Coupling Climatic And Biodiversity Changes With Volcanisms During The Palaeozoic-Mesozoic Upheavals In South China

In a tectonic perspective, the East Tethys likely represnted a transition from ocean basin extension to closing during the late Palaeozoic-Mesozoic interval.In the Middle Permian of Palaeozoic, extensive marine basalt was exposed in the Emeishan area on the southwest edge of Yangtze Platform, the Jianshajiang River, Songpan-Ganze Terrance and Amne Machin area, suggesting Eastern Tethys was probably in an extension stage. The disappearance of red radiolarian chert in eastern Kunlun and the of the super continent Pangea may suggest a closing stage of the East Tethys. The subduction of oceanic crust in the closure of Tethys likely linked to the activities of volcanic arc, which might be the source of wide-spreaded ash layers observed around the Permian-Triassic boundary in South China.Two major volcanisms accompanied such transion from extension to closure and triggered significant environmental changes, large carbonate carbon isotope perturbation and mass extinctions. Comparing these two volcanic events and their environmental-ecological consequenses thus enable us to obtain a better understanding on the co-evolution of Earth system. This conforms recent developments of the Earth System Science.Extensive basalt was widely developed at the Emeishan region, Songpan-Ganzi Terrance and Amne Machin area during the Middle-Late Permian transition, among which the Emeishan Flood Basalt was the most volumnous and could potentially trigger significant environmental changes. The Permian Emeishan LIP was a relatively small LIP that erupted on marine setting during its early stage.This unique setting enables us to conduct comprehensive conodont biostratigraphic and facies analysis studies on Maokou Limestones that were underlying and/or intercalated the initial lava flows. The studies sections span the localities from the LIP central area via LIP periphery to several hundred kilometers beyond the LIP margin. The results suggested the initial onset of the Emeishan volcanism was probably around middle Capitanian Jinogondolella altudaensis conodont zone, which was seen at Xiong Jia Chang and Pingdi sections of Guizhou Province. The large scale eruption only occurred later around J. xuanhanensis zone, which was seen at six sections of Sichuan and Yunnan Province. This phenomenon is also observed in other LIP (e.g. Parana-Etendeka) where the initial eruptions were small and increased in extent and volume with time. The onset of Emeishan volcanism coincided with-5～-6‰negative shift of carbonate carbon isotope around J. altudaensis zone, the extinction of keriotheca-walled fusulinaceans (e.g., Neoschwagerinidae), and the changeover of calcareous algae. Together with former studies, we conclude that the initial and main stages of Emeishan volcanism were within the Capitanian [～263Ma (million years ago)] rather than around the Gaudalupingian-Lopingian boundary (-259Ma). Our conodont data provide not only important constraints on biostratigraphic controls of Emeishan volcanism, but also the correlation criteria for further studies. The Guadalupian Crisis predates a6‰carbon isotope negative excurtion suggesting subsequent severe disturbance of the ocean-atmosphere carbon cycle.The Siberian flood basalt unleashed around the Permian-Triassic interval probably played critical roles in the end-Permian mass extinction and likely trigered a green house state in the aftermath of this crisis. The Early Triassic interval (～250Ma) came at the end of an interval of major climatic change in the aftermath of the end-Permian mass extinction. The Carboniferous-Permian interval saw major and prolonged glaciations in southern polar latitudes but this ice age terminated by the end of Early Permian. Small, mountain glaciers may have persisted a little longer but after the Middle Permian there were few if any glaciers left. The ocean at this time was dominated by diverse and abundant organisms such as the ammonoids, brachiopods, corals, foraminifera, and radiolarians. On land, the dominant animals of the Late Permian were the herbivorous pareiasaurs, and the top predators were the gorgonopsids. They lived in a terrestrial ecosystem with vegetation that was dominated by seed-bearing gymnosperms. Most of these marine and terrestrial organisms were lost in the end-Permian mass extinction and replaced by the low diversity and monotonous assemblages in the Early Triassic:a shrub-like tree fern named Dicroidium and a pig-sized herbivorous reptile named Lystrosaurus dominated on land while mollusks, notably a bivalve called Claraia, appeared in a great number in the ocean. The Early Triassic world took an unusually long time to recover from the mass extinction, diversity in most ecosystems remained low for5million years. The interval was characterized by a global absence of coal burial, deep-sea radiolarian chert formation and metazoan reefs and the prevalence of small and dwarfed animals. Conditions must have been harsh at this time and new Mesozoic marine communities were only gradually established in the Middle Triassic.Calculating the temperature changes during this extinction has proved difficult because calcite-based thermometers of fossils like brachiopod suffered great losses in the end-Permian mass extinction and are very rare in the Early Triassic. Other shelled-fossils, such as bivalves, mainly consisted of aragonite or high-magnesium calcite that is metastable during the diagenasis and thus unreliable for reconstructing the original oxygen isotopic composition that is used to determine temperatures. Conodont bio-apatite is very resistant to post-depositional changes and is ideal for temperature reconstructions in deep time. Equally fortunate, conodonts, suffered little loss in the mass extinction and so they are able to provide a continuous temperature record across the mass extinction.Conodont oxygen isotope ratios suggest that the equatorial sea surface temperatures of the latest Permian were around25℃, very similar to those of today (the annual mean value of the equatorial sea surface temperature is roughly in25-30℃range). The temperatures quickly increased to32℃at the beginning of Early Triassic and continued to increase, reaching a thermal maximum within the Griesbachian. Many Permian survivors, such as the conodont Hindeodus and the ammonoid Otoceras, went extinct at the end of the Griesbachian and these losses may have been caused by the late Griesbachian Thermal Maximum. The following substage, the Dienerian, saw a3-4℃temperature decrease which coincides with a transient recovery pulse in which several groups began to diversify. The early and middle parts of Smithian, represent a relatively stable high temperature plateau but the late Smithian saw a further2℃temperature increase to produce sea surface temperatures that exceeded40℃during the late Smithian. This was the hottest interval in the Early Triassic and one of the hottest intervals ever recorded. The Spathian, saw an initial cooling trend followed by relatively stable temperatures in the middle part and further cooling at the end of this stage and stabilization of temperatures.High temperatures would be expected to exert major impacts on marine lives. This is much more clear in the aftermach of the end-Permian mass extinction. Temperatures above45℃cause protein denaturation for most animals and their response, to produce heat-shock proteins, can only delay death for a short interval. However, for marine animals, the thermal limit is even lower because aerobic metabolic demands increase with temperature whilst oxygen solubility decreases in seawater and bodily fluids as temperature increases. Thus, most marine organisms cannot long survive when temperatures exceed35℃. This is most clearly for creatures with high performance and consequently high oxygen demand, such as ammonoid cephalopods and fish. The former suffered a major diversity decline in the late Smithian Thermal Maximum that was silimar great compared to their losses during the end-Permian mass extinction. Nektonic vertebrates, such as fishes and ichthyosaurs, show both high mobility and low oxygen-dependent thermal tolerance. They could vacate the equator and migrate to more comfortable areas when temperature increases. This is clearly shown in the fossil record. In the late Permian, fishes were globally distributed. They became rare in equatorial waters during the Griesbachian and Dienerian and became very scarce in low latitudes during the Smithian. In contrast, at higher latitudes during the Smithian, in places such as Spitsbergen and British Columbia, the fish fossil record is exceptional with abundant and diverse faunas present. Interestingly, ichthyosaurs first appeared in northern high latitudes during the Smithian and yet they did not appear in low latitude waters until the late Spathian several million years later. Fishes and ichthyosaurs returned to the equator during the late Spathian to early Anisian (Middle Triassic) and showed a globally distributed pattern that had not seen since the late Permian.The active migration of vertebrates with temperature oscillations can be examined on land. Most abundant and fairly diverse Early Triassic tetrapod fossil assemblages are known from high latitudes such as South Africa and Antarctica in the south and the Russian Federations in the north. In contrast, Early Triassic terrestrial strata from low latitudes is widespread (e.g. the Buntsandstein of central Europe) and has been studied intensively for200years and yet its vertebrate remains are exceptionally rare; only in the Spathian and Middle Triassic do they start to become common. Again the prohibitively high equatorial temperatures at this time can explain this pattern:life in the tropics was unsustainable due to the heat.The uneven recovery in the aftermach of the end-Permian mass extinction is a matter of intense debets and has puzzled geologists for years. Groups with swimming abilities and high origination rates, such as conodont and ammonoids, recovered much faster. Their diversity shows a clear inverse relationship with temperature fluctuations:high temperatures corresponding to low diversity whist low temperatures corresponding to high diversity. In contrast, sessile groups such as brachipods and gastropods show much lower diversity. High temperatures in the Early Triassic supressed both marine plants and invertibrates in the low latitudes. For example, calcareous algae were exceptionally rare in South China in the first two substages of Early Triassic. Calcareous algae did not become more common until the Spathian, a stark contrast to the high latitudes records—coeval algae thrived in Greenland, Spitsbergen, British Columbia. Sessile groups such as brachipods and gastropods lack the abilities to avoid hostile environments thus show much lower diversities than the free-living groups and recovered much slower in the aftermath of P-T mass extinction.Our high temperature scenario offers alternative explanations for the coal gap and Lilliput effect in the Early Triassic. The middle30s℃sea surface temperatures are likely to correspond to continental temperatures of40-45℃. Such high temperatures would have been inimical to most plants and animals because the photorespirations dominate over photosynthesis in C3plants when temperatures exceed35℃whist the upper thermal tolerance of ectotherms is below45℃. Furthermore, high temperatures enhance the activities of decomposers (e.g., bacteria and fungi). The low plant mass and high decomposition rate under high temperature conditions were likely responsible for the suspension of global peat formation in the Early Triassic. The gymnosperm forests and peat formation first returned at high latitudes in the late Spathian to Anisian whist this return was much later in the equatorial. In equatorial South China, peat-forming conditions were not restored until the Late Triassic, about15million years after their disappearance. The prevalence of small species, both on land and in the ocean, is a stand-out feature in the Early Triassic low latitudes fossil records-a phenomenon that has been named the "Lilliput effect". The prevalence of small size can be explained by the observation that many organisms decrease their body size as temperatures increase. Together with increased juvenile mortalities, hothouse environments will produce fossil records that are dominated by the small. Similar size reductions among mammals are also observed during the Pal eocene-Eocene transition—another hothouse interval some55million years ago.In conclusion, the volcanisms (especially the LIP activities) were probably the dominate factors on climate changes during the Palaeozoic-Mesozoic transition. The rapid warming during the Permian-Triassic transition was likely induced by the massive carbon release of the Siberia Traps. Sillisic volcanisms may have contributed to the maintaince of the～5Myr warmhouse climate in the Early Triassic. Transient warming events in greenhouse worlds normally represent enhanced effects of positive feedbacks, accelerated and magnified initial carbon ejections (in this case, eruptions of the Emeishan LIP and the Siberia Traps), before negative feedbacks restore the carbon cycle to a steady state. This is likely a key reason that carbon ejections predates two thermal maxima in the Early Triassic. However, it is noteworthy that the climate evolution of the Earth involves more complex feedbacks and compensations of carbon cycles. Chemical weathering is likely to be high under the hothouse conditions. High atmosphere convections and run-off rate in middle-high latitudes will bring more nutrients to oceans thus stimulates primary productivities. These processes will either consume CO2directly or convert CO2into organic carbon and ultimately draw down the atmospheric pCO2level.In conclusion, this study:1). firstly established a high-resolution temperature record for the latest Permian to early Middle Triassic interval and suggest that the sudden temperature raise in P-T boundary interval coincide with the carbon ejections of the Siberia Traps. The following Early Triassic is one of the hottest green house interval during the Phanerszoic. Minor silicic volcanisms could contribute to maintain such presistant warm house conditions for5Myr (million years);2). suggested high temperatures controlled the nature and pace of exinction and recovery in the Early Triassic and likely a diving factor for the Lilliput effects and the dynamic immigrations of faunas and flores;3). established a hypothesis that the high temperatures were a contributing factor for the coal and coral reef gaps;4). established high-resolution conodont biostratigraphic framework for the Emeishan flood province and suggest that the major volcanic activities occurred in the Middle Permian;5). established a cause-and-effect link between the eruptions of Emeishan Basalt and the Middle Permian mass extinction—the extinctions of keriotheca-walled fusulinaceans and the changeover of calcareous algae coincided with the main phase of volcanism;6). careful obseversions on the Maokou-Emeishan contacts across the Emeishan LIP offered no evidence for a "kilometer-scale pre-volcanic uplift". Rather, a dynamic topography variation during the plume acitivies is prosposed for the Emeishan Large Igneous Province (LIP).