Composition and evolution of strontium isotope from the fossiliferous layers of the Luoping biota, Yunnan

The ⁸⁷Sr/⁸⁶Sr curve of seawater is relatively stable in the Anisian period of Middle Triassic according to data available, which is different from the situation of frequent changes in the Early Triassic. The recovery of marine ecosystem from the endPermian mass extinction experienced an accelerating phase during the Anisian period. The Luoping biota is one of the typical representatives of the Triassic biota. The authors studied the composition and evolution of the strontium isotope in 26 carbonate samples from the standard section, i.e., Dawazi section, of the Luoping biota, in order to know the change of strontium isotope from the fossiliferous layers which contain the Luoping biota. The results show three remarkable characteristics:(1) The average value of Sr is more than 2000×10^-6 from the carbonate, which is mainly caused by the aragonite and diagenesis; (2) the low value of Mn and the Mn/Sr ratios less than 0.4 show that the these ⁸⁷Sr/⁸⁶Sr ratios can represent the composition of Sr isotope of the coeval seawater; (3) the ratios of ⁸⁷Sr/⁸⁶Sr of all samples range between 0.707969 and 0.708253, with the average ratio 0.708025. It is notable that the ratios of the ⁸⁷Sr/⁸⁶Sr are larger than the data for the same geological time published from Europe. Nevertheless, they are similar to Sr isotopic values from other areas in southern China. The authors therefore hold that the strontium isotope composition of the fossiliferous layers of the Luoping biota was not directly connected to global geological events, but affected by the regional event, such as the indosinian movement.