the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 May 2024
Spatial and temporal variability of sea surface temperatures and monsoon dynamics in the northwestern Arabian Sea during the last 43 kyr
Abstract. In this study, we present the first well-dated, high-resolution alkenone-based sea surface temperature (SST) record (SL167) from the northeastern Oman Margin (Gulf of Oman) in the northwestern Arabian Sea. The SST reconstructions spanning the last 43 kyr reveal fluctuations of approximately 7 °C (20.1 °C to 27.4 °C) and demonstrate a higher sensitivity to climate variations compared to similar core locations in the Arabian Sea. SSTs remained low during Heinrich events (H2, H3, H4), the Younger Dryas, early and late Holocene, and were high during Dansgaard-Oeschger interstadials (D-O 11, D-O 4 - 9, Bølling-Allerød (B-A), and mid-Holocene. SST was predominantly influenced by the SW monsoon during warmer periods and the NE monsoon during cold intervals. The dynamics of strengthening and weakening monsoon periods were likely controlled by shifts in the Intertropical Convergence Zone prompted by changes in solar radiation in the Northern Hemisphere. The last glacial maximum exhibited no intense cooling probably due to stronger NW winds and an eastward shift of the SST gradient in the Gulf of Oman, resulting in a brief and moderate cooling period. Strong SW winds during the early Holocene transported cold water masses from Oman upwelling into the Gulf of Oman, lowering SSTs. A rapid temperature increase of approx. 2 °C during the mid-Holocene was induced by an abrupt eastward shift of the SST gradient.
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RC1: 'Comment on egusphere-2024-1072', Anonymous Referee #1, 02 Jul 2024
The article by Maier et al. presents a 43 ka-long alkenone-based SST record in the NE corner of Oman peninsula. The core location is hence potentially under the influence of the Mediterranean region, and the Asiatic and African landmasses, so many different climatic regimes could interfere, manifest, or be masked by a multitude of processes. Then it is not surprising, and very interesting, to see a SST record that look anything like what has been already published in the literature in the broader NW Indian ocean sector.
I find the authors have done a good job in interpreting their curious SST record, and I liked their choice of showcasing their record along with other alkenone-based records that are used in Figure 4 to appreciate the potential contrasted influences that surely played some role in shaping their SST record. I think the article could be published after minor revisions.
First, I suggest the authors to introduce more clearly the complexity of their SST record that comes from such a locality, and develop more on that in the conclusion. As it stands the conclusion only summarizes the main findings, but I think there is room to finish the conclusion within a broader regional context than the Arabian Sea and the Gulf of Oman only.
Other remarks:
What I found really missing was a stronger description of the age model, perhaps through adding an additional figure. Before starting to read the article the only d15N record looks so much like a series on Bond cycles for the MIS3 that I first thought that the Heinrich events were mislabeled. I had to check in the original Burdanowitz paper to be convinced by your age model, and I think it is really missing in your own article.
On your SST interpretation, with which I have no problem, it is sometimes hard to follow when you describe your curious SST record with other ones in chapters 5.1.2, 5.1.3 and 5.1.4. Particularly, for example, when you discuss the seasonality of climate patterns such as the monsoon and ITCZ along with other SST records in the northern Arabian Sea. I think the authors will find guidance if you also show the monthly SST for all individual sites that you show in Figure 4, to better highlight how changes in other SST could be partly driven by changes in seasonal/atmospheric processes that you describe. For example, the paper by Bassinot et al. (https://doi.org/10.5194/cp-7-815-2011) shows how seasonal changes in wind could enhance/dampen upwelling in regions situated in the western/eastern parts of the Arabian Sea. Even if you consider your own alkenone as being reflective of mean-annual SST I think more discussion on how seasonal features can deeply affect the regional dynamics could be more apparent in your discussion.
I liked the proposition that the Persian Gulf outflow waters could have played a role on your Holocene record. On that, perhaps you could be interested in reading the article by Naderi et al. (https://doi.org/10.1002/jqs.3614) could help having an illustration of what has been happening there and in the surrounding land.
Finally, I am not sure the whole discussion on the wavelet analysis really adds a value to your discussion. I am not sure whether the sun is something had really a discernible impact on your SST record, given the other processes you list during the discussion, but I don’t have strong recommendation to remove it either. Anyway listing the 525, 505, 493 etc. periodicities does not add something the reader will really focus on, and there is still the possibility that your sediment sampling could add wavelet artifacts on this long list of periodicities. I find it is a shame to discuss it, it dilutes your discussion.
Other minor remarks:
- There is a series of typos (and bugs at reporting the chapter numbering). Pleaser get a profound last read over the manuscript prior to submitting your revised version.
- Productivity-mediated records in your core highlighted in Figure 4 could perhaps be better used while discussing the dynamics of your core.
Citation: https://doi.org/10.5194/egusphere-2024-1072-RC1 - AC1: 'Reply on RC1', Jan Maier, 12 Nov 2024
-
RC2: 'Comment on egusphere-2024-1072', Anonymous Referee #2, 15 Oct 2024
General Comments
In their manuscript Maier and colleagues present a new alkenone SST record from the Arabian Sea. Their record shows substantial millennial scale SST variability of 4C during both the last glacial and the Holocene, with the exception a quiescent period during the LGM. However, the glacial/interglacial offset is small, only around 1C. This result is in stark contrast to other Arabian Sea SST records which show large glacial/interglacial changes, and small millennial scale variability. This intriguing result potentially offers new insights into either the spatial variability of SSTs in the Arabian Sea and their sensitivities to elucidate different components of the climate system, or as a sensitive recorder of the Oman Upwelling Zone or more broadly the east-west SST gradient in the Arabian Sea.
While the paper provides good detail on specific changes in the record and potential mechanisms for these changes, it misses the big picture. The knowledge gap and research question are not stated and it’s not clear what the jump is in our understanding of the regional climate system provided by this paper. It is hard to assess the validity of changes through time when the timing of SST changes in the presented record are rarely stated or given uncertainties. The lack of glacial/interglacial offset is not explored in detail. The impact of changes in bathymetry due to deglacial sea-level rise is only lightly touched upon. While individual Heinrich events are discussed, a common response is not, and no distinction is made between the response to Heinrich events versus Greenland stadials. I have further misgivings over the event chronology used as a comparison, which is based on Mediterranean cosmogenic exposure dates with external error of 4kyr, rather than using the INTIMATE chronology to focus on Greenland stadials and interstadials, or a U/Th dated speleothem which might be able to distinguish Greenland stadials and Heinrich events. The spectral analysis section is reasonable, but the focus is on what frequencies are found, when perhaps the most interesting result is a 5kyr window of no periodicity surrounding the LGM. Overall this is an exciting record that lacks a quantified robust discussion. In its current form it is not ready for publication.
Specific Comments
- Line 20 states that the dynamics of monsoon periods are influenced by northern hemisphere solar radiation, but the paper is largely about Heinrich events and Dansgaard-Oeschger interstadials and other abrupt millennial scale events, not solar radiation.
- Knowledge Gap/Research Question: It’s not clear what the purpose of this study is. What is it trying to find out? At the paragraph break line 51-52 we need to understand what is the unknown which this study intends to investigate. A one or two sentence summary of Gaye et al. 2018 of key results might be useful, as this might outline what the knowledge gap is.
- Age model uncertainty: Dating uncertainty and age model uncertainty need to be stated in section 3: i.e. a key summary of the Burdanowitz et al., 2024 age model.
- Greenland stadials vs Heinrich events: This paper uses the nomenclature of Dansgaard-Oeschger interstadials and Heinrich events. It is not clear why Heinrich events in particular are different from Greenland stadials in this record. I suggest switching to a Greenland stadial nomenclature throughout the manuscript. This would also allow for more precise timings from the INTIMATE chronology to be used in preference to the Mediterranean cosmogenic dataset currently used.
- The timings of the shaded bars in figures 4 and 5 are inaccurate. The 4.2 ka event is not 5-4 kyr BP. The 8.2 ka event is not 9-8 kyr BP. The YD is not 13-12 kyr BP. H1 is not 18-15.5. H2 is tricky to tie down, but I think it is more likely to be where DO-2 is currently labelled (this SST drop would help the case being made that Heinrich events are associated with SST decreases). I believe H3 is more likely to be GS5.1 than GS5.2. I don’t know how much of this is a plotting issue and how much is an issue with the choice of external event chronology.
- Glacial/Interglacial change: The lack of a large change in SST from glacial to interglacial (about 24.5 to 25.5 C by my eye) compared to nearby records (2-4 C) is one of the remarkable results of this paper. Yet it is not discussed. The role of changing bathymetry is similarly only lightly discussed.
- Mid-Holocene cooling: What is the timing of the SST decrease at 5ka relative to the end of the Green Sahara? And does this have any mechanistic implications?
- 2 ka event: I am skeptical here without a detailed description of timing and uncertainty. A climate anomaly between 5 and 4 kyr BP is no longer sufficient to be labelled as the 4.2 ka event. It needs to be coincident with the Carolin et al., (2016) window of 4.26 to 3.97 kyr BP. I think the discussion of the event starting at line 312 is reasonable. But line 316 goes against the previous discussion. I would remove references to 4.2 from elsewhere in the paper (such as the results and conclusions), but keep the brief discussion around 312 as this part is nuanced and reasonable. Unless of course, specific data can be provided to support the assertions made.
- Spectral Analysis: There is a significant reduction in cyclicity between 22 and 17kyr BP (ish). This quiescent LGM might warrant further investigation.
- The 7200 year cycle is a potential subharmonic of precession, modulated by obliquity.
- Might the high number of potential cyclicity peaks around 500 years be one periodicity on top of an uncertain chronology?
Technical Corrections
- Line 29: insert modern: “of the total modern annual precipitation”
- Line 41: “Northern Hemisphere glacial ice-sheets”
- Line 48: no need for a comma between ‘both’ and ‘the’.
- Line 49: “in the past” is redundant and could be deleted.
- Line 65: The ‘Study Area’ section is more of a ‘Modern Climate Dynamics’ section.
- Line 65-85: Better disambiguation between spatial and temporal SST gradients is needed.
- Line 80: This sentence might need a rewrite to make it clearer. At present (4-5C) as a quantification of several hundred kilometers makes no sense.
- Line 93: New Paragraph?
- Line 101: Need a linking sentence or signpost sentence to help the read transfer into this new paragraph.
- Line 119: This sentence is a bit confusing. I suggest ‘Alkenones were measured at 2cm for the upper 162cm and 4cm resolution below 162cm by combining consecutive subsamples due to lower organic content’.
- Line 120: suggest ‘3 to 18g of sediment‘.
- Line 122: space needed between sentences.
- Line 123: This sentence is out of place. Methods should be written in chronological order.
- Line 144: This is precision not accuracy
- Line 151: The resolution of the evenly spaced dataset needs to be stated alongside the resolution of the actual data, so that this choice can be evaluated.
- Line 164: Figure reference needed here.
- Line 187: ‘mitigated’ is not correct here. Is ‘reduced’ sufficient?
- Line 214: ‘westerlies’
- Line 250: 17ka is not earlier than 19ka
- Line 258: its not clear what is meant by the NW winds moving in the opposite direction to the SW monsoon.
- Line 267: suggest ‘minor decrease in Central Arabia’
- Line 299: Lead with the evidence, then the interpretation, not the other way round.
- Line 308: New paragraph.
- Line 320: period missing at the end of the sentence.
- Line 337: D-O interstadial?
- Line 338: ‘Implies’ is incorrect here. Suggests or is linked to might be better.
Citation: https://doi.org/10.5194/egusphere-2024-1072-RC2 - AC3: 'Reply on RC2', Jan Maier, 12 Nov 2024
-
RC3: 'Comment on egusphere-2024-1072', Anonymous Referee #3, 26 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1072/egusphere-2024-1072-RC3-supplement.pdf
- AC2: 'Reply on RC3', Jan Maier, 12 Nov 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1072', Anonymous Referee #1, 02 Jul 2024
The article by Maier et al. presents a 43 ka-long alkenone-based SST record in the NE corner of Oman peninsula. The core location is hence potentially under the influence of the Mediterranean region, and the Asiatic and African landmasses, so many different climatic regimes could interfere, manifest, or be masked by a multitude of processes. Then it is not surprising, and very interesting, to see a SST record that look anything like what has been already published in the literature in the broader NW Indian ocean sector.
I find the authors have done a good job in interpreting their curious SST record, and I liked their choice of showcasing their record along with other alkenone-based records that are used in Figure 4 to appreciate the potential contrasted influences that surely played some role in shaping their SST record. I think the article could be published after minor revisions.
First, I suggest the authors to introduce more clearly the complexity of their SST record that comes from such a locality, and develop more on that in the conclusion. As it stands the conclusion only summarizes the main findings, but I think there is room to finish the conclusion within a broader regional context than the Arabian Sea and the Gulf of Oman only.
Other remarks:
What I found really missing was a stronger description of the age model, perhaps through adding an additional figure. Before starting to read the article the only d15N record looks so much like a series on Bond cycles for the MIS3 that I first thought that the Heinrich events were mislabeled. I had to check in the original Burdanowitz paper to be convinced by your age model, and I think it is really missing in your own article.
On your SST interpretation, with which I have no problem, it is sometimes hard to follow when you describe your curious SST record with other ones in chapters 5.1.2, 5.1.3 and 5.1.4. Particularly, for example, when you discuss the seasonality of climate patterns such as the monsoon and ITCZ along with other SST records in the northern Arabian Sea. I think the authors will find guidance if you also show the monthly SST for all individual sites that you show in Figure 4, to better highlight how changes in other SST could be partly driven by changes in seasonal/atmospheric processes that you describe. For example, the paper by Bassinot et al. (https://doi.org/10.5194/cp-7-815-2011) shows how seasonal changes in wind could enhance/dampen upwelling in regions situated in the western/eastern parts of the Arabian Sea. Even if you consider your own alkenone as being reflective of mean-annual SST I think more discussion on how seasonal features can deeply affect the regional dynamics could be more apparent in your discussion.
I liked the proposition that the Persian Gulf outflow waters could have played a role on your Holocene record. On that, perhaps you could be interested in reading the article by Naderi et al. (https://doi.org/10.1002/jqs.3614) could help having an illustration of what has been happening there and in the surrounding land.
Finally, I am not sure the whole discussion on the wavelet analysis really adds a value to your discussion. I am not sure whether the sun is something had really a discernible impact on your SST record, given the other processes you list during the discussion, but I don’t have strong recommendation to remove it either. Anyway listing the 525, 505, 493 etc. periodicities does not add something the reader will really focus on, and there is still the possibility that your sediment sampling could add wavelet artifacts on this long list of periodicities. I find it is a shame to discuss it, it dilutes your discussion.
Other minor remarks:
- There is a series of typos (and bugs at reporting the chapter numbering). Pleaser get a profound last read over the manuscript prior to submitting your revised version.
- Productivity-mediated records in your core highlighted in Figure 4 could perhaps be better used while discussing the dynamics of your core.
Citation: https://doi.org/10.5194/egusphere-2024-1072-RC1 - AC1: 'Reply on RC1', Jan Maier, 12 Nov 2024
-
RC2: 'Comment on egusphere-2024-1072', Anonymous Referee #2, 15 Oct 2024
General Comments
In their manuscript Maier and colleagues present a new alkenone SST record from the Arabian Sea. Their record shows substantial millennial scale SST variability of 4C during both the last glacial and the Holocene, with the exception a quiescent period during the LGM. However, the glacial/interglacial offset is small, only around 1C. This result is in stark contrast to other Arabian Sea SST records which show large glacial/interglacial changes, and small millennial scale variability. This intriguing result potentially offers new insights into either the spatial variability of SSTs in the Arabian Sea and their sensitivities to elucidate different components of the climate system, or as a sensitive recorder of the Oman Upwelling Zone or more broadly the east-west SST gradient in the Arabian Sea.
While the paper provides good detail on specific changes in the record and potential mechanisms for these changes, it misses the big picture. The knowledge gap and research question are not stated and it’s not clear what the jump is in our understanding of the regional climate system provided by this paper. It is hard to assess the validity of changes through time when the timing of SST changes in the presented record are rarely stated or given uncertainties. The lack of glacial/interglacial offset is not explored in detail. The impact of changes in bathymetry due to deglacial sea-level rise is only lightly touched upon. While individual Heinrich events are discussed, a common response is not, and no distinction is made between the response to Heinrich events versus Greenland stadials. I have further misgivings over the event chronology used as a comparison, which is based on Mediterranean cosmogenic exposure dates with external error of 4kyr, rather than using the INTIMATE chronology to focus on Greenland stadials and interstadials, or a U/Th dated speleothem which might be able to distinguish Greenland stadials and Heinrich events. The spectral analysis section is reasonable, but the focus is on what frequencies are found, when perhaps the most interesting result is a 5kyr window of no periodicity surrounding the LGM. Overall this is an exciting record that lacks a quantified robust discussion. In its current form it is not ready for publication.
Specific Comments
- Line 20 states that the dynamics of monsoon periods are influenced by northern hemisphere solar radiation, but the paper is largely about Heinrich events and Dansgaard-Oeschger interstadials and other abrupt millennial scale events, not solar radiation.
- Knowledge Gap/Research Question: It’s not clear what the purpose of this study is. What is it trying to find out? At the paragraph break line 51-52 we need to understand what is the unknown which this study intends to investigate. A one or two sentence summary of Gaye et al. 2018 of key results might be useful, as this might outline what the knowledge gap is.
- Age model uncertainty: Dating uncertainty and age model uncertainty need to be stated in section 3: i.e. a key summary of the Burdanowitz et al., 2024 age model.
- Greenland stadials vs Heinrich events: This paper uses the nomenclature of Dansgaard-Oeschger interstadials and Heinrich events. It is not clear why Heinrich events in particular are different from Greenland stadials in this record. I suggest switching to a Greenland stadial nomenclature throughout the manuscript. This would also allow for more precise timings from the INTIMATE chronology to be used in preference to the Mediterranean cosmogenic dataset currently used.
- The timings of the shaded bars in figures 4 and 5 are inaccurate. The 4.2 ka event is not 5-4 kyr BP. The 8.2 ka event is not 9-8 kyr BP. The YD is not 13-12 kyr BP. H1 is not 18-15.5. H2 is tricky to tie down, but I think it is more likely to be where DO-2 is currently labelled (this SST drop would help the case being made that Heinrich events are associated with SST decreases). I believe H3 is more likely to be GS5.1 than GS5.2. I don’t know how much of this is a plotting issue and how much is an issue with the choice of external event chronology.
- Glacial/Interglacial change: The lack of a large change in SST from glacial to interglacial (about 24.5 to 25.5 C by my eye) compared to nearby records (2-4 C) is one of the remarkable results of this paper. Yet it is not discussed. The role of changing bathymetry is similarly only lightly discussed.
- Mid-Holocene cooling: What is the timing of the SST decrease at 5ka relative to the end of the Green Sahara? And does this have any mechanistic implications?
- 2 ka event: I am skeptical here without a detailed description of timing and uncertainty. A climate anomaly between 5 and 4 kyr BP is no longer sufficient to be labelled as the 4.2 ka event. It needs to be coincident with the Carolin et al., (2016) window of 4.26 to 3.97 kyr BP. I think the discussion of the event starting at line 312 is reasonable. But line 316 goes against the previous discussion. I would remove references to 4.2 from elsewhere in the paper (such as the results and conclusions), but keep the brief discussion around 312 as this part is nuanced and reasonable. Unless of course, specific data can be provided to support the assertions made.
- Spectral Analysis: There is a significant reduction in cyclicity between 22 and 17kyr BP (ish). This quiescent LGM might warrant further investigation.
- The 7200 year cycle is a potential subharmonic of precession, modulated by obliquity.
- Might the high number of potential cyclicity peaks around 500 years be one periodicity on top of an uncertain chronology?
Technical Corrections
- Line 29: insert modern: “of the total modern annual precipitation”
- Line 41: “Northern Hemisphere glacial ice-sheets”
- Line 48: no need for a comma between ‘both’ and ‘the’.
- Line 49: “in the past” is redundant and could be deleted.
- Line 65: The ‘Study Area’ section is more of a ‘Modern Climate Dynamics’ section.
- Line 65-85: Better disambiguation between spatial and temporal SST gradients is needed.
- Line 80: This sentence might need a rewrite to make it clearer. At present (4-5C) as a quantification of several hundred kilometers makes no sense.
- Line 93: New Paragraph?
- Line 101: Need a linking sentence or signpost sentence to help the read transfer into this new paragraph.
- Line 119: This sentence is a bit confusing. I suggest ‘Alkenones were measured at 2cm for the upper 162cm and 4cm resolution below 162cm by combining consecutive subsamples due to lower organic content’.
- Line 120: suggest ‘3 to 18g of sediment‘.
- Line 122: space needed between sentences.
- Line 123: This sentence is out of place. Methods should be written in chronological order.
- Line 144: This is precision not accuracy
- Line 151: The resolution of the evenly spaced dataset needs to be stated alongside the resolution of the actual data, so that this choice can be evaluated.
- Line 164: Figure reference needed here.
- Line 187: ‘mitigated’ is not correct here. Is ‘reduced’ sufficient?
- Line 214: ‘westerlies’
- Line 250: 17ka is not earlier than 19ka
- Line 258: its not clear what is meant by the NW winds moving in the opposite direction to the SW monsoon.
- Line 267: suggest ‘minor decrease in Central Arabia’
- Line 299: Lead with the evidence, then the interpretation, not the other way round.
- Line 308: New paragraph.
- Line 320: period missing at the end of the sentence.
- Line 337: D-O interstadial?
- Line 338: ‘Implies’ is incorrect here. Suggests or is linked to might be better.
Citation: https://doi.org/10.5194/egusphere-2024-1072-RC2 - AC3: 'Reply on RC2', Jan Maier, 12 Nov 2024
-
RC3: 'Comment on egusphere-2024-1072', Anonymous Referee #3, 26 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1072/egusphere-2024-1072-RC3-supplement.pdf
- AC2: 'Reply on RC3', Jan Maier, 12 Nov 2024
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