Journal article Open Access

Silicon isotopic systematics of deep-sea sponge grounds in the North Atlantic

Hendry, Katharine R; Cassarino, Lucie; Bates, Stephanie L; Culwick, Timothy; Frost, Molly; Goodwin, Claire; Howell, Kerry L

ABSTRACT

Reconstruction of silica cycling in the oceans is key to a thorough understanding of past climates because of the inherent links between the biogeochemistry of silicifiers and sequestration of organic carbon. Diatoms are one of the most important phytoplankton groups in determining export production from surface waters, and rely largely on upwelling deeper waters as a source of dissolved silicon, an essential nutrient for their growth. Quantification of changes in deep water dissolved silicon concentrations in the past allows a more robust understanding of changes in surface nutrient supply and whole-ocean silicon cycling, but cannot be achieved using surface-derived geochemical archives. In the last few years, there has been increasing focus on the use of geochemical archives in siliceous skeletal elements, or spicules, from seafloor-dwelling sponges to fill this gap. The stable silicon isotopic composition of spicules has been shown to be a function of ambient dissolved silicon, providing a potential archive for past changes in bottom water nutrients. However, biomineralisation processes impact silicon isotope fractionation and silica formed by atypical processes (derived from carnivorous sponges, hypersilicified spicules, and giant basal spicules) result in anomalous geochemical signatures. Furthermore, there is considerable scatter in the calibration between spicule silicon isotopes and dissolved silicon in seawater, even when the atypical groups have been removed. Here, we explore this variability further, by examining aggregation and assemblage-level differences in isotopic fractionation, using silicon isotopic measurements of specimens from two monospecific sponge groups (Pheronema carpenteri and Vazella pourtalesi), and one mixed-species population (genus Geodia) from the North Atlantic. Our new data reveal that variability within the monospecific aggregations is less than mixed-species assemblage, pointing towards a genetic control in isotopic fractionation. However, there is still variability within the monospecific aggregations, which cannot be explained by macroscale environmental differences: such variability is likely a reflection of the physiological health of the individuals, or highly localised heterogeneities in sponge habitats.

Other challenges remain in the interpretation of spicule silicon isotopes as proxies for dissolved silicon changes through time, especially when investigating periods of Earth history that extend back considerably further than the residence time of dissolved silicon in the oceans. Despite all the questions still surrounding the use of sponge silicon isotopes in palaeoceanographic applications, they are still the only known archive of bottom water dissolved silicon. Continued efforts to understanding sponge biomineralisation and to incorporate silicon isotopes into oceanic models will help to improve further the reliability of the archive.

HIGHLIGHTS

• Sponge silicon isotopes are an archive of bottom water dissolved silicon concentrations.

• Atypical biomineralisation impacts sponge silicon isotope fractionation.

• New data from sponge grounds show isotopic variation within monospecific aggregations.

• Single and mixed sponge species isotopic variation is likely due to individual health.

• Results show potential recycling of silica within sponge aggregations.

 

Received 20 December 2018, Revised 15 February 2019, Accepted 15 February 2019, Available online 2 March 2019. This research was funded by European Research Council project ICY-LAB (ERC-2015-STG grant agreement number 678371), and EU Horizon 2020 project SponGES (H2020-BG-2015-2 grant agreement number 679849). K. L. Howell is funded by the EU Seventh Framework Programme EUROFLEETS2 (FP7/2007–2013 grant agreement number 312762). Thanks to Christopher D. Coath for assistance with mass spectrometry, and Paco Cárdenas for advice on Geodia identification. Many thanks to Ellen Kenchington and Lindsay Beazley for collection of Vazella pourtalesi sponges and co-located seawater samples, and for supplying details of their previous benthic surveys on Orphan Knoll which assisted with cruise planning. Canadian funding was received from the International Governance Strategy (IGS) fund of the Department of Fisheries and Oceans Canada Project "Marine Biological Diversity Beyond Areas of National Jurisdiction (BBNJ): 3-Tiers of Diversity (Genes-Species-Communities)" to E. K. We also thank Joana Xavier, Manuel Maldonado and Hans Tore Rapp for support through the SponGES project. Many thanks to two anonymous reviewers for their constructive comments. Data availability: An electronic copy of the new data from this study is available at https://doi.pangaea.de/10.1594/PANGAEA.898677.
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