Selective transport and deposition of magnetic particles during speleothem growth

During the last decades, advances in the field of speleothem’s magnetism opened a new door to investigate high-resolution and short-lived features of the Earth’s magnetic field. Due to the rapid precipitation of calcite/aragonite, the lock-in time of the detrital remanent magnetization resulting from the physical alignment of the magnetic minerals parallel to the Earth’s magnetic field is acquired almost instantaneously. The magnetic particles trapped into the speleothem usually originate from the soils capping the cave and are transported into the cave by dripwaters. Authigenic magnetic particles may also precipitate under conditions likely to prevail during speleothem growth. Here we investigate the magnetic mineralogy of a stalagmite from the Gruta da Ceramica of Central Portugal. We also analyzed the host carbonate, the cave sediments and the soils capping the cave. We measured concentration- and grainsize-dependent magnetic proxies, including natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization, mass specific magnetic susceptibility, FORC and hysteresis curves.  Results show that magnetic and hematite are the main magnetic carriers in all samples. A gradual enrichment of hematite relative to magnetite is observed following the transportation path from the soils to the cave sediments up to the stalagmite. The higher contribution of hematite relative to magnetite in the speleothem may reflect precipitation of authentic hematite during speleothem growth or the selective transport of finer particles from the soil to the cave.  

 

Acknowledgments: This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (refs. PTDC/CTA-GEO/0125/2021), (PIDDAC) – UIDB/50019/2020, UIDP/50019/2020 and LA/P/0068/2020