OXYGEN ISOTOPIC RECORD OF DICYNODONT TUSKS ACROSS THE PERMO-TRIASSIC BOUNDARY IN THE KAROO BASIN, SOUTH AFRICA

Eruption of the Siberian Traps through a thick sequence of coal-bearing strata is widely invoked as a primary cause of the Permo-Triassic mass extinction. One possible connection between eruptions and extinctions is dramatic greenhouse warming forced by the release of volcanic CO₂, remineralized organic carbon, and volatilized sedimentary organics. Warming, in turn, could have promoted oceanic anoxia by slowing ocean circulation, lowering seawater O₂ saturation levels, and/or increasing O₂ utilization in ocean interiors through warming’s effects on weathering and productivity. Support for warming has been provided recently by documentation of a large decrease in the δ¹⁸O values of phosphate oxygen in conodonts which could correspond to 8-10°C of tropical warming within ~100,000 years of the boundary.

To test for evidence of warming in a setting other than Tethyan marine sequences, we measured phosphate δ¹⁸O values on dicynodont tusks of 60 individuals from the Permo-Triassic sections in the Karoo Basin of South Africa. Tusk δ¹⁸O values are relatively constant (5.6‰_VSMOW ± 0.8‰ n = 36) across 400 m of section representing deposition during the 3 million years before the boundary. Values then increase gradually by >3‰ over 80 m of section in the earliest Triassic (to 8.7‰ ± 1.1, n = 6 in the youngest horizon sampled). Pedogenic carbonates show a similar pattern whereas diagenetic carbonates do not. This result argues against a purely diagenetic explanation for the increase in tusk δ¹⁸O values. The shifts are consistent with increased aridity (supported by sedimentological and taphonomic observations), warming, and an increase in δ¹⁸O value of precipitation. This conclusion has implications for understanding interactions of Earth systems across the Permo-Triassic event including the response of climate and the hydrologic cycle to rapid increases in CO₂ concentrations. Assumptions can be examined through isotopic analyses of tooth enamel (to reduce the probability of diagenetic alteration compromising interpretation) and numerical experiments of earliest Triassic climates using models that track the isotopic composition of precipitation and the balance between evaporation and precipitation.