Diagenetic overprint on negative δ13C excursions across the Permian/Triassic boundary: A case study from Meishan section, China
The Meishan Section D from China is the global stratotype section and point (GSSP) of the Permian-Triassic boundary (PTB). The succession has been divided into 115 beds with the PTB being placed in the middle of Bed 27. In this section and the laterally equivalent Section A, the diagenetic features of Beds 24 to 62 are examined in this study to determine if the negative shift in δ13Ccarb across the Permian-Triassic boundary is diagenetic in origin or not. Bed 24 is formed of skeletal packstones, whereas Beds 25 to 62 are formed of claystones or argillaceous dolomitic limestones. The dolomite crystals are typically < 30 μm long, subhedral to euhedral, and held in a very fine groundmass formed of calcite and clay minerals. The cathodoluminescent features and backscattered images show that the dolomite crystals are zoned, with a non-luminescent low-calcium calcian dolomite (LCD) core being encased successively by an irregular dull-orange calcite zone, a euhedral bright-orange high-calcium calcian dolomite (HCD) cortex, and an outermost ferroan-HCD zone. Available evidence indicates that the zoned dolomite crystals formed episodically. The dolomite crystals, following their initial syndepositional formation with euhedral LCD cores and HCD cortices, were subsequently modified by calcitization mediated by meteoric water and overgrowth of Fe-rich HCD during burial. 
 
The limestones in Bed 24 display δ13C values ranging from + 1.72 to + 2.92‰, δ18O values from − 7.74 to − 4.84‰. In Beds 25 to 62, the bulk samples have δ13C values from − 0.76 to + 1.28‰ and δ18O values from − 8.58 to − 3.99‰. There is a positive correlation between these isotopic compositions and dolomite content. It is estimated that the first-formed dolomite and the precursor limestones have δ13C value of + 2.19‰ and δ18O value of − 1.14‰. The dramatic negative δ13C excursions of calcite in Beds 26 and 28 are related to meteoric diagenesis, whereas the negative δ13C excursions of bulk samples in the dolomite-bearing beds are ascribed to the diagenetic modifications that post-dated the initial formation of dolomite. The results show that the δ13C signals recorded in the GSSP that spans the PTB are a mixture of original and diagenetic signals. For the dolomite-bearing bulk samples, diagenesis contributes ~ 1.7‰ to the negative shift in the δ13C. Hence, diagenetic alteration needs to be considered before using δ13Ccarb to estimate the dissolved inorganic carbon in the ancient oceans and to calculate the global carbon cycle.