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

摘要

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 (δ~(13)C_(carb) and δ~(18)O_(carb)) 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 verbeeti-Afghanella schenki Zone (Vv-AsZ.),Neoschwageinafusiformis Zone (JVfZ.), Verbeekina verbeeki Zone (Vv Z.), and Colania douvillei Zone (Cd Z.), in ascending order. Analytical results showed that the δ~(13)C_(carb) 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 δ~(13)C_(carb) values in the Kaerimizu section ranged between + 0.5 and + 4.0‰ with average values of δ~(13)C_(carb) of+2.7 ± 1.0‰ on the basis of δ~(13)C_(carb)-2~(18)O_(carb) characterization. This interval shows a monotonous decrease in δ~(13)C_(carb) 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 δ~(13)C_(carb) (over + 3.0‰) of seawater predominated in shallow mid-superocean during the middle Middle Permian.