The effect of speleothem surface slope on the remanent magnetic inclination

Abstract

Speleothems are of interest for high‐resolution reconstruction of the Earth's magnetic field. However, little is known about the influence of speleothem morphologies on their natural remanent magnetization (NRM) record. Here we report on a high‐resolution paleomagnetic study of a dome‐shaped speleothem of middle Holocene age from southern Portugal, with special attention to the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). To assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization, we compare magnetic directions and AMS and AARM fabrics from subhorizontal to gradually subvertical calcite growth layers collected in a transversal cross section of the speleothem. A linear correlation is observed between magnetic inclinations, calcite laminae slope, and AARM k1 inclination. The AMS fabric is mostly controlled by calcite crystals, with direction of the minimum axes (k3) perpendicular to laminae growth. Magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation (PSV) model. After extrapolating magnetic inclinations to the horizontal, the corrected data better fit the PSV model but are still lower than the predicted magnetic inclinations, suggesting that inclination shallowing affects the entire speleothem. We suggest that speleothem morphology exerts a critical role on the magnetic inclination recording, which is controlled by the Earth's magnetic field but also influenced by particle rolling along the sloping surfaces. These observations open new avenues for reconstructing high‐resolution paleomagnetic secular variation records from speleothems and provide new insights into their NRM acquisition mechanisms.

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

“…Lava flow and archeological artifacts record a thermal remanent magnetization of the last time they cooled below Curie temperature, which is generally more stable than sediment magnetization. In a previous study (Ponte et al, 2017), we have demonstrated that magnetic PONTE ET AL. More recently, speleothems have been explored as potential archives of the Earth's magnetic field.…”

“…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.…”

“…More details about dating are available in Veiga-Pires et al (2011), Ghaleb et al (2014), and Ponte et al (2017). Here we used the same specimens as described in Ponte et al (2017) and augmented the sample collection by collecting 54 additional samples along 9 growth layers, giving a total of 102 specimens distributed along 15 growth layers dated from 4,100 to 3,300 years BCE ( Figure 1). Preliminary paleomagnetic data of 48 specimens distributed along six growth layers are provided in Ponte et al (2017).…”

“…This can be resolved or at least minimized by considering samples located in the horizontal surface, where particle rolling is minimum (Bilardello, 2013), or by applying a correction as calculated in Ponte et al (2017). This can be resolved or at least minimized by considering samples located in the horizontal surface, where particle rolling is minimum (Bilardello, 2013), or by applying a correction as calculated in 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.…”