Possible causes for a negative shift in the stable carbon isotope ratio before, during and after the end-Permian mass extinction in Meishan, South China

Abstract

A 2–4‰ negative shift of δ13C has been reported from many Permian/Triassic (P/Tr) boundary sections around the world. Our carbon isotopic measurements of bulk carbonate and organic matter from the Meishan section, South China show that a –3‰ shift of δ13C spanned ∼30 ka over the end-Permian mass extinction. Carbon isotopic excursions (δ13Ccarb, δ13Corg) coupled with Δ13C (=δ13Ccarb–δ13Corg), marine kerogen %, 87Sr/86Sr of siliciclastics and total organic carbon show geochemical signals underwent six geochemical variation zones (GVZ) over the P/Tr transition. Of these, GVZ 1 corresponds to the lower–middle parts of Bed 24e-1, GVZ 2 to the upper part of Bed 24e-1, GVZ 3 to Beds 24e-2, 24e-3, 25-2 and 25-2, GVZ 4 to Bed 25-3, GVZ 5 to Bed 26, and GVZ 6 to Beds 27–29. A −1‰ shift of δ13C within ∼13 ka before the main extinction was probably generated by possible dissociation of methane hydrate by warming. Subsequent mixing of anoxic stratified ocean led to another −1‰ shift of δ13C within ∼5 ka during the extinction. The −1‰ shift of δ13C within ∼13 ka after the main extinction has probably resulted from massive denudation of soil and sediments caused by the collapse of forests and possible dissociation of methane hydrate at the end of the Permian. Furthermore, a dramatic drop of Δ13C values from 29–30‰ in Bed 24e-3 to 25‰ in Bed 25 indicates an abrupt proliferation of phototrophic sulfur bacteria immediately after the main episode of the end-Permian mass extinction. The Δ13C value increases to 29–30‰ in Bed 26 again, and this is probably due to massive denudation of soil and sediment caused by the collapse of forests between 0 and 27 ka after the main extinction, as revealed by negative shift of δ13C. Low Δ13C value recorded in Beds 27–29 indicates that phototrophic sulfur bacteria may have thrived again and lasted more than 150 ka after the main extinction in the low-latitude Paleo-Tethys.