Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America

Plinio Jaqueto,Janine Carmo,Marília H Shimizu,Ivo Karmann,Francisco W Cruz,Nicolás M Stríkis,Joshua M Feinberg,Ricardo I F Trindade,Valdir F Novello

doi:10.3389/feart.2021.634482

Abstract

Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.

Highlights

Speleothems, together with ice-cores, are among the best continental records used in paleoclimate and paleoenvironment reconstruction thanks to their precise chronology, widespread geographic distribution, and continuous growth
This study presents a database of stalagmite magnetic properties, including parameters related to magnetic mineral composition and concentration and grain size
We compared these values to those of the overlying soil and host carbonate. These results expand our knowledge of speleothem magnetic properties in tropical-subtropical regimes far beyond what could be achieved through the examination of one or more speleothems from a single cave. This database provides robust evidence that the main magnetic component in Brazilian stalagmites is derived from soils overlying karst systems and that processes affecting pedogenic enhancement are reflected in these speleothems

Summary

Introduction

Speleothems, together with ice-cores, are among the best continental records used in paleoclimate and paleoenvironment reconstruction thanks to their precise chronology, widespread geographic distribution, and continuous growth. The dissolution phase operates as a cascading system, comprising the atmosphere, the soil ecosystem, the epikarst, and the cave system itself (Fairchild and Baker, 2012). It involves the transformation of carbon dioxide gas [CO2(g)] that dissolves into water to form the species [CO2(aq)] reacting with water to form carbonic acid (H2CO3). This “weak” acid will progressively dissolve the limestone (CaCO3), and this reaction will produce calcium (Ca2+) and bicarbonate (HCO3−).

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