The most severe mass extinction of terrestrial plants is associated with the Permian/Triassic boundary, which occurred at 250 Ma. During this time, plant life shifted from a dominance of Lycopods, Club Mosses, and Sphenopsids to a dominance of Tree Ferns and Conifers. Numerous plant species steadily died off, however there was not a large taxa decrease overall. What was formerly taken to be a mass extinction may truly be a die back of plant species. The three stages of recovery show that various examples of species that died off at the Permian/Triassic boundary gradually re-emerged within the fossil record. The die back of plants associated with the P/T boundary corresponds with a large increase in Hydrogen Sulfide in the air, as well as with a period of Anoxia within the oceans. The formation of Pangaea is most likely a viable source for the change in climate and ocean circulation which could have led to these two events. Evidence for the change in climate is shown by a change in river morphology to braided rivers which are consistent with geomorphic consequences stemming from a rapid and major die-off of rooted plant life, and a coal gap of 5 million years, which provides verification of the anoxia which occurred at this time. Although there is support for plant die back and a huge ecological change, there is a lack of definite proof of global mass plant extinction.
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Paleozoic extinctions reveal evidence for the largest extinction events in the marine and terrestrial biospheres. The Permian Triassic extinction eliminated over 90% of marine species, and an approximation of nearly 70% of all terrestrial vertebrate families were eradicated. This extinction event was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 Ma, and occurred in several short pulses. It is commonly accepted that it occurred in one to three extinction pulses over time. This purging of was punctuated by a final pulse that affected many benthos,
nekton, and zooplankton, as well as many other marine species, which suggest a die back in marine primary producers. An associated long term die back pattern in the late Permian terrestrial biosphere may be assumed from the biodiversity levels of these vertebrates and insects, which also endured an extreme loss of taxa. Plants have always been perceived to have an elevated persistence capacity as opposed to many genera of animals and plankton during the Permian Triassic extinction, however, they are often looked at for records of past terrestrial ecosystem collapse and die back due to their role in global biomass storage and their sensitivity to ecological change.
Over the course of the Permian Triassic extinction event, there was a large die back in plants, but not much of a taxa decrease. As compared to other extinction events such as the K/T impact boundary and the Triassic Jurassic plant turnover and diversification event, there was a much smaller decrease and much less extreme die back of terrestrial plants. Over the course of the Permian Triassic boundary, taxa dominance shifted taxa dominance in terrestrial plants. In the European part of the mega-continent Pangaea, conifer forests were dominant during the latest Permian in a tropical setting. These forests became extinct during the crisis and for 4-5 Mya, small lycopsids experienced an adaptive radiation and dominated the landscapes.
Throughout the Permian period, the Earth was slowly experiencing an increase in heat and aridity, and as a consequence large areas on the super-continental landmass became areas of desert. The red colors of many soils formed in this period reflect this hot and dry climate. The increase in aridity and high temperatures on earth was due to the fact that the super continent Pangaea was forming at this time. Pangaea was formed by the collisions of Gondwana, Euramerica, and Siberia, and later on the collision of Laurasia. During this time, swampy tropical forests gave way to more arid environments, which caused a vast change in plant taxa abundance. The desiccated continental interiors experienced great seasonality, and the Polar Regions were barren tundra. Glaciers were also present at these high latitudes, yet as the Permian shifted to the Triassic, earthâ€™s temperatures increase 4 to 5 degrees Celsius, and many of these glaciers melted. The core of Pangaea was occupied by an intercontinental sea titled Tethys, while the entire super continent was surrounded by a primary ocean by the name of Panthalassa. In order to adapt to this new climate, dissimilar plant taxa became dominant at this time, while plants and animals that were less suited to adapt to the abrupt alteration in environment died off, or decreases.
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The boundary could have transpired due to a conglomeration of multiple factors, including the formation of Pangaea, conversely for plants, some factors may have been more crucial in determining and affecting their die back than others. A large contributing factor to the plant die back that we see at the boundary could be the increased levels of Hydrogen Sulfide that occur in the atmosphere at this time. Simple calculations show that
if the deep ocean Hydrogen Sulfide concentrations increased beyond a decisive threshold while the earth is going through a time of oceanic anoxia, the chemo cline separating deep waters concentrated with sulfides from the oxygenated surface waters could have risen rapidly to the surface of the worldâ€™s oceans. A severe anoxic event at the end of the Permian could have made sulfate-reducing bacteria the dominant force in oceanic ecosystems Atmospheric photochemical modeling has shown us that the result of this is high fluxes of Hydrogen Sulfide to the earthâ€™s atmosphere, which can be deadly and dangerous to terrestrial plants, as well as terrestrial and marine animals. This change of H2S would be liable to have led to noxious levels of H2S in the atmosphere, and can lead to events such as acid rain, and increased Ultraviolet Radiation, both of which can cause vast ecological change.
Because Hydrogen Sulfide levels were most likely affected by a period of anoxia in the worldâ€™s oceans, the isotopic sulfur record shows a period of time when the oceans were largely anoxic. During the upper Permian and developing into the Lower Triassic, a fundamentally stagnant, anoxic, stratified ocean began to form. Oceanic anoxic events are strongly linked to lapses in key oceanic current circulations, to climate warming and greenhouse gases. Many P/T boundary cherts were found in Japan that represent sediments deposited in the deep sea sections of the Panthalassa Ocean. These cherts show that at the time of the Permian Triassic boundary, there was a large rise in rocks and sediments containing pyrite (FeS2), rather than hematite (Fe2O3). Hematite contains iron, and often has a reddish color, showing that oxygen was supported at the time. During the early to middle Permian, and the middle to late Triassic, hematite was present
in ocean sediments, however at the time of the P/T boundary, hematite was virtually nonexistent and the rocks took on a gray or black color due to the lack of oxygenation and abundance of pyrite. The biotic response to the change in ocean stratification was very severe, as the onset timing of the oceanic stratification coincides with the decline of the biota at the end of the Guadalupian, and the final die back of plants and animals on land corresponds with the climax of oceanic super anoxia.
A major event that occurred during the same time period as the P/T extinction event was the explosive eruption of the Siberian Traps. The eruption of the Siberian Traps is the largest volcanic eruption known to date. It lasted for nearly 1 million years, and is a likely cause for the plant extinction event. The real power of the Siberian Traps was the climate altering potential by the emission of ash and gases. The Siberian Traps is recognized as having a large proportion of pyroclastic deposits relative to other flood basalts. This indicates an explosive nature with much ash and gasses being released into the atmosphere. The majority of these basalts was erupted over an extremely short time interval on a geologic timescale, beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year. Due to the rapid release of CO2 and sulfur into the air, and the expansive ash clouds that most likely occurred, the eruption o the Siberian traps are likely candidates for the shift in plant taxa and plant die back. The change in atmosphere and climate that this volcanic eruption caused is a very likely contributing factor. Combined with rainout of H2SO3 and other acids, the increase atmospheric CO2 could also cause increased weathering.
Evidence for a large plant die back across the Permian Triassic boundary is shown by a large coal gap in sediment and soils from the time of the P/T boundary. Early Triassic coals are unknown, and Middle Triassic coals are rare and thin. The Early Triassic coal gap began with extinction of peat-forming plants at the end of the Permian (ca. 250 Ma), with no coal known anywhere until Middle Triassic (243 Ma). Permian levels of plant diversity and peat thickness were not recovered until Late Triassic (230 Ma).
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