The Central American Volcanic Arc is the counterpart MARGINS Focus Site to IBM and has been the subject of several recent volatile (i.e., N2, He-C) studies [Snyder et al., 2001; Fischer et al., 2002; Shaw et al., 2003; Snyder et al., 2003; Zimmer et al., 2004; Li and Bebout, 2005; Elkins et al., 2006; de Leeuw et al., 2007]. The Central American (CA) margin shows distinct along-arc segmentation, and a characteristic “chevron pattern” in crustal thickness, slab dip and many geochemical indices [Carr, 1984; Carr et al., 1990; Morris et al., 1990; Eiler et al., 2005; Barnes et al., 2007]. These geochemical variations have been interpreted as reflecting a stronger slab signature in the center of the arc under Nicaragua, where slab dip is steepest [Carr et al., 1990]. The weak slab signal in Costa Rican volcanics has been interpreted to be the result of erosion and accretion of the uppermost sediment column at the trench [Leeman et al., 1994].
6.3.1. Central America Volatile Systematics
 This tectonic and geochemical segmentation is reflected in volatile studies, which reveal significant differences between the northern/central arc in Guatemala and Nicaragua, and the southern arc in Costa Rica. Volcanic and hydrothermal gases in Nicaragua and Guatemala display a similar chemistry to most IBM gases, having high N2/He, low CO2/N2,exc. and positive δ15N [Fischer et al., 2002; Elkins et al., 2006]. These characteristics indicate a large contribution of sediment-derived nitrogen in Nicaraguan gases, averaging 71% [Elkins et al., 2006]. Calculated outputs range from similar to, to significantly higher than, estimated sedimentary inputs, suggesting that some AOC-derived nitrogen may be required to balance output fluxes, and that nitrogen is efficiently recycled to the atmosphere, with little or no sedimentary nitrogen being carried into the mantle [Elkins et al., 2006] (Figure 7). Despite the similarity in volatile chemistry, this conclusion regarding nitrogen recycling is entirely opposite to that proposed here for the IBM margin, where mass balance considerations suggest large-scale transport of nitrogen into the deep mantle. This observation suggests that major differences exist between the two arcs, either in the relative amount of the sedimentary inputs or in the way in which nitrogen is processed within the subduction zone (see section 6.3.2).
 Importantly, the AOC has not been considered as a significant carrier of nitrogen in the CA margin. Li and Bebout's  study of the sedimentary input to the margin acknowledged that the AOC could contain some nitrogen, but without AOC samples from the margin they estimated only 5 ppm N in the uppermost 2 km of crust and 0.5 ppm N in the remaining 3.5 km of crust. In comparison, N concentrations in the upper 470 m of AOC drilled at Site 801 near the Mariana arc range from 1.3 to 18.1 ppm [Li et al., 2007]. Thus, CA nitrogen inputs are dominated by the sedimentary units (∼80% of subducted nitrogen), while in IBM the sediments are estimated to carry only ∼33% of the total subducted nitrogen [Li et al., 2007].
 Given uncertainties in AOC N contents, we apply the IBM AOC nitrogen input flux to Nicaragua and add it to the sedimentary input to investigate whether assuming generic AOC nitrogen concentrations has any effect on the recycling calculations. These revised fluxes are shown in Figure 7 (in red). Interestingly, the effect on the AOC flux is relatively small, and the sedimentary flux still dominates in Nicaragua (∼67% of subducted nitrogen). This is due to the high nitrogen concentrations (up to 2382 ppm N) [Li and Bebout, 2005] in the sediments subducting at the CA margin.
 Volatile chemistry of Costa Rica is strikingly different from that of Nicaragua and Guatemala. Costa Rican gases have low N2/He, high CO2/N2,exc. and negative δ15N, reflecting a low sediment-derived nitrogen contribution, averaging only 37% [Zimmer et al., 2004]. The output/input ratio of nitrogen is less than unity, suggesting that more nitrogen is subducted than is released through the arc [Zimmer et al., 2004]. This could indicate that a large amount of nitrogen is transported into the deep mantle, possibly due to limited fluid availability under the arc. However, the lack of a strong slab signature is normally interpreted as reflecting a lack of sedimentary material reaching the subarc due to sediment offscraping or fore-arc devolatilization [Zimmer et al., 2004].
 Thus, in contrast to IBM, nitrogen systematics in CA appear to be dominantly controlled by along-strike variation in external forcing functions. However, the major difference in nitrogen systematics between the two margins is in the recycling efficiency of nitrogen. In CA, little or no sedimentary nitrogen is carried into the deep mantle, despite the high nitrogen concentration in the sediments, and the margin acts as a “subduction barrier” for nitrogen [Fischer et al., 2002]. In contrast, the most extreme output/input ratios in IBM suggest that as much as 89% of sedimentary nitrogen, and by inference the majority of AOC nitrogen, may be delivered past the zone of arc magma generation and into the mantle.
6.3.2. Nitrogen Release Beneath the Arc
 Despite the similarity in gas chemistry between IBM and Nicaragua samples, the nitrogen output fluxes are strikingly different. The annual sediment-derived flux to the atmosphere for Nicaragua is 14.8 × 105 mol yr−1 km−1 N2, while for IBM it is 0.26 × 105 mol yr−1 km−1 N2. Thus, despite the output flux of nitrogen being sediment dominated in both margins (71% and 75% in CA and IBM, respectively) the trench length normalized output flux from Nicaragua is almost 60 times greater than that of IBM. Comparing Nicaragua to the flux solely from the Izu-Bonin segment of the arc, which has a stronger sediment signature (0.47 × 105 mol yr−1 km−1 N2), results in a factor of ∼30 difference between Nicaragua and Izu-Bonin. These comparisons suggest a significant difference in the efficiency of nitrogen release and/or transport from sediments beneath these two arc segments.
 The observations above could be explained if the CA margin simply subducts 30–60 times more nitrogen than the IBM margin. However, published sediment subduction fluxes [Sadofsky and Bebout, 2004; Li and Bebout, 2005] show that the CA margin only subducts ∼5x more nitrogen (11.8 × 106 versus 2.5 × 106 g yr−1 km−1), or an order of magnitude less than the ∼60x discrepancy in output flux. Two further, potentially linked explanations exist for the discrepancy. First, nitrogen may be carried in different mineral phases, which breakdown differently, in the sediments at each margin. Second, the different thermal regimes of the two margins may facilitate the breakdown of mineral phases (whether they are the same or not) and release of nitrogen more readily at the CA margin than IBM.
 Nitrogen in subducted sediments occurs mainly as ammonium cations (NH4+), produced during diagenesis of organic matter in oceanic sediments [Busigny et al., 2003]. During subduction metamorphism, NH4+ substitutes for K+ and is incorporated into micas and feldspars [Bebout and Fogel, 1992; Bebout et al., 2007]. There is no evidence to suggest that nitrogen in sediments at either margin is not carried as ammonium cations [Sadofsky and Bebout, 2004; Li and Bebout, 2005]. Therefore, it appears as though an explanation involving different mineral phase hosts for nitrogen is an unlikely possibility for the different fluxes. This leaves the differing thermal regimes, as illustrated by two-dimensional, finite element thermal models of the two margins, as the most plausible explanation for the different behavior of nitrogen. The rapid subduction of old, cool oceanic lithosphere at the IBM margin results in a relatively “cool” subduction environment and lower slab-mantle interface model temperatures, ∼540°C for Izu-Bonin at depths of ∼100 km [Peacock, 2003]. In contrast, the CA margin subducts young, warm lithosphere, which results in a warmer subduction environment and higher slab-mantle interface model temperatures, ∼620–800°C for Central America at equivalent depths [Peacock et al., 2005].
 Studies of metasedimentary palaeoaccretionary suites, such as the Catalina Schist, suggest that in “cool” subduction zones, fluid mobile elements may be far more efficiently retained in sediments, at least to subarc depths if not beyond, while in warmer subduction zones, these elements are more likely to be lost beneath the arc or fore arc [Bebout and Fogel, 1992; Bebout, 1996; Bebout et al., 1999, 2007] (Appendix B). Therefore, we suggest that the greater sediment-derived nitrogen flux at the CA margin relative to IBM is a result of the more efficient release of nitrogen from sediments, as a consequence of the warmer thermal regime in the Central America subduction zone. In light of recent experimental studies [Spandler et al., 2007], we acknowledge, however, that the exact mechanism(s) by which such fluid mobile elements are stripped from the slab is still uncertain.
 A recent N and He-C study of the Sangihe arc, Indonesia [Jaffe et al., 2004; Clor et al., 2005], the only other such study of nitrogen systematics, demonstrates clear along-arc variations in slab volatile contributions to arc volcanoes. Although tectonic complexity prevents the identification of specific causes for these differences, along-arc variation in thermal regime, specifically slab temperature, is suggested as one possible explanation [Jaffe et al., 2004; Clor et al., 2005].
 Thus, the most significant finding of our study is that the thermal regime is likely the dominant factor controlling the recycling efficiency of nitrogen (and volatiles in general) at a given arc; this has important implications for the global nitrogen budget. The thermal structure of IBM is more typical of the majority of western Pacific subduction zones than CA [Peacock, 2003], suggesting that these subduction zones could act as conduits for large-scale recycling of nitrogen into the mantle, rather than back to the atmosphere, with all of the associated implications for terrestrial nitrogen cycling and ocean island basalt sources discussed above. The data required to test and/or reinforce this conclusion, such as the flux and nitrogen concentration and isotopic composition of fluids discharged in the fore arc and back arc of each margin, are sorely lacking and should be considered a priority for future studies.