Quantifying ancient precipitation with annual resolution records of speleothem magnetism (Invited)

Speleothems can potentially record a nearly continuous record of environmental conditions in the form of chemical and isotopic variations. In particular, oxygen isotopic compositions and concentrations of labile elements have traditionally been interpreted as proxies for ancient precipitation intensity.

More recently, rock magnetic protocols have been applied to speleothem samples to quantify the ratio of magnetically soft pedogenic magnetite to other detrital particles. This ratio may reflect precipitation due to the higher rate of pedogenic magnetite formation during times of higher soil saturation. If confirmed as robust, this proxy may provide a unique record of local rainfall that avoids the complexities of the oxygen isotopic record, which is sensitive to far field effects. At the same time, newly developed micrometer-scale magnetic imaging using the quantum diamond microscope (QDM) permits the recovery of magnetic signals at annual resolution inaccessible to other techniques.

We present QDM and oxygen isotopic measurements on a stalagmite sample from the Pau d’Alho Cave, Mato Grosso, Brazil. By performing backfield isothermal remanent magnetization (IRM) acquisition experiments over a 10 cm region, we quantify variations in remanent coercivity over a 600 year interval ending in 1860 CE. Our preliminary measurements over a 1 x 0.5 mm sub-region reveal that the magnetic hardness of the sample varies coherently at~8 year intervals (Fig. 1), indicating that the characteristic magnetic minerals being deposited in the speleothem varied on this timescale. Ongoing experiments over the full 10 cm interval will identify periodicity at multiple timescales and permit comparison with the oxygen isotopic record and historical precipitation records to assess the robustness of the magnetic proxy. Future analysis of other well-dated samples from the Pau d’Alho Cave can potentially yield annually resolved precipitation records in Mato Grosso state over the past 1,300 years, permitting novel tests of climate models describing the distribution of precipitation in Brazil.