Testing the strength of the speleothem magnetism as a rainfall proxy using a high-resolution record from central-eastern Brazil

Common speleothem-based paleoclimate proxies such as d¹⁸O and trace element ratios are capable of high temporal resolution but may be influenced by multiple climatic forcings that occasionally result in ambiguity in their interpretation. The concentration of ferrimagnetic, Fe-oxide particles in speleothem laminae has been recognized as a potential tracer for local precipitation. However, the mechanism by which magnetic particles are enriched is poorly understood for most speleothems and the strength of the correlation between magnetism and rainfall has not been quantified using historical rainfall measurements. 

We use the quantum diamond microscope (QDM) to magnetically image both the central column and flank laminae of a speleothem with an imparted saturation isothermal remnant magnetization (SIRM). The QDM is able to image at sub-annual resolution the laminae of a recently deposited speleothem collected in the Onça cave system from Peruaçu National Park. The speleothem chosen spans the period 1913 - 2018 with U/Th age precision of +/- 4 years, allowing comparison with the locally maintained rainfall record. Following imaging of the speleothem the magnetic field intensity was integrated along laminae and binned annually.

We compared SIRM data with oxygen and carbon isotope from a speleothem collected in a well-ventilated cave room where variations in cave temperature and humidity drive significant kinetic isotope effects. Our results demonstrate a statistically robust negative correlation (R²>0.33, P<10^-9) between the SIRM intensity in the speleothem central column and the observed local annual rainfall record. The correlation is stronger than those found between d¹⁸O or d¹³C and rainfall over the same period and hold true whether using data from the flank or central column. The results point to SIRM as an alternative for paleoclimate reconstruction using speleothems deposited in non-isotope equilibrium conditions.

Based off these results, high-resolution magnetic imaging along with U/Th precision dating present an opportunity to reconstruct local precipitation rates at sub-decadal intervals, providing a paleoclimate proxy uniquely sensitive to rainfall in this cave system.