Facies changes and diagenetic processes across the Permian–Triassic boundary event horizon, Great Bank of Guizhou, South China: a controversy of erosion and dissolution

Abstract

The Permian–Triassic boundary interval in shallow shelf seas of South China shows Upper Permian limestones overlain by lowermost Triassic microbialites. Global sea‐level rose across the Permian–Triassic boundary, but an irregular top‐Permian erosion surface across a 10 km north–south transect of the Great Bank of Guizhou contains evidence of sea‐level fluctuation. The surface represents the ‘event horizon’ of mass extinction, below the biostratigraphic Permian–Triassic boundary defined by first appearance datum of conodont Hindeodus parvus. An Upper Permian foraminiferal grainstone beneath this surface contains geopetal sediments, etched grains, and pendent and meniscus cements interpreted here as vadose. However, these latter diagenetic processes occurred before the event horizon and were followed by erosion of the final Permian surface. This erosion cuts previous fabrics but lacks evidence of weathering or bioerosion. A few centimetres below is an earlier grainstone that was also eroded but lacks proof of sub‐aerial processes. Samples therefore reveal one, or possibly two, small‐scale relative sea‐level changes before the Triassic transgression in this area, and these may relate to local tectonics. The final Permian surface is subject to at least four interpretations: (i) sub‐aerial physical erosion and dissolution by carbon dioxide‐enriched fresh water or carbon dioxide‐enriched mixed water, prior to Triassic transgression; (ii) sub‐aerial physical erosion overprinted by dissolution related to carbon dioxide‐enriched sea water in the Early Triassic transgression; (iii) submarine dissolution affected by acidified sea water due to rapid increase in volcanically‐derived carbon dioxide and oxidized methane released from marine clathrates; (iv) submarine dissolution due to acid anoxic waters rising across the continental shelf, unrelated to atmospheric carbon dioxide or oxidized methane. Field and petrographic evidence suggests that (i) is the simplest option; and it is possible that (ii) and (iii) occurred, but none are proved. Option (iv) is unlikely given the evidence and modelling of supersaturation of upwelled waters with respect to bicarbonate.

Rapid carbonate depositional changes following the Permian-Triassic mass extinction: Sedimentary evidence from South China

“…Payne et al (2010) documented Ca isotope evidence for the ocean acidification hypothesis, which can account for the greater effect on heavily calcified animals (Knoll et al, 2007(Knoll et al, , 1996 and extensive submarine erosional surfaces in the extinction interval, as well as the additional oolites and microbialites in the post-extinction carbonate supersaturated oceans Baud et al, 2005). However, these observed sedimentary phenomenon also can be attributed to Mg/Ca seawater chemistry anomaly and rapid regression (Hönisch et al, 2012;Kershaw et al, 2012;Collin et al, 2009;Kump et al, 2009;Wignall et al, 2009). The saturation rate in the post-extinction (over 20 Ka.)…”

“…The preferential extinction of heavier calcified fossils was demonstrated as a biotic response to ocean acidification (Knoll et al, 2007(Knoll et al, , 1996 while extensive erosion truncation surfaces within the subaerial carbonate P-T boundary successions were documented as the sedimentary response . However, truncation surfaces and strata loss between Permian bioclastic limestone and basal Triassic microbialite also could be attributed to the exposure of shallow carbonates during the end-Permian regression (Yin et al, 2014;Kershaw et al, 2012;Collin et al, 2009;Wignall et al, 2009). The controversial explanations for these sedimentary changes have inspired us to synthesize these scenarios and propose other hypotheses for fluctuations of the carbonate deposition during this time.…”

Microbial–algal community changes during the latest Permian ecological crisis: Evidence from lipid biomarkers at Cili, South China

“…2A; cf. Collin et al, 2009). The carbonate carbon isotopic profile for Cili section supports the inference of continuous sedimentation across the facies contact (Luo et al, 2011a).…”

Diagenetic overprint on negative δ13C excursions across the Permian/Triassic boundary: A case study from Meishan section, China

“…1B-D) because (1) it is the global stratotype section and point (GSSP) for the Permian-Triassic boundary with continuous succession through the Permian/Triassic transitional period, (2) the ages of critical intervals across the PTB are known (Shen et al, 2011;Burgess et al, 2014) and the conodont biostratigraphy is well established (Jiang et al, 2007), and (3) samples are available for the study. Specifically, this study provides a more complete picture of the diagenetic features of the Meishan section and provides the basis for comparison with other global Permian-Triassic boundary sections (Heydari et al, 2000(Heydari et al, , 2001Collin et al, 2009;Wu et al, 2014). The results indicate that diagenetic alteration needs to be considered before the δ 13 C carb is used to estimate the dissolved inorganic carbon in the ancient oceans and calculate the global carbon cycle.…”

“…In the platform succession, the missing conodont zones at the PTB reflects the depositional hiatus associated with the Late Permian drop in sea level between the Clarkina meishanensis and Hindeodus changxingensis zones (Yin et al, 2014). The unconformity at the PTB has long been attributed to subaerial exposure and karst development that took place during a sealevel lowstand (Wu et al, 2006;Collin et al, 2009;Wignall et al, 2009). Thus, it seems probable that the δ 13 C and δ 18 O values in the uppermost Permian beds may have been modified by meteoric diagenesis.…”