Marcasite at the Permian-Triassic Transition: A Potential Indicator of Hydrosphere Acidification

Abundant pyrite in black shales at the Permian-Triassic Boundary (PTB) from several localities around the world has been regarded as evidence of oceanic anoxia during the end-Permian mass extinction (EPME). However, a significant amount of the “pyrite” in these rocks is not actually pyrite but marcasite, the orthorhombic polymorph of FeS₂. Marcasite is particularly sensitive to changes in pH and fO₂, which theoretically enables it to be utilized as a proxy for geochemical changes in the marine environment. Moreover, its abundance in these PTB rocks suggests that major geochemical changes occurred at this time in the global ocean. In this chapter, we present stable isotope and trace element compositional data for both marcasite and pyrite, which were identified in the stratigraphic interval representing the EPME, along with bulk-rock chemostratigraphic data for the Kockatea Shale, Perth Basin, Australia. We compare marcasite-pyrite intergrowth textures in PTB sedimentary sequences from Meishan D (MD), Opal Creek (OC), Ubara (Ub), and the Kockatea Shale (KS), which represent both shallow continental shelves of the ancient Tethys Ocean (KS, MD) and the abyssal plains of the Panthalassic Ocean (Ub, OC) at the end of the Permian. The textures, trace element geochemistry, and sulphur isotopic analyses of these samples favor a synsedimentary to early diagenetic origin for the sulfides, further supporting the notion that the EPME was at least partially caused by a large-scale pH drop in the global ocean at the PTB. A review of other sedimentary sequences around the globe, including pre- and post-Permian sections, indicates that sedimentary-diagenetic marcasite may be more abundant in the rock record than previously recognized, which carries implications for the geochemistry of the marine environment through geologic time.