Resupply of mesopelagic dissolved iron controlled by particulate iron composition

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Abstract

The dissolved iron supply controls half of the oceans’ primary productivity. Resupply by the remineralization of sinking particles, and subsequent vertical mixing, largely sustains this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence as to what controls the subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (that is, replenishment over oxygen consumption) were 10- to 100-fold higher in low-dust subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting and may lead to the differential remineralization of biogenic versus lithogenic iron. At high-dust sites, these processes together increase the iron remineralization length scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter the vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wide-ranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces.

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Fig. 1: In situ particle remineralization measurements at contrasting biogeochemical sites.
Fig. 2: Summary of downward particle fluxes and composition from (TM-)RESPIRE at SAZ (green), ALG (red) and ION (blue).
Fig. 3: Bacterial particle remineralization and \(R_{{\rm{Fe}}/{\rm{O}}_2}\) at SAZ (green), ALG (red) and ION (blue).
Fig. 4: Synthesis of key processes that together set the PFe remineralization length scale expressed as a function of the relative proportion of sinking biogenic/lithogenic PFe.
Fig. 5: Results of model simulations using lithogenic particle-dependent modulation of iron remineralization.

Data availability

Modis chlorophyll a concentrations (ALG and ION sites) were obtained with the Giovanni online data system, developed and maintained by the NASA GES DISC. MODIS chlorophyll a concentrations corrected using an improved regional algorithm for the Southern Ocean (SAZ site) are publicly available via the Australian Integrated Marine Observing System (IMOS) Ocean Portal (www.imos.org.au). After publication, the dataset generated and analysed during the current study (mostly available in Supplementary Information) will be made available (that is, open access) through the IMAS/UTAS data portal (http://www.imas.utas.edu.au/data).

Code availability

The NEMO-PISCES model we use in this work is freely available (http://www.nemo-ocean.eu/) under the CeCILL free software licence (http://www.cecill.info/index.en.html). We used a modified version of the PISCES biogeochemical model. These modifications concern the representation of the particulate iron remineralization and this is not yet present in the freely available NEMO release, but will be provided upon contacting A.T.

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Acknowledgements

We thank the captains and crew of the RV Investigator and RV Pourquoi Pas?, the CSIRO and DT INSU teams for the design and preparation of the mooring line and C. Young and P. Waller for building the TM-RESPIRE. The CSIRO Hydrochemistry team, S. Albani, N. Bhairy, E. Cavan, X. Chan, G. De Liège, J. Derrick, K. Desboeufs, F. D’Ortenzio, A. Dufour, M. Garel, J. Guittonneau, N. Haentjens, S. Helias Nunige, S. Jacquet, P. Jansen, N. Leblond, D. Lefèvre, H. Planquette, C. Ridame, G. Rougier, V. Tallendier, C. Tamburini, A. Tovar-Sanchez and T. Trull, are thanked for their help at sea and/or samples analysis. This project was funded by a Marie Sklodowska-Curie Postdoctoral European Fellowship awarded to M.B. (European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. PIOF-GA-2012-626734 (IRON-IC project)). This study is a contribution to the PEACETIME project (http://peacetime-project.org), a joint initiative of the MERMEX and ChArMEx components supported by CNRS-INSU, IFREMER, CEA and Météo-France as part of the programme MISTRALS coordinated by INSU. This study was also partly funded by the Australian Research Council by a Laureate awarded to P.W.B. (FL160100131) and a Discovery project awarded to M.J.E. and P.W.B. (DP170102108).

Author information

M.B. designed and carried out the study, analysed the data and wrote the manuscript. C.G. and P.W.B. helped in the design of the study and co-led the cruises. P.W.B. worked on the different versions of the manuscript. M.J.E., T.W. and E.C.L.-C. helped in the different instrument deployments and sampling. M.J.E. and T.W. helped in the analysis of samples. A.T. provided and helped interpret the PISCES model output. H.W. and G.S. analysed the ligand samples. All the authors commented on and contributed to the improvement of the manuscript.

Correspondence to M. Bressac.

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Bressac, M., Guieu, C., Ellwood, M.J. et al. Resupply of mesopelagic dissolved iron controlled by particulate iron composition. Nat. Geosci. (2019) doi:10.1038/s41561-019-0476-6

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