Explosive volcanism plays a fundamental role in the exchange of mass and energy between the solid Earth and the oceans and atmosphere. Large-scale silicic eruptions that generate voluminous pyroclastic flows (ignimbrite deposits) represent the largest type of volcanic event known on Earth. Individual eruptions may discharge up to several thousand cubic kilometers of magma during a single event (Chesner et al., 1991). Such events are of great interest, more over, because of their potential for global environmental change (Crowley et al., 1993). Large ignimbrite eruptions occur relatively infrequently (one event every 20,000 yr erupts a volume >1000 km3), but significant uncertainties exist regarding variations in the rate of this type of volcanism on Earth or in a given volcanic province. Just as variations in mid-ocean ridge and plume activity have become the focus of intense study, there is a compelling need to examine long-term variations in volcanism associated with subduction zones, especially considering the potential that such events have for profound societal impact. Recognizing that the Earth's current state is not necessarily typical of all tectonic regimes in the past, it is important to study the long-term geochemical and tectonic evolution of the Earth system.
Unraveling the land-based record of volcanism in a major ignimbrite province is hampered by the paucity and poor precision of many radiometric age dates, lack of mappable stratigraphic levels, and poor preservation. Major explosive volcanic eruptions are, however, well preserved in the marine environment adjacent to subduction zones as volcanic ash, or tephra layers, interbedded with deep-sea sediments. The marine tephra record is condensed in thickness compared with the terrestrial one, but it is more likely to be complete, and easily dated by biostratigraphic methods and radiometric dating of phenocrysts found in the tephra layers.
Recent drilling during Leg 165 of the Ocean Drilling Program (ODP) in the Caribbean Sea (Fig. 1) yielded more than 2000 tephra layers, with individual layers up to 90 cm thick. This led to the discovery of three major episodes of Cenozoic explosive volcanism (Sigurdsson, Leckie, Acton, et al., 1997). These episodes, one during the early to mid-Miocene, another from the mid- to late Eocene and the earliest one in late Paleocene to early Eocene time, represent periods during which the rate of large-scale ignimbrite-forming eruptions was much greater than at present (Fig. 2). The excellent recovery of this record, in seven holes, allows for the study of the most complete sequence of long-term temporal changes in subduction zone volcanism through the Cenozoic Era. This unique tephra record can be used in order to quantify the nature and rates of volcanism during these three important episodes. In this paper we discuss the Leg 165 tephra record and relate it to the complex history of volcano-tectonic activity at the margins of the Caribbean plate. We also present new 40Ar/39Ar dates of biotite and feldspar crystals from a large number of Caribbean marine tephra layers, to provide further constraints of the timing of volcanism in the region.