Foraminiferal Evolution and Formation of Oolitic Limestone near Permian-Triassic Boundary at Yangou Section, Jiangxi Province
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摘要: 江西乐平沿沟剖面均一的碳酸盐相岩沉积, 完整地记录了二叠系-三叠系界线附近生物演变过程.通过对该剖面系统的化石切片研究, 共鉴别出有孔虫27属39种, 包含类4属7种.该剖面有孔虫的演变在二叠纪-三叠纪之交呈现出单幕式的消减过程, 对应于华南两幕生物灭绝中的第1幕.有孔虫的个体数和多样性在21-2层顶部发生了突然降低.4个不同有孔虫组合的演替关系, 揭示了有孔虫在灭绝事件中的演变过程, 即由灭绝前的高丰度、高分异度组合转变为灭绝后低丰度、低分异度组合.通过对二叠系-三叠系界线处鲕状灰岩的化石和沉积特征研究, 发现Hindeodus changxingensis带内的鲕状灰岩中仍存在较为丰富的有孔虫等化石.剧烈的海平面变化以及鲕状灰岩形成使下伏地层中的化石颗粒发生再沉积作用, 是形成这一特殊现象的主要原因.通过对鲕状灰岩中化石和沉积特征的深入研究, 论证了该鲕状灰岩形成于二叠纪-三叠纪之交大灭绝之后, 是当时特殊的海洋生态系产物, 反映了当时较高的海水回旋速率、微生物繁盛、海水碳酸盐超饱和等异常的环境条件.Abstract: The homogeneous carbonate Permian-Triassic boundary section at Yangou records the integrated extinction process of the shallow marine area, which killed over 90% marine invertebrates. This study of foraminifers explores the extinction pattern and deposition progress during the Paleozoic-Mesozoic transition. In total, 39 species in 27 genera of foraminifers have been identified, including 7 species in 4 genera of fusulinids. The richness and diversity remarkably decrease at the top of Bed 21-2, marking the disappearance horizon. 4 distinct assemblages evolved within the Permian-Triassic boundary beds: the assemblages with low richness and low disparity replaced those with high richness and high disparity. The diversity and assemblage analysis exhibit a rapid decline process at the top of Bed 21-2. Whereas, the mixing of pre-extinction fauna, fusulinids, with post-extinction fauna, Hindeodus changxingensis, within the oolitic beds, implies that the fossil could be reworked to the shoal by the sea level changes. Meanwhile, the coating of foraminifers and aragonite-calcite cement around the grains indicate a catastrophic ocean with high water velocity, elevated microbes and carbonate super-saturation.
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Key words:
- Permian-Triassic /
- foraminifer /
- extinction /
- Yangou Section /
- paleontology
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0. 引言
在二叠纪-三叠纪之交的生物集群灭绝事件中,超过90%的海洋无脊椎动物灭绝(Raup, 1979; Raup and Sepkoski, 1982; Erwin, 1994).这次显生宙中最大的灭绝事件被认为是由缺氧(Wignall and Twitchett, 1996; Song et al., 2012)、海底甲烷释放(Retallack and Jahren, 2008)、超大规模的西伯利亚火山喷发(Renne and Basu, 1991; Campbell et al., 1992; Reichow et al., 2009)、陆地风化作用加强(Algeo and Twichett, 2010; Algeo et al., 2011) 剧烈的海平面变化(Hallam and Wignall, 1999; Yin et al., 2014)和海洋酸化(Payne et al., 2010)等灾难性的环境事件引发的,这些环境巨变总体上可归结为泛大陆聚合过程中地球内部多圈层相互作用的结果(殷鸿福和宋海军,2013).也有人认为是地外星体撞击地球引发的全球灾难(Becker et al., 2001; Kaiho et al., 2001).无论是何种原因,二叠纪-三叠纪之交生物和环境事件及这两者之间的关系都非常复杂,研究精确的生物灭绝模式和时间,以及建立同环境事件的联系,对分析探讨环境转变、灭绝原因有重要作用.
近年来,在二叠纪-三叠纪生物灭绝研究中,关于生物灭绝模式和时间存在着多种不同的观点,特别是有单幕式(Jin et al., 2000; Shen et al., 2011)和多幕式(Yin et al., 2012; Song et al., 2013)等,在国际上引起了广泛关注和争论.有孔虫作为一种海洋广泛分布的低等生物,具有数量多、种类丰富、易于保存、对环境转变反应迅速等特点,是研究二叠纪-三叠纪生物灭绝事件的良好材料.目前二叠纪-三叠纪有孔虫研究有丰富的分类学材料(Groves et al., 2005, 2007;宋海军等, 2006, 2012; Song et al., 2007),也有关于灭绝模式和过程的统计学分析(童金南和匡文,1990; Tong and Shi, 2000; Groves et al., 2007; Song et al., 2009a, 2009b, 2013).然而针对不同相区的跨越灭绝线和二叠系-三叠系界线剖面的有孔虫系统研究较少,仅有浅海内陆盆相的浙江煤山剖面(宋海军等,2006; Song et al., 2007, 2009a)、重庆凉风垭剖面(童金南和匡文,1990)、极浅水的贵州打讲剖面(Song et al., 2009b)等少量资料.由于相变迅速或者化石数据不连续等因素,其他相区仅有关于某些层位有孔虫动物群的系统分类学研究(顾松竹等, 2002, 2005;尚庆华等,2003).江西乐平沿沟剖面是华南目前正式报道的唯一一条二叠纪最末期有鲕状灰岩发育的均一碳酸盐岩相二叠系-三叠系界线剖面,剖面类型与当时位于西特提斯洋沿岸的一些欧洲的剖面类似,如土耳其的Kemer Gorge剖面(Marcoux and Baud, 1986)、意大利的Sutrio、Monte Ruche、Dierico和Tesero剖面(Buggisch and Noe, 1986)等.沿沟剖面二叠系-三叠系界线附近的有孔虫的详细研究,对于探讨二叠纪-三叠纪过渡期有孔虫的灭绝模式,恢复古-中生代之交的地球生物学转变过程有重要意义.
1. 地质背景及样品来源
沿沟剖面位于江西省东北部、乐平市沿沟煤矿NNE方向1.5km处水库旁一个采石场内,接近景德镇市与乐平市交界处(图 1a).沿沟地区在二叠纪-三叠纪之交位于扬子台地,为连续的碳酸盐岩沉积(图 1b),牙形石、腹足类、介形类、腕足类产出丰富且保存完整(朱相水等,1994; Sun et al., 2012).
研究剖面地层包括长兴组上部和大冶组底部,实测地层厚度21.1m,共划分为47层.其中,1-20层(共11.1m)为长兴组灰白色厚层巨厚层灰岩,以0.5m左右厚度为标准分层,含丰富的钙藻、有孔虫、腕足类等生物化石.长兴组与之上的大冶组整合接触.21-47层为大冶组,主要为深灰色中薄层灰岩和泥质灰岩,夹少量粘土岩,部分层位富含牙形石、腹足类、介形虫等化石.
根据精细的牙形石生物地层研究结果(图 2),沿沟剖面二叠系-三叠系界线附近地层自下而上划分为7个牙形石带,分别是:Clarkina changxingensis带,Clarkina yini-C.zhangi带,Hindeodus changxingensis带,Clarkina taylorae带,Hindeodus parvus带,Isarcicella staeschei带和Isarcicella isarcica带(Sun et al., 2012).以Hindeodus parvus的首现为标志,二叠系-三叠系界线位于21-4层底部.碳同位素结果和浙江煤山剖面、四川上寺剖面有良好的可对比性(Song et al., 2012).在沉积微相研究中,在21-1和21-2层发现了鲕状灰岩.二叠系最顶部的鲕状灰岩的发现不仅说明了该剖面当时位于碳酸盐岩台地浅滩相,而且在鲕状灰岩中产有丰富的有孔虫化石,因而该鲕状灰岩的成因环境很值得深入探讨,其可能对灭绝时期的特殊海洋环境条件有重要指示作用.
图 2 沿沟剖面二叠系-三叠系界线附近(20层-28层)的牙形石和有孔虫化石分布牙形石数据引自Sun et al.(2012).①Clarkina yini-Clarkina zhangi带;②Hindeodus changxingensis带;③Clarkina taylorae带;④Hindeodus parvus带Fig. 2. The distribution of foraminifers and conodonts at Yangou section (Bed 20 to Bed 28)该剖面化石保存良好,其晚二叠世的有孔虫在朱相水等(1994)的地层学研究中有过初步报道.笔者在对该剖面研究过程中发现了更为丰富的有孔虫、藻类等微体化石,并对其中二叠系-三叠系界线附近的有孔虫化石进行了系统鉴定.为了更全面地揭示二叠纪-三叠纪之交的生物演变过程,此次研究对灭绝线上下地层进行了加密采样,在20层到26层中共制作薄片69张,平均2.5cm/张,每张薄片规格为2.2×2.2cm2.1-19层的有孔虫及其他生物化石面貌与20层基本相同,本文将不做详细论述.
2. 结果
2.1 沿沟剖面有孔虫多样性
此次研究中共观察到有鉴定意义的有孔虫化石622枚,其中类有孔虫化石97枚.主要包括Palaeofusulina sinensis, Nankinella sp., Reichelina cribroseptata, Colaniella nana, Glomomidiella nestellorum, Nodosinelloides aequiampla, Pachyphloia ovate, Rectocornuspira kalhori, Earlandia sp.等共27属39种,其中类有孔虫4属7种,具体化石多样性和丰度及产出层位见图 2和表 1.
表 1 沿沟剖面二叠纪-三叠纪之交有孔虫分布统计Table Supplementary Table Distribution of foraminifers near the Permian-Triassic boundary at Yangou section2.2 有孔虫灭绝率和灭绝过程
各层位的化石个体数和多样性变化显示(图 3),乐平沿沟剖面有孔虫丰度和多样性的快速下降主要发生在21-2层顶部:21-2层之前,各个层位有孔虫化石个体数在12~115之间,7~21个种;在21-3层之上,每个层位最多产出28枚化石,大多都在1~5枚之间,多样性也很低,多数层位只有1~2种有孔虫化石.根据有孔虫分布图(图 2)可以看出,仅有2个种可以延伸至21-3层之上的地层,而在21-4层有3个新生种出现.类有孔虫只产于在21-2层之下的地层中,在20层顶部有1个种消失,其他7个种均21-2层消失.以上结果显示,乐平沿沟剖面的有孔虫消减主要发生在21-2层顶部.
2.3 有孔虫群落演替
根据有孔虫的分布规律和组合特点,可以将该剖面的有孔虫划分为4个不同的化石组合(图 4).
图 4 沿沟二叠纪-三叠纪之交有孔虫群落组成a.Palaeofusulina sinensis-Glomomidiella nestellorum组合;b.Glomomidiella nestellorum组合;c.Earlandia sp.-Globivalvulina bulloides组合;d.Earlandia sp.-Rectocornuspira kalhori组合.Earlandia sp.-Rectocornuspira kalhori组合:产于21-4层到46层,其中发现有孔虫化石5属5种,共43枚,以Earlandia sp.和Rectocornuspira kalhori含量较高,相对丰度分别达到了55.8%和35.4%,同时还有Nodosaria sp.和Diplosphaerina inaequalis,相对丰度分别达到了5%和2.8%,另外还有约0.5%的Globivalvulina bulloides.该组合依据2个优势种命名,以低分异度和低丰度为特征Fig. 4. The composition of foraminifer assemblages near the Permian-Triassic boundary at Yangou sectionPalaeofusulina sinensis-Glomomidiella nestellorum组合:产于剖面第20层中.该组合中类有孔虫丰富,其中Palaeofusulina sinensis, Reichelina cribroseptata和Reichelina pulchra分别占到了个体总数的20%、6%和6%.非有孔虫也类别较多,共鉴别出11属17种,其中Glomomidiella nestellorum最为丰富,个体数相对含量达到了20%,其他种含量较少,均未超过5%.该组合根据其中的2个优势种命名,总体特点是高分异度和高丰度.
Glomomidiella nestellorum组合:产于21-1层和21-2层.该组合有孔虫分异度依然很高,有21属31种,丰度极高,在(20张2.2×2.2cm2)的薄片中共观察到总个体数达376枚,达到此次研究中有孔虫总数(622)的60%.其中Glomomidiella nestellorum占有绝对优势,个体数达到了213枚,占该组合的57%.类有孔虫在该组合中有4属7种,其中Reichelina cribroseptata个体最多,达到了19枚,但是由于该组合总个体数庞大,类有孔虫相对丰度就很低.该组合依据优势种命名,以高分异度、极高丰度和大量的Glomomidiella nestellorum为主要特征.该化石组合产出于鲕状灰岩,很有可能混入了非原地沉积颗粒.因此该化石组合中所包含分子可能并不完全属于该组合,包括该组合的优势分子也可能在保存层位上发生了一定的位移.该组合的优势分子在时代上与上、下地层中的化石分子有显著不同,并且其保存特征有明确的环境指示意义(即与鲕粒灰岩的环境指示特征一致),因此本文仍将其作为单独的化石组合.
Earlandia sp.-Globivalvulina bulloides组合:产自剖面21-3层,但该地层中只观察到有孔虫2属2种,分别是Earlandia sp.和Globivalvulina bulloides,个体数量也极少,分别含有2枚和1枚,相对丰度计算结果分别为67%和33%.该组合即以这2个化石类型命名,以极低的分异度和丰度为特征.
根据沿沟剖面二叠系-三叠系界线地层中不同的有孔虫化石组合特征,可以发现乐平沿沟剖面的有孔虫化石组合在二叠纪-三叠纪之交发生了明显的演变.在20层至21-2层,有孔虫分异度高、丰度高,而21-3层至34层,有孔虫分异度和丰度都很低(图 5).在21-2层和21-3层界线处,有孔虫化石发生了由Glomomidiella nestellorum组合向Earlandia sp.-Globivalvulina bulloides组合转变的现象.同时,由于Earlandia sp.是一个具有全球性广泛分布的大灭绝后灾难分子(Groves et al., 2005, 2007; Song et al., 2009b),因此,本剖面的化石记录指示二叠纪末的主灭绝事件发生在21-2层顶部.有孔虫的第2幕灭绝事件在沿沟剖面没有体现,这与前人的研究是一致的(朱相水等,1994).Song et al.(2013)发现在主幕灭绝之后有孔虫主要残存于较深水区域,因而浅水相剖面主要表现为单幕式的灭绝过程,如贵州打讲剖面(Song et al., 2009b),较深水剖面主要表现为双幕式的灭绝过程,如浙江煤山剖面(Song et al., 2009a).
图 5 沿沟剖面二叠系-三叠系界线附近地层中有孔虫化石图中线条比例尺长度为0.1mm.1~2.Nankinella sp.;1.LY201028,弦切面,产自20层;2.LY20544,弦切面,产自20层;3.Sphaerulina sp., LY21522,旋切面,产自21层;4.Palaeofusulina minima, LY20038,轴切面,产自20层;5.Palaeofusulina simplex, LY21125,近轴切面,产自21层;6~9.Palaeofusulina sinensis,LY205113,弦切面,产自20层;7.LY20016,近轴切面,产自10层;8.LY20514,轴切面,产自20层;9.LY205131,轴切面,产自20层;10.Climacammina moelleri, LY205328,纵切面,产自20层;11.Colaniella nana, LY20518,斜切面,产自20层;12~13.Cribrogenerina spp.;12.LY205130,纵切面,产自20层;13.LY205316,纵切面,产自20层;14.Pachyphloia robusta, LY212114,纵切面,产自21-2层;15~16.Palaeotextularia spp.;15.LY20532,纵切面,产自20层;16.LY212130,纵切面,产自21-2层;17~18.Robuloides lens;17.LY211211,纵切面,产自21-1层;18.LY212315,纵切面,产自21-2层;19.LY212326,纵切面,产自21-2层Fig. 5. Foraminifer fossils from the Permian-Triassic boundary strata at Yangou section3. 讨论
根据有孔虫丰度和分异度变化以及有孔虫群落演替特征,可以发现乐平沿沟剖面最明显的有孔虫组合演替发生在21-2层顶部,即Hindeodus changxingensis带顶部,类有孔虫的消失也发生在该层位.Metcalfe et al.(2007)认为,在全球范围内,Hindeodus changxingensis带应该在主幕灭绝线之上.全球界线层型剖面浙江长兴煤山剖面的有孔虫灭绝的主幕(第1幕,类有孔虫灭绝)发生在Clarkina yini带顶部(Song et al., 2009a),明显早于沿沟剖面.
那么沿沟剖面的有孔虫等生物到底是何时灭绝?鲕状灰岩中的有孔虫这种特殊的化石保存方式反映了什么具体的环境问题?从前文的讨论中可以看出,沿沟剖面的Hindeodus changxingensis带的鲕状灰岩成因成为解决这些问题的关键.
3.1 二叠纪-三叠纪之交的鲕状灰岩与主幕灭绝后的化石颗粒再沉积
此次研究中还发现了一个特殊的现象,一些有孔虫作为鲕状颗粒的核心保存下来(Tian et al., 2014).既有类Reichelina spp.、Palaeofusulina spp.,也有大量Glomomidiella nestellorum等非有孔虫作为核心(图 6-21,6-22),被外围一个圈层包裹形成鲕状颗粒(或者叫包壳).由此推断,该时期的鲕状灰岩中的化石颗粒可能是一种二次沉积产物.鲕状灰岩的沉积时间与Hindeodus changxingensis带相同,当时主幕灭绝事件应该已经发生,里面共生的大量的灾前有孔虫化石,以类有孔虫为代表,可能来源于其下伏的老地层.
图 6 沿沟剖面二叠系-三叠系界线附近地层中有孔虫化石图中比例尺长度为0.1mm.1~5.Reichelina cribroseptata;1.LY20223,近旋切面,产自20层;2.LY212415,斜切面,产自21-2层;3.LY21114,近旋切面,产自21-1层;4.LY211210,弦切面,产自21-1层;5.LY21122,近轴切面,产自21-1层;6~9.Reichelina purlchra;6.LY20041,近轴切面,产自20层;7.LY20522,轴切面,产自20层;8.LY20527,弦切面,产自20层;9.LY205121,弦切面,产自20层;10.Agathammina pusilla,LY21121,横切面,产自21-1层;11~12.Diplosphaerina inaequalis;11.LY2255,球切面,产自22层;12.LY23123,球切面,产自23层;13~14.Rectocornuspira kalhori;13.LY2321,球切面,产自23层;14.LY2285,纵切面,产自22层;15~16.Earlandia sp.;15.LY2321,球切面,产自23层;16.LY2311,纵切面,产自23层;17~18.Globivalvulina bulloides;17.LY205310,纵切面,产自20层;18.LY2251,纵切面,产自22层;19~22.Glomomidiella nestellorum;19.LY20515,球切面,产自20层;20.LY21146,球切面,产自21-1层;21.LY211432,球切面,产自21-1层;22. LY212414,球切面,产自21-2层;23.Hemigordius saidi LY211118,纵切面,产自21-1层;24~25.Ichthyofrondina palmate;24.LY20532,纵切面,产自20层;25.LY201014,纵切面,产自20层;26~27.Neodiscus milliloides;26.LY205134,纵切面,产自20层;27.LY21223,纵切面,产自21-2层;28.Neoendothyra reicheli,LY211313,纵切面,产自21-1层;29~31.Nodosinelloides aequiampla;29.LY21116,纵切面,产自21-1层;30.LY211213,纵切面,产自21-1层;31.LY211428,纵切面,产自21-1层;32~33.Nodosinelloides camerata;32.LY21112,纵切面,产自21-1层;33.LY211428,纵切面,产自21-1层;34.Pachyphloia iranica,LY20516,纵切面,产自20层;35.Pachyphloia ovate,LY2010313,纵切面,产自20层;36~37.Pachyphloia schwageri;36.LY21123,纵切面,产自21-1层;37.LY212517,纵切面,产自21-2层;38~39.Pachyphloia spp.;38.LY205323,纵切面,产自20层;39.LY205410,纵切面,产自20层Fig. 6. Foraminifer fossils from the Permian-Triassic boundary strata at Yangou section与煤山剖面相比,沿沟剖面没有识别出Clarkina meishanensis带,指示当时可能存在一定的地层缺失,即沿沟地区在Hindeodus changxingensis带与Clarkina yini带之间可能发生过短暂的暴露,导致了Clarkina meishanensis带的缺失,也造成了Clarkina yini带内的灾前有孔虫被剥蚀再沉积到Hindeodus changxingensis带.
以牙形石带为时间尺度,可以将当时的地球生物学过程划分为以下3个不同的阶段(图 7):(1)在Clarkina yini带的顶部,大部分有孔虫(包含全部的类有孔虫)灭绝;(2)在Clarkina meishanensi带-Hindeodus changxingensis带时期,海洋中有少量的残存有孔虫和灾后的灾难有孔虫Earlandia sp.等生存.但是,由于早期海平面下降,一些较浅水的碳酸盐岩岩隆区(尤其是长兴期礁相区)沉积停顿并遭受侵蚀(因而缺失Clarkina meishanensis带及其之上的部分地层),导致一些抗风浪作用能力较强的灭绝前化石颗粒被搬运到台地边缘,作为碳酸盐岩颗粒,或者被微生物膜包壳成为鲕状颗粒再次沉积下来;(3)在Hindeodus parvus带内,由于迅速的海平面上升,碳酸盐岩岩隆区重新被海水淹没,并发育钙质微生物岩等碳酸盐沉积建造,但沿沟地区由于海水的加深,不再适合鲕粒发育,而沉积了泥晶碳酸盐岩.
类似的二叠系-三叠系界线地层序列在欧洲多处地区也有发现,其沉积相和有孔虫灭绝形式与沿沟剖面基本相同(Groves et al., 2007),说明二叠纪-三叠纪之交在特提斯洋的周缘地区,存在以鲕状灰岩浅滩相发育为特色的地球生物学过程.虽然各个欧洲剖面上缺乏精确的牙形石带对比,目前无法确认其灭绝时间是否相同,但是有理由推测,这些在二叠纪-三叠纪之交有鲕状灰岩发育的剖面,发生了和沿沟地区类似的地球生物学过程.
3.2 二叠纪-三叠纪之交的鲕状灰岩的环境意义
鲕状灰岩在欧洲多条二叠系-三叠系界线剖面存在,在越南地区(Algeo et al., 2007)和北美西部(Saltzman and Sedlacek, 2013)也有报道.但它们都出现在二叠纪末的大灭绝线之上,表明它的形成与大灭绝事件有重要联系.沿沟剖面二叠纪最末期鲕状灰岩的发现,进一步表明鲕状灰岩是一种特殊的、全球广布的二叠纪末灭绝后的特殊沉积物(Groves, 2004).
在沿沟剖面,21-1层和21-2层为鲕状灰岩,此次研究中笔者共观察到保存较好的有孔虫化石376枚,鉴定出20属31种,即Glomomidiella nestellorum组合.其中个体数超过总数一半的Glomomidiella nestellorum(57%)是一种近球形的有孔虫, 大小集中在0.4~0.6mm左右,与之共生的鲕状颗粒和有孔虫大小相当,为亮晶胶结,说明当时的海水动力条件比较强.除了Glomomidiella nestellorum外,该组合中发现的类化石也比较多.这些壳外形一般呈纺锤形,且与椭球形、球形比较相近.虽然有学者认为该层位的鲕状颗粒不是高水动力的产物,而是作为生物颗粒减少的一种碳酸盐岩沉积补偿形式(Groves et al., 2007),但是沿沟剖面该鲕状灰岩所体现的较好分选和亮晶胶结,反映了高的海水回旋速率(在波浪和潮汐等作用下,海水以回旋形式流动时的速率,被认为与鲕粒形成有重要关系,参见Sumner and Grotzinger, 1993)与高水动力条件,应该有直接的联系.
在沿沟剖面的鲕状灰岩中还可以观察到,部分有孔虫作为鲕状颗粒的核心(即在有孔虫壳体外形成单圈层的包壳,图 8,照片1~3),另一部分有孔虫作为单独的化石颗粒被保存下来(图 8,照片4~6).碳酸盐岩中,颗粒周缘的包壳的形成被认为和微生物活动有密切的联系(Flügel, 2004),包壳的有孔虫可能反映了当时微生物相当繁盛.在部分未被包壳的有孔虫颗粒周缘发育有文石-高镁方解石栉壳圈(图 8,照片4~6,箭头D),以纤维状、垂直于颗粒外壳为特征,文石-高镁方解石的形成需要碳酸盐超饱和海水环境(Flügel, 2004).颗粒边缘的文石-高镁方解石栉壳圈的形成,反映出当时浅海的海水处在碳酸盐度超饱和状态.在其他剖面,也发现了能反映相似环境条件的沉积学证据.例如在湖南慈利、贵州打讲等二叠系-三叠系界线微生物岩发育的剖面,二叠纪末的主幕灭绝线之上直接覆盖有凝块石(王永标等,2005;王钦贤等,2009; Yang et al., 2011),浙江煤山剖面的27层发育有明显的硬底构造和微生物构造(曹长群和郑全峰,2009).这些都说明了主幕灭绝事件之后,发生了海水饱和度异常和微生物繁盛等一系列地球生物学事件.
鲕粒灰岩所反映出的高海水回旋速率、微生物繁盛、碳酸盐超饱和等特殊的环境条件,与其他一系列的地球生物学事件有重要联系.有研究表明,二叠纪-三叠纪之交全球范围内发生了剧烈的海平面变化(Hallam and Wignall, 1999; Yin et al., 2014),加之陆地上的风化作用也突然增强(Algeo and Twichett, 2010; Algeo et al., 2011),浅海台地有明显的剥蚀或喀斯特化现象(Collin et al., 2009; Wignall et al., 2009).以上环境条件为海洋微生物繁盛提供了充足的陆源营养物质(Xie et al., 2010),也是鲕粒大规模发育的重要原因.
全球广泛分布的早三叠世鲕粒被认为可以指示生物灭绝后缺氧、微生物繁盛、海水超饱和、高二氧化碳分压、低硫酸盐浓度等灾难环境(Lehrmann et al., 2012; Li et al., 2013; Woods, 2013; Tian et al., 2014).虽然二叠纪末期鲕状灰岩和早三叠世的鲕粒存在一定的差别,但是此二者有可能形成于相似的沉积环境.因为有其他证据表明二叠纪末期全球范围内开始出现缺氧(Wignall and Twitchett, 2002)、微生物繁盛(Xie et al., 2005)、碳循环剧烈波动等异常的环境条件(Tong et al., 2007; Song et al., 2012),所以二叠纪最末期的鲕状灰岩可能指示了灾难环境的开始,而随着环境进一步恶化,鲕状颗粒进一步发育、圈层增多、结构复杂化、粒径变大.
4. 结论
(1) 江西乐平沿沟剖面上二叠纪-三叠纪之交有孔虫在21-2层顶部发生了明显的多样性剧减和化石组合演替.但二叠纪有孔虫主幕灭绝发生在20层顶部,21-1和21-2层中的部分有孔虫为再沉积产物.
(2) 该剖面上的鲕状灰岩是二叠纪末大灭绝事件后特殊的海洋环境的产物,指示了较高的海水动能条件、微生物繁盛、海水碳酸盐超饱和等异常的海洋环境.
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图 2 沿沟剖面二叠系-三叠系界线附近(20层-28层)的牙形石和有孔虫化石分布
牙形石数据引自Sun et al.(2012).①Clarkina yini-Clarkina zhangi带;②Hindeodus changxingensis带;③Clarkina taylorae带;④Hindeodus parvus带
Fig. 2. The distribution of foraminifers and conodonts at Yangou section (Bed 20 to Bed 28)
图 4 沿沟二叠纪-三叠纪之交有孔虫群落组成
a.Palaeofusulina sinensis-Glomomidiella nestellorum组合;b.Glomomidiella nestellorum组合;c.Earlandia sp.-Globivalvulina bulloides组合;d.Earlandia sp.-Rectocornuspira kalhori组合.Earlandia sp.-Rectocornuspira kalhori组合:产于21-4层到46层,其中发现有孔虫化石5属5种,共43枚,以Earlandia sp.和Rectocornuspira kalhori含量较高,相对丰度分别达到了55.8%和35.4%,同时还有Nodosaria sp.和Diplosphaerina inaequalis,相对丰度分别达到了5%和2.8%,另外还有约0.5%的Globivalvulina bulloides.该组合依据2个优势种命名,以低分异度和低丰度为特征
Fig. 4. The composition of foraminifer assemblages near the Permian-Triassic boundary at Yangou section
图 5 沿沟剖面二叠系-三叠系界线附近地层中有孔虫化石
图中线条比例尺长度为0.1mm.1~2.Nankinella sp.;1.LY201028,弦切面,产自20层;2.LY20544,弦切面,产自20层;3.Sphaerulina sp., LY21522,旋切面,产自21层;4.Palaeofusulina minima, LY20038,轴切面,产自20层;5.Palaeofusulina simplex, LY21125,近轴切面,产自21层;6~9.Palaeofusulina sinensis,LY205113,弦切面,产自20层;7.LY20016,近轴切面,产自10层;8.LY20514,轴切面,产自20层;9.LY205131,轴切面,产自20层;10.Climacammina moelleri, LY205328,纵切面,产自20层;11.Colaniella nana, LY20518,斜切面,产自20层;12~13.Cribrogenerina spp.;12.LY205130,纵切面,产自20层;13.LY205316,纵切面,产自20层;14.Pachyphloia robusta, LY212114,纵切面,产自21-2层;15~16.Palaeotextularia spp.;15.LY20532,纵切面,产自20层;16.LY212130,纵切面,产自21-2层;17~18.Robuloides lens;17.LY211211,纵切面,产自21-1层;18.LY212315,纵切面,产自21-2层;19.LY212326,纵切面,产自21-2层
Fig. 5. Foraminifer fossils from the Permian-Triassic boundary strata at Yangou section
图 6 沿沟剖面二叠系-三叠系界线附近地层中有孔虫化石
图中比例尺长度为0.1mm.1~5.Reichelina cribroseptata;1.LY20223,近旋切面,产自20层;2.LY212415,斜切面,产自21-2层;3.LY21114,近旋切面,产自21-1层;4.LY211210,弦切面,产自21-1层;5.LY21122,近轴切面,产自21-1层;6~9.Reichelina purlchra;6.LY20041,近轴切面,产自20层;7.LY20522,轴切面,产自20层;8.LY20527,弦切面,产自20层;9.LY205121,弦切面,产自20层;10.Agathammina pusilla,LY21121,横切面,产自21-1层;11~12.Diplosphaerina inaequalis;11.LY2255,球切面,产自22层;12.LY23123,球切面,产自23层;13~14.Rectocornuspira kalhori;13.LY2321,球切面,产自23层;14.LY2285,纵切面,产自22层;15~16.Earlandia sp.;15.LY2321,球切面,产自23层;16.LY2311,纵切面,产自23层;17~18.Globivalvulina bulloides;17.LY205310,纵切面,产自20层;18.LY2251,纵切面,产自22层;19~22.Glomomidiella nestellorum;19.LY20515,球切面,产自20层;20.LY21146,球切面,产自21-1层;21.LY211432,球切面,产自21-1层;22. LY212414,球切面,产自21-2层;23.Hemigordius saidi LY211118,纵切面,产自21-1层;24~25.Ichthyofrondina palmate;24.LY20532,纵切面,产自20层;25.LY201014,纵切面,产自20层;26~27.Neodiscus milliloides;26.LY205134,纵切面,产自20层;27.LY21223,纵切面,产自21-2层;28.Neoendothyra reicheli,LY211313,纵切面,产自21-1层;29~31.Nodosinelloides aequiampla;29.LY21116,纵切面,产自21-1层;30.LY211213,纵切面,产自21-1层;31.LY211428,纵切面,产自21-1层;32~33.Nodosinelloides camerata;32.LY21112,纵切面,产自21-1层;33.LY211428,纵切面,产自21-1层;34.Pachyphloia iranica,LY20516,纵切面,产自20层;35.Pachyphloia ovate,LY2010313,纵切面,产自20层;36~37.Pachyphloia schwageri;36.LY21123,纵切面,产自21-1层;37.LY212517,纵切面,产自21-2层;38~39.Pachyphloia spp.;38.LY205323,纵切面,产自20层;39.LY205410,纵切面,产自20层
Fig. 6. Foraminifer fossils from the Permian-Triassic boundary strata at Yangou section
图 7 沿沟及邻区二叠纪-三叠纪之交沉积演变示意
DJ.打讲剖面;YG.沿沟剖面;MS.煤山剖面.测年数据引自Shen et al.(2011a),牙形石带和照片引自Jiang et al.(2007)及Sun et al.(2012)
Fig. 7. The model of the depositional progresses during the Permian-Triassic transition
表 1 沿沟剖面二叠纪-三叠纪之交有孔虫分布统计
Table 1. Distribution of foraminifers near the Permian-Triassic boundary at Yangou section
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