the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Aug 2024
Global analysis of in situ cosmogenic 26Al/10Be ratios in fluvial sediments indicates widespread sediment storage and burial during transport
Abstract. Since the 1990s, analysis of cosmogenic nuclides, primarily 10Be, in quartz-bearing river sand, has allowed for quantitative determination of erosion rates at a basin scale. Paired measurements of in situ cosmogenic 26Al and 10Be in sediment are less common but offers insight into the history of riverine sediment moving down slopes and through drainage basins. Prolonged sediment burial (>105 years), a violation of assumptions underlying erosion rate calculations, is indicated by higher 26Al-based than 10Be-based erosion rates due to preferential loss of shorter-lived 26Al by decay when quartz is shielded from cosmic rays.
Here, we use a global compilation of 26Al and 10Be data generated from quartz-bearing fluvial sediment samples (n = 624, including 121 new measurements) and calculate the discordance between erosion rates derived from each nuclide. We test for correlations between such discordance and topographic metrics for drainage basins, allowing us to infer the likelihood of sediment burial during transport in different geomorphic settings. We find that nearly half of samples (n = 276) exhibit discordance (> 1σ uncertainty) between erosion rates derived from 10Be and 26Al, indicating sediment histories that must include extended burial during residence on hillslopes and/or in the fluvial system after or during initial near-surface exposure. Physical basin parameters such as basin area, slope, and tectonic activity exhibit significant correlation with erosion rate discordance whereas climatic parameters have little correlation.
Our analysis suggests that 26Al/10Be erosion rate discordance occurs more regularly in basins larger than 1,000 km2, particularly when such basins have low average slopes and are in tectonically quiescent terrains. Sediment sourced from smaller, steeper basins in tectonically active regions is more likely to have similar 10Be and 26Al erosion rates indicative of limited storage and limited burial during residence in the hillslope and fluvial sediment system. The data and analysis we present demonstrate that paired 26Al and 10Be analyses in detrital fluvial samples can provide a window into watershed processes, elucidating landscape behavior at different spatial scales and allowing a deeper understanding of both sediment routing systems and whether erosion rate assumptions are violated. Large lowland basins are more likely to transport detrital sediment that has experienced prolonged sediment storage and burial either on hillslopes and/or in fluvial networks; thus, erosion rates from such basins are lower limits due to nuclide decay during storage. Conversely, samples from smaller upland basins are more likely to provide reliable erosion rates.
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RC1: 'Comment on gchron-2024-22', Regis BRAUCHER, 14 Sep 2024
The paper of Halsted et al. presents a statistical analysis of 624 samples from fluvial sediments where both 10Be and 26Al have been measured (among all samples, 121 new 26Al measurements are presented).From these measurements and the determination of denudation rate for both nuclides, the authors state that when the two denudation rates are equal within uncertainties the sediment undergone a simple history and for more than 276 samples with denudation ratios below 1 the authors argue that burial must be involved.This paper is well written and fairly present all calculations and tests performed on this dataset. I think it is worth being published in Geochronology providing some precisions and corrections.
See attached files .
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AC1: 'Reply on RC1', Christopher Halsted, 24 Sep 2024
We thank Dr. Braucher for his review, which we found constructive and will certainly improve our manuscript during revisions. In particular, the sensitivity test he conducted to compare different methods of Al blank correction was very informative and helpful. During our revisions, we plan to address his comments as follows:
- I think the title should be modified as the authors have only work on the denudation ratios, not on the concentration ratios as it is referred.
- We agree that the title of the manuscript should be altered to 'Denudation Ratios' instead of 'Cosmogenic Ratios'. We also plan to add extra material to the discussion in which we provide more information about comparing denudation ratios to the more traditional 26Al/10Be ratios, which we think will aid the reader who may not be used to seeing denudation ratios.
- Perhaps a nasty question; Except the dataset, how this paper differs from Wittmann et al. (2020)? It seems that the two papers have the same conclusion: in large floodplain the probability to have a discordant denudation ratio between the two nuclides is greater than in rapid eroding settings with fast transport.
- We do feel that this paper represents a significant addition to the work conducted by Wittmann et al. (2020), both in the scope of the dataset size and in the types of basins analyzed. We include basins of all sizes in this study and the compilation here has substantially more basins, including previously unpublished data and newly-processed Al data from more than 100 basins (Figure 2 demonstrates the scope of the compilation well). We agree that our results support the conclusions of Wittmann et al. (2020), but we feel that the substantially increased size of this compilation, combined with the greater variety of basin sizes, represents a significant contribution on its own.
- In the abstract it is mentioned lines 32-33 that the denudation ratio study will bring a deeper understanding of sediment routing and whether erosion rate assumptions are violated. I did not see this in the present paper; I think that the authors should work on this to propose a paper that will complement the work of Wittmann et al. . If there is a length limitation in the manuscript for this, the introduction and the background sections can be reduced.
- We agree that we could clarify what exactly we have determined from our results in the abstract. We will be more explicit about our focus on hillslope and floodplain storage in the abstract and expand the discussion on these points as well.
- I have a major concern regarding the newly presented data. Line 221-222 you mention that you correct the ams ratios by subtracting the blank ratio. This is not correct for 26Al. To do this the amount of 27Al in the samples must be the same as the one in the blank. This can be accepted for beryllium as the 9Be added in roughly the same for all samples including blanks. For 27Al the natural amount is highly variable as shown in the following figure presenting the 27Alvariation in your 121 Al samples. Therefore, you must consider subtracting the 26Al atoms (determined from the amount of 27Al added in the blank and the corresponding measured AMS ratio), form the to the 26Al amount in the sample.
- After reviewing the sensitivity analyses provided by Dr. Braucher, and correcting for 1.5 mg of spike added, we observe that the exact Al blank correction procedure matters very little for the overall conclusions of this paper. The median change in calculated 26Al concentration using the different blank correction methods is less than 1%. We will discuss our procedures with all co-authors during our revisions, but we are confident that the choice in blank correction procedure does not change the results of the study. We are grateful to Dr. Braucher for conducting this sensitivity analysis and will include the analysis in Table S2 upon re-submission.
- In Table S2 there were a couple of errors highlighted by Dr. Braucher and which we will fix. For example, one of the initial project batches (CH-05) contained bedrock samples, but these were removed as the project evolved into a fluvial-focused study. We mistakenly kept the blanks associated with samples in CH-05 in Table S2, even though there are no samples from this batch in the study. We will remove these blanks.
- Line 231: as you only compare cosmogenic data why do you add the production rate uncertainties?
- We included production rate uncertainties in denudation rate calculations because we comparing samples from around the world and thus there is variation in production rate uncertainties within the LSDn scaling scheme
- Regarding the statistical analyses, I think you should move the “Morphometric and Climatological Basin Parameters – Detailed sources and procedures”. From the supplement to the main text as you are using many databased from different authors.
- This is a good idea and we will do so
- We will fix all requested figure and line edits
- In supplement add the advantage of the tests you used (why Spearman’s Rank correlation, etc...); this will help.
- Certainly, we will add more information about the statistical tests used
- Section 5.2 : Here you can try to develop more how the denudation ration discordance may help. From this section one can only keep in mind that the “true” denudation rate may be given by 10Be (105 – 106 years) and the denudation ratio (or the concentrations ratio, using a “banana plot”) discordance can be used to show potential sediment sequestration implying a decay in 26Al concentrations.
- This a good suggestion and we plan to expand section 5.2 to explain the denudation ratio discordance metric in more detail.
- Reference : Wittmann et al (2020) is mentioned twice.
- Good catch, we will remove the repeated reference
Citation: https://doi.org/10.5194/gchron-2024-22-AC1 - I think the title should be modified as the authors have only work on the denudation ratios, not on the concentration ratios as it is referred.
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AC1: 'Reply on RC1', Christopher Halsted, 24 Sep 2024
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RC2: 'Comment on gchron-2024-22', Timothée Jautzy, 01 Oct 2024
The paper by Halsted et al. presents a very large dataset of cosmogenic nuclides (10Be and 26Al) drawn from the literature, along with 121 new 26Al measurements. The samples correspond to riverbed sand covering a fairly wide portion of the globe. The authors propose an original indicator (although I do not clearly see its relevance at this stage of the paper) to characterize the complexity of the burial history of the sediments studied. Finally, simple yet well-supported statistical analyses allow for testing potential relationships between the burial indicator and a series of morphometric and climatic parameters.
The results indicate that (1) almost half of the samples show a complex exposure/burial history, and (2) there is a significant relationship between the catchment area and the complexity of sediment transport. These results thus support the recent (and quite similar) study by Wittmann et al. (2020), which obtained similar findings.
In my opinion, the value of this study lies in the size of the dataset (624 samples), the simplicity of the approach, and the clarity of the message.
The article is well-written, and the figures are clear and of good quality. In my view, this paper deserves to be published, with a few minor revisions. Below are some general comments, followed by more specific remarks throughout the text.
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AC2: 'Reply on RC2', Christopher Halsted, 06 Nov 2024
We thank Dr. Jautzy for this review, which offers helpful insight into revisions that will improve the manuscript. Our responses to all comments are shown in italics in the accompanying document attached to this reply. We will add more description and rationale behind using denudation ratios vs. concentration ratios, and we will make more clear in our revision the significant differences between our work/results and those of Wittman who studied only very large basins.
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AC2: 'Reply on RC2', Christopher Halsted, 06 Nov 2024
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