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Calcium carbonate dissolution patterns in the ocean

Nature Geoscience volume 14, pages 423–428 (2021)Cite this article

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Abstract

Calcium carbonate (CaCO3) minerals secreted by marine organisms are abundant in the ocean. These particles settle and the majority dissolves in deeper waters or at the seafloor. Dissolution of carbonates buffers the ocean, but the vertical and regional distribution and magnitude of dissolution are unclear. Here we use seawater chemistry and age data to derive pelagic CaCO3 dissolution rates in major oceanic regions and provide the first data-based, regional profiles of CaCO3 settling fluxes. We find that global CaCO3 export at 300 m depth is 76 ± 12 Tmol yr−1, of which 36 ± 8 Tmol (47%) dissolves in the water column. Dissolution occurs in two distinct depth zones. In shallow waters, metabolic CO2 release and high-magnesium calcites dominate dissolution while increased CaCO3 solubility governs dissolution in deeper waters. Based on reconstructed sinking fluxes, our data indicate a higher CaCO3 transfer efficiency from the surface to the seafloor in high-productivity, upwelling areas than in oligotrophic systems. These results have implications for assessments of future ocean acidification as well as palaeorecord interpretations, as they demonstrate that surface ecosystems, not only interior ocean chemistry, are key to controlling the dissolution of settling CaCO3 particles.

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Fig. 1: Conceptual view of the oceanic CaCO3 cycle.
Fig. 2: Regional profiles of reconstructed CaCO3 dissolution rates and saturation horizons.
Fig. 3: Reconstructed regional CaCO3 settling fluxes and depth distributions of their sinks.
Fig. 4: Global CaCO3 settling flux profile and depth distribution of its sinks.

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Data availability

The TTD ages85 are made available as GLODAPv2 affiliated data on the NOAA Ocean Carbon Data System website at https://www.ncei.noaa.gov/access/ocean-carbon-data-system/oceans/ndp_108/ndp108.html. Seawater chemistry data are available from the GLODAPv2.2016 in refs. 58,59, sediment-trap data are available from ref. 83, 14C-derived ages are available from ref. 71, biome distributions are available from ref. 74 and sediment fluxes are available from refs. 18,39,40.

Code availability

The MATLAB script to reproduce the regional CaCO3 dissolution rate and reconstructed settling fluxes is available at https://github.com/osulpis/pelagic_dissolution.

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Acknowledgements

We thank G. Gebbie for providing the seawater 14C-age dataset. We thank all who contributed to the creation of GLODAPv2. We thank M. P. Humphreys, W. M. Berelson, S. Dong and A. V. Subhas for useful comments on an earlier version of the manuscript and the three journal reviewers for constructive feedback. O.S. and J.J.M. were supported by the Dutch Ministry of Education via the Netherlands Earth System Science Centre (NESSC). A.D. was supported by the Swiss National Science Foundation (#200020_172476) and by the UniBE international 2021 fellowship programme of the University of Bern.

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Authors and Affiliations

  1. Earth Sciences, Utrecht University, Utrecht, The Netherlands

    Olivier Sulpis & Jack J. Middelburg

  2. NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway

    Emil Jeansson & Siv K. Lauvset

  3. Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

    Ashley Dinauer

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Contributions

O.S. and J.J.M. designed the research. O.S. performed the data analysis with inputs from A.D. and wrote the manuscript with contributions from all authors. E.J. and S.K.L. contributed the TTD-age analysis and advised on its usage. All authors interpreted the results and edited the manuscript.

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Correspondence to Olivier Sulpis.

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Extended data

Extended Data Fig. 1 Depth-latitude distributions of seawater properties in the Atlantic Ocean.

a, Seawater Alk*, b, seawater age and c, potential density (σ0) are shown in depth-latitude diagrams separated according to three biogeochemically-distinct regions on which we base our analysis, corresponding to the (left) subtropical South Atlantic, (center) Equatorial Atlantic and (right) subtropical Noth Atlantic regions shown in Extended Data Fig. 2.

Extended Data Fig. 2 Oceanic regions.

Geographical boundaries of the 10 regions used for our study. Black dots correspond to the locations of GLODAPv2 seawater chemistry and age data. Green stars correspond to the locations of sediment-trap data. The numbers indicate the 10 regions: 1, subpolar North Pacific, 2, subtropical North Pacific, 3, Equatorial Pacific, 4, subtropical South Pacific, 5, subpolar North Atlantic, 6, subtropical North Atlantic, 7, Equatorial Atlantic, 8, subtropical South Atlantic, 9, Indian Ocean and 10, Southern Ocean. Polar regions are in blue, subtropical regions are in yellow and equatorial regions are in orange.

Extended Data Fig. 3 Alk* versus seawater age plots from the Atlantic Ocean.

Plots corresponding to randomly selected density bins from the Suptropical North Atlantic (a,d,g), Equatorial Atlantic (b,e,h) and Subtropical South Atlantic (c,f,i). The spatial distribution of the Alk* and seawater age data is shown in Extended Data Fig. 1. Red lines are linear fits from which CaCO3 dissolution rates are estimated, computed as half of the slope, expressed in μmol kg−1 a−1. For each regional density, the mean depth (± 1σ) and the σ0 range are reported.

Extended Data Fig. 4 CaCO3 dissolution rate profiles from the Atlantic Ocean.

Black points correspond to discrete CaCO3 dissolution rate estimates computed as shown in Extended Data Fig. 3, using a constant sigma increment (σ0 = 0.01). Yellow stars represent the 9 CaCO3 dissolution rate estimates originating from Extended Data Fig. 3. Red lines are cubic smoothing splines used to interpolate discrete dissolution rate estimates over depth and obtain regionally harmonized depth profiles.

Extended Data Fig. 5 CaCO3 burial and dissolution rates at the seafloor.

The sum of (a) the burial rate at the seafloor and (b) the dissolution rate at the sediment-water interface represents the CaCO3 flux reaching the seafloor. The three Atlantic regions on which the Methods section focuses are contoured in a black thick line.

Extended Data Fig. 6 Atlantic seafloor CaCO3 sinks and reconstructed settling fluxes.

In each panel, the orange dashed line represents the CaCO3 dissolution rate at the sediment-water interface depth profile (from Extended Data Fig. 5a), the grey dashed line represents the CaCO3 burial rate in sediments (from Extended Data Fig. 5b), the solid black line is the sum of seafloor dissolution and burial at each depth and the solid turquoise line is the reconstructed CaCO3 settling flux.

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Sulpis, O., Jeansson, E., Dinauer, A. et al. Calcium carbonate dissolution patterns in the ocean. Nat. Geosci. 14, 423–428 (2021). https://doi.org/10.1038/s41561-021-00743-y

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