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One-third of Southern Ocean productivity is supported by dust deposition

Nature volume 629, pages 603–608 (2024)Cite this article

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Abstract

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate1,2,3. Yet, this process has never been quantified across the Southern Ocean and at annual timescales4,5. Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5–40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial–interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate6,7,8.

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Fig. 1: Southern Ocean surface nitrate and dust deposition.
Fig. 2: Seasonal variability of nitrate in the epipelagic zone.
Fig. 3: Annual surface nitrate drawdown (ΔNO3) and ANCP as a function of dust deposition.
Fig. 4: ANCP50m derived from present-day and LGM dust deposition.

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Data availability

Supporting data used in the analysis are available at https://doi.org/10.5281/zenodo.10374127 (ref. 64). ACCESS-AM2 2015–2019 dust fields are available at https://doi.org/10.5281/zenodo.8303317 (ref. 56). Figures were created in MATLAB and Adobe Illustrator. Source data are provided with this paper.

Code availability

Analysis scripts are available at https://doi.org/10.5281/zenodo.10374127 (ref. 64). The ECHAM6-HAMMOZ model code and all required input data are maintained and made available at https://redmine.hammoz.ethz.ch after signing a software license agreement that can be downloaded from https://redmine.hammoz.ethz.ch/attachments/291/License_ECHAM-HAMMOZ_June2012.pdf.

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Acknowledgements

We would like to acknowledge the Argo Program, which is part of the Global Ocean Observing System (https://www.seanoe.org/data/00311/42182/), the Southern Ocean Observing System (SOOS) and the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project funded by the National Science Foundation, Division of Polar Programs (NSF PLR-1425989 and OPP-1936222), supplemented by NASA. The BGC-Argo data were collected and made freely available by the International Argo Program and the national programmes that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). This research was undertaken with the assistance of resources and services from the National Computational Infrastructure (project jk72), which is supported by the Australian Government. This research was partially funded by the Australian Government through the Australian Research Council’s Discovery Projects funding scheme (project DP190103504). A.R.B. and S.L.F. are supported by the Australian Antarctic Program Partnership (AAPP) as part of the Antarctic Science Collaboration Initiative (ASCI000002). J.W. and P.G.S. are also supported by the Australian Research Council Centre of Excellence for Climate Extremes (CLEX, CE170100023). We thank S. Krätschmer for providing the LGM dust deposition simulations used in this study. We thank M. Mazloff and T. Rohr for their valuable insights and constructive feedback.

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Authors and Affiliations

  1. Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Hobart, Tasmania, Australia

    Jakob Weis, Zanna Chase, Peter G. Strutton, Andrew R. Bowie & Sonya L. Fiddes

  2. Australian Research Council Centre of Excellence for Climate Extremes (CLEX), University of Tasmania, Hobart, Tasmania, Australia

    Jakob Weis, Peter G. Strutton & Sonya L. Fiddes

  3. Australian Research Council Centre for Excellence in Antarctic Science (ACEAS), University of Tasmania, Hobart, Tasmania, Australia

    Zanna Chase & Peter G. Strutton

  4. Australian Antarctic Program Partnership (AAPP), University of Tasmania, Hobart, Tasmania, Australia

    Christina Schallenberg, Andrew R. Bowie & Sonya L. Fiddes

  5. Environment, CSIRO, Hobart, Tasmania, Australia

    Christina Schallenberg

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Contributions

J.W., Z.C., C.S., P.G.S. and A.R.B. conceived the study. J.W. conducted the analysis and wrote the manuscript with contributions from all co-authors. S.L.F. provided ACCESS dust deposition model outputs. All authors contributed to the interpretation of the results.

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Correspondence to Jakob Weis.

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Extended data figures and tables

Extended Data Fig. 1 Regional extents of the 50 dust regimes.

The 50 dust regimes (R1–R50) defined in this study from low dust (top left) to high dust deposition (bottom right) and 2012–2022 BGC-Argo nitrate observations included in each regime. Lower and upper annual mean dust flux limits (mg m−2 d−1) delineating each regime are indicated in the titles. Note that dust regime boundaries are partially overlapping. Dust limits increase exponentially from low-dust to high-dust regimes, due to the exponential decline of dust with distance from the source regions (see Fig. 1b), ensuring that regimes are similar in regional extent and number of float observations.

Source Data

Extended Data Fig. 2 Monthly nitrate climatologies of the 50 dust regimes.

Monthly 0–200 m nitrate climatologies calculated from float observations in each of the 50 dust regimes (Extended Data Fig. 1). Plotted on the x-axis is the difference in nitrate concentration relative to the winter surface nitrate maximum, illustrating the seasonal nitrate depletion in the epipelagic zone. The horizontal grey bar above each panel indicates the 50-m-averaged maximum seasonal nitrate difference between the winter maximum to the summer minimum (solid profiles, drawdown values are indicated in the title).

Source Data

Extended Data Fig. 3 Surface nitrate seasonality in each dust regime.

a, 50-m-mean surface nitrate depletion between the seasonal surface nitrate maximum (triangles) and minimum (circles) in each of the 50 dust regimes (Extended Data Fig. 1). b, Histogram of the winter nitrate maximum (blue bars) and summer minimum months (red bars), defining the start and end of the productive period, in each dust regime. In >80% of the regimes, the productive period begins in August or September (41 out of 50) and ends between January and March (44 out of 50).

Source Data

Extended Data Fig. 4 Fe:C ratios derived from net community production and dust-derived soluble iron fluxes.

200-m-integrated net community production (NCP200m = ANCP200m divided by the productive period length) regressed against dust deposition fluxes (lower x-axis) in each of the 50 dust regimes. Southern Ocean basin-averaged Fe:C uptake ratios, indicated in the key, were inferred from the inverse of the regression slope (dotted line) and bioavailable soluble iron (sFe) fluxes (upper x-axis, derived from dust using 3.5 weight-% dust-iron content16 and 5–15% fractional iron solubility38). Observations exceeding 7 mg dust m-2 d-1 (open markers) were excluded from the regression and the Fe:C calculation due to the assumed limitation of productivity by iron-scavenging and self-shading on NCP under high dust loads. See the methods for further information.

Source Data

Extended Data Fig. 5 Covariance analysis between ANCP and mixing, latitude and temperature.

Linear regressions of ANCP50m (black, left y-axis) and ANCP200m (blue, right y-axis) against, a, seasonal mean mixed layer depths, b, the seasonal shoaling of the mixed layer, c, latitude and, d, seasonal mean temperatures (50 and 200-m-averaged). R2 and p-values are indicated in the key. ANCP increases northwards, whereas insolation decreases northwards during the productive period (austral spring and summer). Furthermore, the insolation difference in the observed latitude range is minor, which precludes a direct influence of light on the observed increase in ANCP. Temperature limitation factors (Tlim, box next to panel e) were calculated to estimate the maximum possible temperature-induced increase in productivity across dust regimes (see methods), indicating that temperature differences can only account for a minor fraction of the observed ANCP increase.

Source Data

Extended Data Fig. 6 Latitudinally binned 200-m-integrated ANCP.

ANCP200m averaged over 5° latitudinal bins from this study (yellow markers, mapped on the right) compared against corresponding literature values26,31 (black markers). Each set of markers refers to the same 5° latitude bin indicated by the x-axis ticks. Oxygen-derived ANCP estimates from Arteaga, et al.31 were calculated based on respiration rates integrated from 100 to 500 m. Regions accounted for in the latitudinally binned and basin-integrated estimates reported in the main text, highlighted in colour, exclude the sea ice zone, shelf regions and high dust regions ( > 7 mg dust m−2 d−1) and cover 76 million km2. High dust regions were excluded due to the uncertainty associated with the decline of ANCP200m in these regions (see Fig. 3).

Source Data

Extended Data Fig. 7 Maximum ANCP supportable by the winter nitrate inventory.

Upper ANCP limit (ANCPmax in equation 6), derived from the 50-m-integrated winter nitrate inventory at the start of the productive period (August/September climatology, 2013–2021 B-SOSE61). These values are considered to be the upper limit of ANCP50m before productivity is limited by nitrate and cannot be further sustained by dust-iron addition (see Methods). ANCPmax values were used to cap present-day and LGM ANCP estimates in high-dust and low-nitrate regions, respectively. Adjusted regions are indicated by red and white dots in Fig. 4.

Source Data

Extended Data Fig. 8 ECHAM6.3 simulated LGM dust deposition fluxes and difference relative to present-day fluxes.

a, Southern Ocean dust deposition fluxes during the Last Glacial Maximum (LGM) obtained from the ECHAM6.3-HAM2.3 coupled atmosphere-aerosol model27. Markers indicate sediment core locations referred to in Extended Data Table 1. b, LGM dust fluxes divided by ACCESS-AM2 present-day dust fluxes mapped in Fig. 1b. Across the pelagic ice-free Southern Ocean, ECHAM LGM dust fluxes are, on average, by a factor of 11.4 ± 6.6 higher than ACCESS present-day dust fluxes.

Source Data

Extended Data Fig. 9 Cumulative areas impacted by dust, nitrate limitation and high dust loads.

The grey shades indicate the cumulative area of, a, present-day and, b, LGM dust influence on the Southern Ocean with decreasing dust deposition, illustrating the expansion of dust from the source regions. The total area of the study region, the pelagic ice-free Southern Ocean south of 30° S, is 78 million km2. The overlayed blue, red and yellow shades indicate, respectively, the cumulative area that is impacted by nitrate limitation, high dust loads (> 7 mg m-2 d-1), or both. Therefore, the blue and yellow shade combined represent high-dust regions and the red and yellow shade combined represent nitrate-limited regions.

Source Data

Extended Data Table 1 Glacial–interglacial productivity variability comparison
Full size table

Source data

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Weis, J., Chase, Z., Schallenberg, C. et al. One-third of Southern Ocean productivity is supported by dust deposition. Nature 629, 603–608 (2024). https://doi.org/10.1038/s41586-024-07366-4

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