全 文 ：Received 1 Dec. 2003 Accepted 10 Jun. 2004
Supported by the Knowledge Innovation Program of The Chinese Academy of Sciences (KSCX1-08-02).
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (10): 1141-1148
On the Fossil Pediastrum from the Gaoximage Section, Hunshandak
Sandy Land and Its Ecological Significance Since 5 000 a BP
XU Zhao-Liang1, LI Chun-Yu1, 2, KONG Zhao-Chen1
(1. Laboratory of Systematic and Evolutionary Botany, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China;
2. Graduate School of The Chinese Academy of Sciences, Beijing 100039, China)
Abstract: The fossil Pediastrum was investigated from the Gaoximage section ( 42°5714 N, 115°223
E, Alt 1 253 m ) in the Hunshandak Sandy Land, Inner Mongolia (Nei Mongol) of China in order to gain a
better understanding the changes of palaeovegetation, palaeoenvironment and palaeoclimite since 5 000
aBP when the pollen-spore was studied. The results show that the fossil Pediastrum assemblage can be
divided into three periods: Ⅰ. 5 000-3 500 a BP, the fossil Pediastrum occurred rarely. Based on this
result it can be surmised that the lake might be fresh water with poor organics.Ⅱ. 3 500-1 500 a BP, the
fossil Pediastrum likes volcanic emerged. It probably indicate moist conditions and the grasslands were
developed during this time; and Ⅲ. 1 500-200 a BP, the fossil Pediastrum disappeared. It can show that the
lake water become salty and shrunk, and the weather could become drier and drier. This result shows that
Pediastrum, a kind of facies fossil, happened simultaneously in the pollen-spore assemblage during the
second period. Sometimes Pediastrum can reach more than 60 per cent of the total spore pollen assemblage,
indicating that Gaoximage lake was not only fresh water with abundant eutrophic materials but also occu-
pied a large area during 3 500-1 500 a BP.
Key words: Hunshandak Sandy Land; since 5 000 a BP; fossil Pediastrum
Each year in China thousands of square kilometers of
land undergo desertification. This increasingly conspicu-
ous problem threatens the human living environment —
dust and sand storm. Among investigators studying the
causes of desertification, some considered that it was a
natural factor ( Fang, 1987; Ren, 1990), which others thought
that the main cause was desertification induced by human
action (Zhu and Wang, 1992; Lin, 1998; Sun and Ding, 1998).
However, most scientists deemed that the result was due
to a combination between nature and human action factors
(Mensching and Ibrahim, 1985; Yang, 1997; Dong et al.,
1998).
Hunshandak Sandy Land is one of the four largest sandy
land regions in central and east Inner Mongolia (Nei
Mongol). It is one of the first class sensitive areas of global
changes and it is also an agro-pastoral ecotone. In the last
few years, desertification due to agro-pastoral development
and the climate becoming arid has caused the grassland
deterioration and species reduction. This not only hinders
local economic development but also contributes to the
origin of dust and sand storms, which seriously threatens
economic construction and human living conditions around
the region. In order to develop the local and surrounding
region’s economy and to control the ecological environ-
ment it is very important to investigate the changes in
vegetation and environment the Hunshandak Sandy Land
in the past.
Pollen and spore assemblages have been investigated
from the ancient soil of Hunshandak Sandy Land since
5 000 a BP. A good result of such a study was done by Li
and others (2003). However, numerous examples of the fa-
cies alga fossil Pediastrum had been found at the time dur-
ing 3 500-1 500 a BP. In this study, the genus Pediastrum
has been reported from Gaoximage section in the
Hunshandak Sandy Land, Zakesitai, Zhenglanqi of Inner
Mongolia, China. It provides new evidence for studying
the evolution of the ecological system of Hunshandak
Sandy Land.
1 General Vegetation and Geological Setting
Hunshandak Sandy Land, also called Xiaotenggeli
Sandy Land, is one of the four largest sandy lands in the
middle and east of Inner Mongolia, China. It covers a re-
gion at least 300 km long and more than 50 km wide, and
occupies about 20 000 square kilometers. In this area, Ulmus
pumila occurs sparsely in the whole sandy land. Some
sandy plants such as Agropyron, Aneurolepidium, Bromus,
and others are dominant in the east part of this area, and
the community composed of Betula sp., Salix spp., Populus
spp., and Ostryopsis davidiana shrubs can be seen in the
Acta Botanica Sinica 植物学报 Vol.46 No.10 20041142
shady side of the large stable dunes. Generally, in the middle
and east of the land the shrubs composed of Spiraea,
Lonicera, Rosa and the herb Polygonum divaricatum can
be found in the shady side of large dunes. Artemisia
desertorum is dominant in the west.
The Gaoximage section is located about 3 km southwest
of Zagesitai Village in Zhenglanqi, on the second terrace of
Gaoximage lake (Fig.1), which dried up due to the continu-
ous dry weather recently. In order to obtain useful informa-
tion from the lake, the section trenched to dug is meters in
depth. A vertical column of the sedimentological logs is
drawn, and the stratigraphy on order is black silt peat, yel-
low silt, dark gray clay, gray silt, gray silt clay and gray silt.
Studies at the 14C Dating Laboratory of the Geological In-
stitute of Chinese National Earthquake Bureau have pro-
vided the carbon 14 data for the section (Table 1).
2 Materials and Methods
In total, 87 samples (separate by a space of 2 cm, about
35 yrs in age) were selected from the column logs of the
section, and each sample weighs 10 g with Lycopodium
spp. tracer spores (6.56× 104 grains/mL) added. The
samples were treated with hydrochloric acid to remove cal-
cium and carbonates and hydrofluoric acid to remove
silicates, heated gently to the boiling point, followed by a
heavy liquid (specific gravity = 1.9) to separate of organics
from additional unwanted inorganic traces. Then acetic acid
and a fresh mixture (ca. nine parts acetic anhydride and one
part sulfuric acid) were respectively added to remove un-
wanted organics and to clean the residues on fossil surface.
After each step mentioned above, the sample was repeat-
edly washed with water, up to neutral. Finally, the fossils,
pollens and algae were identified and counted under the
microscope of OLYMPUS. In each sample, the pollens of
terrestrial plants (tree, shrub and herb) were counted up to
200 grains except those of little pollen in the samples, and
then its percentage and concentration were calculated in
each sample. The percentage and concentration of
Pediastrum were still calculated (as that of pollen above).
3 Results and Discussion
The data (Figs.2, 3) show that species and abundance
of pollen and spore were rare, only Betula, Artemisia,
Ephedra, Polypodium, Leguminosae and other herbs, and
the fossil Pediastrum can occasionally be found. The pollens
of Betula and Artemisia are dominant from 5 000-3 500 a
BP. This result shows that the vegetation could be sparse
and the Betula forests could sparsely exist in that region.
The pollen-spore content and species increased sharply
from 3 500 to 1 500 a BP. The percentage of herb pollengrains
was more than trees. Among the herbs, the percentage of
Artemisia is higher than that of Chenopodiaceae. The per-
centage of Artemisia gradually decrease with age which
Chenopodiaceae increased (Li et al., 2003). At that time
Pediastrum appeared reaching 60% or more of the assem-
blage from 3 500-1 500 a BP. During 1 500-200 a BP the
pollen spore content was reduced in contrast with the pe-
riod from 3 500 to 1 500 a BP. However, herbs dominated in
the pollen-spore assemblage in which the percentage of
Chenopodiaceae and Ephedra increased and the Artemisia,
ferns and aquatic plants were reduced (Li et al., 2003).
Pediastrum disappeared.
The genus Pediastrum is a widely distributed free float-
ing green algae. There are approximately 40 living species
in the world ( Smith, 1955 ), but only about 10 species occur
in China (Hu, 1980; Bi and Wang, 1987). All Pediastrum
grow in the permanent or semipermanent lakes, pools, and
ditches, and often present in the plankton of fresh-water
lakes, sometimes in considerable quantity. Based on the
result of the investigation on the changes of the nutritional
concentration, nitrogen and phosphorus are always liable
to be limiting factors for the development of Pediastrum in
natural waters when other conditions are suitable. However,
the low nitrogen concentration (0.95× 10-6-13.00×
10-6) was one factor causing better growth of Pediastrum.
When phosphate is about 1.0× 10-6, optimum growth of
Pediastrum occurs. Some, such as P. boryanum, like other
organisms favour relatively lower inorganic salts of calcium,
sodium, and magnesium. Generally, the magnesium and
calcium content in many natural water are above the upper
limit suitable for its growth. However, Pediastrum can re-
sist high concentrations of potassium. Sodium chlorite is
regularly present in natural water, and even rains contain a
certain amount of it, which indicates that it is not essential
for the growth of Pediastrum. Soft water, therefore, seems
to be more suitable than hard water, if the sodium concen-
tration is not too high, less than 4×10-6 (Zhu, 1942; 1943).
Silicate (SiO2) is toxic to Pediastrum although it is not rare
in the natural water. Temperature is an important factor for
Table 1 14C data of Gaoximage section, Zhenglanqi, Inner
Mongolia
Lithology Depth (cm)
14C ages Data corrected by
(a BP) tree rings (a BP)
Silt 6 7 - 7 2 1 850+80 1 820+120
Grey clay 108 -11 3 2 850+100 2 950+155
Dark sit peat 165 -17 0 3 810+80 4 810+120
Dark silt peat 192 -19 7 5 010+90 5 735+135
XU Zhao-Liang et al.: On the Fossil Pediastrum from the Gaoximage Section, Hunshandak Sandy Land and its Ecological
Significance Since 5 000 a BP 1143
the growth of Pediastrum. Generally, it can grew when the
temperature is between 4 ℃ and 36 ℃; however, optimal
growth is about 23 ℃. Most species of Pediastrum, grow
from March to October every year, when the water tem-
perature is at 15 ℃ or higher.
Fossil Pediastrum is known from different deposits by
a number of scientists (Davis, 1916; Wilson, 1953; Borge
and Erdtman, 1954; Evitt, 1963; Alhonen and Ristiloma, 1973;
Zhu, 1978; Wan, 1992), and the earliest report of a fossil
Pediastrum can be traced to the early of 20th century
（Davis, 1916）. In China, fossil Pediastrum has been re-
ported from Heilongjiang, Inner Mongolia, Xinjiang,
Fig.1. Geographic map of sample sites.
Acta Botanica Sinica 植物学报 Vol.46 No.10 20041144
Fig.2. Pediastrum and pollen percentage diagram of Gaoximage section.
XU Zhao-Liang et al.: On the Fossil Pediastrum from the Gaoximage Section, Hunshandak Sandy Land and its Ecological
Significance Since 5 000 a BP 1145
Fig.3. Pediastrum and pollen concentration diagram of Gaoximage section (grains/g).
Acta Botanica Sinica 植物学报 Vol.46 No.10 20041146
Bohaiwan Region, Jiangsu, Anhui, Zhejiang, Jiangxi,
Shanghai, Guangdong and Fujian. In the literature, all of
the fossil Pediastrum sites, mentioned above, are from
outcrop and well cores of lake and marine sediments, and
always occur in abundance associated with fossil spores
and pollen. Evitt (1963) studied the fossil Pediastrum in
Cretaceous marine sediments in Pakistan and California in
order to consider its value as an independent indicator of a
marine depositional environment. He claimed that the re-
mains of Pediastrum coenoba were transported from fresh
water, in which the organisms lived. Alhonen and Ristiluoma
(1973) investigated fossil Pediastrum in a Flandrian core of
southern Finland for biostratigraphic purposes and to un-
derstand its paleoecology. They believed that the
occurrence of planktonic Pediastrum in strata indicated
the water was eutrophic and reflected a more or less pro-
ductive environment of sedimentation. The increase in the
relative frequency of these algae may also be considered to
be related to a rising water level. Zhu et al. (1978) described
15 species and two varieties (including five new species)
from Dainan Formation (Lower Tertiary) of northern Jiangsu,
China, and clarified their views that the deposits were
lacustrine. Wan (1992) found two species belonging to
Pediastrum in connection with other algae and pollen
spores from south of Hailar Basin, Heilongjiang, China, and
also discussed the environment in which the fossil algae
lived. It was always abundant from the continental fresh
water basin in the Tertiary and Quaternary, and relatively
rare in the strata before the Cretaceous.
In summary, the occurrence of fossil Pediastrum from
the Gaoximage section in the Hunshandak Sandy Land,
Inner Mongolia, China, shows that the body of Gaoximage
lake water was fresh with eutrophic materials in which the
planktonic algae thrived, 3 500-1 500 a BP period, although
the lake has dried up due to the present continuously dry
weather.
Systematic Palaeontology
Chlorophyta
Chlorophyceae
Chlorococcales
Hydrodictyaceae
Pediastrum Meyen, 1829
Description Thallus coenobium, free-floating, generally
composed of 2-128 (2n) polygonal cells (mostly 8-32), ar-
ranged in one cell, occasionally two or more cells in the
thickness, stellate pores present or absent; coenobium cells
two kinds of shape, marginal and central, marginal cells
with one, two or three processes; central cells without any
process, closely connected with neighbor cells by periph-
eries or processes; cell wall smooth, granulate or finely
reticulate; asexual reproduction yields zoospore and
autospore, sexual reproduction yields gametes.
Pediastrum boryanum (Trup) Men. 1840
Fig.4
Description Coenobium 45-65 mm in diameter with aver-
age 53 mm (13 coenoba measured) without stellate pores;
cells multiangular, pentagonal or hexagonal; marginal cells
with two obtuse processes, 15-8 mm long, 12-5 mm wide
with average 12.5 mm in length, 9.0 mm in width (16 cells
measured), central cells 22-10 mm long, 20-6 mm wide with
average 14.6 mm in length, 12.6 mm in width (11 cells
measured); cell wall thin, finely granulate.
Comparison The morphology of fossil P. boryanum is the
same as that of living P. boryanum, and it is most abundant
in the samples from Gaoximage section, Hunshandak Sandy
Land. It can reach to 90% of the Pediastrum assemblage.
Its abundance due to is probably the relatively thick cell
walls of P. boryanum.
Pediastrum muticum Kuet, 1849
Fig.5
Description Coenobium spherical without stellate pores,
plate-like in shape, 70-40 mm in diameter with average 56
mm (6 coenoba measured); cells multiangular, pentagonal
or hexagonal, marginal cells with two lobes, 17-10 mm in
diameter with average 14 mm (20 cells measured); Cell wall
with spinules. Others as genus.
Comparison P. muticum is rarely preserved in the stratum
of Gaoximage section, and its percentage is less than 10%.
This is probably due to its relatively thin cell wall and rela-
tives rarely occurred. Its morphological character is similar
to the fossil P. muticum from the Dainan Formation, Lower
Fig.4. Reconstruction of the Pediastrum boryanum.
XU Zhao-Liang et al.: On the Fossil Pediastrum from the Gaoximage Section, Hunshandak Sandy Land and its Ecological
Significance Since 5 000 a BP 1147
Tertiary, of northern Jiangsu of China (Zhu et al., 1978).
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(Managing editor: HAN Ya-Qin)