the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Feb 2026
Holocene hydro-climatic variability and multi-frequency analyses at Lake Sidi Ali (Morocco)
Abstract. The North African desert margin is considered one of the most sensitive areas to future climate changes. Improved knowledge about Holocene climatic variability and environmental responses on millennial to centennial scale will help to refine scenarios related to future climate changes. During the last two decades, the recovery and compilation of Holocene records from the subtropical North Atlantic and the Mediterranean realms have improved our knowledge about the millennial-scale variability of the Western Mediterranean palaeoclimate and the Saharan dust cycle. However, the understanding on periodicities as well as potential coupling and forcing mechanisms remains poor. To detect periodicities in Holocene climatic variability and geomorphological processes, we use a Holocene sediment record from Lake Sidi Ali in the semiarid to sub-humid Middle Atlas with a robust 210Pb /137Cs and pollen-concentrates-based 14C chronology. We use a high-resolution core scanning-XRF record, in order to distinguish between lake-internal (e.g., chemical precipitation) and lake-external (e.g., detrital input) processes. Redfit and Wavelet time series analyses reveal distinct periodicities of millennial to centennial scale. By a correlation analysis of extracted, highly significant, frequency analysis spectra, three XRF-based “Redfit Proxy Groups” (RPGs) which potentially reflect different hydro-climatic forcing mechanisms were derived. Subsequently, we integrated environmental and climatic proxies from the same core (Cedrus pollen abundance, magnetic susceptibility, δ18O and δ13C values of ostracod shells, grain-size endmembers and total organic carbon) and used their wavelet domain to improve the interpretation. Finally, we identified two main periodicity regimes that affected, on the one hand, the hydrological regime and, on the other hand, the lake productivity and catchment erosion dynamics. For RPG 1 (Ca, Sr, Ca/Ti, Sr/Ti), we identified 2 ky and 1 ky periodicities, which we interpret as precipitation/evaporation related proxies in the context of North Atlantic and solar forcing. For RPG 2 (Fe, Ti, K, Si/Ti), we observe 3.5 ky and 1.5 ky periodicities, which we interpret as driven by lake productivity or detrital input.
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Status: open (until 03 Apr 2026)
- RC1: 'Comment on egusphere-2026-414', Anonymous Referee #1, 07 Mar 2026 reply
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RC2: 'Comment on egusphere-2026-414', Anonymous Referee #2, 20 Mar 2026
reply
This is an interesting and potentially valuable study from Schmidt et al and I am confident that it would make a great addition to Climate of the Past. The manuscript applies an innovative frequency-domain approach to high-resolution lacustrine geochemical data from an underrepresented palaeoclimatic region, and the attempt to group XRF proxies based on shared spectral behaviour is original and promising. The Lake Sidi Ali record has clear potential to contribute meaningfully to discussions of Holocene hydroclimatic variability in northwest Africa. I really enjoyed reading this manuscript and learnt a lot about the methodology.
Comments:
The abstract is detailed and methodologically informative, which I appreciate. However, the main scientific findings and their broader significance could be communicated more clearly. The key conclusions are present, but they appear rather late and are somewhat overshadowed by methodological detail. I suggest ending with one or two more direct sentences that state the principal findings and why they matter for clarity.
The statement of aims is slightly inconsistent in structure. The manuscript first presents aims (i)-(iii), then follows with an additional summarising aim introduced differently. It would read more clearly either to frame (i)-(iii) explicitly as objectives and have the final sentence as the overarching aim, or to keep all aims in the same format.
Line 97 - “with individual trees exceeding 800 years in age” may read more smoothly than “trees reaching up to more than 800 years”.
Table 1- Because the temporal resolutions are given as approximate values throughout, it may be clearer to title the relevant column “Approximate temporal resolution” rather than repeating an approximation symbol in each cell.
Figure 2 is a helpful conceptual illustration. However, the label “significant spectrum” in panel C is difficult to read at page size. Please consider improving legibility, for example through font size, or revised labelling.
Please justify more clearly why a third-order polynomial regression was selected for detrending. Polynomial detrending can remove genuine low-frequency climate variability as well as edge effects, so it would be useful to explain why this choice is appropriate here, or ideally to show that the principal spectral results are not overly sensitive to the detrending method.
Grouping proxies based on correlations among significant Redfit spectra is innovative, but the robustness and limitations of this procedure deserve further discussion. For example, how sensitive are the groups to the chosen significance threshold, detrending approach, interpolation method, or age uncertainty? A short sensitivity analysis, or at least a fuller discussion, would strengthen confidence in the grouped interpretations.
The manuscript would benefit from a clearer explanation, earlier in the paper, of why these particular elements and element ratios were selected and what process hypotheses they are expected to represent at Lake Sidi Ali. While the later discussion develops these interpretations, readers would benefit from more guidance before reaching that section. I would agree with Reviewer 1 and suggest that a PCA could be used here as I have no doubt that the chosen elements are grouped together, but visually it would be good to see. Perhaps this could go into the supplementary information if not needed in the main manuscript.
Figure 5 presents an interesting summary of proxy evolution through time, but I found it difficult to extract the main message. If the figure is retained in its current form, I suggest adding annotations or a clearer explanation in the caption and main text to direct readers toward the major patterns and take-home points.
More generally, I encourage the authors to distinguish carefully between proxies that show similar frequency behaviour and proxies that can be confidently interpreted as reflecting the same environmental process. The grouping is compelling, but the mechanistic interpretation remains an inference and should be framed as such.
The manuscript usefully notes that age uncertainty constrains the robustness of shorter periodicities and that comparisons with lower-resolution non-XRF proxies are partly visual. These caveats are important and could be made more visible where the shorter-period features and cross-proxy comparisons are discussed in the Results and Discussion.
You change between hydro-climatic and hydroclimatic. I believe either is fine, but best be consistent throughout.
Citation: https://doi.org/10.5194/egusphere-2026-414-RC2
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- 1
Summary:
The manuscript presents the analysis of several types of geobiochemically derived proxy data extracted from a lacustrine record in Morocco spanning the Holocene. The proxies are mostly believed to represent broad hydroclimatological conditions in the region. The proxies are grouped into three categories based on their cross-correlation. The time series is subject to a spectral analysis to identify the main periodicities. These periodicities are interpreted, mostly heuristically, as driven by large-scale hydroclimatic conditions or by more local lacustrine processes.
Recommendation: This careful and comprehensive analysis is well reflected in the manuscript, and it is also compared to other records from this region. I have a few suggestions, mostly concerning the statistical analysis, that the authors may want to consider
1) The grouping of the proxies seems a bit simplistic: just subjectively grouped by their cross-correlations. A more objective grouping could be achieved simply by principal component analysis. This would have the additional advantage of providing a single time series for each component that could then be compared to a series of reconstructed solar activity, for instance. (see point 2)
2) The physical interpretation is essentially heuristic. The authors visually identify the more important spectral ranges, thereby attributing those variations to known forcings that exhibit similar spectral ranges. Whereas this is common in this type of study, this approach seems a bit superficial, as a common spectral band is a necessary but not sufficient condition: also, the phase of the variations is important for attributing them to, say, solar forcing.
Calculating the PCA components would simplify that comparison. Figure 4 attempts this type of attribution by visually identifying the maxima and minima of the individual time series, but this Figure also illustrates that a confirmation bias is dangerously possible. One can always find maxima or minima that, to some extent, agree.
In this case, to claim the identification of solar forcing, the series of one of the PCs should closely agree with the reconstruction of solar activity of Usoskin et al. 2016 (their figure 3c). Such an agreement would be a much more robust confirmation
Particular points
3) ' Further, Sha et al. (2019)
presented a speleothem record from the SW High Atlas Mountains, that seems not to be influenced by Atlantic air masses but rather tropical-monsoonal air masses'
The use of commas to introduce subordinate clauses is sometimes incorrect throughout the manuscript, but this is an example that impacts the meaning. With the comma, the sentence indicates that the Atlas Mountains are not influenced by the Atlantic air masses. Without the comma, it is the record that is not influenced by the Atlantic air masses. I guess the authors meant the latter case, so the comma should be deleted.
4) 'Currently, the lake is seprated '
separated
5) 'The red noise background is sensitive to the specific persistence of the data and
therefore higher frequencies are often covered by the red noise'
I cannot understand what this sentence would mean. Perhaps they mean that the existence of red noise in a time series blurs the high-frequency variations?
But red noise is not a 'background', it is a property of the time series. Whether or not high frequency variations are blurred or not depends on the relative spectral energy in their respective bands
6) We used the most conservative critical false alarm level (Schulz and Mudelsee, 2002)'
I guess the authors mean that they used the most conservative choice offered by the software. The most conservative option possible is always that no peaks is considered significant.
7) ' Zielhofer et al. (2017a) argued for the Sidi Ali record, that..'
Delete comma
8) 'The latter is in accordance to the 1 ky periodicity of the hydroclimatic δ18O signal '
in accordance with
9) 'Stewart et al. (2017) presented a Bromine record from a peat bog in Scotland, which shows a
distinct 2 ky cycle pattern'
... Scotland that shows a .....
Again, delete the comma. I guess that Scotland does not present a 2 k cycle pattern.