The use of stalagmite geochemistry to detect past volcanic eruptions and their environmental impacts

Abstract

Our knowledge of past volcanic eruptions and their climatic impact is still far from complete. Establishing a clear link be-tween volcanism and anomalous tree ring growth is difficult (Pearson et al., 2005), and ice core chemical signals do not match the historical record of eruptions or do not al-low recognition of distal eruptions (Zielin-ski, 2000; Oppenheimer, 2003). Evidence for a causal connection between past volcanism and climate anomalies relies on the robust correlation between climate proxies and chemical fingerprinting of past eruptions within the same well-dated archive. Stalagmite geochemistry has the potential for providing precisely dated and spatially well-distributed records of past volcanic eruptions and of their envi-ronmental impacts (Frisia et al., 2005). The chemical signal that is the most likely proxy of past volcanic eruptions is Sulfur (S) concentration variability in the carbonate. The S emitted from volcanic eruptions to the atmosphere oxidizes to sulfate and reaches the soil as wet and dry deposition, or as ions adsorbed onto ash particles. The propagation of the at-mospheric sulfate signal from the surface to the cave depends on the transmissiv-ity of the karst aquifer (i.e., the hydraulic conductivity multiplied by the thickness of the aquifer). Caves cut in fissured lime-stone overlain by relatively thin soil with large interconnected pores ensure fast transmission, and in a few days or a few months after the eruption, the growing calcite will form from waters with a higher-than-background concentration of sulfate. Sulfur in the form of sulfate (S-sulfate) will be incorporated either in the calcite crys-tal lattice, or as micro-inclusions in the sta-lagmite. A volcanically induced increase in local acidity levels may also mobilize S adsorbed onto ash or associated with wet deposition and make it available to the ecosystem. This may result in a lagged response in the speleothem compared to the atmosphere, associated with the mineralization of sulfate in organic form followed by re-oxidation to sulfate. Vol-canic eruptions are, therefore, likely to be recorded in stalagmites by both a narrow S-sulfate peak coinciding with the year of a large eruption, or by a broader S-sulfate peak in the years immediately following the eruption (Frisia et al., 2005; Wynn et al., 2008).