Marine dissolved organic carbon (DOC) is thousands of radiocarbon years old in the deep ocean, and its radiocarbon (Δ14\Delta^{14}C) content decreases while its concentration declines along the deep ocean conveyor from the North Atlantic to the Pacific. Druffel, Griffin, Coppola and Walker (2016) established the Atlantic-sector baseline for this pattern — a prebomb deep North Atlantic DOC age of ‘≥‘`\geq`4900 14^{14}C years, intermediate values in the South Atlantic, and a distinct southwest/southeast basin contrast — and posed three mechanistically unresolved questions that remain open: (1) whether the extremely old (18,000–23,000 14^{14}C year) black carbon measured within marine DOC entered the ocean already pre-aged (for example from hydrothermal or fossil-fuel sources) or acquired its great age by radiocarbon decay within the deep sea; (2) the relative roles of aging during transport and selective remineralization of younger DOC in the global DOC age increase and concentration decrease along the conveyor; and (3) the cause of the southwest/southeast South Atlantic deep DOC Δ14\Delta^{14}C contrast. A decade of follow-up work has documented candidate source branches (hydrothermal thermogenic dissolved black carbon, fossil soot, labile riverine input) and established water-mass transport as the primary control on deep DOC Δ14\Delta^{14}C in several basins, but no study has quantitatively partitioned pre-aged input versus in-situ aging for the ancient black-carbon pool, uniquely determined the global conveyor mechanism, or explained the Atlantic basin contrast. The problem is to determine these mechanisms with quantitative, data-constrained attribution.

Background

Dissolved organic carbon (DOC) is the organic carbon dissolved in seawater — roughly 630±30 petagrams of carbon, a pool comparable in size to atmospheric CO2_2 and the largest reservoir of exchangeable reduced carbon in the ocean. DOC is produced mainly by phytoplankton photosynthesis in the surface ocean, yet in the deep ocean its bulk radiocarbon content corresponds to ages of about 4,000–6,000 14^{14}C years, far older than the time deep water takes to circulate. Radiocarbon content is reported as Δ14\Delta^{14}C, the per-mil deviation of the 14^{14}C/12^{12}C ratio from a modern standard; more negative Δ14\Delta^{14}C means older apparent 14^{14}C age. Nuclear-weapon testing in the 1950s–1960s injected 'bomb' 14^{14}C into the surface ocean, so modern deep-water samples can contain a small postbomb (younger) DOC fraction superimposed on the prebomb pool.

The deep ocean circulates as a conveyor belt: North Atlantic Deep Water (NADW) forms in the Nordic and Labrador seas, flows south through the Atlantic, mixes into the Southern Ocean Circumpolar Deep Water, and spreads north into the Indian and Pacific oceans over roughly 1,500 14^{14}C years. Along this path, DOC concentration decreases and DOC Δ14\Delta^{14}C declines — a correlation documented basin-scale by Druffel et al. (2016) for the Atlantic (regression of deep DOC 14^{14}C age against concentration, r2r^2 = 0.97). Their Atlantic measurements showed that at least 10% of deep North Atlantic DOC is of postbomb origin, that the prebomb component is ‘≥‘`\geq`4900 14^{14}C years old (about 900 years older than previously estimated), that deep South Atlantic values are intermediate between the North Atlantic and the Southern Ocean, and that below 3,500 m the southeast Atlantic DOC is significantly 14^{14}C-younger (−464±4 permil) than the southwest basin (−482±3 permil).

Black carbon (BC) is the carbonaceous residue of incomplete combustion of biomass and fossil fuels; in seawater it is quantified as dissolved BC (DBC) using benzene polycarboxylic acids (BPCAs) as molecular markers — oxidation products of condensed aromatic rings whose own Δ14\Delta^{14}C can be measured individually. BC isolated from marine DOC carries 14^{14}C ages of 18,000–23,000 years, far older than the ocean's overturning timescale, making it the most extreme known component of the DOC pool. Standard DOC isolation methods for isotope analysis include ultrafiltration, which separates high-molecular-weight DOC (HMW, typically >1 kDa) from low-molecular-weight DOC (LMW), and solid-phase extraction (SPE), which retains the hydrophobic, aromatic, generally 14^{14}C-depleted fraction (SPE-DOC). The persistent background pool that survives multiple overturning circuits is called refractory DOC (RDOC).

Against this background, Druffel et al. (2016) left three linked mechanistic questions open. First, the origin of the ancient BC: 'Whether this ancient DOC was put into the ocean preaged (e.g., hydrothermal sources), or had aged by decay within the deep sea, is an open question.' Second, the conveyor trend: the observations are consistent with (i) DOC aging during transport within the deep circulation, (ii) selective remineralization of the 14^{14}C-younger DOC fraction during transport, or (iii) both — the authors believed both operate but could not partition them, nor uniquely determine what drives the accompanying concentration decrease. Third, the South Atlantic basin contrast: the southwest/southeast offsets could reflect dissolution of high-Δ14\Delta^{14}C particulate organic carbon (POC) from the surface, mixing-in of Southern Ocean waters, or both, and the authors 'suggest, but are not able to determine definitively' the first. These three questions share one scientific objective: a quantitative mechanistic account of the sources, aging, and removal terms that produce the observed deep-ocean DOC Δ14\Delta^{14}C distribution. Candidate mechanisms discussed in the literature include hydrothermal vent inputs of 14^{14}C-depleted DOC, chemoautotrophic production at ridge flanks, riverine and atmospheric pre-aged BC inputs, in-situ 14^{14}C decay during transport, selective removal of younger DOC, and dilution-limitation of microbial degradation (the hypothesis that refractory DOC persists partly because individual compounds are too dilute for microbes to exploit).

Problem Statement

Determine, with quantitative data-constrained attribution, the mechanisms that produce the deep-ocean dissolved organic carbon radiocarbon structure established for the Atlantic and global ocean by Druffel et al. (2016). Specifically: (1) resolve whether the extremely old (18,000–23,000 14^{14}C year) black-carbon component of marine DOC entered the ocean already pre-aged — for example from hydrothermal (thermogenic) or fossil-fuel sources — or acquired its great age by 14^{14}C decay within the deep sea, including the quantitative contribution of each identified source branch; (2) partition the observed basin-scale increase in DOC 14^{14}C age and decrease in DOC concentration along the deep ocean conveyor between radiocarbon decay during transport, selective remineralization of the younger DOC fraction, and any additional terms (dilution-limited degradation, particulate organic carbon dissolution, hydrothermal or chemoautotrophic inputs) sufficiently to reproduce the observed deep DOC Δ14\Delta^{14}C and concentration distributions; and (3) identify the mechanism responsible for the southwest/southeast South Atlantic deep DOC Δ14\Delta^{14}C contrast (−482±3 vs −464±4 permil below 3,500 m). Resolving the question requires identifying which candidate mechanisms are excluded, quantifying the retained terms with stated uncertainties, and reconciling them with the existing Atlantic, Pacific, Indian, and Southern Ocean DOC Δ14\Delta^{14}C data, including the bomb-14^{14}C transient in the North Atlantic and the heterogeneous age structure of dissolved black carbon.

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