Trace-element concentrations in erodible soils

TRACE-ELEMENT

CONCENTRATIONS LOTHAR

Institut

fiir Meteorologic,

School of Oceanography.

(First rrcrired

University

D-6500

Mainz,

FRG

A. RAHN

of Rhode

in 19 .Yorembrr

SOILS

Sc~Crz

Johannes-Gutenberg-CniversitBt. KENNETH

Graduate

IN ERODIBLE

Island, Kingston,

Rhode

19SO and injnoi/orm 9 februarv

Island 02881. CX.4. 1951)

Abstract-concentrations of 40 elements in 11 desert soils from Africa and North America have been determined as a function of particle size by neutron activation. Concentrations generally increase with decreasing particle size down 10 radius 10-20lm. below which they remain nearly constant. This increase IS greatest in highly weathered and wind-eroded soils and is negli@ble in cultivated soils. In the plateau region below 1&20pm. most elements were within a factor of 2-3 of crustal rock proportions. suggesting that bulk crustal rock is an acceptable reference material for calculating aerosol-crust enrichment factors. The coincidence ofthe plateau region with the aerosol size range implies that the composition ofmineralaerosol should not change markedly during long-range transport; this is borne out by observation. Elements associated with highly resistive minerals such as zircon and rutiie can sometimes become um~sually enriched in the radius range l@-3Opm.

fraction of soil. Historically, the composition of either bulk rock or bulk soil has been used because data are readily available, yet neither of these is really satisfactory. Rock is inappropriate because it is unweathered; bulk soil is inappropriate &cause the composition of its aerosol-size range may differ from the bulk. For example, Rahn rt 01.(1979) found that the composition of particles smaller than radius 16pm in desert soils differed from that of the larger particles; neither size range corresponded in composition to mean crustal rock. We have studied the elemental composition of several desert soils as a function of particle size, with special attention paid to the aerosol-size range. In other words, we are dealing with the nearest surrogate to crustal aerosol discussed above. This paper presents an oven--iew of the methods and results.

aerosol-size

INTRODCCTIOS

The continents are strong sources of mineral aerosol, particularly in arid and semi-arid regions. Recent estimates of the amount of mineral aerosol generated in the particle-size range capable of long-range transport are on the order of hundreds to thousands of million tons annually (Peterson and Junge, 1971; Robinson and Robbins, 1971: Junge, 1979; Schiitz, 1980; for example). Mineral dust is one of the major constituents of tropospheric aerosol, and is found even in background regions far from continents. Based on low aerosol-crust enrichment factors, fully one-half the elements in a typical aerosol appear to be associated with its mineral component (Rahn, 1976; Rahn et al., 1979). As part of understanding the sources of elements in an aerosol, it is customary to estimate the contributions from the crust, the sea, volcanoes, pollution, etc. In order to evaluate the contribution from the crust, the precise elemental composition of mean mineral aerosol is needed. This information is not yet available; some surrogate composition must be used. Possibilities include (in order of increasing appropriateness) bulk crustal rock, bulk soil and the Table 1.

Location

SAMPLESAND

ANALYSIS

A suite of 11 samples from 5 sites in the Sahara Desert and the arid southwestern United States was chosen for analysis (Table 1). The samples differed in degree of weathering and erosion, in mineral composition, and in particle-size distribution. Each

The suite of samples

Sample No.

Remarks

Hamada-el-Hamra (Libyan Sahara)

s-2, IO. 39

Rock desert, active wind erosion

Sebha oasis

s-12, 23. 29, 33

Dune sands from Ubari sandsea, hinhlv weathered and blown-out material - .

CTF. FL

Cultivated

Texas

T

Active wind erosion

Arizona

A

Active wind erosion

Sudanese

Sahara

171

soil

171

IOTH-\R SWCTZ

and KEMETH.~.

sampI was fractionated into 9 sizz :angss by w2t and dry -techniques. Dry sieving producsd fractions of radius range 400-160. 16CHO. 80-31 and 32-16,~m. Sedimentation in distilled water, for Xmin-20 h. produced fractions of radius range 16-8. S--t, 42, t-l and < 1pm. Even though the wet sedimsntation in Xtterberg cylinders dissolved a portion of some elements associated with nitrates. sulfates. carbonates, etc., and probably disaggregated some clusters of particles, it was the only way to fractionate the samples within the critical aerosol size range other than a sophisticated wind-tunnel experiment, which was not within the scope of this invsstipation. This sedim2ntation did not seem to affect the main conclusions of the experiment: only 20”” or less of most elsments dissolved (nearly independent of the element). whereas all important conclusions could be drawn from concentration variations exceeding a factor of two; th2 data of Figs 13

R.-\HY.

1 ,.,.,I

.I..:! I

I

IO RADIUS

100

IO

100

RADIUS r,,uum

.I......!

I

10

RACIUS (, /ii-

100 r, ,um

Fig. 2. Concentrations of Fe and Al LSparticle radius in the 11 soils of this study, normalized to the concentration in the 160400~m range. show that no major discontinuities of concentration pattern with particle size could be seen at the juncture between wet and dry sieving (8-161~11 vs 16-33pm). Elemental composition of the samples was determined by instrumental neutron activation. Shortand long-lived nuclides together yielded data for about 40 elements, 30 of which had analytical uncertainties of less than 20 1, (Table 2). The accuraq- of the results was checked by co-analyzing four standard geochemical refirence materials.

RESCLTS ASD DISCUSSIOS RAOIIJS

r, ,um

Fig. 1. Elemental concentrations vs particle radius in soils from two arid regions, normalized to the concentration in the i604MJpm range.

Out of all the data produced in this study. a few general principles have emerged. and will now be discussed. From the largest particle sizes downward. elemental concentrations genera& increase. first slowly down to radius 5Opm. then rapidly betu-2en 50

Trace-slcment

concentrations

I

E

in erodlble

“‘11

sotis

j

“‘,‘I

I

F:

I

.

Hf

^-F

I 10

1 RADIUS

Fig. 3. Enrichment factors crustal rock) vs particle

1

I , , ,,,,,l , , ,/ loo

r, ,urn

of Fe and Hf (relative to Al and bulk radius in seven soils of this study.

Fig. 4.(a) and (bl

I’4

LO~HARSCH~TZ

and

I(E.';zETH.& RAHI

different concentrations. The magnitude of the increase between large and smail particles seems to be a characteristic property of each sampie ia sort of “fingerprin:“) and appears to be related to the extent of its weathering and wind erosion. Examples of this are shown in Fig. 2 for Al and Fe in various soils. The slope is greatest m highly weathered and blown-out dune sands and desert-like soils (S-12, 2-t. 29, 33 and T), moderate in less winnowed soils where particles are currently being produced by weathering (S-2. 10, 39 and A). and nearly flat in the humus-rich. cultivated Sudanese soils CTF and FL. In a given soil. many elements have very similar patterns. The only element determined here which increases in concentration with increasing particle size is Si. One reason for the higher elemental concentrations at smaller particle sizes in soils could be the larger number of mineral species there compared to larger particle sizes. Mineralogical analysis of the Saharan dune and rock-desert samples indicates that only a few minerals like quartz. dolomite and calcite dominate for particle radii greater than 30ym, whereas for smaller particles a much greater variety of species is found, including plagioclase feldspars, K-feldspars, micas and clay minerals. The latter may be particularly important here, as they contain almost all major and minor elements, as well as trace elements adsorbed on the surfaces (Rosler and Lange, 1975). We have Sound it useful to normalize our results to Al via a soil-rock enrichment factor. defined for element X as follows: 100

IO

I RADIUS Fig.

i, pm

and (d).

4.(c)

Fig. 4. Four empirical groups of elements in arid soils, based on patterns of enrichment factors vs particle radius. and 20pm. Below lo-20pm there is a broad plateau of concentration, with a greater or lesser maximum near 10-20pm. Figure 1 shows curves of concentration for various elements in the Sahara 33 and Texas samples, normalized to their concentrations in the 16U-4OO~m range for easier comparison of elements of highly

where the data of Mason (1966) is used for mean crustal rock. These enrichment factors vs particle size may help answer two important questions: (1) To what extent should the elemental composition of mineral aerosol change during long-range transport, as the mean particle size decreases? (2) How valid is the use of bulk crustal rock as reference material in enrichment factor calculations for remote areas? To begin to answer the first question. Fig. 3 shows EF’s of Fe and Hf vs particle size. The EF of Fe shows n3 size dependence for radius < 100 ,um; no changes of the Fe/Al ratio would thus be expected during transport. The EF of Hf. on the other hand, varies

Table 2. Elements determined and their average concentrations Concentration range fppm) > 10’ IO”-10’ IO’-IO’ IO’-10’ IO’-lti l-10
Elements determined from Short-lived nuchdes Long-lived nuclides Si Al, Ca, Mg Na. Ti Ba, .Mn. (Sr) ‘v, ICU). (Cl) Dy, (1)

Fe, K Zr (Zn). Ce. Rb. Cr, La, Hf. (Nd) Th, Ga. Co. SC.Sm. Yb. (As). Cs. Eu, Ta (W). Tb. Lu, (A@. Sb. 1.4~)

Elements in parens indicate lar_eeanalytical uncerramries

Trace-element concentrations

greatly from soil to soil and with size; transport should thus aiter the Hf Al ratio. The general EF pattern for the rest of the elements can be seen from Fig. 1. which presents geometric mean EF’s for each element over all samples vs particle size. Four broad groups are shown: 16 Hf-like elements (the rare earths, Zr, Th. Ti. Ta. and to a lesser extent SC and Mn). 14 Fe-like elements [including Al). 6 elements whose EF’s increase with particle size, and Ag and Au, whose EF‘s decrease strongly with particle size. The case of the Hf-like elements is particularly interesting. Zr and Hf are enriched by factors of 10-100 in the radius range lO-30pm, and in some samples by considerably more than this. Concentrations of Zr in this size range of our dune-like sands reach 10,OOOppm. The identity of the elements in this group suggests that they are associated with heavy resistant minerals. Zircon. which is enriched in Zr, Hf, Th and the rare earths. and rutile. which contains Ti and Ta, are two possibilities. In general, highly weathered and winnowed soils are enriched in heavy minerals (Petrov, 1976): Saharan dune sands are known to be enriched in zircon and rutile (Sindowski. 1936). In sediments. the mass peak of the heavy minerals is at l&jO~rn radius. independent of the overall mineralogy and mass distribution of the sediment (Fiichtbauer and %Iiiller, 1970). This also matches with our samples. Mineral aerosols observed within roughly 1OOOkm of their desert sources will contain sizeable mass at radii > 10~~m,hence may be considerably enriched in the resistant, Hf-like elements compared to bulk crustal rock. At distances beyond IOOOkm. however, most of the mass will be at radii < lOpm, according to the model calculations and measurements summarized by Schiitz (1980) hence will be much more similar to bulk crust in composition. Because of the multielement concentration plateau, further transport will not alter the composition significantly. This explains the results of Rahn er al. (1979) for transport of Saharan dust over the North Atlantic, where no change in composition could be detected over a 24Wkm cruise track which ranged from 600 to 1300km from the west coast of Africa. It is also in accord with Glaccum (197s) and Glaccum and Prosper0 (1980), who found that Saharan dust after transport to the Caribbean was depleted in the coarse-particle quartz and feldspars and enriched in the finer-particle clay minerals compared to the same dust at the west coast of Africa,

in erodible soiis

whereas relative concentrations of the clay-associated hydrolysate elements La, SC.V, Y, Co, MnO and TiO, were only slightly- increased in the Caribbean dust. In order to compare the composition of our desert soils with that of bulk crustal rock (the second question above). we have summarized in Table 3 the geometric mean EF’s of all the elements for the r < 10pm fraction ofall the samples. The great majority of the e!ements have EF’s within a factor of 2-3 of unity. The three’soluble” elements Na. Caand Sr have mean EF’s c 0.5: seven elements (Cu. Zn, As. Sb, 1,Au, Ag) have EF’s 2 3. These latter elements are characteristically enriched in the aerosol as well. but with orderof-magnitude greater enrichments than found here (Rahn, 1976). Table 3 implies that the true enrichments of Na, Ca, etc. in aerosols (relative to our soils) are somewhat greater than values calculated from rock. and that the true enrichments of Cu, Zn. etc. are less than calculated from rock. Thus, the aerosol-size range of the desert soils is quite similar in composition to bulk crustal rock; the latter therefore continues to be an acceptable reference material for calculating aerosol-crust EF‘s. The constancy of composition within the aerosol size range of most soils. combined with the similar composition of the soils studied here, supports the concept of a single reference material for enrichment-factor calculations. It also suggests that differences in bulk compositions of soils will not necessarily appear in aerosols produced from them. especially more than 100 km from the source. This works against using the elemental composition of a mineral aerosol to deduce the identity of a distant source region. For this, other characteristics of soils may be more suitable.

Aclinowledgemunrs-This work was supported by the Ge:man Science Foundation, through its Sonderforschungsbereich 73 (“Atmospheric Trace Substances”). and by the U.S. Office of Naval Research (Contract NOOO14-76-C-0435). Samples were analyzed at the Rhode Island Nuclear Science Center. V. Johannes. Th. Karlewski. K. Groebe and K. Mobus assisted with data processing and analysis. The Sudan. Arizona, and Texas samples were supplied by S. A. Penkett (AERE Harwell. UK). N. Korte (Univ. of Arizona, Tucson), and D. A. Gillette (NCAR. Boulder, CO) respectively. Mineralogical analyses were supplied by E. D. Goldberg. Scripps Institute of Oceanography, La Jolla. CA. Size fractionation and data analysis were performed at the Max-Planck-Institut fiir Chemie, Mainz. FRG. the former affiliation of L.S.

Table 3. Geometric mean enrichment factors for the r < 10pm fraction of desert soils

< 0.5 0.5-3.0 3.0-10 > 10

175

Sr. Ca. Na Rare earths, Si, Hf. Co. Ba, V. Ti. Cs, Zr, Mn. W. Th, Rb. Mg, Ga. Cr. Fe. Cl Cu. Zn. As. Sb. I Au, xg

176

LOTHAR

SCHCTZand KENXTH A.

REFERENCES

Miiller G. l.1970) Sedimenrr und E. Schweizerbartsche Verlagsbuchhandlung. Stuttgart. FRG. Glaccum R. h. 119X The mineralogical and elemental composition of mineral aerosols over the tropical North Atlantic: the influence of Saharan dust. .%I.S. Thesis, Univ. of Miami (USA). 161 pp. Glaccum R. A. and Prosper0 J. M. (1980) Saharan aerosols over the tropical Sorth Atlantic-Mineralogy. Mar. Grol. 37. 295-321. Junge C. (1979) The importance of mineral dust as an atmospheric constituent. In: Snharan Dusr: Mobilixfion, Tronsporr, Deposirion. SCOPE 14. C. Morales, Ed.: 49-60. Wiley % Sons. Chichester, VK. Mason B. (1966) Principles ofGrochemisrr~. Wiley and Sons, New York. Peterson J. T. and Junge C. E. (1971) Sources of particulate matter in the atmosphere. In: .lfan’s Impact on the C’limure. W. H. Matthews. W. W. Kellog and G. D. Robinson. eds. pp. 31%320. MIT Press, Cambridge, M.4. Petrov M. P. (1976) Deserts qf‘rhe World. Wiley and Sons. New York. Fiichrbauer

H. and

Sedimenzgesteine.

R.~H>

Rahn K. A. (19761The chemical composition of the atmospheric aerosol. Technical Report, Cniversity of Rhode Island. Rahn K. A.. Borys R. D.. Shaw G. E.. Schiitz L. and Jaenicke R. i 1979) Long-range impact of desert aerosol on atmospheric chemistry: Two examples. In: Soharan Dust: .%fobili-_arion.Trunsport. Deposirion. Scope: 14. C. Morales. Ed.: 213-266. Wiley and Sons. Chichesier. UK. Robinson E. and Robbins R. (1971) Emissions, concentrations. and fate of particulate atmospheric pollutants. API Publication No. 476. Stanford Research Institute, Menlo Park, California. R&.ler H. J. and Lange H. (1975) Geochemische Tabellen. VEB Dt. Verlag f. Grundstoffindustrie. Leipzig, GDR. Schiitz L. (1980) Long-range transport of desert dust with special emphasis on the Sahara. Ann. .V.I’. .4cod. Sri. 338. 515-532. Sindowski K. H. (1956) SedimentpetrographiKh-bodenkundliche Untersuchungen an einigen Steppen-, Voilwijstenund Extremwiistenbijden und Gesteintn aus der mittleren Sahara [Libyen). In: Forschungen in der zznwalen Sahara. W. Meckelein, Ed., pp. 152-179. G. Westermann. Oldenburg, FRG.