the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jul 2025
Decoupling climate and avalanche activity: Holocene insights from lacustrine sediments in western Norway
Abstract. Long-term reconstructions provide critical insights into the interplay between large-scale climatic systems and snow avalanche frequency over millennial timescales. This study presents a ~10,000-year-long snow avalanche record reconstructed from a continuous sediment sequence in a lake in western Norway. A multi-proxy approach, combining bulk sediment analysis (LOI, DBD, and grain size) with high-resolution scanning techniques (MS, XRF, CT scanning), was used to characterise the sediments. The avalanche layers consist of a sand-rich base, dispersed organic fragments, and a fining-upward sequence of silt and clay. We applied two independent, semi-automatic detection methods to quantify event frequency and sediment influx over time: the rate of change (RoC) of Ti and interactive thresholding of CT greyscale data. Sedimentation in the early Holocene (>10,100 cal yr BP) was dominated by glacial and periglacial processes linked to the study area's deglaciation. A period of low avalanche activity followed during the Holocene Thermal Maximum (>6500 cal yr BP). The avalanche frequency increased during the mid-Holocene (6500–5000 cal yr BP) and intensified further in the late Holocene, with peak activity from 2300 cal yr BP to the present, a period characterised by large inter-centennial fluctuations and abrupt shifts in avalanche frequency. Comparison with regional palaeoclimatic and palaeo-environmental records suggests that the snow avalanche frequency is primarily controlled by large-scale atmospheric circulation patterns, particularly the intensity of the predominant westerly winds and humidity along the western coast of Norway. These fluctuations are closely linked to North Atlantic Sea surface temperature variability, shifts in the North Atlantic Oscillation, and broader synoptic climate dynamics.
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Status: open (until 24 Sep 2025)
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RC1: 'Comment on egusphere-2025-2910', Anonymous Referee #1, 05 Sep 2025
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This well written and interesting paper investigates the sedimentary record of a high-mountain lake in Western Norway. The author focus on the event-layer chronology and put this in the context of avalanche frequency as a result of climate and weather patterns. They identify three intriguing periods in the Holocene when avalanche activity was highly different. These findings are coinciding with patterns of flood and glacial activity. Moreover, there seems to be a clear link to NAO on a centennial time scale, with all of these players influencing the moisture distribution, temperature (snowline!) and atmospheric circulation patterns. The topic is highly relevant for the Journal Climate of the Past and I was pleased to read this well-structured and well written contribution.
I do have some key comments upfront, followed by some detailed comments.
I welcome the applied dual method of identifying event layers (CT thresholding, RoC) minimizing subjectivity. However, my main concern is that the authors interpret all event layers solely as avalanche-induced ("wet snow avalanches entering an ice-free lake"), and do not consider them being deposited by extreme floods caused by precipitation events. There is no doubt that this basin is influenced by avalanches, which leave some traces in the sedimentary record. However, I am missing a discussion on extreme precipitation events, and how the respective layer can be distinguished. The authors mention that "only small, intermittent streams and no evidence of substantial fluvial reworking" occur and "Given their limited drainage area, the contribution of these streams to the influx of allochthonous minerogenic sediments into the lake is minimal". But there are plenty of high-mountain lake-sediment records that are clearly punctuated by flood deposits, in particular also in basins with no permanent inflow. These lakes also lack major deltas or alluvial fans, as is the case of the Lake Vatnasetvatnet, but this does not mean that extreme precipitation events do not deliver minerogenic deposits to the basin. In fact, the paper does not describe in detail the sedimentologic characteristics of the event layers. Fig. 5 is very nice, and displays for one case the detailed sediment structures, which is, however, in my view highly similar to many well-described flood turbidites from comparable lakes. There is the sharp base, a basal sand, a graded unit, disperse organic matter and a fining-upward cap (why is "inverse graded" on Fig. 5 and "fining upward" in the text?). Some high-density inversions with thin layers towards the base of the event deposit (beautifully shown on the CT data) may also indicate the waxing and waning of the flood (well described in literature), a process that I cannot imagine well in an avalanche. So I am very reluctant to assign all these layers to avalanches. As this is the key interpretation of this study, I expect at least a detailed discussion and explanation, showing more sedimentologic features of the layers, why they cannot be deposited by floods. There must be avalanche-specific features of event layers in the literature, but what I see seems highly reminiscent of flood layers. Several types of avalanches are mentioned (ice-free lake, on frozen lake, wet snow, rain-induced), is it also possible that they can be discerned? In the end, all layers are solely considered to be wet avalanches in an unfrozen lake and I am not convinced that this is the case. This uncertainty is also caused by the lack of an instrumental or historic calibration, i.e. a ground truthing that identifies a recent prominent layer and correlates it to a known event. Not sure such time series exist for Lake Vatnasetvatnet, but it should be attempted or at least commented why this is not possible.
What is also is intriguing in this context, is that the avalanche record does match closely the flood records of other Norwegian lakes (Fig. 9). In all three distinguished Holocene periods, the authors mentions matching records of snowmelt floods chronologies, so the pattern is the same. This leaves me uncertain whether avalanches really match flood records, or whether we indeed look rather at an avalanche or a combined record.
As last general comment, I would say that I rather recognize four Holocene periods with characteristic event layer patterns: The 6500 to 4200 cal BP window is summarized in one epoch, but I do recognize a period 5500-4200 as an interval with intriguing lack of event layers, and 6500-5500 as an interval with very high activity (Fig. 9).
Some Detailed/technical comments
55: The authors provide as examples of sediment-based avalanche reconstructions four Norwegian and one Alpine case studies. Considering the discussion above on distinction between flood and avalanche deposits, a summary of key criteria and key sedimentary structures would be very helpful here.
173: Why is this discussion of the cirque (barely shown on Fig. 2) so important? This is not marked as avalanche track. It is interesting for the Late Glacial situation, ok, but if you mention it, the cirque should be shown fully on a slightly larger map.
Figure 5 is very well done, nice. I fully agree with the interpretation of the sharp lithologic transition coinciding with the disappearance of the glacier in the catchment, as supported by the age model. But I got a bit lost in the description of the stratigraphy. Four cores were retrieved, two were merged into one composite section (VABG) in the western basin (122, 109), this is fine. The shorter one was taken to obtain the sediment-water interface, but why was it not sampled at same location as the piston core? Then two cores were recovered from the eastern basin (209, 309), but only one is shown in Figure 5 (209) but 309 is discussed as well in the text. It is all a bit hard to follow.
330: "and represent instantaneous deposits, i.e., event layers". This statement is here a bit premature. You describe here the sediment, and start right away with an interpretation of depositional environment but one needs to develop the case why these are event deposits. A more detailed description of sedimentary features will help.
363: "For CT thresholding, we classified depths with more than 40% minerogenic sediment as event layers": So the 40 % are a strong quantitative criteria, but it is not clear how this value was obtained by the CT data.
364: "Layers >3 mm thick were marked as instantaneous deposits (slumps) in the rBacon age-depth model to improve chronological accuracy and better reflect true depositional ages (Fig. 8)". The term "slump" is not an appropriate term here, as it a remobilized slope deposits. This is maybe a fix term in the Bacon software, but it should not be used here, these are not slumps.
The GPR profile are not as clear as they are interpreted. It would be useful to make them larger: do we really see these 6 m of sediments? Maybe reflection seismic surveys would give better picture of the sediment architecture, but I guess the authors make the best out of their available site survey data. On Fig. 2a, the cores should be labelled as well. I wonder why were cores were not taken on GPR profiles? Consequently, the cores were obviously projected on the radar lines, this should mentioned.
1-sigma is rather confident for the age model, usually, such lacustrine age models are done with a 2-sigma confidence interval. This just as a remark, in this case, it would not change the interpretation a lot, I guess.
In summary, I like the paper but would like to see the key issue addressed, i.e. the distinction between flood and avalanche deposits. Overall, I look forward to see this published after moderate (I only can chose minor or major, so I made minor) revisions.
Citation: https://doi.org/10.5194/egusphere-2025-2910-RC1 -
RC2: 'Comment on egusphere-2025-2910', Anonymous Referee #2, 06 Sep 2025
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I thoroughly enjoyed reading the manuscript entitled “Decoupling climate and avalanche activity: Holocene insights from lacustrine sediments in western Norway”. The manuscript is well-written and presents multi-proxy reconstruction of Holocene snow avalanche activity using lacustrine sediments from Lake Vatnasetvatnet in western Norway. The study employs sedimentological, geochemical, and CT scanning techniques to reconstruct ~10,000 years of snow avalanche activity from lake sediment records, which seems methodologically sound. Avalanche event layers are identified using two semi-automated approaches: (1) rate of change in Ti concentrations and (2) thresholding of CT grayscale data. Based on these methods, the authors distinguish three main phases of Holocene avalanche activity: (1) low activity during Early Holocene (>6500 cal yr BP), (2) increased activity during the mid-Holocene (6500–4200 cal yr BP), and (3) the highest frequency during Late Holocene (~4200 cal yr BP to the present). These patterns are interpreted as being primarily influenced by large-scale atmospheric circulation (i.e., variability in North Atlantic sea surface temperatures and fluctuations in the North Atlantic Oscillation). This study represents a valuable contribution to palaeoclimatology and natural hazard research, particularly in a region where long-term avalanche dynamics are poorly constrained.
Strengths
The manuscript demonstrates several notable strengths: (1) the 10,000-year record are particularly impressive in avalanche research, (2) combined use of LOI, DBD, grain-size analysis with magnetic susceptibility, XRF, and CT scanning greatly enhances the robustness of event layer identification, (3) the application of dual detection methods (Ti-based rate of change and CT thresholding), helps to minimize subjectivity in distinguishing event layers, (4) the chronostratigraphy is supported by AMS dating and rBacon modeling, and (4) the comparison of avalanche frequency with regional climate systems provides valuable context and strengthens the broader climatic interpretation of the findings.
Limitations/improvement
- Despite these strengths, several areas of the manuscript could benefit from further clarification and refinement: (1) The use of the term decoupling in the title seems not fully explored. The manuscript primarily demonstrates co-variability between avalanche activity and climate proxies rather than a clear decoupling. (2) More explicit discussion of uncertainties, especially in threshold calibration and sediment source attribution would be nice. (3) The novelty as highlighted in the manuscript may seem overstated. Similar reconstructions studies exist, and this study can be discussed as more on refinement than on conceptual breakthroughs.
Other Comments
All event layers are interpreted as avalanche deposits, but what is the likelihood that some of these layers were instead formed by fluvial processes, erosion, or landslides coupled with fluvial activity? While the surrounding landscape may not support a large fluvial system, smaller streams, in combination with upstream erosion and landsliding during climatic extremes, could plausibly produce similar deposits. Would more detailed analyses of the sedimentary characteristics help to distinguish between these potential depositional processes? For example - It was discussed that >3m layers are related to the slumps - are those sudden deposits derived from slumps upstream? Clarification on why these layers are slump-related would be helpful. Given that, I felt some discussion on alternative sediment sources, such as debris flows, rockfalls, fluvial pulses as well as vegetation dynamics and anthropogenic influences (i.e. during the late Holocene) would be appreciated. The role of solar forcing is discussed but not quantitively modeled. A more rigorous climate-forcing attribution or discussion would strengthen the manuscript.
Ti as a proxy for minerogenic input is well-established, but its specificity to avalanche deposits may require further discussion. I assume, Ti can also reflect fluvial or colluvial processes too. Similarly, the selection of thresholds (i.e., 95th percentile for Ti RoC and the 40% CT thresholding) lacks sufficient justification. A sensitivity analysis or probabilistic modeling approach could help validate these thresholds and reduce subjectivity.
Although the manuscript discusses different avalanche types (e.g., wet vs. dry), no attempt is made to differentiate them within the sedimentary record. If feasible, distinguishing between avalanche types based on sediment characteristics would add valuable depth to the interpretation.
The manuscript should clarify how slope gradients were calculated - if derived from a digital elevation model (DEM), the type and horizontal resolution should be specified. The identification of two avalanche tracks is well-supported, but the possibility of lateral migration or undocumented tracks should be acknowledged.
Fig. 1C - The horizontal blue lines likely stem from ESRI data artifacts. I would suggest regenerating the map using better DEM data. Also, the catchment boundary should be clearly labeled and described.
Fig. 2B (GPR Profiles): if possible, labeling the water-sediment and sediment-bedrock interfaces would help readers assess the effectiveness of the GPR method.
Fig. 5 –26th layer appears missing.
Fig. 7: More visual clarity would be helpful, for example through clear labeling of the cores .
To further enhance the manuscript, I recommend incorporating additional sedimentological and geochemical indicators to better distinguish avalanche deposits from other high-energy events. A probabilistic framework for threshold selection and age-depth modeling could improve the robustness of the reconstruction. Integrating climate modeling and linking avalanche frequency to regional climate simulations would provide valuable insights into future scenario analyses. Finally, incorporating land-use history and vegetation dynamics more explicitly would help contextualize the Late Holocene trends. In conclusion, this is a well-written, well-structured and timely study with significant potential to advance our understanding of long-term avalanche dynamics. However, I think the interpretation linking depositional layers exclusively to avalanche events requires more sedimentological justification and differentiation from other depositional processes. I believe addressing the points outlined above would greatly enhance the clarity, robustness, and impact of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-2910-RC2 -
EC1: 'Comment on egusphere-2025-2910', Gerilyn (Lynn) Soreghan, 07 Sep 2025
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Both reviewers complimented the authors on a nicely done manuscript, and I agree. It is clearly presented, and interesting, with good illustrations. Both reviewers provide constructive comments for improvement. I expect to be encouraging the authors to submit a revised version once they carefully consider and address reviewers’ comments.
Specifically, e.g. Reviewer 1’s comments about possible alternative interpretations for the event layers (e.g. extreme precipitation events). Related to this, I am curious if there is any evidence for wildfires (charcoal), and — if so— any relationship between (some) event beds and charcoal concentrations. This is simply my own curiosity.
I also agree with Reviewer 1 that further illustration and discussion of the sedimentology of the event layers would be helpful, and discussion of differentiating avalanche events from flood events.
Reviewer 2 notes some inconsistency between the title and the manuscript arguments. Reviewer 2 also comments on the issue of differentiation between avalanche and flood/fluvial/landslide events, so the authors should address this, with further discussion of key sedimentological criteria.
Citation: https://doi.org/10.5194/egusphere-2025-2910-EC1
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