|关键词: 岩石磁学 磁性地层 吕梁山 红黏土
|英文基金项目:Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (2019QZKK0707); Key Research Program of Frontier Sciences, Chinese Academy of Sciences (QYZDB-SSW-DQC021); Youth Innovation Promotion Association, CAS
|Magnetostratigraphy of Late Miocene—Pliocene red clay in Liulin area
LIU Shihang, ZHANG Peng, AO Hong, YAO Jiao
1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
|Background, aim, and scope The Chinese Loess Plateau (CLP, ~300000 km2, extending from 100°E to 115°E and from 34°N to 41°N) contains the longest (spanning at least the last 25 Ma), relatively continuous high-resolution (up to hundreds of years) wind-blown dust deposits, including the well-known Quaternary loess-paleosol sequence and latest Oligocene—Pliocene red clay. It is one of the most valuable Cenozoic climate archives on the earth. The enormous dust accumulation reflects dust transportation from arid central Asia to the semi-arid CLP primarily by the near-surface winter monsoon. The eolian red clay started to accumulate on the western CLP as early as 25—22 Ma, but it is much younger on the western CLP, generally younger than ~8.2 Ma. The long red clay sequence is devoid of suitable material for radiometric dating, so magnetostratigraphy has become the routine method of choice to establish a precise age model. A robust precise magnetostratigraphy that can be correlated unequivocally to the geomagnetic polarity time scale (GPTS) is a basis to explore links between regional and global climate changes. Here, we report paleomagnetic results of a new red clay section at Liulin on the eastern CLP to establish a precise age model that allows in-depth study of regional environmental changes from both global and regional perspectives. Materials and methods In this paper, the Liulin red clay section locates in the western side of Lüliang Mountain and has a thickness of 60 m, from which we collected 600 paleomagnetic samples directed with a compass and 2646 undirected samples. All 2646 undirected samples were powdered and then packed into non-magnetic cubic boxes (2 cm×2 cm×2 cm) for low frequency magnetic susceptibility (χlf) measurements in the laboratory. χlf was measured with a Bartington Instruments MS2 magnetic susceptibility meter at 470 Hz. After thermal demagnetization of the 600 paleomagnetic samples, remanence was measured using a 2-G Enterprises Model 755-R cryogenic magnetometer housed in a magnetic shielded space (＜150 nT). The temperature dependent susceptibility (χ—T) curves for selected samples were measured in an argon atmosphere from room temperature to 700℃ and back to room temperature using a MFK1-FA susceptometer equipped with a CS-3 high-temperature furnace (AGICO, Brno, Czech Republic). The acquisition curves of the isothermal remanent magnetization (IRM) and Hysteresis loops for selected samples were determined using a Princeton Measurements Corporation (Model 3900) vibrating sample magnetometer (VSM) with stepwise increasing field applied to a maximum field of 1.0 T. Results The mineral magnetic measurements, including the χ—T curves, IRM acquisition curves and hysteresis loops, suggest that magnetite and maghemite are dominant magnetic minerals in the Liulin red clay, with minor contribution from high-coercivity minerals (gothite and/or hematite). From the 600 demagnetized samples, 363 samples yielded reliable characteristic remanent magnetization (ChRM) components based on strict selection criteria. This resulted in a set of 158 normal and 205 revered polarity ChRM directions passing the reversals test. Virtual geomagnetic pole (VGP) latitudes calculated from the 363 ChRM directions are used to establish the Liulin magnetostratigraphy, which includes 6 normal and 7 reversed magnetozones. Discussion The lithology and magnetostratigraphy of Liulin red clay sequences were compared with that of other red clay sections in the adjacent area. The high-resolution magnetostratigraphy studies suggest that the Liulin red clay section records the polarity sequence from C3r to C2An.1r, ranging in age from ~5.8 Ma to 3.1 Ma. The strata before the Miocene—Pliocene boundary (MPB) has higher magnetic susceptibility values than the strata deposited after MPB, which is consistent with summer monsoon intensification across the MPT as suggested by previous studies. Several layers of gravel developed in the early red clay deposition, which may reflect proximal materials transported from the Lüliang mountain to the studies area by high-energy rainfall flow under high precipitation conditions. Conclusions Rock magnetic studies show that magnetic minerology of the Liulin red clay sequences are dominated by magnetite and maghematite. Magnetostratigraphic study suggests that the age of the Liulin red clay profile ranges from ~5.8 Ma to 3.1 Ma. Recommendations and perspectives Results presented in this study establish a precise magnetochronology for the Liulin red clay on the eastern CLP and provide a basis to reconstruct high-resolution paleoclimate history recorded in the Liulin red clay and to explore regional climate dynamics from global perspectives with detailed land-sea correlations.
|Key words: rock magnetism magnetostratigraphy Lüliang Mountain red clay