Origin of hematite in a stalagmite from the Cerâmica Cave, Central Portugal

Magnetic particles trapped into speleothems usually originate from the soils capping the cave and are transported into the cave by dripwaters. However, 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 Cerâmica Cave, Central Portugal, characterized by brown to red calcite laminations. We also analyzed the host carbonate, the cave sediments, and the soils capping the cave. We measured concentration- and grain size-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 the precipitation of authentic hematite during speleothem growth or the selective transport of finer particles from the soil to the cave. 

 

This project is funded by Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), and PTDC/CTA-GEO/0125/2021, and is part of the Ph.D. of Ana Raquel Bras (2024.03482.BD).