the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Feb 2026
Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas
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 (MoEF and UEF), and high MoEF/UEF 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 MoEF-UEF 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.
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Status: open (until 23 Apr 2026)
- CC1: 'Comment on egusphere-2026-863', Giacomo Medici, 04 Mar 2026 reply
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RC1: 'Comment on egusphere-2026-863', Thomas Algeo, 12 Mar 2026
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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.
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RC2: 'Comment on egusphere-2026-863', Yongda Wang, 27 Mar 2026
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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., MoEF, UEF) 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
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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.