Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity

Carbon (C) and nitrogen (N) concentrations and isotopic compositions were determined for sediments from Ocean Drilling Program legs 170 and 205 offshore of Costa Rica, in an attempt to characterize C-N flux into the Central America (CA) convergent margin and identify signatures of diagenesis and changing productivity in this sediment section. Samples from sites 1039 and 1253 (outboard of the trench) contain 62 to 2382 ppm total nitrogen (TN) with delta N-15(Air) values of +2.4 to +8.5 parts per thousand, 0.04-2.65 wt% total organic carbon (TOC) with delta C-13(VPDB) values of -25.4 to -20.8 parts per thousand, and 1.1-87.3 wt% carbonate with delta C-13 values of +0.1 to +3.2 parts per thousand and delta O-18(VSMOW) values of +21.3 to +34.2 parts per thousand. Total organic C and TN concentrations strongly depend on lithology, with carbonate-rich samples containing smaller amounts of both. Total organic C and TN concentrations and isotopic compositions also vary systematically within single units, perhaps reflecting small degrees of diagenetic alteration but mostly significant increase in productivity since the early Pliocene. Sediment subduction feeds 1.3 x 10(10) g yr(-1) N (mean delta N-15 = +5.7 parts per thousand), 1.4 x 10(11) g yr(-1) TOC (mean delta C-13 = -22.0 parts per thousand) and 1.5 x 10(12) g yr(-1) oxidized C (mean delta C-13 = +1.9 parts per thousand) into the 1100 km CA convergent margin. Incorporating possible inputs in altered oceanic crust (AOC) and by tectonic erosion, the C-N inputs appear to be far larger than the arc outputs. A small part of this excess C and N is probably returning toward the surface by devolatilization, along structural heterogeneities in the forearc, and the remaining inventory is likely recycling into the deeper mantle.