Stable carbon isotope stratigraphy across the Permian–Triassic boundary in shallow marine carbonate platforms, Nanpanjiang Basin, south China

摘要

A distinct negative δ~(13)C excursion is documented in two Permian–Triassic sections (Heping and Taiping) in shallow marine carbonate platform deposits in the Nanpanjiang Basin, south China. These sections span from the Changhsingian to the Dienerian and are characterized by a distinct marine boundary facies change from massive, skeletal lime packstone in the Changhsingian to distinctive calcimicrobial framestone in the Griesbachian Hindeodus parvus Zone. The δ~(13)C_(org) and δ~(13)C_(carb) excursions occur directly after the onset of the calcimicrobial framestone (herein termed the 'Permian–Triassic boundary event') and before the first occurrence of H. parvus. The isotope shifts are associated with a sharp drop in species abundance and diversity and coincide with a decrease in total organic carbon (TOC) content. The shift towards depleted values in δ~(13)C_(org) and δ~(13)C_(carb) at the Permian–Triassic boundary event, together with low TOC contents, persists throughout the Griesbachian H. parvus Zone. These data document a corresponding negative shift of δ~(13)C_(org) and δ~(13)C_(carb), values and low TOC contents with the onset of growth of calcified microbial framestones (a postextinction 'disaster facies') immediately below the base of the Griesbachian H. parvus Zone. Based on paleontological evidence, the first occurrence of the 'disaster facies' follows the extinction event, which implies that the ~(13)C-depleted values above this facies postdate the event. This suggests that two separate events had to account for the initiation of the extinction and the δ~(13)C excursion. However, the consequences that led to the negative isotopic shift might be linked to the intriguing recovery lag of Early Triassic ecosystems. Based on data from PTB sections worldwide of a greater δ~(13)C offset in high compared with low latitudes, we propose that methane eruptions from thermal destabilization of high-latitude clathrate deposits may have led to the negative δ~(13)C shift and may have caused long-term adverse ecological conditions.