Reconstructing changes in nitrogen input to the Danube-influenced Black Sea Shelf during the Holocene

Neumann, Andreas; van Beusekom, Justus E. E.; Bratek, Alexander; Friedrich, Jana; Möbius, Jürgen; Sanders, Tina; Wolschke, Hendrik; Dähnke, Kirstin

doi:https://doi.org/10.5194/egusphere-2025-1803

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https://doi.org/10.5194/egusphere-2025-1803

Preprints

05 May 2025

| 05 May 2025

Reconstructing changes in nitrogen input to the Danube-influenced Black Sea Shelf during the Holocene

Andreas Neumann, Justus E. E. van Beusekom, Alexander Bratek, Jana Friedrich, Jürgen Möbius, Tina Sanders, Hendrik Wolschke, and Kirstin Dähnke

Abstract. The western Black Sea shelf, where Danube River contributes the largest river discharge into the Black Sea, is particularly sensitive to river-induced eutrophication, which peaked in the 1980s and 1990s due to human-induced nutrient input. Nutrient input to the western Black Sea shelf and eutrophication is decreasing since the mid-1990s due to the collapse of eastern European economies after 1989 and ongoing mitigation measures to reduce nutrient emissions. The assessment of nutrient inputs to the Black Sea prior to the 1960s however is complicated by the scarcity of information on earlier Danube nutrient loads. Thus, to define what pristine conditions have looked like to provide a reference for nutrient reduction targets remains challenging. In this study, we aim to trace modern and historical nitrogen sources to the western Black Sea Shelf during the last ~5,000 years with special focus on the past 100 years, using sedimentary records of TOC, TIC, nitrogen, and δ¹⁵N.

Our results demonstrate that the balance of riverine nitrogen discharge into the Black Sea on the one hand, and nitrogen fixation in the pelagic system on the other, seem largely determined by climate effects. Specifically, this balance of riverine N input and N fixation is not only controlled by the concentration of nutrients discharged by rivers, but also by the freshwater volume, which controls the intensity of thermohaline stratification and thereby the timing and intensity of nutrient recycling from the deep basin back into the euphotic epipelagic. Based on analytical data of geochemical and isotopic properties of dated sediment cores, we identified a gradient from the nearshore sediment directly at the Danube Delta, where riverine N is dominant to offshore sediment in 80 m water depth, with pelagic N fixation being dominant in the past. Our results based on stable isotopes also demonstrate the increased deposition of nitrogen from human activities in all stations across the shelf and the concomitant changes in deposition rates of organic matter as indication for perturbations in the epipelagic community due to the human-induced eutrophication. Finally, our stable isotope data indicate that human-induced eutrophication can be traced back to the 12^th century CE, which raises the question which point in time is a feasible reference for nutrient reduction goals as the Danube nutrient loads was not pristine since at least in the 800 years.

Received: 18 Apr 2025 – Discussion started: 05 May 2025

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Andreas Neumann, Justus E. E. van Beusekom, Alexander Bratek, Jana Friedrich, Jürgen Möbius, Tina Sanders, Hendrik Wolschke, and Kirstin Dähnke

Status: final response (author comments only)

RC1: 'Comment on egusphere-2025-1803', Anonymous Referee #1, 03 Jun 2025

Neumann et al. present a multi-proxy study investigating nitrogen inputs to the northwestern Black Sea Shelf, based on geochemical and isotopic analyses of 4 sediment cores spanning the past ~5000 years. Specifically, the authors use sediment δ¹⁵N and bulk C and N content to infer changes in nitrogen sources (riverine vs N-fixation) and to trace the onset of anthropogenic eutrophication. The study claims, based on linear interpolation of DIN values, that human-induced eutrophication of the Danube began around the 12^th century. While I think that the study could be of interest to Biogeosciences readers and that the methodology used was generally sound and does support the author’s conclusions, I also think that major revisions are necessary to bring the manuscript to publishable form.
General comments:
Overall, the manuscript seems to me to be a bit “premature.” There are a number of spelling and grammar mistakes, as well as some sentences that read quite awkwardly. I will make some suggestions for improvements in my later comments, but these are not exhaustive. Furthermore, at least two references are cited in the manuscript but not included in the bibliography (e.g., Stuiver et al. 1998 – line 131, Siani et al. 2000 – line 132) and the formatting of the bibliography is very inconsistent.
Beyond the cosmetic issues, I found the manuscript somewhat difficult to follow in that, while there is a stated research aim of identifying natural and anthropogenic nitrogen sources over the past 5000 years, the discussion started with a section of sedimentary signatures of major events in the Black Sea. The abstract also mentioned the difficulty of determining a pristine reference state for nutrient reduction, but this idea was not fully developed in the manuscript. I would recommend breaking down the overarching research aim into smaller sub-aims or sub-questions to help organize the discussion more clearly.
I also miss some reference to the novelty of the study. There has already been a lot of work published on nitrogen inputs into the Black Sea and eutrophication of the Danube. Indeed, in the conclusions, the authors acknowledge that many of the results are confirming previous findings. I do think that the finding of early eutrophication of the Danube is novel but that this can be more clearly stated.
Finally, I have some methodological concerns that I will describe in detail in the “Specific comments.”
Specific comments:
Abstract:
Line 30: “This raises the question of what point in time could serve as a realistic reference for nutrient reduction goals, given that the Danube has not had pristine nutrient levels for at least 800 years.” - This is an interesting point but is not brought up in the manuscript at all. If defining a pristine reference state is one of the study aims, I think that this should be discussed further.
Introduction:
Lines 72-83: Citations needed here.
Methods:
Line 131-133: I am assuming by the reference to Stuiver et al. 1998 that the IntCal98 was used for the calibration. However, in Table 2, it seems that for all the calibrated dates, 500 was just subtracted from the ¹⁴C age. Was that really the result of the calibration? Additionally, what was the rationale for using IntCal98? As there has been many updates to the curve, I do not think it makes sense not to use the most recent version, i.e., IntCal20. However, it would be more appropriate to use Marine20 as that is specifically for marine sediments.
Furthermore, given that linear interpolation assumes constant sedimentation, using a Bayesian approach, such as that of Bacon, could provide a more robust chronology. Given that the manuscript relies heavily on the chronology, the methods here should be brought in line with current practice.
Results:
Line 207: Were these zones identified solely based on observation or were any statistics used?
Line 241-243: The description of the model development should be in the methods section.
Technical comments:
Introduction:
Line 40: Replace “as displayed in” with “as seen”
Line 61: Replace “represents” with “is”
Line 64: Should be Maselli & Trincardi, 2013?
Line 68: Replace “measurements” with “measures”
Methods:
Line 100: Station 2 depth is 27 m in Table 1 but 22 m here. Which is it?
Line 118: Replace “better” with “less”
Line 127 and later: Italicize scientific names
Discussion:
Line 312: Should be Fulton et al. 2012?
References:
Ensure all references have the same style of formatting.

Citation: https://doi.org/10.5194/egusphere-2025-1803-RC1
RC2:
'Comment on egusphere-2025-1803', Anonymous Referee #2, 14 Jul 2025

Neumann et al. reports the modern and historical nitrogen sources to the western Black Sea Shelf during the last 5000 years using sedimentary records of TOC, TIC, nitrogen, and δ¹⁵N. Their analysis of field samples revealed that climate significantly influences riverine nitrogen discharge and offshore nitrogen fixation. Additionally, the study highlighted the importance of freshwater volume in controlling vertical stratification and nutrient transport within the water column. The stable isotope results specifically indicated that increased nitrogen deposition from human activities and human-induced eutrophication began as early as the 12th century CE.

This is a strong manuscript. It's well-organized and clearly written. The results are thoroughly analyzed, and the conclusions are generally well-supported. I only have a few minor comments.

Fig. 1: It would be helpful to include a sub-map that shows the study area within a broader geographical context.
Line 194: seems TOC/N ratio is higher than 8 at 35 cm depth, better check it
Line 200: at station 6, ‘the organic carbon and nitrogen content decreases in the upper 3 to 7 cm of the cores’, which is not the case from the figure. Better check it.
Line 203: It’s hard to see the trends, better to provided detailed numbers or fitting lines
Line 210: I recommend the authors mark the four zones in Fig. 3, which would be helpful to understand Fig. 4

Citation: https://doi.org/10.5194/egusphere-2025-1803-RC2
- AC1: 'Reply on RC2', Andreas Neumann, 03 Sep 2025
  
  Thanks for reading our manuscript and pointing out details that can be further improved. We have copied-in your comments in bold and italics, and our reply is following directly after each comment
  >> Fig. 1: It would be helpful to include a sub-map that shows the study area within a broader geographical context.
  We agree and will add a sub map to make it easier to put the study area into a broader geographical context.
  >> Line 194: seems TOC/N ratio is higher than 8 at 35 cm depth, better check it
  We agree, this is indeed an error. We will change the sentence to “The molar TOC / N ratio decreased from 15 at the sediment surface to 8 at 18 cm sediment depth, and slightly increased to 10 at 35 cm.”
  >> Line 200: at station 6, ‘the organic carbon and nitrogen content decreases in the upper 3 to 7 cm of the cores’, which is not the case from the figure. Better check it.
  We agree that this part is not very clear. We will change it to: “At stations 4 and 6, the organic carbon and nitrogen content decreased from the surface down to 7 cm depth in core 4, and down to 3 cm depth in core 6, respectively (Fig. 3). The TOC content of these cores was in the range of 1.1 to 4.0 %, and the total N content was in the range of 0.10 to 0.42 %. (Fig. 3)”
  >> Line 203: It’s hard to see the trends, better to provided detailed numbers or fitting lines
  We would like to not add additional elements such as trend lines to the already quite condensed Figure 3. Instead, we suggest to change the text to: “In contrast, the molar TOC / N ratios decreased with sediment depth at station 4 from a TOC/N ratio of 12.1 at the surface to 8.5 in 27 cm sediment depth. At station 6, the TOC/N ratio slightly increased from a value of 11.0 at the surface to a value of 12.4 at 25 cm depth (Fig. 3 K, L).”
  >> Line 210: I recommend the authors mark the four zones in Fig. 3, which would be helpful to understand Fig. 4
  We agree and will mark the four Zones in Fig. 3 by adding dashed horizontal lines and corresponding labels to the sub-plots. However, this will also include to present Figure 4 and the associated text first, where we introduce the 4 Zones. If it is not an issue that we present derived data such as the four Zones (Fig. 4) before we present the profiles data first as an introduction, then we can make the changes as suggested.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1803-AC1
RC3:
'Comment on egusphere-2025-1803', Anonymous Referee #3, 19 Jul 2025

This paper made an interesting reconstruct of N loading in Black Sea Shelf region and some major climate events using N isotope and C and N content in sedimentary records. A ~0.8 ky BP onset of eutrophication in the study regions is suggested by authors. I think the method used here is mostly sound. And data and results are interesting and will attract the readers of Biogeosciences. However, the presenting and discussion of this manuscript in current form are preventing readers from better understanding the core story (see following comments).
Specific comments:
Introduction:
Containing large anoxic water body, Black Sea is likely affected by water column denitrification (WCD) and other N loss processes. WCDs can critically affected N isotope signals and nutrient pool and is likely to affect N sources analysis. The authors should include Black Sea WCD in the Introduction and Discussion section.
Another concern is the novelty. Fulton et al., 2012 did some similar analysis regarding N fixation analysis in Black Sea with similar proxies. Authors should clarify what is the major novelty compared with Fulton et al., 2012 and prior research.
Line 73-75 Define delta N isotope notation first.
Methods section:
Section 2.4 and 2.5: The age calculation methods based on Pb and Cs isotope should also be included in methods sections.
Results section:
Section 3.1: I did not see the age results from the Pb/Cs isotopes. I think the dating results for station 1 and 2 should also be included in Figure/Table.
Section 3.2: I suggest adding more subsections here.
Line 234 Figure 5: How is the f calculated? Should be included in method section.
Section 3.3: I think this section is more suitable as a part of discussion section.
Discussion:
Section 4.1:
This section is particularly hard to follow. The raw data in Figure 3 are hard to connect to these events described here. I think at least a time series plot should be presented here (e.g., like fig.3 in Fulton et al., 2012). And the major events/periods should be marked in figures.
Line 265: Consider briefly explaining what 'Unit IIb' and other jargons refers to, as not all readers may be familiar with this regional stratigraphic nomenclature.
Line 276: Why did no detect of H₂S agree with time frame of zone 2 samples?
Section 4.2
This is a very long section discussing about N sources. It could be broken down into subsection according to time periods. And I am wondering the N loss processes’ impacts on N isotopic signals.
Section 4.3
The section aims to discussion the age offsets in Inorganic ¹⁴C and has little connection in core topic. I think these contents can be moved to age result section.
Typo note: There are a lot of typos. E.g., subscript/superscript typos in lines 130, 197, 206, 210 and so on.

Citation: https://doi.org/10.5194/egusphere-2025-1803-RC3
- AC2: 'Reply on RC3', Andreas Neumann, 03 Sep 2025
  
  Thanks for reading our manuscript and pointing out details that can be further improved. We have copied-in your comments in bold and italics, and our reply is following directly after each comment.
  >> Containing large anoxic water body, Black Sea is likely affected by water column denitrification (WCD) and other N loss processes. WCDs can critically affected N isotope signals and nutrient pool and is likely to affect N sources analysis. The authors should include Black Sea WCD in the Introduction and Discussion section.
  We will add water column denitrification as a significant process of nitrogen turnover to the discussion. We will further add references to the studies of Moebius & Daehnke 2015 (Limnology & Oceanography, https://doi.org/10.1002/lno.10068) who demonstrated that most of the Danube N load is assimilated into plankton biomass and not transported as DIN within the Danube River Plume. As part of organic matter, the assimilated N is much more stable, especially in oxygen-deficient environments. We thus argue that water column denitrification had no dominant impact on the isotopic composition of the organic matter in the shelf sediment.
  >> Another concern is the novelty. Fulton et al., 2012 did some similar analysis regarding N fixation analysis in Black Sea with similar proxies. Authors should clarify what is the major novelty compared with Fulton et al., 2012 and prior research.
  It is correct that Fulton et al. 2012 did a similar analysis. In our study we use Fulton et al. 2012 as a reference for our own results to ensure that our own results are consistent with previous observations. A novelty of our study is that we sampled the upper shelf at the Danube Delta while Fulton et al. have sampled the deep basins. While the sediment record of the deep basins integrates signals over the whole Black Sea, this is not necessarily the case in our study area where the riverine signals can be dominant over basin wide signals. The most significant novelty is the reconstruction of Danube N loads with a high temporal resolution for the past 200 years, which was not attempted by Fulton et al. 2012. And since we present a gradient from high influence of Danube to low influence, it is now possible to select future study sites depending on whether signals from the Danube or the Black Sea are to be observed.
  We will make this clearer in the respective sections of introduction and discussion.
  >> Line 73-75 Define delta N isotope notation first.
  We agree and will add a short introduction of the notation first.
  >> Section 2.4 and 2.5: The age calculation methods based on Pb and Cs isotope should also be included in methods sections.
  We did not present the method for age calculation based on Pb and Cs for two reasons: 1) We used the results from Constantinescu et al. (2023) for the stations 1 and 2, and chose to refer to the original study to keep our manuscript concise. 2) We did not attempt to apply an age model for the results of core 4 as the sedimentation rate is apparently already so low that the unsupported, short-lived isotope Pb-210 is basically decayed within the first few cm sediment layer. We thus use the low penetration depth of unsupported Pb-210 and Cs-137 as an indication that the deposition of isotopically enriched N started much earlier than the start of industrialization and that the apparent deep occurrence of enriched N is not simply the result of burial of modern N by e.g. bioturbation, because otherwise modern Pb-210 and Cs-137 would also be present and those depths. In the revision, we will make this clearer.
  >> Section 3.1: I did not see the age results from the Pb/Cs isotopes. I think the dating results for station 1 and 2 should also be included in Figure/Table.
  We did not present explicit age results for core 4 as we did not attempt to derive an Pb/Cs based age model for core 4. The reason is that the modern, unsupported Pb-210 and Cs-237 are decayed already in the uppermost sediment layer, and we concluded that an age model would not be feasible. Instead, we use the low penetration depth of unsupported Pb-210 and Cs-137 as an indication that the deposition of isotopically enriched N started much earlier than the start of industrialization and that the apparent deep occurrence of enriched N is not simply the result of burial of modern N by e.g. bioturbation, because otherwise the co-occurring modern Pb-210 and Cs-137 would also be present and those depths. In the revision, we will make this clearer.
  We chose to not present the age results from cores 1 and 2 as we used the results of Constantinescu et al. (2023) as stated in the method section and the results section. We would like to not show the results of Constantinescu et al. (2023) to prevent any confusion of the reader and to not evoke the impression that these are our own results.
  >> Section 3.2: I suggest adding more subsections here.
  We agree and will restructure the section by adding more subsections to increase the readability.
  >> Line 234 Figure 5: How is the f calculated? Should be included in method section.
  From the subset of the date that we have assigned to Zone 2, we used the largest value of the total N content as the reference for the calculation of f, which consequently plots at the coordinate origin (0, 0) of the plots. We will add a brief explanation to the methods section to make this clearer.
  >> Section 3.3: I think this section is more suitable as a part of discussion section.
  We disagree because the results of the correlation analysis and the resulting plots have to be presented. However, our usage of the word “reconstruct” in the section (L 243, L 245) does indeed signal an interpretation and thus belong to the discussion section. We will split this section and shift the following part to the discussion:
  “The N content-based Models 1 and 2 reconstruct the observed DIN loads of the 1955 – 2015 period reasonably well (Fig. 6 B, D). The δ15N-based Model 3 reconstructs the observed DIN load less accurately (Fig. 6 F). For the period 1800 – 1950, all three models reconstructed that the Danube DIN load was 236 to 318 kt / yr in 1800 CE”
  The last sentence “For the period 1800 – 1950, all three models reconstructed that the Danube DIN load was 236 to 318 kt / yr in 1800 CE and increased gradually with 0.2 to 0.5 kt / yr2” will remain in the results part.
  >> Section 4.1: This section is particularly hard to follow. The raw data in Figure 3 are hard to connect to these events described here. I think at least a time series plot should be presented here (e.g., like fig.3 in Fulton et al., 2012). And the major events/periods should be marked in figures.
  We think that this is a good idea and will add a timeseries figure with marks for the events we mention in this discussion section.
  >> Line 265: Consider briefly explaining what 'Unit IIb' and other jargons refers to, as not all readers may be familiar with this regional stratigraphic nomenclature.
  We agree and will follow this suggestion.
  >> Line 276: Why did no detect of H2S agree with time frame of zone 2 samples?
  Organic matter is remineralised faster in oxic conditions than in anoxic conditions, and the Rayleigh plots in Fig. 5 indicate kinetic fractionation that is characteristic for oxic remineralisation. The absence of a H2S proxy supports this conclusion as the presence of H2S in the photic zone would also imply oxygen-free bottom water. We will add this explanation to the discussion.
  >> Section 4.2 This is a very long section discussing about N sources. It could be broken down into subsection according to time periods. And I am wondering the N loss processes’ impacts on N isotopic signals.
  We agree to split this section according to time periods as suggested. With respect to the effect of N-loss processes on the N isotopic signal, we already discuss how oxygen exposure enhances remineralisation (e.g.line 341 ff) and how euxinic conditions preserve biomass and the N-signature within (e.g. line 304 ff).
  >> Section 4.3 The section aims to discussion the age offsets in Inorganic 14C and has little connection in core topic. I think these contents can be moved to age result section.
  We agree and will move this section to the age results part.
  
  >> Typo note: There are a lot of typos. E.g., subscript/superscript typos in lines 130, 197, 206, 210 and so on.
  These typos unfortunately escaped our attention, and we will correct the typos in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1803-AC2

Andreas Neumann, Justus E. E. van Beusekom, Alexander Bratek, Jana Friedrich, Jürgen Möbius, Tina Sanders, Hendrik Wolschke, and Kirstin Dähnke

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Short summary

The North-Western shelf of the Black Sea is substantially influenced by the discharge of nutrients from River Danube. We have sampled the sediment there and measured particulate carbon and nitrogen to reconstruct the variability of nitrogen sources to the NW shelf. Our results demonstrate that the balance of riverine nitrogen input and marine nitrogen fixation is sensitive to climate changes. Nitrogen from human activities is detectable in NW shelf sediment since the 12^th century.


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