Timing of the Brunhes-Matuyama transition constrained by U-series disequilibrium

Bassam Ghaleb,Julie Carlut,Larbi Boudad,Jean-Pierre Pozzi,Christophe Falguères,Geoffroy Mahieux,Louis Rousseau

doi:10.1038/s41598-019-42567-2

Abstract

U-series disequilibrium measurements carried out on thermogenic travertine samples from a 12.6 m-long core and a 10 m-thick section from southeastern Morocco yielded finite ages ranging from 500 ka to the present-day, as well as two clusters determined to be older than 500 ka. The calculation of initial 234U/238U activity ratios in all samples younger than 500 ka shows high, reasonably constant values, with an average of 5.172 ± 0.520 (one standard deviation). Assuming that this value prevailed for periods older than 500 ka, we derived ages of up to approximately 1.2 Ma using the initial 234U excess decay. Our results indicate that the two older clusters have ages of 776 ± 14 ka for samples from between 8 and 10.1 m and 1173 ± 22 ka for deeper samples respectively. The palaeomagnetic record of the core shows normal polarity inclinations from the surface to around 9 m followed by reverse polarity inclination and antipodal declinations. The inversion is attributed to the Brunhes-Matuyama transition. 234U excess ages for the interval corresponding to the part of the core where the polarity inversion occurred are in the range of 735 ± 51 to 794 ± 54 ka, with an arithmetic mean value of 776 ± 14 ka for the B-M transition. This age is in good agreement with that determined previously using other dating methods.

Highlights

IntroductionThe U-series method is widely used to date biogenic CaCO3 (corals, mollusk shells), as well inorganic CaCO3 precipitates (speleothems, travertine)[1,2] and references in[3,4]
The U-series method is widely used to date biogenic CaCO3, as well inorganic CaCO3 precipitates[1,2] and references in[3,4]
Travertine deposits result from carbon dioxide-rich waters dissolving carbonate rocks at depth and depositing calcium carbonate when pressure and CO2 decrease at the surface[10]

Summary

Introduction

The U-series method is widely used to date biogenic CaCO3 (corals, mollusk shells), as well inorganic CaCO3 precipitates (speleothems, travertine)[1,2] and references in[3,4]. The first method uses 230Th-234U-238U from samples younger than 500 ka and the second is based on the decay of initial 234U excess in samples aged between 500 and 1200 ka. Using this combination of the these two approaches, the present study extends the dating limit of U-series to approximately 1.2 Ma which gives us the ability to add new time constraints on the Brunhes-Matuyama magnetic reversal, which are independent of the K-Ar and 40Ar/39Ar radiometric dating methods or orbital tuning techniques that are usually used to date this transition

Methods

Results

Conclusion