Marine productivity variations and environmental perturbations across the early Triassic Smithian-Spathian boundary: Insights from zinc and carbon isotopes
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On the basis of proposing the existence of a karst carbon cycle and carbon sink at a watershed scale, this paper provides four pieces of evidence for the integration of geology and ecology during the carbon cycle processes in the karst dynamic system, and estimated the karst carbon sink effect using the methods of comparative monitoring of paired watersheds and the carbon stable isotope tracer technique. The results of the soil carbon cycle in Maocun, Guilin, showed that the soil carbon cycle in the karst area, the weathering and dissolution of carbonate rocks under the soil, resulted in a lower soil respiration of 25% in the karst area than in a non-karst area (sandstone and shale), and the carbon isotope results indicated that 13.46% of the heavy carbon of the limestone is involved in the soil carbon cycle. The comparative monitoring results in paired watersheds, suggesting that the HCO3- concentration in a karst spring is 10 times that of a rivulet in a non-karst area, while the concentration of inorganic carbon flux is 23.8 times. With both chemical stoichiometry and carbon stable isotopes, the proportion of carbon in karst springs derived from carbonate rocks was found to be 58.52% and 37.65% respectively. The comparison on carbon exchange and isotopes at the water-gas interface between the granite and carbonate rock basins in the Li River showed that the CO2 emission of the karst water is 10.92 times that of the allogenic water from the non-karst area, while the carbon isotope of HCO3- in karst water is lighter by 8.62‰. However, this does not mean that the karst water body has a larger carbon source effect. On the contrary, it means the karst water body has a greater karst carbon sink effect. When the karst subterranean stream in Zhaidi, Guilin, is exposed at the surface, carbon-rich karst water stimulated the growth of aquatic plants. The values of carbon stable isotopes in the same species of submerged plants gradually becomes heavier and heavier, and the 512 m flow process has a maximum range of 15.46‰. The calculation results showed that 12.52% of inorganic carbon is converted into organic carbon. According to the data that has been published, the global karst carbon sink flux was estimated to be 0.53-0.58 PgC/a, equivalent to 31.18%-34.41% of the global forest carbon sink flux. In the meanwhile, the karst carbon sink flux in China was calculated to be 0.051 PgC/a, accounting for 68% of its forest carbon sink flux.
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The biological extinction that occurred at the Permian–Triassic boundary represents the most extensive loss of species of any known event of the past 550 million years. There have been a wide variety of explanations offered for this extinction. In the present paper, a number of the more popular recent hypotheses are evaluated in terms of predictions that they make, or that they imply, concerning the global carbon cycle. For this purpose, a mass balance model is used that calculates atmospheric CO 2 and oceanic δ 13 C as a function of time. Hypotheses considered include: ( i ) the release of massive amounts of CO 2 from the ocean to the atmosphere resulting in mass poisoning; ( ii ) the release of large amounts of CO 2 from volcanic degassing; ( iii ) the release of methane stored in methane hydrates; ( iv ) the decomposition and oxidation of dead organisms to CO 2 after sudden mass mortality; and ( v ) the long-term reorganization of the global carbon cycle. The modeling indicates that measured short-term changes in δ 13 C at the boundary are best explained by methane release with mass mortality and volcanic degassing contributing in secondary roles. None of the processes result in excessively high levels of atmospheric CO 2 if they occurred on time scales of more than about 1,000 years. The idea of poisoning by high levels of atmospheric CO 2 depends on the absence of subthermocline calcium carbonate deposition during the latest Permian. The most far-reaching effect was found to be reorganization of the carbon cycle with major sedimentary burial of organic matter shifting from the land to the sea, resulting in less burial overall, decreased atmospheric O 2 , and higher atmospheric CO 2 for the entire Triassic Period.
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The existence of unusually large fluctuations in the Neoproterozoic (1,000–543 million years ago) carbon-isotopic record implies strong perturbations to the Earth's carbon cycle. To analyze these fluctuations, we examine records of both the isotopic content of carbonate carbon and the fractionation between carbonate and marine organic carbon. Together, these are inconsistent with conventional, steady-state models of the carbon cycle. The records can be well understood, however, as deriving from the nonsteady dynamics of two reactive pools of carbon. The lack of a steady state is traced to an unusually large oceanic reservoir of organic carbon. We suggest that the most significant of the Neoproterozoic negative carbon-isotopic excursions resulted from increased remineralization of this reservoir. The terminal event, at the Proterozoic–Cambrian boundary, signals the final diminution of the reservoir, a process that was likely initiated by evolutionary innovations that increased export of organic matter to the deep sea.
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Significance The Central Atlantic magmatic province (CAMP) is coincident with the end-Triassic extinction event and several negative carbon isotope excursions (CIEs). Sill emplacements in Brazil would have generated extensive volatiles and degassing due to the contact metamorphism of evaporites, organic-rich shales, and hydrocarbons. Thermogenic carbon release from contact metamorphism represents a plausible source for 12 C; however, this has not yet been explored from a carbon cycle approach. This study explores the effects of thermogenic carbon release from CAMP using carbon cycle modeling and shows that it represents a credible source for the negative CIEs at the end-Triassic. It strengthens the hypothesis that the subvolcanic part of a large igneous province is of major importance for understanding carbon cycle disruptions.
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Abstract The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO 2 ( p CO 2 ) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a 12 C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric p CO 2 and δ 13 C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian p CO 2 and δ 13 C proxy data, and that thermogenic carbon (δ 13 C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.
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High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.
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Significance Mass extinctions are major drivers of macroevolutionary change and mark fundamental transitions in the history of life, yet the feedbacks between environmental perturbation and biological response, which occur on submillennial timescales, are poorly understood. We present a high-precision age model for the end-Permian mass extinction, which was the most severe loss of marine and terrestrial biota in the last 542 My, that allows exploration of the sequence of events at millennial to decamillenial timescales 252 Mya. This record is critical for a better understanding of the punctuated nature and duration of the extinction, the reorganization of the carbon cycle, and a refined evaluation of potential trigger and kill mechanisms.
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