A substantial fraction of the calcium carbonate (CaCO3) exported from the sunlit surface ocean dissolves in the upper few hundred meters, in water that is thermodynamically supersaturated with respect to calcite and aragonite, where abiotic dissolution should not occur. Kwon, Dunne and Lee (2024) argued from a data-constrained global model that this shallow dissolution is controlled by biological export production (remineralization of sinking particulate organic carbon), enhancing the ocean's capacity to absorb CO2. The unresolved problem is to establish what mechanisms actually drive dissolution in supersaturated water, how large the shallow dissolution flux is (current global estimates conflict by roughly a factor of three), what the relative contributions of the candidate mechanisms are, and how the flux responds to temporal variability and climate change. Recent experimental work has quantified individual candidate mechanisms (microzooplankton grazing; microbially acidified microenvironments in sinking particles), but no study has closed the global budget, reconciled the divergent estimates, or validated the temporal sensitivity.

Background

Marine calcifying plankton — coccolithophores (calcifying algae bearing minute calcite plates, or coccoliths), planktic foraminifera, and shelled pteropods — produce calcium carbonate (CaCO3) that sinks through the water column as particulate inorganic carbon, a flux known as the carbonate pump. Seawater is said to be supersaturated with respect to a CaCO3 mineral when the ion product of calcium and carbonate exceeds the mineral's solubility product; saturation decreases with depth and pressure, and the depth where saturation reaches one is the saturation horizon (the water-column expression of the sedimentary lysocline, the depth below which carbonate preservation on the seafloor declines). By the thermodynamics of solubility, sinking CaCO3 should dissolve only below the saturation horizon, in deeper, more corrosive water. Total alkalinity (roughly, the excess of bases over acids in seawater, dominated by carbonate and bicarbonate ions) increases when CaCO3 dissolves, and the dissolved alkalinity raises the seawater CO2 buffer capacity — the ability of seawater to absorb CO2 with a given change in pH and partial pressure — so the depth at which CaCO3 dissolves matters directly for how much atmospheric CO2 the ocean takes up. The twilight zone (mesopelagic ocean, here the upper 300 m below the sunlit euphotic zone) is also where most sinking particulate organic carbon (POC, the biologically produced organic detritus exported from the surface layer) is remineralized by heterotrophic microbes and zooplankton.

Since Milliman, Troy, Balch, Adams, Li and Mackenzie (1999) reported that sediment-trap CaCO3 fluxes attenuate well above the chemical lysocline and asked whether dissolution there is biologically mediated, the oceanographic community has debated how much CaCO3 dissolves in supersaturated shallow water. Friis, Najjar, Follows and Dutkiewicz (2006) argued that such shallow dissolution had been overestimated. Sulpis, Jeansson, Dinauer, Lauvset and Middelburg (2021), using water-column alkalinity and tracer data, attributed about 44% of global CaCO3 dissolution to supersaturated waters, and Subhas and colleagues (2022) documented intense shallow carbonate cycling in the North Pacific, where measured dissolution rates and excess alkalinity could be reconciled by organic-matter respiration acidifying particle microenvironments. Kwon, Dunne and Lee (2024) then provided a global-scale, data-constrained argument that the efficiency with which exported CaCO3 dissolves within the upper 300 m is tied to the export and remineralization of POC — implicating heterotrophic respiration in sinking aggregates and zooplankton guts, which can create localized undersaturated microenvironments even though the surrounding water is supersaturated — and estimated consequences for the CO2 buffer capacity of low-latitude thermocline waters (a regional increase of up to about 5%) and for natural CO2 outgassing (about a 20% reduction across the 30°S-30°N band). Their Discussion states explicitly that the mechanistic drivers are still not clearly understood, that the shallow dissolution estimate is only loosely constrained by observations, and that it remains unvalidated whether the biological controls on the spatial patterns also govern temporal variability and future climate conditions. In parallel, a data-constrained ocean circulation inverse model by Wu, Huang, DeVries, Liang, Wang and Zhang (2025) attributed only about 16.5% of global dissolution to supersaturated water and argued that excess-alkalinity methods overestimate it by neglecting cross-isopycnal transport and three-dimensional mixing, so the magnitude of the phenomenon itself is contested. A 2025 review by Ziveri and colleagues emphasizes that current climate models, which represent pelagic CaCO3 with a single bulk pool, are unlikely to capture these dynamics correctly. The unresolved question is thus both mechanistic (which processes dissolve CaCO3 in supersaturated water, and with what relative contributions and rate scalings) and quantitative (how large the shallow dissolution flux is, and how it will respond to climate change).

Problem Statement

Establish the drivers and magnitude of CaCO3 dissolution in the supersaturated waters of the upper ocean — the dissolution that occurs above the calcite and aragonite saturation horizons, predominantly within the upper 300 m of the twilight zone — and determine its consequences for the ocean's CO2 buffer capacity. Concretely, an answer must resolve three linked questions that Kwon, Dunne and Lee (2024) left open and that subsequent literature has not settled: (1) Mechanism: which processes dissolve CaCO3 in bulk-supersaturated seawater — heterotrophic respiration creating acidic microenvironments within sinking aggregates, zooplankton (including microzooplankton) grazing and gut processing, particle size and mineral-type effects, POC lability, zooplankton-mediated fragmentation and egestion, and organic-matter inhibition or facilitation of dissolution — and what are the relative contributions and rate parameterizations of these processes, covering either all listed mechanisms or a subset demonstrated to dominate, established from experimental, observational, and model-based evidence rather than correlation alone? (2) Magnitude: what fraction of the CaCO3 exported from the euphotic zone dissolves within the upper ocean (the dissolution efficiency) and what share of global CaCO3 dissolution occurs in supersaturated water (the shallow dissolution share), each with quantified uncertainties, reconciling the currently conflicting global estimates of the shallow dissolution share (data-constrained circulation-model estimates near 16.5% versus excess-alkalinity and model estimates near 44-54%) by identifying the specific observational, methodological, or transport biases responsible? (3) Sensitivity: does the biological (POC-export and remineralization) control that governs the spatial patterns of shallow dissolution also govern its temporal variability and its response to future climate-driven changes in export production, ocean acidification, and circulation, and hence how will the associated CO2 buffer-capacity feedback evolve? The completion boundary is a mechanism-attributed, observationally consistent global budget of shallow CaCO3 dissolution — consistent with alkalinity and tracer distributions, sediment-trap fluxes, suspended particle concentrations, and sedimentary burial — together with demonstrated predictive capability for its temporal and climate response. Each of the three questions (1)-(3) is individually required.

References

  1. E. Y. Kwon, J. P. Dunne, K. Lee, 'Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity', Science Advances 10(13), eadl0779 (2024). DOI: 10.1126/sciadv.adl0779, https://doi.org/10.1126/sciadv.adl0779
  2. J. D. Milliman, P. J. Troy, W. M. Balch, A. K. Adams, Y.-H. Li, F. T. Mackenzie, 'Biologically mediated dissolution of calcium carbonate above the chemical lysocline?', Deep Sea Research Part I 46(10), 1653-1669 (1999). DOI: 10.1016/S0967-0637(99)00034-5, https://doi.org/10.1016/S0967-0637(99)00034-5
  3. K. Friis, R. G. Najjar, M. J. Follows, S. Dutkiewicz, 'Possible overestimation of shallow-depth calcium carbonate dissolution in the ocean', Global Biogeochemical Cycles 20(4) (2006). DOI: 10.1029/2006GB002727, https://doi.org/10.1029/2006GB002727
  4. O. Sulpis, E. Jeansson, A. Dinauer, S. K. Lauvset, J. J. Middelburg, 'Calcium carbonate dissolution patterns in the ocean', Nature Geoscience 14(6), 423-428 (2021). DOI: 10.1038/s41561-021-00743-y, https://doi.org/10.1038/s41561-021-00743-y
  5. A. V. Subhas, S. Dong, J. D. Naviaux, N. E. Rollins, P. Ziveri, W. Gray, J. W. B. Rae, X. Liu, R. H. Byrne, S. Chen, C. Moore, L. Martell-Bonet, Z. Steiner, G. Antler, H. Hu, A. Lunstrum, Y. Hou, N. Kemnitz, J. Stutsman, S. Pallacks, M. Dugenne, P. D. Quay, W. M. Berelson, J. F. Adkins, 'Shallow calcium carbonate cycling in the North Pacific Ocean', Global Biogeochemical Cycles 36(5), e2022GB007388 (2022). DOI: 10.1029/2022GB007388, https://doi.org/10.1029/2022GB007388
  6. C. L. Smith, E. L. Harvey, M. D. Johnson, A. V. Subhas, 'Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle', Science Advances 10(45), eadr5453 (2024). DOI: 10.1126/sciadv.adr5453, https://doi.org/10.1126/sciadv.adr5453
  7. Y. Wu, Y. Huang, T. DeVries, H. Liang, G. Wang, S. Zhang, 'The dominant sink of oceanic calcium carbonate occurs in undersaturated seawater', Proceedings of the National Academy of Sciences 122(43) (2025). DOI: 10.1073/pnas.2507771122, https://doi.org/10.1073/pnas.2507771122
  8. P. Ziveri, G. Langer, S. Chaabane, J. de Vries, W. R. Gray, N. Keul, I. A. Hatton, C. Manno, R. Norris, S. Pallacks, J. R. Young, R. Schiebel, S. Zarkogiannis, G. Anglada-Ortiz, S. Bianco, T. de Garidel-Thoron, M. Grelaud, A. Lucas, I. Probert, P. G. Mortyn, 'Calcifying plankton: From biomineralization to global change', Science 390(6771) (2025). DOI: 10.1126/science.adq8520, https://doi.org/10.1126/science.adq8520
  9. B. Borer, A. V. Subhas, M. G. Hayden, R. J. Woosley, A. R. Babbin, 'Microbially enhanced dissolution of calcite in sinking marine particles', Proceedings of the National Academy of Sciences 123(11) (2026). DOI: 10.1073/pnas.2510025123, https://doi.org/10.1073/pnas.2510025123
  10. J. Toullec, 'Evaluation of the particulate inorganic carbon export efficiency in the global ocean', Biogeosciences 23(12), 4361-4377 (2026). DOI: 10.5194/bg-23-4361-2026, https://doi.org/10.5194/bg-23-4361-2026