Towards an understanding of magnetic mineralogy in speleothems from
South America
Abstract
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.