Marine dissolved organic carbon (DOC, ~660 Pg C) carries a systematic relationship between molecular size and radiocarbon age: serially ultrafiltered size fractions from the North Central Pacific show progressively higher radiocarbon contents (Δ14\Delta^{14}C) with increasing molecular size at all sampled depths. Walker et al. (2016) showed that a two-component size–age model reproduces ultrafiltered DOC concentrations and surface Δ14\Delta^{14}C, but fails for deep Δ14\Delta^{14}C at low concentration factors, and posed four linked unresolved questions: (1) which molecular and biochemical features make particular DOC size classes radiocarbon-depleted, i.e. what is the structural basis of the size–age continuum; (2) what relative roles hydrothermal inputs, chemosynthesis, and particle solubilization play as deep-ocean DOC Δ14\Delta^{14}C sources, and whether a two-component size–age description suffices at depth; (3) whether preformed surface DOC size distributions control subsequent DOC reactivity at depth; and (4) how molecular size, composition, and biological reactivity jointly govern the formation of recalcitrant DOC (RDOC) and the operation of the microbial carbon pump. A decade of follow-up work has delivered new tools (single-molecule imaging, coupled ultrafiltration–solid-phase extraction, nanofiltration) and new candidate-source observations (hydrothermal Δ14\Delta^{14}C–δ13\delta^{13}C anomalies, basin-scale deep Δ14\Delta^{14}C variability, bathypelagic injection of young particle-derived material), but none of the four questions is resolved: the radiocarbon-depleted structures remain unidentified, deep-source contributions remain unquantified, the preformed-distribution hypothesis remains untested directly, and the structural-recalcitrance-versus-dilution debate over RDOC persistence continues. All four linked questions therefore remain open.

Background

Marine dissolved organic carbon (DOC) is the carbon contained in the organic molecules dissolved in seawater that pass a ~0.2–0.7 µm filter; with roughly 660 petagrams of carbon it rivals atmospheric CO2_2 as an active carbon reservoir. Its turnover is traced with radiocarbon: the quantity Δ14\Delta^{14}C expresses the 14^{14}C/12^{12}C ratio of a sample relative to a nineteenth-century wood standard, corrected for fractionation and decay, so highly negative Δ14\Delta^{14}C indicates carbon that has been isolated from the atmosphere for thousands of years. Deep-ocean DOC has a radiocarbon age of roughly 4,000–6,000 years, and this has classically been interpreted through a two-pool model in which recently produced, semi-labile surface DOC is added to an old, chemically and isotopically uniform background pool of recalcitrant DOC (RDOC, the fraction that resists microbial degradation on ocean-mixing timescales).

A long-standing empirical framework is the size–reactivity continuum (Benner and Amon, 2015): on average, high-molecular-weight (HMW, roughly >1 kDa) dissolved organic matter is enriched in identifiable biomolecules and is preferentially consumed, while low-molecular-weight (LMW) material is on average more degraded and more persistent. Walker et al. (2011) isolated DOC size fractions by serial large-volume ultrafiltration (tangential-flow filtration through membranes with nominal molecular-weight cutoffs, where successive concentration factors yield fractions of progressively higher average molecular size) and found size versus Δ14\Delta^{14}C relationships in marine DOC. Building on this, Walker et al. (2016, Geophysical Research Letters) measured Δ14\Delta^{14}C in a series of increasing-size ultrafiltered DOC (UDOC) fractions from 20 m, 670 m, and 915 m in the North Central Pacific, and found Δ14\Delta^{14}C increasing systematically with molecular size at every depth — the most detailed evidence then available for a size–age continuum. Two-component Keeling plots (linear regressions of Δ14\Delta^{14}C against inverse DOC concentration, whose intercept estimates the isotope signature of newly added carbon) were consistent with a two-component mixture within each size fraction. A forward ultrafiltration model, parameterized with independently measured size distributions, then predicted UDOC concentrations and surface UDOC Δ14\Delta^{14}C at all concentration factors, and located the most radiocarbon-depleted material in a narrow ~200–500 Da band (consistent with the ~330–550 Da "island of stability" for refractory molecules proposed by Lechtenfeld et al., 2014). At depth, however, modeled Δ14\Delta^{14}C at low concentration factors was consistently more positive than measured. Walker et al. (2016) additionally called for new DOC size-fractionation methods isolating the more recalcitrant (14^{14}C-depleted) fractions, coupled with molecular-level studies, to expose the structural changes underlying RDOC formation, and pointed to the coupling of molecular size, composition, and biological reactivity as central to testing microbial carbon pump processes.

A leading candidate mechanism for RDOC formation is the microbial carbon pump (MCP): the concept that marine bacteria transform labile DOC into progressively more refractory molecules that accumulate in the background pool; its rates and biological pathways were unknown, and rest on short-term laboratory experiments, while RDOC production acts on 50–1000 year timescales. Competing explanations for DOC persistence include intrinsic structural recalcitrance of specific molecules and the dilution hypothesis, in which many individually labile molecules escape degradation merely because each is too rare to sustain microbial uptake. How molecular size, chemical composition, and biological reactivity couple to each other and to radiocarbon age — across the surface ocean, the mesopelagic, and the deep sea — is the overarching unresolved question within which the present problem sits.

Problem Statement

Establish a mechanistic and quantitative account of how molecular size, chemical composition, biological reactivity, and radiocarbon age are coupled in marine dissolved organic carbon through the ocean water column, resolving the four linked issues posed by Walker et al. (2016):

  1. Identify the molecular and biochemical features that make particular DOC size classes radiocarbon-depleted — in particular the structural basis of the ~200–500 Da low-Δ14\Delta^{14}C band implied by the ultrafiltration model — thereby establishing (or refuting) the structural basis of the observed size–age continuum.

  2. Determine the relative contributions of candidate deep-ocean DOC Δ14\Delta^{14}C sources — hydrothermal DOC, chemosynthetic production, and solubilization of sinking particles — and establish whether a two-component size–age description suffices at depth (the modeled deep values at low concentration factors are consistently more positive than measured).

  3. Directly test the hypothesis that preformed (surface-established) DOC molecular size distributions control subsequent DOC reactivity at depth.

  4. Determine how size, composition, and biological reactivity jointly govern the formation of recalcitrant DOC and the operation of the microbial carbon pump, including the rates and biological pathways by which labile DOC is transformed into refractory molecules on 50–1000 year timescales.

All four issues are mandatory: each must be resolved on its own terms with a mechanistic and quantitative account.

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