Ice Nuclei Activity in Stalagmites: A Case Study of Medieval Climate Optimum and Little Ice Age Periods

Primary biological aerosol particles (PBAP, also called bioaerosols) are airborne particles from integral or pieces of biological sources. The most abundant PBAP are bacteria, which have been shown in laboratory studies to have shown efficient ice nucleation activity (INA) in the atmosphere. Therefore, a significant amount of INA bacteria was found at ground level and in soil, indicating that those bacteria would percolate the soil layers to the underground and caves. Earth’s climate presented a high variability last millennia with two very distinct periods: the optimal medieval, from X to XII century (900–1100 DC), here after called Medieval Climate Optimum (MCO) and the Little Ice Age, from the XVI to XIX centuries (1560–1800), referred to as LIA. The MCO presented warmer climates, with less rainfall in Central Brazil and higher temperatures associated with more savanna types. In turn, LIA presented colder climates, with more rainfall in Central Brazil dominated by C3 plants and trees. Both periods are represented by a stalagmite from the Pau D’Alho cave, located at Central Brazil. The study goal aims to verify IN activity on the different layers of the Pau D’Alho stalagmite, focusing on MCO and LIA periods. This is a first study verifying INA in a stalagmite, for which it was necessary to develop a new methodology to approach it. The methodology was performed through smashing the stalagmite and taking the solution sample to a freezing bath, with and without diluting the preparation. The first results show, for temperatures warmer than − 10 °C, MCO presented 15, frozen tubes against only 6 from LIA, both without dilution. If they are bacterial, they likely were moved from land surfaces via percolation. Tests are colder temperatures might be effective in differentiating the activity in MCO stalagmites from that in LIA stalagmites.