Fig. 3.

Total alkali (Na₂O + K₂O) and silica compositions of tephra from GISP2 ice during the 43 BCE event compared with tephra from Okmok II and other potential source volcanoes. Filled circles show measurements (this study) of GISP2 and Okmok reference tephra. Shaded regions show tephra measurements from other potential first century BCE source volcanoes: Etna, Italy (21, 22); Chiltepe from Apoyeque, Nicaragua (23, 24); Masaya Triple Tuff (MTL), Nicaragua (23); A-2000, Askja, Iceland (25); White River Ash northern lobe (WRAn), Churchill, Canada (26); Furnas, Azores (27). Inset shows a tephra shard from the GISP2 sample. See SI Appendix, Fig. S5 for additional comparisons and analytical precision.