Subduction of seafloor sediment and altered oceanic crust carries large volumes of pore water and volatiles structurally bound in minerals into modern convergent margins. Most of the pore water is probably released and expelled back to the surface at shallow depth (<5 km) (Moore and Vrolijk, 1992). In active forearc sedimentary wedges, pore fluid overpressures are created by rapid sediment burial, while thrust faults and décollement provide pathways for fluid mobility (Bolton et al., 1999; Silver et al., 2000; Saffer et al., 2000; Vannucchi and Tobin, 2000). The escape of fluid from depth toward the surface can mobilize not only water-soluble elements but also thermogenic gases (e.g., methane and ammonia) feeding benthic ecosystems (Ruppel and Kinoshita, 2000; Hensen et al., 2004). At greater depths, metamorphic devolatilization can result in metasomatism of the forearc mantle wedge and contribute to arc magmatic volatiles and budgets of volatiles in the deeper mantle (e.g., Bebout, 1995; Mottl et al., 2004). Many questions remain concerning the sources and mobility of fluids in forearc sedimentary wedges, including the extent to which the transport of relatively deeply sourced fluids to shallower parts of prisms can be traced geochemically (see Kastner et al., 1991; Moore and Silver, 2002).
In the Middle America Trench (MAT), fluid dewatered from greater depth and expelled along the décollement and major fault zones (Bolton et al., 1999; Vannucchi and Tobin, 2000; Saito and Goldberg, 2001; Saffer, 2003; Bohrmann et al., 2002) produces pore water anomalies in chlorinity, salinity, cation concentrations, and isotopic ratios (Kimura, Silver, Blum, et al., 1997; Silver et al., 2000; Ruppel and Kinoshita, 2000; Kopf et al., 2000; Morris, Villinger, Klaus, et al., 2003; Saffer and Screaton, 2003). In this paper, we present data for concentrations and isotopic compositions of carbonate, total organic carbon (TOC) and total nitrogen (TN) in the variably deformed sediments from near the toe of the wedge (within 2 km of the trench) of the Central America forearc sampled at Ocean Drilling Program (ODP) Sites 1040, 1254, and 1255 offshore Costa Rica. We consider not only the C-N record of infiltration along structures by fluids mobilized from greater depths but also downhole variations in C-N reservoirs as functions of sediment/organic sources and diagenetic history. Uncertainty remains regarding the sources of the sediments incorporated into the wedge and whether the wedge represents either modern or older sediment offscraping (see discussion by Silver, 2000).