HIGH-PRECISION 40Ar/39Ar GEOCHRONOLOGICAL CONSTRAINTS ON THE EVOLUTION OF MESOZOIC ECOSYSTEMS IN EAST ASIA: RATES AND DATES

The Cretaceous terrestrial Jehol Biota and the Jurassic Haifanggou-Lanqi fossils (HLF) in East Asia include abundant and varied vertebrates, invertebrates and plants with a simply spectacular degree of preservation. Systematic dating of abundant tuffs from classic localities and newly discovered Jehol biota-bearing formations will provide a robust geochronological calibration for the occurrence of early mammals (including the earliest known therians and eutherians), primitive birds (including the earliest known beaked birds) and several groups of feathered dinosaurs (Troodontidae, Dromaeosauridae, Compsognathidae). Determining high-precision ages for the HLF is critical to establishing its role in floral evolution, particularly of the angiosperms, and to improve our knowledge of Mesozoic terrestrial ecosystems that existed in NE China. Within our pilot study in the Berkeley Geochronology Center, eight tuff samples and two basalt samples collected from the Jehol and the HLF related formations near the classic outcrops in western Liaoning, NE China yielded high-precision 40Ar/39Ar ages, indicating a 7 Ma span from 129-122 Ma for the Yixian Formation. As participants of EARTHTIME, we seek to test the interlaboratory comparisons between the Ar lab at LDEO and the Berkeley Geochronology Center. Thus, we are in the process of reanalyzing five of the ten samples at LDEO. Initial age results for the Jehol Biota and the HLF are promising, but they only begin to tap the potential for elucidating the spatial and temporal relationships of the localities that produce these significant fossils. With additional work (in progress) we expect to build a geochronologic framework for understanding the range of the biota and stratigraphic correlation of the fossil beds in different provinces and areas. Our precise chronology for these remarkable fossils will allow an unprecedented look at evolutionary rates and processes for this key interval of earth history.