GEOLOGY | Volume 46 | Number 4 | www.gsapubs.org 1 Congruent Permian-Triassic δ 238 U records at Panthalassic and Tethyan sites: Confirmation of global-oceanic anoxia and validation of the U-isotope paleoredox proxy Feifei Zhang 1 *, Thomas J. Algeo 2,3,4 , Stephen J. Romaniello 1 , Ying Cui 5 , Laishi Zhao 3 , Zhong-Qiang Chen 4 , and Ariel D. Anbar 1,6 1 School of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287, USA 2 Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA 3 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, China 4 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, Hubei 430074, China 5 Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA 6 School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, USA ABSTRACT Oceanic anoxia has been proposed as a proximate cause of the end- Permian mass extinction (EPME), but evaluation of this hypothesis is hampered by limited detailed knowledge of its timing and extent. The recent development of uranium isotopes ( δ 238 U) in carbonates as a global-ocean redox proxy provides new insights into this problem. Three earlier δ 238 U studies of Tethyan sections inferred development of extensive oceanic anoxia at the EPME. However, recent work raises concerns that diagenetic alteration may inluence the reliability of δ 238 U records in bulk carbonate sediments. Here, we evaluate this pos- sibility through δ 238 U analysis of a Permian-Triassic carbonate atoll section from the Panthalassic Ocean (Kamura, Japan) and compari- son with existing δ 238 U proiles from the Tethys Ocean. The Kamura section exhibits a large negative δ 238 U shift across the EPME horizon identical both in timing and magnitude to those in Tethyan sections, demonstrating beyond a reasonable doubt that the negative seawater δ 238 U shift at the EPME was a global event, and that it was recorded by shallow carbonate facies globally. The robustness of the U-isotope proxy is further shown by the fact that a common global signal at the EPME was preserved despite major differences in the burial histories of Panthalassic and Tethyan sections, the former having been tectonically subducted and heated to greenschist metamorphic grade, whereas the latter accumulated in stable cratonic settings and experienced milder burial effects. Finally, we use leaching experi- ments to demonstrate that, although small-scale δ 238 U heterogeneity is common in both modern and ancient carbonates, it probably does not signiicantly affect bulk-carbonate δ 238 U trends. INTRODUCTION The existence of oceanic anoxia during the Permian-Triassic Bound- ary (PTB) crisis, the most severe biotic crisis in Earth history due to its ~90% species-level mortality rate, was inferred by previous studies on the basis of petrographic, geochemical, and biomarker data (Grice et al., 2005; Isozaki, 1997; Meyer et al., 2008; Wignall and Twitchett, 1996). However, reliance on local redox proxies has resulted in divergent views regarding the timing, extent, and intensity of oceanic anoxia that remain unresolved to date. For example, the onset of anoxia was inferred to be several million years before the end-Permian mass extinction (EPME) in some studies (Isozaki, 1997; Wignall and Twitchett, 1996) but no more than ~100 k.y. prior to the EPME in others (Algeo et al., 2012; Shen et al., 2012), and different studies have argued both for (Grice et al., 2005) and against (Loope et al., 2013) the presence of anoxia in shallow-marine facies. Recent work favors a more complex scenario, characterized by widespread expansion of anoxia at intermediate depths (~200–1000 m) prior to the EPME (Winguth and Winguth, 2012; Feng and Algeo, 2014) followed by episodic upward chemocline excursions into the ocean-sur- face layer commencing at the EPME (Algeo et al., 2008). The recent development of uranium isotopes (δ 238 U) in marine carbon- ates as a globally integrative paleoredox proxy has provided new insights into this problem. Three recent studies of PTB sections document a large (0.4–0.5‰) negative δ 238 U excursion, suggesting a close relationship between ocean-redox changes and the EPME (Brennecka et al., 2011; Elrick et al., 2017; Lau et al., 2016). However, the reliability and global signiicance of these records have been challenged on the basis of two issues: (1) potential inluences of diagenetic alteration on bulk carbonate δ 238 U records (Hood et al., 2016; Romaniello et al., 2013), and (2) geo- graphic limitation of existing U-isotope studies of the PTB to the Tethys Ocean, which represented only ~10%–15% of contemporaneous global- ocean area. Thus, despite the congruency of previously published Tethyan δ 238 U records, an independent test of uranium isotopes from a Panthalassic section with a dissimilar diagenetic history is needed to verify oceanic redox changes during the Permian-Triassic transition. Here, we report the irst Permian-Triassic carbonate δ 238 U record from an open Panthalassic Ocean site (Kamura, Japan), evaluate its redox implications relative to existing Tethyan δ 238 U records, and address concerns about preservation of marine δ 238 U signals by bulk carbonate sediments. URANIUM ISOTOPE SYSTEM The power of uranium isotopes as a global-ocean redox proxy derives from the long residence time (~500 k.y. for the modern; Dunk et al., 2002) and well-mixed character of U in seawater. Seawater δ 238 U responds to redox changes because reduction of dissolved hexavalent U [U(VI)] to tetravalent U [U(IV)], which is rapidly removed to anoxic sediments, results in a detectable fractionation of U isotopes, sequestering heavy isotopes in the reduced species (Andersen et al., 2014). Thus, the δ 238 U of U(VI) dissolved in seawater decreases as the areal extent of bottom- water anoxia increases (Brennecka et al., 2011), providing a direct proxy of global-ocean redox changes. Marine carbonate sediments have been *E-mail: fzhang48@asu.edu; zhff414@hotmail.com GEOLOGY, April 2018; v. 46; no. 4; p. 1–4 | GSA Data Repository item 2018092 | https://doi.org/10.1130/G39695.1 | Published online XX Month 2018 © 2018 Geological Society of America. For permission to copy, contact editing@geosociety.org.