What can caves tell us about past volcanic eruptions?

Explosive volcanic eruptions can have significant impacts on the Earth system. Capable of ejecting huge quantities of rock, dust, gas and ash into the atmosphere, the extent of these impacts can be global and long-lived. The aerodynamic structure of fine volcanic ash particles means they can be transported hundreds, even thousands of miles away from their source volcano. Assessing if, when, and how these large volcanic eruptions occurred, and how they affected different regions, is critically important to prepare for future events. However, there remains a problem…

Many volcanic eruptions are yet to be discovered, so existing records significantly underestimate how many volcanic eruptions have occurred in the past.

To track down these events, we need to look for evidence in unconventional places.

Novelist Prof. Joseph Campbell wrote that “the cave you fear to enter holds the treasure you seek”. Referring to their dark, desolate, and mysterious nature, this quote encapsulates how caves have long been a source of mystery and intrigue. Extending many hundreds of meters below the Earth’s surface, they form over many thousands of years from the incessant erosion of rock by water. It is tempting to consider these environments void of anything scientifically valuable; however, this could not be further from the truth. In fact, they contain a rich array of biological, geochemical, and hydrological information that can be used to understand our planet’s history.

Caves contain several unique features and formations. Stalagmites are one of the most common, and most well-known: elongated formations rising from the cave floor, formed by the accumulation of water from the surface dripping through the cave ceiling. As this drip-water flows through cracks and fractures in the cave walls and ceiling, it also carries minute amounts of dissolved minerals and substances (known as trace elements).

These trace elements are the scientists’ ‘treasure’.

The trace element composition of the drip water encodes a geochemical signal unique to the surface environment it originated from. In other words, peaks in trace elements recorded in a stalagmite suggest a rapid increase in the concentration of these elements dissolved within the dripwater – many of which will originate from the soils above the cave. Given that we can also measure how old a stalagmite is, we can get a good indication of when these geochemical changes occurred.

Figure 1: A conceptual diagram illustrating the dominant transport pathways of mass from an eruption to a stalagmite. Pathway 1 is via aeolian transport of ash directly into a cave environment. Pathway 2 is via leaching of tephra and subsequent percolative transport into the cave through the cave roof rock. Insets show specific components of pathway 2. 2a shows leaching of trace elements from fall deposits. 2b shows percolation of leached material in the form of colloids, particulates or solutes through fractures and the permeable karstic limestone. 2c depicts that drip water reaching stalactite tips and dripping onto stalagmite growth surfaces below. This is schematic and not drawn to scale.

Why would a volcanologist be interested in these processes?

Suppose that a volcanic eruption deposited ash directly above a cave site. The surfaces of volcanic ash surface particles are coated in a wide variety of trace elements not typically found in cave environments, such as iron, lead, magnesium, and aluminum. If the ash comes into contact with surface waters, these elemental coatings are removed (or ‘leached’) into the water, and directly alter the chemistry of waters flowing into the cave. By this logic, stalagmites could be used to determine when these events occurred, and how far the ash they produced was able to travel.

Several stalagmites do record distinct ‘spikes’ in their trace element composition coinciding with large volcanic eruptions (1–3). For example, a stalagmite from northern Turkey shows clear peaks in bromine, sulphur, and molybdenum coinciding with the Minoan eruption of Santorini volcano approximately 1600 years ago (2). On a more recent timescale, stalagmite ATM-7 from Actun Tunichil Muknal cave in Belize records clear trace element peaks coinciding with eruptions such as El Chichón (1982 CE), Colima (1991 CE), and Rincón de la Vieja (1998 CE) (3).

However, every cave system is different.

These studies do not tell us whether ALL stalagmites subject to volcanic ash deposition would record similarly clear signals. They all focus on samples grown in regions that experience pronounced seasonality in rainfall (e.g., a clear ‘wet’ and ‘dry’ season), and in close proximity to active volcanoes.

We sought to test the complete opposing case in this study, by choosing a site lacking a clearly seasonal rainfall pattern, and far from any active volcanoes. Enter: stalagmite NIED08-05 from Niedźwiedzia Cave, south-west Poland.

The results?

This stalagmite did show clear peaks in one (or several) of the sixteen trace elements we measured at several points across the 2600-year-record. Although some of these peaks did loosely coincide with known eruption events, we can’t clearly link these enrichments are directly caused by volcanic eruptions.

To visualize why this was the case, imagine you are listening to your favourite song while listening in a crowded coffee shop. People are talking, mugs are clinking, and the shop has ambient background music on. As the volume of the customers and coffee machines increases, they progressively increase the volume to ensure the music is heard over the noise. It becomes increasingly hard to discern the tune of the song you are listening to; to the point you can no longer ascertain which beat is your song, and which is their background music. Eventually, it is near impossible to say with any confidence which track the beat is coming from.

In NIED08-05, our song is a volcanic-produced signal, and the background music are other processes taking place above and within the cave such as storms, soil erosion, and weathering of rocks.

What do these results tell us?

They tell us that as a general rule, not all stalagmites are suitable as records of volcanic ash fall. Yet despite this slightly disheartening result, testing this hypothesis under the most challenging conditions for the preservation of a volcanic signal suggests that stalagmites grown...

- in close proximity to active volcanoes

- under a tropical, highly seasonal climates

- in a cave overlain by thin vegetation and soil cover

…may preserve volcanic signatures with greater success than those grown in temperate environments, such as Niedźwiedzia Cave.

The Bottom Line:

This study can directly enhance future research by helping to identify caves best suited for detection of past volcanic events, and ultimately contribute to improving global records of volcanic eruptions.

PAPER IN FOCUS Paine, A. R. et al. The trace-element composition of a Polish stalagmite: Implications for the use of speleothems as a record of explosive volcanism. Chemical Geology 570, (2021) REFERENCES CITED:

1. Frisia, S., Badertscher, S., Borsato, A., Susini, J., Göktürk, O.M., Cheng, H., Edwards, R.L., Kramers, R.L., Tüysüz, O., Fleitmann, D. The use of stalagmite geochemistry to detect past volcanic eruptions and their environmental impacts. PAGES News 16, 25–26 (2008).

2. Badertscher, S. et al. Speleothems as sensitive recorders of volcanic eruptions - the Bronze Age Minoan eruption recorded in a stalagmite from Turkey. Earth and Planetary Science Letters 392, 58–66 (2014).

3. Jamieson, R. A., Baldini, J. U. L., Frappier, A. B. & Müller, W. Volcanic ash fall events identified using principal component analysis of a high-resolution speleothem trace element dataset. Earth and Planetary Science Letters 426, 36–45 (2015).