Andy Baker

A few weeks ago my colleague Micha Campbell published a paper all about this – you can read it here. As the title states, combustion completeness and sample location determine wildfire ash leachate chemistry. So what is that chemistry?

Lets start with the story of the paper. After the 2019/2020 Australian wildfires, there was a lot of wildfire ash on the landscape. At the same time, we were working out how cave stalagmites can record a record of past fire history (see McDonough et al 2020 in the papers tab on this site and this review paper). It turns out that the fire history record was a chemical one that came from ash fraction that is soluble. So, we got interested in understanding what was in ash and how that varies, with a particular focus on cave regions.

Lots of volunteer sampling of ash later, and lots of lab analyses later …. at ANSTO our ash samples were dissolved in water and then the dissolved fraction was analysed for its inorganic contituents e.g. soluble metals and nutrients. And with collaborators at GNS Science, we also analysed some of the ash samples to see if the contained some biomarkers that are commonly used as fire proxies in other environmental archives such as ice cores.

We also also analysed some soils from the same sites, so that we could check if the soluble ash chemistry is different from the soluble soil chemistry

What did we discover?

First, the soluble ash chemistry is different from the soluble soil chemistry. This makes sense – the ash is the remains of the surface vegetation, which then bioaccumulates soluble metals and nutrients. This is important for our interest in using cave stalagmites to reconstruct past fire. The water reaching the stalagmite will pass through the soil, and we can now be confident that after a fire, that chemistry will change.

The ash chemistry varies with location and also fire severity. That is, we recorded the ash colour, and the more severe fires with white ash had a different chemistry to grey or black ash. Importantly, the metals and nutrients present in our soluble ash extracts did not match those expected based on published data for their volatilisation temperatures based on experimental data where vegetation is burnt. We now have a better understanding of which elements are the most relevant when developing past fire records from stalagmites.

And we also analysed some of the ashes for anhydrosugars and polycyclic aromatic hydrocarbons (PAHs), commonly used fire biomarkers. We found them, but not consistently and in low abundance. How will these be preserved in the stalagmite record? Some research teams are investigating this right now, however anhydrosugars are highly soluble and would be expected to be biodegraded both on the land surface and also while being transported from the ash to the cave. And PAHs are more persistent and sparingly soluble, so the question there is how long do they persist in the environment.

What was Micha’s conclusion for reconstructing past fires using stalagmites. She identified Na, K, Rb, Mn, P, As, Se, V, Cl, and S, as they varied between black and white ash, can be readily measured in stalagmite clacite, and had higher concnetrations in the soluble ash extracts than the soluble soil extracts. Onwards!