Constraining Sources of Subducted and Recycled Carbon Along the Sunda Arc

Abstract:

From sediment subduction rates and C contents at ODP/DSDP sites 765 and 211, we estimate the rate of C subduction along ~2000 km of the East Sunda Arc to be ~0.4 Tg C yr^-1, representing a significant source of subducted volatiles [1]. However volatile recycling efficiency and the provenance of recycled volatiles in this region remain poorly understood. With new δ¹³C measurements of both carbonate and organic carbon from sites 211 and 765, we present the most detailed study yet of the spatial variability of subducted C and recycled CO₂ provenance along the strike of the arc. Furthermore we demonstrate the importance of oceanic crustal carbonate as a C source in a subduction zone that is otherwise carbonate starved.

Carbonate content throughout the sediment column decreases dramatically between site 765, approximately 250 km from the Australian continental margin, and site 211, approximately 300 km southwest of the trench and outboard of the Sunda Strait between Sumatra and Java. Continental and shelf carbonate input from the Australian margin dominates shallow deposits at site 765, but underlying pelagic sediments are thought to contribute the majority of inorganic C to the arc. The paucity of carbonate in sediments at site 211 suggests that along this segment essentially all carbonate subducted is derived from altered ocean crust, presenting an opportunity to study the effects of crustal carbonate input.

While previous C provenance studies relied on globally-averaged δ¹³C values for organic and inorganic C in subducted sediments, we present new estimates based on measured δ¹³C_VPDB of carbonate (average of ~2‰ in subducted sediments) and organic carbon (-22.5 to -23‰ average) along with previously published efflux data [2]. These estimates suggest that the arc-averaged ratio of carbonate to organic C subducted along the East Sunda Arc is nearly identical to the inorganic to organic C ratio represented in volcanic and hydrothermal CO₂ output, suggesting that differential devolatilization of carbonate and organic C is limited. Our calculated CO₂ recycling efficiency of 10 to 20% – which does not include fore-arc outgassing – agrees with geochemical models predicting up to 80% of subducted C may be carried into the deep mantle [3].

[1] Hilton et al., 2002; [2] Halldórsson et al., 2013; [3] Cook-Kollars et al., 2014