| 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. |
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