【作者】 雷勇；

【导师】 冯庆来；

【作者基本信息】 中国地质大学 ， 古生物学与地层学， 2013， 博士

【摘要】 二叠纪与三叠纪之交的生物大灭绝是自显生宙以来地球上发生的最大一次集群灭绝事件,距今约252Ma。这次事件造成当时海洋生态系统中大约90%以上的物种灭绝。许多学者将这次灾难事件归因于环境的突变,如海洋的大范围缺氧、西伯利亚火山喷发以及海平面骤降等。而在统计生物多样性变化的时候,多数学者只关注海洋动物物种多样性的变化,对于遍布海洋的浮游植物却是知之甚少。甚至于一些研究浮游植物的学者也只是参与争论二叠纪与三叠纪之交的“真菌高峰”事件。而近年来,一系列关于二叠纪末期蓝细菌爆发的报道则引发了大家对海洋初级生产者的注意。因此,本文主要研究了华南地区二叠纪与三叠纪之交的疑源类,同时结合前人有关蓝细菌以及其它动物化石的数据,综合分析了二叠纪末生物大灭绝期间浮游植物群落的演替及其对生态环境的影响。首先,笔者总结了二叠纪的疑源类化石。结果表明：相对于孢粉化石,二叠纪疑源类的研究程度较低,但是其物种多样性远远超出前人的预计。尽管具大型膜壳的疑源类化石在古生代中期以后就很少出现,但是具小型膜壳的类型在二叠纪的地层中仍然相当丰富。二叠纪的各个阶都可以发现20-30个疑源类化石属。而某些常见的属(如Micrhystridium和Veryhachium)则被报道超过40余次。然而,我们还需要更多的数据才能找到二叠纪疑源类物种多样性的变化规律,而且需要对具小型膜壳的种类倍加留意。本文选取的七个剖面均位于华南地区二叠纪与三叠纪之交的地层中,包括中寨剖面、凉风垭剖面、煤山剖面、上寺剖面、甘溪剖面、峡口剖面以及东攀剖面。在这些剖面里找到了丰富且保存完好的疑源类化石。大部分化石的膜壳较小,其直径在20微米左右,如：Micrhystridium、Veryhachium以及Leiosphaeridia。同时也在上寺等剖面发现了一些具大型膜壳的疑源类化石,如Dictyotidium (膜壳直径达80微米)。而这些化石基本都在其它晚二叠世的地层中报道过。由于出现了十余种归属于Micrhystridium和Veryhachium的化石,且发现将其划归到物种级别较为困难。因此,本文从化石膜壳形态学的角度将Micrhystridium/Veryhachium complex类群划分为五类。其中,Veryhachium cylindricum亚属代表具椭圆形膜壳的类型；Veryhachium trispinosum亚属囊括具三角形膜壳的类群；而Veryhachium lairdii亚属则包含所有四边形膜壳的种类；Micrhystridium pentagonale亚属涵盖五边形或者六边形膜壳的类群;而Micrhystridium breve亚属则包括具球形膜壳的物种。为了研究华南地区二叠纪末期疑源类在海洋中的空间分布特征,文中选定六个剖面来分析。这些剖面囊括从近岸浅海到远岸深水的沉积环境。根据疑源类属种的生物多样性和物种的丰度,并结合前人的相关资料分析表明：(1)近岸地区的疑源类物种多样性较低,一般只有2-4种(如中寨和东攀剖面)；而相对远岸的大陆架地区物种多样性较高,可以达到10种左右(如煤山、上寺以及凉风垭剖面)；但是随着离岸距离的增加,大陆坡地区的疑源类物种多样性同样是较低的(如甘溪和峡口剖面)。(2)在疑源类化石属一级上,Leiosphaeridia、Reduviasporonites以及Micrhystridium从近岸浅水环境到远岸深水地区均有分布；Dictyotidium和Veryhachium更多地出现于开阔的海域。(3)Schizosporis则只在上寺和煤山剖面的二叠纪与三叠纪之交的地层附近出现(海平面较低时),可能指示一种相对近岸浅水的环境。(4)在物种级别上,具短突起的Micrhystridium breve在近岸环境中占据绝对优势,且对营养盐的浓度较为敏感,其生物量的变化与Fe和Al元素的含量变化较为一致。而膜壳较大(直径约80微米)的球形疑源类如Dictyotidium reticulatum一般出现在浅水区域,膜壳较小(直径约30微米)的Leiosphaeridia则更多地分布在水体较深的环境中。同时,膜壳光滑的Leiosphaeridia minutissima在近岸占据优势,而膜壳具颗粒纹饰的Leiosphaeridia microgranifera则在远岸较为丰富。(5)虽然链状疑源类Reduviasporonites出现在多个剖面,但是众多学者认为的二叠纪末期的“真菌高峰”在扬子板块的海相地层中并没有出现。需要指出的是,个体较大的Reduviasporonites chalastus(单胞直径约40微米)常分布于近岸环境;而个体较小的Reduviasporonites catenulatus(单胞直径约15微米)则更多地出现在远岸深水的区域。基于疑源类化石与动物化石的生物多样性变化规律,文中将华南二叠纪与三叠纪之交的地层划分为较为明显的三个阶段。阶段1对应的牙形石生物带为Clarkina yini-Clarkina Zhangi带,此阶段的疑源类化石生物多样性是三个阶段中最高的,发现疑源类达10属28种。与第一阶段相比,阶段2的疑源类化石生物多样性出现大幅降低,只出现7属11种。其对应的牙形石生物带为Clarkina meishanensis带、Hindeodus changxingensis带、Clarkina taylorae带以及Hindeodus parvus带。而阶段3对应Isarcicella staeschei带和Isarcicella isarcica带,只有一种疑源类化石Leiosphaeridia minutissima零星地出现在凉风垭剖面和甘溪剖面。二叠纪末期的生物大灭绝之前(阶段1),海洋生态系统总体保持动态平衡,动物以及浮游植物都具有较高的生物多样性。但是,由于环境的突变(如大规模的火山喷发、海平面的骤降以及温度升高等),海洋生态系统的平衡被打破。大多数的动植物由于对环境的不适应而惨遭灭绝,而生命力极强的蓝细菌却在这适宜的环境中大面积爆发,成为最主要的初级生产者。然而,蓝细菌的爆发却给环境带来了持续性的恶化,首先是加大了海洋透光带中光衰减的程度；其次是加速了海水中的氧气消耗;同时也为海洋动物提供了有毒的且贫营养的食物。这三点影响对其它浮游植物以及大多数的海洋动物来说都是致命的。但是,不同物种对于这些影响的反应却是不一样的。总体来讲,疑源类等其它类型的浮游植物基本从近岸海域中消失,只分布在远岸的水域。而个体较大的动物则基本消失,体型较小的动物凭借其对蓝细菌以及恶劣环境较强的适应性而得以残存(阶段2)。然而此次生物危机并未结束,随着温度的进一步升高以及无机营养盐的持续性输入,蓝细菌的爆发向远岸以及深水环境中扩张(阶段3；上寺剖面28层以及煤山剖面29层),致使大多数在前一阶段残存的种类也不能在这更为恶劣的环境中幸免而遭到灭绝,只剩下个别适应力极强的灾难种得以存活并繁盛(如双壳类的Claraia)。大规模的火山喷发可能是二叠纪末期生物大灭绝的导火索,其导致了生态环境的巨大变化,如温度升高等。本文认为：高温以及陆地风化的加强直接促使蓝细菌的持续性爆发。而蓝细菌的不断增殖则加深了海水的缺氧程度以及造成了其它浮游植物的大量减少。经过以上的变化,海洋的缺氧、高温以及食物的缺乏则是海洋动物灭绝的直接原因。更多还原

【Abstract】 The end-Permian extinction event (252Ma ago) is considered the greatest mass extinction in the history of the Earth, with over90%of all marine species becoming extinct. Many authors try to explain the causes of this extinction, linking it to environmental catastrophes, such as ocean anoxia, Siberian trap volcanism, sea-level changes, etc., but the causal factors of the extinction still remain controversial. Most studies documented the extinction of marine metazoan groups, and usually ignore the fluctuations of the diversity of primary producers in the marine environments at the Permian-Triassic boundary (PTB), although the presence of a possible "fungal spike" in the Late Permian let to much debate. Recently, a series of papers analyzed the cyanobacterial changes during the PTB interval in South China. On the other hand, a few papers documented in the PTB strata the biodiversity changes of the acritarchs, which are considered to represent the major part of the organic-walled microphytoplankton in the Palaeozoic. The objective of the present study is to document the different phytoplankton communities (including acritarchs and cyanobacteria) in the Chinese PTB strata and to try to analyze the relationships between the mass extinction and the phytoplankton community changes at the PTB, South China.Firstly, we present a synthesis of the Permian fossil record of acritarchs at a global scale. The revision shows that Permian acritarch descriptions have largely been neglected, compared to other palynomorph groups, such as spores and pollen grains. While larger organic-walled cysts, as known from the Lower and Middle Palaeozoic, are usually absent, many smaller acritarchs are commonly found in Permian palynological assemblages. During most of the Permian stages, acritarch show a genus richness of about20to30genera. Some genera, such as Micrhystridium and Veryhachium, have been reported in over40publications. Nevertheless, many Permian acritarchs still need to be documented in detail, and additional systematical studies, in particular of the very small taxa, are needed to fully understand the diversity and significance of Permian acritarch.The description of new material includes the analyses of diverse and well-preserved latest Permian phytoplankton assemblages from seven sections of the Yangtze Block (South China) from the localities Zhongzhai (Guizhou Province), Liangfengya (Chongqing City), Meishan (Zhejiang Province), Shangsi (Sichuan Province), Xiakou and Ganxi (Hubei Province) and Dongpan (Guangxi Province). Most of the species have been reported previously from other Late Permian sections elsewhere in the world. The South Chinese phytoplankton taxa are generally very small in size, usually displaying diameters of about20μm, and commonly include the genera Micrhystridium, Veryhachium and Leiosphaeridia. However, larger taxa with vesicles often exceeding80μm in diameter, such as Dictyotidium, are also abundant in the Shangsi section.The taxonomical descriptions of the acritarch assemblages include a reevaluation of the classification of two of the most common taxa. Due to the presence of large populations of Micrhystridium and Veryhachium, a simple classification scheme for the Micrhystridium/Veryhachium complex is proposed, based on the geometrical shape of the vesicle. We propose dividing the complex into five groups:the Veryhachium cylindricum group, representing all ellipsoidal specimens; the Veryhachium trispinosum group, with triangular shape vesicles; the Veryhachium lairdii group, with rectangular central bodies; the Micrhystridium pentagonale group, including all pentagonal specimens; and the Micrhystridium breve group, representing all spherical forms.Subsequently, in order to analyze the spatial (palaeoecological) distribution of the organic-walled microphytoplankton in the Late Permian, the palynological material from six of the investigated sections from the Yangtze Block, South China, displaying different sediment facies types (from neritic to offshore palaeoenvironments, including basinal facies) has been investigated. Based on the diversity and relative abundance of acritarch species and genera, the new data from the Chinese Late Permian sections provide similar patterns as those described from other geological periods:(1) low diversities with2to4acritarch species occur in nearshore environments, whereas the higher diversities (more than ten acritarch species) appear in the offshore environments;(2) at the generic level, the genera Leiosphaeridia, Reduviasporonites and Micrhystridium are distributed widely, from nearshore facies corresponding to shallow water environments to offshore facies corresponding to deeper water settings, whereas some genera, such as Dictyotidium and Veryhachium, have a narrower distribution, occurring on the continental shelf and towards the basin, indicating open marine environments;(3) the genus Schizosporis only occurs around the PTB, when the sea level declined, probably indicating nearshore environments with shallow water settings;(4) at the specific level, the species Micrhystridium breve, displaying short spines, and Leiosphaeridia minutissima are indicative of neritic facies, whereas the other species of Micrhystridium and Veryhachium with longer spines (e.g., Micrhystridium stellatum and Veryhachium hyalodermum) and Leiosphaeridia microgranifera indicate more open marine environments. Big spherical acritarch species (over80μm in diameter), such as Dictyotidium reticulatum, indicate shallow water environments;(5) in the PTB strata, the relative abundance of the enigmatic Reduviasporonites, interpreted by some authors as a fungal spore, is never higher than14%, indicating that a’spike’of Reduviasporonites did not occur in the Yangtze area. Reduviasporonites chalastus (40μm in length) obviously dominates in shelf environments of shallow water, whereas the smaller Reduviasporonites catenulatus (15μm in length) is more common in deeper water.Another part of the present study concerns the interpretation of the phytoplankton changes in the investigated interval. Based on the analysis of the organic-walled microphytoplankton in the PTB strata of South China three different stages of acritarch communities around the PTB can be identified. Diverse (28species attributed to10genera) and abundant acritarch assemblages occur in Stage1, corresponding to the Clarkina changxingensis and Clarkina yini conodont biozones. Subsequently, moderately diverse acritarch assemblages (11species in7genera) are present in Stage2at the Permian to Triassic transition, corresponding to the Clarkina meishanensis, Hindeodus changxingensis, Clarkina taylorae and Hindeodus parvus conodont biozones. Only one species (Leiosphaeridia minutissima) has been recorded in the Stage3possibly related to low biomass production, corresponding to the Isarcicella staeschei and Isarcicella isarcica conodont biozones.During the end Permian, the marine ecosystem was balanced before the mass extinction (Stage1; diverse and abundant metazoan and phytoplankton communities). However, this balance was perturbated by dramatic environmental changes (e.g., large-scale volcanic eruptions, drastic sea-level changes and higher sea-water temperatures), which most probably promoted cyanobacterial blooms at a worldwide scale, as recorded by the presence of lipid biomarkers and microbialites. The cyanobacterial proliferation possibly had very negative effects on the metazoan diversities, aggravated ocean anoxia, and reduced the abundance and diversity of other primary producers (increased light attenuation). Many species of different metazoan groups became extinct during the Stage2when the marine environments were perturbated (anoxic conditions and food-limitation), when only few primary consumers developed, in particular those with small sizes or/and those that were insensitive to the cyanobacterial toxin. Along with the rising temperature and intense inorganic nutrients (Fe and P), the cyanobacterial blooms proliferated in offshore and deep waters (bed28in Shangsi and bed29in Meishan), when the acritarchs (high quality food for metazoan) became rare. Many species of the primary consumers that survived the end Permian mass extinction disappeared during the earliest Triassic, probably due a degradation of the environmental conditions (high temperature, ocean anoxia and food deficiency). Only a few disaster taxa dominated in the ocean (e.g. the bivalve Claraia).If the end Permian mass extinction was triggered by large-scale volcanic eruptions, important ocean anoxia, large food deficiency and high temperatures, we consider that the cyanobacterial blooms enhanced the ocean anoxia and food deficiency during the PTB. Nevertheless, the relationship between the mass extinction and cyanobacterial blooms are more complex than that we previously thought.更多还原