Speleothem environmental magnetism: some examples from Brazil

Speleothems are classically used to reconstruct the Holocene paleoenvironment in the continents through geochemical proxies. In recent years, these cave formations have been also scrutinised for the significance of their environmental magnetism signal. In Brazil, we have been studying a set of caves encompassing different latitudes, climate and vegetation cover. In all these caves, the magnetic mineralogy composition is shown to be fairly homogeneous across the speleothem record, usually comprising magnetite as the main magnetic mineral, but also showing some amount goethite and hematite. Comparison of magnetic data from the speleothem, the soils above the cave, the sediments inside the cave and the bedrock (epikarst), show that the magnetic mineralogy deposited during speleothem precipitation essentially comes from the soil covering the cave, and is similar to that of sediments deposited inside the cave. In spite of its homogeneous composition, there are significant variations in the magnetic mineral content (magnetic enrichment) across the speleothem stratigraphy. Three main hypothesis for the enrichment were put forward in the literature: (i) episodic flooding events, (ii) higher precipitation, (iii) changes in soil and vegetation coverage. Here we describe some case studies from speleothems collected in caves from Brazil, where oxygen and carbon isotopes are compared to the environmental magnetism parameters of speleothems at different sampling resolutions in order to investigate the speleothem magnetic enrichment hypotheses, and discuss the potential of these techniques for paleoenvironmental studies.