the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Jan 2026
Seasonal variations in flooding inferred from lake sediments in Western Norway
Abstract. Holocene flood reconstructions from western and southern Norway indicate a non-stationary behaviour through time, with a major regime shift around 4000 years ago. Under the influence of global warming, flood frequency, seasonality, and magnitude are changing worldwide. The full range of natural variability in flood frequency and seasonality remains poorly constrained, underscoring the need for ultra-high-resolution records to place recent changes in a long-term context. This study presents an 8000-year flood record from Lake Vangsvatnet, Western Norway, that combines high-resolution computed tomography (CT) scanning, X-ray fluorescence (XRF), grain-size analysis, and radiocarbon dating to differentiate between seasonal flood deposits (spring snowmelt versus autumn rainfall). The 11-metre sediment core reveals distinct flood layers (n = 230), linked to varying hydrological conditions, and validated by historical discharge data (1892 CE–2016 CE). The record shows fluctuating flood frequencies, with peaks at 6000–5300 cal yr BP and 1400 cal yr BP to present. A seasonal shift from rainfall-dominated to snowmelt-dominated floods occurred around 3100 cal yr BP, coinciding with regional cooling. The last 500 years exhibit the highest flood frequency of the entire record. These findings highlight the sensitivity of flood regimes to climatic and, in the most recent centuries, human influence. Under future warming, reduced snowpack may diminish spring floods, while intensified autumn and winter rainfall could increase flood risks.
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Status: open (until 02 Mar 2026)
- RC1: 'Comment on egusphere-2025-6279', juan pablo corella, 01 Feb 2026 reply
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RC2: 'Comment on egusphere-2025-6279', Anonymous Referee #2, 06 Feb 2026
reply
The manuscript entitled: "Seasonal variations in flooding inferred from lake sediments in Western Norway", submitted by Nielsen et al. provides a detailed and interesting paleoflood reconstruction from lake sediments in Western Norway. Overall, the manuscript is very well written in a simple, fluent and clear way that makes it easy to follow the authors' logic, and to clearly distinguish measurements from interpretation, as would be expected from papers submitted to CP. Nevertheless, although I think the presented work is novel, interesting and solid enough for publication in CP, the manuscript could benefit from some small improvements that would increase its impact. I would recommend that the authors address the comments below prior to accepting the manuscript for publication.
Comments:
1. Your introduction feels somewhat too local. I think you should provide a couple of sentences that also highlight the role of paleoflood interpretation from a global perspective. Plenty of works have been published on that topic worldwide, so you can give the readers some broader, more global context (see works by V. Baker, for example).
2. A crucial aspect to clarify is the possible role of lake level in sedimentation patterns at your coring site. Some of the observations you use as cardinal aspect of your discussion and interpretation rely, to a large extent, on lake level that would shift the coastline and alter sedimentation patterns. Thus, you need to provide some statement based on some previous studies in order to clarify to which extent would changing lake levels over the Holocene changed, and if so, to which extent would that affect sediment sorting and deposition at the coring site. Even if you consider this effect to be of negligible impact, you should state that as a premise of the presented work.
3. Another crucial aspect of your claims relies on the interpretation of the four sediment types you mention in the text (B, O, OM, and M in L402-408) - could you improve your statements about their interpretation? Since no thinsections were done, it is hard to judge how robust the claims made about their mode of formation are. Is it in agreement with previous studies that you can cite?
4. You mention using Munsell color, but no data is presented - is it worth correlating this with your RGB values? chemical composition? for the various types of layers?
5. L 184: You mention switching the technical parameters of the XRF - would this affect your measurement in terms of saturation or limit of detection? I noticed you only present key elements such as Fe, Ti and Ca, so maybe this is not an issue. Either way you should clarify if such a change would alter the measurement and how you address that if so. Please also state the list of elements you measured.
6. Table 2 - what is "sorting"? how was this calculated? presenting the mean of grain size is often misleading, and presenting the peak of the distribution would make more sense. If the distribution is bimodal please present both peaks in the table.
7. You present only Fe, Ti, and Ca - is that all you could measure? commonly Al and K are discussed as well. What about Si?
8. Fig. 5 presents some interesting potential. I would recommend to plot some correlation plot between few selected elements, e.g., Ca. vs. Fe/Ti (Al, Si, K?) and color\shape the layers according to their types in order to highlight the major differences between the identified layers and the background (B, O, M, etc.)
9. Fig. 5 - the image of the core is really not visible. You can create another figure to highlight the key types of layers you identify. Ideally this would include some high resolution imaging as well, such as thinsections.
10. Fig. 6 presents some bimodal GS distributions - I think you should address that. Is this expected? what do you think this means? Is it correlated with a specific period or type of events? Please mention and address that in the text.
11. Fig. 7 - I recommend adding the RGB image aligned with the presented CT
12. Fig. 8 - how does this interpretation fit against your LOI values? can you correlate the CT of the event layers with the LOI?
13. Discussion - I think the the concept of nonstationrity is very important, and serves a critical insight from the presented study. I therefore think you could improve the impact of the discussion by mentioning additional examples from places where it was described worldwide (even if not coeval), rather than limiting your discussion to Sweden. I wonder if you should clearly state that in your title, something like: Lake sediment record reveals nonstationary flood occurrence over the last 8 ka in Western Norway
Minor comments:
a. Fig. 1 has no legend for altitude. depicting landcover somehow could also be useful for the reader (maybe in another panel)
b. L115: This is a strange sentence: "The annual precipitation at Voss is currently 1369 mm" - are you referring to mean annual precipitation? a specific year?
c. L117 - is the station code really 62.5?
d. L194 - you are citing a paper in review. I am not sure what the journal policy is. perhaps better refer to a preprint.
e. L243-244 "The glacier component..." - is this your result? or this some interpretation?
f. L580 - the mean of what? please explain what this number is referring to.
g. L307 - you mention OM layer instead of MO
Citation: https://doi.org/10.5194/egusphere-2025-6279-RC2
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Referee report on
“Seasonal variations in flooding inferred from lake sediments in Western Norway”
General assessment
This manuscript presents a very solid and carefully conducted palaeoflood reconstruction from western Norway. The study follows a robust and now well-established methodological framework for the identification and interpretation of flood deposits in lacustrine sediments, and contributes an important new high-resolution record to the growing network of comparable palaeoflood archives in Norway.
A major strength of the paper is the combination of CT-based event detection, sedimentological criteria, and validation against both instrumental and historical discharge records from the Vosso river, which provides an exceptional framework for interpreting the sedimentary archive. The attempt to discriminate flood seasonality (snowmelt versus rainfall-driven floods) using sedimentological proxies is convincing and builds coherently on previous work (e.g. Hardeng et al., 2022).
Overall, the manuscript represents a valuable contribution within the scope of the journal. The scientific approach is sound, the results support the interpretations, and the authors are generally careful and transparent when discussing uncertainties and limitations. In my opinion, the manuscript requires only minor revisions.
Major strengths
Specific comments and suggestions (minor revisions)
The reference list and in-text citations do not appear to follow a consistent ordering scheme (neither alphabetical nor chronological). This should be corrected throughout the manuscript.
Given that the lake inlet is associated with a complex deltaic system and that the coring site is located relatively close to the delta, it would be useful for the authors to briefly discuss whether there is any geomorphological evidence for inlet or delta shifting during the Holocene. Even a short discussion acknowledging whether such changes are known, likely, or unknown would strengthen the interpretation, as inlet position can influence sediment focusing and the preservation of flood layers. It would be advisable to check previous works from Bruno Wilhelm
The manuscript successfully places the lake record record within the context of other Norwegian palaeoflood archives. It may be useful to further emphasize that intercomparisons are inherently complicated by differences in lake sensitivity, threshold behaviour, and sediment availability. Some lakes preferentially record only the most extreme floods, while others may also record more moderate events, and this should be clearly highlighted when discussing regional coherence.
The authors document the presence of lateral collapses and mass movements along the lake margins based on CHIRP data. In this context, it would be useful to more explicitly discuss why the identified event layers—particularly the thicker mixed (MO) layers—are not interpreted as the distal expression of mass movements, delta collapses, or slope failures.
While the interpretation of MO layers as large or prolonged flood events is plausible, a slightly expanded discussion of their texture, structure, and grain-size characteristics would help to further exclude gravitational processes and strengthen the sedimentological interpretation. Similar processes have been documented in Lake Geneva (Kremer et al., 2015) and other swiss lakes.
In Section 4.4, the authors state that the highest sedimentation rates occur in the upper 0–30 cm and are “< 5 mm yr⁻¹”. Given the otherwise very robust chronological framework (numerous ¹⁴C dates and short-lived radionuclide dating), it would be preferable to provide a more precise estimate or a narrower range for these sedimentation rates.
The authors identify the last ~500 years as the period with the highest flood frequency in the record. Although this is already discussed, it may be worth emphasizing even more strongly that this increase could partly reflect enhanced sediment availability and human impact rather than a purely hydrological signal. The climatic and anthropogenic influences are likely strongly intertwined during this interval, which limits a straightforward climatic interpretation.
The authors’ honesty in acknowledging that the human impact cannot be quantified is appreciated. As a possible way forward, the comparison with independent indicators of land-use change (e.g. regional pollen records, arboreal vs. non-arboreal pollen ratios) could be briefly mentioned.
Recommendation
Minor revisions
The manuscript is scientifically sound, clearly written, and well suited for publication in Climate of the Past once the minor points raised above are addressed.