Fluctuations of redox conditions across the Permian–Triassic boundary : new evidence from the GSSP section in Meishan of South China

Oceanic anoxia has long been considered as one of the main causes for the end-Permian mass extinction. However, the results obtained by different researchers are rather divergent from different sections, or even on the same section using the same redox proxy. This study aims to examine the causes for some of these divergent results using high-resolution pyrite framboid sampling at the Meishan GSSP section in South China. Detailed microfacies analysis shows that the uppermost Late Permian strata comprises two significantly different sedimentary facies: one characterized by silicious muddy limestone and recognized as representing autochthonous background sediments; the other distinguished by bioclastic grainstone, interpreted to be allochthonous in origin and have been transported from the nearby platform margin. These two different sedimentary facies represent two distinctly different redox conditions. Together with the facies analysis, a statistical analysis of pyrite framboids was carried out to evaluate the redox evolution across the Permian-Triassic boundary. Abundant framboids with average diameters of about 6μm are found in background sediments beneath the extinction boundary, indicating generally anoxic bottom water conditions. But this condition was punctuated by transient intervals of rapid oxygenation interpreted to have been caused by intrusion of intermittent turbidity flows. Our study also showed that anoxic conditions persisted into the immediate aftermath of the mass extinction, thereafter it was quickly followed by a relatively long period of oxic conditions (with rare framboids). However, the redox conditions returned to anoxia (with abundant pyrite framboids averaging about 5μm in diameter), accompanied by a rapid global transgression. The oxygenation manifested near the Permian-Triassic boundary coincides with the negative excursion of carbon isotope. This would imply that, contrary to previous interpretations, this great δ13C negative excursion was probably not caused by the upwelling of anoxic deep ocean waters.

The drowned apostles: the longevity of sea stacks over eustatic cycles

Cliffed rocky coasts are erosional environments, the remnants of which can be preserved as sea stacks as the shoreline retreats. These sea stacks form spectacular landscapes, such as the iconic Twelve Apostles in Victoria, Australia. However, they are ephemeral features formed on a centennial scale, continually eroding and collapsing, meaning that coasts characterised by sea stacks often have fewer features than when first described. The question arises then as to the longevity of such features and whether they can be preserved over eustatic cycles. The modern Twelve Apostles, of which 8 are still standing, are comprised of the Miocene Port Campbell Limestone and reach 45 m above sea level. Recent multibeam sonar data show five features around 6 km offshore, in 40-50 m water depth that appear to be relict sea stacks. Based on the morphology and geology of both the modern and drowned Apostles, it is inferred that the drowned and modern stacks evolved in a similar manner. While the modern sea stacks have an average height of 45 m, the drowned stacks have an average height of 4 m, suggesting a much greater age and also the possibility of multiple exposures to subaerial processes. The drowned stacks lay 655 m seaward of a drowned cliff averaging 14 m high which likely represents a former interstadial shoreline. This is much greater than the 91 m average distance between stack and cliff for the Modern Apostles, which may imply a more prolonged period of erosion along the drowned coastline.