Brunhes-Matuyama Magnetic Polarity Reversal Tracing using Chinese loess10Be

Tuesday, 16 December 2014
Weijian Zhou1,2, Warren Beck3, Xianghui Kong1,2, Zhisheng An1, Xiaoke Qiang1, Zhenkun Wu1,2, Feng Xian1,2 and Hong Ao1, (1)IEE Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China, (2)Xi’an AMS Center jointed by Institute of Earth Environment and Xi’an Jiaotong University, Xi'an, China, (3)University of Arizona, Tucson, AZ, United States
The geomagnetic polarity reversal is generally considered to occur synchronously around the world, and is commonly used as a time marker. However, in the case of the most recent reversal, the Brunhes-Matuyama (B-M) reversal (~780 ka), comparison of paleomagnetic studies in Chinese loess-paleosol sequences versus marine sediments revealed a marked discrepancy in timing of this event (Tauxe et al., 1996; Zhou and Shackleton, 1999), leading to the debate on uncertainties of paleoclimatic correlation between the Chinese loess-paleosol sequences and marine sediments (Wang et al., 2006; Liu et al., 2008; Jin and Liu, 2011). 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 (Masarik and Beer, 1999). This allows us to reconstruct past geomagnetic field intensity variations using 10Be concentrations recorded in different sedimentary archives. We carried out the 10Be studies 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 S7corresponding to MIS 19. These results have proven that the timing of B-M reversal recorded in Chinese loess is synchronous with that seen in marine records (Tauxe et al., 1996) and 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 magnetic polarity reversal recorded in these same Chinese loess-paleosol sequences, demonstrating that loess magnetic overprinting has occurred.

1.Jin, C.S.,et al., 2011,PALAEOGEOGR PALAEOCL, 299, 309–317
2.Liu, Q.S., et al., 2008, EARTH PLANET SC LETT, 275, 1–2,102–110
3.Masarik, J., and Beer, J., 1999, Jounal of Geophysical Research, 104, 12099-12111.
4.Tauxe, L., et al., 1996, EARTH PLANET SC LETT, 140, 133–146
5.Wang, X.S., et al., 2006,PHYS EARTH PLANET IN, 159, 109–117
6.Zhou, L.P., and Shackleton, 1999,EARTH PLANET SC LETT, 168,117-130