QUANTITATIVE CONSTRAINTS ON END-PERMIAN AND EARLY TRIASSIC OCEANIC ANOXIA USING HIGH-RESOLUTION δ238U RECORDS (Invited Presentation)

The end-Permian mass extinction (EPME) at ~252 Ma represents the most severe biotic crisis in Earth history, marked by ~90% marine species-level mortality and followed by a protracted (~5-Myr-long) interval of biotic recovery during the Early Triassic. Oceanic anoxia has been proposed as a proximate cause of the EPME and the subsequent delayed recovery of marine ecosystems. However, the lack of quantitative constraints on the timing, duration, and extent of oceanic anoxia during the end-Permian and Early Triassic makes the links between redox conditions and contemporaneous biotic changes uncertain.

The recent development of uranium isotopes (δ²³⁸U) in carbonate rocks as a global paleoredox proxy has opened up new opportunities to explore the development of oceanic anoxia during the Permian-Triassic crisis. Here, we combine published δ²³⁸U records from Tethyan sites with that from a Panthalassic section at Kamura, Japan to construct a high-resolution δ²³⁸U profile that provides quantitative contains on the timing and extent of anoxia during the Permian-Triassic transition. The correlated sections show excellent agreement with clear evidence for elevated δ²³⁸U (−0.2 to +0.1 ‰) prior to the EPME and a sharp drop to low δ²³⁸U (−0.6 to −0.8 ‰) in the post-EPME latest Permian to earliest Triassic. LOWESS modeling of our U-isotope dataset indicates marine anoxia began ca. 70 kyr before the EPME and expanded rapidly at the EPME to cover 20–60% of the global ocean floor. The temporal coincidence of the rapid expansion of oceanic anoxia and the EPME suggests that redox changes played an important role in the marine biocrisis.

During the Early Triassic, our high-resolution δ²³⁸U data from Zal, Iran shows multiple negative shifts in δ²³⁸U values in the uppermost Permian and Lower Triassic. U isotope modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, reaching distinct maxima during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. The patterns of redox variation documented by the δ²³⁸U record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic, emphasizing the probable importance of oceanic anoxia as a killing agent.