.2918.97135.6017.33470.935.27
    123.7562.6767.627.2410.8515.24101.4153.0725.92328.6415.25395.3711.72
    123.8582.3989.8424.4532.1319.53113.1856.9825.05261.7919.12430.2615.56
    124.00102.7385.0845.5263.7015.67114.3857.9431.14275.3017.51434.8916.20
    127.0070.3285.6143.5331.306.9955.42108.3418.53253.5910.36350.6116.62
    128.0057.95103.5933.8339.898.4963.90115.3218.75227.9710.64389.9114.83
    130.2055.1290.3744.9933.609.6764.50120.0918.10243.089.56404.7334.21
    132.5056.6977.1431.1232.878.9265.39120.5116.46216.468.79336.217.36
    133.5045.1591.3233.6331.6111.1770.18116.0718.31235.888.51379.8214.78
    134.0051.9773.6537.6933.088.3960.01123.6916.57230.578.73320.7619.09
    135.0057.3286.4529.7729.307.2159.91132.6922.23213.569.86341.2521.89
    137.70131.47103.8052.9256.1517.49133.2494.9921.14253.3914.41399.2622.15
    注：微量元素单位为ug/g；表中Ni和Rb元素含量引自文献[2]。

    Table 2.  Trace element variations in the bulk rocks of the Norian⁃Rhaetian transition from the Xujiahe Formation

4.   测试结果与分析

    4.1.   主微量元素含量

  • 须家河剖面须家河组样品主、微量元素含量表见表1、2。其中,诺利末期（剖面：105.5~129.5 m）样品SiO2的含量介于37.93%~80.8%（平均值为56.62%）；Al2O3的含量介于5.49%~19.95%（平均值为12.71%）；Fe2O3的含量介于1.67%~7.70%（平均值为4.32%）；MgO的含量介于0.62%~4.45%（平均值为2.35%）；CaO的含量介于0.27%~24.69%（平均值为8.26%）；Na2O的含量介于0.62%~1.24%（平均值为0.86%）；K2O的含量介于1.13%~4.17%（平均值为2.59%）；Rb的含量介于46.84~231.51 ug/g（平均值为116.47 ug/g）；Sr的含量介于39.62~304.75 ug/g（平均值为127.22 ug/g）。

    NRB时期（剖面：129.5~135 m）样品SiO2的含量介于61.47%~66.45%（平均值为63.82%）；Al2O3的含量介于7.74%~8.53%（平均值为8.03%）；Fe2O3的含量介于2.83%~3.55%（平均值为3.13%）；MgO的含量介于1.25%~2.44%（平均值为1.87%）；CaO的含量介于7.35%~10.13%（平均值为8.62%）；Na2O的含量介于1.04%~1.21%（平均值为1.15%）；K2O的含量介于1.84%~2.00%（平均值为1.90%）；Rb的含量介于59.91~70.18 ug/g（平均值为64.00 ug/g）；Sr的含量介于116.07~132.69 ug/g（平均值为122.61 ug/g）。由于瑞替早期样品数量过少（图2）,此阶段的古气候变化在本文不做讨论。

    • 4.2.   化学风化指标对古气候的指示

  • 根据风化物质的化学成分,化学风化指标可用来量化风化程度,主要用于示踪古气候记录[35⁃38]。本文总结了常见的化学风化指标,并列出了详细的计算公式（表3）[35⁃36,39⁃44]。这些指标涉及不同的元素,因此可以综合对比讨论化学风化对沉积物成分的影响[45]。例如,钾交代作用对化学蚀变指数（CIA）影响很大,但对化学风化指数（CIW）没有影响[39]；Na消耗指数（τNa）对沉积再旋回和分选导致的石英富集十分灵敏,但这通常不影响CIA值[45⁃46]。须家河组NRT时期的四个代表性风化指标：CIA、CIW、斜长石蚀变指数（PIA）、硅铝比指数（R）在地层垂向上的变化趋势如图2所示。

    序号指标名称表达公式参考文献
    1硅铝比指数(R: Ruxton Ratio)=SiO2/Al2O3Ruxton[35]
    2化学蚀变指数(CIA: Chemical Index of Alteration)=Al2O3/（Al2O3+Na2O+K2O+CaO*）×100Nesbitt et al.[36]
    3化学风化指数(CIW: Chemical Index of Weathering)=Al2O3/（Al2O3+Na2O+CaO*）×100Harnois[39]
    4斜长石蚀变指数(PIA: Plagioclase Index of Alteration)=（Al2O3-K2O）/（Al2O3-K2O+Na2O+CaO*）×100Fedo et al.[40]
    5Na消耗指数(τNa: Chemical weathering depletion)=（Na/Zr）样品/（Na/Zr）母岩-1Rasmussen et al.[41]
    6损失化学风化指标(LCWP: Loss chemical weathering proxy)=（CaO*+Na2O+MgO）/TiO2Yang et al.[42]
    7成分变异指数(ICV: Index of compositional variability)=（Fe2O3+K2O+Na2O+CaO*+MgO+MnO+TiO2）/Al2O3Cox et al.[43]
    注：除τNa和LCWP公式使用质量百分数外,其他公式氧化物参数均使用摩尔为单位。表中CaO*指硅酸盐矿物中的CaO,在无法单独获得硅酸盐矿物中CaO含量时,有必要对CaO含量进行校正。用McLennan et al.[44]提出的方法计算CaO*：CaO剩余=CaO-P2O5×10/3,若计算后的CaO剩余<Na2O,令CaO*=CaO剩余；若CaO剩余>Na2O,令CaO*=Na2O。

    Table 3.  Summary of chemical weathering indices and their computational formulae

    研究表明,高CIA值（80~100）指示炎热潮湿气候条件下强烈的化学风化作用；中等CIA值（70~80）反映温暖潮湿的气候条件下中等化学风化作用；较低的CIA值（50~70）指示寒冷干燥气候条件下较弱的化学风化作用[36,47⁃48]。Ruxton[35]提出了一种简单的风化指数,Chittleborough[49]将其称为Ruxton比（R）,Ruxton[35]认为R最适用于均匀酸性到中性母岩的风化剖面,风化过程中Al2O3含量恒定,并产生高岭土风化产物。R将硅损失量与全元素损失量联系起来,当R大于10时,表明母岩基本没有受到风化作用,R值等于0时,表明母岩受到剧烈的风化作用[35]。本次研究的样品受到了K交代作用的影响（详情见5.1.1）,因此有必要对CIA进行K校正（用CIAcorr表示K校正后的CIA）。诺利末期（剖面：105.5~129.5 m）,CIAcorr（58~81）和R值（3.8~16.9）的平均值分别为73、8.2,表明研究区此时整体上受到了中等强度的化学风化作用,气候条件为温暖湿润气候；在NRB时期（剖面：129.5~135 m）,CIAcorr（59~64）和R值（13.1~13.9）的平均值分别为61和13.5,表明研究区在该时期受到较弱的化学风化作用,存在降温趋势。

    • 4.3.   微量元素对古气候指示

  • Long et al.[50]提出Rb/Sr值可以用来反映风化作用的程度。大多数岩石的Rb/Sr比值随化学风化程度的增加而增大。这是因为Rb+是一种碱性微量元素,相对于选择性浸出的Sr2+,它仍然保留在风化后的岩石中[44,51]。因此,可以利用Rb/Sr比值来评价源区化学风化强度：Rb/Sr>1指示化学风化程度较高；Rb/Sr<1指示化学风化程度中—低[52]。诺利末期样品（剖面：105.5~129.5 m）的Rb/Sr值介于0.2~2.6,平均值为1.2,表明该时期总体风化程度较高；在NRB时期（剖面：129.5~135.0 m）Rb/Sr比值集中在0.5~0.6,平均值为0.5,指示较低的风化程度。此外,沉积物中Ti元素含量的变化能够反映陆源物质的输入程度,Ti含量越高反映陆源物质越丰富,指示一种温暖湿润的气候条件[53⁃54]。图2中可以看出Ti含量变化所指示的气候状况与前文分析的气候变化趋势一致。

    样品在剖面的105.5 m、110.7~122.8 m、119.0 m、123.2~124.4 m、137.7 m处CIA和Rb/Sr值相对较高,R值相对较低；在106.6~108.0 m、115.3~116.6 m、121.5 m、126.4~134.5 m处CIA和Rb/Sr值相对较低,R值相对较高（表4）。结合前文分析,认为NRT时期古气候频繁波动,存在温暖潮湿气候和凉爽干燥气候交替出现的现象（图2）。

    高度/mCIAcorrCIWPIAICVτNaLCWPRRb/SrTi%
    105.508089861.00.75.05.11.70.5
    107.007381771.50.78.17.31.00.4
    107.407280761.60.68.97.20.70.4
    109.057988851.00.83.66.52.00.5
    109.507382781.00.83.016.91.60.4
    110.207887841.00.92.89.01.70.6
    111.008089871.00.84.45.52.10.5
    112.507988851.10.74.95.52.00.5
    114.507482781.30.67.06.71.10.4
    115.007079741.30.66.78.11.10.4
    115.506977721.30.75.99.21.00.4
    116.506977731.20.66.09.81.10.3
    117.007483791.20.75.26.81.30.5
    119.007988851.00.75.65.61.40.4
    119.507281761.40.67.27.00.60.4
    120.007078741.50.68.26.80.40.3
    120.506977722.00.511.67.50.30.3
    121.007280762.00.511.26.70.40.3
    121.505865592.20.612.513.40.20.2
    122.207179751.50.78.27.80.50.3
    122.507078742.00.612.47.10.40.3
    123.408191890.60.84.13.82.30.4
    123.458190880.60.74.25.22.60.4
    123.757786830.70.82.510.31.90.4
    123.857786830.80.72.89.42.00.5
    124.007786820.90.83.48.82.00.5
    127.006269631.50.78.613.90.50.3
    128.005966591.80.611.113.80.60.3
    130.206168611.50.78.913.90.50.3
    132.506067601.90.611.113.10.50.3
    133.506471651.40.77.413.20.60.3
    134.005966591.90.611.513.70.50.2
    135.006168611.70.69.713.70.50.2
    137.707684801.10.84.97.31.40.4

    Table 4.  Geochemical data and proxies in the bulk rocks of the Norian⁃Rhaetian transition from the Xujiahe Formation, Sichuan Basin

5.   讨论

    5.1.   化学风化指标的适用性分析

  • 本次研究所使用的风化指标有CIA、CIW、PIA、R等（表3）。然而,沉积后成岩蚀变、沉积再旋回以及水动力分选都会对样品的化学成分产生影响[40,45,55⁃56]。因此,有必要对化学风化指标的适用性进行评估。

    • 5.1.1.   沉积后成岩蚀变对样品的影响

  • 沉积后成岩蚀变会改变沉积物的主要化学成分,从而影响化学风化指标的真实性,因此需要评价其对样品的影响[57]。如果仅岩石成分是风化的控制因素,样品的风化趋势应平行于A-CN-K图中的A-CN界线[37,44,52,58]。然而,所研究的样品略微偏向K顶点（图3a[59]）,表明存在K交代作用[40,58]。富钾孔隙水和黏土矿物在沉积后成岩作用过程中,会使样品富集K2O,从而导致CIA值偏低[40]。因此,需要对CIA进行K校正[57]。本文利用Panahi et al.[60]提出的方法校正CIA：

    CIAcorr=Al2O3/(Al2O3+Na2O+K2O*+CaO*)         ×100 (2)
    K2O*=[m×Al2O3+m×(CaO*+Na2O)]/(1-m) (3)
    m=K2O/(Al2O3+CaO*+Na2O+K2O) (4)

    式中：K2O*是指发生K交代作用前沉积物中K2O的含量,m代表母岩中K2O的比例。前人研究认为晚三叠世四川盆地物源为南秦岭造山带[61⁃62],因此母岩中各元素含量取自南秦岭上地壳岩石中各元素的平均含量[59]。

    Figure 3.  (a) Plots of A⁃CN⁃K; (b) diagram of ICV⁃CIA; (c) correlations between Al/Si and loss chemical weathering proxy (LCWP); (d) correlations between Al/Si and τNa

    因为CIW、PIA指标不受K交代作用的影响[39⁃40],因此可以用CIW和PIA值来检验CIAcorr值[57],CIAcorr与CIW（r2=1）、PIA（r2=0.998 7）之间都具有很强的相关性（图4a,b）,表明K校正消除了K交代作用带来的影响。

    Figure 4.  Correlations between the K⁃corrected chemical index of alteration and other weathering indices

    • 5.1.2.   沉积再旋回和水动力分选（岩性差异）对样品的影响

  • 再旋回的原岩在被二次风化后,会使CIA值偏大,因此不能准确地指示古气候和源区风化程度[43,46,55,63]。Cox et al.[43]提出了成分变异指数（ICV）,可以用来评估碎屑岩的成分成熟度,从而判断岩石序列是首次沉积的产物还是源于再旋回的沉积物：ICV>1,表明沉积物中含有很少的黏土矿物,属于构造活动背景下的初始沉积[55,64]；ICV<1,则表明沉积物中含有大量的黏土矿物,代表其可能在初始沉积条件下经历了较强烈的风化作用或经历了再旋回沉积作用[55,65]。从图3b中可以看出大部分样品的ICV>1,表明沉积物整体属于构造活动背景下的初始沉积,受到较弱的沉积再旋回作用的影响。此外,四川盆地上三叠统为泥炭沼泽—陆相湖泊—河流沉积序列[66],下伏地层为元古代—中三叠世海相碳酸盐岩[19]。因此,下伏地层不太可能为盆地提供再旋回硅质碎屑物质。综上,沉积再旋回作用对四川盆地晚三叠沉积体系影响不大。

    沉积物在搬运过程中可能受到水动力分选作用,使得黏土矿物及云母类矿物富集于细粒沉积物中,而石英、长石和锆石等矿物富集于粗粒沉积物中[45⁃46]。矿物在不同粒级沉积物中富集导致化学元素也呈现类似规律,Si和高场强元素（U、Th、Zr、Hf）趋于富集在石英和重矿物中,大部分主微量元素则趋于富集在细粒沉积物中[67],从而影响化学风化指数。在相同风化条件下,细粒沉积物比粗粒沉积物具有更高的CIA值[68]。Al/Si比值可作为碎屑岩粒度（岩性）和水动力分选的指标[69⁃70],此次样品其变化范围为0.1~0.5。损失化学风化指数（LCWP）和τNa是基于分选敏感元素Zr和Ti计算的风化指数[45],与Al/Si存在弱相关性（图3c,d）。因此,在研究区沉积物的形成过程中,岩性差异对样品的影响较弱。

    综上,分析了沉积后成岩蚀变、沉积再旋回以及水动力分选对样品的影响后,表明本文的实验数据可用于反映物源区的化学风化程度和古气候。此外,大多数风化指标（CIW、PIA、R、LCWP）与CIAcorr都具有良好的相关性（图4）,指示这些指标均适用于指示须家河组沉积岩的源区化学风化强度[71]。

    • 5.2.   NRB时期全球降温事件

  • 本文系统地研究了须家河组NRT时期的主、微量元素,并运用元素地球化学指标重建了特提斯东缘四川盆地广元地区须家河组NRT时期的古气候,即在诺利末期CIAcorr、Rb/Sr相对较高,R值相对较低（平均值分别为73.0、1.2、8.2）,而NRB时期CIAcorr、Rb/Sr相对较低,R值相对较高（平均值分别为61.0、0.5、13.5）,揭示了诺利末期温暖潮湿的气候条件和NRB存在的一次降温事件（详情见3.2、3.3）。本次降温事件可能是全球性的,且能够被同位素地球化学（牙形石δ18O）、古植物学及沉积学的证据所支持[3,5,16,72⁃73]。

    • 5.2.1.   同位素地球化学证据

  • Trotter et al.[5]利用牙形石的氧同位素数据定量化分析诺利—瑞替期的古海水温度,从而判别当时西特提斯地区该时期的古气候状况。其在意大利Lagonegro盆地的中—上诺利阶（Alaunian-Sevatian）界线附近发现了牙形石氧同位素的一次大幅度负偏,估计当时的表层海水温度上升了大约7 ℃（变暖事件W3）,而后温度逐渐缓慢下降直到诺利阶和瑞替阶界线的附近,反映了短暂的降温事件（图5）[2,4⁃5,13,29,34,74⁃77]。同样,来自加拿大Cordillera地区的牙形石氧同位素数据显示中—上诺利阶界线附近表层海水温度可达35 ℃,支持了诺利中—晚期气候条件较热的观点[78]。

    Figure 5.  Geological events and geochemical curves across the Norian⁃Rhaetian transition

    • 5.2.2.   古植物学证据

  • 中生代大多数蕨类植物生长在沼泽、河岸、林下等温暖潮湿的气候环境下[79],而大多数苏铁类植物能够适应干旱的气候条件[80]。此外,银杏树和针叶树都属于落叶乔木,通常分布于相对干燥的温带地区,具有较强的抗寒能力[3]。Li et al.[15]在四川盆地合川地区须家河组进行了孢粉学研究,发现晚三叠世植被经历了由低地蕨类林向乔木较多的混合林的变化（蕨类逐渐被银杏、针叶植物及苏铁类植物取代）,反映了晚三叠世早期的气候条件相对温暖湿润,而诺利末期至瑞替期总体呈降温干燥趋势。此外,Lu et al.[3]在川东北七里峡剖面上三叠统须家河组一段中发现的植物化石以Dictyophyllum、Clathropteris及Cladophlebis等蕨类植物为主,到了须家河组二段蕨类植物则被已适应干旱环境的Neocalamites所取代,成为该时期植物群落的优势种,Lu et al.[3]认为晚三叠世的古气候总体为温暖湿润,但在NRB时期出现了短暂的凉爽干燥的气候环境。在古植物的研究中,化石木Xenoxylon的出现是凉爽气候条件的标志[14,81⁃82],川西北须家河组二段中化石木Xenoxylon（图2）的发现也佐证了NRB时期凉爽的气候特征[14]。同样,Li et al.[16]对四川盆地宣汉地区须家河组（诺利—瑞替阶）进行了详细的孢粉研究,发现诺利晚期至NRB时期蕨类占比从~50%急剧下降到~30%,而针叶树和苏铁/银杏类占比分别上升了~15%和~10%,揭示NRB时期存在降温事件。

    • 5.2.3.   沉积学证据

  • Berra et al.[72⁃73]发现特提斯西缘阿尔卑斯山脉中部的上三叠统沉积序列在NRB附近突然从碳酸盐沉积向细粒硅质碎屑沉积转变,且伴随着早期白云石化作用的终止,Berra et al.[72⁃73]将其归因于全球降温导致的海平面迅速下降,如图5所示[77]（以长瑞替期为时间轴）。此外,Lu et al.[3]在特提斯东缘四川盆地七里峡剖面须家河组（诺利—瑞替阶）进行了沉积特征分析,认为须一段主要为泥岩和页岩（占94.4%）,属于温暖气候条件下的潟湖相和泥炭沼泽相,而上覆须二段底层为中—细粒的长石砂岩,可能为凉爽干燥气候背景下的河口湾相。因此基于沉积学证据,Lu et al.[3]推测NRB时期存在短暂的降温事件。

    • 5.3.   NRT时期的古气候驱动机制探讨

    5.3.1.   超级季风

  • 本次运用化学风化指标（如：CIAcorr、CIW、PIA、R、Rb/Sr、Ti等）揭示了四川盆地须家河组NRT时期的古气候演化特征。在此期间风化指标数值呈高低交互出现,反映气候波动频繁,存在温暖潮湿气候和凉爽干燥气候交替出现的现象（图2,5）,其可能与晚三叠世盛行于泛大陆上的超级季风有关。Olsen et al.[83]分析了Newark超群6 700 m（持续时间~33 Ma）的岩心,揭示了季风的各种周期性变化。最小有0.2~0.3 mm的微层理,呈现暗色和浅色互层,是季风气候的年纹层,其中较大的沉积韵律可相当于季风两万年的岁差周期[83]。本次研究剖面时间尺度为~0.3 Ma,这种较短时间尺度内的干湿交替,季风可能呈现相当于万年的岁差周期。晚三叠世泛大陆聚合,古气候带呈带状分布[84],季风环流增强[85⁃89],导致气候波动频繁[90⁃91]。同时,四川盆地晚三叠世的风暴沉积[87]、植物化石[3]和树木年轮[92]的研究结果也一致表明,我国中纬度地区晚三叠世为季风性气候,且为受季风驱动的高能量海—陆相环境。此外,西特提斯地区意大利[73]、匈牙利[93]、瑞典[94]以及南半球澳大利亚[95]的古气候研究,表明晚三叠世巨型季风达到顶峰,且控制了整个泛大陆和特提斯沿岸的气候循环。季风性气候容易形成干—湿交替的循环气候[88,92],在季风增强的过程中会带来特提斯洋和泛大洋的丰富水分,导致泛大陆和特提斯沿岸气候湿润,反之季风减弱则会导致气候较为干旱[57]。因此,本文认为晚三叠世的超级季风可能对NRT时期全球气候波动产生了重要的影响[96⁃97],且可能导致了NRB时期的降温事件。

    • 5.3.2.   火山活动

  • 研究表明,火山活动亦是全球气候变化的主要驱动力之一,大规模火山喷发会释放出大量的温室气体,造成气候变暖[98]。例如,二叠纪—三叠纪界线事件时期全球增温事件就归因于西伯利亚大火山岩省爆发释放的大量温室气体[99⁃100]。Schaller et al.[74]根据北美Newark裂谷盆地成土碳酸盐同位素与古土壤中有机质的碳同位素,发现CO2浓度从中诺利期的~2 000×10-6上升到晚诺利期的~4 000×10-6（图5）。同时,在诺利末期,泛大洋深海盆地中的硅质岩和泥岩序列所记录的187Os/188Os值急剧下降[1,101⁃102]。此外,Trotter et al.[5]和Sun et al.[78]的牙形石氧同位素数据显示,在中—上诺利阶（Alaunian-Sevatian）界线附近表层海水温度可达35 ℃,与古新世—始新世极热事件记录的相同纬度地区的温度相当[78]。再者,一些主要的海洋生物群体,例如菊石,牙形石,双壳等在中—晚诺利期开始衰落,并逐渐开始灭绝[5]（图5）。同时,Sun et al.[78]还报道了Alaunian时期珊瑚和海绵的低生物多样性。种种迹象表明诺利—瑞替期地球环境发生了显著的变化,可能与同时期Angayucham大火成岩省的侵位有关[2,4,75⁃76,103]（图5）。然而Angayucham大火成岩省爆发时间被粗略定在214±7 Ma[76],与研究剖面（~205.7至~206 Ma）存在一定的时间差距,但关于Angayucham大火成岩省的认识非常有限,不排除其延长至NRB附近。再者,Jin et al.[2]在须家河剖面中发现了至少三套汞元素的富集层位,且伴随着多次有机碳同位素负漂（图2,5）,认为该时期可能存在多期次大规模的火山活动[2]。基于此认识,我们不排除火山活动对四川盆地NRT时期的古气候系统产生了一定影响的可能性,但火山活动的具体时限、量级及其如何与古气候变化相耦合,这些问题仍不明晰,未来需要进一步研究。

    • 5.3.3.   野火事件

  • 野火的发生受控于诸多因素,如气候干湿程度、大气氧含量和植被,同时大规模的野火事件也会反过来影响区域或甚至是全球的气候[104]。在晚三叠世,四川盆地多个地区均有野火事件的记录。在宣汉地区,上三叠统须家河组有较为零星的木炭化石（丝炭）产出,表明野火有一定的复燃时间,可能存在某些氧气水平较低的时期或对气候周期性变化（即较潮湿的时期）的响应[87]。此外,Song et al.[89]在广元和合川的两个陆相须家河组剖面也发现了以多环芳烃形式存在的野火生物标志物证据,该野火的可能成因是晚三叠世古热带辐合区的南移导致的区域气候干旱化[89]。在本次研究剖面M2煤层（图2,5）中也记录了一次野火事件,据黑碳含量变化特征,Jin et al.[2]推测此次野火类型为阴燃火,形成于湿润气候的背景。晚三叠世的野火事件不仅在四川盆地存在记录,在全球其他地区也存在相关记录。Ziegler et al.[105]以及Tanner et al.[106]在美国新墨西哥州北部上三叠统Chinle组发现了木炭化石并认为其与古野火相关。美国犹他州东南部上三叠统Chinle组产出的树干化石上存在火痕[107]且地层中存在丰度很高的丝炭[106],表明该地区曾发生过野火事件。德国西南部的上三叠统砂岩中也有木炭化石产出,同样指示古野火事件的存在[108]。此外,瑞典上三叠统Höganäs组[109]以及来自波兰Kamień Pomorski剖面和Lipie Śląskie剖面地层[110]的报道,均表明同时期的地层中有丝炭记录。虽然晚三叠世的野火频发,但目前我们对野火事件仍缺乏精细的时间限定和全球统一比对；此外,晚三叠世野火事件能否驱动同期全球的古气候变化仍不清楚。因此,我们不排除晚三叠世野火事件可能对同期古气候变化产生一定的影响,但其与气候环境的互馈机制仍有待进一步研究。

6.   结论

  • （1） 四川盆地须家河组诺利末期（剖面：105.5~129.5 m）整体上受到中等强度的化学风化作用,气候环境以温暖湿润为主；诺利—瑞替期界线（剖面：129.5~135 m）受到较弱的化学风化作用,存在短暂的降温事件。

    （2） 晚三叠世盛行于泛大陆上的超级季风的变化是导致四川盆地须家河组NRT时期气候频繁波动和NRB降温事件的主要原因,且同时期的火山活动和野火事件也对气候变化产生一定的影响,但火山活动和野火事件发生的准确时限以及其如何驱动同时期的气候变化,还需要进一步研究。

Reference (110)

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