the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2023
Distinct oxygenation modes of the Gulf of Oman during the past 43,000 years – a multi-proxy approach
Abstract. Climatic conditions and its change shape the strength and extent of the oxygen minimum zone (OMZ). The presence and variability of the OMZ in the Arabian Sea is of importance for their ecosystem. The state of oxygenation has, for instance, an impact on the pelagic and benthic faunal community or the nitrogen and carbon cycles. The understanding of the dynamics of the OMZ, its marine environmental is of importance due to its climate feedbacks. In this study, we combined three independent proxies to reconstruct the oxygenation state of the water column and bottom water in the Gulf of Oman for the past about 43 ka for the first time. We used nitrogen isotopes (δ15N) as well as the alkane ratio (lycopane + n-C35)/n-C31 and benthic foraminiferal faunal analysis to reconstruct the strength of the OMZ in the water column and bottom water oxygenation, respectively. Our results show that the Gulf of Oman experienced strong pronounced OMZ and bottom water deoxygenation during the Holocene. Contrary, during the Last Glacial Maximum (LGM)/ Marine Isotope Stage (MIS) 2 the Gulf of Oman was very well ventilated with a highly diverse benthic foraminiferal community. This may have been caused by stronger wind-induced mixing and better ventilation by oxygen-rich water masses. Our results also show moderate oxygenation during MIS 3 with deoxygenation events during most of the warmer Dansgaard-Oeschger (D/O) events. We propose two distinct oxygenation modes for the Gulf of Oman: 1) a stable period of either strongly pronounced water column OMZ and bottom water deoxygenation or well-oxygenated water column and bottom water conditions and 2) an unstable period of oscillating oxygenation states between moderately oxygenated (stadials) and deoxygenated (interstadial, D/O events) conditions.
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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.
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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.
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Interactive discussion
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RC1: 'Comment on egusphere-2023-2664', Anonymous Referee #1, 23 Dec 2023
Burdanowitz and co-authors present a study exploring the OMZ dynamics in the Gulf of Oman across the past 43,000 years. They employ a multi-proxy approach, using bulk sedimentary nitrogen isotopes, n-alkane ratios, and benthic foraminiferal analysis to reconstruct Oxygen Minimum Zones and bottom water oxygen conditions during different climatic periods. From this, the authors gain insights into varied oxygenation modes of the Gulf of Oman, delineating stable periods characterised by either pronounced OMZ or well-oxygenated conditions and unstable phases exhibiting oscillating oxygenation.
The manuscript is well written. The data is well documented and presented. The references are up-to-date.
While the paper provides valuable insights into the oxygenation dynamics of the Gulf of Oman (and Arabian Sea), there are some potential limitations and caveats to consider:
The main points of the study come from the d15N records, thus a more in-depth discussion of the robustness of the proxy would be good. For example the authors could show total organic nitrogen in a supplementary plot as a time series, but also as a cross plot with d15N to make sure none of the signal is driven by preferential degradation/ secondary overprints in the water column.
How do the authors know that d15N was a local signal and was not transported from further south/ further north? How do the core sites nitrate d15N look like nowadays? Maybe check Martin and Casciotti, 2017. Is the offset between the sites also observed/maintained in the past? Could a changing offset mean less/more water mass transport/mixing of d15N signal or is it all attributed to local changes in water column denitrification (in connection with O2)?
Minor comments:
Text:
Line 11-12: This sentence doesn’t make sense. Do you mean: The understanding of the dynamics of the OMZ, its marine environmental is of importance due to its climate feedbacks.
Line 14: “for the first time”: oxygenation has been reconstructed in previous studies, what part of this is novel?
Line 15: “bulk sedimentary nitrogen isotopes”
Line 23: Mention the proposed mechanism for the unstable period of oscillating oxygenation
Line 29: N20 -> N2O
Line 34: how does it affect the diversity? making it more/less diverse?
Line 41: what does a “dry” Persian Gulf mean —> exposed due to lower sea level?
Line 57: Add a sentence about why the d15N is elevated with denitrification. Use a more denitrification/d15N specific reference, e.g. Brandes et al., 1998, Cline and Kaplan, 1975 or review by Capone et al., 2008
Line 74: strictly speaking the location is at the rim of the Gulf of Oman, why not calling it Oman margin? Can you show somewhere that hydrographically the Gulf of Oman (rim) is distinct from the Oman margin.
Line 86: Based on their depths, wouldn’t the RSW also count as intermediate waters?
Lines 91-99: when talking about productivity and seasons add the panels in refs for Figure 2. e.g. “In contrast, highest productivity in the northern Arabian Sea and Gulf of Oman is observed during the winter season (Figure 2d).
Line 110: which planktonic foraminifera?
Line 145: add references for ACL and CPI.
Line 155: add how oxygen concentration would affect diversity index.
Line 175: The authors initially discuss the lipid biomarker results, thus I would suggest to flip the order in the methods section (first describe lipid biomarkers and then d15N).
Line 185: also add a results section about the long-term trend of lycopane and refer to Fig. 5d
Line 187: did you also quantify TON to test the robustness of d15N?
Line 201+: This section would benefit from a few subtitles to structure the discussion a bit better and to help the reader to follow the argumentation.
Line 205: D/O 6 and 7 might not be observed in bottom water reconstruction due to lower resolution?
Line 205: “most intense” in what direction? Maybe choose another word for this?
Line 206: superscript -1
Line 206: highlight Younger Dryas with a bar in Fig. 5.
Line 214: Consider moving RC27 to panel b instead of c to be in order of discussion.
Line 244: start new paragraph
Line 244: “Gulf of Oman d15N record”
Line 246: “was not present”
Line 322: What does LTG stand for? latitudinal insolation gradient? in the line below it is abbreviated as LIG
Figures:
Fig. 1: a) source for directions of wind pattern? satellite?
b) is this an annual average or seasonal data?
Fig. 3: 14C (superscript 14)
Fig. 4: lycopane instead of lycopene
Fig. 5: brown line in (d) appears to be grey.
why is arrow with OMZ not extended to panel a?
Fig. 6: what does “mostly based on …” mean?
space missing after Fleitmann et al. (2009).
It would help the reader to either have a small map inserted in this figure with the core locations or add “northern Arabian Sea/ Somali Upwelling/ Oman Upwelling/ Gulf of Oman” to the core sites.
Fig. 7&8: Can the authors describe the colours in panel b?
Table:
Table 1: superscript 14 in 14C
Citation: https://doi.org/10.5194/egusphere-2023-2664-RC1 - AC1: 'Reply on RC1', Nicole Burdanowitz, 24 Jan 2024
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RC2: 'Comment on egusphere-2023-2664', Anonymous Referee #2, 03 Jan 2024
The paper by Burdanowitz et al. aims to reconstruct oxygen conditions in the Gulf of Oman over the last 45,000 years using a multiproxy approach (bulk sediment δ15N, lipid biomarker analysis, and a benthic foraminifera index). The authors propose two distinct modes of oxygenation during this time, with the Holocene reflecting stable conditions, and other periods (e.g., MIS 3) displaying a high degree of fluctuations between oxygenated and deoxygenated conditions. The research contributes to our understanding of the connections between bottom water oxygenation in the region and global climate events.
The authors employ established methodology to produce a robust, high resolution record, and they present their findings in a cohesive manner. Overall, the manuscript is well-written, engaging, and well-organized, and is within the scope of the journal. I present some minor suggestions where additions may help clarify some ideas.
Major points:
Point 1: the δ15N sedimentary record is good, but the caveats of the method should at least be mentioned. Robinson et al. (2012) addresses when it is appropriate to use this method, versus when degradation of material becomes and important factor. The shallow water depth of the core used in this study is one factor that argues against alteration of the signal, but a time period of low accumulation could account for some changes in δ15N and should be addressed. Other useful references are Tesdal et al., (2012) and Junium et al., (2015).
Point 2: the use of acronyms in the manuscript was confusing. Some acronyms were introduced (e.g., EBFOI) only for them to be written in full several paragraphs later. Other acronyms were only introduced in the last paragraph of the discussion (LTG and LIG, lines 322 and 323) and were not necessary. Some acronyms were not written out in full at all (ISM, line 265; AS, line 245). My suggestion would be to limit acronym use to terms that are frequently used throughout the paper, and to write out the full names of indices/events that are referred to only once or twice.
Minor points and typos:
Line 9: “Climatic conditions and its change” -> “Climatic conditions can change”
Line 10: “for their ecosystem” -> “for its ecosystem”
Line 17: “Contrary -> “In contrast”
Line 28: What is the largest sink? OMZs are the largest sink for nitrogen?
Line 29: N2O not N20
Line 59: Why is the isoprenoid hydrocarbon lycopene rarely used? Any reasons you can point to?
Section 2: RSW is also important in the southern Indian Ocean and has been detected in the Agulhas Current region (see Marshall et al., 2023); not necessarily relevant to your paper, but if you want to emphasize the importance of RSW, a sentence or two could be added here.
Line 111: “The age-depth model (Figure 3)”
Section 3.2 and 3.3: Any references used for δ15N and column chromatography methods would be good here.
Line 130: “were” -> “was”
Line 150: “were counted at each depth from representative splits”
Line 155: Could you explain the Shannon Index here briefly in one sentence?
Line 157: A table showing the species and their habitat preference (oxic, suboxic, dysoxic) would be very helpful, even if for the supplemental.
Line 231: “trough” -> “through”
Line 233: “led” -> “lead”
Line 235: This could use a reference: “the sea level was in general lower during MIS 3 than during the Holocene.”
Line 240: “In total, the interplay and the bipolar seesaw structure of the northern and southern hemispheric climate signals may lead to the feature that some of the D/O events as well as Heinrich events are not represented in the record.” This isn’t very clear to me - do you mean the interplay between northern and southern signals? What specific combination of climate factors could result in the dampened signal in the region?
Line 245: You could reference your figure after mentioning the triple peak for the first time, as well as writing which proxies show the triple peak most clearly.
Lines 228 – 277: This paragraph is quite long and puts forward several ideas. Maybe it could be broken up into two or three paragraphs (e.g., break at line 246 or 263).
Line 276: To what does “this” refer to? The fact that no lycopanes were preserved? Why does the presence of this species aid the interpretation (because of its habitat preference)?
Line 289: “environmental” -> “environmentally”
Line 299: Periods just before this were written to one decimal point, and the ones here are written with two; it might be clearer to round these to one point.
Line 308: What does an interstadial AMOC mode mean in this context? A weaker circulation?
Line 310: This phrase sounds odd, as it is not explored further. Perhaps a better way to phrase it would be “this pattern may or may not be the reason for the absence of a strong D/O 6 signal…”
Line 334: the phrasing could be changed - “the very well ventilated conditions” -> “the well-mixed/the strong ventilation/the oxygenated conditions”
Line 344: the phrasing could be changed – “with swinging back and forth oxygen conditions” -> “with fluctuating high and low oxygen conditions.”
Citation: https://doi.org/10.5194/egusphere-2023-2664-RC2 - AC2: 'Reply on RC2', Nicole Burdanowitz, 24 Jan 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2664', Anonymous Referee #1, 23 Dec 2023
Burdanowitz and co-authors present a study exploring the OMZ dynamics in the Gulf of Oman across the past 43,000 years. They employ a multi-proxy approach, using bulk sedimentary nitrogen isotopes, n-alkane ratios, and benthic foraminiferal analysis to reconstruct Oxygen Minimum Zones and bottom water oxygen conditions during different climatic periods. From this, the authors gain insights into varied oxygenation modes of the Gulf of Oman, delineating stable periods characterised by either pronounced OMZ or well-oxygenated conditions and unstable phases exhibiting oscillating oxygenation.
The manuscript is well written. The data is well documented and presented. The references are up-to-date.
While the paper provides valuable insights into the oxygenation dynamics of the Gulf of Oman (and Arabian Sea), there are some potential limitations and caveats to consider:
The main points of the study come from the d15N records, thus a more in-depth discussion of the robustness of the proxy would be good. For example the authors could show total organic nitrogen in a supplementary plot as a time series, but also as a cross plot with d15N to make sure none of the signal is driven by preferential degradation/ secondary overprints in the water column.
How do the authors know that d15N was a local signal and was not transported from further south/ further north? How do the core sites nitrate d15N look like nowadays? Maybe check Martin and Casciotti, 2017. Is the offset between the sites also observed/maintained in the past? Could a changing offset mean less/more water mass transport/mixing of d15N signal or is it all attributed to local changes in water column denitrification (in connection with O2)?
Minor comments:
Text:
Line 11-12: This sentence doesn’t make sense. Do you mean: The understanding of the dynamics of the OMZ, its marine environmental is of importance due to its climate feedbacks.
Line 14: “for the first time”: oxygenation has been reconstructed in previous studies, what part of this is novel?
Line 15: “bulk sedimentary nitrogen isotopes”
Line 23: Mention the proposed mechanism for the unstable period of oscillating oxygenation
Line 29: N20 -> N2O
Line 34: how does it affect the diversity? making it more/less diverse?
Line 41: what does a “dry” Persian Gulf mean —> exposed due to lower sea level?
Line 57: Add a sentence about why the d15N is elevated with denitrification. Use a more denitrification/d15N specific reference, e.g. Brandes et al., 1998, Cline and Kaplan, 1975 or review by Capone et al., 2008
Line 74: strictly speaking the location is at the rim of the Gulf of Oman, why not calling it Oman margin? Can you show somewhere that hydrographically the Gulf of Oman (rim) is distinct from the Oman margin.
Line 86: Based on their depths, wouldn’t the RSW also count as intermediate waters?
Lines 91-99: when talking about productivity and seasons add the panels in refs for Figure 2. e.g. “In contrast, highest productivity in the northern Arabian Sea and Gulf of Oman is observed during the winter season (Figure 2d).
Line 110: which planktonic foraminifera?
Line 145: add references for ACL and CPI.
Line 155: add how oxygen concentration would affect diversity index.
Line 175: The authors initially discuss the lipid biomarker results, thus I would suggest to flip the order in the methods section (first describe lipid biomarkers and then d15N).
Line 185: also add a results section about the long-term trend of lycopane and refer to Fig. 5d
Line 187: did you also quantify TON to test the robustness of d15N?
Line 201+: This section would benefit from a few subtitles to structure the discussion a bit better and to help the reader to follow the argumentation.
Line 205: D/O 6 and 7 might not be observed in bottom water reconstruction due to lower resolution?
Line 205: “most intense” in what direction? Maybe choose another word for this?
Line 206: superscript -1
Line 206: highlight Younger Dryas with a bar in Fig. 5.
Line 214: Consider moving RC27 to panel b instead of c to be in order of discussion.
Line 244: start new paragraph
Line 244: “Gulf of Oman d15N record”
Line 246: “was not present”
Line 322: What does LTG stand for? latitudinal insolation gradient? in the line below it is abbreviated as LIG
Figures:
Fig. 1: a) source for directions of wind pattern? satellite?
b) is this an annual average or seasonal data?
Fig. 3: 14C (superscript 14)
Fig. 4: lycopane instead of lycopene
Fig. 5: brown line in (d) appears to be grey.
why is arrow with OMZ not extended to panel a?
Fig. 6: what does “mostly based on …” mean?
space missing after Fleitmann et al. (2009).
It would help the reader to either have a small map inserted in this figure with the core locations or add “northern Arabian Sea/ Somali Upwelling/ Oman Upwelling/ Gulf of Oman” to the core sites.
Fig. 7&8: Can the authors describe the colours in panel b?
Table:
Table 1: superscript 14 in 14C
Citation: https://doi.org/10.5194/egusphere-2023-2664-RC1 - AC1: 'Reply on RC1', Nicole Burdanowitz, 24 Jan 2024
-
RC2: 'Comment on egusphere-2023-2664', Anonymous Referee #2, 03 Jan 2024
The paper by Burdanowitz et al. aims to reconstruct oxygen conditions in the Gulf of Oman over the last 45,000 years using a multiproxy approach (bulk sediment δ15N, lipid biomarker analysis, and a benthic foraminifera index). The authors propose two distinct modes of oxygenation during this time, with the Holocene reflecting stable conditions, and other periods (e.g., MIS 3) displaying a high degree of fluctuations between oxygenated and deoxygenated conditions. The research contributes to our understanding of the connections between bottom water oxygenation in the region and global climate events.
The authors employ established methodology to produce a robust, high resolution record, and they present their findings in a cohesive manner. Overall, the manuscript is well-written, engaging, and well-organized, and is within the scope of the journal. I present some minor suggestions where additions may help clarify some ideas.
Major points:
Point 1: the δ15N sedimentary record is good, but the caveats of the method should at least be mentioned. Robinson et al. (2012) addresses when it is appropriate to use this method, versus when degradation of material becomes and important factor. The shallow water depth of the core used in this study is one factor that argues against alteration of the signal, but a time period of low accumulation could account for some changes in δ15N and should be addressed. Other useful references are Tesdal et al., (2012) and Junium et al., (2015).
Point 2: the use of acronyms in the manuscript was confusing. Some acronyms were introduced (e.g., EBFOI) only for them to be written in full several paragraphs later. Other acronyms were only introduced in the last paragraph of the discussion (LTG and LIG, lines 322 and 323) and were not necessary. Some acronyms were not written out in full at all (ISM, line 265; AS, line 245). My suggestion would be to limit acronym use to terms that are frequently used throughout the paper, and to write out the full names of indices/events that are referred to only once or twice.
Minor points and typos:
Line 9: “Climatic conditions and its change” -> “Climatic conditions can change”
Line 10: “for their ecosystem” -> “for its ecosystem”
Line 17: “Contrary -> “In contrast”
Line 28: What is the largest sink? OMZs are the largest sink for nitrogen?
Line 29: N2O not N20
Line 59: Why is the isoprenoid hydrocarbon lycopene rarely used? Any reasons you can point to?
Section 2: RSW is also important in the southern Indian Ocean and has been detected in the Agulhas Current region (see Marshall et al., 2023); not necessarily relevant to your paper, but if you want to emphasize the importance of RSW, a sentence or two could be added here.
Line 111: “The age-depth model (Figure 3)”
Section 3.2 and 3.3: Any references used for δ15N and column chromatography methods would be good here.
Line 130: “were” -> “was”
Line 150: “were counted at each depth from representative splits”
Line 155: Could you explain the Shannon Index here briefly in one sentence?
Line 157: A table showing the species and their habitat preference (oxic, suboxic, dysoxic) would be very helpful, even if for the supplemental.
Line 231: “trough” -> “through”
Line 233: “led” -> “lead”
Line 235: This could use a reference: “the sea level was in general lower during MIS 3 than during the Holocene.”
Line 240: “In total, the interplay and the bipolar seesaw structure of the northern and southern hemispheric climate signals may lead to the feature that some of the D/O events as well as Heinrich events are not represented in the record.” This isn’t very clear to me - do you mean the interplay between northern and southern signals? What specific combination of climate factors could result in the dampened signal in the region?
Line 245: You could reference your figure after mentioning the triple peak for the first time, as well as writing which proxies show the triple peak most clearly.
Lines 228 – 277: This paragraph is quite long and puts forward several ideas. Maybe it could be broken up into two or three paragraphs (e.g., break at line 246 or 263).
Line 276: To what does “this” refer to? The fact that no lycopanes were preserved? Why does the presence of this species aid the interpretation (because of its habitat preference)?
Line 289: “environmental” -> “environmentally”
Line 299: Periods just before this were written to one decimal point, and the ones here are written with two; it might be clearer to round these to one point.
Line 308: What does an interstadial AMOC mode mean in this context? A weaker circulation?
Line 310: This phrase sounds odd, as it is not explored further. Perhaps a better way to phrase it would be “this pattern may or may not be the reason for the absence of a strong D/O 6 signal…”
Line 334: the phrasing could be changed - “the very well ventilated conditions” -> “the well-mixed/the strong ventilation/the oxygenated conditions”
Line 344: the phrasing could be changed – “with swinging back and forth oxygen conditions” -> “with fluctuating high and low oxygen conditions.”
Citation: https://doi.org/10.5194/egusphere-2023-2664-RC2 - AC2: 'Reply on RC2', Nicole Burdanowitz, 24 Jan 2024
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Nicole Burdanowitz
Gerhard Schmiedl
Birgit Gaye
Philipp Munz
Hartmut Schulz
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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