During September and October 2002, Ocean Drilling Program (ODP) Leg 205 visited the Middle America Trench off the Nicoya Peninsula, Costa Rica. Sites 1253, 1254, and 1255 (Fig. F1) were drilled along a transect trending roughly northeast-southwest across the deformation front associated with subduction of the Cocos plate beneath the Caribbean plate. Site 1253 is located 0.2 km seaward, Site 1254 1.5 km arcward, and Site 1255 0.4 km arcward of the deformation front (Fig. F1). The ~380-m-thick incoming sedimentary section is completely underthrust beneath a margin wedge and is composed of ~200 m of pelagic carbonates overlain by ~180 m of hemipelagic claystone (Fig. F1B) (e.g., Morris, Villinger, Klaus, et al., 2003). Samples from Site 1253 used in this study are calcareous mudstones from near the base of the incoming sedimentary section, between 380.07 and 394.91 meters below seafloor (mbsf) (Table T1). The single sample from Site 1254 is from near the décollement base at 366.74 mbsf (Table T1). Samples from Site 1255 are from underthrust sediments at depths of 134.89–152.38 mbsf (Table T1). Samples from Sites 1254 and 1255 are composed of hemipelagic mudstones. Exact sample location data are given in Table T1.