Laki 1783-84 AD tephra linked mercury enrichment in peat at Brackloon Wood, Mayo, Ireland.

Mercury is a significant volcanic volatile species from effusive and explosive activity¹. Understanding its emission to the atmosphere from volcanic activity, aids our understanding of the global mercury cycle and its environmental impacts. Sedimentary and ice core records can be archives of these mercury enrichments^2,3.

The Laki 1783-84 AD fissure eruption in Iceland was significant due to its voluminous outpouring of basaltic lava, copious sulphur emissions and widespread environmental effects locally and across the Northern Hemisphere^4,5. Extreme weather events were recorded in Europe and North America, owing to a veil of sulphur dioxide that remained at the tropopause for over a year⁵. Due to the phreatomagmatic and thus explosive nature of Laki, a significant eruption plume was produced⁴. As such, cryptotephra shards have been located at distal locations from Iceland including ice cores in Svalbard and Greenland^6,7 and in an Irish woodland peat at Brackloon Wood, Co. Mayo⁸. There is evidence to suggest significant heavy metal emission to the atmosphere during the Laki eruption, however these records are currently restricted to Greenland ice cores⁹. Previous heavy metal findings linked to Laki do not include mercury, despite its significance as a volcanic volatile, and a potentially environmentally damaging heavy metal. Therefore, to expand our knowledge of the Laki 1783-84 AD eruption plume, its associated emissions, and environmental consequences we returned to the woodland peat site in Brackloon Wood, Co. Mayo, Ireland.

Analysis of a 50 cm peat core using an AMA 254 mercury analyser was combined with a novel technique to find tephra using BSE (back scattered electron) imaging and geochemical discrimination using SEM-EDX (scanning electron microscopy-energy dispersive x-ray). The Laki tephra is successfully located using this method and coincides with a visible enrichment in mercury relative to background concentrations and organic matter. An age-depth model developed using the tephra layer and two radiocarbon dates indicate a strong likelihood that such enrichments are a product of volcanic emission. Such a finding can expand our understanding of heavy metal deposition during Laki 1783-84 AD away from the poles and to our knowledge, demonstrates the first direct exploration of mercury enrichment in distal peat for this eruption. As a secondary test of volcanic volatile enrichment, trace element analysis of the same bulk peat will be conducted to explore enrichments in other volcanic volatiles such as sulphur, cadmium, lead, copper and zinc.

 

1. Pyle, D. M. & Mather, T. A. Atmos. Environ. 37, 5115–5124 (2003).

2. Schuster, P. F. et al.,. Environ. Sci. Technol. 36, 2303–2310 (2002).

3. Roos-Barraclough, F. et al.,. Earth Planet. Sci. Lett. 202, 435–451 (2002).

4. Thordarson, T. & Self, S. Bull. Volcanol. 55, 233–263 (1993).

5. Thordarson, T. & Self, S., J. Geophys. Res 108, 4011 (2003).

6. Kekonen, T. et al., Polar Res. 24, 33–40 (2005).

7. Wei, L. et al., Geophys. Res. Lett. 35, L16501 (2008).

8. Reilly, E. & Mitchell, F. J., Holocene 25, 241–252 (2015).

9. Hong, S. et al., Earth Planet. Sci. Lett. 144, 605–610 (1996).