the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Dec 2025
Geomorphological activity and stability of surfaces and soils formed in hyperarid alluvial deposits (Atacama Desert, Chile)
Abstract. The hyperarid conditions of the central Atacama Desert, characterized by extremely low precipitation and high evaporation rates, create a unique environment where geomorphic stability is generally considered to be exceptionally high. Terrestrial cosmogenic nuclide-based surface exposure ages suggest that many surfaces underwent limited to no geomorphic changes since the Neogene or early Pleistocene. However, a number of recent studies reveal more recent landscape-scale geomorphic activity and link this to slightly wetter episodes during the Quaternary. In order to determine drivers of geomorphic activity, we performed a multi-proxy analysis of five profiles situated in alluvial deposits along a climatic transect from the coastal plain to the upper reaches of the Coastal Cordillera (~ 0–2000 m a.s.l.), combining single-grain feldspar luminescence dating with geochemistry, granulometry, and field observations. Alluvial deposits are prone to heterogeneous bleaching; therefore, we tested the bleachability of the feldspars and found a shallow effective bleaching depth (< 2 cm) and high near-surface bleachability. The five profiles studied could be grouped into geomorphological active and stable sites, based on our multi-method approach. Profiles located on geomorphologically active surfaces show evidence of recent sediment deposition and, in one case, vertical grain transport. In contrast, stable surfaces preserve reworking signals relating to bioturbation at the coast and desert pavement formation in the hyperarid Coastal Cordillera. While no clear chronological trend along the west–east climatic gradient could be found, two phases of widespread geomorphic surface activity – ~ 50 ka and during the last ~ 5 ka – coincide with regionally wetter intervals compiled from other studies. Our findings highlight the value of single-grain luminescence data for reconstructing local sediment dynamics and soil reworking in arid environments, and the need to account for both depositional and post-depositional processes in paleoenvironmental interpretations.
- Preprint
(2390 KB) - Metadata XML
-
Supplement
(3087 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-5895', Anonymous Referee #1, 15 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5895/egusphere-2025-5895-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-5895-RC1 -
RC2: 'Comment on egusphere-2025-5895', Anonymous Referee #2, 20 Feb 2026
Review of manuscript ID egusphere-2025-5895
The study presented in this manuscript uses a combination of quantitative techniques to establish processes associated with late Quaternary geomorphic activity in the Atacama across a climatic gradient. The climate gradient is established using exposures of soil/sediment in the upper 80 cm of unconsolidated alluvial deposits. The authors principally employ and rely upon geochemical analysis and interpretations from single-grain luminescence signals to (1) categorize their study sites by active/inactive geomorphic processes, and (2) assess the palaeoclimatic implications for their observations. I review two main criticisms of this manuscript that warrant further examination prior to publication in EGUsphere.
Site Selection and Characterization
The authors develop their hypothesis that relies upon the use of a climosequence of soils formed in alluvial fan sediments along an east-west transect in a single catchment that drains to the Pacific Ocean. Several aspects of this site selection minimize the significance of the results, as currently presented.
First, the manuscript does not adequately document the relationships of the alluvial fan surfaces to one another in a way that enables systematic interpretation of results. As shown schematically in Figure 2 of the manuscript, each sampling site appears to be associated with its own set of geomorphic factors that complicate interpretations. For example, fans appear to be produced from catchments of widely varying order and stream carrying capacity, as documented in the text. This, coupled with the lithological variations in the fan sediments that were documented, can be expected to produce a host of different stratigraphic relationships in the alluvial deposits unique to each location. Although the authors acknowledge the complexity of the lithologic variability within the catchment, the potential impacts from those variations as they relate to overall preservation of alluvial fan units are not explicitly addressed in the background or in their interpretations of the results. The authors would greatly improve upon their interpretations by incorporating some characterization and subsequent discussion of these characteristics and how they influence their laboratory results.
Similarly, the geomorphic and stratigraphic context of each site is not established in the same detail across sites. The authors rely heavily upon prior work by Walk et al (2023) where careful mapping was performed to produce a geomorphic framework for P1. I expected to see a similar level of characterization for the other four sampling sites so that claims relating to deposition/erosion/soil-forming processes could be addressed using a combination of field methods as well as quantitative methods. Only a cursory level of detail is presented in the Supplementary Materials and only addresses details pertaining to the sampling site itself. One of the principal shortcomings in this regard is the lack of field observations to support the hypotheses presented in the paper.
Interpretations of results
The authors performed geochemical analyses on mineral sample materials to address pedogenic processes that result in net mobilization of cations through the profile. There are two fundamental concerns that I have with this approach that influence their interpretations. First, the bulk mineral matter in the fine-earth fraction, rather than soil solution extracts, are evaluated from a geochemical perspective. The authors note a diversity of lithologies that contain some mixture of volcanic, intrusive, and metasedimentary rocks, undoubtedly with variable components of the cations under evaluation. If pedogenic processes were to be targeted directly, especially in this dry environment where salt mobility drives pedogenesis, then soil solution extracts would provide more direct and reliable evidence in support of claims related to pedogenetic processes compared with the bulk mineral content. Second, and perhaps more important, is the fact that the authors assert that the geochemical differences observed among profiles limit their ability to compare between sites, yet their following PCA analysis appears to do just that.
The PCA results indicate clusters associated with variable amounts of geochemical constituents that presumable relate to lithologic variations rather than geomorphic variations. The use of soil solution extracts in this approach would have been the ideal approach to negate the lithologic influence, but there is limited discussion of this potential shortcoming in the manuscript. Furthermore, the interpretations derived from their PCA results appear to drive their overall conclusions regarding geomorphic stability, as indicated by their annotation on the horizontal axis of Figure 5b.
The rather widespread references throughout the manuscript to post-depositional mixing and the mechanisms attributed to these interpretations is somewhat problematic in the sense that the geomorphic and stratigraphic observations are not utilized to their full extent to evaluate this hypothesis. For example, the authors reference proximity to a fault that has not been documented in this manuscript in terms of its activity and recency or the direct relationship that such faulting would have in terms of plausible geomorphic processes. Despite this, they use this logic to invoke soil mixing based on the observation of heterogenous grain sizes in their profile. In my view, the system and its dynamics have not been evaluated to the degree to which such a hypothesis can be made or tested. Similarly, the authors reference relationships between heterogeneous grain size distributions and root density to arrive at the interpretation of mixing so that they can explain their luminescence results. This, to me, is somewhat reversed logic and the luminescence should be used to test the hypotheses that are based on field evidence and observation.
Overall, I believe that a significant restructure and organization is warranted for publication. I recommend that more careful geomorphic documentation of each site is required to provide the basis for stratigraphic observations. These stratigraphic observations should then be presented, again in greater detail, as evidence which is used to formulate the hypothesis of geomorphic activity/stability across the climate gradient, and which is supported by the appropriate literature. Only then, in my opinion, can the geochemical and luminescence observations be used to test the hypotheses. The results from the luminescence analyses are quite interesting and have the potential to significantly improve our understanding of arid-land alluvial processes in relation to external forcings, so it is critical that the appropriate context be developed to arrive at meaningful and broadly applicable results. I have provided additional line-by-line comments in the attached marked-up pdf.
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 253 | 153 | 21 | 427 | 42 | 21 | 25 |
- HTML: 253
- PDF: 153
- XML: 21
- Total: 427
- Supplement: 42
- BibTeX: 21
- EndNote: 25
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1