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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 22
Biogeosciences, 15, 6959-6977, 2018
https://doi.org/10.5194/bg-15-6959-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Biogeosciences, 15, 6959-6977, 2018
https://doi.org/10.5194/bg-15-6959-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Nov 2018

Research article | 21 Nov 2018

Silicon isotopes of deep sea sponges: new insights into biomineralisation and skeletal structure

Lucie Cassarino1, Christopher D. Coath1, Joana R. Xavier2,3, and Katharine R. Hendry1 Lucie Cassarino et al.
  • 1University of Bristol, School of Earth Sciences, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK
  • 2CIIMAR – Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Avenida General Norton de Matos, 4450-208 Matosinhos, Portugal
  • 3Department of Biological Sciences and K.G. Jebsen Centre for Deep Sea Research, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway

Abstract. The silicon isotopic composition (δ30Si) of deep sea sponges' skeletal element – spicules – reflects the silicic acid (DSi) concentration of their surrounding water and can be used as natural archives of bottom water nutrients. In order to reconstruct the past silica cycle robustly, it is essential to better constrain the mechanisms of biosilicification, which are not yet well understood. Here, we show that the apparent isotopic fractionation (δ30Si) during spicule formation in deep sea sponges from the equatorial Atlantic ranges from −6.74‰ to −1.50‰ in relatively low DSi concentrations (15 to 35µM). The wide range in isotopic composition highlights the potential difference in silicification mechanism between the two major classes, Demospongiae and Hexactinellida. We find the anomalies in the isotopic fractionation correlate with skeletal morphology, whereby fused framework structures, characterised by secondary silicification, exhibit extremely light δ30Si signatures compared with previous studies. Our results provide insight into the processes involved during silica deposition and indicate that reliable reconstructions of past DSi can only be obtained using silicon isotope ratios derived from sponges with certain spicule types.

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Using a simple model, we show that the silicon isotopic composition of sponges can be used to estimate the silicic acid concentration of seawater, a key parameter linked to nutrient and carbon cycling. However, our data illustrate that skeletal type and growth rate also control silicon isotopic composition of sponges. Our study demonstrates the paleoceanographic utility of sponges as an archive for ocean silica content provided that suitable skeleton types are selected.
Using a simple model, we show that the silicon isotopic composition of sponges can be used to...
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