The carbon biogeochemical cycle across the Permian - Triassic boundary strata and its implications: isotope record from the Changhsingian Stage at Meishan, south China

Abstract

The new dataset of the carbon and oxygen stable isotope values across the sedimentary sequence of the Changhsingian Stage at Meishan, south China documents the carbon biogeochemical cycle in detail. The cycle began with storage of organic carbon (C_org), mainly as biogenic carbon dioxide in clathrates, and carbon dioxide from anaeorobic methane oxidation (AMO) in the stratified sea water masses, which is reflected by a rapid increase in the δ¹³C values. Storage of Corg demanded a large biomass productivity, and thus consumption of a vast amount of carbon dioxide, which would have been associated with climatic oceanic water cooling. Oceanic water cooling and enhanced volcanism is actually reflected by a gradual increase in the δ¹⁸O values. The longterm stasis between C^_org burial and reburial was then achieved, as reflected by the stable course of the δ¹³C curve. At the same time, cool water circulation caused a continuous increase in the δ¹⁸O values. At the end of the cycle, several factors including earthquakes and other tectonic movements marking the onset of major Siberian volcanism, together with a local temperature increase resulted in a change of ocean circulation, caused mixing of stratified sea water masses. This was followed by destruction of the existing clathrates, degassing of carbon dioxide, oxidation of organic matter, and a rapid decrease in the oceanic δ¹³C values. Apart from the carbon dioxide that was released to the atmosphere, part of the methane must have been oxidised, creating additional carbon dioxide as well as water vapour. The carbon dioxide and water vapour must have initiated a greenhouse effect and caused thermal stratification of seawater which, in turn, initiated reconstruction of the redox in the Early Triassic. 

The end of the cycle was associated with the end-Permian mass extinction. The global warming associated with methane release from clathrates led to collapse of terrestrial plants. At the same time, the mixing event brought methane and organic matter for oxidation, which caused massive consumption of oxygen and extinction of the terrestrial fauna. In the marine environment, the mixing event brought water masses rich in carbon dioxide, as well as excess hydrogen ions from anoxic zones, onto shelf areas, causing a collapse of marine biota. 

The carbon biogeochemical cycle of the Changhsingian indicated by the initial rapid rise, long-term stasis, and final rapid drop in δ¹³C values, seems to be a rule for a pattern of Phanerozoic δ¹³C values in sedimentary carbonates. This suggests that a process of methane storage and release may have had a bearing on major geochemical perturbations throughout Earth history. However, methane storage, with or without the association of carbon dioxide and other forms of C_org, and its subsequent release, could have occurred in many different scenarios throughout the Phanerozoic. Therefore, the end of the carbon cycles may be associated with mass extinction only when release of methane is coupled with other phenomena, which is the case with the end-Permian event.