Stable Isotopic (δ13C) Evidence for Global Microbial Sequestration of Refractory Dissolved Organic Matter

Abstract Dissolved organic carbon (DOC) in the global oceans is an important long‐term carbon sink. Connections between molecular size, reactivity, and isotopic characteristics show that DOC exists on a continuum from biologically reactive to recalcitrant. The driving mechanisms behind the creation and persistence of recalcitrant DOC remain unknown. We show mean recalcitrant DOC (isolated via solid‐phase extraction; SPE‐DOC) δ 13 C values are 1.3 ± 0.6‰ lower than mean total DOC δ 13 C between depth ranges 0–200 m and 2–4 km on three GO‐SHIP Repeat Hydrography cruises. Lowest observed δ 13 C values correlate with low ∆ 14 C and proximity to deep ocean hydrothermal systems. These data support the hypothesis that reworking of DOC through the microbial carbon pump is a key driver of the ocean's long‐term carbon sink. Mass‐balance modeling shows deep‐ocean DOC not captured by SPE is enriched in 13 C , highlighting the need for continued research on non‐retained DOC to predict mechanisms that drive ocean carbon storage.