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Published online 2 June 2005
Published in Soil Sci Soc Am J 69:1094-1101 (2005)
DOI: 10.2136/sssaj2004.0078
© 2005 Soil Science Society of America
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Pedology

Pedostratigraphy and Carbonate Accumulation in the Last Interglacial Pedocomplex of the Chinese Loess Plateau

Z.-D. Fenga,b,* and H. B. Wangb

a National Lab. of Western China's Environmental Systems, MOE, Lanzhou Univ., Lanzhou 730000, China
b Dep. of Earth and Environmental Studies, Montclair State Univ., Upper Montclair, NJ 07043, USA

* Corresponding author (fengz{at}mail.montclair.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This paper examines the geographic variations of carbonate concentrations in last interglacial pedocomplex (S1) profiles along a northwest (NW)–southeast (SE) transect across the Chinese Loess Plateau. The average S1 carbonate concentration is the same as that of overlying loess (L1) and underlying loess (L2) at the northwesternmost site (Lanzhou), but variations in carbonate concentration within S1 pedocomplex are greater than the within-loess (L1 and L2) variations, indicating that the carbonate translocation within the S1 pedocomplex had occurred. Carbonate was leached to greater depths to form more pronounced peaks in the three paleosols (S1S1, S1S2, S1S3) within the S1 pedocomplex at the Dingxi section. Farther SE to the Qinan section, the three paleosols (S1S1, S1S2, S1S3) have more distinguishable carbonate leaching and accumulation zones than at the Dingxi section. The S1 carbonate concentration at the Tianshui section suggests that the three paleosols were partially welded, as evidenced by the S1S3 and S1S2 paleosols sharing a major portion of the paleosol profile. Soil-forming events occurred repeatedly in a single paleosol profile and the Ck horizon might have served as the carbonate accumulation zone for all three paleosols (S1S1, S1S2, S1S3) at the southeasternmost site (Lantian). Land surface stability complicated carbonate concentration processes in loessial paleosols and extra caution should be observed when interpreting climate implication of carbonate concentration data.

Abbreviations: asl, above sea level • NW, northwest • SE, southeast


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL FORMATION is a function of time and environmental factors including climate, vegetation, topography, and parent material (Buol et al., 1989). The time factor has two implications: soil-forming duration and land-surface stability (Foth, 1978). If the duration of soil formation is too short for soil to reach the equilibrium with the climate and its associated vegetation, the soil (e.g., Entisols and Inceptisols) will not be a reliable indicator of the climate. If the duration of soil formation is too long for the soil to preserve the imprints of equilibrium status, the soil (e.g., Ultisols) may not reflect the climate and its associated vegetation (Birkeland, 1999). Second, soil formation requires land-surface stability. A stable land surface regarding soil formation implies the downward soil development, that is, soil develops in an already deposited parent material. If the soil develops when all or part of the parent material is being added (Almond, 1998), it is an accretionary soil that does not reach its equilibrium with the climate and its associated vegetation. For example, if soil-formation rate is approximately equal to the rate of dust influx in a grassland, a mature Mollisol with a well-developed B horizon will not form. Instead, a thick cumulic pedocomplex may be formed consisting of multiple A horizons with little or no B horizon development (Almond, 1998; Feng et al., 1994a, 1994b, 1994c; Feng and Johnson, 1995; Feng, 1996, 1997; Feng and Chen, 1999).

If the time factor is known and the topography and parent material are constants, soil formation is basically a function of climate and the associated vegetation (Johnson and Watson-Stegner, 1987). The effort to quantify the factor relationships has been persistently attempted since the beginning of soil science (Birkeland, 1999; Retallack, 1994). Among other soil characteristics, carbonate enrichment in arid and semiarid soils has been widely investigated (Arkley, 1963; Gile et al., 1965, 1966; Kemp, 1995; Machette, 1985) to quantify its relationships with the climate and the associated vegetation. The formation of pedogenic carbonate is a function of carbonate-bicarbonate equilibria. Under soil-forming conditions, CO2 (either from the atmosphere or released by biological activities) in the soil air reacts with soil water to form HCO–3. The percolating water carries both the HCO–3 and weathering-released or/and dust-deposited Ca2+ to form carbonate-rich horizon at a certain depth where the solution (i.e., percolating water) becomes saturated with respect to carbonate (Machette, 1985; Marion et al., 1985; Schlesinger, 1985; Van der Hoven and Quade, 2002). The depth of carbonate leaching is basically a function of mean annual precipitation (Arkley, 1963; Zhao, 2000).

Many previous studies noted carbonate concentrations in loessial soils (modern and past) of the mid-continental USA (e.g., Aandahl, 1982; Olson and Nettleton, 1998; Ruhe, 1973, 1983, 1984; Ruhe and Olson, 1980; Ruhe et al., 1955; Schaetzl and Sorenson, 1987). Recent studies (Feng et al., 1994a, 1994b, 1994c; Feng and Johnson, 1995; Feng, 1997) closely examined carbonate concentrations and associated properties of paleosols and their relations to the soil-forming environments in the Great Plains of the USA. As for the calcic loessial soils in the Chinese Loess Plateau, Zhao (1991)(1993, 1994, 2000) was the first who systematically studied carbonate-concentrating processes and their relations to the soil-forming environments. Guo and his colleagues (Guo and Liu, 1993; Guo et al., 1996a, 1996b) carefully examined carbonate-related paleosol micromorphology in the Chinese loess sequence and attempted climatic interpretations of the carbonate-related micromorphology. Kemp and his colleagues (Kemp, 1995, 2001; Kemp et al., 1995, 1997) have comprehensively studied the geographic patterns of the carbonate-related micromorphology of the last interglacial pedocomplex S1 in the Chinese Loess Plateau. Chen et al. (1997)( 1999) and Fang et al. (1996)(1999) went even further to reconstruct the abrupt climatic events from the last glacial loess (L1) and also from the last interglacial pedocomplex (S1) using carbonate concentration as a summer monsoon proxy. Yang et al. (2001) used carbonate-influenced light-reflectance spectra as a proxy for reconstructing summer monsoon intensity to support the last interglacial summer monsoon instability proposed by An and Porter (1997).

Because of the interaction between the winter and summer monsoons, there is a modern SE–NW gradient of climate in the Chinese Loess Plateau (Li et al., 1988). The mean annual precipitation decreases gradually from SE to NW with the mean annual temperature increasing southward, whereas aridity (the ratio of evaporation to precipitation) increases gradually toward the NW. The native vegetation closely follows the aridity trend (Fig. 1) . Assuming that the climatic gradients and the associated vegetation existed during the last interglacial as they do today in the Chinese Loess Plateau, the net loess accumulation should have decreased and soil development should have intensified southeastward. Consequently, the last interglacial pedocomplex (S1) should gradually change from NW to SE and the pedocomplex characteristics should reflect the changes as noted by some researchers (Evans and Rokosh, 2000; Kemp et al., 1997; Rokosh et al., 2002; Zhu et al., 2001).



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  		Fig. 1. Aridity (the ratio of evaporation to precipitation) and native vegetation distributions in the Chinese Loess Plateau.

 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We investigated systematic geographic variation of carbonate accumulation in the last interglacial pedocomplex (S1) and its climatic significance along a NW-SE transect across the Chinese Loess Plateau extending from Lanzhou (36°03' N, 103°53' E) to Lantian (34°06' N, 108°39' E) near Xi'an (Fig. 1) in hope that carbonate-related soil-forming processes will be more fully considered when reconstructing high-resolution climatic records from loessial paleosols. The S1 pedocomplex of the five selected sections was described in the field based on characteristics of soil horizon (Birkeland, 1999; Buol et al., 1989; Catt, 1986, 1990; Foth, 1978) and then sampled at 2-cm intervals for laboratory analyses. At all sections investigated, sampling began in the basal L1 loess unit that overlies the S1 pedocomplex. Carbonate content was measured with a modified gas evolution method (Machette, 1986) using a Bascomb Calcimeter. Grain size of bulk samples was measured using a Malvern Co. Ltd. Mastersizer 2000 laser (Worceshire, UK) diffraction particle-size analyzer (Chen et al., 1997). Magnetic susceptibility (SI) was measured by the procedure of Thompson and Oldfield (1986) using a Bartington MS 2B susceptibility meter (Bartington, Oxford, UK).

In the northwestern part of the Loess Plateau (e.g., Lanzhou section at Jiuzhoutai: 36°03' N, 103°53' E, 1518 m above sea level [asl]), the 8-m thick S1 pedocomplex consists of three separate paleosols (Derbyshire et al., 1995, 1997; Kemp et al., 1995, 1997). Based on thermoluminescence (TL) dates and the comparability of S1 magnetic susceptibility signature with last interglacial marine isotopic signature (Chen et al., 1999, 2000; Feng et al., 2004a, 2004b; Kukla and An, 1989; Kukla et al., 1988, 1990), three Entisol-like paleosols with typical A-C profiles are interpreted as marking marine isotope substages 5a (S1S1), 5c (S1S2), and 5e (S1S3) and two interbedded loess units as demarcating substages 5b (S1L1) and 5d (S1L2) (Fig. 2) . Southeastward near Dingxi (35°35' N, 104°37' E, 1898 m asl) the S1 pedocomplex is 5-m thick with two Entisol-like paleosols (A-C profiles) corresponding to the marine isotope substages 5a and 5c, which are better developed than those at the Lanzhou section. Marine isotope substage 5e is marked by a Mollisol-like paleosol with both an A horizon and a Bk horizon. Farther to the SE near Qinan (34°52' N, 105°40' E, 1226 m asl) the S1 pedocomplex is 4-m thick with three Mollisol-like paleosols corresponding to the three odd-numbered marine isotope substages (5a, 5c, and 5e). The S1S1 and S1S2 paleosols are separated by the S1L1 loess unit. The loess unit S1L2 is not present between the S1S2 and S1S3 paleosols (see Fig. 2). The S1 at the Tianshui section (34°35' N, 105°45' E, 1142 m asl), about 50 km south of the Qinan section, is a pedocomplex without the interbedded loess units. That is, both S1L1 and S1L2 are absent. This 4.5-m thick S1 pedocomplex has a 0.8-m thick A horizon at the top, a 2.5-m thick Bt horizon in the middle, and 1.2-m thick Bk horizon at the bottom. The S1 at Lantian section (34°06' N, E108°39', 800 m asl) near Xi'an is a pedocomplex containing a well-developed Bt horizon with a Ck horizon that is separated from the Bt horizon by a 0.8-m thick BC horizon (Feng et al., 2004a).



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  		Fig. 2. The geographic variations of the S1 pedocomplex profiles from northwest to southeast across the Chinese Loess Plateau: all three paleosols (S1S1, S1S2, S1S3) and two interbedding loess units (S1L1 and S1L2) are preserved at the Lanzhou and Dingxi sections; the S1L2 loess unit was annexed by the S1S2 paleosol development at the Qinan section and both the S1L2 and S1L1 loess units were annexed by later paleosol development at the Tianshui sections; the S1 pedocomplex at the Lantian section is a single composite paleosol profile.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Lanzhou Section
Figure 3 shows that carbonate concentration in the last interglacial pedocomplex S1 (15.8%) is not distinctively different from that of overlying L1 (16.0%) and underlying L2 (15.0%). Yet, the within-pedocomplex (S1) variations in carbonate concentration are greater than the within-loess (L1 and L2) variations, indicating that carbonate translocation (leaching and accumulation) within the pedocomlex profile had occurred during the last interglacial even at this northwestern section. Although variations in median grain size (the correlation coefficient between median gain size and magnetic susceptibility r = –0.65, sample number n = 600) and in the <10-µm fraction (the correlation coefficient between percentage of <10-µm fraction and magnetic susceptibility r = 0.74, n = 600) account for the major portion of the variations in magnetic susceptibility (Feng et al., 2004b), carbonate concentration is also a major factor in shaping the magnetic susceptibility curve (the correlation coefficient between carbonate concentration and magnetic susceptibility r = –0.51, n = 600). Two other notable features deserve mentioning. First, carbonate concentration highs always occur in the layers immediately below the magnetic susceptibility highs and <10-µm fraction highs expressing the three paleosols (S1S1, S1S2, S1S3), suggesting that the carbonate was leached downward, although only to a limited depth and degree (marked as I, II, III in Fig. 3). Second, carbonate concentration is sensibly responding to the minor susceptibility highs and lows within the major susceptibility ({chi}) peaks expressing the three paleosols (S1S1, S1S2, S1S3) within the S1 pedocomplex. For example, the susceptibility peak expressing S1S1 corresponds to a carbonate concentration low (marked as 1 in Fig. 3), and the two secondary susceptibility lows that are superimposed on two first-order susceptibility highs (S1S2, S1S3) correspond to two carbonate concentration highs (marked as 2, 3 in Fig. 3). It should also be noted that the clay content highs and carbonate concentration highs appear approximately at the same layers, again implying that carbonate leaching really happened but only to a limited depth.



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  		Fig. 3. Lanzhou Section: field-observed pedostratigraphy and laboratory data. Md (µm): median size; <10 µm (%): percentage of <10-µm fraction; <2 µm (%): percentage of <2-µm fraction; CaCO3 (%): percentage of carbonate; and {chi}: magnetic susceptibility (10–7 m3 kg–1). Three shaded strips (I, II, III) are paleosol zones (S1S1, S1S2, S1S3). Three dashed lines (1, 2, 3) indicate the relationship between carbonate concentration peaks and their lowered magnetic susceptibility within the three major magnetic susceptibility peaks corresponding to the three paleosols (S1S1, S1S2, S1S3).

 
Dingxi Section
The overall correlation coefficient between magnetic susceptibility ({chi}) and carbonate concentration (%) at the Dingxi section is close to zero (r = 0.01) and no magnetic susceptibility ({chi}) peak exists for the S1S1 paleosol although the frequency-dependent magnetic susceptibility ({chi}fd) does express the S1S1 paleosol well (Fig. 4) . To explain the lack of a magnetic susceptibility peak in the S1S1, we made two mathematical predictions of the magnetic susceptibility: one based on the correlation between the {chi} and {chi}fd for the entire measured section (r = 0.89, n = 300), and the second based on the linear relationship of the {chi} with two major modulators, >63-µm fraction (negative) and 2- to 10-µm fraction (positive) for the entire measured section. Both predictions yield a susceptibility peak corresponding to the S1S1 paleosol. We noticed that the correlation coefficient between the carbonate concentration (%) and the frequency-dependent magnetic susceptibility ({chi}fd) is negative in the lower portion (2.5–6 m) (r = –0.59, n = 175) and two major carbonate concentration highs in the lower portion (marked as III and II in Fig. 4) correspond well to two lows in the susceptibility. However, the correlation coefficient between the carbonate concentration (%) and the frequency-dependent susceptibility ({chi}fd) turns into a positive value in the upper portion (0–2.5 m) (r = 0.42, n = 125) and the carbonate high in the upper portion (marked as I in Fig. 4) is responsive to the frequency-dependent susceptibility high expressing the S1S1 paleosol where the susceptibility peak is absent. Our predicted susceptibility peak and calculated contribution (negative) by the carbonate high (marked as I in Fig. 4) indicate that the susceptibility peak expressing the S1S1 was suppressed by the carbonate dilution effect.



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  		Fig. 4. Dingxi Section: field-observed pedostratigraphy and laboratory data. Md (µm): median size; <10 µm (%): percentage of <10-µm fraction; <2 µm (%): percentage of <2-µm fraction; CaCO3 (%): percentage of carbonate; {chi}: magnetic susceptibility (10–7 m3 kg–1); and {chi}fd: frequency-dependent susceptibility (%). Three shaded strips (I, II, III) are paleosol zones (S1S1, S1S2, S1S3). Three arrows within the three shaded strips indicate carbonate leaching and accumulation in the three paleosols.

 
Qinan Section
The clay (<2 µm) content is correlative with the susceptibility (r = 0.49, n = 250) at the Qinan section. Like at the Dingxi section, the carbonate concentration seems not correlative with the first-order variations in the susceptibility ({chi}) that can be accounted for by the >63-µm fraction-dictated median grain size (negative) and 2- to 10-µm fraction-dictated <10-µm fraction (positive), but the carbonate variations correspond negatively with the second-order variations in the magnetic susceptibility (Fig. 5) , especially in the upper portion (0–3.5 m deep). Although the best developed paleosol S1S3 is not proportionally expressed by the magnetic susceptibility mainly due to dilution of the coarse fractions, the carbonate concentration, as well as the clay content and <10-µm fraction to a lesser degree, expresses the S1S3 paleosol reasonably well. Both the S1S3 and S1S2 paleosols have distinguishable carbonate leaching and accumulation zones as indicated by arrows in Fig. 5. The carbonate leaching and accumulation are somewhat complicated in the S1S1 paleosol. That is, a weakly weathered layer at the bottom of the L1 generated one carbonate concentration peak (marked as 1 in Fig. 5), and the S1S1 itself might have generated two peaks, one in the S1S1 Bk horizon (marked as 2 in Fig. 5) and the other below the Bk horizon (marked as 3 in Fig. 5).



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  		Fig. 5. Qinan Section: field-observed pedostratigraphy and laboratory data. Md (µm): median size; <10 µm (%): percentage of <10-µm fraction; <2 µm (%): percentage of <2-µm fraction; CaCO3 (%): percentage of carbonate; and {chi}: magnetic susceptibility (10–7 m3 kg–1). Three shaded strips (I, II, III) are paleosol zones (S1S1, S1S2, S1S3). The arrows indicate carbonate leaching and accumulation.

 
Tianshui Section
Carbonate concentration, as well as soil morphologic features (e.g., clay coatings and granular structures throughout the 3.7 m thick B horizon), suggests that the S1 at Tianshui section is a partially welded pedocomplex with both S1L1 and S1L2 loess units being absent (Feng et al., 2004a). That is, both the S1S3 and S1S2 paleosol development formed the Bk horizon as arrows indicated (associated with 3 and 2) in Fig. 6 and shared a major portion of the paleosol profile. The carbonate-enriched layer at depth of 1.6 to 2.0 m may mark the bottom of the S1S1 paleosol or the leaching depth of later stage of the S1S1 development. The S1S1 paleosol development might have annexed the uppermost portion of the S1S2 paleosol as the arrows indicate (associated with 1) in Fig. 6. Soil annexation and welding not only altered S1L1 and S1L2 loess units into paleosols but also altered the A horizons of the S1S3 and S1S2 into later B horizons, forming thick accretionary B horizons. Carbonate coats the illuvial clay on the burrow walls and ped-faces in the Bt horizon, and engulfs the entire matrix and fills burrows in the Bk horizon that contains carbonate nodules.



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  		Fig. 6. Tianshui Section: field-observed pedostratigraphy and laboratory data. Md (µm): median size; <10 µm (%): percentage of <10-µm fraction; <2 µm (%): percentage of <2-µm fraction; CaCO3 (%): percentage of carbonate; and {chi}: magnetic susceptibility (10–7 m3 kg–1). Three groups of arrows (associated with three dashed lines 1, 2, 3) indicate carbonate leaching and accumulation for all three paleosols (S1S1, S1S2, S1S3).

 
Lantian Section
The three soil-forming events (S1S1, S1S2, S1S3) occurred in a single paleosol profile at the Lantian section (Fig. 7) . A noticeable feature at this section is that the carbonate concentration is low in the Bt and BC horizons (<5%) and high in the Ck horizon (about 20%), implying that the Ck horizon might have served as the carbonate accumulation zone for all three soil-forming events. The clay content is expectedly high in the Bt horizon. The coarsening grain size trend in the lower portion of the S1 pedocomplex profile (vertically shaded area in Fig. 7) implies that the S1 developed into the underlying older loess L2. Two features regarding the carbonate concentration deserve mentioning: (1) the carbonate in the Ck horizon exists primarily as nodules; and (2) ped-faces of the Btk horizon are nearly thoroughly coated with carbonate powder and small fluffy threads although the total carbonate content is rather low in the Bt horizon at this southeasternmost site.



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  		Fig. 7. Lantian Section: field-observed pedostratigraphy and laboratory data. Md (µm): median size; <10 µm (%): percentage of <10-µm fraction; <2 µm (%): percentage of <2-µm fraction; CaCO3 (%): percentage of carbonate; and {chi}: magnetic susceptibility (10–7 m3 kg–1). The vertically shaded area indicates the chronological discord between parent material (older loess L2) and paleosol (later developed S1). The horizontally shaded area is carbonate accumulation zone for all three paleosols (S1S1, S1S2, S1S3).

 
The following observations can be made from the abovementioned carbonate data at the five sections investigated. At the northwestern section (Lanzhou), the average S1 carbonate concentration is nearly the same as that of overlying L1 and underlying L2 loess units. But, the within-S1 variations in carbonate concentration are greater than the within-loess (L1 and L2) variations, indicating that the carbonate translocation (leaching and accumulation) within the S1 pedocomplex had occurred, although to a limited depth and degree. Carbonate was leached to greater depths to form more pronounced carbonate peaks in the three paleosols (S1S1, S1S2, S1S3) at the Dingxi section than those at the Lanzhou section. The carbonate concentration high corresponding to the frequency-dependent magnetic susceptibility peak and predicted susceptibility peak expressing the S1S1 suggests that carbonate can accumulate in the susceptibility-enhanced weathered layer due to limited leaching.

Further SE at the Qinan section, both the S1S3 and S1S2 paleosols have distinguishable carbonate leaching and accumulation zones. Nevertheless, carbonate leaching and accumulation in the S1S1 paleosol are more complicated. That is, the carbonate concentration peak in the S1S1 and two other peaks (one overlying the S1S1 and the other underlying the S1S1) may be indicative of carbonate accumulations not only during S1S1 formation but also before and after S1S1 formation. All five S1 subunits (S1S1, S1L1, S1S2, S1L2, S1S3) are preserved at both the Lanzhou and Dingxi sections, but the S1L2 is absent at the Qinan section. The carbonate concentration at the Tianshui section, as well as clay-coating characterized soil morphologic features (Feng et al., 2004a), suggests that the three paleosols (S1S1, S1S2, and S1S3) were partially welded with both S1L1 and S1L2 paleosols being absent. Soil annexation and welding not only altered the S1L1 and S1L2 loess units into paleosols but also altered the A horizons of the S1S3 and S1S2 paleosols into later B horizons, forming thick accretionary B horizons. At the Lantian section, three soil-forming events occurred in a single paleosol profile and nearly all carbonate was leached to a Ck horizon mainly consisting of carbonate nodules, suggesting that the Ck horizon might have served as the carbonate accumulation zone for all three soil-forming events.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Carbonate concentration in the S1 pedocomplex is not only a function of the last interglacial paleoclimate, but is strongly controlled by land-surface stability. For example, no composite paleosol or loess annexation occurs at the Lanzhou and Dingxi sections. The S1S2 paleosol development completely annexed the S1L2 loess unit at the Qinan section. At the Tianshui section, not only were both the S1L1 and S1L2 loess units completely annexed, but also the uppermost part of the S1S2 paleosol was annexed by the S1S1 development. The S1S2 and S1S3 paleosols were nearly completely welded into one paleosol profile. The S1 pedocomplex at the Lantian section is a single composite paleosol profile, formed during three soil-forming events (S1S1, S1S2, S1S3). In other words, the single S1 paleosol at the Lantian section results not only from more intensive cyclic pedogenesis but also from much more stable land surface (i.e., limited eolian deposition) during the entire last interglacial (i.e., marine isotope Stage 5) spanning 55000 yr (i.e., from 128000–73000 yr BP). It can be concluded that carbonate concentrations in loessial paleosols is not only a function of climate and its associated vegetation but also a function of land surface stability. Therefore, the climate interpretation of carbonate concentration is not straightforward considering the complexity of factors affecting land surface stability.


   ACKNOWLEDGMENTS
 
This research is financially supported by a U.S. National Science Foundation grant (BCS-0078557) and a Chinese Education Ministry grant (No. 2000-65).

Received for publication February 27, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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