Constraining the Fore-Arc Flux Along the Central America Margin

Abstract:

The transport of carbon to the deep mantle via subduction zones is interrupted by outputs via the fore-arc, volcanic front, and back-arc regions. Whereas output fluxes for the front and back-arc locales are well constrained for Central America (CA) [1], the fore-arc flux via cold seeps and groundwaters is virtually unknown. We present new He and CO₂ data for the inner fore-arc of Costa Rica and western Panama to complement our study [2] of offshore CO₂fluxes on the outer-forearc.

On the Nicoya Peninsula, the Costa Rica Pacific coastline (including the Oso Peninsula) and the Talamanca Mountain Range, as well as coastal seeps in Panama, coupled CO₂-He studies allow recognition of mantle (³He/⁴He up to 6R_A) and crustal inputs to the volatile inventory. We associate the crustal component with CO₂ derived from limestone (L) and organic sediments (S) on the subducting slab, and see a decrease in the L/S ratio trench-ward with the lowest values akin to those of diatomaceous ooze in the uppermost sequence of the subducting sediment package. This observation is consistent with the removal of the uppermost organic-rich sediment from deep subduction by under-plating. As the input carbon fluxes of the individual sedimentary layers are well constrained [3], we can limit the potential steady-state flux of carbon loss at the subaerial fore-arc to ~ 6 × 10⁷ gCkm^-1yr^-1, equivalent to ~88% of the input flux of C associated with the ooze, or <4% of the total incoming sedimentary C.

This study confirms that the greatest loss of slab-derived carbon at the CA margin occurs at the volcanic front with recycling efficiencies between 12% (Costa Rica) and 29% (El Salvador) of the sedimentary input [1]. It also demonstrates the utility of the coupled He-CO₂approach for mass balance studies at subduction zones.

[1] De Leeuw et al., EPSL, 2007; [2] Furi et al., G-cubed, 2010; [3] Li and Bebout, JGR, 2005.