Subduction and recycling of nitrogen along the central American margin

We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta(15)N values are positive (i.e., greater than air) and N-2/He ratios are high (up to 25,000). In contrast, Costa Rican N-2/He ratios are low ( maximum 1483) and delta(15)N values are negative ( minimum -3.0 per mil). The results identify shallow hemipelagic sediments, subducted into the Guatemalan mantle, as the transport medium for the heavy N. Mass balance arguments indicate that the subducted N is efficiently cycled to the atmosphere by arc volcanism. Therefore, the subduction zone acts as a barrier to input of sedimentary N to the deeper mantle.