New insights into the origin of magnetic inclination shallowing in stalagmites

Speleothems can provide high-resolution records to reconstruct the ancient Earth's magnetic field. However, little is known about the influence of speleothem morphologies on the natural remanent magnetisation record. A previous study by Ponte et al. (2017) showed that the magnetic inclination recorded in stalagmites decreases according to the slope of the calcite laminae. Magnetic inclination is 5º lower at the bottom of the stalagmite than at the top. The authors suggest that magnetic particles are re-orientated due to particle rolling during the deposition onto the flank of the stalagmite, resulting in magnetic inclination shallowing. In this scenario, magnetic inclinations are expected to be deeper on the other flank of the stalagmite. Still only half of the stalagmite was available for the study of Ponte et al. (2017). Here, we present new data on a cone-shaped stalagmite from Central Portugal, labelled LM1. We use alternating field demagnetisation, anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetisation (AARM), and X-ray microtomography to investigate the orientation of the calcite and magnetic fabrics along the flanks of three detrital-rich calcite layers and their relationship with paleomagnetic directions. We show that magnetic inclination varies up to more than 10° depending on the slope of the calcite laminae. In contrast, magnetic declination is almost constant, corroborating previous results by Ponte et al. (2017). Orientation of the k1 of the calcite fabric determined by AMS shows a striking correlation with the k1 of the magnetic fabric obtained by AARM, suggesting that the orientation of the magnetic particles is mainly controlled by the growth direction of the calcite crystals, which is perpendicular to the surface of the stalagmite. However, magnetic inclinations are not symmetrical between both flanks, suggesting that the particle rolling hypothesis is invalid in this case. In addition, the fact that the maximum variations of the remanent magnetic inclination do not exceed 10º indicates that not all the magnetic carriers are reoriented according to the calcite fabric and that the mechanisms responsible for the acquisition of the detrital remanent magnetisation are more complex than previously thought.     

 

Acknowledgments: PTDC/CTA-GEO/0125/2021 Foundation of Sciences and Technology (Portugal) and BU037P23 Junta de Castilla y León (Spain). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101005611 for Transnational Access conducted at DMEX-UPPA-FRANCE.