Mineralogical and magnetic evidence of flooding events recorded in a stalagmite from the Moinhos Velhos-Contenda Cave, Portugal

The Moinhos Velhos-Contenda Cave, located in the Estremenho Karst Massif, Central Portugal, is the second-largest and the most visited cave of Portugal. Here we provide new paleomagnetic, rock magnetic and mineralogical data from a stalagmite (GMA-III) collected in Moinhos Velhos Cave, at about ~60m depth. Besides, cave sediments, surface soils and another (clean/whitish) stalagmite (GMA-IV) was collected. This speleothem is mushroom-shaped with dark-reddish laminations, hypothesized to be related to flooding periods. However, how the flooding events are recorded in the calcite laminae of the stalagmite and how the respective conditions affected the quality of the paleomagnetic signal is poorly constrained. After stepwise alternating field demagnetization, the viscous remanent magnetization was cleaned below 10mT, followed by a natural remanent magnetization pointing to the origin. After 100mT, more than 90% of the magnetization was cleaned, suggesting low to medium coercive magnetic minerals as main magnetic carriers of the natural remanence. Sample-based mean directions show an average declination of ~353º and a mean inclination of 38.5º. Isothermal Remanent Magnetization (IRM) curves shows the presence of two components – the first one with a mean coercivity of ~30mT and a DP of ~0.26, typical of pedogenic/detrital magnetite, contributing to ~94% of the total remanence; and the second one with a mean coercivity of ~245mT and a DP of ~0.30, interpreted as being hematite, contributing to ~6% of the total remanence. Plot of k_ARM/IRM versus the mean demagnetization field (MDF) show three distinct clusters, corresponding to the GMA-III and GMA-IV stalagmites, while the third cluster corresponds to the soils and cave sediments. All clusters are close to the pedogenic-detrital zone of the diagram. Notably, the GMA-III stalagmite has significantly lower values of k_ARM/IRM than the GMA-IV stalagmite. Cisowski test on GMA-III samples shows an interaction degree close to 0.5, suggesting the presence of non-interacting single-domain magnetite. Under binocular microscope, the reddish horizons show the presence of opaque materials and dissolution features, suggesting the occurrence of hiatus of precipitation. Analysis of these horizons under scanning electron microscopy (SEM) coupled to energy dispersive spectra show the presence of large (~50-100 micrometers) crystals of Ti-bearing iron oxides, quartz, and carbon-rich particles. Sylvite is also observed in the form of small (~2 micrometer) crystals and filling fractures, suggesting evaporitic conditions. Polarized light observations indicate that Ti-iron oxides correspond to titano-hematite, probably with a detrital origin. Oil immersion objective microscopy observation of carbon-rich particles classifies them as fossil organic matter particles, with grain size between 60 to 300 micrometers. The association of hematite, quartz grains and organic matter particles deposited onto dissolution surfaces confirm a detrital origin, presumably during flooding periods. Although magnetite was not observed under SEM and optical microscope, it carries the natural remanent magnetization, suggesting that hematite deposited by flood events has no or few implications in the quality of the paleomagnetic directions.

 

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 Rafael Dinis (FCT-10216.2022.BD).