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

Alexander Joseph Hammerstrom, Rita Parai, Richard Carlson, Stephen Turner

Subduction zone fluxes control the long-term evolution of Earth’s interior, surface reservoirs, and climate. To
better constrain these fluxes, we present new δ⁸⁸Sr measurements of lavas and sediments from Nicaragua, a key
site of carbonate subduction. δ⁸⁸Sr data are interpreted alongside existing trace element data to assess the value
of Sr stable isotopes as a quantitative subduction tracer. Trace element systematics (1) constrain ambient mantle
enrichment and extent of melting (Nb vs. Yb), (2) show that the arc’s Sr budget is dominated by melts of subducting
altered ocean crust (AOC) (Yb/Sr vs. ⁸⁷Sr/⁸⁶Sr), and (3) verify that incompatible element compositions are
controlled by variable recycling of hemipelagic vs. carbonate sediments (Th/Sr vs. ⁸⁷Sr/⁸⁶Sr). Other trace element
ratios, such as Nd/Sr, require varying AOC melt compositions. A full forward trace element model confirms
the viability of this interpretation. The new δ⁸⁸Sr data build on and corroborate these findings. The range of δ⁸⁸Sr
in arc lavas cannot be explained by sediment proportions or ambient mantle compositions, but instead requires
δ⁸⁸Sr heterogeneity in the subducting oceanic crust. Notably, AOC δ⁸⁸Sr appears to co-vary with the proportion of
hemipelagic 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, cap
carbonate layers and control their transfer to the arc. Within this framework, we estimate 45% to 60% of subducted
carbonate-derived carbon is returned to the arc, consistent with volcanic gas–based estimates.