The late Early Triassic sedimentary-facies evolution and carbonate
carbon-isotope marine record (delta(13)C(carb)) of ammonoid-rich, outer
platform settings show striking similarities between the South
ChinaBlock (SCB) and the widely distant Northern Indian Margin (NIM).
The studied sections are located within the Triassic Tethys Himalayan
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 from a humid and hot climate
during the Smithian to a dryer climate from the Spathian onwards. By
analogy with comparable carbon isotope perturbations in the Late
Devonian, Jurassic and Cretaceous we propose that high atmospheric CO(2)
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 CO(2) flux possibly originating
from a short eruptive event of the Siberian igneous province. This
increase in atmospheric CO(2) concentrations could have additionally
reduced the marine calcium carbonate oversaturation and weakened the
calcification potential of marine organisms, including ammonoids, in
late Smithian oceans. (c) 2006 Elsevier B.V. All rights reserved.