Subduction of oceanic lithosphere, with its blanket of biogenous and terrigenous sediments, is an essential part of the global pathway by which the solid earth is recycled. This part of the cycle is also the least documented and, therefore, the least understood, in terms of the mass balance between subducting oceanic lithosphere, the portion of the lithosphere that descends into the mantle, and the portion that emerges elsewhere (Kimura, Silver, Blum, et al., 1997). Part of the subducted lithosphere may be accreted to or may underplate the overriding plate, released through pore water kinetics and chemistry and ultimately ejected or released as volcanic solids, liquids, and gases or emplaced as magma plutons. An understanding of the nature of this recycling—particularly the amounts and rates at which the lithosphere is underplated or accreted—is important to understanding the processes of earthquake generation at subduction zones (Kimura, Silver, Blum, et al., 1997). In addition, determining the mass balance of carbonate material that enters the subduction zone and leaves as carbon dioxide and other carbon compounds is important in aiding our understanding of the complex cycle of carbon dioxide in our atmosphere, biosphere, and lithosphere and the impacts to trends in global temperatures. To this end, Ocean Drilling Program (ODP) Leg 170 drilled and cored oceanic lithosphere in a transect across the Middle America Trench, off of the Nicoya Peninsula, Costa Rica (Fig. F1). A comprehensive investigation has been undertaken to arrive at an understanding of the fate of subducted materials. One study of importance to this endeavor is the derivation of accurately age-dated fossils from the Leg 170 cores.