Did Paleogene North Atlantic rift-related eruptions drive early Eocene climate cooling? B David W. Jolley a, * , Mike Widdowson b a Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK b Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK Received 17 November 2003; accepted 9 September 2004 Available online 5 November 2004 Abstract The delivery of volcanogenic sulphur into the upper atmosphere by explosive eruptions is known to cause significant temporary climate cooling. Therefore, phreatomagmatic and phreatoplinian eruptions occurring during the final rifting stages of active flood basalt provinces provide a potent mechanism for triggering climate change. During the early Eocene, the northeast Atlantic margin was subjected to repeated ashfall for 0.5 m.y. This was the result of extensive phreatomagmatic activity along 3000 km of the opening northeast Atlantic rift. These widespread, predominantly basaltic ashes are now preserved in marine sediments of the Balder Formation and its equivalents, and occur over an area extending from the Faroe Islands to Denmark and southern England. These ash-bearing sediments also contain pollen and spore floras derived from low diversity forests that grew in cooler, drier climates than were experienced either before or after these highly explosive eruptions. In addition, coeval plant macrofossil evidence from the Bighorn Basin, Wyoming, USA, also shows a comparable pattern of vegetation change. The coincidence of the ashes and cooler climate pollen and spore floras in northwest Europe identifies volcanism as the primary cause of climate cooling. Estimates show that whilst relatively few phreatomagmatic eruptive centres along the 3000 km opening rift system could readily generate 0.5–1 8C cooling, on an annual basis, only persistent or repeated volcanic phases would have been able to achieve the long-term cooling effect observed in the floral record. We propose that the cumulative effect of repeated Balder Formation eruptions initiated a biodiversity crisis in the northeast Atlantic margin forests. Only the decline of this persistent volcanic activity, and the subsequent climatic warming at the start of the Eocene Thermal Maximum allowed the growth of subtropical forests to develop across the region. D 2004 Elsevier B.V. All rights reserved. Keywords: Phreatomagmatic eruptions; Eocene; SO 2 ; Cool climate; Climatic forcing; North Atlantic; Palynology; Biodiversity crisis 1. Introduction During the early Paleogene, the arrival of the Iceland plume beneath the Greenland–northwest European lithosphere led to widespread eruption of 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.09.007 B Taxonomic Note: The author citations of the pollen and spore taxa discussed in the text are as those contained in Jolley, 1996 and Gruas-Cavagnetto, 1978. * Corresponding author. E-mail addresses: d.jolley@sheffield.ac.uk (D.W. Jolley)8 M.Widdowson@open.ac.uk (M. Widdowson). Lithos 79 (2005) 355 – 366 www.elsevier.com/locate/lithos