Grain size record of microparticles in the Muztagata ice core ...

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Science in China: Series D Earth Sciences 2006 Vol.49 No.1 10—17

DOI: 10.1007/s11430-004-5093-5

Grain size record of microparticles in the Muztagata ice core WU Guangjian1,2, YAO Tandong1,2, XU Baiqin1,2, LI Zheng2, TIAN Lide1,2, DUAN Keqin2 & WEN Linke2 1. Laboratory of Environment and Process on Tibetan Plateau, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China; 2. Key Laboratory of Cryosphere and Environment, Jointly Established by Cold and Arid Regions Environmental and Engineering Research Institute and Institute of Tibetan Plateau Research, Chinese Academy of Sciences and Chinese Academy of Meteorological Sciences, China Meteorological Administration, Lanzhou 730000, China Correspondence should be addressed to Wu Guangjian (email: [email protected])

Received September 29, 2004; accepted August 11, 2005

Abstract The dust transport and sediment characteristics are discussed based on analysis of microparticle size and size distribution in the Muztagata ice core at 6350 m a.s.l. The finer particles with diameter of 1―5 μm are the dominant fraction in number, while middle and coarse particles mainly contribute to the total volume. The lognormal distribution characteristics can be seen for some high concentration samples, showing that model size and standard variation are greater than that in the Greenland ice cores. However, size-volume distribution of some low concentration samples is abnormal. Those distributions reflect the dust deposit process in high mountain glaciers at mid-low latitudes and show differences from those in polar ice sheet. Keywords: Muztagata, ice core, microparticle, dust, size distribution.

Dust grain size is a proxy for wind strength that entrains it. Mineral aerosol blown from arid continent to remote sites has a broad diameter range, from less than 0.5 μm to larger than 75 μm[1]. For a long time, geologists reveal the transport and sediment characteristics using grain size and size distribution. In loess research, grain size is widely used as the proxy for winter monsoon strength[2,3]. Fractions entrained by the westerlies and winter monsoon can be discerned by grain size and indicate atmospheric circulation changes[4,5]. In deep sea sediments, grain size of aeolian dust also was taken as a signal for the wind strength and transport dynamics[6]. In ice core researches, grain size and its distribution characteristics were used to reflect the www.scichina.com

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wind strength and dust storms in the source areas[7 9]. Glaciers are excellent medium for atmospheric dust deposition. Many studies have been carried out on dust in polar ice cores. For dust deposited in the mountain ice cores on the Tibetan Plateau, it is the valuable record of environmental changes and atmospheric activities for the Central Asia dust source areas, and its grain size also reflects transport mechanism, which might be different to that in the Greenland, though they have the same source. Those ice cores with an enough altitude would minorly be affected by local factors and represent atmosphere conditions. In this paper, grain size and its distribution of microparticles in a Muztagata ice core is discussed and compared to ―

Grain size record of microparticles in the Muztagata ice core

those in polar. All the grain size means diameter without special annotation. 1

Sampling and measurement

The Mt. Muztagata (75°04′E, 38°17′N) is situated at Eastern Pamir Plateau. Located in the central of the Asian arid regions and circumfused by deserts, those are important part of dust source areas in the Northern Hemisphere, the Muztagata is a favorable site for dust research, especially using microparticles in ice cores recovered there (Fig. 1). There are two main atmospheric circulations over the Mt. Muztagata, the westerlies and the South Asia monsoon, both are justified by the location and shape of the Tibetan Plateau. The Tibetan Plateau monsoon also should be mentioned and might also have a substantial effect. A 43.1-m-long ice core and a 1.2 m depth snowpit were recovered at 6350 m altitude in July and August, 2001. This core was kept frozen before sample incision, with sampling intervals of 4―6 cm, which can reflect seasonal variations. Oxygen isotope and microparticle were analyzed at the Laboratory of Ice Core and Cold Regions Environment. The microparticles were measured using a Beckman Multisizer 3 Coulter Counter with a 50 μm diameter aperture. Since the Coulter Counter counts particles by volume, the particle sizes given here are spherical equivalent diameters. According to the Coulter Principle, the measurement range is from 1 μm to 30 μm (2%―60%

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of the aperture diameter). The particles were counted in 300 channels between 1 and 30 μm diameter on a logarithmic size scale. The result was the number of particles in each channel, while mean diameter, number and mass concentration (per melted water equivalent volume) were calculated using this. Because of high concentration, all the samples were diluted 2―20 times by using ISOTON II electrolyte before measurement. Background of the measurement, with only 400―800 particles per milliliter, mostly finer than 5 μm and a very minor contribution to total number or mass, is negligible. Particle mass, in μg/kg, was calculated from the volume assuming a mean particle density of 2.6 g/cm3[10]. 2

Mean grain size

Based on the measurement results, it can be obviously seen that the finer particle fraction is dominant in the Muztagata ice core. The 1―5 μm fraction takes dominant fraction of the total particles (1―30 μm), both for high and low concentration samples. And, the ultra-finer particle fraction, with diameter less than 1 μm, has greater amount, though they are beyond the measurement limitation. In the range of 1−5 μm, particles exist almost in all measurement channels. While the particles decrease quickly in large channels, showing that coarse particles become thin with diameter increasing. However, given that very coarser particles

Fig. 1. Location of Muztagata ice core (black dot) and dust source areas around the Tibetan Plateau.

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are scare in most samples and with a great uncertainty but have a significant contribution to mass, the number and mass concentration are calculated using the particles ranging from 1 to 15 μm only. The particles within the 15―30 μm diameter are not used to the calculation of mean diameter and concentration, but are included in the fitting of size distribution. Grain size of microparticle includes mean number diameter (MND) and mean mass diameter (MMD), the former is the arithmetic average of particles with different size, while the latter depends on the fraction and volume of total particles[7]. Sample M-10-3 has the greatest MND (2.54 μm) and MMD (3.96 μm), while sample M-25-1 has the smallest MND (1.58 μm) and MMD (1.80 μm). The average MND and MMD of the whole ice core is 1.88 μm and 2.60 μm, respectively. At deposit sites far away from the source areas, such as Greenland, it has been pointed out that the MMD is sensitive to dust events in the source areas, because during dust storm some coarse particles can be blown up to upper air and be transported to a far distance; while the MND reflects the entraining wind strength[7]. For mountain glacier in the mid-low latitudes, strong turbidities and/or turbulences can carry many local coarse particles to deposit on it. The MND and MMD for ice core and snowpit samples in Muztagata have a striking correlation (Fig. 2). The linear correlation coefficient of MND and MMD is 0.91 and 0.93 for ice core samples and snowpit samples, respectively. The MMD has more frequent fluctuations

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and greater change amplitudes than MND, indicating that coarse particles have greater contribution to the MMD than fine ones. Sharply seasonal variations in particle grain size can be seen in the record (Fig. 3). The grain size and oxygen isotope record have similar variations in general trend, suggesting that dust storms and/or strong wind often occur in warm season. However, though dust storm events seldom occur in cold season, there are still some grain size peaks appear with the low oxygen isotope ratio. Grain size peaks often rise when oxygen isotope ration increases, which are corresponding to late spring and early summer. During the periods with high oxygen isotope ratio, temperature fluctuation is minor, while particle record has more peaks than oxygen isotope, suggesting that dust storms/strong winds are more frequent (with two or more grain size peaks) in warm season, such as in 1999, 1998, and 1985. The systemic correlation between isotope ratio and grain size change amplitude is not very clear. The deposit patterns (the dry and wet deposit) can also affect particles deposited in ice cores. Based on the limited meteorological data from Central Asia, the blowing dust or dust storms in Tashkent and Biskek usually occurred through April to October, temporally coinciding with the period of particle peaks in Muztagata. Meteorological data during 1954―2002 in north China indicate that the month with the most frequent dust storms is from March to May, especially in April[11]. Analysis on main ions in the upper 5 m of

Fig. 2. Correlation between mean number diameter (MND) and mean mass diameter (MMD). (a) Samples from Muztagata ice core; (b) samples from snowpit at 6350 m.


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Fig. 3. The oxygen isotope, MMD, and MND record in Muztagata ice core. MMD and MND are after 5 points smoothing, asterisks mean annual layers.

a Guliya ice core revealed that high dust load on the northwestern Tibetan Plateau mainly occurred through February to May, coinciding with the major period of dust storm in west China[12]. The microparticle record from the Muztagata ice core shows that dust storms in Central Asia are approximately similar to those in northwest China. Size fraction changes in loess can reflect a spatial variation of winds that carry loess, showing an important climatic application[4]. Affected by the Mongolian High, the winter monsoon played a key role in eolian dust transport and deposit in northwest China, along with the perennial westerlies. However, Muztagata might mainly receive dust from the Central Asia by westerlies, and possibly also by Indian monsoon from south Asia deserts. 3

Volume distribution

Based on researches on aerosol in atmosphere and ice cores, the dust volume distribution obeys the lognormal equation[13,7] that may be expressed as the following:

Vtotal dV ⎡ 1 ln d − ln d m 2 ⎤ = a × exp ⎢ − ( ) ⎥, a = . d ln d ln σ 2π ln σ ⎣ 2 ⎦ The d means the fitted size range, Vtotal is the total volume of the lognormal distribution, σ is the lognormal standard deviation, dm is the mode size. A total of 68% of the total lognormal volume (or mass) is in the size interval [exp(ln μ − lnσ), exp(lnμ+lnσ)]. Logarithms with base of 10 can also be used, and radius r can be used instead of diameter d. To use dlnd instead of d is a matter of convenience. Because of wide range of particle diameter, the logarithm coordinate axis will be more convenient. The mass distribution[14] is essentially the same as the volume distribution because of the linear correlation between volume and mass. The measurement results show that some channels have no counts, meaning that particles in this channel have no contribution to the total volume. While in the lognormal distribution, the volume distribution is supposed to be continuous. The vacant count in some channels will yield errors in fitting process. For those low concentration samples, a single coarse particle will take a large fraction to total volume. Here, we

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only computed some high concentration samples. Before the lognormal fit process, the measurement channels were reduced from 300 to 50. The fitting results of two typical samples, M-9-3-3 and M-10-3, are shown in Fig. 4. The channel setting has a minor effect on the fit result, because the standard deviation σ and mode size dm are essentially unchanged. The decisive coefficient R2 will increase with lesser channels because of the less data used in the fitting and the smoothing of original record. Since σ and mode size dm have little changes, the fitted curve will be consistent. In GRIP and NGRIP ice core, strong positive correlation exists between mode size and mass concentration, the latter increase correspondingly with the former[7,15]. Based on fitting result of some high concentration samples (n=74), it indicates that the mode size becomes coarser while σ becomes small with mass concentration increasing. However, these trends are not clear and both σ and mode size has a very low correlation coefficient with total mass concentration. For those samples in Muztagata ice core, σ concentrates around 2 μm, suggesting minor difference during the sorting process. And, the very coarse particles (diameter >15 μm) have a complex contribution to the total mass without systemic correlation. Many high mass concentration samples have a minor even no fraction of the >15 μm particles. This indicates that size distribution of microparticles in mountain ice cores is different to those in Greenland, though only the high concentration samples are discussed here. ⎡ 1 ⎛ln d − ln 7.79 ⎞2 ⎤ dV M-9-3: = 30622365 × exp ⎢ − ⎜ ⎟ ⎥, d ln d ⎠ ⎦⎥ ⎣⎢ 2 ⎝ ln1.65 2 R = 0.915; ⎡ 1 ⎛ln d − ln 9.37 ⎞2 ⎤ dV M-10-3: = 77854006 × exp ⎢ − ⎜ ⎟ ⎥, d ln d ⎠ ⎥⎦ ⎢⎣ 2 ⎝ ln1.68 2 R =0.889. 4

Comparison with other ice cores

The modern MND of microparticle in the Muztagata ice core ranges from 1.58 to 2.54 μm. In the GISP2 ice core, the MND ranges from 1.18 to 1.32

Fig. 4. Typical lognormal distribution for two high concentration samples.

μm during the last glacial maximum[16]. It can be seen that the microparticle is coarser than that in the Greenland ice sheet, though they have the same dust source areas[16,17]. Many results hold that the microparticles in ice cores obey the lognormal distribution with only one mode size, such as in GRIP, GISP2, EPICA Dome C, and Penny ice core. The mode size is about 2 μm in GRIP[8], 2―3 μm in GISP2[7], 1―2 μm in NGRIP[15]. In the Penny ice core in Baffin Island, particle mode size is a little coarser than Greenland, extending to 6.7 μm during the Holocene because of nearby snow cover melted and dust source areas appeared[9]. In EPICA Dome C in Antarctica, the mode size approximately kept at 2 μm during the last glacial maximum and the Holocene[18]. The Muztagata shows some difference from those Greenland ice cores. In high concentration samples, mode sizes, from 3.34―9.37 μm, are coarser than those in GRIP and GISP2. The Muztagata also has greater standard deviations (1.55―3.16 μm), showing a broader size range and more important contribution of coarse particles. However, many low concentration samples do not show such a lognormal distribution, so they will not be discussed in this paper (Fig. 5).


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Fig. 5. The lognormal distribution for microparticles in EPICA Dome C and GISP2.

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Discussion and conclusion

Based on the measurement and fitting results, some characteristics of grain size and size distribution in Muztagata ice core can be discussed. After a distance far enough, about 1000―2000 km away, the mean size will be matched with the wind ― strength and keep stable[19 21]. Dust with this size is named equilibrium grains. For dust from the Central Asia arid areas, the equilibrium grains have a mode size of ~2 μm (volume distribution) in the Northern Pacific deep sea sediment[6] and Greenland ice cores. The finer part in Chinese loess also centered between 2―6 μm[5], similar to those equilibrium grains carried by westerlies. The equilibrium grain size also changes with atmospheric environment, such as the difference between LGM and Holocene. Gravity can sort the floating dust during its transport process. Heavy and coarse particles will be preferentially removed, making the dust finer with further transport distance. Particles far from source areas, such as those in the Northern Pacific deep sea sediments and Greenland ice cores, are well sorted and more proximate to lognormal distribution. This is an important cause for the difference for particle size and size distribution between polar and mountain ice cores. The Muztagata ice core is close to dust source areas, and dust in it experienced a very poor sorting process, compared to those in Greenland. Particles in Muztagata are not the equilibrium

grains because of poor sorting with greater standard deviation. For those high concentration samples, sometimes coarse particles do not exist continuously. The majority of mass is the middle size particles (5― 10 μm), so lognormal distribution can be gained, like those in polar ice cores. In the Dunde ice core, north Tibetan Plateau, microparticles also show the lognormal distribution, mainly being the middle size grains (2.5―5.04 μm) take the majority of total volume[22]. The lognormal distribution is usually used to fit aerosol size distribution in atmosphere, but it is not the only distribution, nor the best one. The normal (Guassian) distribution can also be fitted fairly well for high concentration samples in Muztagata ice core. The lognormal fitting result also shows some departures from measurement, such as samples M-10-3 and M-9-3. This discrepancy also exists in polar ice core and other aerosol samples, such as the GRIP[8] and EPICA-Dome C[18]. The Sahara dust deposited at Canary Islands and Puerto Rico shows no lognormal characteristics in size distribution[1]. Some samples show bi-mode size in the Penny ice core[9], or their volume distribution shows almost a special line in a Tibetan Plateau ice core[14]. In EPICA Dome C ice core, samples can be fitted by the Weibull distribution[18]. For those low concentration samples in Muztagata, few or individual coarse particles take a very large fraction of the total volume. This will significantly make deviation between the lognormal fitting

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and the measurement results. For example, in Muztagata ice core, a particle with 16.15 μm diameter (1/1928 in total number) takes 26.18% fraction in the total mass, while a particle with 15.63 μm diameter (1/1310 in total number) takes 30.44%. Though contamination in measurement is inevitable, the coarse particles do exist in Muztagata ice core samples. It has also been pointed out that particles in mountain ice cores had complex volume distribution and might not always obey the lognormal formula, such as the Hispar glacier in Karakum and Ngozumpa glacier in eastern Himalaya[14]. The special characteristic of size distribution of microparticle from mountain ice cores might lie in the proximity to source and transport process. The complete size distribution for eolian dust may have 3 modes: (1) centered between 0.02―0.5 μm radius, which might be the background aerosol and have no relation to soil; (2) centered between 1―10 μm radius, which mainly soil-derived aerosols under all condition; and (3) centered between 10―100 μm radius, which might be the soil parent particle and appear only under heavy dust loading conditions[13]. Grain size correlates both with transport wind strength and its source. The size distribution can provide some tentative evidence for dust source tracing. For example, eolian and hemipelagic sediments in the Northern Pacific have the different mode sizes[23], and the very similar size distribution of red clay and loess in China might suggest that they have the same source[24]. The Chinese loess, as the typical eolian dust deposit, contains coarse and fine parts in its size distribution. The coarse part with large kurtosis and narrow distribution was carried by the winter monsoon, while the fine part with small kurtosis and poor sorting was carried by westerlies, representing background aerosol on the Loess Plateau and being similar to those in the North Pacific[4,5]. However, it should be pointed out that the transport dynamic for loess and microparticle in ice core is different. The background aerosol, with diameter less than 1 μm, is beyond the Coulter Counter measurement limitation, should have a very huge number. The soil parent particle, with radius between 10―100 μm and equivalent to the coarse component of loess, seldom appears in ice core. While the mineral aerosol emitted form soil, equivalent to the finer component of loess,


is the principal part of microparticles in ice. This might suggest that dust in Muztagata ice core is mainly carried by upper westerlies from remote source areas, like the fine part of loess. For those coarse particles, they might come from nearby local sources. The Dunde ice core also has two types of source area, the far and the nearby[22]. Since the Muztagata lies in the westmost of China, its dust source areas might be the Central Asia (in narrow terms) deserts, such as Sary-Isikotrau, Kyzylkum, Karakum, Caspian Sea, etc. The arid regions in west China might have very minor contribution. However, more studies are needed to do for the detailed and precise source tracing. The mean size of dust and the fraction of coarse particles will decrease when the altitude increases. For particles in the Muztagata ice core at 6350 m, the finer particles less than 5 μm take the main number fraction of total 1―30 μm particles. However, proximate to the Central Asian dust source areas, strong dust storms and turbulences will carry some coarse particles, such as those >20 μm, up to the 6350 m or even higher altitudes, yielding an important effect on total volume and size distribution. The wider range and existence of coarser particles might be one of the important characteristics for microparticles in mountain ice cores. Acknowledgements This work was supported by the National Basic Research Program of China (Grant Nos. 2001CB711001 and 2005CB422004), the Collective Innovation of the the National Natural Science Foundation of China (Grant No. 40121101), the Program of the NNSFC (Grant No. 40301009), and the Chinese Academy of Sciences (Grant No. KZCX3-SW-339).

References 1. Maring, H., Savoie, D.L., Izaguirre, M. A. et al., Mineral dust aerosol size distribution change during atmospheric transport, Journal of Geophysical Research, 2003, 108(D19), doi: 10.1029/ 2002JD002536. 2. Porter, S.C., An, Z.S., Correlation between climate events in the North Atlantic and China during the last glaciation, Nature, 1995, 375: 305―308. 3. Derbyshire, E., Kemp, R.A., Meng, X.M., Variations in loess and palaeosol properties as indicators of palaeoclimatic gradients across the Loess Plateau of Northern China, Quaternary Science Reviews, 1995, 14: 681―697. 4. Sun Donghuai, An Zhisheng, Su Ruixia et al., Eolian sedimentary records for the evolution of monsoon and westerly circulations of

Grain size record of microparticles in the Muztagata ice core northern China in the last 2.6 Ma, Science in China, Series D,

central Greenland NGRIP ice core during the last glacial period,

2003, 46(10): 1049―1059.

Journal of Geophysical Research, 2003, 108(D3): doi10.1029/

5. Sun, D.H., Bloemendal, J., Rea, D.K. et al., Grain-size distribu-

16. Biscaye, P.E., Grousset, F.E., Revel, M. et al., Asian provenance

vironments, and numerical partitioning of the sedimentary com-

of glacial dust (stage 2) in the Greenland Ice Sheet Project 2 Ice

ponents, Sedimentary Geology, 2002, 152: 263―277.

Core, Summit, Greenland, Journal of Geophysical Research, 1997,

10. 11.

12.

102(C12): 26765―26781.

processes of the mineral component of abyssal sediment: Lessons

17. Bory, A.J.-M., Biscaye, P.E., Sevensson, A. et al., Seasonal vari-

from the North Pacific, Paleoceanography, 1995, 10(2): 251―

ability in the origin of recent atmospheric mineral dust at North-

258.

GRIP, Greenland, Earth and Planetary Science Letters, 2002, 196:

7. Zielinski, G.A., Mershon, G.R., Paleoenvironmental implications

9.

2002JD002376.

tion function of polymodal sediments in hydraulic and aeolian en-

6. Rea, D.K., Hovan, S.A., Grain size distribution and depositional

8.

17

123―134

of the insoluble microparticle record in the GISP2 (Greenland) ice

18. Delmonte, B., Petit, J.R., Maggi, V., Glacial to Holocene implica-

core during the rapidly changing climate of the Pleistocene-

tions of the new 27000-year dust record from the EPICA Dome C

Holocene transition, Geological Society of American Bulletin,

(East Antarctica) ice core, Climate Dynamics, 2002, 18: 647―

1997, 109(5): 547―559. Steffensen, J.P., The size distribution of microparticles from selected segments of the Greenland Ice Core Project ice core representing different climatic periods, Journal of Geophysical Research, 1997, 102(C12): 26755―26763. Zdanowicz, C.M., Zielinski, G.A., Wake, C.P., characteristics of modern atmospheric dust deposition in snow on the Penny Ice Cap, Baffin Island, Arctic Canada, Tellus, 1998, 50(B): 506―520. Sugimae, A., Elemental constituents of atmospheric particulates and particle density, Nature, 1984, 307: 145―147. Zhou Zijiang, Zhang Guocai, Typical severe dust storms in northern China during 1954−2002, Chinese Science Bulletin, 2003, 48(21): 2366―2370. Li Zhongqin, Yao Tangdong, Xie Zhichu, Modern atmospheric environmental records in Guliya ice cap of Qinghai-Xizang Plateau, Chinese Science Bulletin, 1996, 40: 874―875.

660. 19. Janecek, T.R., Rea, D.K., Quaternary fluctuations in the Northern hemisphere trade winds and westerlies, Quaternary Research, 1985, 24: 150―163. 20. Rea, D.K., Leinen, M., Janecek, T.R., Geological approach to the long-term history of atmospheric circulation, Science, 1985, 227: 721―725. 21. Chuey, J.M., Rea, D.K., Pisias, N.G., Late Pleistocene paleoclimatology of the central equatorial Pacific: a quantitative record of eolian and carbonate deposition, Quaternary Research, 1987, 28: 323―339. 22. Liu Chunping, Yao Tandong, Xie Shucheng, Characteristics of microparticle variation and records of atmospheric environment in Dunde ice core, Marine Geology and Quaternary Geology, 1999, 19(3): 105―113.

13. Patterson, E.M., Gillette, D.A., Commonalities in measured size

23. Sun Youbin, Gao Shu, LI Jun, Preliminary analysis of grain-size

distributions for aerosols having a soil-derived component, Jour-

populations with environmentally, Chinese Science Bulletin, 2003,

nal of Geophysical Research, 1977, 82(15): 2074―2082. 14. Wake, C.P., Mayewski, P. A., Li Z. et al., Modern eolian dust

48(2): 184―187. 24. Yang, S.L., Ding, Z.L., Comparison of particle size characteristics

deposition in central Asia, Tellus, 1994, 46(B): 220―233.

of the Tertiary ‘red clay’ and Pleistocene loess in the Chinese

15. Ruth, U., Wagenbach, D., Steffensen, J.P. et al., Continuous re-

Loess Plateau: implications for origin and sources of the ‘red

cord of microparticle concentration and size distribution in the

clay’, Sedimentary, 2004, 51: 77―93.