Dissolved organic carbon (DOC) in Pacific Deep Water between ~2,000 and 3,500 m in the North Pacific reaches radiocarbon activities (Δ14C) of −550‰ to −570‰ — among the lowest bulk-DOC values in the open ocean — accompanied by anomalously low δ13C near ~3,400 m. Druffel et al. (2019), who documented these values along a central Pacific GO-SHIP section, could not attribute them to any source or process: preferential removal of young DOC is excluded by the concentration data, heterotrophic recycling of bulk DOC cannot lower Δ14C, and an old, 13C-depleted hydrothermal or chemosynthetic source appeared unlikely along a ridge-free transect. Follow-up work through 2026 has sharpened rather than settled the candidates — DOC Δ14C correlates with the hydrothermal tracer excess ³He in the eastern Pacific, hydrothermally derived black carbon is transported on basin scale, and microbial-carbon-pump isotope signatures pervade refractory DOC — but the magnitude of any hydrothermal old-DOC source and the composition of that DOC remain open. The problem is to identify and quantify the source(s) and process(es) responsible for the anomalously 14C- and 13C-depleted DOC in the deep North Pacific, and to determine how the answer changes the inferred residence time of the ocean's refractory DOC reservoir.

Background

Dissolved organic carbon (DOC) is the organic carbon that passes through a sub-micrometer filter in seawater; at roughly 660 petagrams of carbon it is the largest pool of reduced carbon in the ocean and is comparable in size to atmospheric CO2. Its radiocarbon content is reported as Δ14C, the per-mil deviation of the 14C/12C ratio of the carbon from a modern standard after standard fractionation corrections, and its stable carbon isotope composition as δ13C, the per-mil deviation of 13C/12C from a reference standard. Because organic matter newly produced in the surface ocean carries near-modern radiocarbon, the discovery that DOC in the deep central North Pacific is on average about 6,000 14C years old (Williams & Druffel, 1987) established that a large refractory DOC (RDOC) fraction survives multiple cycles of ocean mixing; surface DOC is approximated as a roughly 1:1 mixture of this old deep component and newly produced DOC containing bomb-produced 14C from nuclear testing. Why such old carbon persists, and how uniform its radiocarbon signature really is, remain central puzzles of marine organic geochemistry.

In a basin-scale survey along a central Pacific GO-SHIP repeat-hydrography section (GO-SHIP: the international program that reoccupies oceanographic transects about once per decade), Druffel et al. (2019) measured DOC concentration, Δ14C, and δ13C from the Southern Ocean to the North Pacific. They found that 14C aging of DOC and of dissolved inorganic carbon in Antarctic Bottom Water were similar, implying that deep circulation is the primary control on DOC 14C content — but they also observed minimum DOC Δ14C values of −550‰ to −570‰ in Pacific Deep Water (PDW, the water mass occupying roughly 2,000–3,500 m in the North Pacific, fed by northward-spreading upwelled deep water and returning southward) between about 10°N and 53°N, with correspondingly low δ13C values at ~3,400 m. These values are distinctly more depleted than the surrounding deep waters. The authors ruled out preferential microbial utilization of young DOC, because DOC concentrations were higher, not lower, in the anomalous samples; heterotrophic recycling of bulk DOC could shift δ13C by only about 1‰ and leaves Δ14C unchanged; and an input of old, 13C-depleted carbon from off-axis hydrothermal systems hosting chemoautotrophic production — documented elsewhere to supply DOC with Δ14C as low as −835‰ and δ13C as low as −34.5‰ (values reported in Druffel et al., 2021) — was considered possible but unlikely along that particular transect, which crosses no active ridges north of the equator. Their conclusion was that more data are needed to attribute the anomalously low DOC Δ14C and δ13C values to any one source or process.

Subsequent work has made the candidate mechanisms concrete but has not decided between them. Along the eastern Pacific P18 section, DOC Δ14C correlates with excess ³He — the helium isotope excess relative to atmospheric solubility, a standard tracer of hydrothermal input — and the lowest δ13C values occur at stations near the East Pacific Rise (EPR), the globe's major ridge system (Druffel et al., 2021). Hydrothermally derived dissolved black carbon (DBC, the aromatic combustion-like fraction measurable by benzenepolycarboxylic acids) is transported on basin scale in the eastern Pacific, with deviations from the normal DBC–apparent-oxygen-utilization relationship correlated with excess ³He (Yamashita et al., 2023). At the same time, solid-phase-extractable DOC (SPE-DOC, the fraction retained on standard PPL adsorbents) is systematically about 1.3‰ depleted in δ13C relative to bulk DOC across three ocean basins, a signature attributed to the microbial carbon pump — the ensemble of microbial transformations believed to convert labile organic matter into refractory DOC (Lewis et al., 2021, 2024). Refractory DOC itself appears to have different Δ14C signatures at different depths and in different oceans despite similar chemical characteristics (White et al., 2023), and hydrothermal systems demonstrably alter deep central-Pacific DOC concentrations through particulate-iron scavenging along the same ~150°W corridor (Hansell et al., 2025). Attribution matters quantitatively: the larger any external old-DOC source to the deep ocean, the shorter the inferred residence time of the 640 PgC refractory DOC pool, a first-order number in the marine carbon cycle.

Problem Statement

Identify the source(s) and process(es) responsible for the anomalously low DOC Δ14C values (−550‰ to −570‰, with correspondingly low δ13C at ~3,400 m) observed between ~2,000 and 3,500 m in Pacific Deep Water in the North Pacific (about 10–53°N along the central Pacific GO-SHIP section), and quantify the magnitude of any old-DOC input entering the deep ocean. Quantitatively discriminate among the candidate mechanisms — in-situ microbial reworking/recycling of pre-aged bulk DOC (including the microbial carbon pump), lateral or long-range transport of hydrothermally derived DOC (chemoautotrophic or thermogenic, e.g., from the East Pacific Rise), and other ancient-DOC inputs such as sedimentary production — and state what the attribution implies for the residence time of the ocean's refractory DOC reservoir, together with the pool size assumed. The attribution must be consistent with the published central and eastern Pacific profiles (Druffel et al., 2019, 2021) and with the absence of active ridge systems along the central North Pacific transect itself.

References

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