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Pedogenic carbonate isotopes as evidence for extreme climatic events preceding the Triassic-Jurassic boundary: Implications for the biotic crisis?

David M. Cleveland^1,, Lee C. Nordt¹, Steve I. Dworkin¹ and Stacy C. Atchley¹

¹ Department of Geology, Baylor University, Waco, Texas 76798, USA

The Triassic-Jurassic boundary is associated with widespread marine and terrestrial extinctions, but there is disagreement regarding the existence and extent of climatic changes that may have driven the biotic crisis. Here, we apply quantitative isotopic climate proxies in order to construct two age-equivalent, relatively continuous temperature and pCO₂ records that span the eight million years preceding the Triassic-Jurassic boundary and that supersede previous terrestrial records in temporal resolution. The ¹⁸O data suggest that mean annual temperatures (MAT) increased by 7–9 °C from the late Norian to the Rhaetian in association with the peak increases in pCO₂ levels. The ¹³C data suggest relatively low late Norian pCO₂ levels (<500–1000 ppmV), increased Rhaetian levels (>1500 ppmV), and at least two periods of extreme pCO₂ levels (~3000 ppmV) preceding the Triassic-Jurassic boundary. These estimates are consistent with a recent Late Triassic climate model that suggests the effects of increased pCO₂ levels on Pangea would cause severe climatic consequences, including a global MAT increase of 6 °C (>10 °C in some regions). While it is possible that periods of increased aridity could have resulted in erroneously high estimates of both temperature and pCO₂ levels, it is likely that climate was still fluctuating during the end of the Triassic. Although our data precede the Triassic-Jurassic boundary, many studies conclude that the mass extinction took place over a more prolonged period beginning in the Late Triassic. Thus, climate may have been a significant driving mechanism of the Late Triassic extinctions.

Key Words: Late Triassic • paleoclimate • stable isotopes • pedogenic carbonates • extinction

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