Refractory Dissolved Organic Matter has Similar Chemical Characteristics but Different Radiocarbon Signatures With Depth in the Marine Water Column

The >5,000‐year radiocarbon age (14C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result, 14C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar 14C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ14C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same 14C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ14C gradient for RDOC. The observed Δ14C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean. Plain Language Summary Ocean water contains a large reservoir of carbon that is stored as dissolved organic carbon (DOC). It is currently unknown how or on what timescales this reservoir interacts with the global carbon cycle. As such, the sensitivity of this reservoir to perturbations in the climate system remains unconstrained. The average radiocarbon age of this reservoir suggests that some components are unreactive (refractory) and spend an average time of >5,000 years in the ocean. In this study, we find that refractory DOC as isolated by our analytical approach shares chemical characteristics in the surface and deep ocean—a finding that is demonstrated with confidence for the first time. However, while this refractory DOC is depleted in radiocarbon, its old “age” is not the same throughout the water column and may be changing with time. Furthermore, these results suggest that the refractory DOC reservoir in the sur