Astronomical forces as a potential cause for the end-guadalupian biotic crisis: A case study from central Alborz, Iran

Abstract

The Middle Permian extinction event is considered one of the most devastating events of all Phanerozoic biotic crises, yet it is not fully understood and has attracted a lot of controversial comments. This event is found at the end of the Capitanian carbonate strata in the Ruteh Formation (late Wordian–Capitanian) at the Alborz Mountains in Iran. It is characterized by the decimation of marine organisms, particularly calcareous algae and microproblematic fossils. The current study attempts to interpret the cause of this crisis in the framework of astronomically-forced climatic changes. Based on cyclostratigraphy analysis, the evidence of the Milankovitch cycles, including long eccentricity, short eccentricity, obliquity, and precession are recorded in the Ruteh Formation succession. The establishment of a floating astronomical time scale using the long eccentricity cycles led to an estimation of a duration of 1.6 and 2.43 million years for the Wordian and Capitanian stages, respectively. Integrated facies studies and a sedimentary noise model indicate that the Ruteh Formation is composed of three ∼1.2 Myr sequences, corresponding to third-order sequences, and ten 405-kyr sequences, corresponding to fourth-to fifth-order sequences. The boundaries of the ∼1.2 Myr sequences indicate regression in sea level and low sedimentation rate. Considering the paleogeographic situation of the Alborz Mountains, the sedimentation of the Ruteh Formation in the Middle Permian was affected by the Inter-Tropical Convergence Zone and interactions between the ocean and atmosphere during the Pangea assemblage, leading to an increase in temperature. With the sum of these observations, we conclude that factors such as high temperature and a decrease in sedimentation rate created unfavorable conditions for the mentioned biota at the end of the Capitanian.