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The Hauterivian–Aptian Isachsen Formation at Glacier Fiord, Axel Heiberg Island, in the Sverdrup Basin of the Canadian Arctic Archipelago was deposited contemporaneous with initiation of the High Arctic Large Igneous Province (HALIP). New palynological biostratigraphy and paleoenvironmental reconstruction, in coordination with the emerging geochronology of HALIP igneous rocks, permits exploration of the effects of volcanism on Arctic vegetation during the Early Cretaceous. Four informal terrestrial palynofloral zones are defined and used to reconstruct vegetation change over the Isachsen Formation's ca. 17 million year history and explore the role of the HALIP in these changes. Climate warming during the Hauterivian promoted expansion of a hinterland community dominated by members of the Pinaceae. By the middle Barremian, this community was replaced by mixed heathland and mire, represented by up to 70% fern spores in the uppermost Paterson Island Member, that may be, in part, in response to environmental disturbance associated with volcanic flows of the HALIP. Above the fern spore spike, dinoflagellate cyst assemblages suggest an early Aptian age and a marine setting for mudstones of the Rondon Member in which Ocean Anoxic Event 1a is recorded. An interval of floral instability is recorded in the overlying Walker Island Member, characterized by fluctuations in Pinaceae and Cupressaceae pollen and fern spores, possibly as a result of post-OAE 1a temperature variabilty and landscape disturbance associated with lava flows of the HALIP that were repeatedly extruded onto the subsiding delta plain during deposition of the member. [ABSTRACT FROM AUTHOR]