Sponge skeletons as an important sink of silicon in the global oceans

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Abstract

Silicon (Si) is a pivotal element in the biogeochemical and ecological functioning of the ocean. The marine Si cycle is thought to be in internal equilibrium, but the recent discovery of Si entries through groundwater and glacial melting have increased the known Si inputs relative to the outputs in the global oceans. Known outputs are due to the burying of diatom skeletons or their conversion into authigenic clay by reverse weathering. Here we show that non-phototrophic organisms, such as sponges and radiolarians, also facilitate significant Si burial through their siliceous skeletons. Microscopic examination and digestion of sediments revealed that most burial occurs through sponge skeletons, which, being unusually resistant to dissolution, had passed unnoticed in the biogeochemical inventories of sediments. The preservation of sponge spicules in sediments was 45.2 ± 27.4%, but only 6.8 ± 10.1% for radiolarian testa and 8% for diatom frustules. Sponges lead to a global burial flux of 1.71 ± 1.61 TmolSi yr−1 and only 0.09 ± 0.05 TmolSi yr−1 occurs through radiolarians. Collectively, these two non-phototrophically produced silicas increase the Si output of the ocean to 12.8 TmolSi yr−1, which accounts for a previously ignored sink that is necessary to adequately assess the global balance of the marine Si cycle.

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Fig. 1: Digestion kinetics of BSi and LSi pure materials.
Fig. 2: Diatom and sponge BSi skeletons during digestion in 1% sodium carbonate at 85 °C.
Fig. 3: Contributors of BSi in superficial sediments.

Data availability

The authors declare that all other data supporting the findings of this study are available within the article and its Supplementary Information. Further additional data are available at the institutional repository of the Spanish National Research Council (CSIC), http://hdl.handle.net/10261/184130.

The map layers that contain the ocean geomorphology features were downloaded from www.bluehabitats.com, except for coral reefs, which were obtained from the World Resource Institute (www.wri.org). The extension of radiolarian-rich sediments was calculated using the map layers available at the www.earthbyte.org/webdav/ftp/papers/Dutkiewicz_etal_seafloor_lithology/.

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Acknowledgements

We thank the British Ocean Sediment Core Research Facility (BOSCORF-NOC) for providing access to cores 1, 12, 14 and 16. We also thank E. Kenchington, C. Campbell, K. Jarrett and J. Murillo (BIO) for making the data and sediment of cores 2 and 4 available. A. Ehrhold (IFREMER) is thanked for core 3, M. A. Mateo (CEAB) for core 7 and T. Whiteway (Australian Geosciences) for core 15. R. Ventosa and M. Abad are thanked for helping with the DSi autoanalyser determinations, B. Dursunkaya for helping with the digestion experiments and P. Talberg and L. Cross for providing strains of the Thalassiossira diatom. J. Krause is especially thanked for comments and insight on the manuscript. This study, which spanned five years, benefitted from funding by two grants of the Spanish MINECO (CTM2012-37787 and CTM2015-67221-R). Financial support by the European Union’s Horizon 2020 research and innovation program to the SponGES project (grant agreement 679849) is acknowledged.

Author information

M.M. designed the study and experiments. Sediment and BSi digestions were conducted by M.M., M.L.A., C.S., M.G.-P. and C.G. Sediment cores were collected by G.E. Light microscopy determination of the BSi was conducted by M.L.A., C.S., M.G.-P. and C.G. under supervision of M.M. SEM and energy dispersive X-ray spectrometry analyses were conducted by M.M. and C.S. Data analyses were conducted by M.M. and M.L.A. and ArcGis mapping by M.G.-P. M.M. wrote the manuscript, with invaluable inputs made by M.L.A. and the rest of co-authors at different stages of the process.

Correspondence to Manuel Maldonado.

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Supplementary information

Supplementary information

Supplementary data tables 1–4, Supplementary figs 1–5, Supplementary dicussion, Supplementary methods and Supplementary references.

Supplementary data table 1

Supplementary Methods data (Table 1).

Supplementary data table 2

Supplementary Methods data (Table 2).

Supplementary data 1

Supplementary data for burial depth and Si preservation.

Supplementary data 2

Supplementary data for deposition rate and sponge and radiolarian burial rate.

Supplementary data 3

Supplementary data for global ocean Si burial and preservation.

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