An Early–Middle Guadalupian (Permian) isotopic record from a mid-oceanic carbonate buildup: Akiyoshi Limestone, Japan

In order to understand the oceanographic changes before the Guadalupian–Lopingian (Permian) boundary mass extinction event, we investigated the isotopic compositions of the inorganic carbon and the oxygen (δ13Ccarb and δ18Ocarb) of the Guadalupian (Middle Permian) shallow marine carbonates deposited on a seamount-top in the superocean Panthalassa. The drilled samples were obtained at Kaerimizu in the Akiyoshi area, SW Japan. We focused on the Roadian-Wordian (Middle Guadalupian) interval that spans over 7 fusuline zones; i.e. the Parafusulina kaerimizuensis Zone (Pk Z.), Afghanella ozawai Zone (Ao Z.), Neoschwagerina craticulifera robusta Zone (Ncr Z.), Verbeekina verbeeki–Afghanella schenki Zone (Vv–As Z.), Neoschwageina fusiformis Zone (Nf Z.), Verbeekina verbeeki Zone (Vv Z.), and Colania douvillei Zone (Cd Z.), in ascending order. Analytical results showed that the δ13Ccarb values stayed almost constant around + 3.0‰ PDB in the Pk Z., Ao Z. and the lower half of the Ncr Z., and those in the upper-section gradually decreased down to −2.0‰, of which the lowest was found in the Cd Z.We statistically extracted the samples with presumably better preserved δ13Ccarb values in the Kaerimizu section ranged between + 0.5 and + 4.0‰ with average values of δ13Ccarb of + 2.7 ± 1.0‰, on the basis of δ13Ccarb–δ18Ocarb characterization. This interval shows a monotonous decrease in δ13Ccarb values from ca + 4.0‰ to + 2.0‰. This indicates that the primary productivity might be generally high in the Wordian mid-oceanic domain but slightly declined in the Late Wordian. The studied Early-Middle Guadalupian interval is chemostratigraphically correlated with the other mid-Pansalassan paleo-atoll limestone e.g. Iwato Formation in Japan, suggesting that the relatively high δ13Ccarb (over + 3.0‰) of seawater predominated in shallow mid-superocean during the middle Middle Permian.