- Email: [email protected]
Controls on dune colour in the Namib sand sea: preliminary results
Joumd
of @ian
Earth Sdsnus. -#II
VoL 22. No. 3. pp. 349-353, 0 1996 Elmvim
Printed in Great Bdti.
scicm
19% Lid
Au rkhb awewed
0899436W96
SiS.00
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Controls on dune colour in the Namib sand sea: preliminary results JOHN WALDEN’,
School
KEVIN WHITE2 and NICK A. DRAKE3
of Geography
and Geology, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife, Scotland, KY16 9ST, UK *Department of Geog ra p h y , University of Reading, Whiteknights, Reading, RG6 2AB, UK 3Department of Geography, King’s College London, Strand, London, WC2R 2I.S UK (Received
7 December
1995: revised version received 1 February 1996)
Abstract - The red colouration observed in many sand seas is a consequence of fine grained haematite and clay coatings surrounding quartz grains and a number of hypotheses have been proposed to account for this. The marked increase in dune redness with increasing distance inland in the northern area of the Namib sand sea is investigated using satellite imagery and mineral magnetic measurements. These results suggest that source materials exert at-least a partial control on dune colour in this field setting, but that evidence for the influence of other factors (e.g. climatic gradient and/or abrasion of coating during transport) is also present. RtsumC - La coloration rouge observee dans de nombreuses mers de sable est due a la presence d’hematite finement grenue ainsi qu’a des revetements argileux autour de grams de quartz. Differentes hypotheses ont ete proposees comme explication. Dans la region septentrionale de la Mer de Sable du Desert de Namib, l’augmentation nette de l’intensite de la couleur rouge avec la distance vers l’mterieur des terres est etudiee a l’aide d’images satellitaires et de mesures du magn&isme de certains mineraux. Ces resultats suggerent que dans ce contexte de terrain, la source des materiaux soit au moms partiellement a l’origine de la couleur des dunes mais que d’autres facteurs (p.ex. le gradient climatique et/au l’abrasion des revetements lors du transport) peuvent egalement jouer un role.
INTRODUCTION
coating consisting of clay minerals and the iron oxide mineral haematite, typical of ‘red bed’ type sediments (Walker, 1967; van Houten, 1973; Turner, 1980; Gardner and Pye, 1981). In the Namib sand sea, however, a number of hypotheses have been suggested (Logan, 1960; Besler, 1980; Lancaster, 1989) to explain the observed colour variation and as yet no consensus view has developed. The main hypotheses currently suggested are: i) the increasing age of the sands inland allowing greater time for weathering processes to develop the haematite coatings around quartz grains; ii) pre-existing red coatings are lost progressively due to abrasion during sand transport either as materials experience longer periods of abrasion or in zones of greater transport energy; iii) derivation from different sand source materials (either different concentrations of Fe-bearing minerals or residual cement coating); and iv) regional climatic gradients (warmer and wetter inland) providing a control on rates of weathering processes which generate the haematite coatings. The types, concentrations and behaviour of Fe oxide minerals within many environmental systems can often provide valuable insights into the way such systems function (Schwertmann and Taylor, 1989). The exact
The Namib sand sea is a large, complex dune system (Fig. l), which is thought to have been developing since the Late Pliocene (Ward, 1987). Underlying the vast majority of the contemporary sand sea is the Tsondab sandstone. The Tsondab is a red sandstone cemented with Fe oxides and CaCO, and is of Tertiary age. It is generally 50-100 m thick, but locally reaches thicknesses of over 200 m (Ward, 1987). East of approximately 15’20’ E, the Tsondab outcrops in many inter-dune areas. To the west of this, however, in the northern margin of the sand sea, the surface of the Tsondab sandstone is capped by the Karpfencliff conglomerate (a calcreted fluvial conglomerate), also of Tertiary age, and it is these materials which form inter-dune outcrops (Ward, 1987). Although a large body of geomorphological research literature already exists dealing with various aspects of the environmental history of the contemporary sand sea (e.g. Lancaster, 1989), there are many questions unanswered. One feature which had been observed by all workers in the area is the marked colour variation in the dune sand with pale coastal dunes and strongly coloured red dunes at the sand sea’s eastern margin. Previous work has shown that the colour variation was due to progressively higher concentrations inland of a 349
I. \VALDEN cf (71.
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Figure 1. Location map of the Namib Desert showmg the studv area covered by the Landsat scene (see Fig. 3)
pattern of dune colour may therefore hold some basic environmental information and further investigation could prove useful. METHODS Producing an accurate map of Fe oxide concentration is difficult given the inaccessible nature of the terrain. The synoptic capability of satellite imagery has been used in other field settings to map Fe oxide concentration as a proxy for the distribution and relative concentration of Fe oxides at large spatial scales (White et al., 1992) and an approach including remotely sensed data would seem to offer considerable advantages in the current field setting. Comparing such maps with field data poses another problem due to the low concentrations of Fe oxides being measured (typically less than 1%). Mineral magnetic measurements have proved themselves to be a versatile and sensitive means of quantifying Fe oxide assemblages within soil or sediment samples (Oldfield, 1991). The spatial scales and resolutions at which each technique operates are very different and therefore have different advantages and disadvantages. With a combination of these approaches it might be possible to develop a better spatial (perhaps quantitative)
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Figure 2. (a) Munsell Redness values, showing a gradual mcrease with dlstancc from the coast. Figure 3 shows the localities of the places shown on the transect. (b) Transect of Fe oxide concentration hased upon Landsat data showing a gradual increase with distance from the coast and the major break associated with the contact between the Karpfencliff conglomerate and the Tsondab sandstone. The data points represent image data at the field sampling locations. (c) Transect of ‘hard’ IRM values for the ‘non-magnetic’ extract showing two dishnct gradational increases in canted antiferromagnetic mineral concentrations on either side of the contact between the Karpfencliff conglomerate and the Tsondab sandstone.
awareness of the pattern itself and thus examine the various hypotheses concerning the Fe oxide distribution in the Namib sand sea. A simple Fe oxide ratio of Thematic Mapper band 3 (visible red) divided by the sum of all the visible bands (1, 2 and 3), provides crude, but rapid estimates of surficial material redness. This map was used to direct field sampling strategy (within logistical constraints imposed by limited accessibility and fieldwork time). Fieldwork was carried out in April 1993. Samples were collected from over 30 locations in a transect running along the northern margin of the sand sea. At a number of sites, several samples were collected to allow comparisons of within and between site compositional variability. All sampling sites were located using GPS, allowing their position within the satellite image to be established accurately.
351
Controls on dune colour in the Namib sand sea
iv 14’3U E
0
15”WE
15”W E
Walvis Bay
23”OO S
50 km
I
Figure 3. Monochrome dis~~lavof Fe oxide distributions based upon the processing of Landsat data. The brighter pixels contain larger q&ntities of Fe oxide (=haimkite coatings on quartz grains). ’
A series of three sub-samples from each field sample was prepared for mineral magnetic analysis. The first was a simple ‘bulk’ sample. The other two consisted of a second bulk sample split into ‘magnetic’ and ‘nonmagnetic’ fractions using a simple bar magnet. While only a crude separation was likely to be achieved, the intention was to isolate the magnetite signal (hopefully extracted by the magnet) from the haematite signal represented by the quartz grain coatings. This latter signal is essentially what the satellite sensors are able to detect and express as dune ‘redness’. Each sample was subjected to a range of standard magnetic measurements (White and Walden, 1994; Walden et al., in prep.), but here only the high field acquisition of isothermal remanent magnetisation (or ‘hard IRM) is quoted. This was calculated as the mass specific remanence acquired between fields of 300 and 1000 mT and is proportional to the concentration of canted antiferromagnets (mainly haematite and goethite) in the sample. Further image processing work was carried out after the field work was completed to refine the ‘haematite’ map. The quality of the images derived from the original Landsat scene was improved in two ways, firstly by increasing the accuracy of the image to ground correction by using more ground control points and
secondly by implementing a linear mixture model to improve the accuracy of the remote sensing estimates of haematite abundance. Mixture modelling seeks to map the proportions of scene components within each pixel of an image. It assumes additive (linear) mixing of spectrally distinct scene components, knowledge of the number and identity of scene components and the ability to characterize the spectra of pure examples of the scene components (see Settle and Drake, 1993 for a full description of the method adopted). The method improves on the band ratio estimates of haematite by using all six reflective wavelengths, by considering the spectral response of the other spectrally distinct materials in the image and by providing a method that allows for the removal of the effect of shadows. Using the mixture modelling approach, the proportions of yellow (unreddened) quartz sand, calcium carbonate, red (Fe oxide stained) quartz sand and the shadow from a Thematic Mapper image of the sand sea were mapped (path 179, row 76, 11th June, 1992). To remove the effect of the shadow a simple, but very effective technique was adopted whereby the shadow proportions were subtracted from each pixel and the remaining scene component proportions were re-calculated on a pro-rata basis.
352
I. WALDEN it al.
‘Non-magnetic’ extract samples
4
y- 1904+04* 0
0
50
100
150
‘=oiI
200
250
Image Fe-oxide value Figure 4. Plot of ‘hard’ sand samples overlying Tsondab sandstone.
IRM versus image Fe ox~dc values tar the Karpfencliff conglomerate and the
RESULTS The values of Munsell ‘redness’ (Torrent et al., 1980) made on the field samples confirmed the suggestion made by many previous workers who have observed a trend of increasing haematite coatings moving inland from the coast (Fig. 2a). Figure 3 shows the distribution of Fe oxides seen in the dune sands from the coast (ca 14”30’ E) to the inland margin of the sand sea (ca 16“O’ E) as mapped from the Landsat data. It is clear that there is a marked difference between the western and eastern portions of the sand sea. The image processing techniques used here suggest there is a major break in the Fe oxide concentrations to the east of Gobabeb at approximately 15”20’ E. The variation is therefore not simply a smooth trend, as suggested by some previous workers on the basis of field observation. This break is confirmed by Fig. 2b and occurs close to a major boundary in the underlying lithology (Ward, 1987) between the Tsondab sandstone and the Karpfencliff conglomerate. Outcrops of these lithologies are exposed in many inter-dune areas. On either side of the lithological change, however, trends can be seen in the image data. From the eastern margin of the sand sea to this geological boundary, the data show a gradual decreasing trend in Fe oxide concentrations. To the west of this boundary, the trend in Fe oxide concentration values exhibits a much more rapid decrease. The hard IRM data (Fig. 3c) also indicate the importance of the lithological boundary between the Tsondab sandstone and the Karpfencliff conglomerate and suggests that the underlying geology is exerting a strong control on the concentration of canted antiferromagnetic mineral types found within the contemporary dune sands. This control is most likely to be achieved through the provision of source materials of differing mineral composition. Other magnetic variables (Walden EL al., irz prep.) confirm these differences in all three magnetic sample sets.
As with the image data, on either side of this lithological boundary, the ‘hard’ IRM data suggests east-west trends of increasing concentrations of haematite type minerals. These trends are less obviously explainable by the mixing of different source materials and may, therefore, suggest that some or all of the alternative explanations for colour differences outlined above are also operating. As might probably be expected, a comparison of Fig. 2b and 2c suggests that the mineral magnetic and remote sensing analyses are not quantifying the ‘haematite’ concentrations within the dune sands in an identical fashion. While the remotely sensed approach is only sensitive to the high red pigmenting power of fine grained haematite coatings on quartz grain surfaces, the mineral magnetic measurements are, in addition, detecting Fe oxides in other forms (e.g. detrital grains), which have little or no influence of the overall colour of the materials. However, within the two distinct mineral magnetic provinces identified in Fig. 4, the two data sets are clearly related. This may provide a means whereby mineral magnetic measurements of field samples can be used to calibrate remotely sensed estimates of ‘Fe oxides’ (more specifically, haematite coatings in ‘red beds’) provided such data are interpreted with regard to the different sensitivities and abilities of the techniques (Walden et al., in prep.).
CONCLUSIONS These data confirm that a combination of controls is operating within the Namib sand sea to produce the marked east-west colour variation observed by many workers. The underlying lithology appears to exert a strong influence on dune colour, either by providing source materials with differing assemblages and concentrations of Fe-bearing minerals, or by providing sand eroded from Fe oxide cemented sandstone, as suggested by Besler and Marker (1979). It would also seem, however, that these minerals are then being subjected to different environmental conditions (temperature, moisture availability or wind regime) which are, in turn, imposing some sort of gradient on the production (or preservation) of haematite coatings upon the surfaces of the quartz grains from which the dunes are composed. These issues form the focus of ongoing work (White et al., in prep.; Walden et al., in prep.) These results also indicate that mineral magnetic and remotely sensed estimates of haematite concentrations in surface samples from the Namib sand sea are related and may provide complementary information on haematite distributions. As expected, however, the relationships between the data sets are not simple. While, in this case, the satellite data is primarily sensitive to a single form of Fe oxide (haematite coatings on quartz grains), the magnetic data provide a more complete account of the overall Fe oxide assemblage within the sands, albeit at a greatly reduced spatial scale.
Controls on dune colour in the Namib sand sea
Mineral magnetic analyses may therefore represent a useful adjunct to ‘Fe oxide’ maps produced solely from satellite data in other regional contexts. Furthermore, remote sensing techniques may have the potential to extrapolate ‘point’ mineral magnetic measurements over large areas, as long as the complex relationships between the different data are appreciated. Acknowledgements J. W. and K. W. would like to acknowledge the School of Geography, University of Oxford and the Environmental Science Division, University of Wolverhampton for access to some of the equipment used in this work and the Trapnell Fund for a grant in support of fieldwork costs. The authors would like to thank the following; the staff of DERU, Gobabeb, Namibia, particularly Dr. Mary Seely and Peter Jacobson, and to Derek Dutoit of Enviroteach, Namibia, for their considerable help and encouragement in the field; Professor Andrew Goudie and Frank Eckardt of the School of Geography, University of Oxford for their support, encouragement and useful discussions concerning this project; and both Graeme Sandeman of St Andrews University and Peter Hayward of Oxford University for their cartographic support. REFERENCES Besler, H. 1980. Die Dunen-Namib: entstehung und dynamik eines ergs. Stuttgarter Geographische Studien %, 208~.
Besler, H. and Marker, M. E. 1979. Namib sandstone: a distinct lithological unit. Transactions Geological Society South Africa 82155-160.
Gardner, R. A. M. and Pye, K. 1981. Nature, origin and palaeoenvironmental significance of red coastal and desert sand dunes. Progress Physical Geography 5,514534.
Lancaster, N. 1989. The Namib Sand Sea: dune forms,
353
and sediments. 180~. A. A. Balkema, Rotterdam. Logan, R. F. 1960. The central Namib Desert, South-West Africa. 162~. National Academy of Sciences, Washington DC. Oldfield, F. 1991. Environmental magnetism; a personal perspective. Quatema y Science Reviews 10,73-85. Schwertmann, U. and Taylor, R. M. 1989. Iron oxides. In: Minerals in soil environments (Edited by Dixon, J. B. and Weed, S. B.) ~~379-438. Soil Science Society of America, Madison. Settle, J. J. and Drake, N. A. 1993. Linear mixing and the estimation of ground cover proportions. processes
International
Journal Remote Sensing 14,1159-1177.
Torrent, J., Schwertmann, U. and Schulze, D. G. 1980. Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma 23,191-208. Turner, P. 1980. Continental red beds. 562~. Elsevier, Amsterdam. Van Houten, F. B. 1973. Origin of red beds. A review. Annual Reviews Earth Planetary Science 1,39-61. Walker, T. R. 1967. Formation of red beds in ancient and modern deserts. Bulletin Geological Society America 78,353-368.
Ward, J. D. 1987. The Cenozoic Succession in the Kuiseb Valley, Central Namib Desert. Memoir 9, Geological Survey of South-West Africa/Namibia, 124~. White, K. and Walden, J. 1994. Mineral magnetic analysis of iron oxides in arid zone soils, Tunisian Southern Atlas, In: The effects of environmental change on geomorphic processes and biota in arid and semi-arid regions (Edited by Eye, K. and Millington, A. C.) pp43-
65. Wiley, Chichester. White, K., Walden, J. and Rolin, E. M. 1992. Remote sensing of pedogenic iron oxides using Landsat Thematic Mapper data of Southern Tunisia, In: Proceedings of the 18th Annual Conference of the Remote Sensing Society (Edited by Cracknell, A. P. and
Vaughan, R. A.) ~~179-187. Remote Sensing Society, Nottingham.
of @ian
Earth Sdsnus. -#II
VoL 22. No. 3. pp. 349-353, 0 1996 Elmvim
Printed in Great Bdti.
scicm
19% Lid
Au rkhb awewed
0899436W96
SiS.00
+ 0.00
PIk SOtW9-5362(%)00011-S
Controls on dune colour in the Namib sand sea: preliminary results JOHN WALDEN’,
School
KEVIN WHITE2 and NICK A. DRAKE3
of Geography
and Geology, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife, Scotland, KY16 9ST, UK *Department of Geog ra p h y , University of Reading, Whiteknights, Reading, RG6 2AB, UK 3Department of Geography, King’s College London, Strand, London, WC2R 2I.S UK (Received
7 December
1995: revised version received 1 February 1996)
Abstract - The red colouration observed in many sand seas is a consequence of fine grained haematite and clay coatings surrounding quartz grains and a number of hypotheses have been proposed to account for this. The marked increase in dune redness with increasing distance inland in the northern area of the Namib sand sea is investigated using satellite imagery and mineral magnetic measurements. These results suggest that source materials exert at-least a partial control on dune colour in this field setting, but that evidence for the influence of other factors (e.g. climatic gradient and/or abrasion of coating during transport) is also present. RtsumC - La coloration rouge observee dans de nombreuses mers de sable est due a la presence d’hematite finement grenue ainsi qu’a des revetements argileux autour de grams de quartz. Differentes hypotheses ont ete proposees comme explication. Dans la region septentrionale de la Mer de Sable du Desert de Namib, l’augmentation nette de l’intensite de la couleur rouge avec la distance vers l’mterieur des terres est etudiee a l’aide d’images satellitaires et de mesures du magn&isme de certains mineraux. Ces resultats suggerent que dans ce contexte de terrain, la source des materiaux soit au moms partiellement a l’origine de la couleur des dunes mais que d’autres facteurs (p.ex. le gradient climatique et/au l’abrasion des revetements lors du transport) peuvent egalement jouer un role.
INTRODUCTION
coating consisting of clay minerals and the iron oxide mineral haematite, typical of ‘red bed’ type sediments (Walker, 1967; van Houten, 1973; Turner, 1980; Gardner and Pye, 1981). In the Namib sand sea, however, a number of hypotheses have been suggested (Logan, 1960; Besler, 1980; Lancaster, 1989) to explain the observed colour variation and as yet no consensus view has developed. The main hypotheses currently suggested are: i) the increasing age of the sands inland allowing greater time for weathering processes to develop the haematite coatings around quartz grains; ii) pre-existing red coatings are lost progressively due to abrasion during sand transport either as materials experience longer periods of abrasion or in zones of greater transport energy; iii) derivation from different sand source materials (either different concentrations of Fe-bearing minerals or residual cement coating); and iv) regional climatic gradients (warmer and wetter inland) providing a control on rates of weathering processes which generate the haematite coatings. The types, concentrations and behaviour of Fe oxide minerals within many environmental systems can often provide valuable insights into the way such systems function (Schwertmann and Taylor, 1989). The exact
The Namib sand sea is a large, complex dune system (Fig. l), which is thought to have been developing since the Late Pliocene (Ward, 1987). Underlying the vast majority of the contemporary sand sea is the Tsondab sandstone. The Tsondab is a red sandstone cemented with Fe oxides and CaCO, and is of Tertiary age. It is generally 50-100 m thick, but locally reaches thicknesses of over 200 m (Ward, 1987). East of approximately 15’20’ E, the Tsondab outcrops in many inter-dune areas. To the west of this, however, in the northern margin of the sand sea, the surface of the Tsondab sandstone is capped by the Karpfencliff conglomerate (a calcreted fluvial conglomerate), also of Tertiary age, and it is these materials which form inter-dune outcrops (Ward, 1987). Although a large body of geomorphological research literature already exists dealing with various aspects of the environmental history of the contemporary sand sea (e.g. Lancaster, 1989), there are many questions unanswered. One feature which had been observed by all workers in the area is the marked colour variation in the dune sand with pale coastal dunes and strongly coloured red dunes at the sand sea’s eastern margin. Previous work has shown that the colour variation was due to progressively higher concentrations inland of a 349
I. \VALDEN cf (71.
350
\
I
STUDY AREA \
:,.-,,’ ;
Walvis Bay
..
--.
, ’ ,_- ’ ’ ,’ ,:: \\ ‘~,_$obab”b ,‘9; $3 h LL.__..’
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1500
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15030
15045
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1600
27%
.oc0
zi, t z
g =
Figure 1. Location map of the Namib Desert showmg the studv area covered by the Landsat scene (see Fig. 3)
pattern of dune colour may therefore hold some basic environmental information and further investigation could prove useful. METHODS Producing an accurate map of Fe oxide concentration is difficult given the inaccessible nature of the terrain. The synoptic capability of satellite imagery has been used in other field settings to map Fe oxide concentration as a proxy for the distribution and relative concentration of Fe oxides at large spatial scales (White et al., 1992) and an approach including remotely sensed data would seem to offer considerable advantages in the current field setting. Comparing such maps with field data poses another problem due to the low concentrations of Fe oxides being measured (typically less than 1%). Mineral magnetic measurements have proved themselves to be a versatile and sensitive means of quantifying Fe oxide assemblages within soil or sediment samples (Oldfield, 1991). The spatial scales and resolutions at which each technique operates are very different and therefore have different advantages and disadvantages. With a combination of these approaches it might be possible to develop a better spatial (perhaps quantitative)
.
I
i
.
-.
‘Oj . . . *. -*:
E
oi_,-.-_
P
14930
West
14045
1500
..
. .
.: _;. - 15'15
Longitude
15030
15'45
...i
16'0
East
Figure 2. (a) Munsell Redness values, showing a gradual mcrease with dlstancc from the coast. Figure 3 shows the localities of the places shown on the transect. (b) Transect of Fe oxide concentration hased upon Landsat data showing a gradual increase with distance from the coast and the major break associated with the contact between the Karpfencliff conglomerate and the Tsondab sandstone. The data points represent image data at the field sampling locations. (c) Transect of ‘hard’ IRM values for the ‘non-magnetic’ extract showing two dishnct gradational increases in canted antiferromagnetic mineral concentrations on either side of the contact between the Karpfencliff conglomerate and the Tsondab sandstone.
awareness of the pattern itself and thus examine the various hypotheses concerning the Fe oxide distribution in the Namib sand sea. A simple Fe oxide ratio of Thematic Mapper band 3 (visible red) divided by the sum of all the visible bands (1, 2 and 3), provides crude, but rapid estimates of surficial material redness. This map was used to direct field sampling strategy (within logistical constraints imposed by limited accessibility and fieldwork time). Fieldwork was carried out in April 1993. Samples were collected from over 30 locations in a transect running along the northern margin of the sand sea. At a number of sites, several samples were collected to allow comparisons of within and between site compositional variability. All sampling sites were located using GPS, allowing their position within the satellite image to be established accurately.
351
Controls on dune colour in the Namib sand sea
iv 14’3U E
0
15”WE
15”W E
Walvis Bay
23”OO S
50 km
I
Figure 3. Monochrome dis~~lavof Fe oxide distributions based upon the processing of Landsat data. The brighter pixels contain larger q&ntities of Fe oxide (=haimkite coatings on quartz grains). ’
A series of three sub-samples from each field sample was prepared for mineral magnetic analysis. The first was a simple ‘bulk’ sample. The other two consisted of a second bulk sample split into ‘magnetic’ and ‘nonmagnetic’ fractions using a simple bar magnet. While only a crude separation was likely to be achieved, the intention was to isolate the magnetite signal (hopefully extracted by the magnet) from the haematite signal represented by the quartz grain coatings. This latter signal is essentially what the satellite sensors are able to detect and express as dune ‘redness’. Each sample was subjected to a range of standard magnetic measurements (White and Walden, 1994; Walden et al., in prep.), but here only the high field acquisition of isothermal remanent magnetisation (or ‘hard IRM) is quoted. This was calculated as the mass specific remanence acquired between fields of 300 and 1000 mT and is proportional to the concentration of canted antiferromagnets (mainly haematite and goethite) in the sample. Further image processing work was carried out after the field work was completed to refine the ‘haematite’ map. The quality of the images derived from the original Landsat scene was improved in two ways, firstly by increasing the accuracy of the image to ground correction by using more ground control points and
secondly by implementing a linear mixture model to improve the accuracy of the remote sensing estimates of haematite abundance. Mixture modelling seeks to map the proportions of scene components within each pixel of an image. It assumes additive (linear) mixing of spectrally distinct scene components, knowledge of the number and identity of scene components and the ability to characterize the spectra of pure examples of the scene components (see Settle and Drake, 1993 for a full description of the method adopted). The method improves on the band ratio estimates of haematite by using all six reflective wavelengths, by considering the spectral response of the other spectrally distinct materials in the image and by providing a method that allows for the removal of the effect of shadows. Using the mixture modelling approach, the proportions of yellow (unreddened) quartz sand, calcium carbonate, red (Fe oxide stained) quartz sand and the shadow from a Thematic Mapper image of the sand sea were mapped (path 179, row 76, 11th June, 1992). To remove the effect of the shadow a simple, but very effective technique was adopted whereby the shadow proportions were subtracted from each pixel and the remaining scene component proportions were re-calculated on a pro-rata basis.
352
I. WALDEN it al.
‘Non-magnetic’ extract samples
4
y- 1904+04* 0
0
50
100
150
‘=oiI
200
250
Image Fe-oxide value Figure 4. Plot of ‘hard’ sand samples overlying Tsondab sandstone.
IRM versus image Fe ox~dc values tar the Karpfencliff conglomerate and the
RESULTS The values of Munsell ‘redness’ (Torrent et al., 1980) made on the field samples confirmed the suggestion made by many previous workers who have observed a trend of increasing haematite coatings moving inland from the coast (Fig. 2a). Figure 3 shows the distribution of Fe oxides seen in the dune sands from the coast (ca 14”30’ E) to the inland margin of the sand sea (ca 16“O’ E) as mapped from the Landsat data. It is clear that there is a marked difference between the western and eastern portions of the sand sea. The image processing techniques used here suggest there is a major break in the Fe oxide concentrations to the east of Gobabeb at approximately 15”20’ E. The variation is therefore not simply a smooth trend, as suggested by some previous workers on the basis of field observation. This break is confirmed by Fig. 2b and occurs close to a major boundary in the underlying lithology (Ward, 1987) between the Tsondab sandstone and the Karpfencliff conglomerate. Outcrops of these lithologies are exposed in many inter-dune areas. On either side of the lithological change, however, trends can be seen in the image data. From the eastern margin of the sand sea to this geological boundary, the data show a gradual decreasing trend in Fe oxide concentrations. To the west of this boundary, the trend in Fe oxide concentration values exhibits a much more rapid decrease. The hard IRM data (Fig. 3c) also indicate the importance of the lithological boundary between the Tsondab sandstone and the Karpfencliff conglomerate and suggests that the underlying geology is exerting a strong control on the concentration of canted antiferromagnetic mineral types found within the contemporary dune sands. This control is most likely to be achieved through the provision of source materials of differing mineral composition. Other magnetic variables (Walden EL al., irz prep.) confirm these differences in all three magnetic sample sets.
As with the image data, on either side of this lithological boundary, the ‘hard’ IRM data suggests east-west trends of increasing concentrations of haematite type minerals. These trends are less obviously explainable by the mixing of different source materials and may, therefore, suggest that some or all of the alternative explanations for colour differences outlined above are also operating. As might probably be expected, a comparison of Fig. 2b and 2c suggests that the mineral magnetic and remote sensing analyses are not quantifying the ‘haematite’ concentrations within the dune sands in an identical fashion. While the remotely sensed approach is only sensitive to the high red pigmenting power of fine grained haematite coatings on quartz grain surfaces, the mineral magnetic measurements are, in addition, detecting Fe oxides in other forms (e.g. detrital grains), which have little or no influence of the overall colour of the materials. However, within the two distinct mineral magnetic provinces identified in Fig. 4, the two data sets are clearly related. This may provide a means whereby mineral magnetic measurements of field samples can be used to calibrate remotely sensed estimates of ‘Fe oxides’ (more specifically, haematite coatings in ‘red beds’) provided such data are interpreted with regard to the different sensitivities and abilities of the techniques (Walden et al., in prep.).
CONCLUSIONS These data confirm that a combination of controls is operating within the Namib sand sea to produce the marked east-west colour variation observed by many workers. The underlying lithology appears to exert a strong influence on dune colour, either by providing source materials with differing assemblages and concentrations of Fe-bearing minerals, or by providing sand eroded from Fe oxide cemented sandstone, as suggested by Besler and Marker (1979). It would also seem, however, that these minerals are then being subjected to different environmental conditions (temperature, moisture availability or wind regime) which are, in turn, imposing some sort of gradient on the production (or preservation) of haematite coatings upon the surfaces of the quartz grains from which the dunes are composed. These issues form the focus of ongoing work (White et al., in prep.; Walden et al., in prep.) These results also indicate that mineral magnetic and remotely sensed estimates of haematite concentrations in surface samples from the Namib sand sea are related and may provide complementary information on haematite distributions. As expected, however, the relationships between the data sets are not simple. While, in this case, the satellite data is primarily sensitive to a single form of Fe oxide (haematite coatings on quartz grains), the magnetic data provide a more complete account of the overall Fe oxide assemblage within the sands, albeit at a greatly reduced spatial scale.
Controls on dune colour in the Namib sand sea
Mineral magnetic analyses may therefore represent a useful adjunct to ‘Fe oxide’ maps produced solely from satellite data in other regional contexts. Furthermore, remote sensing techniques may have the potential to extrapolate ‘point’ mineral magnetic measurements over large areas, as long as the complex relationships between the different data are appreciated. Acknowledgements J. W. and K. W. would like to acknowledge the School of Geography, University of Oxford and the Environmental Science Division, University of Wolverhampton for access to some of the equipment used in this work and the Trapnell Fund for a grant in support of fieldwork costs. The authors would like to thank the following; the staff of DERU, Gobabeb, Namibia, particularly Dr. Mary Seely and Peter Jacobson, and to Derek Dutoit of Enviroteach, Namibia, for their considerable help and encouragement in the field; Professor Andrew Goudie and Frank Eckardt of the School of Geography, University of Oxford for their support, encouragement and useful discussions concerning this project; and both Graeme Sandeman of St Andrews University and Peter Hayward of Oxford University for their cartographic support. REFERENCES Besler, H. 1980. Die Dunen-Namib: entstehung und dynamik eines ergs. Stuttgarter Geographische Studien %, 208~.
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