Vol49_1_2006def 245 ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006 Key words hyperspectral – iron bearing minerals – paleoclimate 1. Introduction The Las Tablas de Daimiel Natural Park frames a lake at the head of the river Guadiana draining to the Atlantic, settled in the large plain of La Mancha in Central Spain (fig. 1). This con- tinental dry plain is subject to a Mediterranean dry, semi-arid climate with dry summer and win- ter and relatively short rainy seasons in fall and spring. Periods of drought are recurrent within Mapping of semi-arid iron bearing red sands on emerged areas around lake marshes (Tablas de Daimiel, Spain) using hyperspectral DAIS 7915 spectrometer data Asunción Riaza (1), Eduardo Garcia-Melendez (2), Mercedes Suárez (3), Andrea Hausold (4), Ulrich Beisl (4) and Harald van der Werff (5) (1) Instituto Geológico y Minero de España (IGME), Madrid, Spain (2) Área de Geodinámica Externa, Facultad de Ciencias Ambientales, Universidad de León, Spain (3) Departamento de Geología, Universidad de Salamanca, Spain (4) DLR_German Aerospace Research Establishment, Remote Sensing Data Centre, Oberpfaffenhofen, Wessling, Deutschland (5) International Institute for Aerospace Survey and Earth Sciences (ITC), Enschede, The Netherlands Abstract Wetlands are particularly sensitive environments receiving attention from the natural sciences community due to their wealth of both flora and fauna, and often considered as natural parks. In the Tablas de Daimiel (La Man- cha, Central Spain), Digital Airborne Imaging Spectrometer data (DAIS 7915) have been analyzed to map geo- logical processes on areas around the receding wetland which have never been flooded by water in the past. Sed- iments permanently exposed to the atmosphere dehydrate and oxide, developing different mineralogical associ- ations arranged on planation surfaces. Such planation surfaces are key in the geological knowledge of recent cli- mate change and landscape evolution. Progressive iron oxide/hydroxide rate and decarbonation can be spectral- ly followed on the Holocene sands framing the current marshy area. Such mineralogical changes are geological- ly registered on flat surfaces at different heights over the receding shore of the paleolake. Interacting erosion and sedimentation processes are responsible for the development of the flat morphological surfaces with increasing dryness. Maps are built for four different morphological units consisting of planation surfaces following chrono- logically the receding marsh during the last 2000 years before the present. Interactive spectral responses of min- eralogical associations are described on the imagery, field and laboratory spectra. Mailing address: Dr. Asunción Riaza, Instituto Geoló- gico y Minero de España (IGME), Rios Rosas 23, 28003 Madrid, Spain; e-mail: a.riaza@igme.es 246 Asunción Riaza, Eduardo Garcia-Melendez, Mercedes Suárez, Andrea Hausold, Ulrich Beisl and Harald van der Werff the lapse of five years. It is a wetland with a lin- ear morphology associated with alluvial river flats and hollows, resulting from the overflood- ing of the rivers Cigüela and Guadiana, joining and developing the Tablas, and the natural dis- charge of the underlying aquifer, through up- welling water at locations termed «ojos» (eyes). The study area is located at the Northern Las Tablas within the floodplain of the Cigüela Riv- er, carrying silty and evaporitic sediments from Tertiary and Triassic gypsum materials underly- ing the upstream river course. The floodplain is formed by silty and clay sediments with organic matter and gypsum. The recent sedimentological record of the quaternary deposits in the area points to recent harsh paleoenvironmental conditions (2500-2300 years before present) with diminishing humidity and lowering of groundwater levels (García An- tón et al., 1986). Intensive use of groundwater for cereal crop irrigation has produced a serious de- scent on the groundwater level and coverage, an already irreversible environmental problem. The spectral behaviour of iron bearing miner- als has long been studied because of their widescale presence on lunar rocks (Hunt et al., 1971). Limonitic alteration was one of the first features attracting the attention of geological re- mote sensing (Rowan et al., 1977; Hunt and Ash- ley, 1979; Segal, 1983; Townsend, 1987). Weath- ering processes produce the same minerals as hy- drothermal alteration processes (Buckingham and Sommer, 1983), and mask the spectral re- sponse of underlying rocks with coatings and in- ternal mineralogical transformations (Lyon, 1997). Space imagery is able to depict subtle varia- tions in the reflectance properties of desert sur- faces, which are indicative of changes in miner- alogical composition. The intensity of the red colour of sands in Namibia (Logan, 1960) has been used as an indication of age in the dunes. In Australia, the intensity of dune reddening result- ed in information on the distance from the source (El Baz and Prestel, 1980). Recent use of spec- tral response of iron bearing minerals in fluvial sediments is addressed to map the contamination of mine waste disposals (Farrand, 1997) using hyperspectral imagery. Traditional studies on la- terite soils are conducted using the relative amount of iron oxides and hydroxides (Madeira et al., 1997) in humid rainforest climate. Weathering of rock and soil surfaces respon- sible for the exterior mineral mixtures mainly Fig. 1. Geological map composed from DAIS images indicating the main morphological units not previously mapped on emerged areas overprinting the Holocene underlying sands. Geographical location of the Tablas de Daimiel wetland and natural park in Central Spain and the northeastern area within DAIS data selected for mapping. 247 Mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral DAIS 7915 data contributing to the spectral response detected by remote sensors has gained attention among the remote sensing geological community (Pontual, 1987; Amos and Greenbaum, 1989; Riaza et al., 1995, 2000, 2001; Lyon, 1997; Younis et al., 1997). Different stages on weathering products and textures have been used for relative dating of lava flows of the same chemical and miner- alogical composition with thermal infrared data (Kahle et al., 1988; Abrams et al., 1991). The mixtures of iron and clay minerals pro- duced by weathering alteration associated with posthercynian erosion paleosurfaces in the Duero Basin (Central Spain) developed under different paleoclimate have been mapped helping to mod- el sediment patterns on various stages of the evo- lution of the sedimentary basin (Riaza et al., 2000). Climate-dependent saline soils, carbon- ate, organic matter and iron oxide surfaces have been mapped along different stages of flooding and emersion in the past 2000 years using hyper- spectral data on prior work (Riaza et al., 2002). Emphasis is focused here towards the areas which have always been over the shoreline. Lakes are environments particularly sensi- tive to climate changes, both actual and paleo- climate. Open system lakes fed by groundwater and seasonal runoff show a fluctuating shore- line. The most recent sediments hosting the Tablas de Daimiel wetland are Holocene sands arranged on several morphological units de- scending towards the marshy area. On a semi- arid climate, the well ventilated and open sand deposits which are emerged, easily lose water. The iron bearing minerals decrease in hydrox- ides (goethite) to gain anhydrous ferric oxide (hematite) (Duchaufour, 1984) with the time of exposure to the atmosphere. As the wetland re- cedes, the whole sedimentological system on the riversides is demantled down towards the river. Four geomorphologic units not previously mapped can be traced through the imagery on the everemerged sands surrounding the marsh, based on their different height from the coast- line, confirmed by fieldwork and aerial photog- raphy interpretation. These units are related to terraces, eolian deposits and desiccated areas. The earlier units are topographically high, and show a bright strong brown colour to the eye be- cause of the comparatively higher hematite con- tents. Further loss of material by erosion and sedimentation produced by rain, runoff, and wind break the planation surfaces, which ap- pear as both continuous and remanent sparse en- cased surfaces showing different stages of land- scape development. 2. Data set DAIS 7915 (Digital Airborne Imaging Spec- trometer, VIS-NIR-TIR) hyperspectral Spec- trometer data were recorded in July 2000, aiming at minimum vegetation vigour and maximum soil exposure. Flight altitude was decided to en- sure the better sensor performance minimizing noise. Non-coherent noise was corrected after in- flight calibration for DAIS (Strobl et al., 1996) to remove the sensor sensitivity effects on the 79 DAIS channels. DAIS data have a spatial resolu- tion of 5 m. Two overlapping flight lines were recorded and georeferenced (Schläpfer and Richter, 2002). Different spectral imaging pro- cessing tools were tested on atmospherically cor- rected data with a Radiative Transfer Code using a midlatitude summer profile (Richter, 1996; Richter et al., 2002) and field spectra from dark and light targets. Black body radiance surface and temperature were calculated using thermal infrared channels from field temperature meas- urements on a water body (Richter, 1996; Richter and Coll, 2002). Field reflectance spectra were collected with a GER spectrometer for thematic analysis on geologically representative targets, to be used for interpretative image processing. Non- consolidated soil samples were collected at dif- ferent stages of the study of the imagery lead by image processing suggestions on sedimentol- ogy and time-dependent geomorphological processes resulting in mineralogical changes. Laboratory reflectance spectra were acquired on these samples using a Perkin-Elmer Lambda 6 spectrometer (400-2400 nm) provided with an integrating sphere using a standard of barium sulphate. Non-consolidated rock samples were dry-sieved before measurement. Munsell soil colour indexes (Munsell, 1990) were assigned in the laboratory to soil samples under a con- stant illumination source. 248 Asunción Riaza, Eduardo Garcia-Melendez, Mercedes Suárez, Andrea Hausold, Ulrich Beisl and Harald van der Werff X-ray diffraction (XRD) analysis was per- formed on selected non-consolidated rock sam- ples to confirm field observations regarding mineralogical contents. Further spectral meas- urements were made on powdered samples af- ter XRD aiming to ascertain spectral features which might appear on fine grained samples. Image processing was conducted using EN- VI (RSI, 2000) hyperspectral modules and ref- erence spectral libraries. The spectral interpretation of imagery, field and laboratory spectra was led by geological knowledge of the area and conventional geolog- ical field and laboratory survey. Interaction of all data and multidisciplinary analysis have been used throughout the study at every stage of progress. 3. Digital image processing DAIS images were processed aiming to sep- arate different sediments and minerals indicative of geological processes of palustrine, fluvial and eolian environments (fig. 1). Mosaics were com- posed with the two flight lines for synoptic view of the area, and for selection of areas of interest (fig. 1). Masks were built for densely vegetated areas using channels 17 (0.7780 nm) and 13 (0.7070 nm) for a red/near infrared ratio. Masks for water were also built using digital number 26 on channel 1 (0.4900 nm) as threshold. A false colour composite with channels 19 (0.8130 nm), 6 (0.5850 nm) and 53 (2.1330 nm) (BRG) was selected gathering information on wavelength ranges related to iron bearing minerals, and car- bonate, for a preliminary estimation of the spec- tral variability in the area. The six thermal infrared channels were used for an early evaluation of lithological variety (Riaza et al., 1998). Both Principal Compo- nents and Minimum Noise Fraction Transforms were useful to identify the main lithological re- gions indicating further hyperspectral image processing. Following field sedimentological observations suggested by previous image pro- cessing, masks were built using thresholds on selected minimum noise fraction transforms from thermal infrared DAIS channels to isolate geological units within the sands (Riaza et al., 2002). Within the areas assigned to each unit, Pixel Purity Index and n-dimensional analysis were run on the 72 VNIR DAIS channels, help- ing to understand the geological spectral mean- ing of the various statistical populations among the sands (Boardman and Kruse, 1994). 4. Laboratory spectra The Holocene red sands surrounding the Tablas de Daimiel wetland are sediments com- posed of sand with quartz as a main component. Comparison with USGS mineral spectral li- brary on individual isolated minerals present on soil was made to identify the spectral features on the laboratory spectra possibly related to the minerals present on the mixture (fig. 2a). X-ray diffraction analysis showed no presence of clays on the red sands, but rather widespread carbonate. Spectra acquired on clay size pow- der from soil samples showed the presence of carbonate on most red sands. Strong brown colour (Munsell, 1990) oc- curs on the earlier and topographically highest early morphological units rich in hematite re- sulting in deeper iron absorption in the visible and higher overall reflectance (fig. 2b). Dark brown colour on soil samples from recent units with more abundant iron hydroxide in the min- eral mixture is associated with spectral lower overall reflectance. The early topographically higher morpho- logical units rich in hematite display a smooth absorption at 850 nm because of the presence of ferric cation (Hunt et al., 1971). Hematite is al- so shown by the inflexion point at 548 nm in the visible (fig. 2b). The recent units display a less steep straight line response in the visible and lower overall reflectance, due to the more abun- dant iron hydroxide. Recent units also show a relatively emphasized shape at 2300 nm due to carbonate (Hunt and Salisbury, 1971). 5. DAIS spectral response Spectral profiles are extracted from the ar- eas finally mapped on DAIS imagery (fig. 1). The spectra shown in this chapter are highly Fig. 3a,b. a) VNIR spectral profiles from DAIS images from areas representative of morphological units on the map in fig. 1. b) TIR spectral profiles from DAIS images from areas representative of morphological units on the map in fig. 1. 249 Mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral DAIS 7915 data mixed areas both in geological and land use terms. The four morphological units are sands containing mixtures of iron bearing minerals which can be spectrally traced by relative tran- sitional mineralogical abundances produced by increasing dehydration and decarbonation with time. The sands are cultivated with vineyard crops, regularly spaced plants which cannot be spectrally removed from data with a spatial res- olution of 5 m. Crops diminish near the natural Fig. 2a,b. a) Spectra of the main minerals present in the sediments of the area of study related to dehydration and oxidation of sands exposed to the atmosphere (United States Geological Survey Spectral Library). b) Lab- oratory spectra from field non-consolidated rock samples measured with a Perkin Elmer Lambda 6 Spectrome- ter on representative morphological units. a b a b 250 Asunción Riaza, Eduardo Garcia-Melendez, Mercedes Suárez, Andrea Hausold, Ulrich Beisl and Harald van der Werff park and the land use changes to an intermittent ploughed and natural vegetation pattern, where the recent and most recent units occur. The fol- lowing spectral observations must be consid- ered trends of spectral behaviour. No single mineral influence is expected to be identified. DAIS images present a distinguishing re- sponse between once flooded and everemerged areas surrounding the wetland (Riaza et al., 2002). Spectra of all the emerged areas, whether sand deposits or continental aprons or fluvial de- posits, show depressions in the visible mainly due to the presence of iron bearing minerals (fig. 3a). The wide depression in the visible related to iron bearing minerals on red everemerged sands is more pronounced, consistent with their strong brown colour (fig. 3a). In all of the mapped units, early emerged sands display the highest overall reflectance. The areas which were once flooded are less expressive in the visible (Riaza et al., 2002) because of the presence of organic matter. The areas emerged display a shoulder be- tween 2.2-2.3 nm non-existent on the flooded areas. This might suggest abundance of clays, which is not confirmed by XRD mineralogical analysis or by sedimentological expectations. Widespread carbonate is more likely to con- tribute to this spectral feature. Four systems of reddish sands arranged on four morphological units have been spectrally identified based on different iron minerals relat- ed to variable time of exposure to the atmosphere and weather conditions. Such morphological units can be distinguished in the field and spec- tra by intensity of reddening that is directly relat- ed to age (figs. 2a and 3a). The intensity of red- ness in semi-arid environments is related to the increasing hematite contents (Duchaufour, 1984) by loss of water in the environment. Goethite de- creases in the iron bearing mineral mixtures with increasing dryness. Such mineralogical changes are spectrally recorded by the imagery. The topo- graphically upper and early morphological units show a shoulder on 0.85 nm which would be re- lated to the ferric ion from hematite (Hunt et al., 1971). This is not present on the lower and recent morphological units. The two recent morphological units display a smooth depression between 0.70-0.74 nm due to the iron hydroxide goethite (Hunt et al., 1971). Vegetation has been discarded as con- tributing to this feature, since vineyard crops are homogeneously distributed. Carbonate is also suggested on the two recent morphological units by a subtle 2.3 nm absorp- tion or weakened shape in comparison with spectra from hematitic surfaces. This absorption is not present on spectral profiles of the two ear- lier units, which have been exposed long enough to be decarbonated. Carbonate is widespread throughout the area both in emerged and flooded areas. Imagery is too noisy on the near infrared to be able to show carbonate-related absorption, which is relatively weak on the laboratory spec- tra. Spectral profiles on DAIS imagery do not de- pict a clear spectral response due to the low en- ergy available for the fourth detector on the 2000-2500 nm wavelength range, but tendencies of more abundant carbonate are suggested on the early more hydrated units. The thermal infrared channels from DAIS images display a varied spectral response for the four morphological units (fig. 3b). The re- cent units rich in hydroxides show a higher ra- diance surface at 8.58 nm decreasing compara- tively to the older units at longer wavelength ranges. Further laboratory spectral analysis is necessary to explain diversity on the spectral re- sponse. 6. Conclusions Morphological units developed over sedi- ments not previously mapped have been out- lined. Such morphological units are planation surfaces which developed during a certain peri- od of time. The four morphological units have been mapped using DAIS images, and their spectral properties described in terms of geo- logical processes resulting in different mineral mixtures. Image processing has helped to lead the identification by conventional geological survey. Further image processing produced the final map. Hyperspectral imagery has been able to dis- tinguish four morphological units descending towards a receding wetland due to different iron bearing mineral contents associated with vari- able times of exposure to the atmosphere and 251 Mapping of semi-arid iron bearing red sands around lake marshes using hyperspectral DAIS 7915 data increasingly dry weather conditions. Miner- alogical variations on iron oxides and hydrox- ides developed when sediments are emerged are easily recorded in the visible wavelength range. The older units rich in hematite and poor in carbonate depict a distinguishing convex shape in the visible with a shoulder at 850 nm due to the ferric ion from hematite. On the contrary, the younger less dehydrated units, display a smooth depression between 700-740 nm likely due to goethite, and a subtle absorption or flat- tened response at 2300 nm because of the pres- ence of carbonate. Both features are displayed by imagery and laboratory spectra. DAIS thermal infrared, particularly sensi- tive to topography, orientated the first image processing to depict spectrally variable areas. A more detailed map of the emerged morphologi- cal units has been drawn from the imagery, with further hyperspectral image processing in the 72 VNIR channels on selectively masked areas for wide geological areas outlined from Ther- mal Infrared. Acknowledgements The Fifth Framework Program, Improve- ment of Human Potential, Access to Research Infrastructures (contr. 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