Particle-size fractionation of eolian dusts during transport and sampling

Marine Geology, 21 (1976) M17--M21

17

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Letter Section PARTICLE-SIZE F R A C T I O N A T I O N OF EOLIAN DUSTS D U R I N G T R A N S P O R T AND SAMPLING

L.R. JOHNSON

Department of Mineralogy, British Museum (Natural History), London (Great Britain) (Received December 12, 1976)

ABSTRACT

Johnson, L.R., 1976. Particle-size fractionation of eolian dusts during transport and sampling. Mar. Geol., 21: M17--M21. In recent years considerable attention has been given to eolian dusts and their contribution to deep-sea sediments. However, particle-sizefractionation during transport and deposition can lead to considerable modification of these dusts, so that misinterpretation can occur.

INTRODUCTION-

METHODS

Many workers have made collections of eolian dust over the oceans to assess its contribution to oceanic sediments. Eolian dust is of considerable quantitative importance in some areas. Most recent collections have used the mesh technique (Delaney et al., 1967; Parkin et al., 1970; Chester and Johnson, 1971a, b) wherein the dust particles collect on the monofilament fibres of a woven terylene mesh suspended in the air stream under study. However, this technique does n o t collect all grain sizes of dust particles with equal efficiency. Theoretically, particles below 6/~m are collected with decreasing efficiency; collection of 1 #m particles is only 15% efficient when compared with particles larger than 6 um (Parkin et al., 1970). In practice, collection is more efficient than this due to the fact that many particles are aggregated. Despite this, the technique has the advantage of enabling large samples t o be collected in a short time. Most early and many recent collections were taken by crudely washing from the exposed surfaces of ships the dust blown on to them. The efficiency of this method of collection with respect to particle size is less well known and probably varies considerably. This latter type of collection was often made during or after "haze" conditions at sea were encountered, that is when the quantity of dust transported in the atmosphere is sufficient to obscure visibility, a p h e n o m e n o n especially noted in the vicinity of the Cape Verde Islands (Darwin, 1846; Game, 1962).

Location (approx.)

11°37'S 16° 55%V 14°22'N 26°24'W Barbados

BM,1973,0,288 E13 BM,M7398

12°35'N 17°35%V 16°00'N 18°00'W 21°00'N 21°00'W

Dusts collected o f f ships' surfaces:

E3 EG25 BM,1969,0,87

Dusts collected by the mesh technique:

Sample number

80 160 400

80 800 8000

Distance from coast ( k m ) (approx. )

Particle-size d i s t r i b u t i o n of samples of eolian d u s t

TABLEI

Jun1973 Apr1969 Dec 1898

Mar 1971 Mar 1971 O c t 1965

Date of collection

57.0 60.0 69.5

75.2 78.3 81.4

0--2

14.8 19.0 14.7

13.2 15.4 9.8

2--4

2.1 0.56 1.3

8--16

11.4 6.3 8.5 7.0 9.9 3.4

8.8 5.6 6.8

4--8

7.0 4.7 2.0

0.6 0.11 0.65

16--32

F r e q u e n c y p e r c e n t a g e of particles in the size range (urn)

3.1 1.1 0.5

0.14 0 0

32--64

0.26 0 0

0 0 0

> 64

0.84 1.15 1.42

2.81 3.37 2.74

Calculated value of (see t e x t )

b-,

M19 PARTICLE-SIZE DISTRIBUTION -- MINERALOGY T h e particle-size distributions, as d e t e r m i n e d b y m i c r o s c o p i c a l counting, of some typical eolian dusts c o l l e c t e d at various times f r o m the N o r t h East T r a d e Winds blowing o f f the n o r t h w e s t coast o f Africa, b y these t w o m e t h o d s are given in Table I. It is evident t h a t dusts c o l l e c t e d f r o m the surfaces of ships are, in general coarser t h a n those c o l l e c t e d b y the mesh t e c h n i q u e . Thus, the f o r m e r m e t h o d o f c o l l e c t i o n m u s t be even m o r e biased in f a v o u r o f c o l l e c t i o n of the larger particles t h a n the mesh t e c h n i q u e . This increased bias is p r o b a b l y mainly due t o the fact t h a t d u s t b l o w n o n t o ships' surfaces is likely t o c o n t a i n m o r e large or dense particles falling u n d e r gravity at the time o f c o l l e c t i o n t h a n the meshes which collect a sample m o r e representative o f the a i r b o r n e dust. This size sorting also leads t o mineral sorting, and, h e n c e , p r e s u m a b l y , chemical sorting. Table II shows t h a t dusts c o l l e c t e d f r o m ships' s surfaces c o n t a i n m o r e q u a r t z and feldspar, and smaller a m o u n t s o f clay minerals t h a n dusts collected b y the mesh t e c h n i q u e . Q u a r t z and feldspar generally c o n s t i t u t e the bulk o f the larger size grades o f eolian dusts and o f deep-sea sediments, and h e n c e are m o r e p r o n e t o gravitational fall-out t h a n the clay minerals which are m o r e closely associated with the finer size grades. It also seems f r o m T a b l e II t h a t the clay m i n e r a l o g y o f the less t h a n 2 ~m f r a c t i o n o f the dusts varies with the m e t h o d o f collection: m o r e m o n t m o r i l l o n i t e is p r e s e n t in samples collected f r o m ships's surfaces t h a n f r o m the meshes. T h e reason f o r this m o n t m o r i l l o n i t e e n r i c h m e n t is n o t clear. I t does n o t a p p e a r t o be a seasonal effect. In the a t m o s p h e r e m o n t m o r i l l o n i t e is possibly aggregated in p r e f e r e n c e t o o t h e r clay minerals, and h e n c e t e n d s t o fall o u t preferentially, or it is of such a small grain size in c o m p a r i s o n t o the o t h e r clay minerals t h a t it is inefficiently c o l l e c t e d b y the mesh t e c h n i q u e .

TABLE II Mineralogy of samples of eolian dust Sample number

Mineralogical composition of

Mineralogical composition of

total sample (%)

the < 2 um clay fraction (%)

clay

quartz

minerals

Kfeldspar

plagioclase feldspar

montmorillonite

illite

kaolinite + chlorite

1.1 1.4 1.5

5.5 4.7 3.4

13 18 24

55 56 35

34 26 41

5.5 1.9

8.2 9.5

34 30

37 49

29 21

Dusts collected by the mesh technique:

E3 EG25 BM,1969,0,87

48 44 62

15 10 11

Dusts collected o f f ships' surfaces:

BM,1973,0,288 BM,M7395

27 40

32.3 17.8

M20 Because the particle-size distributions and mineralogical composition of eolian dust samples depend on the collection method, some d o u b t is cast on the validity of comparisons between dusts and deep-sea sediments when attempting to assess the possible c ont r i but i on of eolian dusts to deep-sea sediments. However, the main mechanism for the removal of eolian dust from the atmosphere over deep-sea areas is rainfall rather than gravitational fall-out (Prospero and Bonatti, 1969; Goldberg and Griffin, 1970). Hence, samples collected by the mesh technique are more representative of material reaching the deep-sea areas, than those blown ont o ships which, as shown above, are enriched in gravitationally settling particles. However, gravitational settling may be i m p o r t a n t in the inshore environment. MATHEMATICAL ASPECTS -- DISCUSSION The mathematical aspects of the fractionation of eolian dusts during transport have been discussed by Parkin (1974), but the effect of gravitational settling is apparent from the particle-size distributions given in Table I. The p r o p o r t i o n of large particles in the samples decreases with increasing distance from land, the decrease being clearly marked for dusts taken from ships' surfaces whilst those collected by the mesh technique only show a slight relationship. Junge (1969) has established t hat continental aerosols between 0.1 and 100 pm in diameter closely follow a particle-size distribution law of the type: d N = K r - ~ d r , where d N is the n u m b e r of particles per unit volume of air in the size range r to r + dr. Integrating between r and 2r, we have N ~ r' - ~ and hence log N ~ (1 - a)log r. Thus, using the particle-size distribution data given in Table I, the values of a can be determined from a plot of log N vs. log r, and these are given in Table I. Junge f ound for most aerosols t hat a lay between 2 and 4, 3 being typical, and this is very close to values found for dust samples collected by the mesh technique over the ocean. T he values of a for samples collected from ships' surfaces are considerably less than 3, but if pl ot t ed vs. downwind distance from the coast, they extrapolate toward this value at some 750 km from the coast. Such a direct c o m p u t a t i o n and comparison of values of a is a little crude: no allowance has been made for the inefficiency of the mesh collection technique, or that samples deposited on ships' surfaces contain gravitationally deposited material as well as some cont ri but i on by the suspended dust load. However, the data does indicate that eolian dust deposited on ships' or other surfaces becomes similar in composition to the " t o t a l " atmospherically transported dust at some 1000 km downwind due to this form of winnowing. This conclusion is also supported by the mineralogical data of Table II. CONCLUSION Some workers (e.g. Game, 1962) have concluded that, because dusts de-

M21 p o s i t e d on ships' surfaces d u r i n g sea hazes differ mineralogically f r o m the u n d e r l y i n g deep-sea sediments, eolian dusts c a n n o t m a k e a n y m a j o r c o n t r i b u tion t o the sediments. T h e a b o v e c o n s i d e r a t i o n s s h o w t h a t this d i s c r e p a n c y m a y well arise solely f r o m the m e t h o d o f c o l l e c t i o n itself, a n d t h e r e f o r e t h a t such a c o n c l u s i o n c a n n o t safely be drawn.

REFERENCES Chester, R. and Johnson, L.R., 1971a. Atmospheric dusts collected off the West African Coast. Nature, 229: 105--107. Chester, R. and Johnson, L.R., 1971b. Atmospheric dusts collected off the Atlantic coasts of North Africa and the Iberian Peninsula. Mar. Geol., 11: 251--260. Darwin, G., 1846. A n account of the fine dust which often fallson vessels in the Atlantic Ocean. Q. J. Geol. Soc. (London), 2: 26--30. Delaney, A.C., Delaney, Audrey C,, Parkin, D.W., Griffin, J.J., Goldberg, N.D. and Reimann, B.E.F., 1967. Airborne dust collected at Barbados. GeoChim. Cosmochim.

Acta, 31: 885--909. Game, P.M., 1962. Observations on a dustfall in the eastern Atlantic. February, 1962. J. Sediment. Petrol., 34: 355--359. Goldberg, E.D. and Griffin, J.J., 1970. The sediments of the northern Indian Ocean. Deep-Sea Res., 17: 513--537. Junge, C.E., 1969. Comments on the concentration of size distribution measurements of atmospheric aerosols and a test of the theory of self-preserving size distribution. J. Atmos. Sci., 26: 603--615. Parkin, D.W., 1974. Trade-winds during the glacial cycles. Proc. R. Soc. London, Ser.A, 337: 73--100. Parkin, D.W., Phillips, D.R., Sullivan, R.A.L. and Johnson, L.R., 1970. Airborne dust collections over the North Atlantic. J. Geophys. Res., 75: 1782--1793. Prospero, J.M. and Bonatti, E., 1969. Continental dust in the atmosphere of the eastern equatorial Pacific. J. Geophys. Res., 74: 3362--3371.