the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jul 2024
A long-term drought reconstruction based on oxygen isotope tree ring data
Abstract. This study investigates the relationship between oxygen isotope ratios (δ18O) in oak tree ring cellulose and past drought variability in Letea Forest, Romania. A δ18O site chronology spanning 1803–2020 was compiled from seven individual time series. δ18O values exhibited a significant negative correlation with moisture-related variables (cloud cover, relative humidity and precipitation) and a positive correlation with temperature and sunshine duration. This confirms that δ18O from tree rings can be a good proxy for moisture availability. The strongest correlation was found between δ18O and the August Standardized Precipitation Evapotranspiration Index for an accumulation period of 9-months (SPEI9) for central and eastern Europe. This highlights SPEI9 as a superior indicator of drought compared to individual parameters like temperature or precipitation. Using a linear regression model, we reconstructed August SPEI9 variability for the past 200 years. The reconstruction captured interannual and decadal variations, with distinct wet and dry periods. Analysis of large-scale atmospheric circulation patterns revealed a link between high δ18O values (indicating dry conditions) and a high-pressure system over the North Atlantic. Conversely, low δ18O values (indicating wet conditions) corresponded to negative pressure anomalies over Europe. Moreover, extreme values of δ18O are also associated with the prevalence of a hemispheric teleconnection pattern, namely wave number 4. This δ18O chronology and the corresponding August SPEI9 reconstruction offer valuable tools for understanding past climate variability and its relationship with large-scale atmospheric and oceanic circulation patterns.
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RC1: 'Comment on egusphere-2024-2144', Marzena Kłusek, 09 Aug 2024
The reviewed manuscript presents a new oxygen stable isotope chronology measured in cellulose extracted from annual growth-rings of oak trees, which originated from the area of Letea Forest, Romania. This chronology is over 200 years old and was used to reconstruct the August Standardised Precipitation Evapotranspiration Index (for an accumulation period of 9-months - SPEI9) in the area of central and eastern Europe. The paper also discusses the interannual and decadal variability evident in the obtained palaeoclimatic reconstruction, resulting from the occurrence of dry and wet periods associated with large-scale atmospheric and oceanic circulation patterns. The new stable oxygen isotope chronology proved to be an effective tool for reconstructing past hydroclimatic variability in the Letea Forest of the Danube River delta and in central and eastern Europe. This makes the reviewed article an important contribution to the issues of past climate research.
The manuscript has an appropriate text structure, is written in clearly formulated language and the individual chapters present the subsequent topics in sufficient detail. The accompanying table and Figures enable a better understanding of the descriptions. The authors also refer to previous studies of a similar type in the neighbouring areas, discussing and comparing the results obtained. The article is of high scientific value and I recommend it for publication.
Some 'specific comments' and 'technical corrections' are contained in the attached version of the manuscript.
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AC1: 'Reply on RC1', Viorica Nagavciuc, 13 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2144/egusphere-2024-2144-AC1-supplement.pdf
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AC1: 'Reply on RC1', Viorica Nagavciuc, 13 Sep 2024
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RC2: 'Comment on egusphere-2024-2144', Anonymous Referee #2, 21 Aug 2024
The authors present a new drought reconstruction for Eastern Europe in this manuscript, based on δ18O tree-ring measurements. Collectively, the authors possess strong knowledge and expertise in the methodological aspects of the manuscript (lab and statistical work). Therefore, I have only a few minor suggestions regarding the methods and analyses performed. However, my main concern relates to the relevance of the study, which I believe the authors might have overlooked. What new insights does their study provide? What knowledge gap are they aiming to fill? Why is this important in the context of what we already know? What does your reconstruction tell us? The manuscript would greatly benefit from better contextualization of the results and a less methodological approach.
The title “A long-term drought reconstruction based on oxygen isotope tree-ring data” exemplifies my concerns. Unfortunately, it is not an appealing title; it sounds like just another reconstruction study. While it may indeed be another reconstruction, 1) it also focuses on a region (Romania, Eastern Europe) that has been largely overlooked and underrepresented, and 2) it shows no strong trend in recent decades, which could be further discussed in the manuscript.
The authors also present very advanced (and nice) analyses, including the link between the δ18O chronology and atmospheric circulation, but the manuscript lacks some explanation and contextualization (beyond their own studies). In this regard, I feel that the introduction could also be reformulated to incorporate more of these atmospheric drivers of δ18O.
Finally, in the comparison with other reconstructions, I agree that the high-frequency domains align, but I am not as certain about the low frequencies, trends, and values. Brázdil et al. (2016) show low SPEI values (<0) since the 1950s, which aligns with most studies indicating a drying trend since the mid-20th century. The present study shows low SPEI values only since 2005, implying that the latter part of the 20th century was relatively humid. Indeed, the reconstruction shows quite high SPEI values compared to the observed values for the period 1980–2000. Perhaps some comments and reflections on this discrepancy would be interesting.
Minor comments.
Line 41: "Complex climatic dynamics" — Provide more detail for readers who may not be familiar with the region's climatic dynamics.
Line 65: It might be worth adding that δ18O records capture significant climatic information, even when collected from sites that are not classically considered “climatically” limited, as in this case.
Line 97: Which oak species are you analyzing here? This is not mentioned anywhere in the manuscript.
Line 104: "Latewood" — The spelling appears to be incorrect.
Figure 1: The resolution is very low, particularly for the climatograph. Is it possible to improve the quality?
Line 130: Why not use VPD? It could be even better than SPEI due to its direct link with leaf-to-atmosphere pressure.
Supplementary Figures: They are usually ordered according to their appearance in the main text, but here they are not. The chronology is labeled as S3, while the atmospheric pattern (cited later in the text) is S1. Please reorder them.
Line 250: While you cite the paper that explains this methodology, this manuscript should stand on its own. It would be better if you could briefly describe the method behind the stability map.
Figure S3 (Chronology): Why are there not consistently 7 cores, but rather sometimes 7, then 6, and then 7 again? Are these missing rings? Please explain.
Line 223: Please reference Figure 4 at the end of the sentence.
Figure 3: This may not be the most appropriate figure to show here. The focus should be on the last rows—correlations between δ18O and monthly SPEI for the five different windows. The rows above, showing correlations between September SPEI with January SPEI (which, of course, is positive), August with January, and so on, may not be needed. Please consider revising this figure.
Section 3.4: This section is not described in the methods but is explained here. I suggest moving the method explanation up and leaving the results and discussion in this section.
Citation: https://doi.org/10.5194/egusphere-2024-2144-RC2 -
AC2: 'Reply on RC2', Viorica Nagavciuc, 13 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2144/egusphere-2024-2144-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Viorica Nagavciuc, 13 Sep 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-2144', Marzena Kłusek, 09 Aug 2024
The reviewed manuscript presents a new oxygen stable isotope chronology measured in cellulose extracted from annual growth-rings of oak trees, which originated from the area of Letea Forest, Romania. This chronology is over 200 years old and was used to reconstruct the August Standardised Precipitation Evapotranspiration Index (for an accumulation period of 9-months - SPEI9) in the area of central and eastern Europe. The paper also discusses the interannual and decadal variability evident in the obtained palaeoclimatic reconstruction, resulting from the occurrence of dry and wet periods associated with large-scale atmospheric and oceanic circulation patterns. The new stable oxygen isotope chronology proved to be an effective tool for reconstructing past hydroclimatic variability in the Letea Forest of the Danube River delta and in central and eastern Europe. This makes the reviewed article an important contribution to the issues of past climate research.
The manuscript has an appropriate text structure, is written in clearly formulated language and the individual chapters present the subsequent topics in sufficient detail. The accompanying table and Figures enable a better understanding of the descriptions. The authors also refer to previous studies of a similar type in the neighbouring areas, discussing and comparing the results obtained. The article is of high scientific value and I recommend it for publication.
Some 'specific comments' and 'technical corrections' are contained in the attached version of the manuscript.
-
AC1: 'Reply on RC1', Viorica Nagavciuc, 13 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2144/egusphere-2024-2144-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Viorica Nagavciuc, 13 Sep 2024
-
RC2: 'Comment on egusphere-2024-2144', Anonymous Referee #2, 21 Aug 2024
The authors present a new drought reconstruction for Eastern Europe in this manuscript, based on δ18O tree-ring measurements. Collectively, the authors possess strong knowledge and expertise in the methodological aspects of the manuscript (lab and statistical work). Therefore, I have only a few minor suggestions regarding the methods and analyses performed. However, my main concern relates to the relevance of the study, which I believe the authors might have overlooked. What new insights does their study provide? What knowledge gap are they aiming to fill? Why is this important in the context of what we already know? What does your reconstruction tell us? The manuscript would greatly benefit from better contextualization of the results and a less methodological approach.
The title “A long-term drought reconstruction based on oxygen isotope tree-ring data” exemplifies my concerns. Unfortunately, it is not an appealing title; it sounds like just another reconstruction study. While it may indeed be another reconstruction, 1) it also focuses on a region (Romania, Eastern Europe) that has been largely overlooked and underrepresented, and 2) it shows no strong trend in recent decades, which could be further discussed in the manuscript.
The authors also present very advanced (and nice) analyses, including the link between the δ18O chronology and atmospheric circulation, but the manuscript lacks some explanation and contextualization (beyond their own studies). In this regard, I feel that the introduction could also be reformulated to incorporate more of these atmospheric drivers of δ18O.
Finally, in the comparison with other reconstructions, I agree that the high-frequency domains align, but I am not as certain about the low frequencies, trends, and values. Brázdil et al. (2016) show low SPEI values (<0) since the 1950s, which aligns with most studies indicating a drying trend since the mid-20th century. The present study shows low SPEI values only since 2005, implying that the latter part of the 20th century was relatively humid. Indeed, the reconstruction shows quite high SPEI values compared to the observed values for the period 1980–2000. Perhaps some comments and reflections on this discrepancy would be interesting.
Minor comments.
Line 41: "Complex climatic dynamics" — Provide more detail for readers who may not be familiar with the region's climatic dynamics.
Line 65: It might be worth adding that δ18O records capture significant climatic information, even when collected from sites that are not classically considered “climatically” limited, as in this case.
Line 97: Which oak species are you analyzing here? This is not mentioned anywhere in the manuscript.
Line 104: "Latewood" — The spelling appears to be incorrect.
Figure 1: The resolution is very low, particularly for the climatograph. Is it possible to improve the quality?
Line 130: Why not use VPD? It could be even better than SPEI due to its direct link with leaf-to-atmosphere pressure.
Supplementary Figures: They are usually ordered according to their appearance in the main text, but here they are not. The chronology is labeled as S3, while the atmospheric pattern (cited later in the text) is S1. Please reorder them.
Line 250: While you cite the paper that explains this methodology, this manuscript should stand on its own. It would be better if you could briefly describe the method behind the stability map.
Figure S3 (Chronology): Why are there not consistently 7 cores, but rather sometimes 7, then 6, and then 7 again? Are these missing rings? Please explain.
Line 223: Please reference Figure 4 at the end of the sentence.
Figure 3: This may not be the most appropriate figure to show here. The focus should be on the last rows—correlations between δ18O and monthly SPEI for the five different windows. The rows above, showing correlations between September SPEI with January SPEI (which, of course, is positive), August with January, and so on, may not be needed. Please consider revising this figure.
Section 3.4: This section is not described in the methods but is explained here. I suggest moving the method explanation up and leaving the results and discussion in this section.
Citation: https://doi.org/10.5194/egusphere-2024-2144-RC2 -
AC2: 'Reply on RC2', Viorica Nagavciuc, 13 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2144/egusphere-2024-2144-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Viorica Nagavciuc, 13 Sep 2024
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