Trace element and stable Sr isotope evidence for seamount-driven variations in subducted sediment and carbon recycling in Central America

Subduction zone fluxes control the long-term evolution of Earth’s interior, surface reservoirs, and climate. Tobetter constrain these fluxes, we present new δ⁸⁸Sr measurements of lavas and sediments from Nicaragua, a keysite of carbonate subduction. δ⁸⁸Sr data are interpreted alongside existing trace element data to assess the valueof Sr stable isotopes as a quantitative subduction tracer. Trace element systematics (1) constrain ambient mantleenrichment and extent of melting (Nb vs. Yb), (2) show that the arc’s Sr budget is dominated by melts of subductingaltered ocean crust (AOC) (Yb/Sr vs. ⁸⁷Sr/⁸⁶Sr), and (3) verify that incompatible element compositions arecontrolled by variable recycling of hemipelagic vs. carbonate sediments (Th/Sr vs. ⁸⁷Sr/⁸⁶Sr). Other trace elementratios, such as Nd/Sr, require varying AOC melt compositions. A full forward trace element model confirmsthe viability of this interpretation. The new δ⁸⁸Sr data build on and corroborate these findings. The range of δ⁸⁸Srin arc lavas cannot be explained by sediment proportions or ambient mantle compositions, but instead requiresδ⁸⁸Sr heterogeneity in the subducting oceanic crust. Notably, AOC δ⁸⁸Sr appears to co-vary with the proportion ofhemipelagic vs. carbonate sediment recycled to the arc. We suggest these variations reflect seamount subduction,where seamounts with heterogeneous δ⁸⁸Sr, emplaced during the transition in marine sediment deposition, capcarbonate layers and control their transfer to the arc. Within this framework, we estimate 45% to 60% of subductedcarbonate-derived carbon is returned to the arc, consistent with volcanic gas–based estimates.