Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas

Yang, Fen; Li, Sen; Grasby, Stephen; Bond, David; Pan, Ming; Sun, Yadong

doi:10.5194/egusphere-2026-863

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https://doi.org/10.5194/egusphere-2026-863

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26 Feb 2026

| 26 Feb 2026

Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas

Fen Yang, Sen Li, Stephen Grasby, David Bond, Ming Pan, and Yadong Sun

Abstract. Marine anoxia has been implicated as a key environmental driver of the end-Permian mass extinction (EPME) and the subsequent prolonged recovery. However, the spatial and temporal extent of oxygen limitation during the EPME interval remains contentious. Here, we present iron speciation, pyrite framboid and molybdenum-uranium (Mo-U) covariation data from two palaeogeographically distinct settings: the Tethyan Chibi section (South China) and the Panthalassian Ursula Creek section (Western Canada) to evaluate redox dynamics across the Permian-Triassic transition. Our data suggest that bottom waters were predominantly dysoxic during the late Changhsingian at both sites. Later, the prevalence of small pyrite framboids, elevated Mo and U enrichment factors (Mo_EF and U_EF), and high Mo_EF/U_EF ratios near the EPME horizon implicate seafloor anoxia as a key trigger for marine extinctions in epicontinental seas. In the post-extinction Early Triassic, iron speciation and Mo_EF-U_EF covariation data reveal a shift to persistently ferruginous conditions in both locations. A global compilation of iron speciation data indicates that anoxic conditions fluctuated between ferruginous and euxinic in epicontinental seas during the Permian-Triassic crisis, with ferruginous conditions expanding significantly in the earliest Triassic. The expansion of a ferruginous seafloor would have limited phosphorus bioavailability, suppressing primary productivity in the immediate aftermath of the EPME, thereby contributing to the slow recovery.

Received: 13 Feb 2026 – Discussion started: 26 Feb 2026

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Fen Yang, Sen Li, Stephen Grasby, David Bond, Ming Pan, and Yadong Sun

Status: final response (author comments only)

CC1:
'Comment on egusphere-2026-863', Giacomo Medici, 04 Mar 2026

General comments
Good research on paleoclimate. Please, follow my suggestions to improve the manuscript.

Specific comments
Line 32. “An extreme greenhouse climate prevailed during the Early Triassic”. Please, add the following references on the Early Triassic climate and stratigraphy.
- López-Gómez, J., Arche, A., Marzo, M., Durand, M. 2005. Stratigraphical and palaeogeographical significance of the continental sedimentary transition across the Permian–Triassic boundary in Spain. Palaeogeography, Palaeoclimatology, Palaeoecology, 229, 3-23.
- Medici, G., West, L.J., Mountney, N.P. 2019. Sedimentary flow heterogeneities in the Triassic UK Sherwood Sandstone Group: insights for hydrocarbon exploration. Geological Journal, 54(3), 1361-1378.
Line 83. Summarize the overall goal of your research on paleoclimate.
Line 83. Describe the objectives of your research by using numbers (e.g., i, ii, and iii).
Lines 85-109. Provide detail on the major Permo-Triassic tectonic phases.
Lines 85-109. Mention the major tectonic lines in the study area.
Lines 111-136. If the result section is divided in sub-paragraphs, it should be the same for the methodology section. Please, fix the issue and expand the methodology.
Lines 357-365. “The Conclusions”. This section is too short. Please, expand.

Figures and tables
Figures 1a-c. The logs are of paramount importance. Please, increase the graphic resolution.
Figures 1a-c. Make the letters larger.
Figure 1c. What about nature of faults? Extensional?
Figure 7. Please, increase the graphic resolution also for this figure.
Figure 7. Make the letters larger also here.

Citation: https://doi.org/10.5194/egusphere-2026-863-CC1
- AC1: 'Reply on CC1', Fen Yang, 23 Apr 2026
  
  We sincerely thank Giacomo Medici for his constructive and positive evaluation of our manuscript, as well as for his detailed comments and suggestions. We provide a point-by-point response below.
  1. Line 32. “An extreme greenhouse climate prevailed during the Early Triassic”. Please, add the following references on the Early Triassic climate and stratigraphy.
  We have reviewed the recommended references and agree that they directly or indirectly support our statements regarding Early Triassic climate and stratigraphy. We will incorporate them into the revised manuscript as suggested.
  2. Line 83. Summarize the overall goal of your research on paleoclimate and describe the objectives of your research by using numbers (e.g., i, ii, and iii).
  We will revise the final paragraph of the Introduction to clarify the overall aim and present the research objectives as follows:
  
  To better constrain marine redox dynamics in epicontinental seas during the Permian-Triassic transition under extreme climatic conditions, this study investigates two Upper Permian to Lower Triassic successions from contrasting palaeogeographic settings: Chibi (equatorial Tethys) and Ursula Creek (northwestern margin of Pangaea). Specifically, this study aims to: (i) characterize the spatio-temporal evolution of redox conditions across this transition using integrated iron speciation, pyrite framboid, and Mo-U covariation data; and (ii) evaluate regional differences and broader patterns in redox shifts (euxinic vs. ferruginous).
  3. Lines 85-109. Provide detail on the major Permo-Triassic tectonic phases and mention the major tectonic lines in the study area.
  We will add the relevant tectonic information to the Geological Setting section as follows:
  The Permian-Triassic interval corresponds to the final assembly of Pangaea, which was centered near the equator and extended across nearly all latitudes, surrounded by the Panthalassa Ocean (Benton and Newell, 2014; Scotese, 2014). This large-scale continental amalgamation marked the termination of the Late Palaeozoic Ice Age and the transition to greenhouse climatic conditions (Crowell, 1995; Chen et al., 2018). Contemporaneously, extensive large igneous province volcanism, including the Emeishan Large Igneous Province and the Siberian Traps, peaked around the Permian–Triassic boundary (He et al., 2007; Shellnutt, 2014; Burgess and Bowring, 2015).
  The South China Craton is bounded by several major tectonic elements, including the Ailaoshan–Songma suture zone, the Jinshajiang suture, the Longmenshan Thrust Belt, and the Qinling–Dabie–Sulu orogen (Wan, 2012; Metcalfe, 2013; Cawood et al., 2018). During the Permian, it was not fully incorporated into Pangaea and largely evolved as a passive continental margin. This tectonic regime persisted until the Middle Triassic, when collision with the Simao–Indochina blocks initiated orogenesis (He and Luo, 2010; Wang et al., 2018). The well-preserved Permian-Triassic boundary successions in South China have been linked to this relatively stable, extension-dominated setting. On the northwestern margin of Pangaea, the Canadian Cordillera developed from remnants of Rodinia (~750 Ma) and attained its present configuration by the Late Cretaceous (Monger and Price, 2002). During the Late Permian, this region was situated at mid-latitudes (~30°N) along a passive continental margin facing the Panthalassa Ocean (Zelt et al., 2006).
  4. Lines 111-136. If the result section is divided in sub-paragraphs, it should be the same for the methodology section. Please, fix the issue and expand the methodology.
  We will revise the Methods section by introducing sub-paragraphs that correspond to those in the Results section (i.e., 3.1 Pyrite framboids, 3.2 Iron speciation, and 3.3 Trace elements and enrichment factors). The methodology will also be slightly expanded and clarified to improve consistency and readability.
  5. Lines 357-365. “The Conclusions”. This section is too short. Please, expand.
  The Conclusions section will be expanded to incorporate broader implications for redox evolution in global epicontinental seas and provide a more comprehensive synthesis.
  6. Figures and tables
  We will improve the resolution and increase the font size of Figures 1a–c and 7 to enhance readability. Regarding Figure 1c, the faults shown are thrust faults rather than extensional faults, and this will be clearly indicated in the revised figure.
  
  Citation: https://doi.org/10.5194/egusphere-2026-863-AC1
RC1:
'Comment on egusphere-2026-863', Thomas Algeo, 12 Mar 2026

This study examines redox proxies (mainly pyrite framboid size, Fe speciation, and Mo-U concentrations) at two Permian-Triassic boundary sites (Chibi South China and Ursula Creek Western Canada) with the goal of adding to our knowledge on redox variation during this biocrisis. The newly generated data are compared with data from other PTB sections globally.
Summary: The study is fairly well-executed: the data and interpretations are robust, the writing and figures are generally good (although some improvement is possible). There are no major/fatal flaws, so the shortcomings that are present can be fixed, with a view toward eventual publication. I rate the manuscript as in need of moderate revision.
My first comment is that there has been a huge amount of work undertaken on redox changes during the PTB event at many sections globally. The authors are going to have to work a bit harder to convince me that the present study really adds something new and important to the literature on this topic.
The text lacks citations of some key papers on redox conditions at the PTB. The authors have extensively cited my work (many thanks for that!), although they have overlooked several key studies of mine examining PTB redox variation in shallow-marine (Algeo et al., 2007, 2008) and deep-ocean sections (Algeo et al., 2010, 2011) [in fact, they cited Algeo et al., 2010 but failed to identify some of its key findings]. To summarize the importance of these papers: The two studies on the Nhi Tao section demonstrated multiple (at least 8) incursions of euxinic watermasses onto this shallow-marine platform in the earliest Triassic. This demonstrates highly dynamic redox conditions in shallow environments at that time. The two studies on the Japanese sections demonstrated that the deep ocean shifted only to dysoxia, and that the pyrite framboids almost certainly came from much shallower depths (presumably the OMZ at 200-1000 m). Thus, the key change in redox in the open ocean was euxinia in an expanded OMZ globally (200-1000 m water depth) with little to no change in deep-ocean redox conditions (minor shift to dysoxia). This was documented by Algeo et al. (2010, 2011) and supported by modeling (e.g., Winguth and Winguth, 2012):
Algeo, T.J., Ellwood, B., Nguyen, T.K.T., Rowe, H. and Maynard, J.B., 2007. The Permian–Triassic boundary at Nhi Tao, Vietnam: evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform. Palaeogeography, Palaeoclimatology, Palaeoecology, 252(1-2), pp.304-327.

Algeo, T., Shen, Y., Zhang, T., Lyons, T., Bates, S., Rowe, H. and Nguyen, T.K.T., 2008. Association of 34S‐depleted pyrite layers with negative carbonate δ13C excursions at the Permian‐Triassic boundary: Evidence for upwelling of sulfidic deep‐ocean water masses. Geochemistry, Geophysics, Geosystems, 9(4).
Algeo, T.J., Kuwahara, K., Sano, H., Bates, S., Lyons, T., Elswick, E., Hinnov, L., Ellwood, B., Moser, J. and Maynard, J.B., 2011. Spatial variation in sediment fluxes, redox conditions, and productivity in the Permian–Triassic Panthalassic Ocean. Palaeogeography, Palaeoclimatology, Palaeoecology, 308(1-2), pp.65-83.
Winguth, C. and Winguth, A.M., 2012. Simulating Permian–Triassic oceanic anoxia distribution: Implications for species extinction and recovery. Geology, 40(2), pp.127-130.
Re the redox proxies used, framboid size and Fe speciation are shown for both study sections. Beyond this, Mo-U are shown for Ursula Creek and Ce/Ce* for Chibi. The problem here is that Ce/Ce* is not a valid redox proxy when any clayey material is present in a section—even a few percent clays invariably result in a lithogenic REE signature from which no redox information can be drawn. The authors should remove Ce/Ce* from the Chibi section and, for the sake of comparability to Ursula Creek, add Mo-U data.
The discussion of marine primary productivity changes around line 340 does not adequately convey what is known about this topic. Primary productivity does not show any consistent pattern globally; there was a lot of regional variation. See Algeo et al. (2013):
Algeo, T.J., Henderson, C.M., Tong, J., Feng, Q., Yin, H. and Tyson, R.V., 2013. Plankton and productivity during the Permian–Triassic boundary crisis: an analysis of organic carbon fluxes. Global and Planetary Change, 105, pp.52-67.
I have been working to provide a logical and consistent framework for tectonics terminology in Chinese studies. I encourage the authors to read this paper:
Algeo, T.J. and Li, H., 2026. Are South China and North China “blocks” or “cratons”?. Palaeogeography, Palaeoclimatology, Palaeoecology, 683, 113464.
I am returning an annotated PDF with additional comments.

Citation: https://doi.org/10.5194/egusphere-2026-863-RC1
- AC3: 'Reply on RC1', Fen Yang, 23 Apr 2026
  
  We sincerely thank Reviewer 1 (Thomas Algeo) for his positive evaluation and constructive comments, which will help us improve the manuscript. Our point-by-point responses are provided below.
  1. My first comment is that there has been a huge amount of work undertaken on redox changes during the PTB event at many sections globally. The authors are going to have to work a bit harder to convince me that the present study really adds something new and important to the literature on this topic.
  We acknowledge that numerous studies have documented redox changes across the Permian‑Triassic boundary from many sections globally, and that our manuscript requires further revision to improve its overall quality, as reflected in the detailed comments provided by Thomas Algeo (including those in his annotated PDF). Nevertheless, we also consider our study offers new and important contributions that are not simply repetitive: 1) Integration of multiple redox proxies from two palaeogeographically distinct settings. Our study combines iron speciation, pyrite framboid distributions, and Mo‑U covariation from one Tethyan section (Chibi) and one Panthalassian section (Ursula Creek). This multi‑proxy approach enhances the robustness of our redox interpretations. Although both sections have been previously studied, iron speciation data were lacking; our new dataset enables a more refined distinction between euxinic and ferruginous conditions, thereby providing improved constraints on marine redox states; 2) Global compilation of iron speciation data with a focus on ferruginous vs. euxinic conditions. We have compiled published global iron speciation data from epicontinental seas spanning the Permian‑Triassic interval. This synthesis allows us to place our results into a broader spatial context and to highlight large‑scale patterns in redox evolution, including the expansion of ferruginous conditions in the earliest Triassic.
  We will revise the manuscript according to the reviewer’s suggestions to further improve the quality and persuasiveness of our paper.
  2. The text lacks citations of some key papers on redox conditions at the PTB. The authors have extensively cited my work (many thanks for that!), although they have overlooked several key studies of mine examining PTB redox variation in shallow-marine (Algeo et al., 2007, 2008) and deep-ocean sections (Algeo et al., 2010, 2011) [in fact, they cited Algeo et al., 2010 but failed to identify some of its key findings]. To summarize the importance of these papers: The two studies on the Nhi Tao section demonstrated multiple (at least 8) incursions of euxinic watermasses onto this shallow-marine platform in the earliest Triassic. This demonstrates highly dynamic redox conditions in shallow environments at that time. The two studies on the Japanese sections demonstrated that the deep ocean shifted only to dysoxia, and that the pyrite framboids almost certainly came from much shallower depths (presumably the OMZ at 200-1000 m). Thus, the key change in redox in the open ocean was euxinia in an expanded OMZ globally (200-1000 m water depth) with little to no change in deep-ocean redox conditions (minor shift to dysoxia). This was documented by Algeo et al. (2010, 2011) and supported by modeling (e.g., Winguth and Winguth, 2012).
  We acknowledge that our Introduction did not adequately cite several key studies by Algeo et al. (2007, 2008, 2010, 2011) and may have oversimplified spatial variations in redox conditions during the PTB. In the revised manuscript, we will update the Introduction to address these issues as follows: 1) incorporate the expanded oxygen minimum zone (OMZ) euxinia scenario, which suggests that euxinic conditions were mainly restricted to intermediate water depths (approximately 200–1000 m), whereas the deeper ocean (>1000 m) remained predominantly dysoxic (Algeo et al., 2010, 2011; Winguth & Winguth, 2012); 2) include evidence for dynamic redox conditions in shallow-marine settings, as documented by repeated incursions of euxinic water masses during the earliest Triassic (Algeo et al., 2007, 2008).
  3. Re the redox proxies used, framboid size and Fe speciation are shown for both study sections. Beyond this, Mo-U are shown for Ursula Creek and Ce/Ce* for Chibi. The problem here is that Ce/Ce* is not a valid redox proxy when any clayey material is present in a section—even a few percent clays invariably result in a lithogenic REE signature from which no redox information can be drawn. The authors should remove Ce/Ce* from the Chibi section and, for the sake of comparability to Ursula Creek, add Mo-U data.
  We will remove the Ce/Ce* data and add Mo-U data from the Chibi section to avoid potential misinterpretation.
  4. The discussion of marine primary productivity changes around line 340 does not adequately convey what is known about this topic. Primary productivity does not show any consistent pattern globally; there was a lot of regional variation. See Algeo et al. (2013).
  We acknowledge that our discussion of marine primary productivity changes was somewhat generalized. In the revised manuscript, we will incorporate Algeo et al. (2013) and revise the relevant section to better reflect the spatial variability of productivity changes, rather than implying a globally consistent trend.
  5. I have been working to provide a logical and consistent framework for tectonics terminology in Chinese studies. I encourage the authors to read this paper:
  Algeo, T.J. and Li, H., 2026. Are South China and North China “blocks” or “cratons”?. Palaeogeography, Palaeoclimatology, Palaeoecology, 683, 113464.
  We have carefully read this article and will follow its recommendations regarding tectonic terminology. In the revised manuscript, we will adopt the term “South China Craton” (rather than “South China Block”) and ensure consistent usage of related terminology throughout.
  6. Detailed comments provided in the annotated PDF
  Many of these comments overlap with those addressed above (Comments 1–5). We will carefully address all remaining comments in the PDF, including wording issues. Following the reviewer’s suggestion, we will also add a final interpretative figure to synthesize our main findings and better illustrate the redox framework and its spatial variability across the PTB.
  
  Citation: https://doi.org/10.5194/egusphere-2026-863-AC3
RC2:
'Comment on egusphere-2026-863', Yongda Wang, 27 Mar 2026

Yang et al. present a multi-proxy analysis of marine redox dynamics across the Permian–Triassic transition, suggesting that anoxic conditions fluctuated between ferruginous and euxinic states during the crisis, and that ferruginous conditions in the earliest Triassic. The authors further propose that the expansion of ferruginous seafloor environments contributed to the delayed biotic recovery following the crisis. This manuscript provides a valuable synthesis of redox data from marine deposits and offers important insights into oceanic redox variations across the Permian–Triassic transition. Overall, I consider this work a significant contribution to our understanding of marine anoxia during the Permian-Triassic transition. However, several issues need to be addressed before the manuscript can be considered for publication.
Main concerns:
1) The references cited in the main text, particularly in the Introduction section, are relatively outdated. Please update them.
2) It would be better to briefly introduce the proxies used in this study (e.g., Mo_EF, U_EF) prior to the Results section.
3) The statement in line 201 lacks objectivity, as this study applies both framboidal pyrite analysis and geochemical proxies.
4) The authors aim to illustrate the spatiotemporal evolution of anoxic conditions in epicontinental seas across the Permian–Triassic transition; however, Fig. 7 does not clearly convey this information. If possible, annotating the marine redox conditions in different regions on paleogeographic maps for the end-Permian, PTB crisis, and earliest Triassic would enhance the clarity of the results.
5) Please indicate all cited sections in Fig. 1.

Citation: https://doi.org/10.5194/egusphere-2026-863-RC2
- AC2: 'Reply on RC2', Fen Yang, 23 Apr 2026
  
  We thank Reviewer 2 (Yongda Wang) for the positive and constructive evaluation of our manuscript. We will carefully revise the manuscript to address the specific issues raised below.
  1. The references cited in the main text, particularly in the Introduction section, are relatively outdated. Please update them.
  In the revised manuscript, we will incorporate recent studies while retaining key classic references.
  2. It would be better to briefly introduce the proxies used in this study (e.g., Mo_EF, U_EF) prior to the Results section.
  In the current manuscript, these redox proxies (pyrite framboid distributions, iron speciation, and Mo-U covariation) are presented in Section 5.1 (Redox evolution), which follows the Results. In the revised manuscript, we will move this section before the Results to improve clarity and readability.
  3. The statement in line 201 lacks objectivity, as this study applies both framboidal pyrite analysis and geochemical proxies.
  In the revised manuscript, we will explicitly include the geochemical proxies used (iron speciation and Mo-U covariation) and revise the sentence as follows: “Among various geochemical and sedimentological proxies for distinguishing redox states, pyrite framboid mean size, iron speciation, and Mo-U covariation have become established as reliable methods (e.g., Wilkin and Barnes, 1996; Algeo and Tribovillard, 2009; Poulton and Canfield, 2011).”
  4. The authors aim to illustrate the spatiotemporal evolution of anoxic conditions in epicontinental seas across the Permian–Triassic transition; however, Fig. 7 does not clearly convey this information. If possible, annotating the marine redox conditions in different regions on paleogeographic maps for the end-Permian, PTB crisis, and earliest Triassic would enhance the clarity of the results.
  We agree with the reviewer that adding clearer spatial annotations of marine redox conditions on paleogeographic maps would improve the clarity and interpretability of Fig. 7. In our previous work (Yang et al., 2024), we illustrated redox changes across extinction intervals on paleogeographic maps of South China. Here, we will attempt to extend this approach to a global‑scale paleogeographic framework, if the data permit, in order to better present the spatiotemporal evolution of redox conditions across the P-T transition.
  Yang, F., Li, S., An, K.Y., Bond, D.P.G., Ao, R., Wu, X.B., Ma, L.L., Sun, Y.D., 2024. Re-Evaluating Water Column Reoxygenation During the End Permian Mass Extinction. Geochemistry Geophysics Geosystems 25.
  5. Please indicate all cited sections in Fig. 1.
  We apologize for the omission of the citation in Fig. 1C. This figure was modified from Wignall and Newton (2003), which will be clearly indicated in the revised figure caption.
  Wignall, P.B., Newton, R., 2003. Contrasting deep-water records from the Upper Permian and Lower Triassic of South Tibet and British Columbia: Evidence for a diachronous mass extinction. Palaios 18, 153-167.
  
  Citation: https://doi.org/10.5194/egusphere-2026-863-AC2

Fen Yang, Sen Li, Stephen Grasby, David Bond, Ming Pan, and Yadong Sun

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Short summary

The end-Permian mass extinction was the most severe loss of life in Earth history, yet the role of ocean oxygen restriction remains debated. We examined rock records from South China and western Canada to track redox conditions before and after the crisis. Oxygen levels were already low prior to the extinction, and widespread seafloor anoxia developed at its peak. Afterwards, ferruginous conditions dominated, reducing nutrient availability, limiting productivity, and slowing life’s recovery.


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