Stratigraphic, geochemical and mineralogical characterization of pyroclastic deposits on the Gronau West Nunatak of East Greenland indicates that both alkaline and basaltic tephras occurred during the eruption of flood basalts associated with the opening of the North Atlantic ocean in the early Tertiary. Within the exposed section of Gronau West Nunatak, c. 1 km thick representing c. 1.6 Ma, 17 of the horizons are phreatomagmatic basaltic tephras. Near the top of the section, an alkaline tuff was dated at 53.8±0.3 Ma by 40Ar/39Ar method. This tuff contains a distinctive mineralogy (sanidine, Mg-katophorite, aegirine) and geochemistry (melt inclusion compositions, and trace and rare earth element abundances) that indicate it was erupted from the Gardiner melanephelinite-carbonatite volcanic complex located 175 km to the SE, which was active from c. 55 to 52 Ma. The alkaline tuff can be correlated to age-equivalent, compositionally similar alkaline pyroclastic horizons reported in North Atlantic sediment cores and in outcrops in Northern Europe, making it an important regional time-stratigraphic marker. This study indicates that North Atlantic explosive volcanic events were prevalent, regionally widespread, and originated in East Greenland during continental rifting, suggesting the need for reappraisal of the impact of North Atlantic volcanism on climate change during the early Tertiary.