Carbon has two stable isotopes, carbon¹² and carbon¹³, of which plants
preferentially incorporate carbon¹² into their tissues as it requires less energy to fix; this means that
sediments with a high plant-derived carbon content (such as coal bed)
will tend to be enriched in carbon¹² relative to sediment without (such
as marine limestones). Furthermore, an increase in carbon in the
atmosphere from burning plant matter, either as forests or coal beds,
will tend to lead to an increase in the relative amount of carbon¹² in
all sediments, known to geochemists as a positive organic carbon isotope excursion, whereas an increase in atmospheric carbon from other sources,
such as volcanic eruptions, will tend to lead to a a drop in carbon¹² in all sediments, or a negative organic carbon isotope excursion.The Permian-Triassic mass extinction was the most severe extinction event of the Phanerozoic, both in marine and terrestrial settings, but the relative timing of these crises is debated. A negative carbon isotope excursion in both carbonate and organic matter is seen at the main extinction horizon and is usually attributed to release of volcanic carbon. Most proposed kill mechanisms for the Permian-Triassic mass extinction are linked to the effects of Siberian Traps eruptions. A spike in mercury concentrations observed at the onset of the Permian-Triassic mass extinction, thought to be derived from Siberian eruptions, provides a chemostratigraphic marker in marine records. A similar mercury enrichment event has also been documented in contemporaneous terrestrial sediments. Marine records show widespread environmental instability prior to the Permian-Triassic mass extinction, and a new study of the Sydney Basin (New South Wales, Australia), suggests that the collapse of southern high-latitude floras occurred significantly before the onset of marine extinctions roughly coincident with onset of northern high latitude marine stress.
In a paper published in the journal Geology on 3 January 2020, Daoliang Chu of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Stephen Grasby of the Geological Survey of Canada, Haijun Song, also of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Jacopo Dal Corso of the School of Earth and Environment at the University of Leeds, Yao Wang, again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Tamsin Mather of the Department of Earth Sciences at the University of Oxford, Yuyang Wu, Huyue Song, Wenchao Shu, and Jinnan Tong, once again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, and Paul Wignall, also of the School of Earth and Environment at the University of Leeds, evaluate the timing and nature of the terrestrial crisis at the End of the Permian in southwest China by examining variations in fossil charcoal abundance from paleo–tropical peatlands to explore changes in wildfire occurrence and the carbon-isotope composition of land plant cuticles, charcoal, and bulk organic matter to track changes in the isotopic composition of atmospheric carbon dioxide. In addition they investigated sedimentary mercury concentrations, and the integration of their record with carbon isotope values permits chemostratigraphic correlation of terrestrial and marine records.
Chu et al. examined the continental Permian-Triassic transition in cored borehole ZK4703, drilled 15 km south of Fuyuan County in Yunnan Province, China, and the Chinahe outcrop section, 30 km southeastern of Xuanwei City, both from the border area between western Guizhou and eastern Yunnan in southwestern China. Latest Permian to earliest Triassic terrestrial strata in this region include, in ascending order, the fluvial-coastal swamp facies of the Xuanwei and Kayitou Formations. The former consists of sandstone, mudstone, and common coal beds. The associated plant fossils belong to the Gigantopteris flora and include Pecopterids (Tree Ferns), Gigantopterids (a morphologically advanced group of Permian Vascular Plants that disappeared in the End Permian Extinction), Lycopsiales (Giant Club Mosses), and Equisetales (Horsetails) taxa, collectively regarded as tropical rainforest-type vegetation. The Kayitou Formation (latest Permian to earliest Triassic age) is similar to the underlying Xuanwei Formation, but lacks coal and is shale dominated. Previous studies showed that the loss of the Gigantopteris flora occurred in the lowest Kayitou Formation.
Late Permian to Early Triassic palaeogeographic map showing locations of the ZK4703 core (25.54151°N, 104.28994°E) and the Chinahe section (26.13077°N, 104.35637°E) in southwestern China, and the Meishan section, south China. Chu et al. (2020).
Organic carbon isotopes, charcoal abundance, fossil plant ranges, total organic carbon, total sulphur concentrations, and aluminium and Mercury contents were assessed through the Permian-Triassic transition in the ZK4703 core and at the Chinahe outcrop. To avoid facies variation issues, only mudstone samples were processed for charcoal. Some charcoal was examined under scanning electron microscope to confirm identification. To ensure that the charcoal concentrations were not affected by variations in the nature or abundance of organic material, its abundance was normalised to phytoclast abundance and total organic carbon.
Location map of the studied section. Chinahe section (26.13077°N, 104.35637°E) is located in the Chinahe Viliage of the Tianba town, Xuanwei City. ZK4703 core (25.54151°N, 104.28994°E), drilled in Anzichong Viliage of Dahe Town, Qujing City. Chu et al. (2020).
The proportion of carbon¹² drops sharply in the lower part of the Kayitou Formation at Chinahe, both in organic matter and charcoal (a negative organic carbon isotope excursion). At the same time bulk organic matter and palynomorphs (pollen fossils) from the ZK4703 core section also show a drop in carbon¹² values.
Cuticle and charcoal particles under binocular microscope and scanning electron microscope. Chu et al. (2020).
The abundant, peat-forming Gigantopteris flora is seen at six levels in the Xuanwei Formation at Chinahe, and is dominated by well-preserved, large leaves. Both diversity and abundance of this flora decline drastically at the very top of the formation at a level that corresponds to the onset of the negative negative organic carbon isotope excursion. Thereafter, the flora consists of a monotonous assemblage of small plants, mostly Annalepis and Peltaspermum.
Typical Gigantopteris flora from the Xuanwei Formation of the Chinahe section. (A) Gigantopteris dictyophylloides; (B) Annularia pingloensis; (C) Lobatannularia sp.; (D) Pecopteris marginata; (E) Gigantonoclea guizhouensis; (F) Pecopteris sp.; (G) Compsopteris contracta; (H) Abundant plant leaf fossils preserved on the same bedding surface. Chu et al. (2020).
At Chinahe, the charcoal abundance is less than 300 particles per 100 g rock prior to the a negative organic carbon isotope excursion, but rises briefly above background levels during the onset of the excursion (1524 particles per 100 g at 25 m log height), and ranges from 400 to 1600 particles per 100 g in the 4 m interval of the uppermost part of the Xuanwei Formation to lower part of the Kayitou Formation. Similarly, in the ZK4703 record there is a sharp increase in charcoal abundance, from under 400 particles per 100 g below 15 m, to over 2400 particles per 100 g above 16.5 m height at the base of the Kayitou Formation. Scanning electron microscope observation shows that the charcoal preserves anatomical details and has similar preservation and structures with variable size, indicating minimal transport sorting. The reported variations in charcoal abundance do not appear to be an artifact of preservation or changes in terrestrial organic delivery, because variations in preserved phytoclasts (microscopic plant fragments) and total organic carbon do not vary with charcoal abundance.
The plant fossil ranges and species richness of the Chinahe section. (1) Peltaspermum sp.; (2) Annalepis sp.; (3) Compsopteris contracta; (4) Fascipteris densata; (5) Cladophlebis permica; (6) Annularia pingloensis; (7) Compsopteris sp.; (8) Lobatannularia heianensis; (9) Pecopteris marginata; (10) Lobatannularia cathaysiana; (11) Pecopteris guizhouensis; (12). Rajahia guizhouensis; (13) Gigantonoclea sp.; (14) Stigmaria sp.; (15) Gigantonoclea guizhouensis; (16) Gigantopteris dictyophylloides; (17) Pecopteris sp.. Chu et al. (2020).
Mudstone total organic carbon concentrations are relatively high in the Xuanwei Formation and modestly enriched concentrations persist into the lower part of Kayitou Formation before dropping at the 27 m log height at Chinahe. Both overall mercury levels and the mercury-total organic carbon ratio rise above background levels immediately above the interval with the onset of the negative organic carbon isotope excursion and increased charcoal abundance. High overall mercury levels and the mercury-total organic carbon ratios can also be observed at higher stratigraphic levels, with a peak value at 19.75 m in the ZK4703 core, which is about 50 times background levels. Overall mercury levels and the mercury-total organic carbon ratio drop to the previous baseline values above 37 m at Chinahe and 25 m in ZK4703. The weak correlation between overall mercury levels and the mercury-total organic carbon ratios suggests that the mercury fluctuations are not affected by changes in total organic carbon. Additionally, the ZK4703 core has low total sulphur contents which show no significant covariation with mercury values. Correlation between Aluminium and Mercury concentrations is also weak, indicating that mercury fluctuations are not controlled primarily by clay content, even if some mercury is probably adsorbed onto clay. Nonetheless, there is secular variability in the mercury/aluminium ratio, with very low background mercury/aluminium values below and above the mercury anomaly and enriched mercury/aluminium values within the interval.
Volcanic emissions represent one of the largest natural inputs of mercury to the atmosphere, and the mercury enrichment seen in many marine Permian-Triassic boundary sequences is thought to record large-scale Siberian Traps eruptions. Volcanic mercury emissions from this source may have been up to 10 000 milligrammes per year (roughly 14 times natural background levels). Thermogenic release of mercury from baking of organic-rich sediments on contact with Siberian Traps intrusions is another potential source of mercury. Terrestrial plants constitute a large mercury reservoir, and so wildfires can also contribute significantly to mercury fluxes to the atmosphere and freshwater environments such as those studied by Chu et al., who propose that the mercury spikes observed in terrestrial and marine successions provide a useful correlative tool between terrestrial and marine records along with carbon isotope ratios.
The onset of the main phase of marine extinctions in South China, at the top of the Clarkina yini Zone, correlates with a peak in mercury concentrations and mercury/total organic carbon ratios, while a second phase of extinctions at the top of the Isarcicella staeschi Zone corresponds to a rise in mercury concentrations and mercury/total organic carbon ratios that peaks in the following Isarcicella isarcica Zone. The relative magnitude of these peaks varies between sections: at Meishan, South China, the lower mercury/total organic carbon ratios peak is the largest, whereas at Guryul Ravine, Kashmir, the second peak is larger. Levels of organic carbon began to decline somewhat before the marine extinctions in the Clarkina changxingensis Zone.
In the terrestrial sections of southwestern China, the floral mass extinction (and charcoal peak) starts with the onset of the negative organic carbon isotope excursion. Mercury concentrations begin to slightly rise at the same time, while mercury/total organic carbon ratios shows a sharp spike at the minimum of the negative organic carbon isotope excursion. The mercury and mercury/total organic carbon ratios peak 4–6 m above the terrestrial extinction level and likely correlate with the rise in mercury/total organic carbon ratios values seen at the end–Isarcicella staeschi Zone that saw diverse taxa disappear from Triassic oceans. Thus, the terrestrial crisis seen in equatorial sections of China appears to predate the main marine extinction phase (which occurred near the low point of negative organic carbon isotope excursion), and likely dates to the late Clarkina changxingensis Zone.
Chu et al.'s results demonstrate that a synchronous onset of the negative organic carbon isotope excursion is present in the bulk organic matter, cuticle, and charcoal carbon isotope records from terrestrial settings. Changes in organic carbon values of land plant cuticles record changes in atmospheric CO₂. Chu et al. suggest that the observed negative organic carbon isotope excursion in cuticles and fossil charcoal reflects an injection of carbon¹³-depleted emissions associated with the Siberian Traps. Interestingly, in our study, the peak in mercury concentrations and mercury/total organic carbon ratios also occurred after the onset of the negative carbon isotope excursion, suggesting a decoupling between the carbon and the mercury records that could result from the source of these two elements, be it volcanic, thermogenic, continental runoff, wildfire, or a combination of different reservoirs. Such decoupling deserves further investigation because it suggests different mechanisms of carbon and mercury release and/or processing in End Permian environments.
Studies have shown insignificant or constant fractionation of carbon isotopes during the burning process, and so charcoal carbon isotope ratios are a direct record of the original wood tissue carbon isotope ratios. The gradual decrease in the proportion of carbon¹³ in terrestrial plant-derived charcoal in the studied successions indicates, as discussed also for cuticle carbon isotope ratios, a change in the proportion of carbon¹³ in the original peat and vegetation due to changes in the proportion of carbon¹³ in the atmospheric CO₂. The charcoal carbon¹³ negative shift is coeval with an increase in charcoal abundance, i.e., with increased wildfire activity suggesting that the latest Permian forests experienced recurring wildfires and regrowth while the atmosphere became more carbon¹³ depleted. Additionally, burning of terrestrial plant biomass can also increase the emission of mercuty into the atmosphere, and then this mercury can be scavenged and buried in sediments.
The intensification of wildfire activity at the time of terrestrial mass extinction provides evidence of the harmful climatic changes in the lead-up to terrestrial crisis. The Gigantopteris coastal swamp flora thrived in humid, warm equatorial locations and was unlikely to have been adapted to intense levels of wildfire, as evidenced by the low charcoal abundance prior to the extinction interval. Thus, the increased wildfires suggest a transition to more unstable conditions punctuated by dry periods that would have been detrimental to coastal swamp floras.
Chu et al.'s study sheds new light on the temporal links between the deterioration in the terrestrial environment and floral extinction, and the geochemical changes that mark the Permian-Triassic mass extinction. Their terrestrial mercury record from the Permian-Triassic transition shows a sharp peak contemporaneous with the disappearance of Permian flora that correlates with marine mercury records. Carbon isotope data from cuticles and fossil charcoal, thought to reflect changes in the carbon isotope composition of the atmosphere, show a negative carbon isotope excursion during the terrestrial flora mass extinction interval. However, this was prior to the increase in mercury concentrations. Charcoal abundance shows that the floral extinctions coincided with an increase of wildfire activity and the carbon-cycle disruption. This likely reflects a change from persistent humidity to an unstable climate with frequent drought episodes. The temporal relationships between the events show that terrestrial disruption occurred shortly (but measurably) before the marine crisis.
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