The Fotan Group from Zhangpu of Fujian yielded abundant plant megafossil and amber, generating a typical Miocene tropical rainforest ecosystem of China. The plant fossils are preserved in diatomite and mudstone, while the amber occurs within coal seams and diatomite that directly underlie the layer hosting plant flora. Determining the age of Fotan fossil horizons can help date the Miocene tropical rainforest in China and constrain its long-term history.
Recently, Dr. ZHENG Daran and Associate Professor SHI Gongle from the ‘Modern terrestrial ecosystem origin and early evolution’ group at Nanjing institute of Geology and Palaeontology of Chinese Academy of Sciences, cooperating with the researchers from the University of Hong Kong and Columbia University, pioneered this age-determining research and provided a robust 40Ar/39Ar age for the Fotan biota based on the basalt samples from Maping and Wuling of Zhangpu, Fujian Province.
For the sample from Maping, incremental heating analysis of groundmass yielded a plateau age of 14.5 ± 1.5 Ma (MSWD = 0.65). Data from the step-heating analysis were plotted on an inverse isochron isotope correlation diagram (36Ar/40Ar vs. 39Ar/40Ar). The isochron age of 15.2 ± 2.0 Ma (MSWD = 1.2) shows well agreement with the plateau age including its error term.
For the sample from Wuling, incremental heating analysis of groundmass yielded a well-defined plateau age of 14.7 ± 0.4 Ma (MSWD = 0.28). The isochron age of 14.7 ± 0.4 Ma (MSWD = 0.96) is identical with the plateau age. The 302 ± 2 40Ar/36Ar intercept from the inverse isochron diagram resembles atmospheric values (i.e., 295.5 or 298.56).
Although the age results for both samples are indistinguishable at the 2σ level, we interpret the more precise 14.7 ± 0.4 Ma ages as the upper limit of the depositional age of the Fotan fossils.
This age constrains interpretations of the early evolution of tropical rainforest ecosystems in Asia, and laid foundation for the systematic study of Zhangpu amber biota.
This research was recently published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, and supported by the National Natural Science Foundation of China and RGC General Research Fund of Hong Kong.
Reference: Zheng, D., Shi, G., Hemming, S.R., Zhang, H., Wang, W., Wang, B., Chang, S.-C.*, 2019. Age constraints on a Neogene tropical rainforest in China and its relation to the Middle Miocene Climatic Optimum. Palaeogeography, Palaeoclimatology, Palaeoecology, 518: 82–88. DOI: https://doi.org/10.1016/j.palaeo.2019.01.019.
Photograph of a outcrop in Fotan, Zhangpu
Stratigraphic column of Fotan Group in Zhnagpu
40Ar/39Ar age result