The globally averaged ¹⁰Be atmospheric production rate is inversely proportional to the geomagnetic dipole moment (GDM). Geomagnetic reversals or excursions are characterized by GDM collapses, so these geomagnetic events can be reconstructed by the ¹⁰Be overproduction they trigger, independently of paleomagnetic measurements. The Matuyama-Brunhes (M-B) transition is the most recent geomagnetic field reversal and is an important time marker for the Quaternary period. During the past few decades, this reversal has been investigated by cosmogenic ¹⁰Be in many geological archives, including basalts, ice core, marine and lacustrine sediments, as well as Chinese loess. However, there has been no research on tracing M-B reversal using lacustrine ¹⁰Be.
Based on the preliminary work on the loess ¹⁰Be, the research team led by Prof. ZHOU Weijian from the Institute of Earth Environment, Chinese Academy of Sciences (IEECAS) recently presented a new ¹⁰Be record spanning the Matuyama-Brunhes (M-B) reversal from the Heqing paleolake basin in southern China. The aim of this study is the identification of the GDM collapse associated with the M-B transition using ¹⁰Be from lacustrine sediments for the first time. Through test and analysis of high resolution ¹⁰Be, ⁹Be data and related proxies, authigenic ¹⁰Be/⁹Be ratio was considered as the most reliable proxy for reconstructing atmospheric ¹⁰Be production rates. The M-B transition is identified as a pronounced maximum of authigenic ¹⁰Be/⁹Be ratio between 768.6-778.5 ka, that is consistent with all other available ¹⁰Be-proxy records from marine, ice and loess archives. However, it is offset by approximately 0-60 cm depth from the magnetic signature of the polarity transition recorded in the same sediments. This offset is attributed to the lock-in effect of the remanence acquisition process, which is similar to the phenomenon that exists in marine sediments. It is suggested that the identification of geomagnetic reversal/excursion events using ¹⁰Be along with paleomagnetic measurements in marine and stable lacustrine sediments (homogeneous clay and silty clay) can help to eliminate the depth offset due to the lock-in effect and obtain more precise geochronologies.
This work has been published on the Quaternary Science Reviews.

Fig. 1.Comparison of ¹⁰Be/⁹Be ratios and paleomagnetic data from the Heqing sediments. (Image by Du et.al)

 Fig. 2 Comparison of ¹⁰Be production rates and relative paleointensities between the Heqing core and other sediments. (Image by Du et.al)

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email:baijie@ieecas.cn