Reconstruction of warm season temperatures in central Europe during the past 60,000 years from lacustrine GDGTs

Zander, Paul D.; Böhl, Daniel; Sirocko, Frank; Auderset, Alexandra; Haug, Gerald; Martínez-García, Alfredo

doi:https://doi.org/10.5194/egusphere-2023-1960

Preprints

https://doi.org/10.5194/egusphere-2023-1960

Preprints

26 Sep 2023

| 26 Sep 2023

Reconstruction of warm season temperatures in central Europe during the past 60,000 years from lacustrine GDGTs

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald Haug, and Alfredo Martínez-García

Abstract. Millennial-scale climate variations during the last glacial period, such as Dansgaard–Oeschger (D/O) cycles and Heinrich events, have been extensively studied using ice core and marine proxy records. However, there is a limited understanding of the magnitude of these temperature fluctuations in continental regions, and questions remain about the seasonal signal of these climate events. This study presents a 60,000-year long temperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) extracted from lake sediments from the Eifel volcanic field, Germany. brGDGTs are bacterial membrane lipids that are known to have strong relationship with temperature, making them suitable for temperature reconstructions. We test several temperature calibration models on modern samples taken from soils and multiple maar lakes. We find a bias associated with water depth and anoxic conditions that can be corrected for by accounting for a brGDGT isomer that is only produced in anoxic conditions. The corrected temperature reconstruction correlates with proxy and model temperature record spanning the same time period, validating the calibration model. However, millennial-scale variability is significantly dampened in the brGDGT record, and in contrast to other northern hemisphere climate records, during several Heinrich stadials, temperatures actually increase. We demonstrate that these apparent discrepancies can be explained by the unique seasonal response of the brGDGT paleothermometer to temperatures of months above freezing (TMAF). Our data support the view that warm season temperatures in Europe varied minimally during the last glacial period, and that abrupt millennial-scale events were defined by colder, longer winters. Our continuous high-resolution temperature reconstruction provides important information about the magnitude of seasonal climate variability during the last glacial period that can be used to test climate models and inform studies of paleoecological change.

Received: 28 Aug 2023 – Discussion started: 26 Sep 2023

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Journal article(s) based on this preprint

08 Apr 2024

Reconstruction of warm-season temperatures in central Europe during the past 60 000 years from lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGTs)

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald H. Haug, and Alfredo Martínez-García

Clim. Past, 20, 841–864, https://doi.org/10.5194/cp-20-841-2024,https://doi.org/10.5194/cp-20-841-2024, 2024

Short summary

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald Haug, and Alfredo Martínez-García

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-1960', Francien Peterse, 27 Sep 2023
Dear Paul Zander and co-authors,
I came across your manuscript based on your Tweet (or whatever that is called these days) and was intrigued by some of the replies that suggested a coupling between the occurrence of the IIIa’’ isomer and redox changes. I just read through your work and have a few questions and comments for you to ponder on (this is not an official review, just an exchange of ideas I hope):
Your Fig. 2 suggests that you also detected IIIa’’ in soils, whereas Weber et al 2015 found that it was exclusively produced in lakes. In fact, they used the absence of IIIa’’ in soils and the presence of IIIa’’ in the lake sediment as their main argument for its aquatic source, later backed up by isotope analysis. Can you comment on the presence of IIIa’’ in the soils studied here?

In their follow up study (Weber et al 2018) is mentioned that IIIa’’ does not occur in all lakes. Based on the isotopic composition of IIIa’’-derived alkanes you could derive preferred production in eutrophic/anoxic lakes (see their Fig 4C-E), but just anoxia is not doing the trick. I can tell you firsthand that IIIa’’ is not detected in Lake Chala, although this lake has anoxic bottom waters (resulting in a continuously laminated sediment record; van Bree et al., 2020, Baxter et al 2023). Maybe the fact that Chala is oligotrophic explains its absence, but I would be careful with using IIIa’’ as marker for ‘just’ anoxic conditions.

Based on all the lake work that has recently appeared in the literature I have started to have serious doubts about the importance of soil-derived GDGTs in lake sediments, especially for lakes that are not connected to any river system. I feel that the vast majority, if not all brGDGTs in lake sediments are produced in situ, and that their production in the water column is strongly impacted by redox conditions, as you already mention in your manuscript, and has also clearly been shown by e.g., Weber et al., 2018, van Bree et al., 2020, Wu et al 2021, and Baxter et al., 2023. Most importantly, these studies have also shown that the producers of the different brGDGT isomers may occupy different niches in the water column. This can make brGDGT production very sensitive to changes in e.g., lake depth (be it climate driven, or by lake basin development or infilling of the lake over time) or mixing regime and affect the reliability of the currently available transfer functions. I know this mostly from working on Lake Chala, where we did very extensive lake monitoring to find this out (see Baxter et al., 2023), and I can imagine that this is not possible for every lake, but we should try to use all available information from these studies in our interpretation of novel lake records. Enfin, the point I wanted to make was that changes in IIIa/IIa may not necessarily reflect soil vs aquatic brGDGT sources, but instead the expansion and shrinking of the niches of their producers within the lake, related to redox. The different zones in the water column occupied by the producers of IIa and IIIa is actually clearly visible in your Fig. 4e.

Following this line or reasoning and letting go of IIIa/IIa as indicator for soil input, this also allows you to interpret the high BIT index values during the Holocene differently, especially since the high BIT index values are not supported by more clastic material (Fig. 7I), but rather with increased productivity, sediment anoxia, and methanogens (Fig. 7GHD). This would fit with a shallower oxycline in the lake, which increases the zone for brGDGT production (in the anoxic zone of the water column; see Weber et al., 2018, van Bree et al., 2020, Wu et al., 2021), and decreases the zone of crenarchaeol production, which, under ideal conditions peaks in the suboxic zone, as is also the case in the modern lake, shown in your Fig. 4C. As Thaumarchaeota exhibit photoinhibition, they cannot thrive in the very upper water layer, and are thus outcompeted during periods with less mixing, more stratification, etc, increasing the BIT index (see Baxter et al., 2021 and 2023 for detailed explanation and illustrating schematics). That this scenario may hold true is also reflected by the absolute abundances of the GDGTs, which show an increase in brGDGTs and isoGDGTs that are produced in the anoxic part of the lake, whereas crenarchaeol remains relatively stable/low during the transition into the Holocene.

I haven’t looked in much detail at the part of the record spanning MIS3, so I am not sure what exactly is going on there, and I can only encourage you to keep the water column structure, mixing regime, and lake basin development in the back of your mind when interpreting the data! Looking forward to hearing your thoughts about all of this.
All the best,

Francien Peterse
PS. Quick last note for the introduction: the presence of brGDGT Ia in culture was already shown by Sinninghe Damsté et al in 2011. Also, I miss a reference to Weijers et al., 2007 as the paper in which the temperature dependency of brGDGTs was first demonstrated and the MBT was defined.

References:

Baxter et al 2021 QSR

Baxter et al 2023 Nature (check methods section on calibrations and extended data Fig. 2)

van Bree et al 2020 BGS

Sinninghe Damsté et al., 2011 AEM

Weber et al 2015 GCA

Weber et al 2018 PNAS

Weijers et al., 2007 GCA

Wu et al 2021 Chemical Geology
Citation: https://doi.org/10.5194/egusphere-2023-1960-CC1
- AC1: 'Reply on CC1 Francien Peterse', Paul Zander, 18 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1960/egusphere-2023-1960-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC1
RC1:
'Comment on egusphere-2023-1960', Maria Fernanda Sanchez Goñi, 13 Oct 2023

This manuscript provides a continuous record from 60 ka to the present of brGDGT temperature reconstruction for the Eifel region (Central Europe). This record is relevant and deserves to be published in Climate of the Past. So far, there is a lack of regional quantitative climate reconstructions for the last glacial period in Central Europe and very few for other European regions. However, before accepting this manuscript for publication, the authors should address some chronological issues and discuss the ELSA record in more detail, taking into account the other quantitative climate reconstructions for Europe. I am not an expert in geochemistry and my review is mainly focused on section 3 of the manuscript "Results and discussion".
Lines 35-40 and lines 450-453: In the Introduction and in Section 3.4, it would be relevant to cite the papers that, based on pollen analysis of deep-sea sediment sequences, have detected for the first time, without chronological ambiguity, the European millennial scale climatic variability in response to the D-O warming and cooling events, including the Heinrich events (e.g. Sanchez Goñi et al. 2000, 2008). The only works cited in the manuscript are the terrestrial pollen records and speleothem sequences, which, even if relatively well dated, cannot be directly compared with the North Atlantic millennial-scale climate variability.
Figure 7: How is it possible to have 50%-100% of tree pollen in the 60-30 ka interval? These values are found during interglacial periods, as shown during the Holocene of the same sequence.
Lines 394-401 in section 3.3 and lines 453-471 in section 3.4: I do not know to what extent the temperature of months above freezing (TMAF) in the Eifel region can be compared with the sea surface temperatures (SST) of the Iberian margin but if the authors do so, I think it is more appropriate to discuss the TMAF in the Eifel region in the light of the surface air temperatures estimated from the pollen records of the Iberian margin, the Alboran Sea (Sanchez Goñi et al., 2002) and the Gulf of Lyon (Sanchez Goñi et al., 2021), which cover the period 50-27 ka, i.e. HS 5 to HS 3 and the associated D-O cycles. Although the latter correspond to winter temperatures, the authors should compare their estimates of summer temperatures with them, especially in the light of the question of seasonality they discuss in Section 3.4, which is the main contribution of this work. For HS 5, pollen-based winter surface air temperatures show pronounced decreases, averaging 5-10°C relative to present, in southern Iberia and south-eastern France, consistent with the severe SST low in the eastern North Atlantic. HS 3 is also marked by cooling in southern Iberia, although less so than HS 5 and HS 4. Furthermore, why are HS 5 and HS 3 marked by warm summers in the Eifel region and HS 4 by cold summers? Are these seasonal changes linked, for example, to orbital parameters?Figure 10 should be implemented with the pollen-based surface air temperatures from western Europe.
Lines 409-422: The authors discuss the mean temperature of the 29-24 kyr b2k interval in the Eifel and compare it with other reconstructions for the LGM without defining their respective chronologies. This interval extends from 29 to 19 ka according to Hughes et al. (2022) but from 23 to 19 ka according to Mix et al. (2001, EPILOG). The problem is that the LGM according to Hughes et al. encompasses three D-O warming events (D-O 4, D-O 3 and D-O 2), HS 2 and the LGM defined by EPILOG. Do they compare the same period in the different reconstructions (Paris Basin and Auel Maar, "...considering the significant age uncertainty in the Auel Maar data", line 372, for example), the assimilation of global palaeoclimatic data and the output of the HadCM3 model? In terms of vegetation in north-western and southern Iberia, the HS and LGM are markedly different (Naughton et al., 2007; Fletcher et al., 2008; Turon et al., 2003), with more Ericaceae and less Amaranthaceae-Chenopodiaceae during the LGM compared with the HS that flank it, indicating drier and colder conditions during the HS, associated with greater seasonality.
Lines 468-471 : It is interesting to add that the 17°C of warming of the coldest month between stadial and interstadial yielded by the LOVECLIM model simulations (Van Vermeerck et al., 2011) is higher compared to the pollen-based estimations of 10°C on average for SW and SE Iberia and 5°C for SE France.
The last part of section 3.4, devoted to the examination of changes in seasonality during the last glacial period, should be improved and strengthened by taking into account the pollen-based reconstructions of winter temperatures available for Western Europe.
Minor comments
Line 212 : Specify in the text the meaning of the abbreviation SM = Schalkenmehrener Maar
Line 214 : Replace Schalkenmehrener Maar with « SM »
Lines 245-247 : Rephrase. A dot is lacking after Figure 5b.
Line 391 : Replace « prescence » with « presence ».
Line 445 : the subject of the sentece is lacking.
References
Fletcher, W.J. & Sanchez Goñi, M.F. (2008) Orbital- and sub-orbital-scale climate impacts on vegetation of the western Mediterranean basin over the last 48,000 yr, Quaternary Research, 70, 451-464.
Hughes et al. (2022) The European glacial landscapes from the Last Glacial Maximum – synthesis. In : European Glacial Landscapes Maximum Extent of Glaciations, Elsevier, Pages 507-516.
Mix, A., Bard, E., Schneider, R., (2001). Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quat. Sci. Rev. 20, 627–657.
Naughton, F., et al. (2007). Present-day and past (last 25 000 years) marine pollen signal off western Iberia. Marine Micropaleontology 62: 91-114.
Sánchez Goñi, M.F., et al. (2000). European climatic response to millenial-scale changes in the atmosphere-ocean system during the Last Glacial period. Quaternary Research 54: 394-403.
Sanchez Goni M.F., et al. (2008) Contrasting impacts of Dansgaard-Oeschger events over a western European latitudinal transect modulated by orbital parameters, Quaternary Science Reviews, 27, 1136-1151.
Sánchez Goñi, M.F., et al. (2021). Muted cooling and drying of NW Mediterranean in response to the strongest last glacial North American ice surges. Bulletin of the Geological Society of America 133, 451-460.
Turon, J.-L., Lézine, A.-M., Denèfle, M., (2003). Land–sea correlations for the last glaciation inferred from a pollen and dinocyst record from the Portuguese margin. Quat. Res. 59, 88–96.

Citation: https://doi.org/10.5194/egusphere-2023-1960-RC1
- AC3: 'Response to review from Maria Fernanda Sanchez Goñi', Paul Zander, 05 Nov 2023
  
  See attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC3
RC2:
'Comment on egusphere-2023-1960', Anonymous Referee #2, 24 Oct 2023

The manuscript by Zander et al. presents a 60 kyr long record of brGDGTs composed by several lake sediment records from the Eifel volcanic field, Germany. By using an array of modern soil and top core sediments from the area, the authors show a clear and unique lacustrine signal in the lake sediments. Additionally this modern dataset performs relatively well when recent temperature calibrations are applied to it, however the authors observe that the performance of this calibrations is enhanced when a correction to the calibration is created based on the abundance of the isomer IIIa’’. When the Raberg et al., 2021 calibration, adjusted with the IIIa’’ correction, is applied to the 60 kyr record; the authors observe a generally good agreement between this record and previously published records for this period. Nevertheless, warmer than expected temperatures during Dansgaard-Oeschger events are interpreted by the authors as the proxy recording generally stable non freezing season temperatures, while the DO influence may have primarily been reflected in the winter season in this area.

I think this is a very well written manuscript with a clear structure and well presented data and results. I do not have any major comment and would be happy to recommend it for publication once the following minor observations are addressed.

Figure 3 A and B: I imagine the authors might have gone for open shapes in these panels to avoid cluttering the figure. However I find it slightly difficult to distinguish the colors when only the outlines are presented. Maybe using some transparency could help avoid cluttering in this figure and would make it easier to read.

Lines 287-288: Could you mention in-text some of the most relevant correlation values? I think it would enhance the reading of the text without having to look for Table 2.

Figure 8: This figure is a little hard to read since all the values from the different bins are presented together and it makes it hard to distinguish which bin the points belong to. Maybe something like removing the horizontal lines could improve this, or adding some space between the bins, would be my suggestion.

Line 445: Sentence is missing the first part.

Lines 459-460: This sentence is a little hard to read, please rephrase it.

Citation: https://doi.org/10.5194/egusphere-2023-1960-RC2
- AC2: 'Response to review from Anonymous Reviewer 2', Paul Zander, 05 Nov 2023
  
  Response to review from Anonymous Reviewer 2
  The manuscript by Zander et al. presents a 60 kyr long record of brGDGTs composed by several lake sediment records from the Eifel volcanic field, Germany. By using an array of modern soil and top core sediments from the area, the authors show a clear and unique lacustrine signal in the lake sediments. Additionally this modern dataset performs relatively well when recent temperature calibrations are applied to it, however the authors observe that the performance of this calibrations is enhanced when a correction to the calibration is created based on the abundance of the isomer IIIa’’. When the Raberg et al., 2021 calibration, adjusted with the IIIa’’ correction, is applied to the 60 kyr record; the authors observe a generally good agreement between this record and previously published records for this period. Nevertheless, warmer than expected temperatures during Dansgaard-Oeschger events are interpreted by the authors as the proxy recording generally stable non freezing season temperatures, while the DO influence may have primarily been reflected in the winter season in this area.
  I think this is a very well written manuscript with a clear structure and well presented data and results. I do not have any major comment and would be happy to recommend it for publication once the following minor observations are addressed.
  Thank you for your positive review, responses are in bold below
  Figure 3 A and B: I imagine the authors might have gone for open shapes in these panels to avoid cluttering the figure. However I find it slightly difficult to distinguish the colors when only the outlines are presented. Maybe using some transparency could help avoid cluttering in this figure and would make it easier to read.
  We will revise the figure to use filled shapes to improve the clarity of the figure.
  Lines 287-288: Could you mention in-text some of the most relevant correlation values? I think it would enhance the reading of the text without having to look for Table 2.
  Yes, thank you for the helpful comment. We will insert the most important correlation results into the text.
  Figure 8: This figure is a little hard to read since all the values from the different bins are presented together and it makes it hard to distinguish which bin the points belong to. Maybe something like removing the horizontal lines could improve this, or adding some space between the bins, would be my suggestion.
  
  We will revise the figure to include more space between bins to improve readability.
  Line 445: Sentence is missing the first part.
  Lines 459-460: This sentence is a little hard to read, please rephrase it.
  The text will be edited to fix the above comments in a revised version.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC2

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-1960', Francien Peterse, 27 Sep 2023
Dear Paul Zander and co-authors,
I came across your manuscript based on your Tweet (or whatever that is called these days) and was intrigued by some of the replies that suggested a coupling between the occurrence of the IIIa’’ isomer and redox changes. I just read through your work and have a few questions and comments for you to ponder on (this is not an official review, just an exchange of ideas I hope):
Your Fig. 2 suggests that you also detected IIIa’’ in soils, whereas Weber et al 2015 found that it was exclusively produced in lakes. In fact, they used the absence of IIIa’’ in soils and the presence of IIIa’’ in the lake sediment as their main argument for its aquatic source, later backed up by isotope analysis. Can you comment on the presence of IIIa’’ in the soils studied here?

In their follow up study (Weber et al 2018) is mentioned that IIIa’’ does not occur in all lakes. Based on the isotopic composition of IIIa’’-derived alkanes you could derive preferred production in eutrophic/anoxic lakes (see their Fig 4C-E), but just anoxia is not doing the trick. I can tell you firsthand that IIIa’’ is not detected in Lake Chala, although this lake has anoxic bottom waters (resulting in a continuously laminated sediment record; van Bree et al., 2020, Baxter et al 2023). Maybe the fact that Chala is oligotrophic explains its absence, but I would be careful with using IIIa’’ as marker for ‘just’ anoxic conditions.

Based on all the lake work that has recently appeared in the literature I have started to have serious doubts about the importance of soil-derived GDGTs in lake sediments, especially for lakes that are not connected to any river system. I feel that the vast majority, if not all brGDGTs in lake sediments are produced in situ, and that their production in the water column is strongly impacted by redox conditions, as you already mention in your manuscript, and has also clearly been shown by e.g., Weber et al., 2018, van Bree et al., 2020, Wu et al 2021, and Baxter et al., 2023. Most importantly, these studies have also shown that the producers of the different brGDGT isomers may occupy different niches in the water column. This can make brGDGT production very sensitive to changes in e.g., lake depth (be it climate driven, or by lake basin development or infilling of the lake over time) or mixing regime and affect the reliability of the currently available transfer functions. I know this mostly from working on Lake Chala, where we did very extensive lake monitoring to find this out (see Baxter et al., 2023), and I can imagine that this is not possible for every lake, but we should try to use all available information from these studies in our interpretation of novel lake records. Enfin, the point I wanted to make was that changes in IIIa/IIa may not necessarily reflect soil vs aquatic brGDGT sources, but instead the expansion and shrinking of the niches of their producers within the lake, related to redox. The different zones in the water column occupied by the producers of IIa and IIIa is actually clearly visible in your Fig. 4e.

Following this line or reasoning and letting go of IIIa/IIa as indicator for soil input, this also allows you to interpret the high BIT index values during the Holocene differently, especially since the high BIT index values are not supported by more clastic material (Fig. 7I), but rather with increased productivity, sediment anoxia, and methanogens (Fig. 7GHD). This would fit with a shallower oxycline in the lake, which increases the zone for brGDGT production (in the anoxic zone of the water column; see Weber et al., 2018, van Bree et al., 2020, Wu et al., 2021), and decreases the zone of crenarchaeol production, which, under ideal conditions peaks in the suboxic zone, as is also the case in the modern lake, shown in your Fig. 4C. As Thaumarchaeota exhibit photoinhibition, they cannot thrive in the very upper water layer, and are thus outcompeted during periods with less mixing, more stratification, etc, increasing the BIT index (see Baxter et al., 2021 and 2023 for detailed explanation and illustrating schematics). That this scenario may hold true is also reflected by the absolute abundances of the GDGTs, which show an increase in brGDGTs and isoGDGTs that are produced in the anoxic part of the lake, whereas crenarchaeol remains relatively stable/low during the transition into the Holocene.

I haven’t looked in much detail at the part of the record spanning MIS3, so I am not sure what exactly is going on there, and I can only encourage you to keep the water column structure, mixing regime, and lake basin development in the back of your mind when interpreting the data! Looking forward to hearing your thoughts about all of this.
All the best,

Francien Peterse
PS. Quick last note for the introduction: the presence of brGDGT Ia in culture was already shown by Sinninghe Damsté et al in 2011. Also, I miss a reference to Weijers et al., 2007 as the paper in which the temperature dependency of brGDGTs was first demonstrated and the MBT was defined.

References:

Baxter et al 2021 QSR

Baxter et al 2023 Nature (check methods section on calibrations and extended data Fig. 2)

van Bree et al 2020 BGS

Sinninghe Damsté et al., 2011 AEM

Weber et al 2015 GCA

Weber et al 2018 PNAS

Weijers et al., 2007 GCA

Wu et al 2021 Chemical Geology
Citation: https://doi.org/10.5194/egusphere-2023-1960-CC1
- AC1: 'Reply on CC1 Francien Peterse', Paul Zander, 18 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1960/egusphere-2023-1960-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC1
RC1:
'Comment on egusphere-2023-1960', Maria Fernanda Sanchez Goñi, 13 Oct 2023

This manuscript provides a continuous record from 60 ka to the present of brGDGT temperature reconstruction for the Eifel region (Central Europe). This record is relevant and deserves to be published in Climate of the Past. So far, there is a lack of regional quantitative climate reconstructions for the last glacial period in Central Europe and very few for other European regions. However, before accepting this manuscript for publication, the authors should address some chronological issues and discuss the ELSA record in more detail, taking into account the other quantitative climate reconstructions for Europe. I am not an expert in geochemistry and my review is mainly focused on section 3 of the manuscript "Results and discussion".
Lines 35-40 and lines 450-453: In the Introduction and in Section 3.4, it would be relevant to cite the papers that, based on pollen analysis of deep-sea sediment sequences, have detected for the first time, without chronological ambiguity, the European millennial scale climatic variability in response to the D-O warming and cooling events, including the Heinrich events (e.g. Sanchez Goñi et al. 2000, 2008). The only works cited in the manuscript are the terrestrial pollen records and speleothem sequences, which, even if relatively well dated, cannot be directly compared with the North Atlantic millennial-scale climate variability.
Figure 7: How is it possible to have 50%-100% of tree pollen in the 60-30 ka interval? These values are found during interglacial periods, as shown during the Holocene of the same sequence.
Lines 394-401 in section 3.3 and lines 453-471 in section 3.4: I do not know to what extent the temperature of months above freezing (TMAF) in the Eifel region can be compared with the sea surface temperatures (SST) of the Iberian margin but if the authors do so, I think it is more appropriate to discuss the TMAF in the Eifel region in the light of the surface air temperatures estimated from the pollen records of the Iberian margin, the Alboran Sea (Sanchez Goñi et al., 2002) and the Gulf of Lyon (Sanchez Goñi et al., 2021), which cover the period 50-27 ka, i.e. HS 5 to HS 3 and the associated D-O cycles. Although the latter correspond to winter temperatures, the authors should compare their estimates of summer temperatures with them, especially in the light of the question of seasonality they discuss in Section 3.4, which is the main contribution of this work. For HS 5, pollen-based winter surface air temperatures show pronounced decreases, averaging 5-10°C relative to present, in southern Iberia and south-eastern France, consistent with the severe SST low in the eastern North Atlantic. HS 3 is also marked by cooling in southern Iberia, although less so than HS 5 and HS 4. Furthermore, why are HS 5 and HS 3 marked by warm summers in the Eifel region and HS 4 by cold summers? Are these seasonal changes linked, for example, to orbital parameters?Figure 10 should be implemented with the pollen-based surface air temperatures from western Europe.
Lines 409-422: The authors discuss the mean temperature of the 29-24 kyr b2k interval in the Eifel and compare it with other reconstructions for the LGM without defining their respective chronologies. This interval extends from 29 to 19 ka according to Hughes et al. (2022) but from 23 to 19 ka according to Mix et al. (2001, EPILOG). The problem is that the LGM according to Hughes et al. encompasses three D-O warming events (D-O 4, D-O 3 and D-O 2), HS 2 and the LGM defined by EPILOG. Do they compare the same period in the different reconstructions (Paris Basin and Auel Maar, "...considering the significant age uncertainty in the Auel Maar data", line 372, for example), the assimilation of global palaeoclimatic data and the output of the HadCM3 model? In terms of vegetation in north-western and southern Iberia, the HS and LGM are markedly different (Naughton et al., 2007; Fletcher et al., 2008; Turon et al., 2003), with more Ericaceae and less Amaranthaceae-Chenopodiaceae during the LGM compared with the HS that flank it, indicating drier and colder conditions during the HS, associated with greater seasonality.
Lines 468-471 : It is interesting to add that the 17°C of warming of the coldest month between stadial and interstadial yielded by the LOVECLIM model simulations (Van Vermeerck et al., 2011) is higher compared to the pollen-based estimations of 10°C on average for SW and SE Iberia and 5°C for SE France.
The last part of section 3.4, devoted to the examination of changes in seasonality during the last glacial period, should be improved and strengthened by taking into account the pollen-based reconstructions of winter temperatures available for Western Europe.
Minor comments
Line 212 : Specify in the text the meaning of the abbreviation SM = Schalkenmehrener Maar
Line 214 : Replace Schalkenmehrener Maar with « SM »
Lines 245-247 : Rephrase. A dot is lacking after Figure 5b.
Line 391 : Replace « prescence » with « presence ».
Line 445 : the subject of the sentece is lacking.
References
Fletcher, W.J. & Sanchez Goñi, M.F. (2008) Orbital- and sub-orbital-scale climate impacts on vegetation of the western Mediterranean basin over the last 48,000 yr, Quaternary Research, 70, 451-464.
Hughes et al. (2022) The European glacial landscapes from the Last Glacial Maximum – synthesis. In : European Glacial Landscapes Maximum Extent of Glaciations, Elsevier, Pages 507-516.
Mix, A., Bard, E., Schneider, R., (2001). Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quat. Sci. Rev. 20, 627–657.
Naughton, F., et al. (2007). Present-day and past (last 25 000 years) marine pollen signal off western Iberia. Marine Micropaleontology 62: 91-114.
Sánchez Goñi, M.F., et al. (2000). European climatic response to millenial-scale changes in the atmosphere-ocean system during the Last Glacial period. Quaternary Research 54: 394-403.
Sanchez Goni M.F., et al. (2008) Contrasting impacts of Dansgaard-Oeschger events over a western European latitudinal transect modulated by orbital parameters, Quaternary Science Reviews, 27, 1136-1151.
Sánchez Goñi, M.F., et al. (2021). Muted cooling and drying of NW Mediterranean in response to the strongest last glacial North American ice surges. Bulletin of the Geological Society of America 133, 451-460.
Turon, J.-L., Lézine, A.-M., Denèfle, M., (2003). Land–sea correlations for the last glaciation inferred from a pollen and dinocyst record from the Portuguese margin. Quat. Res. 59, 88–96.

Citation: https://doi.org/10.5194/egusphere-2023-1960-RC1
- AC3: 'Response to review from Maria Fernanda Sanchez Goñi', Paul Zander, 05 Nov 2023
  
  See attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC3
RC2:
'Comment on egusphere-2023-1960', Anonymous Referee #2, 24 Oct 2023

The manuscript by Zander et al. presents a 60 kyr long record of brGDGTs composed by several lake sediment records from the Eifel volcanic field, Germany. By using an array of modern soil and top core sediments from the area, the authors show a clear and unique lacustrine signal in the lake sediments. Additionally this modern dataset performs relatively well when recent temperature calibrations are applied to it, however the authors observe that the performance of this calibrations is enhanced when a correction to the calibration is created based on the abundance of the isomer IIIa’’. When the Raberg et al., 2021 calibration, adjusted with the IIIa’’ correction, is applied to the 60 kyr record; the authors observe a generally good agreement between this record and previously published records for this period. Nevertheless, warmer than expected temperatures during Dansgaard-Oeschger events are interpreted by the authors as the proxy recording generally stable non freezing season temperatures, while the DO influence may have primarily been reflected in the winter season in this area.

I think this is a very well written manuscript with a clear structure and well presented data and results. I do not have any major comment and would be happy to recommend it for publication once the following minor observations are addressed.

Figure 3 A and B: I imagine the authors might have gone for open shapes in these panels to avoid cluttering the figure. However I find it slightly difficult to distinguish the colors when only the outlines are presented. Maybe using some transparency could help avoid cluttering in this figure and would make it easier to read.

Lines 287-288: Could you mention in-text some of the most relevant correlation values? I think it would enhance the reading of the text without having to look for Table 2.

Figure 8: This figure is a little hard to read since all the values from the different bins are presented together and it makes it hard to distinguish which bin the points belong to. Maybe something like removing the horizontal lines could improve this, or adding some space between the bins, would be my suggestion.

Line 445: Sentence is missing the first part.

Lines 459-460: This sentence is a little hard to read, please rephrase it.

Citation: https://doi.org/10.5194/egusphere-2023-1960-RC2
- AC2: 'Response to review from Anonymous Reviewer 2', Paul Zander, 05 Nov 2023
  
  Response to review from Anonymous Reviewer 2
  The manuscript by Zander et al. presents a 60 kyr long record of brGDGTs composed by several lake sediment records from the Eifel volcanic field, Germany. By using an array of modern soil and top core sediments from the area, the authors show a clear and unique lacustrine signal in the lake sediments. Additionally this modern dataset performs relatively well when recent temperature calibrations are applied to it, however the authors observe that the performance of this calibrations is enhanced when a correction to the calibration is created based on the abundance of the isomer IIIa’’. When the Raberg et al., 2021 calibration, adjusted with the IIIa’’ correction, is applied to the 60 kyr record; the authors observe a generally good agreement between this record and previously published records for this period. Nevertheless, warmer than expected temperatures during Dansgaard-Oeschger events are interpreted by the authors as the proxy recording generally stable non freezing season temperatures, while the DO influence may have primarily been reflected in the winter season in this area.
  I think this is a very well written manuscript with a clear structure and well presented data and results. I do not have any major comment and would be happy to recommend it for publication once the following minor observations are addressed.
  Thank you for your positive review, responses are in bold below
  Figure 3 A and B: I imagine the authors might have gone for open shapes in these panels to avoid cluttering the figure. However I find it slightly difficult to distinguish the colors when only the outlines are presented. Maybe using some transparency could help avoid cluttering in this figure and would make it easier to read.
  We will revise the figure to use filled shapes to improve the clarity of the figure.
  Lines 287-288: Could you mention in-text some of the most relevant correlation values? I think it would enhance the reading of the text without having to look for Table 2.
  Yes, thank you for the helpful comment. We will insert the most important correlation results into the text.
  Figure 8: This figure is a little hard to read since all the values from the different bins are presented together and it makes it hard to distinguish which bin the points belong to. Maybe something like removing the horizontal lines could improve this, or adding some space between the bins, would be my suggestion.
  
  We will revise the figure to include more space between bins to improve readability.
  Line 445: Sentence is missing the first part.
  Lines 459-460: This sentence is a little hard to read, please rephrase it.
  The text will be edited to fix the above comments in a revised version.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1960-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (04 Dec 2023) by Qiuzhen Yin

AR by Paul Zander on behalf of the Authors (20 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 Jan 2024) by Qiuzhen Yin

RR by Maria Fernanda Sanchez Goñi (09 Jan 2024)

RR by Anonymous Referee #3 (02 Feb 2024)

Suggestions for revision or reasons for rejection

I am providing a review of the author replies to previous reviewers’ comments and the adjustments made in the manuscript. I think that the authors have done a good job responding to the main concerns and my additional points are only minor. Notably, temperature reconstruction based on brGDGTs in lacustrine sediments is notoriously hard, so I appreciate the efforts of the authors here. Although they have done an above average job in assessing the brGDGT signals in their record, I feel like there could have been even more. Regardless, given the overall positive feedback in the first round of comments, I’ll leave it to future studies with further insights into lacustrine brGDGT dynamics to possibly extract this information from the data on Pangaea.

L18: We find a bias… -> specify: bias in what?

L42&section 3.4: Millennial scale climate events are also detected in Lake Van (there are multiple papers from this lake, but Stockhecke et al. (2021, https://doi.org/10.1016/j.palaeo.2021.110535) specifically is on biomarkers and D-O events, but just predates the global lake calibration – still, D-O variation is presumably >>1C!)

L142: were one (as is mentioned in the manuscript) or two (as is proposed by Hopmans et al., 2016) silica columns used for GDGT separation by the LC?

Calibration selection: I never really understand the urge to assess all possible calibrations and then continue with the one that looks most like the expected (or desired?) record. How does matching expectations validate the reliability of the record? Especially when most of the tested calibrations come down to (some form of) the MBT…
There is always information in downcore changes in brGDGT (or any biomarker) distributions. So, if one calibration does “not work” it is always worth to investigate which compound is driving this and then to assess why this compound could be behaving the way it is. Notably, the PCAs in Fig. 3 suggest that temperature is likely not recorded well by the MBT’5me, as the compounds that are in the numerator (tetramethylated 5-me brGDGTs) and the denominator (penta- and hexamethylated 5-me brGDGTs) of this ratio do not clearly plot in opposite parts of any of the PCs (specifically, IIa plots with all tetramethylated brGDGTs, which would make the MBT’5me mostly dependent on IIIa, IIIb, and IIIc).
Anyhow, I guess that this manuscript has passed the stage of this kind of comments, so keep things as is. Also, during the review process of this manuscript two new lacustrine brGDGT studies have appeared: one specifically for European lakes (Bauersachs et al., 2024, https://doi.org/10.1016/j.scitotenv.2023.167724) and a global one (O’Beirne et al., 2023, https://doi.org/10.1016/j.gca.2023.08.019) that both address some of the sensitivity issues that are also discussed in the current manuscript. In particular the European calibration study addresses the potential role and presence of IIIa’’ in lake sediments.

L370: Add reference after the statement that BIT is more likely controlled by the niche for crenarchaeol producing Thaumarchaeota (which are currently named Nitrososphaerota btw)

L378: I agree with the interpretation that IIIa/IIa is more likely to reflect contributions from different niches within the water column than soil vs aquatic producers. Please also adjust this in the panels and captions of the figures (for those people that ‘read’ a paper by only looking at the graphs). Also, later in the discussion (e.g., section 3.3) the interpretation of IIIa/IIa as a soil indicator is still maintained. So, which interpretation is considered valid in the end?

General comments:
Replace all apostrophes with the prime symbol throughout the manuscript.
Fig. 4: connect datapoints with straight lines as to not infer trends.
Check the order of the figures to match with their occurrence in the text. For sure Fig. 3 is out of order.

Hide

ED: Publish subject to minor revisions (review by editor) (06 Feb 2024) by Qiuzhen Yin

AR by Paul Zander on behalf of the Authors (13 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Feb 2024) by Qiuzhen Yin

AR by Paul Zander on behalf of the Authors (20 Feb 2024)

Journal article(s) based on this preprint

08 Apr 2024

Reconstruction of warm-season temperatures in central Europe during the past 60 000 years from lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGTs)

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald H. Haug, and Alfredo Martínez-García

Clim. Past, 20, 841–864, https://doi.org/10.5194/cp-20-841-2024,https://doi.org/10.5194/cp-20-841-2024, 2024

Short summary

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald Haug, and Alfredo Martínez-García

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https://doi.org/10.5194/egusphere-2023-1960-supplement

Paul D. Zander, Daniel Böhl, Frank Sirocko, Alexandra Auderset, Gerald Haug, and Alfredo Martínez-García

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Bacterial lipids (branched glycerol dialkyl glycerol tetraethers; brGDGTs) extracted from lake sediments were used to reconstruct warm-season temperatures in central Europe during the past 60 kyr. Modern samples were used to test and correct for bias related to varying sources of brGDGTs. The temperature reconstruction is significantly correlated with other temperature reconstructions but features less millennial-scale variability, which is attributed to the seasonal signal of the proxy.


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