Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record


Large volcanic eruptions are major geohazards, so identifying their frequency in the geologic record is critical for making predictions and hazard assessments. Following the discovery of a thick (18 cm) tephra layer in marine sediments from Integrated Ocean Drilling Program (IODP) Site U1396 between Montserrat and Guadeloupe in the Caribbean Sea, we document here how highprecision Pb isotopes, trace elements, and grain morphological analyses of the tephra can be used, together with volcanological models, to identify a large (Volcanic Explosivity Index ~6) Plinian eruption from BasseTerre, Guadeloupe, at ca. 2.36 Ma. This previously unrecognized eruption is believed to be the largest documented volcanic event in this region since this time. We hypothesize that this large eruption was associated with the final stage in the evolution of an individual volcanic center, which has implications for prediction of geohazards in this setting. INTRODUCTION Reconstructing the history of island arc volcanoes is critical for determining their petrological evolution and potential geohazards, but such studies are difficult because erosion and later eruptions destroy and obscure the subaerial record of volcanoes, with much of the erupted material rapidly transported to the oceans (Le Friant et al., 2015). While marine sediment cores can provide more complete histories of arc volcanism, this approach requires identification of the depositional mode of marine tephra layers and their precise origin from closely spaced and compositionally similar volcanoes. This problem is exemplified in the Lesser Antilles, Caribbean Sea, where volcanism has been active since ca. 40 Ma. To the north of Martinique, the arc is divided into two chains. The eastern, inactive chain is older, with carbonate platforms covering volcanic basement. Tectonic adjustments during the Miocene modified the orientation of the subducting slab, causing westward migration of the volcanic front and initiation of a new arc that includes all the active volcanoes (Pindell and Barrett, 1990). In order to reconstruct a long-term record of volcanism in the northern Lesser Antilles, Integrated Ocean Drilling Program (IODP) Expedition 340 occupied four sites in A.D. 2012 offshore the currently active island of Montserrat. Site U1396 is located on a topographic high (water depth of ~780 m), ~30 km southwest of Montserrat and ~75 km west of Guadeloupe (Fig. 1). Of the three holes cored at the site with the advanced piston corer, two produced continuous (Hole U1396A, 140.5 m; Hole U1396C, 145.9 m) records that extend back ~4.5 m.y. (Hatfield, 2015). The sediments comprise hemipelagic (largely carbonate) sediment and volcaniclastic mud, with more than 150 visible (>0.5 cm thick) tephra layers intercalated in hemipelagic background sediments (Le Friant et al., 2015). The present study uses geochemical, grain morphology, and volcanological techniques to identify the origin and magnitude of the eruption that generated the thickest tephra layer from Site U1396. SEDIMENT CORE OBSERVATIONS Tephra was likely delivered to Site U1396 from different sources, but prevailing wind directions and seafloor topography suggest, however, that these layers were predominantly from volcanoes on Montserrat and Guadeloupe. The erupted material on both islands consists mainly of andesites, with lesser amounts of basalts and basaltic andesites (Harford et al., 2002; Samper et al., 2007), but there is a clear north-south gradation in the Pb isotope ratio of volcanic rocks in the Lesser Antilles that has been present for ~5 m.y. (Lindsay et al., 2005; Labanieh et al., 2010), and Pb isotope analyses of Montserrat (Cassidy et al., 2012) and Guadeloupe lavas (White and Dupré, 1986) show that the two centers are distinguishable (Fig. 2A; see the GSA Data Repository1). The one Dominican Pb isotope datum that lies in the Guadeloupe field is from the <40 ka Morne Anglais center (Lindsay et al., 2005). Of 29 samples of visible tephras analyzed from Site U1396 and shown in Figure 1, 17 fall in the Montserrat Pb isotope field and 12 in the Guadeloupe field. Of the latter, five come from an ~18-cm-thick tephra layer (subcores 6H4-U1396A and 6H2-U1396C). The other samples with a Guadeloupe origin come from tephra layers <5 cm thick. The sedimentary interval of the tephra layer (Fig. 1) is bracketed by the base of the C2r.1n (B) Reunion polarity zone (2.148 Ma) and the base of the C2r.2r (B) Gauss/Matuyama polarity zone (2.581 Ma) (Hatfield, 2015) and has an 1 GSA Data Repository item 2016031, T2.36 geochemical data tables, figure of tephra grain morphological characteristics, and age assignment details, is available online at .htm, or on request from or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, February 2016; v. 44; no. 1; p. 1–4 | Data Repository item 2016031 | doi:10.1130/G37193.1 | Published online XX Month 2015 © 2015 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Antigua Atlantic Ocean Outer arc Inner arc Guadeloupe Nevis St. Kitts

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@inproceedings{Palmer2015DiscoveryOA, title={Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record}, author={Martin R. Palmer and Stuart J. Hatter and Thomas M. Gernon and Rex N. Taylor and Michael J. Cassidy and Peter E. Johnson and Anne Le Friant and Osamu Ishizuka}, year={2015} }