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Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation

Doering, Kristin LU orcid ; Ehlert, Claudia ; Pahnke, Katharina ; Frank, Martin ; Schneider, Ralph and Grasse, Patricia (2021) In Frontiers in Marine Science 8.
Abstract

The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ30Sirad), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ30Sirad records are available. To investigate its applicability as a paleo proxy, we compare the δ30Sirad of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ30Si signatures... (More)

The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ30Sirad), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ30Sirad records are available. To investigate its applicability as a paleo proxy, we compare the δ30Sirad of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ30Si signatures (δ30SiDSi) along the coast between the equator and 15°S. Three different radiolarian taxa were selected according to their specific habitat depths of 0–50 m (Acrosphaera murrayana), 50–100 m (Dictyocoryne profunda/truncatum), and 200–400 m (Stylochlamydium venustum). Additionally, samples containing a mix of species from the bulk assemblage covering habitat depths of 0 to 400 m have been analyzed for comparison. We find distinct δ30Sirad mean values of +0.70 ± 0.17‰ (Acro; 2 SD), +1.61 ± 0.20 ‰ (Dictyo), +1.19 ± 0.31 ‰ (Stylo) and +1.04 ± 0.19 ‰ (mixed radiolaria). The δ30Si values of all individual taxa and the mixed radiolarian samples indicate a significant (p < 0.05) inverse relationship with DSi concentrations of their corresponding habitat depths. However, only δ30Si of A. murrayana are correlated to DSi concentrations under normally prevailing upwelling conditions. The δ30Si of Dictyocoryne sp., Stylochlamydium sp., and mixed radiolaria are significantly correlated to the lower DSi concentrations either associated with nutrient depletion or shallower habitat depths. Furthermore, we calculated the apparent Si isotope fractionation between radiolaria and DSi (Δ30Si ∼ 30ε = δ 30Sirad − δ 30SiDSi) and obtained values of −1.18 ± 0.17 ‰ (Acro), −0.05 ± 0.25 ‰ (Dictyo), −0.34 ± 0.27 ‰ (Stylo), and −0.62 ± 0.26 ‰ (mixed radiolaria). The significant differences in Δ30Si between the order of Nassellaria (A. murrayana) and Spumellaria (Dictyocoryne sp. and Stylochlamydium sp.) may be explained by order-specific Si isotope fractionation during DSi uptake, similar to species-specific fractionation observed for diatoms. Overall, our study provides information on the taxon-specific fractionation factor between radiolaria and seawater and highlights the importance of taxonomic identification and separation to interpret down-core records.

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publishing date
type
Contribution to journal
publication status
published
subject
keywords
core-top calibration, isotope fractionation, Nassellaria and Spumellaria, Peruvian upwelling system, protozooplankton, silicon cycle, silicon isotopes
in
Frontiers in Marine Science
volume
8
article number
666896
publisher
Frontiers Media S. A.
external identifiers
  • scopus:85110462492
ISSN
2296-7745
DOI
10.3389/fmars.2021.666896
language
English
LU publication?
no
additional info
Funding Information: This work is a contribution of the Collaborative Research Centre (SFB) 754 "Climate-Biogeochemistry interactions in the Tropical Ocean" (www.sfb754.de), which was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation). This research was funded through a Mini-proposal granted by the SFB754 in 2017. Publisher Copyright: © Copyright © 2021 Doering, Ehlert, Pahnke, Frank, Schneider and Grasse.
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8c1691de-5235-4ec8-b2ee-1960f75bc9b6
date added to LUP
2022-08-15 16:17:58
date last changed
2022-09-02 18:00:00
@article{8c1691de-5235-4ec8-b2ee-1960f75bc9b6,
  abstract     = {{<p>The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ<sup>30</sup>Si<sub>rad</sub>), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ<sup>30</sup>Si<sub>rad</sub> records are available. To investigate its applicability as a paleo proxy, we compare the δ<sup>30</sup>Si<sub>rad</sub> of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ<sup>30</sup>Si signatures (δ<sup>30</sup>Si<sub>DSi</sub>) along the coast between the equator and 15°S. Three different radiolarian taxa were selected according to their specific habitat depths of 0–50 m (Acrosphaera murrayana), 50–100 m (Dictyocoryne profunda/truncatum), and 200–400 m (Stylochlamydium venustum). Additionally, samples containing a mix of species from the bulk assemblage covering habitat depths of 0 to 400 m have been analyzed for comparison. We find distinct δ<sup>30</sup>Si<sub>rad</sub> mean values of +0.70 ± 0.17‰ (Acro; 2 SD), +1.61 ± 0.20 ‰ (Dictyo), +1.19 ± 0.31 ‰ (Stylo) and +1.04 ± 0.19 ‰ (mixed radiolaria). The δ<sup>30</sup>Si values of all individual taxa and the mixed radiolarian samples indicate a significant (p &lt; 0.05) inverse relationship with DSi concentrations of their corresponding habitat depths. However, only δ<sup>30</sup>Si of A. murrayana are correlated to DSi concentrations under normally prevailing upwelling conditions. The δ<sup>30</sup>Si of Dictyocoryne sp., Stylochlamydium sp., and mixed radiolaria are significantly correlated to the lower DSi concentrations either associated with nutrient depletion or shallower habitat depths. Furthermore, we calculated the apparent Si isotope fractionation between radiolaria and DSi (Δ<sup>30</sup>Si ∼ <sup>30</sup>ε = δ<sup> 30</sup>Si<sub>rad</sub> − δ<sup> 30</sup>Si<sub>DSi</sub>) and obtained values of −1.18 ± 0.17 ‰ (Acro), −0.05 ± 0.25 ‰ (Dictyo), −0.34 ± 0.27 ‰ (Stylo), and −0.62 ± 0.26 ‰ (mixed radiolaria). The significant differences in Δ<sup>30</sup>Si between the order of Nassellaria (A. murrayana) and Spumellaria (Dictyocoryne sp. and Stylochlamydium sp.) may be explained by order-specific Si isotope fractionation during DSi uptake, similar to species-specific fractionation observed for diatoms. Overall, our study provides information on the taxon-specific fractionation factor between radiolaria and seawater and highlights the importance of taxonomic identification and separation to interpret down-core records.</p>}},
  author       = {{Doering, Kristin and Ehlert, Claudia and Pahnke, Katharina and Frank, Martin and Schneider, Ralph and Grasse, Patricia}},
  issn         = {{2296-7745}},
  keywords     = {{core-top calibration; isotope fractionation; Nassellaria and Spumellaria; Peruvian upwelling system; protozooplankton; silicon cycle; silicon isotopes}},
  language     = {{eng}},
  month        = {{06}},
  publisher    = {{Frontiers Media S. A.}},
  series       = {{Frontiers in Marine Science}},
  title        = {{Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation}},
  url          = {{http://dx.doi.org/10.3389/fmars.2021.666896}},
  doi          = {{10.3389/fmars.2021.666896}},
  volume       = {{8}},
  year         = {{2021}},
}