SUITMA 2005 Cairo - Soil classification at a Han Dynasty tomb site, China

Soils of Urban, Industrial, Traffic, Mining and Military Areas
SUITMA 2005 Cairo

Soil classification at a Han Dynasty tomb site, China

SUITMA

This paper is one of the Historical sites theme of the SUITMA 2005 symposium.

Fengrong Zhang,i	Yan Xu,
Danfeng Sun.

• i - College of Resources and Environment - China Agriculture University, Beijing, China.

Abstract

Archaeologists opened an ancient tomb located in the western suburb of Beijing, constructed more than ten meters deep over 2000 years ago. The upper soil horizons from the surface to three meters were thoroughly mixed with fine earth and some rock fragments. The lower layers from three meters down to the top of coffin, buried more than ten meters deep, were quite uniform with fine earth. Clearly, all of the soil material was transported during construction of the tomb. The upper soil horizons from surface to a depth of two meters were described, sampled and characterized. Our objective was to determine the formation and classification of this type of soil. The results indicate that the soil was a coarse loam, the structure was medium crumb, and the consistency was hard when dry. The organic matter content showed an irregular tendency to decrease with depth in the solum, as well as the carbonates. Rock fragments were not deeply weathered. There were a few of mycelium-like carbonate crystals in the middle horizon. There are evidences showing that the soil was in the juvenile stage, far from being in equilibrium with their surroundings. There were not any taxa that could cover the. However, it was fitted to the Technosols, according to the World Reference Base (WRB).

Introduction

In 2000, archaeologists opened an ancient tomb that was located in the west suburb area of Beijing, Shi Jingshan District. The tomb was constructed during the Han Dynasty about 2000 years ago. The Cultural Relics Bureau of Beijing organized a group of professional archeologists that started the excavation at the tomb site on February 2000. The whole excavation lasted for more than a year. Based on the soil characteristics at the site, it is expected to have an understanding of the soil formation process and rate, as well as on the classification of this type of soil. The assumption is that the initial time of formation for this soil is precisely known.

Materials and methods

Shi Jingshan District of Beijing is characterized by a warm and sub-humid climate, and the average annual temperature is 12.1ºC; the hours of solar radiation are 2545 hours per annum; the average annual precipitation is 576 mm and the inter-yearly and intra-yearly variation is very large, and 56% rainfall is in the summer from July to September. The topography ranges from steep hills in the northwest to flat plain in the southeast across Shi Jingshan District. The Han Dynasty tomb lies in a rounded and mild sloped hill. The soil parent material is weathered residue of rocks, loess or loess-like material and quaternary deposits. Siliceous calcareous parent material is mostly in the northern mountainous area, calcareous parent material in the central plain area, and diluvial and alluvial parent material in the southwestern area along Yongding River. Natural vegetation comprises trees (Robinia pseudoacacia L., Ulmus pumila L., Platycladus orientalis L., etc.) and shrubs (Zizyphus jujuba Mill. Vitex negundo L.).

Soil profile characteristics were described according to the “FAO Soil Profile Description Manual”. Soil samples were analyzed for soil organic matter (SOM), calcium carbonate (CaCO₃), pH, total-Fe (Fe_t), active-Fe (Fe_a), dissociative-Fe (Fe_d), and soil mechanical composition.

Soil samples were analyzed by routine methods. The soil pH was measured by the glass electrode method, mechanical composition by the pipette method, CaCO₃ by neutral titrimetric method, soil organic matter by rapid dichromate oxidation, total-Fe by 1,10-phenanthroline photometric method, active-Fe by NH₄-oxalate-oxalic method, and dissociative-Fe by citrate-bicarbonate-dithionite method.

Results and discussions

Soil Characteristics

The tomb excavation work started February, 2000. The tomb was more than 10 m deep. All of the soil materials were collected by mechanical and manual methods. The soil profile description may be presented as follows:

• 0-24 cm: Bright brown when dry (7.5YR5/6); coarse loam; medium crumb; much soil pore space; shrub roots; clearly boundary.
• 24-84 cm: Bright brown when dry (7.5YR5/6); coarse loam; medium crumb; a few of mycelium like carbonate crystals, very hard; schist fragments (<10% V/V); weathering weakly; shrub roots; gradually boundary.
• 84-122 cm: Bright brown when dry (7.5YR5/6); coarse loam; medium crumb; less mycelium like carbonate crystals than the higher layer; blended with a little terra rosa material; blocky structure, very hard, yellow red (5YR4/6); suddenly boundary.
• 122-133 cm: schist fragments of ten-centimeter-thick, regularly paved, weathering weakly.
• 133-150 cm: Bright brown when dry (7.5YR5/6); coarse loam; medium crumb; less mycelium- like carbonate crystals than the higher layer; blended with laterite, blocky structure, very hard, yellow red (5YR4/6); gradually boundary.
• 300-700 cm: Soil color is uniform; coarse loam.

As evidenced by the data shown in Table 1, there is no appreciable mechanical development in the selected profile. The soil mechanical composition data show that the soil is a coarse loam and is in the juvenile stage.

Two of the soil layers (24-84 cm and 133-150 cm) in the profile showed slight increase of clay content compared with the close horizons. The increase of clay content in the layer of 133-150 cm is likely due to clay mineral material mixed during construction of the tomb, as was noted earlier.

Table 1. Physical and chemical analysis of the soil profile

Layer	pH	OM	CaCO3	Mechanical composition, mm %,			Concentration, mg/kg			Fea / Fed	Fed / Fet
Cm		g/kg	g/kg	2.00 - 0.05	0.05 - 0.002	<0.002	Fet	Fea	Fed
0-24	7.50	11.8	5.19	56.24	38.53	5.23	47.3	0.707	11.03	0.064	0.233
24-84	7.78	8.98	5.68	59.98	33.65	6.37	53.3	0.927	11.44	0.081	0.215
84-122	7.75	10.1	4.70	49.26	44.52	6.22	50.9	1.138	11.51	0.099	0.226
122-133	7.70	7.30	5.58	50.47	43.14	6.39	64.1	1.305	11.98	0.109	0.187
133-150	7.70	6.74	8.33	57.41	35.49	7.1	40.8	0.125	12.93	0.01	0.317
300-700	*	*	2.74	60.93	32.58	6.47	*	*	*	*	*

* no data.

It is difficult to confirm the clay forming process for the 24-84 cm layer because the materials are homogeneous. Due to the typical warm and sub-humid climate, high temperature and high humidity appear simultaneously in the research site, which help contain a relatively stable soil temperature and humidity in certain deepness in soil and make the mineral weathering easier, which further causes clay content to increase. Therefore, the clay content increased in the 24-84 cm layer perhaps as the result of more than 2000 years residual clay deposition. However, due to the apparent translocation and refilling during the construction of the tomb, the materials are very complicated, which makes it very difficult to confirm whether clay deposition indeed took place.

CaCO₃ Forming Process

The results of CaCO₃ content analysis of the profiles provided hints for determining the degree of development. Because of the apparent translocation and refilling, the original soil materials are very complicated, which makes it rather difficult in confirming whether the calcification process took place from the CaCO₃ content variation in the soil profile alone. However, the presence of a few of mycelial-like carbonate crystals in the middle horizons proves that calcification process undoubtedly took place. With the periodic precipitation, Ca²⁺ ions did not leach out of the solum, and illuviated to the lower depth. The existence of carbonate crystals was likely to be the result of bicarbonate translocation in the soil profile coupled with a decrease in the partial pressure of CO₂.

Iron Oxide Translocation

During the weathering of soil minerals, iron in the minerals was subject to hydroxylation and oxidation reactions to produce free iron oxides that are the base of soil pigmentation responsible for the soil bright brown color (7.5YR5/6) (Dai et al, 1995). Under the anaerobic conditions of organic matter accumulation, the insoluble Fe₂O₃ and Fe(OH)₃ may be reduced to soluble Fe²⁺, which leads to reduction and leaching.

Iron oxides are strongly related to soil clay particle content（Xu et al, 1986）. Chart 1 clearly shows that the trend of variation of Fe_t with depth is almost identical to that of clay particle content. The low ratios of Fe_a/clay, Fe_d/clay, etc suggest the variation of different layers is not great. The soil is in the juvenile stage.

Chart 1. Distributions of clay content and total-Fe, active-Fe and free-Fe in the soil Profile

Profile Position in the Soil Classification Systems

There are a lot of soils, like the soil described in this paper, which are developed from the artificially transported material. Although the artificially transported material is mainly local soil material, they were thoroughly mixed, sometimes a few artifacts such as coal cinder, bricks, and glass fragments can be found. The soil was far from being in equilibrium with their surroundings. Similarly, such soils in terrace sites, are sharply disturbed and indistinctive from surrounding soils although they have the same soil development condition.

Most of the major soil taxonomic classification systems, including World Reference Base (WRB), Soil Taxonomy (ST) (2003ed.) and CST, emphasize the soil characteristics and classification that are affected by human activities, and classify as anthropogenic soil accordingly. CST places Anthrosols and Primosols as soil order and suborder separately. Anthrosols has two distinctly different suborders, depending on farming measures; whereas Primosols has an Anthric Primosols suborder that consists of Turbi-Anthric Primosols group and Silti-Anthric Primosols group. The CST and ST have not classified such soils that have not anthropic epipedon, but with its soil horizon mixed thoroughly by human and soil material transported from local or ecdemic soils.

WRB proposed a new soil group, Technosols, having a horizon of technic soil material within a depth of 100 cm. Technosols are subdivided into six subgroups: alloic, anthrostratic, compactic, humic, ekranic and toxic. So, the described soil in Han Dynasty tom site is fitted in the alloic subgroups of Technosols in WRB. In China, there are a lot of soils, like the soil described in this paper, which are developed from artificially transported material. Although the artificially transported material is mainly local soil material, they were well mixed, sometimes, a few artifacts such as a coal cinder, bricks, and glass fragments can be found. So it is necessary to give a taxon for these soils in CST and ST.

To establish a proper position for such soil type, we suggest that CST reconsider classification criterions to include a new Primosols soil group, Earth cumuli-Anthric Primosols group which coordinates Turbi-Anthric Primosols group and Silti-Anthric Primosols group. There are quite a number of man-made terraces in China. Soil layer is filled at the bottom of the terrace while soil material is quite homologous and as same as surrounding soils. But, this type of soil is clearly different from the surrounding soils that are not disturbed. These soils should also be given a classified position as mentioned above.

References

• Dai W., Xiang S.Q., 1995, Study on Non-calcific Soil Genetic Classification of the Beijing Low Hill, Chinese Journal of Soil Science, 26(4): 150-152
• The Group of Soil Taxonomy of Institute of Soil Science, Chinese Academy of Sciences, Chinese Soil Taxonomy index, Hefei: Press of University of Science and Technology of China, 2001
• The Soil Survey Staff, Keys to Soil Taxonomy. USDA. Washington. 2003.
• Xu L.Y., Zhou M.C., Xi C.F., 1986. The Soil Characteristics in Northwestern

Beijing Hill, Soil Special Report, 40: 69-90.