Paleoclimate studies on speleothems commonly use oxygen isotopes as a record of precipitation variability and carbon isotopes to document soil, vegetation, and atmospheric processes. Magnetic minerals in speleothems record complementary paleoclimate information but need to be interpreted within the context of the particular geographic and geologic setting in which a karst environment occurs. This study surveys 23 caves in South America (7°N to 25°S latitude). The present-day climate is dominated by a monsoon regime, with variable precipitation between 50 to 800 mm/month covering different biomes, therefore making South America a good candidate to explore the properties of magnetic minerals at the tropical/subtropical climate. We share a database of magnetic properties from 23 stalagmites samples (90 specimens), 4 soil samples (34 specimens) and 2 limestone samples (15 specimens). Measured rock magnetic parameters include magnetic susceptibility, natural, anhysteretic, and isothermal remanent magnetization (NRM, ARM, IRM), as well as low-temperature magnetometry and first-order reversal curves. These data help constrain the types and granulometry of the magnetic mineralogy that commonly occur in South American speleothems, their host carbonates, and their overlying soils. We show that concentration-dependent parameters in soils overlying the caves are two to three orders of magnitude higher than those in stalagmite and limestones. Despite these differences, unmixed coercivities between soil (median value of 19 mT) and stalagmites (median value 20 mT) and substantially different from those of host limestones (median 39 mT). Our results suggest that much of the magnetite in South American speleothems is pedogenic in origin, and may allow magnetic measurements to capture changing soil and vegetation dynamics in the epikarst through time.

Pyrrhotites are a class of geologically important nonstoichiometric iron sulfides with the general composition of Fe1-xS (0 ≤ x ≤ 0.125) and are found in a variety of intergrown polytypes, conventionally separated into the antiferromagnetic hexagonal and the ferrimagnetic monoclinic (4C) varieties. Both structures undergo magnetic phase transitions, where antiferromagnetic polytypes display the λ-transition at ~490K and the 4C polytype shows the Besnus transition at ~30K. However, recent studies have shown the relationship between pyrrhotite polytypes and their magnetic behavior to be more complex and new non-monoclinic polytypes (e.g., 3C) have been described that are also capable of retaining a spontaneous magnetization at room temperature. These advances raise the level of detail needed for the characterization of pyrrhotite in rock magnetic and paleomagnetic studies. This study demonstrates the utility of combining X-ray diffraction data collected as a function of temperature with low- and high-temperature magnetic measurements to characterize natural samples. We analyze two natural samples that contain mixtures of 4C, 5C, and 6C polytypes and describe how their polytypes and magnetic properties vary as a function of temperature across the λ-transition and how and when pyrite and greigite form. We also report the effect on natural samples of an annealing protocol commonly used to elevate the concentration of the 4C polytype in synthetic samples and found that annealing instead transformed some antiferromagnetic pyrrhotite into a form whose diffraction pattern most closely resembles the 4C polytype and displays room temperature spontaneous magnetization, but lacks the characteristic Besnus transition.