the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A Holocene temperature (brGDGT) record from Garba Guracha, a high-altitude lake in Ethiopia
- 1Heisenberg Chair of Physical Geography with focus on paleoenvironmental research, Institute of Geography, Technische Universität Dresden, Dresden, Germany
- 2Geological Institute, Department of Earth Sciences, ETH Swiss Federal Institute of Technology, 8092 Zurich, Switzerland
- 3Plant Ecology and Geobotany dept., Philipps-Marburg University, Marburg, Germany
- 4Department of Geo-environmental Processes and Global Change, Pyrenean Institute of Ecology, CSIC, Zaragoza, Spain
- 5Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK
- 6Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland
- 7Department of Geological Sciences, Brown University, USA
- 1Heisenberg Chair of Physical Geography with focus on paleoenvironmental research, Institute of Geography, Technische Universität Dresden, Dresden, Germany
- 2Geological Institute, Department of Earth Sciences, ETH Swiss Federal Institute of Technology, 8092 Zurich, Switzerland
- 3Plant Ecology and Geobotany dept., Philipps-Marburg University, Marburg, Germany
- 4Department of Geo-environmental Processes and Global Change, Pyrenean Institute of Ecology, CSIC, Zaragoza, Spain
- 5Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK
- 6Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland
- 7Department of Geological Sciences, Brown University, USA
Abstract. Eastern Africa has experienced strong climatic changes since the last deglaciation (15,000 years ago). The driving mechanisms and teleconnections of these spatially complex climate variations are yet not fully understood. Although previous studies on lake systems have largely enhanced our knowledge of Holocene precipitation variation in eastern Africa, few such studies have reconstructed the terrestrial temperature history of eastern Africa from lake archives. Here, we present (i) a new branched glycerol dialkyl glycerol tetraether (brGDGT) temperature calibration that includes Bale Mountain surface sediments and (ii) a quantitative record of mean annual temperature (MAT) over the past 12 cal ka BP using brGDGTs in a sediment core collected from Garba Guracha (3950 m a.s.l.) in the Bale Mountains. After adding Bale Mountain surface sediment (n=11) data to the existing East African lake dataset, additional variation in 6-methyl brGDGTs was observed, which necessitated modifying the MBT'5ME calibration by adding 6-methyl brGDGT IIIa' (resulting in the MBT-Bale Mountain index, r2=0.93, p<0.05). Comparing the MBT’5ME and the new MBT-Bale Mountain index, our high altitude Garba Guracha temperature record shows that significant warming occurred shortly after the Holocene onset. The temperature increased by more than 3.0 °C in less than 600 years. The highest temperatures prevailed between 9 and 6 cal ka BP, followed by a temperature decrease until 1.4 cal ka BP. The reconstructed temperature history is strongly linked to supraregional climatic changes associated with insolation forcing and the African Humid Period (AHP), as well as with local anomalies associated with catchment deglaciation and hydrology.
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Lucas Bittner et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-95', Jonathan Raberg, 24 May 2022
Review of: "A Holocene temperature (brGDGT) record from Garba Guracha, a high-altitude lake in Ethiopia" by Lucas Bittner et al.
Reviewed by: Jonathan Raberg
Overview and recommendation:
Branched glycerol dialkyl glycerol tetraethers are bacterial membrane-spanning lipids whose distribution has been shown to correlate with temperature. This correlation has made the lipids a valuable paleotemperature proxy in a variety of archives, including lake sediments. Here, Bittner et al. contribute to both the refinement of this paleotemperature proxy through a regional study of modern lake surface sediments and to our understanding of East African paleoclimate through an application of the refined proxy downcore. The authors find that a brGDGT isomer that is usually excluded from proxy applications is important in their study region. They also reconstruct the paleotemperature history of a high-altitude lake in a region where such data are lacking and put the results in a broader climatic context. The work is well thought out, well written, and makes a significant contribution to the study of brGDGTs and their application. It is well suited for publication in Biogeosciences. I recommend it for publication with one major revision and some minor revisions.
Major Revision:
An important contribution of this work is the inclusion of the 6-methyl brGDGT IIIa’ into a regional temperature calibration. This compound is normally excluded from such calibrations, but here the authors show that it is important for maximizing r2 in the modern dataset. This compound also shows substantial variation downcore: Figure 4 shows sediments from the AHP have a higher proportion of 5-methyl compounds while those from the Meghalayan have more 6-methyl. The authors note that this shift coincides with prolonged drought conditions and hypothesize that two factors – lake water conductivity/salinity and/or microbial community shifts – could be driving these changes in IIIa’ downcore. While the latter would require much additional analysis beyond the scope of the work, the former hypothesis is easily testable.
In theory, regional drying could lead to an increase in lake water conductivity/salinity, especially for this seasonally closed lake (which could perhaps become permanently closed if lake level dropped?). Calibrations exist for reconstructing such conductivity/salinity changes (Raberg et al., 2021; Wang et al., 2021). I recommend the authors apply calibrations from one or both of these publications to the Garba Guracha record and discuss the results, specifically Equation 12 or S5-7 from Raberg et al. (2021) and/or Equation 10 or 11 from Wang et al. (2021).
Minor Revisions:
Does temperature go below freezing for these lakes? If not, it would be nice to mention that MAT = MAF for these lakes.
Does salinity, conductivity, or pH data exist for these lakes? I wonder if that could help explain their deviation from the East African Lakes dataset.
L68: “Northern” is capitalized while “eastern” is not.
L82: Morrissey et al. (2018) use isoprenoidal rather than branched GDGTs, I believe…
L85-87: A few additional citations that may be of interest are (Weber et al., 2018) and (Van Bree et al., 2020), both of which examined microbial communities and brGDGTs in lacustrine settings. (I see these publications are cited later in the manuscript, but I think they would be relevant here as well.) Halamka et al. (2021) also cultured a brGDGT-producing acidobacterium. Depending on timing, two recent pre-prints (Chen et al., 2022; Toby A Halamka et al., 2022) may also be relevant.
L89: Sentence should end, “…pH values, respectively”
L120: Remove period after “sea level”
L176: Put “(III)” after “hexamethylated” for consistency
L177: Change to “α and/or ω C5…”
L191: Fractional abundance is defined here, but the authors use mostly percentages rather than fractions throughout the text. Would be good to standardize for clarity.
Fig. 2: Inset has “East African Lakes” while caption has “eastern African lakes”. Is there a difference? Colors in caption don’t match those in plot. State whether the PCA uses fractional or absolute abundances.
L218-219: The nomenclature of this sentence is a bit confusing and inconsistent.
Fig. 3: Again, clarify that plots B and C are using % rather than absolute abundance. Check that colors match.
L229: Again, specify that this is %IIIa or f(IIIa)
L230-231: Raberg et al. (2021) and Wang et al. (2021) both show that conductivity/salinity can control isomerization in lake sediments.
Table 1: Define EAL and EALBM in caption
L281: R2 for EALBM doesn’t match that in Table 1
L283-287: Is “Table 6; Eq. 7” in L285 supposed to be “Table 1; Eq. 7”? And are you referring to the calibration using the MIa Set in Table S3 of Raberg et al. (2021) here? They’re not completely comparable as that calibration uses multiple fractional abundances (fIaMI2, fIIa’MI, fIIIaMI, and fIIIaMI2) calculated in the Meth-Isom Set while your Equation 7 in Table 1 would just be the fractional abundance of Ia in the Meth-Isom Set (fIaMI). I calculated the correlation between fIaMI (= your Equation 7) and MAF in the dataset from Raberg et al. (2021) and it has r2 = 0.75 and RMSE = 3.45°C (p-value << 0.01), so you can make a direct comparison between Eq. 7 in Table 1 and those values if you’d like.
Fig. 6: Caption should start with “Correlations of the EALBM datasets”? Colors don’t match.
L298-301: Consider rephrasing this. Also, it was unclear to me where the r2 = 0.97 came from.
Reference:
Van Bree, L. G. J., Peterse, F., Baxter, A. J., De Crop, W., Van Grinsven, S., Villanueva, L., et al. (2020). Seasonal variability and sources of in situ brGDGT production in a permanently stratified African crater lake. Biogeosciences, 17(21), 5443–5463. https://doi.org/10.5194/bg-17-5443-2020
Chen, Y., Zheng, F., Yang, H., Yang, W., Wu, R., Liu, X., et al. (2022). The production of diverse brGDGTs by an Acidobacterium allows a direct test of temperature and pH controls on their distribution. BioRxiv [Preprint]. https://doi.org/10.1101/2022.04.07.487437
Halamka, T.A., McFarlin, J. M., Younkin, A. D., Depoy, J., Dildar, N., & Kopf, S. H. (2021). Oxygen limitation can trigger the production of branched GDGTs in culture. Geochemical Perspectives Letters, 36–39. https://doi.org/10.7185/geochemlet.2132
Halamka, Toby A, Raberg, J. H., Mcfarlin, J. M., Younkin, A. D., Liu, X., & Kopf, S. H. (2022). Production of diverse brGDGTs by Acidobacterium Solibacter usitatus in response to temperature , pH , and O 2 provides a culturing perspective on brGDGT paleoproxies and biosynthesis. EarthArXiv [Preprint].
Raberg, J. H., Harning, D. J., Crump, S. E., De Wet, G., Blumm, A., Kopf, S., et al. (2021). Revised fractional abundances and warm-season temperatures substantially improve brGDGT calibrations in lake sediments. Biogeosciences, 18, 3579–3603. https://doi.org/10.5194/bg-18-3579-2021
Wang, H., Liu, W., He, Y., Zhou, A., Zhao, H., Liu, H., et al. (2021). Salinity-controlled isomerization of lacustrine brGDGTs impacts the associated MBT5ME’ terrestrial temperature index. Geochimica et Cosmochimica Acta, 305, 33–48. https://doi.org/10.1016/j.gca.2021.05.004
Weber, Y., Damsté, J. S. S., Zopfi, J., De Jonge, C., Gilli, A., Schubert, C. J., et al. (2018). Redox-dependent niche differentiation provides evidence for multiple bacterial sources of glycerol tetraether lipids in lakes. Proceedings of the National Academy of Sciences of the United States of America, 115(43), 10926–10931. https://doi.org/10.1073/pnas.1805186115
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RC2: 'Comment on bg-2022-95', Anonymous Referee #2, 07 Jun 2022
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-95/bg-2022-95-RC2-supplement.pdf
Lucas Bittner et al.
Lucas Bittner et al.
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