Mobile evaporite enhances the cycle of physical-chemical erosion in badlands
Abstract. Chemical weathering driven by physical erosion is one of the manifestations of natural processes that strongly affect chemical and solid matter budgets at the Earth’s surface. However, the influence of extreme climatic erosion on chemical weathering dynamics is poorly understood. Badland landscapes formed in highly erodible, homogeneous substrates have the potential to respond to individual events on scales that are rapid enough for direct observation. Here, we assess the geochemical and grain-size composition of suspended sediment and riverine chemistry measurements collected from two catchments during the 2017 Nesat and Haitang typhoons in southwestern Taiwan. During the typhoons, the sodium adsorption ratio covaried with suspended sediment concentration, which we attributed to sodium-induced deflocculation. Evaporite weathering at peak rainfall is succeeded by peak silicate weather at maximum discharge, which dominates the weathering signal of the event. Overall, our observations suggest that initial weathering of near-surface evaporite enhances the physical erosion of silicate rock during extreme rainfall events.
Ci-Jian Yang et al.
Status: final response (author comments only)
RC1: 'Comment on esurf-2022-53', Anonymous Referee #1, 24 Nov 2022
- RC2: 'Reply on RC1', Anonymous Referee #2, 05 Dec 2022
- AC1: 'Comment on esurf-2022-53', Ci-Jian Yang, 31 Jan 2023
Ci-Jian Yang et al.
Ci-Jian Yang et al.
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Overall, I found the study interesting and well written. I think that a lot of work that needs to be done linking event-scale weathering to long-term dynamics, and this paper does a good job at bridging a knowledge gap. I particularly liked the idea that rapid dissolution of evaporites causes enhances physical weathering. To me, this seems a novel and important finding. One area, however, that I feel this study could improve upon is the discussion of the feedback between physical and chemical weathering. Throughout the paper, there were a several inconsistent descriptions of the mechanism driving enhanced chemical weathering. Clarifying this mechanism will enhance the overall impact of the paper.
Lines 56-58 – This sentence is confusing. Do you mean that if you assume the mudstone is primarily comprised of silicate minerals then you expect the silicate weathering rate to co-vary with Na concentrations from evaporite weathering? Or do you mean that you expect concentrations of SiO2 and SSC to covary with Na?
Lines 64-65 – Here you say that you expect that Na enhanced physical erosion will enhance chemical weathering in the dry season by exposure of fresh silicate minerals. Wouldn’t you expect new evaporite minerals to be re-precipitated in the dry season at the surface? I understand that enhanced physical erosion in the wet season could expose fresh minerals in the dry season, but I suspect water throughput, not mineral availability is limiting in the dry season.
Lines 169, 181, 189 – I think the endmember values would be more helpful in a table.
Line 164 – More details on your use of MEADIR would be helpful. After reading through the methods, it looks like you are only using MEADIR to apportion Ca and Mg? It is a little confusing because you assign all of the Na that is not rain or evaporite to silicates, but then give a Ca/Na ratio for carbonates. It seems slightly contradictory, but maybe the math works out such that the Na from carbonates is negligible. Nonetheless, more explanation regarding the mixing model would assuage my worries.
Lines 155, 205- You wrote that SAR > 13 causes soil particles to repel each other, preventing the formation of soil aggregates. In the results and Fig. 2, SAR rarely goes above this threshold. Does this pose an issue for the interpretations that evaporite weathering enhances physical denudation through disaggregation/deflocculation?
Paragraph starting at Line 234- This section is talking about enrichment and dilution of different sources with time. I think this section would benefit of thinking about these enrichments and dilutions as a function of discharge. There are many papers that have looked at concentration-discharge behaviors during flood pulses. Discussion of how these findings compare to others would strengthen this analysis.
Line 262- the title of this section is evaporite dissolution over time, but you discuss carbonate and silicate weathering here as well. Consider re-naming this section to more accurately reflect its contents.
Lines 285- 287 - I thought that you already determined the mixing by MEANDIR. What is gained by also looking at mixing with Sr?
Fig. 5- I find this figure difficult to follow and it is not totally clear to me what new information the figure is conveying that is not already shown in Fig. 3a-c.
Line 338- Is this true? I thought that evaporite-derived solute fluxes were larger than the silicate-derived fluxes.
Lines 346-348- Your mixing calculations should tell you the proportion of Ca and Mg from carbonate and silicate weathering. It is unclear to me how the enriched ratio of carbonate Ca means that it is not a contributor to weathering fluxes.
Line 357 – I am a little lost here. Are you suggesting that increased erosion on the hillslope increases silicate weathering in the river channel via abrasion of sediments in the river channel?
Line 365 – Your flux is comparable in what way? Same magnitude?
Lines 369-370 – I don’t think this is true. In my experience, most global weathering calculations account for evaporite weathering.
Lines 411-412- Again, I do not think your data supports this. Table S3 shows that at baseflow, carbonate-, silicate-, and evaporite-derived fluxes are roughly equal and during the typhoon, silicate- and evaporite-derived fluxes are roughly equal.
Line 9- “manifestations” is an awkward word choice, consider rewording
Line 49- “SW” needs to be defined. Intuitively this means “southwest”, but its better to define here and abbreviate after.
Line 62- I would add the total length of time here. “… a temporal resolution of 3-hours collected over XX hours.”
Line 207- It might be good to define D50 in the methods for grain size.
Line 275- are the values in parentheses correlation coefficients?