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) δ13C values are 1.3 ± 0.6‰ lower than mean total DOC δ13C between depth ranges 0–200 m and 2–4 km on three GO-SHIP Repeat Hydrography cruises. Lowest observed δ13C values correlate with low ∆14C 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 13C, highlighting the need for continued research on non-retained DOC to predict mechanisms that drive ocean carbon storage.