the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Aug 2023
Quantification of allochthonous and autochthonous organic carbon in large and shallow Lake Wuliangsu based on distribution patterns and δ13C signatures of n-alkanes
Abstract. Identification and quantification of allochthonous and autochthonous organic carbon (OC) are crucial for the interpretation of burial behaviors of sedimentary OC of shallow lakes under anthropogenic interferences. In this study, we analyzed distribution patterns and δ13C signatures of mid- and long-chain n-alkanes on various types of surface samples from a typically large and shallow Lake Wuliangsu in the Hetao Irrigation District. The results indicate that n-alkanes among submerged macrophytes, emergent plants, and riverine soil show unique distribution patterns and δ13C signatures, supporting the practicability of quantification on OC of these sources by end-member mixing models. Introducing the δ13C values of n-alkanes into the end-member mixing models could effectively reduce the potential error derived from end-member determination on n-alkane distribution patterns and OC degradation. The model results suggest that the riverine sourced OC from the main channel to Lake Wuliangsu has settled down during the southward migration process. Therefore, Lake Wuliangsu serves as an important trap and sink for the allochthonous OC from the Upper Yellow River Reaches. The model results also show a predominate contribution from the autochthonous OC to Lake Wuliangsu (mostly >85 %), with open-water areas dominated by submerged macrophytes and the rest of areas by emergent plants, largely modulated by water transparency, water depth, and nutrient concentrations. Together with previously published tetraether results, we further proposed that areas dominated by submerged macrophytes might be more favorable for heterotrophic anaerobic bacteria and methanogenic archaea, largely due to active recycling processes for the labile OC derived from submerged macrophytes.
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RC1: 'Comment on bg-2023-134', Anonymous Referee #1, 23 Aug 2023
This paper tried to use n-alkane proxies to reconstruct the contribution of different sources to sedimentary OM, which is significant for studying carbon cycling and carbon burial in lakes. The results from these proxies are generally credible and consistent with the current lake ecosystem systems and the natural background of the lake. This paper was well organized and written. I mainly suggested a further comparison of the results with those deduced from gross OM proxies (mainly gross OM carbon isotope).
Line 30, the shallow lakes are generally defined as the mixing depth is larger than the maximal depth.
Line 45, will phytoplankton contribute to the in situ productivity?
Line 50, Zhu et al., 2018, Li et al., 2019, He et al., 2023. These are not original references about the function and carbon cycling associated with submerged macrophytes, please also adding the initial references.
Line 120，figure 1b should be added after the description of the aquatic macrophytes.
Line 120, all the species of submerged/emergent macrophytes and their relative proportions should be provided.
Line 130, why these two types of macrophytes are selected as representations?
Line 155, please give references for calculating CPI and Paq, even the detailed method was given in supplementary material.
Line 175，the data 33.73% and 37.08% are from ? or why you can get these data?
Line 190, this conclusion should be further evaluated as the n-alkanes might be selectively enriched relative to other OM fraction, resulting in a higher abundance in sediments than in biomass.
Why the abundances of short-chain n-alkanes were not considered in section 3.2?
Line 205, our previous works reported the n-alkane distribution in surface sediments from 30 lakes in middle and lower Yangtze River basin (Environmental Science and Pollution Research 26, 22472-22484, 2019; Organic Geochemistry 122, 29-40, 2018), you can make a comparison with your work.
Line 235, the first sentence in section 3.4 indicated the isotopic compositions of n-alkanes in aquatic macrophytes or sediments?
Line 250, the figure caption indicating sediment samples, but it seems the figure contained macrophyte samples.
Line 295, the calculation methods stated here should be included in the main text.
In 4.2, how about using the gross OM isotope to calculate the contribution from different sources, as the isotopes of all potential contributors can be easily obtained.
A further improvement of the English is needed in some cases.
Citation: https://doi.org/10.5194/bg-2023-134-RC1 - AC1: 'Reply on RC1', Yuxin He, 02 Sep 2023
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RC2: 'Comment on bg-2023-134', Anonymous Referee #1, 04 Sep 2023
The authors have considered all my suggestions in the revised manuscript. I suggested an accept if the editor is satisfied with the English quality in this paper.
Citation: https://doi.org/10.5194/bg-2023-134-RC2 -
AC3: 'Reply on RC2', Yuxin He, 30 Sep 2023
Thank you and we will polish the language in the revised manuscript to ensure the English quality.
Citation: https://doi.org/10.5194/bg-2023-134-AC3
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AC3: 'Reply on RC2', Yuxin He, 30 Sep 2023
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RC3: 'Comment on bg-2023-134', Anonymous Referee #2, 07 Sep 2023
Lacustrine sequences are important archives for past climate changes and anthropogenic activities. Over the past decades, the molecular and carbon isotope compositions have been widely applied for lacustrine sediments. An quite important prerequisite for the paleo applications is the reasonable source identification and proxy interpretation. This study aimed to quantitatively assess the contribution of allochthonous and autochthonous to the surface sediment of a shallow lake. They collected surface sediments from lake and drainage channel, as well as previously published plant data, and built three end-member models. Data in this study is interesting and the major findings will be surely welcomed by readers from the filed of organic geochemistry and paleoclimate. Nonetheless, this version of the paper needs significant improvement in terms of both its language and scientific content before it can be accepted for publication.
I strongly suggest the authors move the model equation to the main text and provide an extensive discussion. Additionally, the definition of femer, friv and fsub is missing, making it difficult to evaluate the accuracy of model #3, which appears to be too simplistic. Furthermore, the representativeness of riverine sediment is a concern since only one sample was analyzed, yet the results indicate a high proportion of riverine OC. The effect of such a major weakness on the uncertainty should be discussed.
This manuscript has too many objectives that are overly ambitious yet not well-supported. Additionally, some objectives are not well-aligned with the title of the manuscript. Furthermore, the equation used to estimate the CPI and carbon isotope for each homolog is overly complex, and the idea that the carbon isotope composition of a single alkane is dependent on the percentage in the total n-alkanes is strange. I suggest that the current subsection 4.4 be removed, as it is beyond the scope of source identification and quantification. Moreover, the correlation between TOC and lipid ratio cannot be interpreted as a causal relationship between OC content and bacterial activities. It is more likely that the OC content is the result of heterotrophic decomposition. Furthermore, it is difficult to confidently assert that the lake is anaerobic without providing evidence. These interesting issues should be explored further, but they should not all be included in one paper without adequate support.
The introduction section lacks a clear logic, making it difficult for readers to understand the advances and knowledge gaps in this field from a broad perspective. The current version is too narrow and hard to comprehend.
Mistakes are frequently found in this document. Here are some examples.
The sample number labeled in Fig. 1 is inconsistent with the text and the other figures. In addition, the rationale to distinguish the offshore lake sediments and other lake sediments is absent.
L90-91: please check the data in Collister et al (1994). In addition, here you need to tell the results of n-alkanes rather than the bulk OM. Other reference citation should be also checked. In the supplement, you cited Eglinton and Hamilton (1967) for the calculation of CPI. This paper did not discuss CPI.
L125: specify the 3-cm as the depth. Similar problem occurs in L129.
L142-143: carbonate
Figure 3: Citation for the plant data is required. In addition, please tell the plant numbers and show the error bar at least for a and b.
L541: delete ‘and’ after Y.H.
Citation: https://doi.org/10.5194/bg-2023-134-RC3 - AC2: 'Reply on RC3', Yuxin He, 27 Sep 2023
Status: closed
-
RC1: 'Comment on bg-2023-134', Anonymous Referee #1, 23 Aug 2023
This paper tried to use n-alkane proxies to reconstruct the contribution of different sources to sedimentary OM, which is significant for studying carbon cycling and carbon burial in lakes. The results from these proxies are generally credible and consistent with the current lake ecosystem systems and the natural background of the lake. This paper was well organized and written. I mainly suggested a further comparison of the results with those deduced from gross OM proxies (mainly gross OM carbon isotope).
Line 30, the shallow lakes are generally defined as the mixing depth is larger than the maximal depth.
Line 45, will phytoplankton contribute to the in situ productivity?
Line 50, Zhu et al., 2018, Li et al., 2019, He et al., 2023. These are not original references about the function and carbon cycling associated with submerged macrophytes, please also adding the initial references.
Line 120，figure 1b should be added after the description of the aquatic macrophytes.
Line 120, all the species of submerged/emergent macrophytes and their relative proportions should be provided.
Line 130, why these two types of macrophytes are selected as representations?
Line 155, please give references for calculating CPI and Paq, even the detailed method was given in supplementary material.
Line 175，the data 33.73% and 37.08% are from ? or why you can get these data?
Line 190, this conclusion should be further evaluated as the n-alkanes might be selectively enriched relative to other OM fraction, resulting in a higher abundance in sediments than in biomass.
Why the abundances of short-chain n-alkanes were not considered in section 3.2?
Line 205, our previous works reported the n-alkane distribution in surface sediments from 30 lakes in middle and lower Yangtze River basin (Environmental Science and Pollution Research 26, 22472-22484, 2019; Organic Geochemistry 122, 29-40, 2018), you can make a comparison with your work.
Line 235, the first sentence in section 3.4 indicated the isotopic compositions of n-alkanes in aquatic macrophytes or sediments?
Line 250, the figure caption indicating sediment samples, but it seems the figure contained macrophyte samples.
Line 295, the calculation methods stated here should be included in the main text.
In 4.2, how about using the gross OM isotope to calculate the contribution from different sources, as the isotopes of all potential contributors can be easily obtained.
A further improvement of the English is needed in some cases.
Citation: https://doi.org/10.5194/bg-2023-134-RC1 - AC1: 'Reply on RC1', Yuxin He, 02 Sep 2023
-
RC2: 'Comment on bg-2023-134', Anonymous Referee #1, 04 Sep 2023
The authors have considered all my suggestions in the revised manuscript. I suggested an accept if the editor is satisfied with the English quality in this paper.
Citation: https://doi.org/10.5194/bg-2023-134-RC2 -
AC3: 'Reply on RC2', Yuxin He, 30 Sep 2023
Thank you and we will polish the language in the revised manuscript to ensure the English quality.
Citation: https://doi.org/10.5194/bg-2023-134-AC3
-
AC3: 'Reply on RC2', Yuxin He, 30 Sep 2023
-
RC3: 'Comment on bg-2023-134', Anonymous Referee #2, 07 Sep 2023
Lacustrine sequences are important archives for past climate changes and anthropogenic activities. Over the past decades, the molecular and carbon isotope compositions have been widely applied for lacustrine sediments. An quite important prerequisite for the paleo applications is the reasonable source identification and proxy interpretation. This study aimed to quantitatively assess the contribution of allochthonous and autochthonous to the surface sediment of a shallow lake. They collected surface sediments from lake and drainage channel, as well as previously published plant data, and built three end-member models. Data in this study is interesting and the major findings will be surely welcomed by readers from the filed of organic geochemistry and paleoclimate. Nonetheless, this version of the paper needs significant improvement in terms of both its language and scientific content before it can be accepted for publication.
I strongly suggest the authors move the model equation to the main text and provide an extensive discussion. Additionally, the definition of femer, friv and fsub is missing, making it difficult to evaluate the accuracy of model #3, which appears to be too simplistic. Furthermore, the representativeness of riverine sediment is a concern since only one sample was analyzed, yet the results indicate a high proportion of riverine OC. The effect of such a major weakness on the uncertainty should be discussed.
This manuscript has too many objectives that are overly ambitious yet not well-supported. Additionally, some objectives are not well-aligned with the title of the manuscript. Furthermore, the equation used to estimate the CPI and carbon isotope for each homolog is overly complex, and the idea that the carbon isotope composition of a single alkane is dependent on the percentage in the total n-alkanes is strange. I suggest that the current subsection 4.4 be removed, as it is beyond the scope of source identification and quantification. Moreover, the correlation between TOC and lipid ratio cannot be interpreted as a causal relationship between OC content and bacterial activities. It is more likely that the OC content is the result of heterotrophic decomposition. Furthermore, it is difficult to confidently assert that the lake is anaerobic without providing evidence. These interesting issues should be explored further, but they should not all be included in one paper without adequate support.
The introduction section lacks a clear logic, making it difficult for readers to understand the advances and knowledge gaps in this field from a broad perspective. The current version is too narrow and hard to comprehend.
Mistakes are frequently found in this document. Here are some examples.
The sample number labeled in Fig. 1 is inconsistent with the text and the other figures. In addition, the rationale to distinguish the offshore lake sediments and other lake sediments is absent.
L90-91: please check the data in Collister et al (1994). In addition, here you need to tell the results of n-alkanes rather than the bulk OM. Other reference citation should be also checked. In the supplement, you cited Eglinton and Hamilton (1967) for the calculation of CPI. This paper did not discuss CPI.
L125: specify the 3-cm as the depth. Similar problem occurs in L129.
L142-143: carbonate
Figure 3: Citation for the plant data is required. In addition, please tell the plant numbers and show the error bar at least for a and b.
L541: delete ‘and’ after Y.H.
Citation: https://doi.org/10.5194/bg-2023-134-RC3 - AC2: 'Reply on RC3', Yuxin He, 27 Sep 2023
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