Fluctuations in mercury and organic carbon in the peatlands of southwest China before the End Permian Extinction.
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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.