geosciences Review Multiple Sulfur Isotope Geochemistry during the Permian-Triassic Transition Masafumi Saitoh School of Geosciences and Civil Engineering, Kanazawa University, Kanazawa 920-1192, Japan; [email protected] Abstract: The end-Permian mass extinction was the largest biodiversity crisis in the Phanerozoic. Based on characteristic negative D33S signals of sedimentary pyrite, previous multiple sulfur isotope studies suggested shoaling of anoxic/sulfidic deep-waters onto a shelf, leading to the shallow- marine extinction. However, the validity of this shoaling model has been controversial. I compiled previously-reported multiple sulfur isotope records during the Permian-Triassic transition interval, and examined a stratigraphic relationship between the extinction horizon, redox oscillation in the depositional settings, and the multiple sulfur isotope record in each studied section. The compilation shows that the negative D33S signals do not correspond clearly to the extinction horizon or to the benthic anoxia/euxinia in the studied sections. The compilation also documents that the multiple sulfur isotope records during the Permian-Triassic transition are substantially variable, and that the negative D33S signals were observed in various types of sediments including shallow-marine carbonates, carbonates/siltstones of relatively deep-water facies, and abyssal deep-sea cherts. Those observations allow me to infer that the negative D33S signal is not a robust indicator of shoaling. Rather, this isotopic signal may reflect substantial sulfur isotope heterogeneity in the sediments controlled by local factors. Citation: Saitoh, M. Multiple Sulfur Keywords: Permian-Triassic transition interval; triple extinction; multiple sulfur isotopes; deep-water Isotope Geochemistry during the shoaling; bioturbation; local sulfur isotope heterogeneity Permian-Triassic Transition. Geosciences 2021, 11, 327. https:// doi.org/10.3390/geosciences11080327 1. Introduction Academic Editors: Jesus Martinez- Frias and Henrik Drake The end-Permian mass extinction was the largest biodiversity decline in the Phanero- zoic (e.g., [1]). Although the major extinction across the Permian-Triassic boundary (P-TB) Received: 7 July 2021 (ca. 251.9 Ma) has been extensively studied since the 1980s (e.g., [2,3]), numerous studies Accepted: 29 July 2021 revealed repeated biotic crises with protracted environmental deterioration during the Published: 1 August 2021 Permian-Triassic transition interval (Figure1a) (e.g., [ 4–6]), including the end-Guadalupian (Middle Permian) extinction (ca. 259.5 Ma) [7–9], and biotic upheavals in the Early Triassic Publisher’s Note: MDPI stays neutral (ca. 251.9 to 246.7 Ma) [10] with the extinction immediately before the Smithian-Spathian with regard to jurisdictional claims in boundary (S-SB; ca. 250.6 Ma) in the Olenekian (Late Early Triassic) (e.g., [11,12]). Several published maps and institutional affil- geologic events around the Guadalupian-Lopingian (Late Permian) boundary (G-LB) have iations. been proposed as the cause of the end-Guadalupian extinction, such as the eruption of the Emeishan flood basalts (e.g., [13,14]), a eustatic sea-level fall [7], a global cooling [15], and oceanic anoxia/euxinia [16–18]. As the cause of the P-TB extinction, a number of geologic phenomena have been proposed, including a bolide impact (e.g., [19]), volcanism of the Copyright: © 2021 by the author. Siberian Traps Large Igneous Province (STLIP) (e.g., [20,21]) concurrent with climate warm- Licensee MDPI, Basel, Switzerland. ing (e.g., [22]), anoxia/euxinia (e.g., [23,24]), hypercapnia [25], and oceanic acidification This article is an open access article (e.g., [26]). distributed under the terms and Previous studies also revealed biotic upheavals following the P-TB extinction in the conditions of the Creative Commons Early Triassic (Figure1a), which were presumably driven by sustained environmental Attribution (CC BY) license (https:// stresses in this interval, such as recurrent STLIP volcanism (e.g., [27]), elevated pCO2 creativecommons.org/licenses/by/ (e.g., [28–30]) and pCH4 (e.g., [31,32]), global warming [33], and ocean acidification [34] 4.0/). Geosciences 2021, 11, 327. https://doi.org/10.3390/geosciences11080327 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 32 Geosciences 2021, 11, 327 2 of 30 [28–30]) and pCH4 (e.g., [31,32]), global warming [33], and ocean acidification [34] and anoxia (e.g., [10,35–37]). Especially, the late-Smithian extinction event was one of the sev- erest biodiversityand crises anoxia during (e.g., the [10 delayed,35–37]). re Especially,covery interval the late-Smithianof the Early Triassic extinction (Figure event was one of 1a) (e.g., [11]), andthe severestwas characterized biodiversity by crisesa turnover during in marine the delayed fauna recovery including interval ammonoids, of the Early Triassic conodonts, and(Figure other invertebrates1a) (e.g., [ 11 (e.g.,]), and [38,39]), was characterized and by a change by ain turnoverterrestrial in ecosystems marine fauna including [11]. Previous studiesammonoids, suggested conodonts, that the and late other-Smithian invertebrates extinction (e.g., generally [38,39]), coincided and by a changewith in terrestrial several geologicecosystems events, such [11 as]. the Previous recurrent studies STLIP suggested activity [40], that thelarge late-Smithian carbon cycle extinction fluc- generally coincided with several geologic events, such as the recurrent STLIP activity [40], large car- tuations [11,41], and extreme greenhouse conditions (late Smithian Thermal Maximum) bon cycle fluctuations [11,41], and extreme greenhouse conditions (late Smithian Thermal [33,42] followed by a major cooling around the S-SB associated with ocean anoxia [12,43] Maximum) [33,42] followed by a major cooling around the S-SB associated with ocean and/or acidification [34]. Nevertheless, the ultimate trigger mechanisms of the “triple” ex- anoxia [12,43] and/or acidification [34]. Nevertheless, the ultimate trigger mechanisms of tinction during the Permian-Triassic transition are not yet fully understood and are still the “triple” extinction during the Permian-Triassic transition are not yet fully understood hotly debated (e.g., [3,44]). and are still hotly debated (e.g., [3,44]). Multiple sulfur isotope geochemistry is useful for tracking the global sulfur cycle Multiple sulfur isotope geochemistry is useful for tracking the global sulfur cycle through Earth history (e.g., [45,46]). Several previous studies analyzed multiple sulfur iso- through Earth history (e.g., [45,46]). Several previous studies analyzed multiple sulfur topes in sedimentary records in the Permian-Triassic transition interval (Figure 1b), and isotopes in sedimentary records in the Permian-Triassic transition interval (Figure1b), and tried to reconstruct the sedimentary sulfur cycle in association with bottom water redox tried to reconstruct the sedimentary sulfur cycle in association with bottom water redox conditions andconditions benthos activity and benthos during activitythe three during extinction the three events. extinction In particular, events. based In particular, on based on an anomalous ansulfur anomalous isotopic sulfursignal isotopicobserved signal in the observed analyzed in sediments, the analyzed initial sediments, studies initial studies proposed shoalingproposed of anoxic/sulfidic shoaling of deep-wat anoxic/sulfidicers onto deep-waters a shelf, leading onto to a the shelf, shallow-ma- leading to the shallow- rine extinction.marine However, extinction. the validity However, of this theshoaling validity model of this has shoaling been questioned model has bybeen later questioned by studies. In thislater article, studies. I briefly In this review article, the I brieflymultiple review sulfur the isotope multiple system sulfur and isotope its initial system and its initial application to theapplication Permian tosedimentary the Permian records sedimentary with the records original with shoaling the original models. shoaling Then, models. Then, I review later multipleI review sulfur later multiple isotope sulfurstudies isotope in the Permian-Triassic studies in the Permian-Triassic transition interval, transition interval, particularly focusingparticularly on a stratigraphic focusing on arelati stratigraphiconship between relationship the extinction between horizon, the extinction redox horizon, redox conditions in theconditions depositional in the settings, depositional and the settings,multiple sulfur and the isotope multiple record sulfur in each isotope stud- record in each ied section. Finally,studied I compile section. all Finally, the previous I compile multiple all the sulfur previous isotope multiple records sulfur during isotope the records during Permian-Triassicthe transition Permian-Triassic and examine transition the validity and examine of the theoriginal validity shoaling of the models. original shoaling models. Figure 1. Cont. GeosciencesGeosciences 2021, 11, x FOR2021 PEER, 11, 327REVIEW 3 of 32 3 of 30 Figure 1. The Permian-TriassicFigure 1. The Permian-Triassic transition interval. transition (a interval.) “triple” (a)extinction “triple” extinction during during the interval the interval with with the the ages of the bound- aries [47,48]; (b)ages paleo-geographic of the boundaries map [47,48]; [49 ](b with) paleo-geographic sections of previous map [49] multiple with sections sulfur of isotopeprevious studies.multiple The location of