the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Jul 2026
Climatic, tectonic and landslide-dam signals preserved in alluvial terraces of the Naryn Basin, Kyrgyzstan
Abstract. Fluvial terraces can act as geomorphic markers of past environmental change. Although they are commonly interpreted in the context of a single driver, multiple climatic and/or tectonic processes may occur simultaneously in nature. Here, we test our ability to interpret terrace sequences with potentially complex formation histories by studying the Naryn Basin in the central Tien Shan, where glacial–interglacial cycles affected land-surface processes, numerous active faults accommodate crustal shortening, and a large landslide temporarily dammed a lake. We investigate the relative contribution of these terrace-formation drivers by mapping terrace profiles, providing new constraints on the terrace chronology, and numerically simulating terrace formation in response to climate change and lake drainage. Tributary terrace profiles record local deformation in response to faults and a regional concave-up pattern that could be explained by lacustrine sedimentation and subsequent base-level fall induced by lake drainage. Terrace ages cluster between 15 and 20 ka, correlating with both deglaciation following the last glacial maximum and lake drainage. Our modelling, together with the spatial distribution of modelled terraces, indicates that lake drainage had a strong, but spatially restricted impact on river long-profile evolution, whereas Pleistocene climate triggered regional terrace formation. By integrating numerical modelling of alluvial river long-profile evolution with detailed mapping and geochronology, we present a robust approach to establish direct links between environmental change and the resulting terrace record.
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Status: open (until 02 Sep 2026)
- RC1: 'Comment on egusphere-2026-3489', Anonymous Referee #1, 10 Aug 2026 reply
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- 1
General comments
This manuscript addresses an important question: how to distinguish climatic, tectonic, and landslide-dam controls on alluvial terrace formation in a complex river network. The integration of terrace mapping, cosmogenic and luminescence geochronology, and network-scale numerical modelling is innovative and has the potential to make a valuable contribution to fluvial geomorphology.
I recommend moderate revision. The manuscript is promising, but several methodological and interpretative issues require further clarification. I have four major comments, followed by several specific minor comments, as detailed below.
Major comments:
Lines 225–245: While there is no universally prescribed integration interval for feldspar IRSL/pIRIR signals, the selected 100-s background interval is relatively long compared with commonly used approaches, such as ~2 s signal integration combined with a ~20 s terminal background. A long background interval may incorporate low-level signal decay, instrumental drift, or other variations (Galbraith, 2002; 2012). Please justify this choice and, if necessary, recalculate the De values using an alternative integration interval, preferably with reference to comparable studies from the region.
Lines 230–235: The MAM is applied to all samples because incomplete bleaching is expected in fluvial settings. However, incomplete bleaching may vary substantially among samples, and inappropriate application of the MAM could lead to age underestimation. Please provide the De distributions and overdispersion values, preferably in the Supplementary Information, and select CAM or MAM according to the characteristics of individual De distributions.
Please also report the aliquot size or mask diameter, because this information is relevant when evaluating De distributions, and the choice of age model.
Table 2: Please add overdispersion values. Sample 22-KG-17 has an unusually high De (~650 Gy) and dose rate (~6 Gy ka−1). Please verify these values and, if correct, briefly explain whether the high dose rate is related to a particular lithology, sedimentary facies, or geochemical setting and so on.
My second major concern is the presentation of time in the modelling results, particularly in Figures 9 and 10.
In Figure 9a, “Time [ky]” represents elapsed model time, whereas Figure 9d shows terrace age relative to the end of the simulation. Similarly, the labels 0, 2, 10, and 20 kyr in Figure 10 represent elapsed model time, whereas the text refers to terrace ages of 20–8.9 ka relative to the end of the model run.
This distinction is not sufficiently clear and may easily confuse readers. I suggest using consistent notation throughout the manuscript. For example:
t = elapsed time since forcing onset;
A (terrace age) = age relative to model termination.
Alternatively, the authors may adopt another notation, but model time and terrace age should be clearly distinguished and consistently presented in Figures 9 and 10.
The Introduction frames the study around three end-member mechanisms: tectonic forcing, climate-driven Qs/Qw changes, and lake drainage. However, the numerical modelling mainly evaluates the latter two, whereas tectonic forcing is treated more qualitatively. The authors should either provide additional quantitative evaluation of the tectonic end-member or reduce the emphasis placed on tectonic forcing in the discussion, as the supporting evidence is less comprehensive than other two.
I also encourage the authors to strengthen the discussion of transient responses and lag times. Figure 9 and 10 show that terrace abandonment may occur several thousand years after the forcing maximum and that the timing varies spatially across the river network. This is an important result because it demonstrates that terrace age does not necessarily correspond directly to forcing age. The broader implications of this diachronous response deserve greater emphasis.
Section 2.2: “Geomorphic features” focuses mainly on the Beshkiol landslide and paleolakes, but provides little regional overview of the terrace system itself. Please add a concise description of the number of terrace levels, typical heights above the modern channel, and their spatial distribution along the trunk river and tributaries.
Sections 5.1–5.2 contain detailed site-by-site descriptions, but some interpretation, particularly the acceptance or rejection of individual 10Be ages, is already mixed into the Results. I suggest moving the regional terrace framework to Section 2 and reserving detailed chronological interpretation for the Discussion.
Minor comments
Lines 315–318: Please explain why 23- and 41-kyr periodicities were selected for the Qs/QW forcing experiments.
Figure 1: Consider adding glacial extent, distinguishing fault types where possible, and showing the spatial extent of Figure 2.
Figure 4: Please clarify the x-axis and improve the legend/caption. In particular, explain the white lines and why some segments differ in thickness or connectivity.
Tables 1 and 2: Consider adding tributary name, terrace level, and height above the modern channel for each sample. Please also add overdispersion values to Table 2.
Figure 5: The symbols for 10Be and luminescence samples are difficult to distinguish. Please use more contrasting colours and/or different symbols. The caption should also indicate the 14C ages and their source.
Line 530: “signals can propagated” should be changed to “signals can propagate.”
Lines 560–565: “Surfaces of the latter range of ages cluster often form…” is grammatically unclear and should be revised.
Lines 605–610 / Figure 10: Consider expressing the x-axis as upstream distance from the landslide dam, with the dam as zero, to make upstream signal propagation more intuitive.
Please standardize terms such as “rock uplift/rock-uplift” and “lake drainage/lake-drainage.”