Permian-Triassic Boundary Sections from Shallow-Marine Carbonate Platforms of the Nanpanjiang Basin, South China: Implications for Oceanic Conditions Associated with the End-Permian Extinction and Its Aftermath

Abstract

Permian-Triassic boundary (PTB) sections from isolated carbonate platforms in the Nanpanjiang Basin of south China contain Upper Permian skeletal packstones with diverse open-marine fossils overlain by a 7-15 m thick boundary horizon composed of calcimicrobial framestone constructed by globular to tufted, calcified cyanobacteria similar to Renalcis. The framestone contains interbeds of limegrainstone with abundant thin-shelled bivalves and brachiopods. The overlying Lower Triassic strata contain microgastropod lime-packstones followed by a thick succession of thin-bedded lime-mudstones. The PTB event horizon is interpreted to occur at the top of the packstone containing diverse, open-marine fauna and Palaeofusulina and coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The conformable biostratigraphic boundary occurs at the first appearance of Hindeodus parvus within the basal meter of the calcimicrobial framestone. Intensively studied PTB sections in south China, such as the GSSP at Meishan, primarily are condensed sections from deep-water, basin environments that contain a thin (Ͻ 30 cm) boundary horizon of claystone and lime-mudstone or marl. The sections reported herein are fundamentally different in that they consist of shallow-marine carbonate facies, contain a thick boundary horizon composed of calcimicrobial framestone, and lack evidence of an abrupt shift in depositional environment or water depth. The calcimicrobial framestone boundary horizon occurs in all of the isolated carbonate platforms in the Nanpanjiang Basin. A similar microbial facies has been found in the basal Triassic H. parvus zone in the Sichuan Basin and in Japan. Distribution of the calcimicrobial framestone suggests that it may represent an anomalous oceanic event that affected a vast area of the equatorial eastern Tethys and Panthalassa during and/or immediately following the end-Permian mass extinction. The persistence of similar calcimicrobial framestone horizons into the Upper Scythian suggests that detrimental environmental conditions associated with the extinction persisted until the end of the Scythian. Further study of these sections promises to provide constraints on causes of the extinction and the environments in the aftermath.

High‐resolution Lopingian (Late Permian) timescale of South China

“…During the last decade, the latest Permian and pre-Lopingian mass extinctions have become two of the most extensively studied subjects (e.g. Wignall and Hallam, 1992;Wignall and Twitchett, 1996;Bowring et al, 1998;Jin et al, 2000a;Shen and Shi, 2002;Lehrmann et al, 2003;Mundil et al, 2004;Twitchett et al, 2004;Grice et al, 2005;Isozaki et al, 2007a;Yin et al, 2007;Cao et al, 2009;Chen et al, 2009a;Wignall et al, 2009a). Debate has centred on whether there is a longlived inter-related event between the latest Changhsingian extinction and the pre-Lopingian crisis (Isozaki et al, 2007b;Yin et al, 2007;Bottjer et al, 2008) or two distinct extinctions including a short catastrophic latest Changhsingian extinction (Jin et al, 2000a;Rampino et al, 2000;Becker et al, 2001) unrelated to the end-Guadalupian extinction Shi, 1996, 2002;Shi et al, 1999;Wang and Sugiyama, 2000).…”

Calcium isotope constraints on the end-Permian mass extinction

“…Both the Strangelove Ocean and acidification scenarios predict an initial decrease in the carbonate depositional flux followed by an increase in carbonate deposition to return the system to steady state, but the perturbation is potentially much larger under the acidification scenario. In contrast, under the ocean overturn scenario one would expect a positive excursion in δ 44∕40 Ca because of the stimulation of carbonate precipitation by upwelling of alkaline deep water (2,5,15). Oceanographic mechanisms allowing for the overturn scenario have been debated (29-31); here we consider its implications for calcium isotopes irrespective of its feasibility from an oceanographic standpoint.…”

“…Microbialites and oolites overlie diverse, fossiliferous limestones of the latest Permian age in carbonate strata deposited across the tropical Tethys (1)(2)(3)(4)(5)(6)(7)(8) and in the Panthalassa Ocean (8,9) (Fig. S1).…”

“…Second, the ocean overturn model proposes that extensive sulfate reduction in anoxic deep waters of the Permian oceans resulted in a buildup of carbonate alkalinity and hydrogen sulfide in deep water prior to the extinction event (11)(12)(13)(14). Upwelling of these alkaline deep waters would have triggered carbonate precipitation on the shelves (2,5,15,16) and caused mass extinction through combined stresses of hypercapnia, anoxia, and hydrogen sulfide poisoning (14,15,17). Third, the ocean acidification model proposes that massive release of 13 C-depleted carbon from a reservoir in the crust (e.g., methane clathrates, coal, and magma) (18)(19)(20)(21)(22) acidified the ocean, reducing carbonate sedimentation and potentially leading to dissolution of carbonate sediments (8).…”

“…The section contains more than 50 m of diverse, fossiliferous packstone and grainstone of the Upper Permian Wujiaping Formation, which contains fusulinid and nonfusulinid foraminifers, calcareous green and red algae, rugose corals, crinoids, brachiopods, calcareous sponges, and gastropods (5,34). The Wujiaping Formation is overlain by a 15-m-thick thrombolitic microbialite deposited in the immediate aftermath of the mass extinction (Hindeodus parvus conodont zone) (35), which contains a lowdiversity assemblage of foraminifers, gastropods, and bivalves with rare echinoderms and calcitic and phosphatic brachiopods (4,5). The microbialite is overlain by 1-2 m of molluscan and brachiopod packstone with rare echinoderms.…”