Loess 10Be evidence for an asynchronous Brunhes-Matuyama magnetic polarity reversal

Thursday, 17 December 2015: 10:35
300 (Moscone South)
Weijian Zhou1,2, Warren Beck3, Xianghui Kong4, Zhisheng An1, Xiaoke Qiang5, Zhenkun Wu4, Feng Xian4 and Hong Ao5, (1)IEE Institute of Earth Environment, Chinese Academy of Sciences, State Key Laboratory of Loess and Quaternary Geology, Xi'an, China, (2)Shaanxi Provincial Key Laboratory of Accelerator Mass Spectrometry and Application, Xi’an AMS Center, Xi'an, China, (3)University of Arizona, Tucson, AZ, United States, (4)Xi’an AMS Center jointed by Institute of Earth Environment and Xi’an Jiaotong University, Xi'an, China, (5)IEE Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China
In Chinese loess the Brunhes-Matuyama (B-M) geomagnetic reversal appears to occur about 25 ka prior to the established axial dipole reversal age found in many marine sediments, i.e., in Chinese loess this magnetic reversal boundary is found in glacial loess unit L8 which is thought to be correlated with Marine Isotope Stage 20 (MIS 20), in marine sediment records, however, this boundary is commonly found in interglacial period of MIS 19[1-2], leading to the debate on uncertainties of paleoclimatic correlation between the Chinese loess-paleosol sequences and marine sediments[3-5]. Based on this issue, here we propose to use the cosmogenic 10Be to address this conundrum. 10Be is a long-lived radionuclide produced in the atmosphere by cosmic ray spallation reactions and carried to the ground attached to aerosols. Its atmospheric production rate is inversely proportional to the geomagnetic field intensity [6]. This allows us to reconstruct past geomagnetic field intensity variations using 10Be concentrations recorded in different sedimentary archives. We carried out both the 10Be studies and paleogeomagnetic measurements in Luochuan and Xifeng sections in Chinese Loess Plateau. Both loess profiles show that 10Be production rate was at a maximum-an indication of the dipole field reversal-at ca. 780 ± 3 ka BP., in paleosol unit S7 corresponding to MIS 19, proving that the timing of B-M reversal recorded in Chinese loess is synchronous with that seen in marine records [1]. These results reaffirmed the conventional paleoclimatic correlation of loess-paleosol sequences with marine isotope stages and the standard loess timescale as correct. However, it is ~25 ka younger than the age (depth) of the paleogeomagnetic measurements, which show that the B-M boundary is in Lin these two Chinese loess-paleosol sequences, demonstrating that loess magnetic overprinting has occurred.

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