the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Dec 2024
Faithful transfer of radiolarian silicon isotope signatures from water column to sediments in the South China Sea
Abstract. Radiolarian silicon isotopes (δ30Sirad) hold significant potential as a proxy for constraining past silicon cycling in seawater. However, the extent to which δ30Sirad signatures in sediments accurately represent the isotopic signals of the overlying water column remains unclear, particularly under the influence of radiolarian shell dissolution during sinking and burial in the sediment record. This study presents the first comparative analysis of δ30Sirad compositions and the radiolarian assemblage community using water column and surface sediment samples collected from the South China Sea. The results indicate that δ30Sirad values range from 1.56–1.83 ‰ (mean = 1.74 ‰) in the water column, and from 1.61–1.85 ‰ (mean = 1.73 ‰) in surface sediments. No significant discrepancies in δ30Sirad values were observed between plankton and sediment samples at each sampling station as evidenced by the paired t-test (p = 0.75), implying that dissolution has a minimal impact on δ30Sirad during the transfer of radiolarian shells to the sediment record. This finding may be enhanced by the dominance of more dissolution-resistant Spumellaria and Nassellaria taxa (>99 % relative abundance) within the radiolarian community, coupled with the scarcity or absence of the readily dissolvable radiolarian taxa in the analysed samples. This study demonstrates the faithful preservation of the δ30Sirad signature and its potential for studying past changes in the marine silicon cycle.
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RC1: 'Comment on egusphere-2024-3686', Anonymous Referee #1, 07 Jan 2025
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Review of manuscript egusphere-2024-3686 submitted for publication in EGUsphere by Qiang Zhang and colleagues: Faithful transfer of radiolarian silicon isotope signatures from water column to sediments in the South China Sea
There is growing interest in how the silicon isotope composition (expressed as δ30Si) of radiolarians can be used to supplement/complement those of the more established proxy archives in diatoms and sponges. As with these two groups, there is a need to understand and account for any post-mortem alteration of the initial isotope signal. Here, Zhang et al compare bulk-assemblage radiolarian δ30Si values from water-column plankton tows and underlying coretop sediments. They demonstrate the two are statistically indistinguishable, lending more confidence to the use of radiolarian δ30Si as a window on to past Si cycling.
In general, this manuscript is very well written and the figures are clear (with possible exception of Fig.3). The referencing is generally appropriate – with some notable absences (see below). The data are generated by appropriate techniques (though some more details may be warranted). Overall, there is little to criticise in terms of the central conclusion – that the radiolarian δ30Si signal is not altered during water column sinking – which is supported by the data (though I have a series of minor comment/suggestions that I detail below). Nevertheless, this is a relatively small dataset and I have the impression that with a slightly expanded dataset much more could be done. I note in the supplement, Fig. S1 contains 22 ‘unpublished’ radiolarian datapoints. By integrating these, and dissolved Si δ30Si data, a much more impactful paper would result. Some suggestions are below, but this is ultimately an editorial decision.
Minor comments and suggestions
L16-27 – the prominence of the SALH in the introduction is a bit strange to me, considering it’s not the focus of the manuscript (and isn’t returned to)
L48: Two papers that deserve citation/discussion here and elsewhere are Closset et al. 2015 (doi: 10.1002/2015GB005180) and Grasse et al. 2021 (doi: 10.3389/fmars.2021.697400) – both present a comparison of plankton tow diatoms and sediment trap (Closset) or core-top (Grasse) material, concluding that the transfer of biogenic silica from surface ocean to depth isn’t associated with a resolvable change in δ30Si. See also Varela et al. 2004 (doi: 10.1029/2003GB002140; sediment trap data) and Fripiat et al. 2012 (doi: 10.5194/bg-9-2443-2012; water column biogenic silica data).
L85 and introduction: In general, there is a growing awareness that ‘bulk’ assemblage δ30Si data have disadvantages as a paleo-archive, and that where possible single-species records are much stronger. Therefore it would be good to see some justification for why this was not attempted here.
Section 2.3: I would suggest more detail is needed here. Specific suggestions include:
- Define what is ‘sufficient’ radiolarian tests (L80; what is the typical mass of Si processed)
- Give a brief overview of Zhang and Swann (L81). Is there potential for larger diatoms or sponge spicules to ‘contaminate’ the sample?
- Confirm that Na (used in dissolution) was successfully removed by the ion exchange chromatography
- Give approximate mass resolution (m/Dm, L93)
- Give details of how Mg measured/which ratio(s) (presumably in ‘dynamic’ mode), and a reference to Cardinal et al. 2004 (doi: 10.1039/b210109b) is probably appropriate
- Give approximate sample introduction rate, concentration, and instrument sensitivity
- Confirm what one analytical replicate represents (just one standard-sample-standard bracket, or (as is usual) three or four?)
L114: It’s not clear what volume the 28025-102443 individuals refer to – in a 1m2 water column? It is also not clear how these numbers are derived – presumably because the volume of water passing through the nets (L66) is known? This could be clarified.
L126: Can an approximate detection limit be given for these elements?
Discussion section: In general, there is no discussion of any spatial pattern in radiolarian assemblage. But I feel there is probably useful insight here. For example, Station 28 is located away from the cluster of other stations, and visually in Fig. 3 looks different. What physicochemical parameters influence the community assemblages? As an aside, I note that the assemblage data is not made available. Could the species in Fig 3 be amalgamated at a higher taxonomic level in order to make the similarities and differences clearer? And/or condensed, via an appropriate multivariate statistical approach, to 2 axes?
L134-140: In general, it’s the fractionation between dissolved Si and biogenic silica that is relevant, not the absolute δ30Si value. So it’s a shame not to see any dissolved Si δ30Si data from the water samples (perhaps this is coming in a later publication?). An existing dataset of δ30Si exists for the SCS (Cao et al. 2012, doi: 10.1016/j.gca.2012.08.039). The overlap isn’t perfect in terms of location or seasonality but its surprising that it’s not mentioned here, given the importance of water δ30Si in setting radiolarian δ30Si, and that it would allow the authors to place constraints on the fractionation of Si isotopes by radiolarians.
L165: Can an indication of the timespan covered by the upper 1cm be given? I presume there are some constraints on sedimentation rates and bioturbation in this well studied region.
Section 4.2: The radiolarian assemblages are similar between water column and core-top, so the inference is that ‘dissolution is expected to have limited impacts on these radiolarian shells’ (L209) – but is this necessarily true? Is is possible weakly silicified parts of the tests are dissolving? This would be interesting to know, as it has different implications for *how* the δ30Si is preserved: if no dissolution occurs, then there’s no real potential for altering the isotopic signature (which therefore means the conclusions here are not transferable to other settings where more dissolution does occur). But if dissolution does occur and the δ30Si remains the same, then either a) different parts of the tests have the same δ30Si or b) a coincidental balance of heavier and lighter parts dissolved. To begin to address this, it would be good to see an independent constraint on the (radiolarian) biogenic silica preservation effeciency, either from the literature, from a comparison of export vs. sediment-trap/burial fluxes, or even a theoretical predicted efficiency based on sinking speeds, water column depth, and dissolution kinetics. Finally, It would be good to see an attempt to engage with what may happen with progressive dissolution in the upper centimeters of the sediment – if there is preferential dissolution of some species here, might that introduce a bias (an apparent fractionation) to the bulk-assemblage δ30Si data?
Citation: https://doi.org/10.5194/egusphere-2024-3686-RC1
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