Environmental impact of tephra fallout: exploring the effects of plinian eruptions in Iceland
The aim of this thesis is to investigate the immediate and longer-term environmental impact of plinian tephra deposition events in the Holocene. The importance of better understanding the potential impacts from such events was recognised following the recent eruption from Eyjafjallajökull in 2010. This study uses a palaeoenvironmental approach under a tephrochronological dating framework to address this, focusing in particular on the Hekla 4 (4153-4402 cal yrs BP) and Hekla 3 (2832-3032 cal yrs BP) plinian eruptions in Iceland. These huge eruptions produced 13.1 and 11.0 km$^3$ of tephra respectively, now preserved as identifiable isochrones across the island. Several peat and soil sites were selected from across Iceland to represent variations in the thickness and characteristics of these two layers, as well as different ecosystems and ecologies. EMPA analyses were obtained from the Hekla 4 and 3 tephras taken from 17 profiles. A further 220 analyses from 13 other layers including Hekla 5, C, B, A, and Katla N and E were also undertaken. The production of 9 radiocarbon dates from bulk peat samples allowed the dating of 6 tephra layers, including the Katla N and E deposits which bracket Hekla 3, and supported the development of soil and sediment accumulation models. Over 25,000 terrestrial pollen grain identifications were carried out along with 160 soil analyses, and 195 cm$^3$ of peat was processed for plant macrofossils. Results from basic ecological surveys of Icelandic woodland and relevant vegetation are presented for context. The data analysis is also supported by an in-depth review of available research on the environmental impact of tephra fall as well as post-depositional tephra processes. This is used to develop a conceptual model of impact, within which the palaeoenvironmental analyses are framed in. Large differences in tephra thickness and characteristics of the Hekla 4 and 3 deposits are observed at various distances from the Hekla volcano. Deposit thickness and particle size are major controls on the environmental impact, particularly for peat formation processes, although other unidentified factors also played a part. Major immediate impacts from the two eruptions appear to be mainly restricted to a proximal zone around the volcano. The results suggest that long-term climate change and, more recently, human activity are more significant drivers of Iceland’s environmental development overall during the Holocene than volcanic impact. The presence of the thick Hekla tephra layers within Icelandic soils may, however, have enhanced climatic deterioration in the late-Holocene. This has implications for disentangling volcanic, climatic, and human signals in Iceland as well as understanding volcanic impact in the palaeoenvironmental record here and elsewhere.