Magneto- and cyclostratigraphy in the red clay sequence:New age model and paleoclimatic implicationfor the eastern Chinese Loess Plateau

The Chinese Loess Plateau red clay sequences display a continuous alternation of sedimentary cycles that represent recurrent climatic fluctuations from 2.58 Ma to the Miocene. Deciphering such a record can provide us with vital information on global and Asian climatic variations. Lack of fossils and failure of absolute dating methods made magnetostratigraphy a leading method to build age models for the red clay sequences. Here we test the magnetostratigraphic age model against cyclostratigraphy. For this purpose we investigate the climate cyclicity recorded in magnetic susceptibility and sedimentary grain size in a red clay section previously dated 11 Myr old with magnetostratigraphy alone. Magnetostratigraphy dating based on only visual correlation could potentially lead to erroneous age model. In this study the correlation is executed through the iteration procedure until it is supported by cyclostratigraphy; i.e., Milankovitch cycles are resolved in the best possible manner. Our new age model provides an age of 5.2 Ma for the Shilou profile. Based on the new age model, wavelet analysis reveals the well-preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern Chinese Loess Plateau. Further, paleomonsoon evolution during 2.58–5.2 Ma is reconstructed and divided into three intervals (2.58–3.6 Ma, 3.6–4.5 Ma, and 4.5–5.2 Ma). The upper part, the youngest stage, is characterized by a relatively intensified summer monsoon, the middle stage reflects an intensification of the winter monsoon and aridification in Asia, and the earliest stage indicates that summer and winter monsoon cycles may have rapidly altered. The use of cyclostratigraphy along with magnetostratigraphy gives us an effective method of dating red clay sequences, and our results imply that many presently published age models for the red clay deposits should be perhaps re-evaluated

Link between global cooling and mammalian transformation across the Eocene-Oligocene boundary in the continental interior of Asia

Evidence in the world&#39;s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene-Oligocene boundary at 33.9 Ma. The remarkable cooling period in the ocean, called the Eocene-Oligocene transition (EOT), is correlated with pronounced mammalian faunal replacement as shown in terrestrial fossil records. For the first time within Asia, a section is magnetostratigraphically dated that also produces mammalian fossils that span the Late Eocene-Early Oligocene transition. Three fossil assemblages revealed through the EOT (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and that changes in mammalian communities were accelerated by aridification in central Asia. Three fossil assemblages (34.8, 33.7, and 30.4 Ma) within the north Junggar Basin (Burqin section) tied to this magnetostratigraphically dated section, reveal that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and that changes in mammalian communities were accelerated by aridification in central Asia. The biotic reorganization events described in the Burqin section are comparable to the Grande Coupure in Europe and the Mongolian Remodeling of mammalian communities. That is, the faunal transition was nearly simultaneous all over the world and mirrored global climatic changes with regional factors playing only a secondary role.</p