the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2024
Characterization of the particle size distribution, mineralogy and Fe mode of occurrence of dust-emitting sediments across the Mojave Desert, California, USA
Abstract. Understanding the effect of dust upon climate and ecosystems needs comprehensive analyses of the physiochemical properties of dust-emitting sediments in arid regions. Here, we analyse a diverse set of crusts and aeolian ripples (n=55) from various dust-hotspots within the Mojave Desert, California, USA, with focus on their particle size distribution (PSD), mineralogy, aggregation/cohesion state and iron mode of occurrence characterization. Our results showed differences in fully and minimally dispersed PSDs, with crusts average median diameters (92 and 37 µm, respectively) compared to aeolian ripples (226 and 213 µm, respectively). Mineralogical analyses unveiled variations between crusts and ripples, with crusts enriched in phyllosilicates (24 vs 7.8 %), carbonates (6.6 vs 1.1 %), Na-salts (7.3 vs 1.1 %) and zeolites (1.2 and 0.12 %), while ripples enriched in feldspars (48 vs 37 %), quartz (32 vs 16 %), and gypsum (4.7 vs 3.1 %). Bulk Fe content analyses indicate higher concentrations in crusts (3.0±1.3 wt %) compared to ripples (1.9±1.1 wt %), with similar Fe speciation proportions; nano Fe-oxides/readily exchangeable Fe represent ~1.6 %, hematite/goethite ~15 %, magnetite/maghemite ~2.0 % and structural Fe in silicates ~80 % of the total Fe. We identified segregation patterns in PSD and mineralogy differences within the Mojave basins, influenced by sediment transportation dynamics and precipitates due to groundwater table fluctuations. Mojave Desert crusts show similarities with previously sampled crusts in the Moroccan Sahara for PSD and readily exchangeable Fe, yet exhibit differences in mineralogical composition, which could influence the emitted dust particles characteristics.
- Preprint
(2015 KB) - Metadata XML
-
Supplement
(720 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-434', Richard Reynolds, 23 Mar 2024
The primary objective of this research was to characterize surficial sediments for mineralogic and particle-size properties from a few sites in the Mojave Desert and to compare these results with those from some Moroccan dryland settings and dust-emitting settings on Iceland. The careful analytical work on the Mojave samples is highly commendable. The manuscript is well organized. In my view, however, many clarifications, corrections, and revisions are required before the manuscript is suitable for publication.
Major comments:
It is not clear why EMIT mineral identification maps are presented, except to convey that it “enriches our understanding of the region’s mineralogic diversity”. A few limited interpretations and a couple of maps (Fig. 3) are provided under Methodology (not under Results), but no comparisons of EMIT results with those of this manuscript or discussion follow. Unless there is detailed mineralogic comparison among EMIT interpretations and those of this study, it appears that mention of EMIT in this manuscript is unnecessary. Sure, EMIT employs highly valuable tools aloft and data reduction methods on the ground, but the “transfer” of mineral identification from surface sediments to actual dust seems to remain an interesting challenge and a topic for further discussion beyond this manuscript.
This work did not characterize dust. The presented results depend greatly on the compositions of sand-size particles, and not directly on actual dust, although some inference can be made from the plots in Fig. 5. The work characterized bulk samples from sites that apparently emitted dust. A meaningful assessment of mineral dust in the samples would have been to dry sieve the bulk samples to separate out the PM63 fraction, that is, the silt plus clay size particles and analyze those. A useful addition would be listing PM63 vol. % derived from PSD data. Because dust emission from sampled sites has not been demonstrated or adequately inferred on the basis of, say, prevalence of dust PM sizes in the samples, I strongly suggest that the title be revised to delete “dust-emitting sediments”; it’s not adequately demonstrated that the samples represented dust. Please see following comments on water treatments of samples.
Because the sample treatment for PSD is extremely important, that method should be described in this manuscript and not simply referred to as Gonzalez-Romero et al. 2023. Relevant methods described by Sperazza should be included.
It appears that samples were treated in liquid: “To analyse mineral-size fractionation (< 10 and 10–63 µm), a fully dispersed size fractionation conducted using Milli-Q-grade water and by shaking the samples was applied prior to separation for 12–24 h.” from Gonzalez-Romero et al. 2023. Wouldn’t that treatment have fully dissolved at least some of the salt minerals (e.g., thenardite, burkeite, others) in any or all samples? To any extent, then, the treated-sample PSDs cannot be those of their natural occurrences (see also Buck et al.).
There is a disconnect between the spatial assessment of dust-producing settings in the Mojave (Fig. 2) and the locations of actual dust sources that were a target of this study. The broad AOD footprint map is a superb rendition of general dust activity, but it does not inform about footprints of actual dust-emission point sources. Much dust emitted from the Mojave is from alluvial settings—dry river beds, the toes of alluvial fans, and disturbed areas (many references, especially the Reheis articles). For one observation, the blue contours apparently attributed to “Soda” do not closely coincide with the areal extent of Soda Lake playa. The blue area corresponds primarily to the floodplain of the Mojave River as it exits Afton Canyon. Dust from this floodplain has been studied in detail for timing and frequency of dust emission, as well as aeolian sediment (including dust) mineral composition and PSD (Urban et al., 2018). All of the major emission from the Soda area is generated from floodplain deposits. Yes, some white dust is emitted from the Soda playa surface but only rarely under certain conditions of surface wetting/drying, and such emission is short-lived in small amounts as the ephemeral salt fluff is quickly dispersed.
With respect to disturbed areas, some major ones are military bases--foremost Fort Irwin (centered ~34.04, -115.86). Dust-source sediment there can accumulate to many cm depths before winds sweep it up and carry it beyond base boundaries. It appears that much of the Fort Irwin footprint coincides with high FoO of AOD (Fig. 2). (The small playa at Ft. Irwin is a dry playa and emits insignificant amounts of dust and only then when heavily traveled).
The rationale for choosing to sample only “ripples” and crusts is not clear. These features are ephemeral and superficial. And these features could appear under somewhat different conditions in different places. For example, there are many different types/compositions of crusts with great variability in wind-erosion vulnerabilities (e.g., see Buck et al., 2011, an excellent, highly significant study). In addition, the formation of ripples depends on wind strengths, PSD, and other sediment properties related to sorting phenomena and so many not be identical from place to place.
I don’t understand the rationale about comparing properties of rippled sediment among the Mojave, Morocco, and Iceland.
At this time, some points can be drawn and will be emphasized later:
- There are many dust sources of variable composition in the Mojave.
- Many major sources are not playa surfaces.
- The overall picture of Mojave dust is that, with any given windstorm, dust from many different sources activate, and so dusts of variable compositions are mixed (see Reheis et al. 2009; Reynolds et al., 2006).
For these reasons as examples, the statements in the manuscript about characterizing the dust-source sediments across the Mojave is way too far a stretch and thus regrettably misleading. No, the authors have characterized just a few samples from limited areas that may not very well represent Mojave dust compositions during major dust storms. In addition, a large fraction of most samples is sand.
Along these lines, the attribution of “hot spots” to the sampled areas is not well justified. Authors have not tightly demonstrated that the sampled sites are sites of major, recurrent emission.
The manuscript claims to have identified variability in PSD and mineralogy as influenced by sediment transport dynamics and groundwater fluctuations (e.g., see abstract). The work did not involve measurement or investigation of sediment transport dynamics and groundwater fluctuations but only cursorily inferred these conditions from some literature.
With respect to understanding emissions and properties of dust from playa surfaces, at least a few years of observation and measurements are required to capture playa-surface/groundwater interactions to understand interlinked dynamics and variability in crust structure, roughness, mineral composition, PSD, and emissivity. Importantly, the most voluminous dust emissions from playa surfaces – wet playas only -- occur when very strong winds (typically > 20 m/s) rip off thin crusts to expose many cm-thickness of soft, fine-grained sediment beneath the crust that then is emitted as dust. The fine-grained sediment is typically a mixture of lithogenic and salt mineral particles.
The original description of wet and dry playas in the Mojave (and vulnerability to dust emission from them) was not cited (Reynolds et al., 2007) despite adopting interpretations presented in that article (and as summarized by Goudie).
With respect to Mesquite lake playa: Mesquite Lake playa is treated as one that has been “significantly” disturbed by human activity, in this case, mining. Out of roughly 1630 hectares of playa surface, only about 65 hectares are currently in the gypsum-mine area. So, a very small fraction (~4%) of the playa surface is “exploited” (line346). Mining gypsum there does not involve “pumping groundwater to separate different salts for economic purposes” (lines 344-345). The mining for gypsum and no other minerals occurs in a dry quarry. There is no evaporation at Mesquite to produce brines. Please see https://www.sbcounty.gov/Uploads/lus/Desert/MesquiteLake/MesquiteLakeIS.pdf
Most large dust events witnessed by this reviewer while at Mesquite have been generated from natural (undisturbed) playa surfaces close to its margins.
Fe occurrences. I understand that the Fe-extraction methods in this study are commonly used by some labs. But mineral identification from chemical extractions involves assumptions and uncertainties. The best ways to identify actual Fe-bearing minerals is a combination of Mössbauer spectroscopy at liq. He temperatures and rock magnetics techniques. It’s fully understandable that one or both such approaches cannot commonly be done. Nevertheless, results on the Fe mineral habitats in Mojave dusts are available in the literature (Reheis et al., 2009; Reynolds et al., 2006). For example, magnetite is ubiquitous in Mojave dusts emitted during regional wind/dust storms because such winds tap many magnetite-bearing dust-source sediments. As written, this manuscript appears to claim that its analyses describe all Fe occurrences “across the Mojave”. Such inference is misleading. For example, this statement is a stretch: “Aeolian ripples have very similar contents and modes of Fe occurrence across the Mojave Desert” (p. 10). The samples described in this manuscript deal with just a miniscule part of the Mojave and only a very limited number of the hundreds to thousands of different dust-point-sources there.
Based on chemical extractions, the authors conclude that their Mojave samples, and by inference, Mojave dust-source sediment contain maghemite and the only occurrence of crystalline Fe oxides (lines 368-369). This inference is highly misleading. Magnetite is present in many Mojave dust-source sediments and is ubiquitous in dust generated by widespread windstorms.
The use of “Mode of occurrence of Fe” is cumbersome. One could write more clearly: “Occurrences of Fe”, or “Mineral habitats for Fe”.
I don’t understand the significance of comparing Mojave (and Moroccan) settings to those in Iceland. The differences, especially in bedrock mineralogy and thus dust-source sediments, are obvious, and thus dust properties will obviously differ greatly. And as implied (line 434-435), all, or nearly all, Icelandic dusts from alluvial (mostly glacial outwash) settings.
The Conclusions section is mostly a discussion of comparisons among Mojave, Moroccan, and Icelandic results. I suggest a separate Discussion section for the comparisons.
Additional comments/suggestions by lines (L)
102. Franklin Lake playa
106. Northerly
107. …natural factors, whereas…
108-109. …training, and livestock grazing
123. Topic sentence needed.
146. Lakes existed for thousands of years after the LGM.
156. What is the Zzyzx “complex”?
170. What is meant by “other notable areas”?
167-169. Suggested re-write (please do not begin a sentence/paragraph with Figure x shows, etc.
As derived from MODIS Deep Blue C6.1 Level 2 data, the regional distribution of the annual Frequency of Occurrence (FoO) of dust events with dust optical depth exceeding 0.1 is illustrated in Figure 2.
209. Suggest minor re-write: …amount of dry ground sample was mixed and dry ground again…
232. …crystalline…, not crystallized.
255. Unclear was is meant by “very reduced dust emissions”.
255. ...this, not these
256-276. Use past tense.
259. … “whereas”, not “while” in this usage.
261-262. Averaged FDPSDs and MDPSDs of aeolian ripples from the Mojave Desert were similar, typically featuring a major size mode between 100-300 μm.
283. “relatively small” is subjective. What are the actual differences?
299-300. How (or by whom) was this interpretation made?
302-304. It has not been shown in this manuscript that mineralogy of crusts and ripples have identified dust source markers.
Also, “size fractionation processes” have not been described in any detail.
305-314. This study did not analyze actual emitted dust. Mineral abundances were determined on heavily treated bulk samples that did not appear to represent dust. In addition, crust types, mineral compositions, and strengths can and do vary greatly and sometimes very quickly depending on the factors of wetting/drying of surfaces and more. Because emissivity and dust properties can be variable under certain recurrent conditions, a snapshot in time with one collection may not represent all important aspects of dust emissivity and properties.
322. Soda Lake is not a hotspot. Please see comments above.
376. Delete “presence”.
395. What is meant by “top sediment”, here and elsewhere?
438-440. Please see comments about Mesquite Lake playa in the foregoing. This setting is not a good example of a disturbed playa. Only a small fraction has been disturbed. And I don’t find evidence in this manuscript that the mined portion is a significant dust producer.
443. The water table does not supply salts. Direct evaporation or vapor discharge from the water table might.
I recommend incorporating the compositional property findings presented in the following articles with citations to provide a much fuller picture of Mojave dust mineralogy, PSD, other properties, and emissivity. The studies by Nield et al. are also highly relevant to playa-surface dynamics and dust emission.
Buck, B.J., King, J. and Etyemezian, V., 2011. Effects of salt mineralogy on dust emissions, Salton Sea, California. Soil Science Society of America Journal, 75(5), pp.1971-1985.
Reheis, M.C., Budahn, J.R., Lamothe, P.J. and Reynolds, R.L., 2009. Compositions of modern dust and surface sediments in the Desert Southwest, United States. Journal of Geophysical Research: Earth Surface, 114(F1).
Reheis, M.C., Goodmacher, J.C., Harden, J.W., McFadden, L.D., Rockwell, T.K., Shroba, R.R., Sowers, J.M. and Taylor, E.M., 1995. Quaternary soils and dust deposition in southern Nevada and California. Geological Society of America Bulletin, 107(9), pp.1003-1022.
Reheis, M.C. and Kihl, R., 1995. Dust deposition in southern Nevada and California, 1984–1989: Relations to climate, source area, and source lithology. Journal of Geophysical Research: Atmospheres, 100(D5), pp.8893-8918.
Reheis, M.C., Budahn, J.R. and Lamothe, P.J., 2002. Geochemical evidence for diversity of dust sources in the southwestern United States. Geochimica et Cosmochimica Acta, 66(9), pp.1569-1587.
Reheis, M.C., 1997. Dust deposition downwind of Owens (dry) Lake, 1991–1994: Preliminary findings. Journal of Geophysical Research: Atmospheres, 102(D22), pp.25999-26008.
Reynolds, R.L., Yount, J.C., Reheis, M., Goldstein, H., Chavez Jr, P., Fulton, R., Whitney, J., Fuller, C. and Forester, R.M., 2007. Dust emission from wet and dry playas in the Mojave Desert, USA. Earth Surface Processes and Landforms: The Journal of the British Geomorphological Research Group, 32(12), pp.1811-1827.
Reynolds, R.L., Reheis, M., Yount, J. and Lamothe, P., 2006. Composition of aeolian dust in natural traps on isolated surfaces of the central Mojave Desert—Insights to mixing, sources, and nutrient inputs. Journal of Arid Environments, 66(1), pp.42-61.
Urban, F.E., Goldstein, H.L., Fulton, R. and Reynolds, R.L., 2018. Unseen dust emission and global dust abundance: Documenting dust emission from the Mojave Desert (USA) by daily remote camera imagery and wind‐erosion measurements. Journal of Geophysical Research: Atmospheres, 123(16), pp.8735-8753.
“In its geomorphology and surficial sediment, the Soda Lake basin is similar to many other dry-lake and associated alluvial settings known to emit dust in the Mojave Desert …. Moreover, many other types of dust sources are active in the Mojave Desert, including broad, open alluvial fans and drainages, agricultural lands, military bases (heavy-vehicle training centers), dirt and gravel roads, and recreational areas for off-road vehicles. Dust emissions from these settings are only rarely observed in satellite retrievals.”
Nield, J.M., Bryant, R.G., Wiggs, G.F., King, J., Thomas, D.S., Eckardt, F.D., and Washington, R., 2015. The dynamism of salt crust patterns on playas. Geology, 43(1), pp.31-34.
Nield, J.M., Neuman, C.M., O’Brien, P., Bryant, R.G., and Wiggs, G.F., 2016. Evaporative sodium salt crust development and its wind tunnel derived transport dynamics under variable climatic conditions. Aeolian Research, 23, pp.51-62.
Nield, J.M., Wiggs, G.F., King, J., Bryant, R.G., Eckardt, F.D., Thomas, D.S. and Washington, R., 2016. Climate–surface–pore‐water interactions on a salt crusted playa: implications for crust pattern and surface roughness development measured using terrestrial laser scanning. Earth Surface Processes and Landforms, 41(6), pp.738-753.
Some recent articles associating atmospheric processing of Fe (Ti) oxides to ocean fertilization.
Hettiarachchi, E. et al., 2019. Bioavailable iron production in airborne mineral dust: Controls by chemical composition and solar flux. Atmospheric environment, 205, pp.90-102.
Hettiarachchi, E. et al., 2020. Atmospheric processing of iron-bearing mineral dust aerosol and its effect on growth of a marine diatom, Cyclotella meneghiniana. Environmental Science & Technology, 55(2), pp.871-881.
Citation: https://doi.org/10.5194/egusphere-2024-434-RC1 -
RC2: 'Comment on egusphere-2024-434', Anonymous Referee #2, 19 Apr 2024
This well-written manuscript presents observations of the Mojave desert, especially including particle size distributions and mineralogy and iron mode of occurrence characterization. The authors identify segregation patterns in PSD and mineralogy difference within the Mojave basins, relevant to understand dust emission dynamics and properties of emitted dust in such regions.
General comments
Although the presented observations are highly informative and relevant, some changes to the discussion could, in my opinion, help increase the applicability and the impact of the presented findings.Firstly, the differences in properties at sampling locations and the dynamics (mostly related to ground water) are discussed, but it is hard to get an overview of their occurrence due to the different map types and lack of information on topography. The relationship to dust emissions does not become clear. The manuscript could benefit from a final spatial overview indicating the location of different dust source types (not just sampling points) and a comparison to the AOD shown in figure 2. Do the dust properties explain the spatial variability of AOD or is meteorology important as well? A map of the dust source types and a discussion on dust emissions/airborne dust can help implement such findings in dust emission models.
Furthermore, the authors choose to compare the observations mostly to observations in the Moroccan Sahara and Iceland. These were part of the same project, but in this manuscript the motivation for this comparison is lacking and hardly informative. I could think of many other regions where dust properties are different. Maybe it is interesting to focus on similar regions and help the reader understand if the conclusions help describe dust properties and emissions elsewhere, or if findings are unique to the Mojave Desert? And are there previous observations in the Mojave Desert that support and complement your current findings?
Specific comments
Section 2.2: Could you please add some discussion on how representative these different sample types are for the region? E.g. how much of the area is covered with crusts or ripples?
Section 3.1: How do observed PSD of samples relate to PSD of emitted dust?
Section 3.2: Do you expect similar mineralogy of emitted dust or is it likely that mineralogy will be shifted because some aggregates are more susceptible to emission.
Line 277-287: It is not clear from the manuscript why this comparison to observations in Morocco and Iceland is relevant here. A comparison of observations to previous field campaigns in the Mojave desert would be useful.
Line 429/435; I would suggest using ‘interactions’ between groundwater and crust formation, rather than relationship. Also, I think the discussion on differences between Mojave Desert and glacier forefields should include the repeated deposition of new sediments by flooding in contrast to the salt enrichment and effects on porosity by the shifting groundwater, if I understand the presented concepts correctly.
Figure 1: Please add height contours to emphasize the playa lake extents and help understand the concepts in figures 7/10.
Figure 2; Showing the gridded map rather than iso-contours should give a better representation of the observations. Only add the arrows for places also indicated in Figure 1 (i.e. no need for Los Angeles).
Figure 5: I find the colours confusing and would suggest using blue for all crust observations vs red for all ripple observations.
Figure 7: Legend is incomplete.
Figure 10: shouldn’t there be a larger difference in the amount of emitted dust in these examples?
Citation: https://doi.org/10.5194/egusphere-2024-434-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 251 | 55 | 15 | 321 | 25 | 6 | 6 |
- HTML: 251
- PDF: 55
- XML: 15
- Total: 321
- Supplement: 25
- BibTeX: 6
- EndNote: 6
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1