Late Early Triassic climate change: Insights from carbonate carbon isotopes, sedimentary evolution and ammonoid paleobiogeography
Résumé
The late Early Triassic sedimentary–facies evolution and carbonate carbon-isotopemarine record (δ13Ccarb) of ammonoid-rich, outer platform settings show striking similarities between the South China Block (SCB) and the widely distantNorthern IndianMargin (NIM). The studied sections are located within the Triassic TethysHimalayan belt (Losar section, Himachal Pradesh, India) and the Nanpanjiang Basin in the South China Block (Jinya section, Guangxi Province), respectively. Carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end-Permian biotic crisis. This study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover (extinction followed by a radiation) occurring around the Smithian–Spathian boundary. Based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity, we demonstrate that the late Early Triassic (Smithian and Spathian)was a time of a major climate change.More precisely, the end Smithian climate can be characterized by a warm and equable climate underlined by a flat, pole-to-equator, sea surface temperature (SST) gradient, while the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions.Moreover, sedimentary evidence suggests a transition froma humid and hot climate during the Smithian to a dryer climate fromthe Spathian onwards. By analogywith comparable carbon isotope perturbations in the LateDevonian, Jurassic andCretaceouswe propose that high atmosphericCO2 levels could have been responsible for the observed carbon cycle disturbance at the Smithian–Spathian boundary. We suggest that the end Smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased CO2 flux possibly originating froma short eruptive event of the Siberian igneous province. This increase in atmospheric CO2 concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms, including ammonoids, in late Smithian oceans.