IRON, CLIMATE, AND MASS EXTINCTION: GEOCHEMISTRY OF GREEN-GRAY AND RED-GRAY SILTSTONES STRADDLING THE VERTEBRATE-DEFINED, PERMIAN–TRIASSIC MASS EXTINCTION IN SOUTH AFRICA

The Permian–Triassic Mass Extinction (PTME) is considered the largest in Earth’s history, yet the terrestrial response is not well understood. Only a few localities in the world are known to expose the terrestrial PTME sequence, and most either preserve records of only a portion of the time span or are extremely inaccessible. The Karoo Basin, South Africa, was an inland closed basin during the latest Paleozoic and the earliest Mesozoic, and hosts abundant exposures of terrestrial deposits that are reported to span the vertebrate-defined Permian–Triassic Boundary (PTB).

The proposed vertebrate-defined PTB is contained in the uppermost Balfour Formation of the Karoo Supergroup, where it is associated with a transition from green-gray to red-gray siltstones. The current PTME model uses the change in these siltstone colors as one indication of a rapid transition from a wet to an arid climate that is directly correlated with the PTME. We collected green-gray and red-gray siltstones from below, at, and above the vertebrate-defined PTB at Old Lootsberg Pass, Eastern Cape Province, and Bethulie, Free State Province. Grain size of green-gray siltstones ranges from coarse to sandy coarse, while red-gray siltstones generally are finer-grained. Petrographic data indicate that sand and coarse silt grains in both siltstones are primarily quartz, but red-gray samples have a higher mud content. Preliminary XRF analyses indicate that these siltstones differ significantly in their total Fe concentrations (wt.%), likely a result of mineralogical differences. Fe/K plots of red-gray samples show two distinct clusters, one with higher Fe. The high-Fe cluster also has a higher K proportion. Most green-gray samples have K levels similar to the low-Fe cluster but have even lower Fe levels.

Traditionally, mudrock coloration is thought to be a function of Fe²⁺/Fe³⁺ ratio and reflects climatic conditions at the time of formation. This current study tests the geochemistry of these green-gray and red-gray siltstones across the vertebrate-defined PTB to determine whether they differ in Fe²⁺/Fe³⁺. If the ratio of Fe²⁺/Fe³⁺ does not differ significantly, re-interpretation is required for the commonly accepted relationship between mudrock coloration and climate in the Karoo Basin’s PTME model.