Carbon Isotope Chemostratigraphy Across the Permian-Triassic Boundary at Chaotian, China: Implications for the Global Methane Cycle in the Aftermath of the Extinction

During the end-Permian extinction, a substantial amount of methane (CH4) was likely released into the ocean-atmosphere system associated with the Siberian Traps volcanism, although fluctuations in the global CH4 cycle in the aftermath of the extinction remain poorly understood. The carbon (C) isotopic composition of carbonate (δ13Ccarb) across the Permian-Triassic boundary (P-TB) was analyzed at Chaotian, South China. The δ13Ccarb values decrease from ca. +1 to –2‰ across the P-TB, possibly caused by a collapse of primary productivity associated with the shallow-marine extinction. The frequent intercalation of felsic tuff layers around the P-TB suggests that a volcanogenic carbon dioxide (CO2) input to the surface oceans may also have contributed to the δ13Ccarb decline. The magnitude of the δ13Ccarb decrease (∼3‰) is substantially smaller than the magnitude of a decrease in C isotopic composition of organic matter (δ13Corg) in the same P-TB interval (∼7‰). This apparent δ13Ccarb-δ13Corg decoupling could be explained by proliferation of methanogen (“methanogenic burst”) in the sediments. A global δ13C compilation shows a large variation in marine δ13Corg records, implying that the “methanogenic burst” according to the Siberian Traps volcanism may have contributed, at least in part, to the δ13Corg variability and to the elevated CH4 levels in the atmosphere. The present and previous observations allow us to infer that the global CH4 cycle may have fluctuated substantially in the aftermath of the extinction.

在二叠纪末期大灭绝期间，大量甲烷（CH₄）可能随着西伯利亚暗色岩火山活动被释放到海洋 - 大气系统中，不过大灭绝之后全球甲烷循环的波动情况仍知之甚少。对中国南方朝天地区二叠纪 - 三叠纪界线（P - TB）处碳酸盐的碳（C）同位素组成（δ¹³C₍carb₎）进行了分析。在二叠纪 - 三叠纪界线处，δ¹³C₍carb₎值从约 +1‰降至 -2‰，这可能是由与浅海生物灭绝相关的初级生产力崩溃所致。二叠纪 - 三叠纪界线附近频繁夹有长英质凝灰岩层，这表明火山成因的二氧化碳（CO₂）输入到表层海洋可能也导致了δ¹³C₍carb₎的下降。δ¹³C₍carb₎下降的幅度（约3‰）远小于同一二叠纪 - 三叠纪界线区间内有机质碳同位素组成（δ¹³C₍org₎）的下降幅度（约7‰）。这种明显的δ¹³C₍carb₎ - δ¹³C₍org₎解耦现象可以用沉积物中产甲烷菌的增殖（“产甲烷爆发”）来解释。一项全球δ¹³C综合研究显示，海洋δ¹³C₍org₎记录存在很大差异，这意味着与西伯利亚暗色岩火山活动相关的“产甲烷爆发”可能至少在一定程度上导致了δ¹³C₍org₎的变化以及大气中甲烷含量的升高。目前和以往的观测结果使我们能够推断，在大灭绝之后，全球甲烷循环可能发生了大幅波动。