the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Apr 2024
Sedimentary organic matter signature hints at the phytoplankton-driven Biological Carbon Pump in the Central Arabian Sea
Abstract. The Central Arabian Sea, a unique tropical basin is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton bloom associated with high biological productivity and particle flux occurs in the northern part of the central Arabian Sea due to summer monsoon-induced open ocean upwelling and winter convection. The core Oxygen Minimum Zone (OMZ) at the intermediate water depths is another important feature of the north-central Arabian Sea and fades southward. In this study, we have attempted to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core top (1 cm) samples representing the recent particle flux signatures were analyzed from 5 locations (21° to 11° N; 64° E) in the central Arabian Sea. The C37 alkenone-based sea surface temperature (SST) proxy indicated cooler SST (27.6 ± 0.25 °C) in the north mostly due to upwelling (summer) and convective mixing (winter) and warmer (0.4 °C) in the south, which usually remains nutrient-poor. This trend was consistent with the satellite-derived average SST values (2017–2020). Lipid biomarker analysis suggested that dinoflagellates were the highest contributor as indicated in dinosterol and its degradative product dinostanol followed by brassicasterol, and C37 alkenone representing diatoms, and coccolithophores, respectively. The stations in the north (21–15° N) that largely experience periodic phytoplankton blooms and is influenced by the thick OMZ revealed the highest contents of organic matter, diatom frustules (diversity and abundance) dominated by large thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia), and phytoplankton organic biomarkers, but lower zooplankton biomarkers (cholesterol and cholestanol). Whereas relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton biomarker contents were found in the south where zooplankton biomarkers and silicious radiolarians were more abundant. The probable impacts of the presence of the OMZ along the sampling transect on particle flux related to the phytoplankton community, zooplankton grazing along with other factors have also been discussed.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1923 KB) - Metadata XML
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Supplement
(381 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-845', Ralf Schiebel, 06 Jun 2024
The manuscript of Medhavi Pandey and coauthors on ‘Sedimentary organic matter signature hints at the phytoplankton-driven Biological Carbon Pump in the Central Arabian Sea’ presents new and significant data on a topic of general interest. However, as the second paper on the same samples and along a similar scientific avenue following the paper of Pandey et al. (2023, Environmental Monitoring and Assessment, 195/1), I would suggest a broader appreciation of the first paper, i.e., to build on the first paper and not just reusing the same data. By doing so, a clearer distinction of the two papers as stand-alone contributions would be possible. By a clearer distinction, the two papers may receive more visibility, and may be better cited in the upcoming literature. Following a clearer distinction of the two papers, the Abstract and Conclusions may need to be reorganized.
I believe that an improvement of syntax and language, as well as the figures, would enhance the overall quality of the paper. Overall, I would suggest to use the present / present perfect tense when presenting the results, which allows a more dynamic reading. Finally, I would suggest to accept the paper for final publication following major improvements. To support revisions of the manuscript, I provide an annotated pdf file.
Finally, the figures and tables should be improved. For example, the panel a of the Fig. 1 does not show the water-depth related differences discussed in the paper, because all of the sampling sites are shown in the same kind of blue indicating the water depth from 2000-4000 m (the Fig. 1 is better in the 2023 paper). Please find detailed suggestions in the annotated pdf file.
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AC1: 'Reply on RC1', Haimanti Biswas, 07 Jul 2024
We gratefully acknowledge the time and effort the reviewers took to evaluate this manuscript, and specifically, the supportive comments and positive criticism from two reviewers. We replied to all points raised by the reviewers and took most of their suggestions, or rebutted. Please see the attached file for detailed response to the comments.
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AC1: 'Reply on RC1', Haimanti Biswas, 07 Jul 2024
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RC2: 'Comment on egusphere-2024-845', Katrin Schmidt, 18 Jun 2024
The manuscript ‘Sedimentary organic matter signature hints at the phytoplankton-driven biological carbon pump in the Central Arabian Sea’ by Pandey et al. looks at the composition of top sediment cores along a productivity gradient in the central Arabian Sea with the aim to evaluate the relative contribution of diatoms, flagellates, coccolithophores and zooplankton to carbon sequestration. This is an interesting study and Figure 5 nicely illustrates how the regional differences in the physical and biogeochemical environment lead to a different community structure of primary producers, different grazer abundances and different export. The manuscript builds on a previous study by the same lead author (‘Interlinking diatom frustule diversity from the abyss of the central Arabian Sea to surface processes: physical forcing and oxygen minimum zone, Environmental Monitoring and Assessment, 195(1), 161, 823 https://doi.org/10.1007/s10661-022-10749-7, 2023), but introduces lipid biomarker results as a novelty. The authors’ main conclusion is that dinoflagellates rather than diatoms or coccolithophores contribute most to the sedimentary flux. This conclusion derives from higher amounts of dinosterol (used for dinoflagellates) compared to brassicasterol (used for diatoms) and alkenone (used for coccolithophores) per unit organic matter in the sediment.
However, brassicasterol is not a reliable marker for diatoms. Rampen et al. (2010, ‘A comprehensive study of sterols in marine diatoms…’ ) analysed the sterol composition of > 100 marine diatoms species and in regard to brassicasterol they wrote: ’As this sterol is only the fifth most common sterol and absent in all radial centric diatoms and some important groups of bi(multi)polar centrics, our data support the statement by Barrett et al. (1995) that 24- methylcholesta-5,22E-dien-3b-ol should not be considered a general biomarker for diatoms. Furthermore, this sterol has also been found in many other groups of algae like Haptophyceae, Cryptophyceae, Chrysophyceae, Bangio[1]phyceae, and in a number of dinoflagellates (Volkman 2003)’. In line with Rampen (2010), I found more brassicasterol in Emiliania huxleyi than in 30 polar diatom species. The occurrence of brassicasterol in coccolithophores has also been described by others (e.g. Ding et al. 2019, ‘Lipid biomarker production …’). Looking at the 8 diatom species that the authors mentioned in Fig. 4 (Coscinodiscus, Thalassiosira etc..), none of them has been found producing brassicasterol in Rampen et al. (2010).
The second point, I would like to bring across: Sterols and other lipid biomarker such as fatty acids have rarely a fixed ratio to carbon or biomass. The production of these components can be highly sensitive to environmental conditions, e.g. light levels, nutrient supply, pH etc. Therefore, even though sediment cores contain more dinosterol than brassicasterol, this does not allow extrapolation to algae cell numbers or biomass. Ratios of two sterols have some potential for regional comparisons of relative abundance or presence vs absence, but not to quantify biomass.
If the authors would like to move forward with sterol biomarkers, I would suggest they analyse the sterol composition of their two main diatom species (Coscinodiscus and Thalassiosira) – either picking sufficient live cells from the sediment or water column, or culture them and grow sufficient biomass. Based on those findings, the samples from top sediment cores could be re-analysed for the ‘right’ sterols. Rampen et al. (2010) found chalinasterol (24- methylcholesta-5,24(28)-dien-3β–ol; m/z 470) in both Coscinodiscus and Thalassiosira (but likely different species). The same should be dome with their common platted dinoflagellates and coccolithophores. This will help to correctly interpret the sterol composition of the sediment cores. For a further read on sterols in microalgae (including the production of dinosterol) I would recommend Volkman (2017, 10.1007/978-3-319-24945-2_19 )and for phyto-vs-zoosterol ratios (Kohlbach et al. 2021, https://doi.org/10.3389/fmars.2020.610248 )
Citation: https://doi.org/10.5194/egusphere-2024-845-RC2 -
AC2: 'Reply on RC2', Haimanti Biswas, 07 Jul 2024
We gratefully acknowledge the time and effort the reviewers took to evaluate this manuscript, and specifically, the supportive comments and positive criticism from two reviewers. We replied to all points raised by the reviewers and took most of their suggestions, or rebutted. Please see the attached file for detailed response to the comments.
-
AC2: 'Reply on RC2', Haimanti Biswas, 07 Jul 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-845', Ralf Schiebel, 06 Jun 2024
The manuscript of Medhavi Pandey and coauthors on ‘Sedimentary organic matter signature hints at the phytoplankton-driven Biological Carbon Pump in the Central Arabian Sea’ presents new and significant data on a topic of general interest. However, as the second paper on the same samples and along a similar scientific avenue following the paper of Pandey et al. (2023, Environmental Monitoring and Assessment, 195/1), I would suggest a broader appreciation of the first paper, i.e., to build on the first paper and not just reusing the same data. By doing so, a clearer distinction of the two papers as stand-alone contributions would be possible. By a clearer distinction, the two papers may receive more visibility, and may be better cited in the upcoming literature. Following a clearer distinction of the two papers, the Abstract and Conclusions may need to be reorganized.
I believe that an improvement of syntax and language, as well as the figures, would enhance the overall quality of the paper. Overall, I would suggest to use the present / present perfect tense when presenting the results, which allows a more dynamic reading. Finally, I would suggest to accept the paper for final publication following major improvements. To support revisions of the manuscript, I provide an annotated pdf file.
Finally, the figures and tables should be improved. For example, the panel a of the Fig. 1 does not show the water-depth related differences discussed in the paper, because all of the sampling sites are shown in the same kind of blue indicating the water depth from 2000-4000 m (the Fig. 1 is better in the 2023 paper). Please find detailed suggestions in the annotated pdf file.
-
AC1: 'Reply on RC1', Haimanti Biswas, 07 Jul 2024
We gratefully acknowledge the time and effort the reviewers took to evaluate this manuscript, and specifically, the supportive comments and positive criticism from two reviewers. We replied to all points raised by the reviewers and took most of their suggestions, or rebutted. Please see the attached file for detailed response to the comments.
-
AC1: 'Reply on RC1', Haimanti Biswas, 07 Jul 2024
-
RC2: 'Comment on egusphere-2024-845', Katrin Schmidt, 18 Jun 2024
The manuscript ‘Sedimentary organic matter signature hints at the phytoplankton-driven biological carbon pump in the Central Arabian Sea’ by Pandey et al. looks at the composition of top sediment cores along a productivity gradient in the central Arabian Sea with the aim to evaluate the relative contribution of diatoms, flagellates, coccolithophores and zooplankton to carbon sequestration. This is an interesting study and Figure 5 nicely illustrates how the regional differences in the physical and biogeochemical environment lead to a different community structure of primary producers, different grazer abundances and different export. The manuscript builds on a previous study by the same lead author (‘Interlinking diatom frustule diversity from the abyss of the central Arabian Sea to surface processes: physical forcing and oxygen minimum zone, Environmental Monitoring and Assessment, 195(1), 161, 823 https://doi.org/10.1007/s10661-022-10749-7, 2023), but introduces lipid biomarker results as a novelty. The authors’ main conclusion is that dinoflagellates rather than diatoms or coccolithophores contribute most to the sedimentary flux. This conclusion derives from higher amounts of dinosterol (used for dinoflagellates) compared to brassicasterol (used for diatoms) and alkenone (used for coccolithophores) per unit organic matter in the sediment.
However, brassicasterol is not a reliable marker for diatoms. Rampen et al. (2010, ‘A comprehensive study of sterols in marine diatoms…’ ) analysed the sterol composition of > 100 marine diatoms species and in regard to brassicasterol they wrote: ’As this sterol is only the fifth most common sterol and absent in all radial centric diatoms and some important groups of bi(multi)polar centrics, our data support the statement by Barrett et al. (1995) that 24- methylcholesta-5,22E-dien-3b-ol should not be considered a general biomarker for diatoms. Furthermore, this sterol has also been found in many other groups of algae like Haptophyceae, Cryptophyceae, Chrysophyceae, Bangio[1]phyceae, and in a number of dinoflagellates (Volkman 2003)’. In line with Rampen (2010), I found more brassicasterol in Emiliania huxleyi than in 30 polar diatom species. The occurrence of brassicasterol in coccolithophores has also been described by others (e.g. Ding et al. 2019, ‘Lipid biomarker production …’). Looking at the 8 diatom species that the authors mentioned in Fig. 4 (Coscinodiscus, Thalassiosira etc..), none of them has been found producing brassicasterol in Rampen et al. (2010).
The second point, I would like to bring across: Sterols and other lipid biomarker such as fatty acids have rarely a fixed ratio to carbon or biomass. The production of these components can be highly sensitive to environmental conditions, e.g. light levels, nutrient supply, pH etc. Therefore, even though sediment cores contain more dinosterol than brassicasterol, this does not allow extrapolation to algae cell numbers or biomass. Ratios of two sterols have some potential for regional comparisons of relative abundance or presence vs absence, but not to quantify biomass.
If the authors would like to move forward with sterol biomarkers, I would suggest they analyse the sterol composition of their two main diatom species (Coscinodiscus and Thalassiosira) – either picking sufficient live cells from the sediment or water column, or culture them and grow sufficient biomass. Based on those findings, the samples from top sediment cores could be re-analysed for the ‘right’ sterols. Rampen et al. (2010) found chalinasterol (24- methylcholesta-5,24(28)-dien-3β–ol; m/z 470) in both Coscinodiscus and Thalassiosira (but likely different species). The same should be dome with their common platted dinoflagellates and coccolithophores. This will help to correctly interpret the sterol composition of the sediment cores. For a further read on sterols in microalgae (including the production of dinosterol) I would recommend Volkman (2017, 10.1007/978-3-319-24945-2_19 )and for phyto-vs-zoosterol ratios (Kohlbach et al. 2021, https://doi.org/10.3389/fmars.2020.610248 )
Citation: https://doi.org/10.5194/egusphere-2024-845-RC2 -
AC2: 'Reply on RC2', Haimanti Biswas, 07 Jul 2024
We gratefully acknowledge the time and effort the reviewers took to evaluate this manuscript, and specifically, the supportive comments and positive criticism from two reviewers. We replied to all points raised by the reviewers and took most of their suggestions, or rebutted. Please see the attached file for detailed response to the comments.
-
AC2: 'Reply on RC2', Haimanti Biswas, 07 Jul 2024
Peer review completion
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Medhavi Pandey
Daniel Birgel
Nicole Burdanowitz
Birgit Gaye
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1923 KB) - Metadata XML
-
Supplement
(381 KB) - BibTeX
- EndNote
- Final revised paper