The ocean's carbonate counter-pump - calcium carbonate (CaCO3) produced by planktonic calcifiers in the surface ocean and redissolved at depth - is a first-order term in the marine carbon cycle, yet where the dissolving CaCO3 actually enters solution remains disputed. Tracer-based studies of the early 2000s inferred substantial dissolution hundreds of meters above the calcite saturation horizon, in water where calcite is thermodynamically supersaturated. Friis, Najjar, Follows and Dutkiewicz (2006) countered with a global model in which dissolution was forbidden above the saturation horizon yet the observed excess-alkalinity (TA*) patterns were reproduced by transport alone, suggesting that the tracer-inferred shallow dissolution rates could be artifacts of circulation. Two decades of subsequent work - in-situ dissolution-rate measurements using carbon-13-labeled calcite, dissolved and solid-phase tracers, improved preformed-property estimates, and data-constrained inverse models with realistic transport - have established that above-horizon dissolution is real: confining dissolution to undersaturated water cannot reproduce observed upper-ocean alkalinity. What remains open is the quantitative and mechanistic core. State-of-the-art data-constrained models still disagree by roughly a factor of three on the fraction of water-column dissolution occurring in supersaturated seawater (about 16.5 +/- 5.5% above the aragonite saturation horizon in a 2025 estimate, versus 54% (range 13-66%) in waters supersaturated with respect to both calcite and aragonite in a 2024 estimate); a trade-off between CaCO3 export magnitude and shallow-dissolution efficiency makes the partition weakly identifiable from alkalinity data alone; and no candidate mechanism - respiration-driven acidification inside sinking aggregates, zooplankton grazing, dissolution of the more soluble aragonite and high-Mg calcite phases, or pressure-dependent dissolution kinetics - has been quantitatively established as sufficient to predict the observed patterns. The problem is to determine the magnitude, spatial distribution, and mechanisms of shallow-depth CaCO3 dissolution in the present ocean, with transport and in-situ terms quantitatively separated and reconciled across all observational lines.

Background

Marine calcifiers - coccolithophores (single-celled algae covered in plates of calcite), planktonic foraminifera (calcite-shelled protists) and pteropods (swimming snails with shells of aragonite, a CaCO3 polymorph about 1.5 times more soluble than calcite) - precipitate particulate CaCO3 in the sunlit ocean. Sinking CaCO3 carries alkalinity downward and releases it on dissolution, opposing the drawdown of atmospheric CO2 by the organic-carbon pump; the strength and depth structure of this carbonate counter-pump help regulate the partitioning of CO2 between atmosphere and ocean.

Total alkalinity (TA) measures the excess of proton acceptors over proton donors in seawater and rises by two equivalents for every mole of CaCO3 dissolved. Dissolution is thermodynamically favored only in undersaturated water: the saturation state is the ratio of the seawater ion product to the solubility product, Ω=[Ca2+][CO32−]/Ksp\Omega=[\mathrm{Ca}^{2+}][\mathrm{CO_3^{2-}}]/K_{sp}, where KspK_{sp} depends on pressure, temperature and salinity; the depth where Ω=1\Omega=1 defines the calcite or aragonite saturation horizon (of order 4-5 km for calcite, with the aragonite horizon lying hundreds of meters to a few kilometers shallower).

In the early 2000s, basin-scale syntheses of the global CO2 survey carried out by the World Ocean Circulation Experiment (WOCE, an international program that surveyed ocean hydrography and tracers along global sections) and the Joint Global Ocean Flux Study (JGOFS, its companion program studying ocean carbon fluxes) (Feely et al. 2002 for the Pacific; Chung et al. 2003 for the Atlantic) reported substantial CaCO3 dissolution far above the calcite saturation horizon. Their diagnostic was excess alkalinity: TA minus the preformed alkalinity that a water parcel carried when it last left the surface (often written TA*), accumulated along surfaces of constant density (isopycnals) and divided by a ventilation age obtained from chlorofluorocarbon (CFC) tracers - inert anthropogenic gases whose known atmospheric history dates a water parcel since surface contact. Feely et al. inferred Pacific dissolution rates of 0.01-1.1 micromol kg-1 yr-1, with rates in North Pacific Intermediate Water (400-800 m) more than seven times the mid-depth and deep average, and a total Pacific dissolution of about 0.31 Pg C yr-1 north of 40 S, roughly 74% of Pacific CaCO3 export; Chung et al. found Atlantic dissolution rising near the aragonite saturation horizon, about 11.1 x 10^12 mol C yr-1 basin-wide. Extrapolating such rates suggested of order 1-2 Pg C yr-1 dissolving within the upper ~1000 m of the world ocean - above the calcite saturation horizon and in places above the aragonite horizon, where inorganic thermodynamics forbids calcite dissolution.

Friis et al. (2006) challenged this interpretation. In a coarse-resolution (2.8 degree) global ocean circulation-biogeochemistry model in which calcite dissolution was switched off everywhere above the saturation horizon, alkalinity transported from permitted deeper dissolution sites nevertheless reproduced the observed basin-scale patterns of TA and TA* and the observed increase of TA* with CFC-12 age along thermocline isopycnals; apparent shallow dissolution rates computed from the model output with the standard observational diagnostic matched the observational estimates (a Pacific mean of about 0.44 micromol kg-1 yr-1 over 200-1000 m). The authors concluded that TA*-based rates may overestimate in-situ shallow dissolution because the observational analyses had 'not adequately accounted for the influence of transport'. They simultaneously acknowledged countervailing weaknesses: the model's upper-ocean ventilation was somewhat too vigorous; its mixing across density surfaces (diapycnal diffusivity) was a globally uniform 0.5 x 10^-4 m2 s-1, larger than tracer-release measurements in most pycnoclines; aragonite production was omitted; the saturation-horizon depth carried an uncertainty of order ten percent because calcite solubility determinations differ by 4-24%; and the empirical preformed-alkalinity regressions and normalization choices underlying TA* and excess-calcium analyses are themselves biased.

The question left open - whether significant shallow-depth dissolution is real, and if so at what magnitude, spatial distribution, and by what mechanisms - has since been attacked with in-situ dissolution-rate incubations of calcite labeled with the carbon-13 isotope (13C, a rare stable carbon isotope whose incorporation into the dissolving CaCO3 tags it against the large seawater carbon background), dissolved and particulate calcium tracers, improved preformed-property estimates, and data-constrained inverse models of ocean transport, but not yet closed.

Problem Statement

Determine the magnitude, spatial distribution, and mechanisms of in-situ CaCO3 dissolution occurring above the calcite saturation horizon (the depth below which seawater is undersaturated with respect to calcite) in the present-day ocean - shallow dissolution in supersaturated seawater - quantitatively separating it from alkalinity redistributed by ocean circulation and mixing from deeper dissolution sites.

The binary question raised by Friis et al. (2006) - whether the tracer-inferred shallow dissolution is real or a transport artifact - has been answered in the affirmative by later literature: in-situ measurements document dissolution in calcite-supersaturated water, and data-constrained inverse modeling shows that confining dissolution to below the aragonite and calcite saturation horizons cannot reproduce observed upper-ocean alkalinity. What is asked here is the surviving quantitative and mechanistic core of that question:

  1. The partition of total water-column CaCO3 dissolution among saturation-state regimes - supersaturated with respect to both calcite and aragonite, supersaturated with respect to calcite but undersaturated with respect to aragonite, and undersaturated with respect to both - together with its spatial pattern across basins and productivity regimes. Current best data-constrained estimates conflict by roughly a factor of three (Kwon et al. 2024: about 54%, range 13-66%, of interior dissolution in waters supersaturated with respect to both minerals; Wu et al. 2025: about 16.5 +/- 5.5% of dissolution above the aragonite saturation horizon, excluding the upper 114 m), and the trade-off between the magnitude of CaCO3 export (the flux of particulate CaCO3 leaving the surface ocean) and shallow-dissolution efficiency leaves the partition weakly identifiable from alkalinity data alone (Liang et al. 2023).

  2. The mechanisms sufficient to predict the observed dissolution patterns, i.e. the quantified roles of respiration-driven acidification inside sinking aggregates, zooplankton grazing and gut chemistry, dissolution of the more soluble phases aragonite and high-Mg calcite (Mg-rich calcite whose solubility exceeds that of pure calcite), and pressure-dependent dissolution kinetics - currently candidate explanations but not quantitatively established.

An answer resolves how much CaCO3 (and hence alkalinity) is regenerated above the saturation horizons, where in the ocean this occurs, and by which processes, with the transport term and the in-situ source term quantitatively separated and the result reconciled across all observational lines.

The two numbered parts are jointly required: an answer that establishes only one of them does not resolve the task. For item 1, a resolution must give the saturation-state partition with a quantified uncertainty together with its spatial pattern across basins and productivity regimes. For item 2, a mechanism counts as sufficient to predict the observed dissolution patterns when it can account for the observed magnitude, spatial pattern and saturation-state partition; a resolution must determine which of the candidate mechanisms meet this test, with the role of each candidate quantified sufficiently to establish its sufficiency or insufficiency. To be reconciled across all observational lines, the result must be consistent with each of the independent observational constraints discussed above - the water-column tracer and excess-calcium budgets, the in-situ dissolution-rate measurements, the dissolution-kinetics experiments and the data-constrained inverse models - and not with one of them alone.

References

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