The atmospheric turbulence in the East Asia monsoon area since 60kaBP: A preliminary. study of the multimodal grain size distribution of Chinese loess

Transcription

1 The atmospheric turbulence in the East Asia monsoon area since 60kaBP: A preliminary study of the multimodal grain size distribution of Chinese loess Xiaoguang Qin, Binggui Cai, Tungsheng Liu Abstract: The multimodal phenomenon in loess and dust-storm deposits has been widely reported in recent years. Our analysis demonstrated that along the dust transportation path it is the interactions among wind, the atmospheric turbulence and the mass of dust grains that resulted in a multimodal grain-size distribution for the suspended dust. The median sizes of the coarse and the medium modes are related to the atmospheric turbulence intensity during the courses of both dust entrainment in the source area and deposition in the depositional area. Based on the numeric characteristics of the three modes of suspended particles, a tentative model has been established and is used to study the fluctuation of the paleo-atmospheric turbulence intensity. From time series of the turbulence intensity and the paleo-atmospheric environmental changes of the Loess Plateau for the past 60ka, we found that there are three different patterns of the atmospheric turbulence in the dust source area and 13 strong turbulence events since 60kaBP. The climatic variability in the dust depositional area is stronger than in the source area. The spatial pattern of turbulence structure in the Holocene, i.e., the turbulence intensity is stronger in the dust source area and weak in the dust depositional area, is different from that in the last glacial period. The strong turbulence periods corresponding to the cooling events such as the Heinrich and the Younger Dryas cold events might not be related to alternation from the glacial period to the inter-glacial

2 period. The turbulence intensity may provide some important evidence in understanding of the dynamic processes of the past global change. Keyword: multimode, turbulence intensity, loess and paleosol 1 Introduction Chinese loess has been one of the most important geological records of paleoenvironmental change. Various proxy indexes derived from grain size distribution of the loess are used to describe the paleo-climate change (Liu, et al., 1966, 1985; An, et al., 1991; Ding, et al., 1994; Xiao, et al., 1995; Rutter, et al., 1996, Kukla, et al., 1989). Grain size variation of the loess was usually explained as the change of the wind strength and thus as a proxy of the Asian winter monsoon (An, et al., 1991; An, Z.S., Kukla,1991; Ding et al., 1992). In recent years, the distance from dust source and chemical weathering were believed to have affected the grain size distribution of loess (Ding, et al., 1999) that both the strength of the winter monsoon and the advance-retreat of the deserts are major factors for the grain size variations in Chinese loess. In addition, the multimodal features in grain size distribution of suspended particles was widely found in Asia loess (Sun, 2000; Zhang et al., 1994; Yang, 2001), modern dust storms of Asia, Africa, and Pacific Ocean (Jan-Berend, 2001; Institute for Hydrospheric-Atmospheric Sciences of Naguya University, 1995;Sun et al., 2000) and aerosol (Patterson et al., 1977; Slinn et al., 1976; Twomey, 1984; Wang et al., 1984) in the worldwide different regions. In particular, all samples of loess-paleosol and dust storms in China reported by Sun (2000), Zhang (1994), and Yang (2001) contain similar three modes in the size range of suspended particles.

3 Various explanations for the multimode of the grain size distribution were made (Jan- Berend, 2001;D.H. Sun, 2000; Zhang, 1994,2001; Yang, 2001;), but no one seems convincing. In the last decade, higher-resolution paleo-climate records were reported worldwide and established the climate instability nature for the last glacial period (Dansgaard W. et al., 1993; Keigwin L D., et al., 1994; Porter et al., 1995; H. Schulz, et al., 1998; Richard B. Alley, 2000). The most important climate fluctuations include Dansgaard-Oeschger cycles (D-O), the Heinrich cold events and the Younger Dryas, found first in Europe and the North Atlantic Ocean are also recorded in the East Asia monsoon area (Lu et al., 1999). An et al. (1997) reported the climate fluctuations of the millennial time scale through grain size series of loess-paleosol sections. Wu et al. (1996) found rapid climate variability by mollusk species on the loess plateau during the last glacial maximum. Ding et al. (1998) noticed that the variation of the grain size of loess is similar to D-O cycles in the Greenland ice core record. Given these granted, this study focuses on numerical characteristics of the grain size of the dust associated with the paleo-atmospheric turbulence intensity. Then, a detailed study is made on the grain size distribution of a loess-paleosol section in Weinan (of the southern Chinese Loess Plateau) and its possible implications for the paleo-climate change since 60ka. 2. A model for estimate of the paleo-atmospheric turbulence intensity 2.1The principle Turbulence is an important property of the atmosphere and directly related to the characteristics of climate change. It has been used to study the soil particle entrainment

4 (Bagnold,1941; Slinn, 1976), the dust transportation (Pye, 1987), and the end-member types of pyroclastic density current (Burgisser and Bergantz,2002). According to the convective diffusion theory, the dust dry deposition flux can be described by the following equation (Slinn, 1977; S. Twomey, 1977; Wang, 1991): N D = K z + ( V f + Vy ) N (1) z z Where, D is the dust deposition flux, kg/m 2 s. z is the height in meter, K z is the coefficient of turbulent exchange at the height z and indicates the degree of vertical air mixing, N z is the dust concentration at the height z, V f is the dust settling velocity by the grain weight, V y is the mean vertical wind velocity. In this formula, the first part describes the influence of the atmospheric turbulence and the second part represents the influence of the particle weight. Because the coefficients of turbulent exchange K z vary with height and the effect of heat in the past cannot be easily quantified, it s assumed that the atmosphere is stable, the vertical wind velocity V y = 0 and the friction velocity near the surface is a relative fixed constant in order to simplify the analysis. The gravitational settling velocity of a suspended particle V f can be calculated by the Stokes equation (Green and Lane, 1964; Pye, 1987) 2 gd ρd V = 1 (2) f 18µ ρ It has been revealed that the three modes in grain size distribution of suspended particles widely exist in the loess-paleosol sediment, dust storms, and aerosol. According to the definitions of the coefficient of turbulence exchange (m 2 /s) and the friction velocity u = u w * (after Xian, 2000) K z u w = z u

5 K z 2 u w u* = z = z (3) u u Where u is the mean horizontal wind velocity, u is the horizontal fluctuating velocity. w is the vertical fluctuating velocity. z is the height. If we neglect the impact of the roughness of Earth s surface and the boundary layer, we could assume that particles that fall on ground do not rise up again, i.e., the dust concentration in the air on the surface N 0 =0 and the mean wind velocity u 0 =0. Then, a new equation (4) can be obtained from equations (1) and (3) when z is small enough, u* N u* ( Nz N0) u* D = z + Vf Nz = + Vf Nz = Nz( + Vf ) (4) u z u u u z 0 z Let V k 2 u* = and the dust settling velocity V d = V k.+ V f. The parameter V k is called the u z turbulent settling velocity and represents the variation in the coefficient of turbulence exchange K z at the height z. The wind velocity in the equation is a random value with the mean u. Therefore, V k is a random value with a mean that obeys a certain distribution function. The dust concentration N z is affected by the grain size distribution of the dust source materials, the wind velocity and the turbulence structure, also obeys a certain distribution function. There are three possible situations in the equation (4), (1) If the first part (the turbulent settling flux of dust) is much larger than the second part (the gravitational settling flux of dust), i.e., D=N z V k, the dust settling flux is only influenced by the turbulence near the ground. This part constitutes the fine particles. Because a turbulence flow consists of eddies of different sizes and velocities, the friction

6 velocity u * is a variable with a mean and a probability density function. The dust settling flux D will in turn obey the distribution of the friction velocity. Slinn (1976) found that for particles of diameter d<100µm the relation between the minimum u * required to move particles along a flat surface and particle s diameter is 1/ 4 10 u * = 28 (5) d It means there is a threshold of particles diameter for particle entrainment. Particles coarser(?) than the threshold will more easily stay in the air. Because the fine particles are less in loess and dust storms (fig 1b,c), a small mode completely dominated by turbulence will occur. (2) If the first part (the turbulent settling flux of dust) is close to the second part (the gravitational settling flux of dust), i.e., D=N z (V k + V f ), the dust settling flux is mainly affected by both the turbulence and the dust concentration near the surface. Let V f V k, then 2 u* D 2 N zvk = 2N. (6) z u z Therefore, the size distribution of dust deposited will be affected by both the distribution of the turbulence and the dust concentration. The particles of this part finally result in a medium-size mode. Slinn (1976) demonstrated that an independent mode occur at the 1-10µm interval due to the influence of turbulence. (3) If the first part (the turbulent settling flux of dust) is much smaller than the second part (the gravitational settling flux of dust), i.e., D=N z V f, the dust settling flux is mainly dominated by dust concentration and the particle weight. This part mainly consists of the coarse particles. Because the concentration of coarse dust is relatively

7 abundant in loess and dust storms and V f is a monotonous function, D represents mainly the size distribution of the dust concentration. The above analysis is also suitable to the dust settling flux at any height. However, the contribution of the first part in equation (1) and (4) might change if the gradient of the dust concentration changes. In the dust source area, the gradient of the dust concentration is a negative value, i.e., the dust concentration at lower height is larger than that at greater height, meaning that dust disperses upward by turbulence and the turbulence will reduce the settling velocity of particles. In the dust depositional area, the gradient of the dust concentration is positive and the dust concentration at lower height is less than that at higher height. The dust dispersion direction of turbulence is downward and the turbulence will accelerate particle settling. Dust will be held up at any altitude by the turbulence if the gradient of dust concentration is negative. Hoven (1957) reported the spectrum of horizontal wind speed and revealed that the spectrum has a maximum at high frequencies, which corresponds to micro turbulent flow of length scales of 1 to 100 m. It may be the reason why the size ranges of the multimode of aerosol and dust storms in different regions are similar. During dust transport process, dust particles are sorted and deposited along the transport path to form three modes. The coarse particles travel in relatively short distance due to their larger settling velocity. The medium particles might stay in air during a relatively long term and are transported to far areas because their mass is close to the turbulence intensity. The fine particles will stay in air for a very long period. It s obvious that a larger transportation distance is important for dust sorting to form three modes. Therefore, the gravitational settling of dust, the turbulence, and the horizontal wind speed

8 are the dynamic factors that result in the three modes of the grain size distribution of suspended particles. The above discussion shows that the coefficient K z of turbulence exchange is an important parameter to record the change of turbulence. The change in the coefficient K z of turbulence exchange can be simplified seen as the oscillation of the turbulence intensity. 2.2 influence factors In addition to turbulence, vegetation, soil moisture, scavenging and landform are possible factors to influence dust deposition process. Vegetation: vegetation both in the source area and the depositional area will influence the dust deposition. In the dust depositional area, the growth status of vegetation might affect the dry deposition by filtrating dust, changing the collection efficiency of surface and altering the surface roughness (Slinn, 1976). Because the dust particles collected by vegetation will finally fall down into soil with litter, the filtration efficiency of the canopy can be ignored. The surface roughness is related to the height z 0 on which the wind velocity u 0 =0. It may be unimportant to the dust deposition if neglecting re-suspension processes. The collection efficiency of surface will increase with the vegetation growth (density, height, etc) and the increase in ground surface s moisture. If assuming the collection efficiency of surface to be a coefficient in the right part of equation (1) and (4), it will not change the relative proportions of the three modes of suspended particles. In the dust source area, the vegetation growth status affects dust entrainment and controls the advance and retreat (or open and close) of the dust source thus changing the

9 source distance. As the dust source distance increases, suspended dust will transport along a longer path and the proportion of the coarse mode will be reduced to result in the median size of the whole dust sample decreasing, such as the distribution of the median size of Malan loess (L 1 ) reported by Liu et al (1985). In this scenario, it is possible to estimate the dust source distance using the median size and the proportions of the three modes. The estimation model of the dust source distance will be discussed elsewhere. Wet deposition: There are two mechanisms of washout: removal by cloud and by rain (Pasquill et al., 1983; Twomey,1977). The collision and aggregation of fine particles to increase particle size will occur if the air moisture is higher. However, the Loess Plateau is a semi-dry area where the air moisture is lower. Therefore, this mechanism could not be important. Rain-wash can efficiently remove almost all particles except the unsolvable particles close to 0.1µm (Wang, 1991). Therefore, all particles in loess that can be efficiently measured will be removed by rain, meaning that the multimode pattern of grain size distribution in the air dust remained in the deposits even after a long-distance transportation. This was seen from Beijing dust storm s wet depositional samples. The stable status of atmosphere: The turbulence is affected by both temperature gradient and wind shear. When the atmosphere is unstable, the air temperature decreases rapidly with height and the turbulence will obtain energy from upward floating air and wind shear. In this way, the turbulence conveys the information about the wind and heat conditions of the atmosphere. When the atmosphere is neutral, the temperature gradient is very close to zero, the turbulent energy completely come from the wind shear, suggesting that the turbulence represents wind shear. If the atmosphere is stable, the temperature

10 gradient is larger than zero and the turbulence is suppressed. Therefore, the actual wind shear is larger than that recorded by turbulence (Pye, 1987; Peng et al., 1994). In fact, equation (4) based on the convection theory is suitable to a neural condition rather than not a stable or instable atmosphere. Studies of modern dust storms in China revealed that the dust storms usually occur when the atmosphere is unstable and the related upward vertical wind velocity in the source area is larger (Ying et al., 1996), thus the turbulence intensity recorded by the grain size distribution of dust might contain the information of both heat and wind. Because it s difficult to estimate the heat and wind as well as the vertical wind velocity at present, however, we assume the vertical wind velocity is zero and the atmosphere is neural for the following discussion. It s clear that various factors, such as wind velocity, atmospheric status (i.e. temperature gradient), landcover types (i.e. vegetation, ice and snow), soil moisture, and landform, may influence the atmospheric turbulence. Only the variation of turbulence is discussed in this paper because more details are unknown until present. 2.3 Turbulence intensity near surface during dust entrainment in the source area: In the dust source area, all suspended particles are carried to high altitudes by turbulence and upward wind when a dust storm begins. Therefore, the weight of coarse particles should be in balance with the turbulence intensity if the vertical wind velocity is assumed to be zero. The median size of the coarse mode represents an average status of turbulence of the dust source area during dust entrainment and its variation is related to the fluctuation of the average turbulence intensity. The proportion of the coarse mode

11 decreases with the increase of the dust source distance, resulting in a decrease of median size for a bulk sample in total. The friction velocity u * is an important parameter of turbulence. S. Twomey (1977) established the relation between the settling velocity and the coefficient of the turbulence exchange at a given height with an assumption that the diffusion time z 2 /K z equals to the dust settling time z/v f : K z =V f z (7) Another estimation method reported by Pye (1987) shows that the gravitational settling velocity of dust has a positive correlation to the coefficient of the turbulence exchange. In the 50~100m sub-layer close to the ground which is the bottom part of the atmospheric boundary layer, the relation between K and z can be described by the mixing length theory (Prandtl, 1935) l=akz (where l is the mixing length. k is the von Karman constant=0.4. a is a coefficient, a=1 for neutral atmosphere (Temperature gradient T / z = 0 ), a>1 for unstable atmosphere ( T / z < 0 ), a<1 for stable atmosphere ( T / z > 0)), K= lw = akzu * (8) The friction velocity in the dust source area can be estimated from the equation, 2 V f 1 gd ρ d u * = = 1 (9) ak ak 18µ ρ 2.4 Turbulence intensity during dust deposition in the depositional area: In the dust depositional area, the weight of the coarse mode is much larger than the turbulence intensity and the weight of the medium mode is close to the turbulence intensity. It means that the median size of the medium mode can be used to analyze the atmospheric environment of the dust depositional area during the dust deposition period

12 by eqs. (7) (8). However, carefulness must be addressed because eqs. (1) and (4) show that the grain size distribution of dust also influence the grain size of the medium mode. Therefore, the median size of the medium mode is an approximate index to represent the turbulence intensity in the dust depositional area during deposition. Compared with the medium mode, the turbulence settling velocity V k directly records the variation of turbulence intensity in the dust depositional area during dust deposition and is related to the coefficient of turbulence exchange K z. Let the proportions of the three modes are c 1, c 2 and c 3, their gravitational settling velocities are V 1f, V 2f and V 3f that can be calculated by equation (2) (V 1f is neglected due to smaller value), their deposition mass in unit time are D 1,D 2, D 3, the whole dust deposition mass is D, their air dust concentrations near the surface are N 1, N 2, N 3, and the total concentration is N. Equations can be established as: D 1 =N 1 V k D 2 =N 2 (V k + V 2f ) D 3 =N 3 V 3f N=N 1 +N 2 +N 3 D=D 1 +D 2 +D 3 (10) c 1 =D 1 /D c 2 =D 2 /D c 3 =D 3 /D To solve the equations, the following equation is derived: N 2 N ( V f c3) Vk + ( V2 fv3 f c1v 3 f c3v2 f c2v3 f ) Vk c1v 3 fv2 D D 3 f = 0 (11)

13 N/D is an unknown parameter, which is influenced by the wind velocity and the settling time. It s impossible to get its exact value, but it can be simply assumed, V k V 2f, because the fine mode can be neglected due to its lower proportion. The equation (12) is therefore derived. N/D = (N 1 +N 2 +N 3 )/D (D 2 /(2V 2f )+ D 3 /V 3f )/D = c 2 /(2V 2f )+ c 3 /V 3f (12) According to equation (11) and (12), the turbulent settling velocity V k of dust can be calculated. 2.5 The air dust concentration excess: Following the equation (10), the following equation can be derived, N i /D = c i /V i (13) where, i is ith mode (i=1,2,3), V i is the total settling velocity of ith mode, ie. V 1 = V k for the fine mode, V 2 = V k + V 2f for the medium mode, V 3 = V 3f for the coarse mode. N i /D is the air dust concentration of ith mode normalized by whole the dust deposition flux. For a single sample, the ratio N i /D directly represents the relative concentration of the ith mode in air. In different samples, the ratio N i /D of the ith mode is related to the contribution of its concentration to whole dust deposition flux. The larger the ratio, the less the contribution of the mode to whole deposition fluxes. Therefore, we call the ratio N i /D the air dust concentration excess (ADCE). Equation (13) also shows that a larger ratio means a larger proportion, or a smaller settling velocity. 3. Samples and Pretreatment

14 Sample site and Sampling method The study section (34 34 N, ) is located at Yangge town of city Weinan, Shanxi province, the southern part of the loess plateau and about 400km south to the Mu Us desert (figure 1a). The loess-paleosol section is exposed on the roadside cliff cut through a flat loess terrace where topographic effect on dust deposition can be neglected. The present study is focused on the upper part of the section that consists of Holocene paleosol S 0 and loess L 1 of the last glacial. The loess layer L 1 in this region is divided into five sub-layers (Liu et al., 1994), including typical 3 loess layers L 1-1, L 1-3 and L 1-5, and 2 paleosol-like layers L 1-2 and L 1-4. However, the paleosol L 1-2 in our section is not well developed. The section was sampled at intervals of 5cm for paleosols (S 0 ) and 20cm for loess (L 1 )(some 10cm) on the upper 7.5m of the section. Time scale Magnetic susceptibility of the samples was measured on an English Bartington MS2 instrument. By comparing the time series of magnetic susceptibility with that of the Weinan section (1994), some important age-controlling points have been obtained. The age between two adjacent control points is linearly interposed according to their depths. Grain size analysis The pretreatment of samples followed a standard method introduced by Lu (1998). First, the sample is putted into 10% H 2 O 2 liquid and boiled for removing organic matter. After cooled to room temperature, 10% HCl is added and then heated to 200 o C. Then distilled water is filled into the sample beaker and placed overnight. After to swill out the distilled water, 10ml of 0.05 mol/l dispersant (NaPO 3 ) 6 is filled into the sample liquid. The sample is stirred for 10 minutes using an ultrasonic machine. Size analysis was

15 performed with an English Malvern Instruments Mastersizer 2000 with measured size range from 0.02 to 2000µm. The repeatability is <1%. For simplification, the data obtained from volume percentage are treated as equal values of weight percentage by assuming a same density for various grain sizes. The grain size distributions of all samples are shown in figures 1b and c. The multimode of all samples is similar but proportions of each mode are different between loess and paleosols. The coarse mode is a dominant component and the proportion of the medium and fine modes are relative smaller, especially in loess samples. In paleosol samples, the medium and fine modes are more prominent. Three independent modes can be separated for all samples. The grain-size range of the modes may be overlaid each other but the range of the median sizes of each mode is relatively fixed. The ranges of >10µm, 2~10 µm, and <1~2µm are named the coarse, the medium, and the fine mode (or end member) respectively. Pye (1987) and Gillette et al. (1974) pointed out that the upper limit of pure suspension is V f /u * = 0.7 (where V f is the settling velocity of particles and u * is the friction velocity) and particles smaller than 50µm will completely be suspensive. Therefore, three modes in the grain size distribution of loess-paleosol samples are suspensive materials and derive from the sorting process of suspended windblown dusts. In two samples, there is another mode of 270~600µm, which size range doesn t overlaid with these three modes and is a saltative particle mode, representing local dust material near sources. This indicates a different dynamic mechanism from the other three modes. For these samples, therefore, the proportions of the three suspensive modes were re-estimated by subtracting the saltative mode s proportion. Method to simulate the multimode of the grain size distribution

16 The lognormal distribution on the cumulative probability coordinate is used to separate the multimode of the grain size distribution (Appendix A). Three parameters of each mode, medium size, proportion, and variance, are obtained for all the samples. 4. Discussion In East Asia, dust storms can result either from the winter monsoon, or the sinking westerly jet (Liu, et al, 1985; Zhang, et al, 1991, 1999; Ying, et al., 1996). In this paper we only focus on the paleo-atmospheric turbulence, not Asia monsoon. Figure 2 shows the variations of the median size of whole sample, the median size of each mode, and the magnetic susceptibility of Weinan L 1 ~S 0 loess section. Figure 2c and 2d represent the fluctuations of the atmospheric turbulence in the dust source area and the dust depositional area since 60 kabp. Figure 2f is the curve of the turbulent settling velocity V k that contains the common features of the medium mode and the fine mode. The turbulent settling velocity V k is positively correlated to the gravitational settling velocity of the medium mode: V k = V 2f (R 2 = ) (14) From this equation, it could be reasonable for an approximation of V k V 2f. 4.1 Strong turbulence periods and cold events In figure 2, the time series of the median size of the three modes in the Weinan loess-paleosol section show more details of climate fluctuations than those of the magnetic susceptibility and the median size of bulk sample. The millennial fluctuations of the turbulence are obvious in the dust source area and the dust depositional area since 60kaBP. The whole trends of the median size of the coarse and medium modes

17 synchronously change, meaning the fluctuation of the atmospheric turbulence in the dust source area is consistent with that in the dust depositional area on a longer time scale. However, the median size of the medium mode (fig 2c) and the turbulent settling velocity (fig 2f) show that the strength and the periodicity of the millennial fluctuations in the dust depositional area are much more obvious than in the dust source area. At least 13 strong turbulence periods (STP) (T1~T13) that can be distinguished. Each cycle is about 4-5ka and each strong turbulence period lasts about 2ka. In the Holocene, two strong turbulence periods are consistent with the cooling events of 4kaBP and 8kaBP that were widely found in other records (Shi, et al., 1992). After the cold event of 8kaBP ended, the turbulence intensity rapidly decreased and the temperature rapidly increased. Before the Holocene, the strong turbulence period T3 is synchronous with the Younger Dryas cold event and the strong turbulence periods of T4, T6, T7, T9 and T11 are compatible with the Heinrich cold events found in North Atlantic Ocean (Heinrich, 1988; Alley, R. B.et al., 1999;H. Schulz, et al., 1998; P. M. Grootes, et al., 1993). The T2, T3, and T4 events of the dust depositional area are stronger than these in the dust source area, suggesting that the dust depositional area is more sensitive to global cooling events than the dust source area. This demonstrates that the variation of the atmospheric turbulence of the Eastern Asian monsoon area is consistence with global change. When the global began to cool, the atmospheric turbulence intensity of Eastern Asian gradually increased and the turbulence intensity in the dust depositional area responded more sensitively to the global climate change than the dust source area.

18 Cold strong turbulence periods occurred both in the last glacial and the Holocene, suggesting that the cold strong turbulence events might not be related to the glacialinterglacial transition. 4.2 The combined patterns of the atmospheric turbulence in the dust source and deposition areas The relation between the gravitational settling velocity of the coarse mode and the turbulent settling velocity of the dust depositional area represents the combination patterns of the turbulence intensity for the two areas (Fig 3a). The data points in the figure can be divided into four classes. The data of the last glacial period are grouped to the three patterns: A, B, and C, denoting respectively stronger, strong, and weak turbulence pattern. Pattern D with strong turbulence in source areas and weak turbulence in the depositional area consists of the data after 8kaBP. 1) In the last glacial period, the turbulence intensity of the dust source area is correlated positively to that of the dust depositional area on a larger spatial and longer time scale. The turbulence strength of the three patterns gradually decreases from A to C. The synchronous variation of the turbulence in the dust source area and the dust depositional area represents a larger scaled climatic fluctuation. 2) In the Holocene, the turbulence intensity in the dust depositional area usually changed early than that in the dust source area. The stronger turbulence pattern before 8kaBP is a continuation of the turbulence environment from the LGM to the deglaciation period. The turbulence structure after 8kaBP formed pattern D with the stronger turbulence intensity in the dust source area (like pattern B) and the weak turbulence

19 intensity in the dust depositional area (like pattern C). The turbulence variation of the dust source area and the depositional area is not synchronous. This may be because the better vegetation cover in the dust depositional area in the Holocene reduced the turbulence intensity near ground. During the Holocene, the turbulence in the dust depositional area first decreased rapidly in the early period, then the turbulence of the dust source area weakened to pattern C. After 8kaBP, the turbulence in the dust-source and the deposition areas gradually increased and changed to pattern B. In late Holocene especially the turbulence pattern of the dust source area fluctuated into the stronger turbulence pattern A, representing an evolutional trend that a new cooling event would occur. 3) In the last glacial period, the turbulence intensity in the dust source area dominate the climatic environment in the three patterns, A, B, and C, because the range of the median size of the coarse mode of these three patterns doesn t overlap each other but the range of the turbulent settling velocity of three patterns overlaps. Pattern A contains the data from the last glacial maximum to the last deglaciation period. In this pattern, both the turbulence intensity of the dust source area and the dust depositional area are much stronger. However, the turbulence of the dust source area doesn t fluctuate much and the turbulence of the dust depositional area gradually decreased from the LGM to the early Holocene, representing the climate maybe change from the south to north. Pattern B contains the data of the strong turbulence periods in the last glacial period. The data usually corresponds to some cooling event, such as the Heinrich events. Pattern C consists of the data of the weak turbulence periods of the last glacial period and is the

20 relative warm periods between cold events. The characteristic of pattern C is that the turbulence of the dust source area is negatively correlated to that of the dust depositional area, representing a local and small-scale change of the atmospheric environment. Pattern B and C frequently and alternately occurred, suggesting stronger instability and periodicity of climate in the last glacial period. The paleosol layer L 1-4 belongs to the weak turbulence pattern C. The sample No.62 in the late L 1-4 occurs at the left bottom of Pattern C, reflecting a special combination of turbulence in which turbulence intensity is weakest. 4.3 The characteristics of the atmospheric turbulence of the dust source area The characteristics of the atmospheric turbulence of the dust source area can be revealed by the relation between the median size and the proportion of the coarse mode in the figure 3b. 1) The data of the dust source area can be classified into four linear groups labeled as II, III, IV, and I. Group II is close to the III and they can be seen as one group. The four groups are almost the same as the four patterns in the figure 3a. 2) Group I is an environment with the strongest turbulence intensity in the dust source area and a lower proportion of the coarse mode, much the same as the stronger turbulence pattern A in figure 3c. The data from the LGM to 8kaBP are contained in this group. Considering the evolutional trend, the turbulence intensity decreases gradually from the LGM to the Holocene. The linear relation is described in the following equation: Y = * X (R 2 = ) (15) Where Y is the median size (µm) of the coarse mode, X is the proportion (%) of the coarse mode, R is the correlative coefficient.

21 3) In group II that consists of all data after 8kaBP, an increase in the median size and proportion shows that the turbulence intensity of the dust source area strengthens gradually from 8kaBP to present, inverse to group I. In this group, the atmospheric turbulence of the dust source area is stronger (like pattern D). The fluctuation between the group I and II in the early and the late periods reflects the instability of the atmospheric environment during the transition periods between different environmental conditions in the dust source area. The fluctuation of the late period represents the trend of climate deterioration after the Holocene optimum. The linear relation of the group is the following: Y = * X (R 2 = ) (16) 4) Group III contains the data in the strong turbulence pattern B of the last glacial period. The proportion of the coarse mode in samples is relatively larger, and the median size is the same as that of pattern B. This group is very close to the group II. In fact the two groups can be seen as one group, meaning that the environment of the dust source area of the period since 8kaBP is similar to the cold events (strong turbulence periods) of the last glacial period. The linear relation of this group can be presented as: Y = * X (R 2 = ) (17) 5) Group IV contains the data of the weak turbulence pattern C of the last glacial period. The data comes from relative warm periods of the last glacial period. The median size of the group is smaller, and the proportion is larger. The linear relation of the group is the following: Y = * X (R 2 = ) (18)

22 In comparison of fig 3a with fig 3b, some data of pattern C (such as No.63 and No.64 in the paleosol layer L 1-4 ) all belong to group III, meaning that there might be other factors affecting the atmospheric environment rather than the turbulence intensity. A special situation is the position of the data No.62. Because of the regular relation among No.62 and pattern A, B, C, it may represent a special turbulence pattern with the minimum median size and the largest proportion 4.4 The characteristics of the atmospheric turbulence in the deposition area Figure 3c shows the relation between the turbulent settling velocity and the proportion of the medium mode, which represents the atmospheric environment change of the dust depositional area. The major characteristics are: 1) The data points can be divided into 4 classes similar to the groups of the dust source area. The four classes are named a, b, c, and d. 2) Class a contains data from the LGM to the early Holocene and is equivalent to group I. The median size decreases and the proportion increases from the LGM to the early Holocene, representing that the turbulence intensity is reduced in the dust depositional area when the turbulence decreases in the dust source area. 3) Class b contains the data of group III and is the strong turbulence pattern of the dust depositional area (Fig 2). Class c consists of the data points of group IV and shows the characteristics of weaker turbulence intensity in the dust depositional area. Frequent fluctuations between class b and class c demonstrate that the rapid fluctuation of the turbulence intensity also exists in the dust depositional area during the last glacial period before the LGM.

23 4) Class d contains the data of the Holocene after 8kaBP. In this class, the turbulence of the dust depositional area is as weak as class c but the proportion of the medium mode is relatively larger. Similar to the dust source area, the turbulence intensity of the dust depositional area oscillates rapidly between class a and d during the early Holocene, representing the climatic instability of the dust depositional area during the climatic transition period. From figs 3b with 3c, both the turbulence intensity of the dust source area and the dust depositional area in the paleosol layer L 1-4 (data No.63 and 64) are stronger. This pattern differs from that of the Holocene paleosol, meaning that the development of the paleosol may not be related to the change in the turbulence intensity. 4.5 Variation of the air dust concentration excess The characteristics of the air dust concentration excess (ADCE) in figure 4 are analyzed: 1) From figure 3a, the ADCE is the largest for the medium mode and the smallest for the coarse mode, reflecting the impact of the gravitational settling velocity, because the contribution of the coarse mode to the dust deposition mass is larger in the Weinan area. 2) The contribution of the coarse mode to the dust deposition mass is the largest in the period from the LGM to the last interglacial (the ADCE is lowest), the smallest in the Holocene (the ADCE is largest), and obviously fluctuates in the last glacial period before the LGM. Variations of the medium mode are same as the fine mode and their ADCE are larger in the weak turbulence periods and smaller in the strong turbulence periods.

24 3) All data points can be classified into three classes with different ADCE of the coarse mode, which correspond to the groups in figure 3b. Groups II and III in figure 3b merged into one class in figure 4b and the data point No.62 seems to be an independent class. In each class, the ADCE of the coarse mode is negatively correlative to the ADCE of the medium mode. In each class, when the median size of the coarse mode is larger (it means a larger turbulence intensity of the dust source area; fig 4c), the median size of the medium mode is also larger (fig 4d), the ADCE of the medium mode is lower (fig 4e), and the ADCE of the coarse mode is larger (fig 4d), meaning that the contribution of the medium mode to the dust deposition mass is larger and that of the coarse mode is lower. The ADCE of the coarse mode is negatively correlative to the ADCE of the medium mode in each class (fig 4b). From class 1, it s noted that the median sizes of the coarse mode and the medium mode decrease gradually from the LGM to Holocene. Therefore, the positive relation between the ADCE of the coarse mode and the median size of the medium mode in each class (fig 4d) represents the systematic change of the wind field (strength or boundary between desert and loess area). 4) The ADCE of the coarse mode is negatively correlated to the median size of the coarse mode (R 2 = ) (fig 4c), that is also shown by the equation (13). The data clusters into three classes (the point No.62 seems to be a single class) similar to three groups in Fig 4b. For each class, the contribution of the coarse mode to the dust deposition mass is relatively fixed, meaning that there are three different atmospheric environments in the dust source area (or four types if data No.62 can be seen

25 as a class). Contributions of the three groups to the dust deposition mass gradually decrease, suggesting the difference among these three environments is remarkable. 5) The whole trend of the ADCE of the coarse mode (N 3 /D) is negatively correlated to the median size of the medium mode (fig 4d), consistent with the positive correlation of the median sizes of the coarse and the medium modes in fig3a. However, the ADCE of each group is positively correlated to the median size of the medium mode, meaning that the smaller the turbulence intensity of the dust depositional area is, the larger the contribution of the coarse mode to the dust deposition mass. It represents the impact of the turbulence of the dust depositional area to the deposition mass of the coarse mode in the relatively stable environment of the dust source area. 6) The ADCE of the medium mode is negatively correlative to its median size on the total trend (fig 4e). There are three parallel straight lines that correspond to the three groups in figure 4b,c, and d. The sample No.62 is in the group No.3, meaning that the environment of its depositional area is similar to that of the weak turbulence period although the environment of its source areas is a special class. The data of the last glacial period is located on the left of figure 4e. The fluctuation direction with a larger slope demonstrates that climate change is strong and there are three environment patterns during the last glacial period. The data of the Holocene are on the right part of figure 4e, representing that the climate of the dust depositional area slightly fluctuates in the Holocene and the contribution of the medium mode to the dust deposition mass in the Holocene is smaller than in the last glacial period due to its weaker turbulence intensity (fig 4e).

26 Comparing figure 4e with figure 4f, the difference of the median size between three classes in fig 4f is more obvious than that in fig 4e, suggesting the dust source area may be a more important factor affecting the ADCE of the medium mode.. In the figure 4g, the ADCE of the medium mode is negatively correlated to the turbulence deposition velocity. The data of the Holocene distributed in a larger range without obvious changes of the turbulence deposition velocity suggest that the classification in figure 4e mainly results from the atmospheric environment of the dust source area rather than the dust depositional area. 7) The atmospheric environment during the Holocene is different from that during the last glacial period. The environment of the dust source area during early Holocene is similar to the LGM and the late Holocene similar to the strong turbulence periods of the last glacial period (fig 4c,d). The atmospheric environment of the dust depositional area in the Holocene is close to the weak turbulence period during the last glacial period (fig 3a and fig 4g). It s believed that the dust depositional patterns are mainly controlled by the environment of the dust source area because the dust depositional area is affected by the summer monsoon and the winter monsoon together. 5. Conclusion (1) The variation of the atmospheric turbulence intensity of the dust source area and the dust depositional area can be estimated from the median size of the coarse mode and the turbulence settling velocity. (2) The climate fluctuations on millennial time scale since 60kaBP are obvious in the Weinan loess area. The periods of strong turbulence and weak turbulence occur

27 alternately. In general, the strong turbulence period corresponds to the global cooling event and the weak turbulence period occurs in the warmer period between two cold events. The Heinrich events, the Younger Dryas event, and the cold events of 8kaBP and 4kaBP may be similar climate fluctuations that are unrelated to alternation between glacial and interglacial periods. (3) There are three patterns of atmospheric environments since 60kaBP. Three patterns are mainly affected by the environment of the dust source area. (4) The atmospheric environments of the dust source area and the dust depositional area change synchronously on a longer time scale. However, there are differences during the Holocene period. The strong turbulence event in the dust depositional area is another independent climate process because it occurs in all patterns of the dust source area. It means that the factors controlling the three patterns of the atmospheric turbulence are different from those resulting in the climate fluctuation of the dust depositional area. The atmospheric environment of the dust depositional area changes earlier than that of the dust source area. The dust source area is located in the north of the loess plateau, i.e., the dust depositional area where is more affected by the summer monsoon. Therefore, it might be implied that the summer monsoon changes earlier than the winter monsoon. (5) The atmospheric turbulence after 8kaBP is obviously different from the three patterns during the last glacial period. In the Holocene, the turbulence intensity is stronger in the dust source area, and is weak in the dust depositional area. The turbulence intensity gradually increased after 4kaBP, meaning that a new cooling event may occur. (6) The median size, the proportion and the ADCE of three modes should be studied together in order to reveal the atmospheric turbulence patterns and their evolution.

28 (7) In order to understand the atmospheric turbulence structure in the East Asian monsoon area in a greater detail, more loess-paleosol sections and higher resolution sampling are necessary. Acknowledgement The study is supported by the projects ( , ) of the National Natural Science Foundation of China, the project G of the Ministry of Science and Technology of China. Many of the ideas presented have evolved during discussions with my colleagues, Houyuan Lu, Jintai Han, Zhaoyan Gu, Naiqin Wu, Luo Wang. Han corrects the English grammar. Juluo Xiao provided the laboratory for the grain size analysis. Jimin Sun provided the data of modern dust storms. I thank Dr. Xiupin Liu and Yu Cao participated the fieldwork. Appendix A. The lognormal distribution is used to fit the cumulative probability distribution of the grain size of samples (Sinclair, A.J., 1976; Zhao et al, 1983). The cumulative distribution of a lognormal probability density function is a straight line on the logarithm probability coordinate. The slope of the line represents the standard deviation and the grain size at the cumulative distribution equal to 50% is the mean size of the sample. For the grain size distribution of two modes, the cumulative value at the inflexion of the cumulative distribution curve indicates the proportion of these two modes. The distributions of data points under or above the inflexion are individually re-calculated on a 100% cumulative probability. The new data points are re-mapped on the cumulative probability coordinate as two new grain size distributions. If all new points of each new

29 distribution distribute on a straight line, the two straight lines indicate two independence modes called as end members. Similarly, three or more modals can be separated from the grain size distribution of a sample. To change the position of the inflexions, the proportions of all modes can be calculated for every position combination of inflexions. Then, the cumulative probability value is estimated by the following formula, n ln x ( t ai ) c 2 1 2σ i F(ln x) = e dt (A1) i= 1 σ i 2π Where, x is the grain size; ln x is the logarithm of the grain size, meaning the grain size distribution can be fitted by a lognormal function. i means the ith mode; n is total number of modes, n=3 for most of loess and dust storm samples; c i (i=1,...,n) is the n c i i= 1 proportion of the ith mode, = 1; σ i is the standard deviation of ln x of the ith mode; a i is the average value of ln x of the ith mode. The difference between the estimated cumulative probability and the original cumulative distribution can be calculated. By choosing the inflexion position combination with the minimum difference, the modes can be separated and three important numerical features (the proportion, the median size and the variation) can be estimated. Reference Alley, R. B. & Clark, P. U. 1999, The deglaciation of the Northern Hemisphere: a global perspective. Annu. Rev. Earth Planet. Sci. 27, An, Z., Liu, T., Lu, Y., Porter, S.C., Kukla, G., Wu, X. & Hua, Y., 1991, The longterm paleomonsoon variation recorded by the Loess-Paleosol sequence in central China, Quaternary International, Vol.7 An, Z.S., and Porter, S. C.,1997,Millennial-scale climatic oscillations during the last interglaciation in central China. Geology 25,

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