Soil Development on Alluvium in the Nepean valley in Western Sydney

Abstract

A soil chronosequence was constructed based on four sites in the Nepean Valley in Western Sydney. These sites where profiled in the field, which placed a limit on the detail of the study of each individual site. A challenge of the validity of the basic assumptions behind this (and many other) chronosequence(s) is also made, particularly the possibility of finding sites where the only soil forming factor that varies is time, due to the fact that topography, vegetation and organisms and especially climate will all change over the long periods of time that chronosequences span.

Introduction

Due to the nature of the Nepean Valley in Western Sydney it can be attempted to explain the sequence of soil development on alluvium over time with a chronosequence. Under the traditional (zonal) approach soil formation is thought to be governed by five different factors; parent material, climate, relief or topography, organisms and time (Charman and Murphy, 2001). The idea behind using a chronosequence is that due to the selection of sites profiled time is the only variable. They transform spatial difference into temporal differences (Hugget, 1998) and enable a progressive view of soil development to be formed.

A soil chronosequence has been established in the Nepean Valley composing of four sites of increasing age and elevation from the riverbed (which is the Nepean river at this point).

Approach/Methods

The four sites for this chronosequence where profiled in the field, to a variety of depths. A field pH was taken, colour was recorded according to the Munsel colour chart and a field texture was taken.

The first site was taken on a levee bank of the Nepean River with an auger. The profile was recorded to a depth of 300 cm with a uniform (Uc) soil with seven layers recorded.

The second site was a road cutting and was able to be sampled to a depth of 170 cm. Four layers where recorded and once again this profile was uniform (Um).

The third site in this chronosequence was a cutting at the Angus Bank sand mine. This profile had 7 layers and extended too greater than 545 cm deep. The profile was uniform with a large variety of colours.

The fourth profile was another road cutting. This cutting was 640cm deep. Due to the site there was a 90cm gap in the profile that could not be textured, assigned a colour or pH. This profile had the greatest range in field textures.

One feature observed at all the sites in this chronosequence was a band of red or dark red rounded nodules up to 2cm in size. In the youngest (first) site in this chronosequence the nodules where found between 180 and 200cm and between 1 and 2mm in size. At the second site the nodules where spherical and found between 50 and 120cm deep. At the Angus Bank sand mine (site three) the nodules where sub angular and located between 30 and 50cm deep with a maximum density at 40cm. Finally at the oldest (fourth) site the nodules where found between 5 and 35cm deep with a maximum size of 2cm.

The sites for this chronosequence were only observed in the field however. This means that in depth particle size analysis, fine clay: total clay ratios, chemical weathering analysis and other lab based calculations that have been used to determine soil development sequences in other chronosequences (Chittleborough et al., 1984 I, Chittleborough et al., 1984 II, Brewer and Walker, 1968) could not be calculated. This means that the rate of clay illuviation the most common process ascribed to soil development over time in literature using chronosequences (Chittleborough et al., 1984 I, Chittleborough et al., 1984 II, Brewer and Walker, 1968) cannot be determined.

Results

The results for site profiles are presented in tables 1 to 4 respectively. Site One was the youngest site of the chronosequence and due to its spatial location (next to the Nepean river) the river is still playing a part in its accretion. Site two was the next oldest. Probably forming during the Pleistocene with an approximate age of several hundred thousand years if this is true. The third site could possibly have been formed by Aeolian processes. This would make it approximately 15,000 to 20,000 years old, which would mean it doesn’t fit in with the rest of this chronosequence. 

 

Site One	Profile	Notes
	Ph 6
A1	7.5YR 3/2 (Dark Brown)
70cm	Sandy Loam
	Ph 6
A2	10YR 5/3 (Brown)
172cm	Sand
	Ph 6
	7.5YR 4/4 (Brown)
180cm	Sandy Loam
		Nodules Found in this layer
	10YR 6/6 (Dark Yellowish Brown)
200cm	Fine Sandy Loam
	7.5YR 4/4
270cm	Medium Sand
	7.5YR 4/4 (Brown)
285cm	Sandy Loam
	Ph 6
	7.5YR 4/4
300cm	Loamy Sand

Table One- Site One

Site 2	Profile	Notes
	Ph 6
A1	5YR 4/3
30cm	Sandy Loam
Clear	Ph 6
Possible A2	5YR 5/4
50cm	Sandy Clay Loam
Gradual	PH 6	Nodules at 60cm
	5YR 5/6
120cm	Sandy Clay Loam
Gradual
	2.5YR 4/8
170cm	Sandy Clay Loam

Table Two- Site Two

Site 3
Layer	Depth	Texture	Moist Colour	Dry Colour	pH	Grain Size	Boundary	Notes
Section A
0	0-8cm	Sandy Loam	2.5YR 3/2	10YR 3/3	5.5	710-1000	Clear
Section B
1	8-20cm	Sand	7.5YR 6/1	7.5YR 5/1	6	500-710	Gradual, Straight	Nodules Between 30-50cm with max density at 40cm
2	20-50cm	Sand	10YR 5/6	10YR 5/8	6		Gradual, Straight
3	50-150cm	Sand	10YR 7/6	10YR 6/6	6	250-710	Abrupt, Wavy
4	150cm+	Sand			6	350-710
Section C
4	200-475cm	Sand	Red- 2.5YR 4/8		6	350-710	Clear, Wavy
			Yellow 2.5YR 7/8
			White 7.5YR 8/1
			Grey 5YR 5/8
Section D
5	480-545cm	Sand	2.5YR 7/8		5.75	500-710	Gradual, Wavy
6	545cm+	Sand	7.5YR 8/1		6	500-710	Clear, Straight

Table Three

Site 4	Profile	Notes
	Ph 4.5
A1	10YR 4/4 (m) (Dark Yellowish Brown)
5cm	Sandy Loam
Sharp	Ph 5	10 R 3/6 (Dark Red) Nodules
A2	10YR 4/6 (m) (Dark Yellowish Brown)
35cm	Sandy Loam
Gradual	Ph 5.5
	5YR 4/6 (Reddish Brown) and 10YR 6/8 (Brownish Yellow)
60cm	Medium Clay
Gap
150cm
	10R 3/6 (Dark Red)
	Sandy Clay
240cm	Ph 4.5
	2.5YR 3/6 (Dark Red)
	Sandy Clay
520cm	Ph 5
	10YR 4/6 (Reddish Brown)
	Medium Clay
640cm
	10YR 6/1 (Reddish Grey)
	Medium Clay

Table Four- Site Four

However if fluvial processes are assumed to have formed the soil found at the third site the age of this site would be estimated as several hundred thousand years old also. The final (fourth) site can be given the most definite age due to the folding at the site. This site has been worked out to be between 5 and 15 million years old.

There was a high degree of textural similarity between the first three sites profiled. With the textures of all the different layers fitting into the first two texture groups, the sands and sandy loams (Northcote, 1968). The major observed differences between sites profiled were colour and depth of the upper layers.

The first site had a consistently darker colour possibly indicating more organic content due to the nearby river. Site two was predominantly red in colour mainly due to the oxygen rich environment oxidising the iron. The third site had a large range in colours ranging from reds to greys to yellow to whites. This could indicate different drainage patterns, oxidation or deposition patterns.

For site one the A1 layer was 70cm thick and the total thickness of the A1 and A2 layers was 172cm. This is much larger than the combined thickness of the A1 and possible A2 layers in site two which came to a total of 50cm. At the third site the A layer was even thinner only measuring 8cm thick. This could possibly represent greater levels of erosion with time and less deposition (site one would probably still be receiving fluvial deposits).

Discussion

Constructing this chronosequence is reliant on some key assumptions that should be explored. With the theory of a chronosequencing (time being the only variable in the soil forming process) you are exchanging the spatial variance for a temporal one to establish a timeline of soil development (Hugget, 1998). The biggest assumption here is that you can find a variety of sites that have been exposed to the same soil forming processes over differing periods of time. This means under the traditional method you need to find site where climate, parent material, topography and organism activity have all been constant for hundreds of thousands or even millions of years (these basic zonalistic assumptions can also be challenged as seen in Paton et al., 1995). Climate is highly unlikely to have remained constant for even a (relatively) short period of time (Hugget, 1998). The period of time that the chronosequence studied spans is up to 15 million years and there have been many major climate changes over that period of time, such as the end of the last ice age approximately 10,000 years ago.

Vegetation (organisms) may change due changes in climate, but they could also change with large periods of time such as millennia (Hugget, 1998). Topography is also likely to change over a period of millennia and if this does change it may not represent a single pedogenic process acting without interruption from geomorphic processes (Hugget, 1998). The site used for this chronosequence all had the same parent material (so this factor is constant) however the range of ages in this chronosequence would be expansive enough to have the possibility of topographic or vegetative change between the youngest and oldest site. Vegetation change has also occurred intensively in the last 200 years with European settlement. Site one has become agricultural land while site two was stripped of vegetation to become a sand mine. Climate would also have been likely to change at least once with the end of the last ice age 10,000 years ago.

The changes in land use could also have affected soil development, and certainly will in the future. Especially at site three where the soil is being mined and transported away.

Hugget (1998) also identified a second set of assumption made when constructing a chronosequence and that is that all changes in the soil processes and events in the soil history are recorded or recorded at equal magnitude. In relation to the studied chronosequence this could mean that although from the field study the first three site appeared similar (uniform sands or loamy sands) there could have been events that occurred at individual sites that have not been recorded in the soil history as accurately.

Conclusion

There were many assumptions made in constructing this chronosequence many of which can be challenged. Most importantly being the consistency of the climate and organisms and vegetation’s effect on the soil development. The brief nature of the study also makes it difficult to measure clay illuviation rates and chemical weathering rates that can be used to compare the different sites with the aim to indicate how soil develops over time.

From the data collected the assumptions that can be made over the development of the soil is that over time the A1 and A2 topsoil decreases in depth (possibly due to erosion rates increasing or less material being deposited, as site one was in the position to receive fluvial deposits). The other conclusion that could be made is that the rates of oxidation increase with time (or perhaps distance from the river), which was illustrated by the redder colouration of the soil in the older chronosequences.

The validity of the assumptions needed to create this chronosequence (and most others) however are substantial and bring into doubt the idea that the only traditional soil forming process that varies in these site is time. If this is true then any conclusion based on the “fact” that these profiles for a continuous temporal sequence is probably untrue.


References

Brewer, R. & Walker, P.H., 1969. Weathering and soil development on a sequence of river terraces. Australian Journal of Soil Research, 7: 293-305 

 
Charman, E.V. & Murphy, B.W. (eds). 2001. Soils: Their Property and Management. Oxford University Press, Melbourne  

 

Chittleborough, D.J., Walker, P.H. & Oades, J.M. 1984. Textural differentiation in chronosequences from Eastern Australia, Descriptions, chemical properties and micromorphologies of soils, Geoderma, 32: 181-202  

 

Chittleborough, D.J., Walker, P.H. & Oades, J.M. 1984. Textural differentiation in chronosequences from Eastern Australia, Evidence from particle-size distributions, Geoderma, 32: 203-226  

 

Hugget, R.J. 1998. Soil chronosequences, soil development, and soil evolution: a critical review. Catena, 32: 155-172  

 

Paton, T.R., Humphreys, G.S. & Mitchell, P.B. 1995. Soils: A new global view, UCL Press Ltd, London  

 

Walker, P.H. & Hawkins, C.A., 1957. A study of river terraces and soil development on the Nepean River, NSW. Journal and Proceedings of the Royal Society of NSW. 91: 67-84