Tephra fall deposits from large-scale explosive eruptions contain information about the source eruptions and the prevailing atmospheric conditions. Studies of marine- and land-based tephra fall deposits have demonstrated several systematic relationships that appear to apply universally to these types of layers. One of the more important is that both thickness and grain size decrease exponentially to distances of >1000 km from the source (Pyle, 1989; 1995). This type of decay reflects the underlying mechanism of sedimentation of particles from the upper umbrella regions of explosive eruption plumes (Sparks et al., 1991; Bursik et al., 1992).
The exponential decay of grain size and layer thickness can be used to place constraints on the size of the source eruptions. Based on arguments presented in the previous section, the atmospheric circulation in the Caribbean area favors a volcanic source that is located to the west of Sites 998, 999, and 1000. For the Miocene peak in explosive volcanism this source is likely to be the extensive Tertiary ignimbrite deposits that extend through Guatemala, Honduras, and Nicaragua. This province was at least 700 km from the Leg 165 sites, based on a reconstruction of the site locations during the Miocene (Fig. 15). Thus, the plate reconstructions place good constraints on the minimum distance from source with which to evaluate the size of the largest feldspar crystals.
Figure 16 presents maximum grain size and thickness relationships for three well-documented marine tephra fall deposits. The layers were produced by the 75-ka Toba eruption in Indonesia (Ninkovich et al., 1978), the 33-ka Campanian ignimbrite eruption in Italy (Cornell et al., 1983), and the Minoan eruption of Santorini ~1620 B.C. (Watkins et al., 1978). The data for these layers provide important recent analogs for the Miocene tephra fall layers in the western Caribbean. For example, the median size of the largest feldspar crystals in the Miocene tephra fall layers of Sites 998, 999, and 1000 are 190, 190, and 260 µm, respectively. If the minimum distance from source was 700 km, then these sizes are in line with eruptions of Campanian scale (Fig. 16). The Campanian eruption produced ~80 km3 dense rock equivalent (DRE) of trachytic magma that was erupted as Plinian pumice fall and widespread pyroclastic flows (Barberi et al., 1978; Cornell et al., 1983). The eruption column is estimated to have been at least 44 km in height (Carey and Sigurdsson, 1989).
Because the western Caribbean sites lie at such great distances from potential sources (>700 km), the size of the feldspar crystals provide a robust signal of the size of the source eruptions. The data suggest very large magnitude source events (more than tens of cubic kilometers in DRE magma volume) that were likely associated with the generation of voluminous ignimbrites and caldera formations. Much of the widespread fallout was probably derived from large-scale co-ignimbrite plumes that were generated from the tops of pyroclastic flows (e.g., Woods and Wohletz, 1991; Sparks and Huang, 1980). The abundance of thick ignimbrites in the Central American Tertiary volcanic province supports this interpretation.