TWO EPISODES OF EXTENSIVE MARINE ANOXIA DURING THE PERMIAN-TRIASSIC BOUNDARY RECORDED BY URANIUM ISOTOPES IN MARINE DOLOMITE

Organic and inorganic geochemical studies in the past decades of the Permian-Triassic boundary (PTB) have significantly advanced our understanding of the pattern and potential causes of the end-Permian mass extinction (EPME; ca. 251.941 Ma), the largest mass extinction event in Earth’s history. Among these geochemical studies, the uranium isotope (δ²³⁸U) record is of particular importance because it provided quantitative constraints on the details of global marine anoxia during the EPME (Brennecka et al., 2011; Lau et al., 2016; Zhang et al., 2018). Here, we present new uranium isotope data of syndepositional dolomite from a well-characterized drill core in Carnic Alps of Austria (Holser et al., 1989). This high-resolution 330-meter core drilled through the marine Permian-Triassic boundary provides unique opportunities to evaluate the reliability of U isotope records in marine dolomites and to explore the detailed timing, duration, and extent of marine anoxia during the EPME.

Although the dolomites δ²³⁸U records show much more scatter than previously studied limestones from other locations (Brennecka et al., 2011; Lau et al., 2016; Zhang et al., 2018), we found two distinct episodes of negative excursions, each positively correlated with the global perturbations in carbonate δ¹³C during the EPME. Our δ²³⁸U record is broadly consistent with previous δ²³⁸U studies at the EPME (Brennecka et al., 2011; Lau et al., 2016; Zhang et al., 2018) while the present study clearly shows two distinct negative δ²³⁸U excursions compared to these prior studies that only recorded one negative excursion, possibly due to the extremely high sedimentation rate in the present studied section. This study thus suggests that syndepositional marine dolomite can reliably preserve primary marine δ²³⁸U signals.

Through modeling uranium isotopes cycling in the ocean, our data suggests that the EPME was characterized by two distinct episodes of extensive marine anoxia that were separated by ~0.35 Ma. We’re currently exploring whether enhanced marine productivity or climatic warming induced ocean stagnation as a primary cause of the two episodes of marine anoxia using an Earth system model of intermediate complexity with carbon cycle (cGENIE).