Magnetic fingerprint of southern Portuguese speleothems and implications for paleomagnetism and environmental magnetism

Abstract

Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.

Age of the Laschamp excursion determined by U-Th dating of a speleothem geomagnetic record from North America

“…Speleothems may grow continuously for thousands of years, and can be dated with very high accuracy and precision using 230 Th dating, a technique that can be reliably used on specimens younger than 700-600 ka (Edwards et al, 2003;Dorale et al, 2004;Cheng et al, 2013). Recent speleothem magnetism studies have shown that magnetic minerals encapsulated in stalagmites (e.g., Strauss et al, 2013;Font et al, 2014) can be used successfully in dating geomagnetic excursions (Osete et al, 2012), as well as for reconstructing hydrologic and climatic variations (Xie et al, 2013;Bourne et al, 2015). Here we present a speleothem geomagnetic record from Crevice Cave, Missouri, USA (Dorale et al, 1998) that captures the changes in geomagnetic field direction and intensity associated with the Laschamp excursion, dated directly on speleothem calcite using a combination of high-precision 230 Th dating and incremental chronometry from annual growth laminae.…”

Speleothems as Magnetic Archives: Paleosecular Variation and a Relative Paleointensity Record From a Portuguese Speleothem

“…Characteristic Remanent Magnetization (ChRM) was calculated based on principal component analysis (Kirschvink, 1980) and Fisher statistics (Fisher, 1953) using the software Remasoft 6.0 (AGICO) software. IRM acquisition was performed using an impulse magnetizer (model IM-10-30) by application of a magnetic field with 100 mT, which is strong enough to saturate most magnetite grains, the main magnetic carrier present in these samples, while hematite and goethite have a low contribution to the total saturation remanence (<14%, Font et al, 2014). For this reason, two different methods were used: normalization of NRM by a ferromagnetic concentration-dependent parameter, namely IRM and ARM in order to account for changes in grain size and concentration, and the pseudo-Thellier method (Tauxe et al, 1995) to also eliminate VRM contribution.…”

“…Osete et al (2012) reported the first relative paleointensity (RPI) record in a speleothem from Cobre Cave in northern Spain and recognized the previously reported low RPI values of the Blake geomagnetic excursion, with a notable relative maximum peak just after the first reversal. Here we report on additional directional (PSV) data in a radioisotopic well-dated speleothem (named SPAIV; dated 4,500-3,200 years BCE by the U-Th method) collected in Algarve (Portugal), previously studied by Font et al (2014) and Ponte et al (2017), and compared them with contemporaneous archaeomagnetic, volcanic, and speleothem data from Spain and Italy, as well as three different PSV models (CALS10k.1b, SHA.DIF.14k, and pfm9k.1a). In front of these recent advances in the field of speleothem magnetism, further examination of their reliability as time series recorders of paleosecular variations (PSVs) and paleointensity is needed.…”

“…The speleothem was orientated in situ using a magnetic compass and has been subsequently cut in several slices in the laboratory. Detailed rock magnetic, mineralogical, and geochemical data are provided in Font et al (2014). More details about dating are available in Veiga-Pires et al (2011), Ghaleb et al (2014), and Ponte et al (2017).…”

“…These karstic deposits have been described as reliable Earth magnetic field recorders (Font et al, 2014;Lascu et al, 2016;Latham et al, 1979Latham et al, , 1982Latham et al, , 1986Latham et al, , 1989Lean et al, 1995;Morinaga et al, 1986Morinaga et al, , 1989Martin, 1990;Openshaw et al, 1997;Osete et al, 2012;Ponte et al, 2017). Paleomagnetic data are commonly obtained from three main sources: marine or lake sediments, volcanic rocks, and archeological objects.…”