A combination of grain-size data from tephra layers in the western Caribbean (Sites 998, 999, and 1000) and consideration of the general atmospheric circulation of the area provides important insights into the location and nature of the source eruptions. Despite the dominance of easterly trade winds in the Caribbean area, a stronger westerly transport component is present at upper levels in the atmosphere, where tephra from explosive eruptions is typically injected. This component allows transport of material from Central American volcanism into the western Caribbean. The Lesser Antilles volcanic arc, located some 1500 km to the east, is too far away from Sites 998, 999, and 1000 to be a source for the abundant tephra layers given the circulation regime of the region and distribution of tephra layers in the eastern Caribbean and western equatorial Atlantic. Support for a western source comes from the abundance of dated Miocene pyroclastic deposits exposed throughout Central America.
Crystal sizes in the western Caribbean tephra layers suggest derivation from highly energetic explosive eruptions that generated voluminous pyroclastic flow and were associated with the formation of calderas. Based on comparisons with younger, well-documented marine tephra layers, many of the source eruptions for Leg 165 tephra layers were of the scale of the Campanian ignimbrite event that generated 80 km3 of magma with an eruption column height of >40 km.
At all three sites there is a trend toward the coarsest crystal size to be associated with the early to mid-Miocene peak in explosive volcanism as defined by the accumulation rate per million years. This trend is not likely to be caused by paleowind variations during the last 20 m.y., as no significant correlation exists between the tephra grain-size record and the independent record of paleowind provided by eolian transport of material to the deep sea. We propose that the coarsening of the layers reflects the occurrence of higher intensity eruptions during the Miocene peak in explosive volcanism. Higher intensity eruption can greatly enhance the dispersal of tephra by generating high-altitude lateral injections of material that are then advected by the prevailing wind.